Perspectives for using sweet potatoes as raw material for bioethanol production in Brazil

Perspectivas de utilização da batata-doce como matéria-prima para produção de bioetanol no Brasil

Abstract

The production of biofuels has stood out on the international scene due to its environmentally friendly attributes and potential for promoting rural development in developing countries. In Brazil, the federal program RenovaBio, corresponding to the National Biofuels Policy, aims to provide an important contribution to fulfilling the commitments determined by Brazil under the Paris Agreement, promoting the adequate expansion of biofuels in the energy matrix, with emphasis on a regular supply and ensure predictability for the fuel market, inducing gains in energy efficiency and reduction of greenhouse gas emissions in the production, commercialization, and use of biofuels. The possibility of expanding raw material sources for bioethanol production benefits Brazilian competitiveness. At the same time, given the vast varieties of raw materials available in this country, it is a challenge to discover which species would be most suitable for this chain. The Ipomoea batatas species (sweet potato) can contribute to the diversification of the energy matrix in Brazil, Latin American countries and around the world. Sweet potatoes have attracted scientific interest due to their high productivity rates, adaptability to diverse soil types, and tolerance to elevated temperatures, facilitating cultivation in flat and sloping terrains. In this sense, this work analyzes the opportunity to produce I. batatas bioethanol as a potential source to contribute to future energy supplies, especially in Brazil. Aspects such as economics, botanical description, extraction, and compositions, in addition to the production of bioethanol using sweet potatoes as raw material, are covered comprehensively throughout the manuscript.

Keywords:
biofuels; ethanol; sweet potato

Resumo

A produção de biocombustíveis tem se destacado no cenário internacional por seus atributos ambientalmente amigáveis ​​e potencial para promover o desenvolvimento rural em países em desenvolvimento. No Brasil, o programa federal RenovaBio, correspondente à Política Nacional de Biocombustíveis, visa dar uma contribuição importante para o cumprimento dos compromissos determinados pelo Brasil no acordo de Paris, promovendo a expansão adequada dos biocombustíveis na matriz energética, com ênfase no fornecimento regular e garantindo previsibilidade para o mercado de combustíveis, induzindo ganhos em eficiência energética e redução de emissões de gases de efeito estufa na produção, comercialização e uso de biocombustíveis. A possibilidade de expansão das fontes de matéria-prima para a produção de bioetanol beneficia a competitividade brasileira. Ao mesmo tempo, dada a vasta variedade de matérias-primas disponíveis neste país, é um desafio descobrir quais espécies seriam mais adequadas para esta cadeia. A espécie Ipomoea batatas (batata-doce) pode contribuir para a diversificação da matriz energética no Brasil, nos países da América Latina e no mundo. A batata-doce tem atraído interesse científico devido às suas altas taxas de produtividade, adaptabilidade a diversos tipos de solo e tolerância a temperaturas elevadas, facilitando o cultivo em terrenos planos e inclinados. Nesse sentido, este trabalho analisa a oportunidade de produzir bioetanol de I. batatas como uma fonte potencial para contribuir com o fornecimento futuro de energia, especialmente no Brasil. Aspectos como economia, descrição botânica, extração e composições, além da produção de bioetanol usando batata-doce como matéria-prima, são abordados de forma abrangente ao longo do manuscrito.

Palavras-chave:
biocombustíveis; etanol; batata-doce

1. Introduction

The production of biofuels has gained prominence around the world due to the need for alternatives to reduce emissions from non-renewable energy sources used in the transport sector (Aravindan, 2023). The rise in oil prices and their correlation with the greenhouse effect has stimulated the search for alternatives (Aravindan, 2023). Biofuels are promising, as they allow the partial or total replacement of conventional fossil fuels (Escalante et al., 2022). Reducing global carbon emissions is currently the most significant challenge facing humanity. One strategy to achieve this goal is to reduce the use of petroleum products or replace them with biofuels. Currently, several countries in Europe and others such as the United States, Brazil, India, China, Canada and Japan are developing policies to encourage the production of biofuels to replace fossil fuels (Duer and Christensen, 2010).

The use of alternative feedstocks for bioethanol production has evolved as a sustainable strategy to reduce dependence on fossil fuels and mitigate environmental impacts (Niu et al., 2024). Currently, there are four main generations of feedstocks, the first generation uses starchy or sugar-rich food crops such as corn and sugarcane, the second generation focuses on lignocellulosic biomass from agricultural and forestry residues, the third generation explores the potential of microalgae, and the fourth generation involves genetically modified organisms and carbon capture technologies (Cavelius et al., 2023). The most significant advances are in the second generation, which offers less competition with food production and takes advantage of abundant residues (Maliha and Abu-Hijleh, 2023).

Technologically, there has been progress in biomass pretreatment methods, in the engineering of more efficient fermenting microorganisms, and in the integration of steps such as simultaneous saccharification and fermentation (SSF) (Igwebuike et al., 2024). These developments aim to increase the efficiency of the process and make it economically viable. In addition to the energy benefits, the production of bioethanol from alternative raw materials contributes to the reduction of greenhouse gas emissions, promotes the recovery of waste and stimulates rural development (Ștefănescu-Mihăilă, 2016). However, there are still challenges such as the need for public incentive policies, investments in research and development and adaptation of industrial infrastructure to consolidate this technology on a large scale.

New sources of biomass are essential for bioethanol production because they offer more sustainable, economical and environmentally responsible alternatives compared to traditional feedstocks (Bušić et al., 2018). First, the use of agricultural, forestry and industrial residues (lignocellulosic biomass) reduces competition with food, unlike first-generation crops such as corn and sugarcane (Novia et al., 2025). In addition, these alternative biomasses make use of materials that would normally be discarded, contributing to the circular economy and reducing the environmental burden (Joyia et al., 2024). From an environmental perspective, these sources have a smaller carbon footprint and help mitigate greenhouse gas emissions (El-Araby, 2024). They also allow for the diversification of the bioethanol production base, promoting greater energy security and independence from unstable markets (Rajeswari et al., 2022). Another important factor is that new biomasses can be cultivated in marginal areas, not competing with fertile agricultural land, which makes it possible to expand biofuel production without compromising food security (Chen and Zhang, 2015). Furthermore, investing in new sources stimulates technological innovation in more efficient conversion processes and opens space for local production chains, strengthening regional economies (Chen and Zhang, 2015). Therefore, the incorporation of new biomasses expands the potential for bioethanol production and strengthens its environmental, economic and social benefits.

Sweet potatoes [Ipomoea batatas (L.) Lam (Convolvulaceae)] stand out as a promising raw material for bioethanol production due to their high starch and sucrose content, which are carbohydrates that are easily converted into fermentable sugars, enabling efficient conversion with less need for complex pretreatments (Jimoh and Amire, 2024). In addition, the crop has a short cycle, high productivity per hectare and low production costs, making it economically viable, especially for small producers (Srinivas, 2009). Its hardiness and adaptability to low-fertility soils and regions with water scarcity make it suitable for cultivation in marginal areas, without directly competing with food production in arable areas (Tang et al., 2022). Sweet potatoes also require fewer agricultural inputs, which reduces their environmental impact, and are compatible with commercial yeasts such as Saccharomyces cerevisiae, allowing their use in existing industrial units with few adaptations (Jacobus et al., 2021). These factors, added to its potential for use in tropical and subtropical regions, highlight sweet potatoes as a sustainable, economically and technically viable alternative to diversify and expand the base of raw materials used in bioethanol production.

Sweet potato is an important food crop in tropical and subtropical regions, being the sixth most widespread crop globally (Wang et al., 2016a). It is highly nutritious, rich in starch, and a source of carotenoids, phenolic acids, hydroxycinnamic acids and carbohydrates. In addition, it has high levels of dietary fiber, minerals, vitamins and anti-inflammatory and antioxidant compounds (Rashid et al., 2020). Sweet potato leaves are used as a source of heterogeneous biocatalysts, which are economical, ecological, recyclable and renewable, with the potential to convert waste into clean energy (Rashid et al., 2020). The synthesized catalyst is used to produce biodiesel using Scenedesmus obliquus (Turpin) (Scenedesmaceae) oil and residual soybean cooking oil (Eldiehy et al., 2022).

The use of sweet potato leaves in the production of heterogeneous biocatalysts for transesterification has emerged as a sustainable and efficient approach in the production of biodiesel (Eldiehy et al., 2022). Recent studies have shown that sweet potato leaf residues, after thermal treatment (such as calcination), can be transformed into solid catalysts with alkaline properties suitable for catalyzing the transesterification reaction of vegetable oils and residual fats (Silva et al., 2018). These biocatalysts have significant advantages, such as reusability, greater thermal and chemical stability, and reduced generation of liquid waste, compared to traditional homogeneous catalysts (Suhendy et al., 2023). In addition, the use of agricultural residues such as sweet potato leaves contribute to the valorization of agro-industrial by-products and to the promotion of circular economy practices, in line with the principles of sustainable development (Nath et al., 2023). Therefore, the use of sweet potato leaves in the production of biocatalysts represents a promising alternative for the production of biodiesel in a more environmentally friendly and efficient way (Eldiehy et al., 2022).

The use of biofuels derived from roots presents benefits in the social, environmental and economic spheres. Globally, the most consumed biofuel in the transport sector is bioethanol, which has several associated benefits, among which it is worth highlighting the reduction in oil imports and the reduction of greenhouse gas (GHG) emissions, which makes it the biofuel safest and cleanest available (Krylova et al., 2008). Bioethanol is considered the most promising alternative to replace oil, standing out as a fuel with a high rate of energy renewability (Susmozas et al., 2020; Bai et al., 2008). The combustion of ethanol causes fewer toxic substances to be emitted into the atmosphere. It can provide an 89% cut in greenhouse gas emissions - GHGs, such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (NO₂) (Susmozas et al., 2020).

The additional use of bioethanol as a transport fuel contributes to cutting CO₂ emissions in two ways: by reducing the consumption of fossil fuels and by recycling the CO₂ that is released when it is burned as fuel (Balat and Balat, 2009; Bai et al., 2008). Carbon emissions are reduced by more than 80% by replacing gasoline with ethanol. Cutting CO₂ emissions contributes to reducing the impacts of climate change. At the same time, the impacts caused by the formation of acid rain related to sulfur dioxide (SO₂) emitted during the combustion of gasoline are mitigated (Balat and Balat, 2009). Thus, bioethanol is expected to face the current global energy crisis better and improve the quality of the environment (Balat and Balat, 2009).

Brazil is recognized as one of the main global producers of bioethanol, ranking second behind the United States (USA) (Aditiya et al., 2016). In Brazil, the main raw material used to produce bioethanol is sugar cane because this crop adapts well to tropical and subtropical climates. The USA uses corn as its main raw material (Hernandez and Kafarov, 2009). Brazil has also studied and explored new sources of bioethanol, such as corn (Zea mays), cassava (Manihot esculenta), baroa potato (Arracacia xanthorrhiza), beetroot (Beta vulgaris), English potato (Solanum tuberosum), sweet potato (Ipomoea batatas), sweet sorghum (Sorghum bicolor), among other lignocellulosic, with various processing methods, acid hydrolysis with fermentation, enzymatic hydrolysis with fermentation, malting with fermentation or just fermentation, (SHF - Separate Hydrolysis and Fermentation), (SSF - Simultaneous Saccharification and Fermentation), (CBP - Consolidated Bioprocessing) and (Steam Explosion) (Ray and Naskar, 2008; Ochaikul and Suwannaposri, 2014; Wang et al., 2024; Chu et al., 2012; Rodríguez et al., 2010; Swain et al., 2013; Dar et al., 2018; Zhao et al., 2018; Tanaka et al., 2019; Suresh et al., 2020).

This review aims to structure the understanding of bioethanol production from sweet potatoes, addressing its potential benefits, advances to increase ethanol production, environmental and economic sustainability. Additionally, it offers insight into the challenges, situation and market share of bioethanol production from sweet potatoes.

2. Botanical Description of Ipomoea batatas

Sweet potatoes originate in Central America and have spread throughout the world due to their high yield potential and wide adaptability to different soils and climates (Yang et al., 2011; Santos et al., 2022). Sweet potato is a dicotyledonous plant of the Convolvulaceae family, recognized for its potential as an alternative source of bioenergy due to its high biomass production and favorable agronomic characteristics (Maino et al., 2019). Studies highlight that sweet potato has a high dry matter and starch content, making it an efficient raw material for fuel ethanol production (Silva, 2022). In addition, the crop is hardy, adaptable to different climatic conditions and soils, and has a relatively short cultivation cycle, factors that contribute to its economic viability and sustainability as a source of bioenergy (Salelign and Duraisamy, 2021; Cardoso, 2018).

Sweet potatoes propagate asexually through branches. Seeds are produced by cross-fertilization (due to the self-incompatibility mechanism), which facilitates allelic recombination under natural conditions. Furthermore, its vegetative reproduction is an advantage in breeding, as it allows the selected superior genotypes to be fixed and reproduced (Mello et al., 2021). The plant cycle is short, 120 to 150 days, with production during any time of the year, and harvests carried out manually, semi-mechanically or mechanized (Andrade Júnior et al., 2009; Mello et al., 2021). The morphological characteristics of sweet potatoes are: branches that can reach 5-10 m and the leaves of the plant are simple, alternate and wide, with varied shapes, colors and margins (Figure 1A). The shape of the leaves can be rounded, reniform, cordiform, deltoid, sagittate or stalked. The roots have different sizes and weights, reaching 2kg at 30-50cm (Figure 1B). The plant has two types of roots: absorbent roots, responsible for absorbing water and extracting nutrients from the soil, and storage roots, which are commercially important due to their high starch content and other nutrient reserves (Wang et al., 2016b; Yu et al., 2022). The size and shape of the storage roots vary mainly according to the cultivar, with the color of the root skin being white, yellowish and purple, and the color of the pulp being white (Brazlândia Branca) (Yu et al., 2022), orange (Beauregard) (Gichuhi et al., 2014), or purple (Purple Potato) (Wang et al., 2013a), with different intensities and pigmentations depending on the varieties and environmental conditions. Its bloom usually has purple interiors and white exteriors (Figure 1C). It produces a defined and biparous inflorescence with hermaphrodite flowers that undergo cross-pollination.

Figure 1
(A) Sweet potato leaves (B) root and (C) flower. Source: Prepared by the authors.

3. Sweet Potato Applications and Perspectives

Traditionally used in human and animal nutrition, sweet potatoes have stood out as a species with multiple uses. Their planting is significant in family farming due to their high nutritional content, use of simple techniques, rapid development even in nutrient-poor soils, resistance to pests and diseases, high productive potential and low production cost (Maino et al., 2019). The easy adaptation of sweet potatoes to different climatic environments allows small farmers to produce food for their subsistence, rich in carbohydrates and highly energetic. This creates opportunities to generate low income, which favors the local economy (Camargo et al., 2016; Maino et al., 2019).

Among the different cultivated genotypes, in general, sweet potatoes have in their root’s carbohydrates, beta-carotene, vitamins C, complex B and E, as well as minerals such as potassium, calcium, iron, phosphorus, magnesium, sulfur and sodium (Leonel and Cereda, 2002; Wang et al., 2016b; Gupta et al., 2024). Table 1 shows the physical and chemical properties of I. batatas. It also presents the most varied benefits to human health, as it acts as an antioxidant, hepatoprotective, anti-inflammatory, antitumor, antidiabetic, antimicrobial, antiobesity and antiaging (Vizzotto et al., 2017; Gupta et al., 2024). It has been widely explored in the manufacture of extruded products, health products, bakery products and weaning foods and supplements (Vizzotto et al., 2017; Gupta et al., 2024).

Table 1
Physicochemical properties of Ipomoea batatas (Leonel and Cereda, 2002; Wang et al., 2016b; El Sheikha and Ray, 2017; Vizzotto et al., 2017; Gupta et al., 2024).

Nutrients	Quantity	Unit (100g)
Water	81.3	g
Energy	106	kcal
Protein	1.5	g
Lipids	0.1	g
Carbohydrates	28.2	g
Dietary fiber	3	g
Calcium	63	mg
Iron	1.3	mg
Magnesium	37	mg
Phosphor	51	mg
Potassium	337	mg
Zinc	0.7	mg
Vitamin A	45	ug
Vitamin B	82.5	mg
Vitamin B1	165	mg
Vitamin B2	47.5	mg
Vitamin B5	1	mg
Vitamin B6	0.21	mg
Vitamin C	15	mg

• Source: Prepared by the authors.

The prospects for the production of biofuels such as biogas, bioethanol and biocatalysts applied to biodiesel production are very promising, driven by the growing demand for renewable and sustainable energy sources (Adewuyi, 2020). Biogas, derived from the anaerobic digestion of organic waste, stands out as an efficient solution for the reuse of agro-industrial waste, sewage and urban solid waste, in addition to contributing to the reduction of greenhouse gas emissions (Alengebawy et al., 2024). Bioethanol, particularly second-generation (2G), has been gaining prominence for using lignocellulosic biomass, such as straw, bagasse and energy crops such as sweet potatoes, which reduces competition with food production and increases the efficiency of the production process (Muktham et al., 2016).

As for biocatalysts in biodiesel production, there are significant advances in the development of heterogeneous catalysts obtained from plant waste (such as leaves, peels and ash), which offer advantages such as reuse, less waste generation and operation under milder conditions (Maroa and Inambao, 2021). The incorporation of biorefinery technologies, which integrate the production of different biofuels and value-added co-products, is also a strategic trend for the economic and environmental viability of the sector (John et al., 2025). These alternatives are aligned with the global objectives of decarbonization, circular economy and energy security, and are supported by public policies, investments in research and technological innovation. The prospects for the production of biofuels such as biogas, bioethanol and biocatalysts applied to biodiesel production are broadly promising, driven by technological advances and policies to encourage renewable energy.

The nutritional value may vary due to the different genotypes of potatoes, especially genetically improved species. The orange-fleshed potato stands out for being an important source of beta-carotene, a precursor substance of vitamin A, whose prolonged deficiency can cause blindness and produce changes in the skin, especially in children (Gichuhi et al., 2014; El Sheikha and Ray, 2017) and white-fleshed potatoes contain the highest percentages of starch (Khoury et al., 2015; El Sheikha and Ray, 2017). Purple pulp cultivars may present anthocyanins, a type of flavonoid with antioxidant action, with levels similar to those of grape, açaí, blueberry, blackberry and plum plants with the potential to become commercial sources of this substance, they are also used to produce food colorings and by the cosmetics industries (Wang et al., 2013a).

Sweet potatoes can be offered in the form of roots, branches and fresh leaves or silage in animal feed (ruminants, pigs, fish and poultry), both in natura and in silage form, the branches have a high content of crude protein and good digestibility, characteristics that allow its use in dairy cattle feed (Zabaleta et al., 2009; El Sheikha and Ray, 2017). However, tuberous roots are sources of starch, sugars and vitamins, with good digestibility but low protein content. So, replacing corn with sweet potatoes has been a great strategy for reducing costs for poultry producers, as it allows for greater production profitability, simplification in the supply of food to birds, ease of management and contribution to environmental preservation (Zabaleta et al., 2009; El Sheikha and Ray, 2017).

Due to their chemical composition, the leaves can supplement animal feed to a large extent (Ranteallo et al., 2023). The leaves are excellent sources of anthocyanins and phenolic acids, in addition to high nutritional value, as they contain B vitamins, beta-carotene, potassium, phosphorus, calcium, magnesium, iron, manganese, copper, zinc, fiber and proteins (Sun et al., 2014; Ranteallo et al., 2023). In a study carried out by Sun et al. (2014), it was found that the protein value found in the leaf (2.99g/100g of dry weight) is higher than that found in the root (1.28-2.3g/100g of dry weight), as well as the micronutrient values.

4. Agroindustry and Sweet Potatoes

Sweet potato has a highly promising potential in the context of biorefinery due to its rich starch composition, high productivity per hectare, wide climate adaptability and full use of the plant (Tedesco et al., 2023). In a biorefinery, sweet potato can be used as a raw material for the production of several value‑added products, including bioethanol, biogas, bioplastics, organic acids and biocatalysts (Rizzolo et al. 2021; Chakraborty et al. 2024). Sweet potato starch can be easily hydrolyzed into fermentable sugars, which are ideal for the production of bioethanol by fermentation processes using yeasts such as Saccharomyces cerevisiae (Duvernay et al., 2013). The residual solid fraction of the fermentation can be directed to anaerobic digestion, contributing to the production of biogas (Mussoline and Wilkie, 2015). In addition, residues such as leaves and peels have been studied as sources of heterogeneous biocatalysts, especially in the form of ash rich in minerals such as calcium, used in processes such as transesterification to obtain biodiesel (Eldiehy et al., 2022). Recent perspectives also point to the use of bioactive compounds present in sweet potatoes, such as polyphenols and carotenoids, in the food and pharmaceutical industries, expanding the concept of integrated biorefinery. Therefore, the full use of sweet potatoes not only strengthens the bioenergy chain, but also contributes to environmental sustainability and the economic viability of biorefineries.

Sweet potato has highly favorable characteristics for bioethanol production, standing out for its high starch content (20-30% on a wet basis), low lignin and excellent digestibility, which facilitates the conversion into fermentable sugars (Zhang et al., 2016a; Lima et al., 2022). The industrial process begins with the pretreatment of the roots, including washing, grinding and thermal gelatinization, a simpler step than that required for lignocellulosic biomass (Woolfe, 1992). Then, enzymatic hydrolysis of starch occurs by enzymes such as amylase and glucoamylase, under controlled conditions of pH, temperature and time, resulting in the release of glucose and maltose. Subsequent fermentation is generally carried out with yeasts such as Saccharomyces cerevisiae, in systems ranging from batch to continuous, and can last from 24 to 72 hours (Mussatto et al., 2010). After this stage, the ethanol is separated by fractional distillation and subsequently dehydrated by molecular sieves or chemical agents, obtaining anhydrous ethanol. The waste generated, such as fibers and process water, can be used as animal feed, organic compost or for energy generation, promoting a more sustainable approach (Lima et al., 2022). Specific advantages of using sweet potatoes include the high conversion of biomass into ethanol (over 100 L/fresh ton), the low processing cost and the lower need for chemical inputs in pretreatment compared to other raw materials (Zhang et al., 2016a; Woolfe, 1992).

Bioethanol production from sweet potato is influenced by a series of factors related to starch composition, efficiency of industrial processes, availability of raw material, and genetic characteristics of cultivars. Different sweet potato genotypes present significant variations in the proportion of amylose and amylopectin, with those with higher amylopectin content favoring enzymatic hydrolysis and resulting in greater conversion of fermentable sugars, increasing ethanol yield (Zhang et al., 2016b). In addition, the choice of fermentation process, such as simultaneous saccharification with fermentation (SSF), can reduce costs and improve the energy efficiency of production, making it more competitive (Mussatto et al., 2010). Raw material logistics are also critical, as roots are perishable and their transportation can increase costs, making it advisable to organize local and adapted supply chains (Lima et al., 2022). Furthermore, genetic improvement of cultivars aimed at high starch content, pest resistance and increased productivity is essential to ensure an adequate and economically viable supply (Woolfe, 1992). The combination of these factors determines the success of sweet potato as a promising raw material in the context of sustainable bioethanol production.

Several products are derived from sweet potatoes, as shown in Figure 2. The fresh roots can be cooked, roasted or fried, and when processed, the preparations are varied, from canned, in the form of purees, sweets, desserts, flour, pasta, production of chips, brown icing and others (Figure 2). Traditionally, sweet potatoes are consumed roasted, boiled or fried, in addition to being part of other recipes, such as soups, purees, cakes, sweets, and breads, among others. Faced with a change in the consumer market that increasingly seeks more practicality, the food industry has invested in the production of French fries in sticks or slices (wavy or smooth), known as chips (El Sheikha and Ray, 2017). Considering the high nutritional value, the segment of cereal bars containing sweet potato flour has grown. In a study carried out with the addition of different percentages (10, 15 and 20%) of sweet potato flour to cereal bars, it was observed that the cereal bar containing 15% sweet potato flour in its formulation presented fiber values (7.0%) and proteins (8.6%) higher than those of conventional industrialized cereal bars and obtained greater sensory preference in relation to other samples with 10 and 20% sweet potato flour in the formulation (Bezerra et al., 2015; El Sheikha and Ray, 2017). In Peru, it is common to use sweet potato flour to replace wheat for the production of bread and, like cassava when transformed into flour using practically the same processing; it can be applied for the same purpose (Antônio et al., 2011; El Sheikha and Ray, 2017).

Figure 2
Different uses of sweet potatoes. Source: Prepared by the authors.

Among the many possibilities for industrial uses of sweet potatoes is the production of biofuel from the root (Torquato-Tavares et al., 2017). Due to their tolerance to adverse climatic conditions, low cost of implementation, and low requirements regarding soil fertility and slope, sweet potatoes can be cultivated in places where more demanding crops, such as sugarcane, cannot be cultivated. From sugar and mainly due to their high energy potential, sweet potatoes have shown themselves to be very promising for ethanol production (Kłosowski et al., 2015). Sweet potatoes have a short cultivation cycle compared to sugar cane and cassava, positioning them as a quick return plant, as shown in Table 2. Sweet potato cultivation is a profitable activity for farmers (Antônio et al., 2011). Furthermore, this crop possesses the capacity to yield substantial amounts of bioethanol in contrast to other starch sources, with potential ethanol production reaching up to 160 liters per ton, surpassing that of sugar cane (Jusuf and Ginting, 2014; Kłosowski et al., 2015; Nur et al., 2023; Zhao et al., 2024). Table 2 presents the productivity of other crops.

Table 2
Comparative cultivation practices and ethanol production in other crops (Jusuf and Ginting, 2014; Kłosowski et al., 2015; Wang et al., 2020).

Cultures	Harvest time	Planting time	Ground	Propagation	Water demand (mm)	Cultivation*	Productiont/ha
Sugar cane	12-13 months	October to July	Medium to clayey texture	Sets or seedlings	1500-2500 mm/ year	Plant fertilization, irrigation systems	80
Sweet potato	4-5 months	All year	Sandy loam to clayey	Seedlings, cuttings, botanical seeds, detached leaf rooting or tissue culture.	750-1000 mm/ year	Nitrogen and potassium fertilization in top dressing	80
Cassava	14-16 months	March-September	Sandy or medium texture	Maniva or seed maniva	1000-1500 mm/ year	Quite demanding on Calcium and Magnesium.	14-21
Corn	3-6 months	September-December, February-March	Clayey, with good structure, like oxisols.	Seeds	500-800 mm/ phenological cycle	Nitrogen fertilization in top dressing	6
Sweet Sorghum	3-5 months	September-November	Various types, from heteromorphic soils to oxisols	Seeds	380-600mm/ Phenological cycle	Nitrogen fertilization in top dressing	60
Wheat	4-5 months	It depends on the planting region. Winter culture.	Clayey	Seeds	400-500mm/ Phenological cycle	Demanding in fertilization, but easy to manage.	2-5
Beet Saccharin	5-6 months	All year round (Except rainy season)	Sandy loam	Seeds	Requires little water. About 40 to 60% of sugar cane.	Demanding on fertilizer.	30-40

• Source: Prepared by the authors.

Sweet potato cultivation for bioethanol production is expanding in several countries. In Brazil, although it is still in its initial stage, research has been dedicated to the development of new technologies to improve the processing of ethanol from this raw material (Wang et al., 2020).

5. Molecular Biology

Several molecular biology studies have deepened the understanding of starch biosynthesis metabolism in sweet potato, aiming to improve the quality and yield of starch in roots (Wang et al., 2019). Recent research has identified differentially expressed genes (DEGs) between sweet potato accessions with high and low starch content, highlighting the regulation of pathways related to starch biosynthesis and plant hormones during the development of storage roots (Jiang et al., 2024). In addition, CRISPR/Cas9 technology was used to edit genes such as IbGBSSI and IbSBEII, responsible for starch synthesis, demonstrating the effectiveness of this approach in improving starch properties and developing sweet potato cultivars with desirable traits (Lin et al., 2025). Studies have also revealed that overexpression of the IbPMA1 gene results in a significant increase in starch and sucrose contents, while its suppression has the opposite effect (Uchôa et al., 2015). This gene is activated by the transcription factor IbbHLH49, which improves source-sink synergy, promoting greater starch accumulation in roots (Jiang et al., 2024). These findings provide valuable insights for genetic improvement programs and biotechnological strategies aimed at optimizing starch production in sweetpotato.

It is possible to use molecular biology tools to develop sweet potato plants with greater efficiency in starch accumulation in the roots. Techniques such as gene editing (e.g., CRISPR/Cas9) and modification of the expression of genes related to starch biosynthesis have shown promising results. For example, manipulation of the IbGBSSI (granule-bound starch synthase I) gene, involved in amylose synthesis, allowed altering the starch composition in sweet potato roots (Wang et al., 2017). Furthermore, overexpression of the IbPMA1 gene, which encodes a plasma membrane proton pump, significantly increased the accumulation of sucrose and starch in the roots. This effect was further intensified when the gene was coexpressed with the transcription factor IbbHLH49, promoting greater efficiency in carbohydrate translocation and storage (Zhou et al., 2020). Such advances demonstrate the potential of biotechnology to increase the productivity and quality of sweet potatoes as a raw material for food and industrial applications, such as bioethanol production.

There are different types of starch polymerization in different sweet potato genotypes, which directly affects their physicochemical properties and yield in industrial processes, such as bioethanol production. Starch is composed of two main macromolecules, amylose (linear chains of α-1,4-glucose) and amylopectin (branched chains of α-1,4 and α-1,6-glucose) (Bhatt et al., 2025). The proportion between these two fractions varies according to the plant genotype. Genotypes with higher amylopectin content tend to present greater gelatinization and better digestibility, favoring enzymatic hydrolysis and, consequently, conversion into fermentable sugars a crucial step in ethanol production (Song et al., 2014). On the other hand, genotypes with high amylose starch content may present lower conversion efficiency, requiring more stringent processing conditions (Zhang et al., 2012; Truong et al., 2018). Furthermore, factors such as molecular weight, degree of branching, and crystalline structure of starch vary among genotypes, affecting the hydrolysis rate and ethanol productivity. Recent research has focused on selecting or modifying genotypes with starch characteristics more suitable for industrial applications.

The different structures of starch polymers, especially the proportion and arrangement of amylose and amylopectin, significantly affect the efficiency of saccharification (enzymatic breakdown of starch into simple sugars) during the industrial processing of sweet potatoes for bioethanol production (El Sheikha and Ray, 2017). Amylopectin, because it has a highly branched and soluble structure, is more easily accessed by enzymes such as α-amylase and glucoamylase (Al-Maqtari et al., 2024). This results in faster and more efficient hydrolysis, increasing the yield of glucose, which will be converted to ethanol in the fermentation stage. Amylose, on the other hand, with its linear structure and tendency to form crystalline aggregates, is less accessible to enzymes, making its digestion difficult and reducing the saccharification rate (Zhang et al., 2012; Tester and Karkalas, 2004). In addition, the granular structure of starch, such as the size and crystallinity of the granules, also influences the process. Smaller granules with low crystallinity facilitate enzymatic penetration. Another important factor is the gelatinization rate; genotypes with starch that gelatinizes at lower temperatures require less thermal energy, reducing industrial pretreatment costs. Therefore, sweet potato genotypes with high amylopectin content, low crystallinity and lower gelatinization temperature are preferable for the bioethanol industry, as they present greater saccharification efficiency and lower energy costs.

As plantations become more expressive, different biotechnological approaches are needed for the genetic improvement of the cultivated species. Advances in the agricultural production of food and non-food products such as biofuels, chemicals and biopolymers are heavily dependent on studies in the area of molecular biology (Mortimer, 2019; Yang et al., 2022). Important advances have been obtained in relation to sweet potato metabolic regulation pathways, such as studies on Cell Wall Integrity - CWIs that determine the relationship between sucrose and apoplastic glucose and regulate related signaling pathways. CWIs also maintain sucrose concentration gradients for transport, storage, and partitioning of sugar between source and sink tissues (Chourey et al., 2006). Cell wall integrity (CWI) signaling plays a significant role in regulating sweetpotato metabolism, especially starch accumulation in tuberous roots (Jiang et al., 2024). Studies indicate that enzymes such as cell wall invertases (CWINs) convert sucrose into glucose and fructose in the apoplastic space (Baez et al., 2022). These monosaccharides are then transported into parenchyma cells by sugar transport proteins such as sucrose transporters (SUTs) and monosaccharide transporters (STPs) (Rickman et al., 2025).

This process facilitates starch accumulation in sweetpotato storage roots. Furthermore, maintaining cell wall integrity is crucial for plant growth and development. Signaling mechanisms detect changes in cell wall structure and initiate adaptive responses to maintain its integrity. These responses include modifications in cell wall composition and adjustments in cellular metabolism, allowing controlled changes in wall structure during development and in response to environmental stresses (Yang et al., 2018). Therefore, cell wall integrity signaling directly influences carbohydrate metabolism in sweetpotato by regulating the activity of enzymes and transporters involved in sugar assimilation and starch biosynthesis (Lin et al., 2025). This regulation is essential to optimize starch accumulation in tuberous roots, positively impacting sweetpotato productivity and quality for food and industrial applications.

CWI activity is highly regulated at both the transcriptional and post-transcriptional levels (Bennett et al., 2021). Anthocyanins extracted from purple-fleshed sweet potatoes have been reported to have several biomedicinal functions (Shan et al., 2014). Acylated anthocyanins are those used in the food coloring industry, where color stability is important (Bennett et al., 2021).

To explore the origins of differences in accumulation between different sweet potato varieties, Li et al., (2021) obtained the transcriptome of three lines: 'Xuzi8' with dark purple flesh, 'Xuzi6' with light purple flesh and 'Xu28' with white. The anthocyanin content in Xuzi8 dark purple flesh sweet potato was the highest, more than 19 times that of Xuzi6 light purple flesh sweet potato, and in Xu28 white flesh sweet potato it was very low. The difference in color between white and purple-fleshed sweet potatoes is due to the accumulation of anthocyanins (Li et al., 2021).

The metabolism of starch accumulation in sweetpotato roots, highlighting genetic, physiological and environmental factors that influence this process. A recent study demonstrated that prolonged soil drought significantly reduces starch accumulation in sweetpotato tuberous roots. This is due to the decrease in the activities of key enzymes, such as ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS) and starch branching enzyme (SBE), in addition to the reduction in the expression of genes related to sucrose transport and metabolism, such as IbSWEET11 and IbSUT4 (Sheng et al., 2023). Furthermore, fractional application of fertilizers has been shown to improve starch production in sweetpotato by regulating the activity and gene expression of enzymes involved in starch synthesis (Du et al., 2020).

At the molecular level, overexpression of the IbVP1 gene, which encodes a vacuolar pyrophosphatase, resulted in sweetpotato plants with higher starch content in storage roots. This was attributed to the upregulation of starch biosynthesis pathway genes, especially AGPase and GBSSI (Fan et al., 2021). Additionally, comparative transcriptomic analyses identified differentially expressed genes involved in starch biosynthesis and sucrose metabolism during sweetpotato storage root development, providing insights into the molecular mechanisms regulating starch accumulation (Zhang et al., 2017). These studies evidence that starch accumulation in sweetpotato roots is a complex process influenced by genetic factors, agricultural management practices, and environmental conditions. Understanding these mechanisms is essential for developing cultivars with higher productivity and quality, as well as optimizing agricultural practices to improve yield and efficiency in starch production.

The Kyoto Encyclopedia of Genes and Genome - KEGG databases have an extensive set of sweet potato metabolic pathways associated with the ribosome, carbon metabolism, amino acid biosynthesis, plant-pathogen interaction, protein processing in the endoplasmic reticulum, plant hormone signal transduction, starch and sucrose metabolism, purine metabolism, spliceosome and RNA transport (Li et al., 2021). KEGG pathway analysis also revealed that 56 unigenes were assigned to the flavonoid biosynthesis pathway, 8 unigenes were annotated to the flavone and flavonol biosynthesis pathway, and 7 unigenes were assigned to the anthocyanin biosynthesis pathway. In addition, assignments and analyzes of the KEGG pathway for the Differentially Expressed Genes – DEG of the three comparisons. Based on sequence homology, DEGs have been classified into three categories: biological process, cellular component, and molecular function (Li et al., 2021).

5.1. Intercropping system with sweet potatoes

In evaluating plant growth, production of sweet potato roots and corn grains in single and intercropping systems, they observed that the corn and sweet potato intercropping system promotes greater dry matter production and food production per area (Redin et al., 2021). Corn presented the highest production of dry matter, number of leaves, stem diameter and plant height when single. Sweet potato cultivars, when intercropped, showed lower dry matter and root production, while corn grain production showed no difference between production systems. In evaluating the agronomic performance of cassava and sweet potatoes intercropping in different arrangements and determining the efficient land use index, it was found that the highest productivity of sweet potatoes was in monoculture. The intercropping proved to be advantageous in relation to monoculture for promoting a value greater than 1 in the UTE index, in addition to better use of environmental resources (Beraldo Rós and São João, 2016).

The intercropping of sweet potatoes with other crops has proven to be a promising strategy for the sustainable production of biomass for bioethanol production (Tedesco et al., 2023). This practice offers several agronomic, economic, and environmental advantages. Among the main benefits is the more efficient use of natural resources, such as sunlight, water, and soil nutrients, promoting greater productivity per planted area (Tedesco et al., 2023). In addition, the intercropping contributes to the reduction of the incidence of pests and diseases, since the plant diversity in the field acts as a physical and biological barrier, hindering the spread of pathogenic organisms specific to sweet potatoes (Silva et al., 2021). The presence of legumes in the intercropping, such as cowpea or peanut, improves soil fertility through biological nitrogen fixation, reducing the need for chemical fertilizers.

There are also benefits to the physical structure of the soil, thanks to the complementary action of the root systems of the different cultivated species. The intercropping system also provides greater soil coverage, reducing erosion and weed growth, in addition to increasing moisture retention. Another positive aspect is the diversification of products, which provides economic security and greater profitability to the producer. In a broader context, crop consortium is aligned with the principles of agroecology, promoting resilient, biodiverse agricultural systems with a lower environmental impact (Altieri et al., 2015). Thus, the adoption of intercropping with sweet potato represents a viable alternative to intensify biomass production at a lower cost and greater sustainability, favoring its application as a raw material for biofuels such as bioethanol.

6. Cultivation in Succession of Sweet Potatoes with Other Crops

In work carried out to evaluate the effects of corn and Crotalaria juncea crops inoculated with Glomus clarum and the performance of sweet potato in succession, it was observed that when inoculated, C. juncea showed greater shoot biomass production and greater root productivity of sweet potatoes when compared to the cultivation preceded by corn (Souza et al., 2019). The pre-cultivation of C. juncea inoculated with the Arbuscular Mycorrhizal Fungus - AMF G. clarum brought benefits to the agronomic performance of sweet potatoes under field conditions (Souza et al., 2019). A study carried out by Goulart et al., (2021) evaluated the performance of sweet potatoes in succession to pre-cultivations with the following species: sunn hemp (C. juncea) in monoculture, sunn hemp intercropping with corn, jack bean (Canavalia ensiformis) in monoculture, sugar bean pork intercropping with corn and spontaneous vegetation. The results demonstrated that sunn hemp in monoculture showed a greater accumulation of dry biomass in two consecutive years of succession. In the second year, this treatment provided greater accumulated amounts of N, K and Mg in plant biomass. It was observed that the treatment with monoculture sunn hemp provided the best performance of sweet potatoes, reaching productivity of 19.9 t ha-¹, in the second year of the succession.

7. Sweet Potato Cultivation on Marginal Lands

Planting sweet potatoes on marginal lands represents a promising strategy for sustainable bioethanol production (Mussoline and Wilkie, 2015). This crop is notable for its adaptability to low-fertility soils and adverse weather conditions, such as drought and heat, common features in marginalized areas (Kulpinski, 2008). In addition, sweet potatoes have a relatively short growth cycle, allowing multiple harvests per year, which is advantageous for maximizing biomass production on less productive lands (Wang et al., 2013a). Sweet potatoes have a high starch content, essential for bioethanol production, and can be grown with low agricultural inputs, reducing production costs (Altieri et al., 2015). Studies indicate that industrial varieties of sweet potatoes can produce between 4,500 and 6,500 liters of ethanol per hectare, outperforming traditional crops such as corn in terms of yield per cultivated area (Silva et al., 2021). In addition, the use of marginal lands for sweet potato cultivation avoids competition with areas destined for food production, contributing to food and energy security (Silva et al., 2021). The production of bioethanol from sweet potatoes on marginal lands also offers environmental benefits, such as reducing greenhouse gas emissions and restoring degraded soils (Mussoline et al., 2017). The crop acts as a plant cover, preventing erosion and improving soil structure, which is essential for the sustainability of agricultural ecosystems. In addition, sweet potatoes can be integrated into family farming systems, promoting the economic development of rural communities and contributing to the diversification of the energy matrix with renewable sources.

Although the term “marginal land” is common, it is a relative term and should be described according to the socioeconomic context of the people and the purpose of use (Mehmood et al., 2017). Therefore, marginal lands refer to areas with degraded soils, where low productivity and profitability make them unsuitable for agricultural practices (Mehmood et al., 2017). Given this, sweet potato cultivation can be a useful strategy for the recovery of these areas and also for the production of food and biomass. The selection of appropriate varieties must be taken into consideration: resistance to adverse conditions, such as nutrient-poor soils, high salinity or water stress. There are sweet potato varieties that are better adapted to different soil and climate conditions. Before starting cultivation, it is important to evaluate soil conditions and, if necessary, carry out improvement practices, such as adding organic matter, correcting pH and controlling erosion. On marginal lands, where water availability may be limited, it is essential to implement efficient irrigation systems to ensure supply. Sweet potatoes are known for their drought resistance and can grow in conditions of low water availability. This makes it a viable option for degraded areas where water may be a limited resource (Eldiehy et al., 2022). Sweet potato is known for its drought tolerance and ability to grow with limited water availability, which is supported by several scientific studies. For example, Gouveia et al. (2019) observed that sweet potato genotypes with higher water use efficiency (WUE) performed better under water stress, maintaining higher biomass and lower stress index. In addition, Yooyongwech et al. (2017) demonstrated that foliar application of paclobutrazol in sweet potato cultivars increased tolerance to water deficit by promoting the accumulation of soluble sugars and proline, stabilizing photosynthetic pigments, and maintaining storage root yield. These studies indicate that sweet potato possesses physiological and biochemical mechanisms that confer resilience under conditions of low water availability, making it a suitable crop for drought-prone regions.

Sweet potatoes can also be a useful plant in the recovery of degraded areas, mainly due to their ecological and cultivation characteristics (Eldiehy et al., 2022). Sweet potato has several ecological characteristics that make it a highly adaptable and interesting crop, especially in contexts of food security and bioenergy production, high tolerance to abiotic stresses, high efficiency in nutrient use, vigorous growth and soil coverage, efficient vegetative multiplication, climatic versatility, short cultivation cycle (FAO, 2008; Gouveia et al., 2019; Lebot, 2010; Scott, 1992; Woolfe, 1992).

Sweet potato roots can penetrate deep into the soil, helping to stabilize it and reducing erosion. This is especially important on marginal lands where topsoil has been removed or damaged (Mehmood et. al., 2017). Furthermore, it can improve soil structure and increase soil fertility. Its roots produce organic matter that enriches the soil, making it more suitable for the growth of other plants (Sallustio et al., 2022). The main advantage of using sweet potato as a raw material for ethanol production is the high efficiency of biomass conversion into fermentable sugars, combined with a relatively short cultivation cycle (90–150 days), which allows multiple harvests per year in tropical and subtropical regions. Sweet potato has a high starch content (20–30% on a wet basis) and a low amount of lignin, which facilitates pretreatment and enzymatic saccharification, reducing operating costs and the use of chemical inputs compared to lignocellulosic biomass (Zhang et al., 2016a; Wang et al., 2018). In addition, its cultivation requires less irrigation and fertilizers, which contributes to environmental sustainability and economic viability, especially in marginal areas or areas with low agricultural suitability. This combination of characteristics favors faster and more economical ethanol production, with yields exceeding 100 liters per ton of fresh sweet potato (Sun et al., 2014).

The use of marginal areas significantly contributes to the sequestration of carbon from the atmosphere, contributing to the mitigation of climate change (Sallustio et al., 2022). It is possible to obtain technically and economically viable sweet potato production on marginal lands for bioethanol purposes, and this represents an important strategic advantage. Sweet potato is a hardy crop, with tolerance to less fertile soils, drought resistance and nutrient use efficiency, characteristics that make it adaptable to areas with low agricultural suitability, where more demanding crops would not thrive (Zhang et al., 2016b; Andrade et al., 2020). From a technical point of view, sweet potato productivity on marginal lands can be optimized through the selection of adapted genotypes, the use of practices such as intercropping, and efficient irrigation and fertilization management. Economically, this alternative reduces competition with lands destined for food production and avoids pressure on areas of high agricultural value, contributing to the sustainability of the biofuels sector (Wang et al., 2018; FAO, 2013). Furthermore, sweet potatoes have low cultivation costs, require few chemical inputs, and have high biomass productivity and starch content, which results in high ethanol yield per hectare, even under adverse conditions (Sun et al., 2014).

In the search for sustainable development, Sallustio et al., (2022) highlighted the importance of identifying and evaluating marginal lands' potential to generate considerable amounts of biomass for the production of biofuels, simultaneously reducing land use conflicts and negative ecological impacts.

8. Raw Material Processes for Bioethanol Production

Ethanol fermentation processes from starchy materials generally involve three stages: the liquefaction of starch by α-amylase and the enzymatic saccharification of low molecular weight liquefaction products such as dextrin to produce glucose and the fermentation of glucose to ethanol (Ko et al., 2020). The energy input of the first stage is estimated to be about 30 to 40% of the total energy during the production of bioethanol from starch to high temperature (about 90 °C) to pre-cook and dissolve the particles (Ko et al., 2020). The development of a simultaneous starch liquefaction, saccharification and fermentation process would reduce energy input and increase substrate utilization efficiency (Kłosowski et al., 2015; Sarian et al., 2023). Many researchers have attempted to combine the two-stage fermentation process into a single step. Co-cultivation methods have also been used, but not on an industrial scale (Cinelli et al., 2015; Sarian et al., 2023) because the two strains used in co-cultures do not always have similar culture requirements, such as pH, temperature, nutrients, and oxygen demand, among others, making it very difficult to optimize conditions for one strain without affecting other strains. Therefore, co-immobilization of different types of microorganisms within the same porous matrix by co-immobilization and combining the two-step fermentation process into a single step can reduce the energy input and solve the problem mentioned above (Castañeda-Ayarza and Godoi, 2021; Katanski et al., 2024).

9. Production of Bioethanol from Sweet Potatoes

Sweet potato bioethanol is produced from the roots, and the production process is shown in Figure 3, including the following steps: washing, grinding, hydration, pre-saccharification, saccharification, fermentation and ethanol distillation.

Figure 3
Flowchart for ethanol production from sweet potatoes (Rahmawati et al., 2019; Chou and Yang, 2020; Kim et al., 2021; Okoro et al., 2022; Komlaga et al., 2022). Source: Prepared by the authors.

Considering that sweet potatoes have a high potential for bioethanol production, there is also a great possibility of using those considered waste, such as those that do not meet commercial standards due to defects in shape, extreme sizes, insect infestations or cuts related to the harvest, which could be used for processing purposes. The color characteristics of the skin or pulp, which vary between cream, yellow, orange, purple and white, is a factor that can influence. Studies already show that sweet potatoes with white pulp are more profitable for ethanol production (Rahmawati et al., 2019). Ethanol derived from sweet potatoes is considered to be of better quality and has a higher value than sugar cane, a ton of roots can generate 160 to 180 liters of ethanol, all in four months, which is how long sweet potatoes are grown, from planting to harvesting (Udeh et al., 2024; Carvalho et al., 2023). This gives three times more than sugar cane per hectare. In a year, sweet potatoes can process 27 thousand liters, while sugar cane eight thousand liters (Hakeem, 2019; Kim et al., 2021; Chou and Yang, 2020; Udeh et al., 2024; Carvalho et al., 2023).

Juice extraction is done with a starch extraction machine where the sweet potato is ground and transformed into brans. Water is added in a proportion of twenty percent (20%) of the original volume or more, depending on the properties of the agitator equipment, submitting the mixture to heat (heating up to 70 °C) to jellyification the starch. The muddy juice is then passed through the rotary vacuum for filtration and the filtrate is concentrated in the evaporation section. It can be sent directly to the fermentation section.

During fermentation, the yeast Saccharomyces cerevisiae converts wort into ethanol (Gou et al., 2024). The sugar present in the wort is converted into ethanol, carbon dioxide and yeast biomass (Gou et al., 2024). Sweet potatoes are an economical substrate for ethanol production as they are rich in fermentable sugars and mineral elements. Wang et al. (2016a) studied the production of bioethanol from sweet potato waste, the results showed that 79.00 g/L of ethanol can be obtained from Surface Plasmon Resonance - high gravity SPRs using hydrolysis enzymatic. The concentrations of glucose and ethanol produced from sweet potato waste were the highest, the processes described have enormous potential for the production of bioethanol on an industrial scale because they are environmentally friendly, highly productive, low cost and easily manipulated (Wang et al., 2016a).

Studied the production of bioethanol from saccharification and fermentation conditions from the co‑immobilization of saccharolytic molds (Aspergillus oryzae and Monascus purpureus) with Saccharomyces cerevisiae, and observed that the immobilized yeast cells showed that at a 10% glucose YPD medium the maximum fermentation rate was 80.23% (Lee et al. 2012). Immobilization increased the ethanol tolerance of yeast cells upon co-immobilization of S. cerevisiae with A. oryzae or M. purpureus. In the co-immobilization of S. cerevisiae with mixed cultures of A. oryzae and M. purpureus in a 2:1 ratio, bioethanol production was greater (Pratiwi et al., 2022a). However, a ratio of A. oryzae and M. purpureus at 1:2 resulted in a bioethanol production rate of 4.08%. Investment costs and energy required for distillation can be minimized by rapid fermentation and high alcohol levels. To improve ethanol fermentation, immobilized cell technology is an attractive and effective means as it leads to higher cell densities. This, in turn, increases reaction rates and productivity (Pratiwi et al., 2022a). Therefore, shorter residence times and smaller reactor sizes can be employed.

Sweet potato, due to its high starch content and ease of conversion into fermentable sugars, is an excellent raw material for bioethanol production, especially when associated with efficient fermentation technologies. In this context, studies such as that of Lee et al. (2012) demonstrate promising strategies using the immobilization of yeasts and saccharolytic fungi, such as Aspergillus oryzae and Monascus purpureus, co-immobilized with Saccharomyces cerevisiae, which is widely used in the fermentation of sweet potato hydrolyzed starch. Cell immobilization increased the ethanol tolerance of yeasts and improved process efficiency, achieving a maximum fermentation rate of 80.23% in a medium with 10% glucose, with cell viability of 95.70% and a final ethanol concentration of 6%. Pratiwi et al. (2022b) observed that the 2:1 ratio between A. oryzae and M. purpureus promoted higher yield in bioethanol production compared to other tested ratios. The application of these techniques to the use of sweet potato starch can significantly increase fermentation efficiency, reducing the investment and energy costs required in distillation, due to the rapid fermentation and high alcoholic contents obtained.

10. Sweet Potato Starch Hydrolysis

Hydrolysis of sweet potato starch by A. oryzae or M. purpureus has a higher yield for a 1-day incubation using 0.3 g of dry A. oryzae mycelium, and for a 2-day incubation using 0.1-0.2 g of dry mycelium, and then gradually decreased (Sanni et al., 2022). In immobilized M. purpureus, gels with 0.05 g of dried mycelium had a maximum Y p/s of 0.57 for a 4-day incubation. This suggests that an increase in the concentration of immobilized mycelium reduces the incubation time and achieves a higher Y p/s. At Y E/s, gels immobilized with 0.1 and 0.2 g of dry mycelium achieved higher ethanol production in substrate consumption compared to 0.05 g of dry mycelium (Sanni et al., 2022).

10.1. Relationship between yeast concentration and ethanol production in co-immobilization

The effects of yeast cell concentrations during co-immobilization on ethanol yield obtained maximum ethanol production and Y E/s were 3.05% (vv⁻¹) and 0.31 for a 13-day incubation with concentration of yeast 5 × 10⁶ cells mL⁻¹, respectively (Lee et al., 2012). Bioethanol production from sweet potato can be significantly optimized by co-immobilizing yeasts and saccharolytic fungi, such as Aspergillus oryzae and Monascus purpureus, with Saccharomyces cerevisiae. This approach allows for the simultaneous performance of liquefaction, saccharification, and fermentation steps, reducing processing time and costs associated with ethanol production.

Studies have shown that the initial concentration of yeast cells during co-immobilization directly influences ethanol yield. For example, Lee et al. (2012) observed that a concentration of 5 × 10⁶ cells mL⁻¹ resulted in a maximum ethanol production of 3.05% (v/v) and a yield of 0.31 after 13 days of incubation. These results indicate that optimizing cell concentration is crucial to maximize the efficiency of the fermentation process.

Furthermore, co-immobilization with fungi such as A. oryzae and M. purpureus can improve the tolerance of yeast cells to ethanol, increasing cell viability and process productivity. Pratiwi et al. (2022b) reported that a 2:1 ratio between A. oryzae and M. purpureus resulted in superior bioethanol production, while a 1:2 ratio led to a production rate of 4.08%. These findings highlight the importance of the ratio between microorganisms in co-immobilization to optimize ethanol production.

The application of these techniques to sweet potato, which is rich in starch, can significantly increase fermentation efficiency, reducing the investment and energy costs required for distillation, due to the rapid fermentation and high alcohol contents obtained.

10.2. Effect of initial ph on ethanol fermentation in co-immobilization

The pH value is a very important factor for amylolytic enzymes as well as ethanol production (Izmirlioglu and Demirci, 2016; Dash et al., 2017). The co-immobilization of A. oryzae and S. cerevisiae showed that a maximum ethanol production of 3.05% (vv−1) could be achieved at an initial pH of 4.0., the initial pH of 4.0 gave the highest ethanol production of 3.17% (vv−1), this highest ethanol yield and production rate were achieved at an initial pH of 4.0 (Izmirlioglu and Demirci, 2016; Dash et al., 2017). From this information, simultaneous fermentation of sweet potato starch to ethanol can be achieved through co-immobilization of S. cerevisiae and A. oryzae, M. purpureus or both. The mixtures of fungi and S. cerevisiae gave the highest ethanol yields and the shortest fermentation times. It is a potential process for producing ethanol from sweet potato starch (Izmirlioglu and Demirci, 2016; Dash et al., 2017).

10.3. Ethanol distillation and dehydration

The alcohol is concentrated to 95% v/v of the fermented wort to produce ethanol (99.6% v/v) for distillation purposes. The volatile compounds present are removed at 78 °C, allowing the alcohol to be distilled at a temperature close to its boiling point. Finally, the alcohol content in °GL of distilled alcohol is established according to standard NBR 13920 (Izmirlioglu and Demirci, 2017).

11. Biotechnological Advances

11.1. Sweet potato breeding programs

The best approach to overcoming the limiting factors of ethanol production is the development of specialty sweet potato cultivars through advances in genetic modification and plant transformation. The genotype and chemical composition are the main factors influencing fermentation efficiency and ethanol yields, as described in previous research works (Kumar, 2014). These investigations determined the main characteristics that increase or decrease yield. The main components such as starch, protein and tannins related to ethanol production from sweet potatoes may be related to the genotype. It was found that genotypes with high lysine content can contain 60% more of this essential amino acid than their equivalents and similar content compared to quality protein corn (QPM) genotypes. The high protein digestibility of these strains is due to the better digestibility of kafirin, which result from the unique and highly invaginated protein bodies. In addition to the change in starch and protein digestibility, the total starch harvested per unit area is an important factor in this conversion. Therefore, the fundamental objective of sweet potato breeding programs will be to develop high yielding hybrids.

11.2. Sweet potato ethanol yield

Considering the high levels of carbohydrates present in sweet potato roots, this crop has stood out as an alternative to the use of sugar cane and corn for ethanol production (Salelign and Duraisamy, 2021). When compared to corn, rice, sorghum and banana crops, sweet potatoes can be considered one of the most efficient plants in terms of the amount of net energy produced per unit of area and per unit of time. This is due to the relatively short production cycle of 4 to 6 months, which allows two harvests per year to be obtained, in addition to the high volume of roots (Kulpinski, 2008; Tang et al., 2022). In research carried out in Tocantins, a greater advantage was observed in the use of sweet potatoes for ethanol production, where family farmers obtained yields between 4,600 liters and 10,000 liters of ethanol per hectare through the implementation of mini plants for each ton of root around 150 kg of protein-rich residue is generated that can be used in animal feed. This ethanol/feed aggregation is quite significant for family farming (Kulpinski, 2008; Tang et al., 2022). In a study to evaluate potential sources of carbohydrates for bioethanol production in the states of Alabama and Maryland (USA) in sweet potato, cassava and corn crops, it was found that for both locations, sweet potatoes produced the highest concentration of root carbohydrates (about 80%), mainly in the form of starch (about 50%) and sucrose (about 30%); compared to sugar cane sucrose of 15% to 23%, while cassava had concentrations of carbohydrates in the root (about 55%), almost entirely as starch (Kulpinski, 2008). In relation to corn, the production of carbohydrates in sweet potatoes was on average 1.5 to 2.3 times more and in cassava around 0.5 to 1.6 times, showing that in the evaluated locations I. batatas have high potential as raw material to replace corn as source of biofuels (Kulpinski, 2008).

In relation to other starchy foods, sweet potatoes have a higher content of dry mass, carbohydrates (around 85.0%, 22.4% being starch), lipids, calcium and fiber than common potatoes, more carbohydrates and lipids than yam and more protein than cassava, due to the presence of amylolytic enzymes, during storage, part of the starch is converted into fermentable sugars at a level ranging from 27 to 28%, used in the production of ethanol (Jusuf and Ginting, 2014). In a study carried out, the influence of starch content on the volume of ethanol produced in the processing of sweet potatoes was observed. It was found that the higher the starch content present in sweet potatoes, the greater the amount of ethanol produced per ton of raw material (Taborda et al., 2015). Therefore, this direct relationship between the starch content present in sweet potatoes and productivity per hectare will impact the total volume of ethanol produced per hectare, as well as the cost of ethanol production. According to Waluyo et al., (2015), when studying sweet potato cultivars for bioethanol production, starch levels were found ranging from 0.9 to 7.4 t/ha, showing that the variations depend on the environmental conditions where they were cultivated, as well as the characteristics of each cultivar evaluated, making it extremely important to select the most promising materials for the manufacture of alcohol.

With technification in production, research shows that it is possible to obtain a yield of 40 to 60 tons of sweet potatoes with 25% starch content (Taborda et al., 2015). Thus, based on a sweet potato production of 40 t/ha and a starch content of 25%, ethanol production could reach 7500 liters per hectare, a value equivalent to ethanol production from sugar cane, showing the high potential of using sweet potatoes as a raw material in the production of ethanol in areas where climatic factors are not favorable to the cultivation of sugar cane (Taborda et al., 2015). Plants with a high carbohydrate concentration and high productivity must be considered when choosing the raw material for ethanol production on a production scale (Table 3).

Table 3
Annual yield of different sources of raw material for ethanol production in a cultivation cycle (Taborda et al., 2015).

Culture	Percentage of sugar/starch	Industry yield (L of alcohol/t)	Alcohol productivity (L/ha/year)
Sugar cane	20	85	6800
Sweet potato	25	190	9300
Cassava	80	180	2700
Corn	72	420	1680
Sweet sorghum	49	60	2500

• Source: Prepared by the authors.

12. Challenges Associated with Bioethanol Production from Sweet Potatoes

The technology for producing ethanol from sweet potatoes has been gaining momentum around the world. However, it is necessary to address some central issues to make sweet potatoes a choice for biofuel production by entrepreneurs and farmers. The barriers or limitations in expanding sweet potato-based ethanol production is found in the need to increase investments in all stages of the production process (Aruwajoye et al., 2020). Furthermore, there is a need for standardization of the product to transform it into a commodity (Salelign and Duraisamy, 2021). In the trends of ethanol production based on sweet potatoes, we can see an expansion in the debate about the use of cultivable areas for food production or for fuel production (Kour et al., 2022). The limitations of the study are the lack of materials on sweet potato-based ethanol, as most of the materials found are directed to sugarcane cultivation (Cao et al., 2022).

13. Benefits of Producing Bioethanol from Sweet Potatoes

Sweet potato has potential as one of the sources of bioethanol, as it had the highest calculated yield of bioethanol per hectare (Kumar et al., 2020). The benefits of the production of ethanol based on sweet potatoes are diverse, with emphasis on the production factors, phenological cycle of 4 to 6 months, large amount of starch, its residues can be reused in different ways, generating employment and income, being viable from a small production scale, in addition to not harming the environment throughout the entire production process, elements these determinants for making the production of sweet potato ethanol an alternative to fossil fuel sources (Koçar and Civaş, 2013; Singh et al., 2022; Okoro et al., 2022). Innovation of suitable improved varieties and cultivation technologies for different agroecological systems is available in Indonesia, where three sweet potato varieties with high yield and starch content have been released (Koçar and Civaş, 2013; Singh et al., 2022). These three varieties yield > 30 t of fresh roots per hectare with starch content of 22.5%, 19.0% and 18.0%, highlighting that sweet potato bioethanol appears to have considerable potential to be used as a substitute for gasoline and diesel oil (Koçar and Civaş, 2013; Singh et al., 2022).

14. Conclusions

The production of bioethanol from sweet potatoes shows promising potential owing to their high yield per hectare, favorable physicochemical properties due to its favorable physicochemical properties, which include high starch content, sweet potato contains between 20% and 30% starch on a wet basis, which is essential for ethanol production, as starch can be converted into fermentable sugars (glucose and maltose). Low content of lignin and structural fibers, this facilitates pretreatment and enzymatic hydrolysis, as less energy and reagents are required to release fermentable sugars compared to lignocellulosic biomasses such as straw or bagasse. Good starch digestibility, sweet potato starch has a molecular structure (proportions of amylose and amylopectin) that favors enzymatic action, resulting in greater efficiency in saccharification and fermentation.

Low inhibitor content, unlike some lignocellulosic biomasses, sweet potato releases fewer inhibitory compounds (such as furfural or acetic acid) during processing, which favors the viability of fermentation. High agricultural yield, under suitable conditions, sweet potatoes can produce more than 30 tons per hectare, which guarantees good availability of raw material per cultivated area and ease of processing, as it is a root with a high moisture content and fewer recalcitrant components, processing is more direct, with lower energy consumption in pre-treatment, use of the roots considered waste and the versatile utilization of various plant parts, roots with low commercial value or outside the market standard, leaves, stem (branches), root peel and processing waste.

Both Brazil and global regions possess considerable potential for sweet potato-based bioethanol production due to their robust adaptability to drought conditions, various low-nutrient soils, and resistance to pests. Importantly, this crop offers significant economic and environmental advantages, such as, high yield per hectare, low production cost, multiple uses, use of roots outside the commercial standard, job creation, capacity for cultivation on marginal lands, low use of pesticides, reduction of greenhouse gas (GHG) emissions, high energy efficiency and short cycle and crop rotation. With a cultivation cycle of 4 to 5 months, sweet potatoes can be harvested 1 to 3 times annually, yielding higher energy outputs compared to crops such as sugar cane, beet, corn, wheat, and others.

Advanced technologies for ethanol production using genetic breeding, genetic engineering or biotechnology in sweet potatoes to create efficient cultivars and develop efficient ethanol-tolerant strains of yeast and bacteria will meet the future demand for bioenergy. Different countries' policy structures for renewable energy need to be reevaluated and revisited according to energy demand and supply. Farmers should be incentivized through adequate subsidies to cultivate sweet potatoes. These factors will significantly contribute to the consolidation of the species as a bioenergetic culture.

Acknowledgements

The authors are grateful to “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (Finance code 001), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG)” and “Programa Cooperativo sobre Proteção Florestal (PROTEF) do Instituto de Pesquisas e Estudos Florestais (IPEF)” for support.

Data Availability Statement

The entire data set that supports the results of this study was published in the article itself.

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Edited by

• Editor:
  Takako Matsumura Tundisi

Publication Dates

• Publication in this collection
  29 Aug 2025
• Date of issue
  2025

History

• Received
  22 Jan 2025
• Accepted
  20 June 2025