Bioactive, technological-functional potential and morphological structures of passion fruit albedo ( Passiflora edulis )

Agroindustrial disposal generates tons of passion fruit albedo during pulp and juice processing. The reduction of this environmental pollution potential and its use involves research such as this one, which aims to determine the bioactive compounds in the passion fruit albedo ( Passiflora edulis ), its technological, functional potential and its morphological structures, aiming at food application. The methodologies applied followed internationally accepted and recommended guidelines. The results showed that the albedo flour obtained a yield of 9.76%, and from this, a semifine powder flour was prepared (75.8%), with 4.95% moisture, 40.72% reducing sugars, water activity of 0.37, pH of 5.29, total titratable acidity of 0.99 and total soluble solids of 1.5 °Brix. In addition, the presence of bioactive compounds such as polyphenols (18.88 mg AGE/100 g), flavonoids (13.51 mg/100 g), anthocyanins (1.74 mg/100 g) and chlorophyll b (69.65 mg/100 g) was evidenced, highlighting a considerable amount of vitamin C (377.36 mg/100 g) and pectin concentration (40.5%). In addition, the presence of bioactive compounds such as polyphenols (18.88 mg AGE/100 g), flavonoids (13.51 mg/100 g), anthocyanins (1.74 mg/100 g) and chlorophyll b (69.65 mg/100 g) was evidenced, highlighting a considerable amount of vitamin C (377.36 mg/100 g) and pectin concentration (40.5%).


Introduction
Originating in tropical America, passion fruit belongs to the Passifloraceae family and the Passiflora genus.It is estimated that this genus is composed of more than 500 species, of which more than 150 are native to Brazil, considered one of the largest centers of diversity.However, only three species are more produced in Brazil and in the world: yellow passion fruit (Passiflora edulis f. flavicarpa), purple passion fruit (Passiflora edulis sims.) and sweet passion fruit (Passiflora alata).Yellow passion fruit, also known as sour or acid passion fruit, is the most cultivated fruit worldwide and is responsible for more than 90% of the production of Brazilian orchards.
The advance in production and processing in the fruit sector in Brazil for the extraction of juices, pulps and oils generates a high surplus of materials, which are considered byproducts.The passion fruit for obtaining pulp produces approximately 54 thousand tons of residues per year, such as seeds and rinds (Albuquerque et al., 2019); however, the increase in consumption increases the generation of residues, which can increase the cost of the adequate destination of the byproducts, in addition to causing negative environmental impacts (Santos et al., 2021).
In this context, research has been carried out to use byproducts (passion fruit peel albedo) as functional ingredients in the processing of bread products associated with cereal flours (corn, wheat and rice) (Nascimento et al., 2020;Maia et al., 2018;Souza et al., 2020a), development of vegetal probiotic beverage of passion fruit (Guedes et al., 2021), development of cookies (Lima et al., 2022), development of biscuit (Weng et al., 2021).The enrichment of these products with passion fruit residue flour in the research by Souza et al. (2020b) showed important minerals (calcium, sodium and potassium) in the bark composition, as well as a viable alternative for application in bakery products intended for consumption by patients with celiac disease.
In addition, passion fruit peel represents 40% to 60% of the fruit's weight and is composed of flavedo and albedo.Flavedo contains large amounts of bioactive compounds, such as flavonoid luteolin and fibers, which have the ability to reduce

Bioactive, technological-functional potential and morphological structures of passion fruit albedo (Passiflora edulis)
LDL and increase HDL levels, which are important fractions in supporting the treatment of diseases such as diabetes and obesity (Ribeiro et al., 2018a;Santos et al., 2021).The albedo (white part) contains a high content of niacin (vitamin B3), iron, calcium and high content of dietary fiber, mainly pectin (Ribeiro et al., 2018b;Faleiro & Junqueira, 2016;Santos et al., 2021).
Research has been carried out with the use of food products made from the introduction of these byproducts to add positive effects on metabolic parameters (blood glucose control, lipid profiles and antioxidant status of human tissues) (Sousa et al., 2021;Marques et al., 2016;Lima et al., 2016;Panelli et al., 2018).Thus, in view of the above, this research aimed to determine the bioactive compounds, functional-technological potential and morphological albedo structures of passion fruit (Passiflora edulis) for food applications.

Raw material
The passion fruit (Passiflora edulis) used in this research was purchased at the Pará Supply Center, located in the metropolitan region of the city of Belém (Pará, Brazil).The sample was transported in low-density polyethylene plastic bags to the Food Science Laboratory of the Federal University of Pará (Belém, Pará, Brazil).All samples were stored at 7 °C until used for analysis.

Passion fruit albedo flour obtention
Passion fruits were selected, washed in running water, sanitized with sodium hypochlorite solution (20 ppm for 15 min) and washed again to remove excess solution.Then, the albedos were separated, crushed and subjected to the drying process in an air circulation oven (TH-520, Thoth) at 55 °C and an air velocity of 1.0 m/s for 12 h.The flour was obtained by grinding the dehydrated albedo in a knife mill (Start FT 50, Fortinox) for 5 minutes.The sanitized fruits, peel, pulp, albedo and flour obtained were weighed for yield calculation, which was determined by gravimetry.

Physico-chemical analysis
The recommended methods of the Association of Official Analytical Chemists (Association of Official Analytical Chemists, 2010) were adopted to determine the moisture content, water activity (a w ), reducing sugars (RS), hydrogenic potential (pH), titratable total acidity (TTA) and total soluble solids (TSS) of fresh albedo and passion fruit albedo flour.The moisture content was determined gravimetrically by sample drying at 105 °C.A w , pH and TTS were determined by direct measurement on a digital thermohygrometer (neo Series 3TE Novasina, Labmaster-aw), benchtop potentiometer (mPA210, MS Tecnopon) and benchtop digital refractometer (1809001, Guimis), respectively.TTA was determined by the titrimetric method, while the RA content was determined by the Lane-Eynon method.All of the experiments were performed in triplicate.

Determination of the concentration of bioactive compounds
To determine the concentration of bioactive compounds, an aqueous extract was prepared according to the methodology proposed by Vieira (2011).The extract was prepared in a 1 : 20 ratio (sample: solvent) using distilled water as a solvent, homogenized for 30 min and filtered in a vacuum filtration system.

Flavonoids and anthocyanins
These compounds were determined according to the methodology described by Lees & Francis (1972) with a UV-Vis spectrophotometer (IL-592, KASUAKI) at a wavelength of 374 nm for flavonoids and 535 nm for anthocyanins.

Total polyphenols
The total polyphenol content was determined according to the Folin Ciocalteu assay, as described by Aliakbarian et al. (2011), using a UV-Vis spectrophotometer (IL-592, KASUAKI) at a wavelength of 725 nm.The results were calculated using the standard curve of gallic acid with the equation of the straight line y = 0.0017x (R 2 = 0.9966).

Chlorophylls
Chlorophylls were quantified according to the methodology proposed by Vinha et al. (2014), with readings at wavelengths of 453, 505, 645 and 663 nm in a UV-Vis spectrophotometer (IL-592, KASUAKI).Chlorophyll a and chlorophyll b were calculated using Equations 1 and 2, respectively.

Technological functional properties
The main technological functional properties of passion fruit albedo flour were evaluated based on the following analyses: water absorption capacity (WAC): determined by the methods described by Sosulski et al. (1976) and Ramos & Bora (2004, 2005).Water Solubility Index (WSI): determined according to the methodology proposed by Anderson et al. (1970) with adaptations.Oil absorption capacity (OAC): determined by a combination of the methods described by Ramos & Bora (2004, 2005) and Gong et al. (2016).Emulsifying capacity (EMC) and foam stability (FS): determined according to the methods described by Ramos & Bora (2004, 2005) and Coffmann & Garciaj (1977).Gelling property (GP): determination according to the methodology of Lawal & Adebowale (2005).

Flour morphology
The morphological surface analysis of the partially defatted granules of the Passion fruit albedo flour was performed using scanning electron microscopy (SEM).The granules were coated with a 20-nm thick Au/Pd layer for 150 s under an 85-90 µA current using a sputter coater (SC7620; Quorum Technologies Ltd, Ontario, Canada).The electron micrographs were obtained using a scanning electron microscope (model VEGA3; TESCAN USA, Inc., Warrendale, PA).

Absorption spectroscopy in the infrared region
The absorption spectra were obtained by infrared absorption spectroscopy with Fourier transformation in a spectrometer (IRPrestige-21, Shimadzu, Japan) with records in the absorption spectral frequency range from 4000 to 500 cm -1 .Sample incorporation was performed in potassium bromide (KBr) pellets with Scan 100 and resolution 4 cm -1 .All bands were analyzed using the Origin 8.0 software program.

Passion fruit albedo flour yields
The passion fruit albedo flour (PFAF) yield was 9.76% and considered relatively low due to the high albedo moisture content (89.06%) lost during the drying process.This result is superior to that found by Talma et al. (2019) for mature mesocarp flour (4.6%).
In the granulometric analysis, the data show a semifine structure with 47.5% of the mass retained in the 60 mesh sieve and 75.8% between two sieves (60 and 80 mesh) of 250 and 180 μm.Thus, PFAF is classified as a semifine powder, with particles that pass through a sieve with a mesh opening of 355 µm and, at most, 40% through a sieve with a mesh opening of 180 µm, according to the powder classification of the Farmacopeia Brasileira (Agência Nacional de Vigilância Sanitária, 2010).
Regardless of the particle size, it will have technological and functional applicability, as smaller particles allow the formation of a softer texture, that is, less hardness, while larger granules provide an increase in crispness and greater hardness (Souza et al., 2020a).However, this last characteristic can be controlled by homogenizing the size of the particles, as it occurs more in the larger particles that will be less hydrated than the smaller ones, resulting in different degrees of cooking of the product.

Physicochemical characteristics
The physical-chemical data of fresh passion fruit albedo and its flour prepared in this study are described in Table 1.
The drying process caused the loss of water and, consequently, a reduction in the moisture content (from 89 to 5%) and water activity (from 0.983 to 0.374).A higher value compared to this study was presented by Silva et al. (2019) when evaluating the water activity of the albedo of the same species in natura (0.995), but the flour dried at 70 °C of this residue presented a lower value (0.285), confirming that the increase in temperature influences the level of water reduction in this product.
According to Silva et al. (2019), the dehydration of passion fruit byproducts is extremely important for their conservation, since most of the water is free and can be used in chemical, enzymatic and microorganism growth reactions.Thus, the drying of the albedo was effective for the production of a low-perishable flour with a longer shelf life, preventing the proliferation of microorganisms in this raw material and thus ensuring against the production of toxins in the albedo pulp (Miranda et al., 2017).
The moisture content of albedo was reduced 18 times compared to fresh albedo (89.05%).This reduction was also reported in the study by Miranda et al. (2017) for the same byproduct of yellow passion fruit with 95.77% moisture, resulting in flour with 4.33% moisture.Considering the dehydrated flours of fruit byproducts in an oven, higher moisture contents were obtained in the albedo of dried yellow passion fruit at 70 °C (5.13%) (Silva et al., 2019), dried orange passion fruit peel at 60 °C (14.56%) (Reis et al., 2018), dried açaí residue at 60 °C (9.06%) (Borges et al., 2021) and dried mango peel at 60 °C (14.14%) (Chagas et al., 2020).
The moisture content in the flour (5%) is within the levels allowed for flour (15%) (m/m) according to current Brazilian legislation (Brasil, 2005).In addition, considering the humidity reached after the drying process, flour is considered a good alternative for the elaboration of food products, as it does not tend to form lumps with other ingredients, which would impair homogenization (Fernandes et al., 2008).
The passion fruit albedo flour had an average pH value of 5.293, characterizing it as a weakly acidic product; however, it retained the neutrality (pH of 6.273) of the albedo in natura.Nempeque et al. (2021) obtained inverse pH values for Tahiti lemon peel (4.26) and its flour (9.80) compared to the study.This significant difference provides the flour with greater stability by hindering the development of microorganisms, such as bacteria that generally prefer a pH close to neutrality (6.5-7.0).However, even with this change in pH, the flour is still susceptible to the development of fungi adapted to acidic pH (4.5-5.0)(Abud & Narain, 2010).
Albedo flour showed a total titratable acidity (0.99% citric acid) higher than in natura albedo (0.06% citric acid), so the application of heat was able to increase the acidity of the flour.In studies by Miranda et al. (2017) and Coelho et al. (2018), the total titratable acidity was higher than in this study for flour of yellow passion fruit byproducts, such as albedo (1.74 g/100 g of citric acid) and peel (4.65 g/100 g of citric acid).In addition to influencing the rate of microbial growth, depending on the presence and nature of acids in foods (Ribeiro et al., 2018a), pH variations also affect stability, color and flavor and allow the detection of certain acids of a nutritional nature for the body.human, such as citric acid and ascorbic acid (Miranda et al., 2017).
A higher content of total soluble solids was obtained in the flour (1.5 °Brix) compared to the albedo in natura (0.25).Machado et al. (2017) found a much higher value in yellow passion fruit peel flour (28.73 °Brix).This difference in values may be related to the degradation of starch by time and temperature applied during the drying process, resulting in soluble molecules, especially sugars.
The flour showed a high percentage of reducing sugars (40.72%) compared to the value found in albedo in natura (4.68%).In the study developed by Miranda et al. (2017), the value was much lower (1.52%) than that found in the albedo flour of the same passion fruit species.The higher amount of sugars present in the albedo flour may come from the drying process, since the temperature also has the effect of causing hydrolysis in the reducing sugars.
The passion fruit albedo flour presented good yield and physicochemical characteristics that suggest good quality and conservation of this product according to Brazilian legislation.In addition, the presence of bioactive compounds was evidenced mainly in the flour, such as traces of polyphenols, flavonoids, anthocyanins and chlorophyll b, highlighting a considerable amount of vitamin C. FAM presented fiber bundles in its morphology and relevant concentration of pectin, which are essential for gastrointestinal health.Thus, passion fruit albedo flour presented relevant substances (pectin and bioactive compounds) that combined with healthy habits can prevent and treat various chronic diseases, highlighting cardiovascular diseases.
Furthermore, the processing used in this research was able to produce a flour with different properties, characteristics and aspects that allow its high technological potential in several applications of interest to the food industry.Thus, the use of passion fruit albedo for flour production showed potentially applicable results to human health and the food industry, in addition to enabling a reduction in environmental disposal.
The albedo flour presented 40.5% calcium pectate, a value much higher than the pectin contents in the yellow (37.67%), purple (32.85%) and orange (21.55%) passion fruit peel flours (Reis et al., 2018).Yellow passion fruit dry albedo powder (30.78%) (Inayati et al., 2018) and yellow passion fruit mesocarp flour (26.6%) (Lawal & Adebowale, 2005).The higher pectin content can be explained by the different extraction methods applied, considering in the present research the use of citric acid, while in the comparison studies, tartaric acid, hydrochloric acid (0.02 N) and nitric acid were used, respectively, as well as different times and temperature processes.
Pectin present in flour can have food applications, from traditional as a gelling agent in the preparation of jams, jellies and marmalades, as well as applications to a variety of products including fruit drinks, soft drinks, dairy products, confectionery, and bakery fillings (Picot-Allain et al., 2020), providing these products with a higher content of fibers and bioactive compounds.Pectin has also been used to nanoencapsulate bioactive compounds, thereby increasing their shelf life and stability (Rehman et al., 2019).The consumption of this flour with high levels of pectin can help in the prevention and reduction of diseases, such as gastric ulcers, by protecting the mucous membranes of the gastric wall; diabetes mellitus 2, by reducing fasting blood glucose and glycated hemoglobin; dyslipidemia, by decreasing the level of triglycerides and reducing the absorption of fat and cholesterol (Abboud et al., 2019).
According to Picot-Allain et al. ( 2020) and Sundarraj & Ranganathan (2017), the quality, physicochemical characteristics and extraction yield of pectin depend on the extraction method used and are influenced by the extraction time, the type of acid, pH, temperature of the medium and even the liquid-solid ratio.

Concentration of bioactive compounds
A diet is not limited to its nutrient content but also provides other compounds that protect against oxidative stress, such as bioactive compounds, which are mainly found in plant foods such as fruits and their byproducts.Table 2 presents the values of bioactive compounds in fresh passion fruit albedo and in its oven-dried flour.
The content of phenolic compounds was six times higher in the flour (18.88 mg AGE/100 g) than in the in natura albedo (2.99 mg AGE/100 g), which suggests that dehydration caused the concentration of phenolic compounds in the albedo of yellow passion fruit, with the temperature submitted for the elaboration of this product being adequate.Bakar et al. (2018) and Sulaiman et al. (2011) explained that this increase in concentration may be related to the drying process, as the oxidative enzymes and phenolic compounds separate, causing a collapse in the vacuoles and thus increasing the availability of these compounds.In addition, hydrolytic and oxidative enzymes can be denatured by oven drying at 60 °C, allowing the preservation of phenolic compounds.
Phenolic compounds are secondary metabolites produced in fruits and are not evenly distributed in tissues (Miller et al., 2020).In yellow passion fruit, the concentrations in the peel (1061.87mg AGE/100 g) were much higher than those in the seed (346.69 mg AGE/100 g) (Reis et al., 2018).Different values when compared to our study can be attributed to the analysis method used in our research (spectrophotometry), which differed from the method used by Reis et al. (2018) (highperformance liquid chromatography).In addition, other fruits present higher amounts in their byproducts and flours than in this study, as in the skins melon (364.01 μg g -1 ) (Miller et al., 2020) and pomelo (241 mg AGE/100 g) (Rodrigues et al., 2021) as well as in the peel flours yellow passion fruit albedo (0.64 mg GAE/g) (López-Vargas et al., 2013), Tanduk banana (54.60 mg GAE/g) (Bakar et al., 2018), and mango (83.72 mg AGE/g) (Chagas et al., 2020).
In the concentration of flavonoids in albedo flour (13.51 mg/100 g), there was an increase of 14% in relation to albedo in natura in relation to albedo in natura (0.75 mg/100 g) after treatment at 55 °C for 24 h.This behavior is already expected due to the loss of moisture, even if part of the compounds is lost during drying by the action of temperature.In research on passion fruit byproducts, flavonoids were present in all parts of different species (Reis et al., 2018), as well as in yellow passion fruit albedo flour (3.18 mg RE/g) (López-Vargas et al., 2013).This lower amount than the flour in our study may be associated with losses in flavonoid content during the process, by the equipment used, during extraction, processing or storage of food, as well as influencing pH, temperature, enzymes and metal ions (Havsteen, 2002).
The anthocyanin content found was low both in fresh passion fruit (0.08 mg/100 g) and in flour (1.74 mg/100 g).This pigment was not detected by Reis et al. (2018) in fruits of the same species, while the purple passion fruit peel revealed a large amount of anthocyanins (103,686.48mg/100 g) as well as the orange passion fruit pulp, while in the other parts, it showed an insignificant amount of these compounds.
Vitamin C (ascorbic acid) is the most abundant vitamin in citrus fruits, located mainly in the peel, so these fruits are the main natural source of vitamin C. Significant values are also found in other fruit byproducts, as observed in this research.The flour presented a much higher amount (377.36 mg/100 g) than that found in fresh albedo (94.34 mg/100 g).These results were higher compared to the research by Miller et al. (2020) with melon rinds that obtained in the flour (29.3 mg/100 g) content four times higher than that found in the flavedo/albedo (18 mg/100 g).Therefore, the flour had a significant content of vitamin C, and when considering its importance as an essential nutrient for human health, it can be used as an indicator of food quality, with its antioxidant properties and protection of the organism from oxidative stress.
Although flour has low values for some bioactive compounds (anthocyanins and flavonoids) compared to other flours derived from fruit byproducts, it can still have beneficial effects on the health of consumers.Research with another genotype is necessary, as is evaluating effects such as geographic, crop year, maturation and storage conditions.There are some characteristics that can influence the content of phenolic compounds, anthocyanins, flavonoids, carotenoids and other bioactive compounds in all fruits (Cardeñosa et al., 2016).
Chlorophyll is the pigment responsible for flavedo (fruit peel) green color, represented mainly by chlorophyll a and chlorophyll b.The content of this pigment tends to be high during the fruit development period and to decrease with ripening.The mean value of the total amount of chlorophyll a (1.35 mg/100 g) is approximately twice as low as the mean value of chlorophyll b (2.16 mg/100 g) in albedo in natura.These values are opposite to those reported in the literature for fruits and vegetables, which are generally found in a 3 : 1 ratio (chlorophyll a:chlorophyll b) (Rodriguez-Jimenez, 2018).In the flour, the chlorophyll b content increased more than thirty times in relation to the in natura albedo, while chlorophyll a was not detected.This behaviour in the flour may be associated with the temperature to which this product was subjected to dehydration, since chlorophyll a is thermally more unstable than chlorophyll b (Pareek et al., 2017).In the study by Miller et al. (2020), the melon rind showed higher values than this research.The content of chlorophyll a (57.50 ± 6.27 µg/g) was almost twice as high as that of chlorophyll b (30.36 ± 4.32 µg/g).

Functional and technological properties of passion fruit albedo flour
Passion fruit albedo flour showed a water absorption capacity (WAC) of 862.4% (8.6 g H 2 O/g flour).This result is higher than that verified by Miranda et al. (2017) for passion fruit albedo flour (501.56%), by Oliveira et al. (2016) for purple passion fruit albedo flour (549.84%) and by Chagas et al. (2020) for mango peel flour (717.19%).These differences between albedo flours from the same fruit in different studies may be related to the processes (such as milling, drying, heating or extrusion cooking) to which they were submitted.These processes can modify the physical properties of the fiber matrix, also affecting the hydration properties (Elleuch et al., 2011).
In addition, the high WAC of passion fruit albedo flour is due to the high amount of soluble fiber (common in fruits such as bananas, passion fruit and grapes and their skins), mainly due to the higher pectin content (Table 1).Pectin is a potent hydrocolloid with high porosity and the ability to trap a large amount of water in its structure (Santana et al., 2017).This is a relevant feature, as it can help retain moisture, allowing the addition of more water to the dough and improving material handling, for flour applicability in products such as meat, bakery and confectionery (Silva et al., 2021).
The WSI of yellow passion fruit albedo flour was 12.5%, which is much lower than that obtained by Weng et al. (2021) for yellow (35.09%) and purple (31.73%) passion fruit peel flours.This result indicates that the soluble molecular content of the flour is lower than that of the flours from the peels of both purple and yellow passion fruit.Solubility is a characteristic related to the amount of soluble solids in the dry sample and plays a critical role in the texture, color and sensory and nutritional characteristics of the product (Yuliana et al., 2014).
The passion fruit albedo flour had an average oil absorption capacity (OAC) of 252.7%, which corresponds to 2.53 g oil per g of flour.The value found was lower than the value found in passion fruit peel flour (298%) by Duarte et al. (2017) and higher than that found for mango peel flour (86.47%) studied by Chagas et al. (2020).However, a similar value was found in studies by Miranda et al. (2017) and Borges et al. (2021) for passion fruit albedo flour (262.62%) and açaí residue flour (2.47 g/g), respectively.
Albedo flour presented a considerable OAC, possibly associated with a considerable amount of fiber.Bau et al. (2012) observed that fibers have hydration properties and the ability to retain oil.According to Mozafarpour et al. (2019), a higher ORC can be caused by an increased protein concentration caused by reduced moisture.This concentration forms smaller protein aggregates that improve the functionality of the protein, conferring binding of protein parts of the flour to the oil molecules (Santana et al., 2017).
The elaborated flour absorbs a relevant amount of oil, which acts to retain flavor and enhance the texture of foods (Santana et al., 2017).Thus, flour can be a good flavor retainer and play a role in industrialized foods, such as meat products and emulsified products (cake doughs, mayonnaise or salad dressings, soups and processed cheeses) (Porte et al., 2011).
The passion fruit albedo flour presented an emulsifying capacity (EMC) of 22.87%, and during this analysis, it was possible to verify that the FAMDE retains a large amount of water, providing a smaller amount for the formation of the emulsion.This occurrence may explain the higher value found in yellow passion fruit peel flour (35.56%) (Sohaimy et al., 2018) and defatted castanets almond flour (30.50%) (Santos et al., 2020).
The EMC and emulsion stability (ES) are properties dependent on the proteins (quality and solubility) present in the studied food.According to Shi et al. (2022), EMC and ES are determined by the ability of proteins to undergo changes in their composition and rearrangement at the air-water interface, causing the formation of a cohesive viscoelastic film by intermolecular interactions.For Damoraran & Parkin (2010), a concentration of 2 to 8% of proteins is required to form foams.However, the concentration of protein alone is not enough to form foams, as other factors can compromise these results, such as pH, temperature, particle size, and oil-to-protein ratio.
The flour produced in this research was not able to foam in a significant volume (0.068%), and consequently, it was not possible to evaluate the foam stability.Similar behavior was observed by Chagas et al. (2020) in mango peel flour.In the studies of Santana et al. (2017) and Santos et al. (2020), good foaming was observed in passion fruit peel flour (35.56%) and in defatted castanets almond flour (52.3%), respectively.This result suggests the impossibility of using passion fruit albedo flour as an ingredient in food products, such as sparkling drinks, ice cream, meringues and mice (Silva et al., 2021).
Gelation is an extremely important functional property in the application of a particular ingredient in food production.This occurrence results in the formation of a three-dimensional network of carbohydrates modified or not by thermal processes, together with partially denatured protein and lipid molecules (Adebowale & Lawal, 2003).The results of the qualitative test for the gelling capacity of the oven-dried yellow passion fruit albedo flour are shown in Figure 1.
Figure 1 shows the formation of a fragile gel in the presence of 12% flour and a resistant gel at a concentration of 14%.Therefore, this is the minimum concentration necessary to be used in the manufacture of products that depend on the formation of gel.Santana et al. (2017) found a fragile gel at concentrations of 14 to 20% for commercial passion fruit flour.On the other hand, oat and white wheat flours showed good gel formation capacity at low concentrations, which demonstrates that these flours can be used in the formulation of porridges, creams and sauces.

Morphology of passion fruit albedo flour
Scanning electron microscopy (SEM) is an instrument used to detect the surface phenomena of solid materials.The results provide information on morphology, including shape and size.The micrographs shown in Figures 2a-2b reveal the structure of the passion fruit albedo flour granules.The structural morphology of the flour (Figure 2a) reveals the presence of residual structures from the processes of grinding, drying and releasing the lipid portion, resulting in a material that looks like fibrous bundles.In general, processing was responsible for creating amorphous and fibrous structures in which it is not possible to clearly distinguish other structures in the sample.
In Figure 2b, the preservation of structuring plant parenchyma is observed, keeping the fibrillar networks in their natural state.The plant cell wall constituents with elongated structures and thin thickness perceived in micrographs are related to dietary fibers (Costa, 2004).The flour has a high content of dietary fiber; therefore, this suggests that the structures observed in the micrographs are characteristic of the existing fibers.
The structural conformations shown in these micrographs can be related to some aspects that guided the responses obtained in the functional technological potential of flour, such as CAA, CAO and gelling capacity.Miranda et al. (2017) visualized the surface of passion fruit albedo flour, with a fibrous structure in the form of overlapping cross-linked material bundles and a compacted membrane.Thus, the observed behavior confirmed that the high fiber contents potentiated the technological applications of the studied flour in CAA and CAO.
The application of SEM and the functional and technological properties made it possible to determine the structure and its respective functions.In addition, this knowledge allows a glimpse of its application possibilities in the various food industrial segments with greater efficiency, time and cost reduction.Furthermore, the nutritional and functional importance associated with pectin yield highlights the quantitative emphasis on fibers, one of the most important aids in the prevention of cardiovascular diseases, acting on cholesterol metabolism among other functions, which make it a functional ingredient.

Absorption spectroscopy in the infrared region
Other data of considerable relevance are the chemical groups present in this raw material.Figure 3 shows the Fourier transform infrared spectroscopy of passion fruit albedo flour.
The greatest highlight is located in the spectral peak in the range of 3350 cm -1 , which is characteristic of hydrogen bonds with the presence of OH compounds of hydroxyl groups frequent in D-glucose units, a reducing sugar of great energetic importance in human food (Valencia et al., 2015;Melo et al., 2015).This fact confirms the high levels found in this flour, as shown in Table 2.
A frequency range of approximately 1637 cm -1 to 1055 cm -1 is present, which is characteristic of carbonyl functional groups (C=O), methyl esters, ketones, frequent aldehydes in long-chain fatty acids and elongation (NH) of amide groups present in proteins (Araujo et al., 2021;Martinez et al., 2021).

Conclusions
The passion fruit albedo flour presented good yield and physicochemical characteristics that suggest good quality and conservation of this product according to Brazilian legislation.In addition, the presence of bioactive compounds was evidenced mainly in the flour, such as traces of polyphenols, flavonoids, anthocyanins and chlorophyll b, highlighting a considerable amount of vitamin C. The flour presented fiber bundles in its morphology and relevant concentration of pectin, which are essential for gastrointestinal health.Thus, passion fruit albedo flour presented relevant substances (pectin and bioactive compounds) that combined with healthy habits can prevent and treat various chronic diseases, highlighting cardiovascular diseases.
Furthermore, the processing used in this research was able to produce flour with different properties, characteristics and aspects that allow its high technological potential in several applications of interest to the food industry.Thus, the use of passion fruit albedo for flour production showed potentially applicable results to human health and the food industry, in addition to enabling a reduction in environmental disposal.

Figure 1 .
Figure 1.Qualitative test for the gelling capacity of oven-dried yellow passion fruit albedo flour ([-] indicates no gel formation, water and material flow occurs; [±] indicates weak gel formation, viscous material flow occurs; [+] indicates gel formation, material flow does not occur).

Figure 2 .
Figure 2. Micrographs of the surface structure of yellow passion fruit albedo flour.(a) Overview of structures and (b) highlighted view of fibers.

Figure 3 .
Figure 3. Fourier transform infrared spectroscopy of passion fruit albedo flour.

Table 1 .
Physical-chemical characteristics of fresh passion fruit albedo and its oven-dried flour.

Table 2 .
Bioactive compounds from fresh passion fruit albedo and its oven-dried flour.