Processing quality of potato tubers produced during autumn and spring and stored at different temperatures

Qualidade de processamento de tubérculos de batata produzidos no outono e na primavera e armazenados em diferentes temperaturas

RESEARCH PESQUISA

Processing quality of potato tubers produced during autumn and spring and stored at different temperatures

Qualidade de processamento de tubérculos de batata produzidos no outono e na primavera e armazenados em diferentes temperaturas

Sergio T de Freitas^I; Engil Isadora P Pereira^I; Ana Cecília S Gomez^II; Auri Brackmann^III; Fernando Nicoloso^III; Dilson A Bisognin^III,*

^IUniv. of California, Depart. of Plant Sciences, One Shields Ave, Davis, CA, 95616 - USA; [e-mail]; [e-mail]

^IIUniversity of the Republic, Depart. of Vegetable Production, Montevideo, 12900 - Uruguay; [e-mail]

^IIIUFSM-Depto. Horticultura, 97105-900 Santa Maria-RS, Brazil; [e-mail]; [e-mail]

ABSTRACT

The objective of this work was to access processing quality of potato clones (Solanum tuberosum) Asterix, SMINIA793101-3, and Missaukee cultivated during spring and autumn growing seasons and stored at 4, 8, 12, or 25ºC. Clones grown in spring had shorter dormancy than clones grown in autumn. Potato tubers grown in spring and autumn and stored at 4ºC, as well as tubers grown in autumn and stored at 8ºC had no sprouting for six months. Among clones grown in autumn, the longest dormancy period during storage at 12ºC was observed in the clone SMINIA793101-3, and at 25ºC was in the clone Asterix. In potato tubers grown in spring, the longest dormancy period during storage at 8ºC was observed in the clones SMINIA793101-3 and Missaukee, at 12ºC was in the clone SMINIA793101-3, and at 25ºC was in the clones Asterix and Missaukee. Potato tubers grown in spring had higher dry matter, starch content, and respiration rates, and lower reducing sugar, and total polyphenol content. The only exception was the Missaukee that had similar dry matter content in both growing seasons. In general, storage at 4ºC and 8ºC were more effective in reducing respiration rates and maintaining higher dry mater content of potato tubers. Chip darkening, reducing sugars, and total polyphenol content increased during storage of potato tubers. Darker chip color, higher reducing sugars and polyphenol content were obtained during storage at temperatures of 4ºC and 8ºC. These results suggest that the best storage temperature depends on genotype, tuber physiological age, and growing conditions.

Keywords:Solanum tuberosum, dry matter, respiration, chip color, reducing sugars, polyphenols.

RESUMO

O objetivo deste trabalho foi determinar a qualidade de processamento dos clones de batata (Solanum tuberosum) Asterix, SMINIA793101-3, e Missaukee cultivados durante outono e primavera e armazenados a 4, 8, 12, ou 25ºC. Clones produzidos na primavera tiveram menor período de dormência que clones produzidos no outono. Tubérculos produzidos na primavera e outono e armazenados a 4ºC, assim como tubérculos produzidos no outono e armazenados a 8ºC não apresentaram brotação por seis meses. Entre os clones cultivados no outono, o maior período de dormência durante o armazenamento a 12ºC foi observado no clone SMINIA793101-3, e a 25ºC foi no clone Asterix. Em tubérculos produzidos na primavera, o maior período de dormência durante o armazenamento a 8ºC foi observado nos clones SMINIA793101-3 e Missaukee, a 12ºC foi no clone SMINIA793101-3, e a 25ºC foi nos clones Asterix e Missaukee. Tubérculos produzidos na primavera continham maiores teores de massa seca e amido, maiores taxas respiratórias, e menores concentrações de açucares redutores e polifenóis totais. A única excessão foi o clone Missaukee que apresentou teores de matéria seca similares em ambas as épocas de cultivo. De modo geral, as temperaturas de armazenamento, 4ºC e 8ºC, foram mais efetivas na redução da respiração e na prevenção da redução dos teores de matéria seca de tubérculos. Escurecimento de chip, concentração de açúcares redutores e polifenóis totais aumentaram durante o armazenamento dos tubérculos. Os maiores valores de escurecimento de chip, açúcares redutores e polifenóis totais foram obtidos com o armazenamento a 4ºC e 8ºC. Os resultados sugerem que a melhor temperatura de armazenamento depende do genótipo, idade fisiológica do tubérculo, e condições de cultivo.

Palavras-chave:Solanum tuberosum, matéria seca, respiração, cor de chip, açucares redutores, polifenóis.

Processing quality of potato tubers is determined by high dry matter, and low reducing sugar and phenol contents (Kadam et al., 1991; Abong et al., 2009). High dry matter content increases chip yield, crispy-consistency, and reduces oil absorption during cooking (Pedreschi et al., 2005; Rommens et al., 2010). Low reducing sugars and phenol contents are required to avoid dark color and bitter taste of processed products, which negatively affect consumer acceptance (Wiltshire & Cobb, 1996; Wang-Pruski & Nowak, 2004).

Dry matter of potato tubers and chip color are genetically controlled and influenced by environmental conditions during growing season and storage temperature (Kawchuk et al., 2008). Potato tubers grown in spring have higher dry matter content and shorter dormancy than tubers grown in autumn (De Freitas et al., 2006; Bisognin et al., 2008a). High storage temperatures hasten the reduction in sprout inhibitors and the increase in sprout promoters (Bhargava, 1997). Once sprouting is started, high rates of starch hydrolysis and reducing sugars accumulation are required to supply carbon and energy used for sprout growth and development, which negatively affect tuber processing quality by reducing dry matter and increasing dark color of potato chips (Wiltshire & Cobb, 1996; Bisognin et al., 2008a). Storage temperatures lower than 6ºC increase the activity of the enzymes that convert starch into fructose and glucose (Matsuura-Endo et al., 2004). Since the respiration rate and sprouting are inhibited by low temperatures, the consumption of reducing sugars is low and their content accumulates in the tubers, resulting in dark chip color (Coelho et al., 1999; Chapper et al., 2004; Knowles et al., 2009). In spite of apparent differences in storage behavior, the actual processes leading to reducing sugar accumulation are likely to be the same for different cultivars (Hertog et al., 1997).

In the southern region of Brazil, potato tubers can be cultivated in spring and autumn (Bisognin et al., 2008a). During spring growing season, the average environmental temperature, photoperiod, and irradiation increase from 15ºC, 12 hs, and 4.11 MJm²/day to 22ºC, 15 hs, and 24 MJm²/day, respectively (Bisognin et al., 2008b). During the autumn growing season, the average environmental temperature, photoperiod, and irradiation decrease from 28ºC, 13 hs, and 19.9 MJm²/day to 12ºC, 11 hs, and 4.63 MJm²/day, respectively (Bisognin et al., 2008b). These opposite environmental conditions during spring and autumn growing seasons can potentially result in different processing quality of potato tubes during storage at different temperatures.

The objective of this work was to access the processing quality of potato clones Asterix, SMINIA793101-3, and Missaukee cultivated during spring and autumn growing seasons and stored at different temperatures.

MATERIAL AND METHODS

Tubers of the potato (Solanum tuberosum) clones Asterix, SMINIA793101-3, and Missaukee (Douches et al., 2010) were produced in the field in autumn (February to July) and spring (August to December) growing conditions of Julio de Castilhos (29º13'37"S and 53º40'54"W), southern region of Brazil. Asterix and SMINIA793101-3 were chosen because of adaption to the environmental conditions, and Asterix is also widely cultivated in the southern region of Brazil. Missaukee was used as non-adapted clone that have high processing quality attributes (Douches et al., 2010). Seed production and crop management followed standard technical recommendations used in the region (Bisognin, 1996). After harvest, tubers were cured at 20ºC and 85±5% relative humidity for 15 days. Tubers of each clone were then randomized and stored at 4, 8, 12, and 25ºC with 85±5% relative humidity. During storage, temperature and relative humidity were constantly monitored with an automated system (Fockink, Panambí, Brazil). The treatments were combinations among three clones x two growing seasons x four storage temperatures. The experiment followed a completely randomized design, with four replications per treatment and ten tubers per replication.

Number of days to reach one hundred percent sprouting was determined by counting the number of days that each replication required to reach one hundred percent sprouting (ten sprouted tubers). Dry matter, respiration rate, chip color, reducing sugars (glucose + fructose), total polyphenols, and starch content were analyzed at 0, 90, and 180 days of storage. The dry matter content was quantified in tubers dried in a forced air circulation oven at 60ºC, until constant weight. The respiration rate was calculated based upon the CO₂ production of 500 g of potato samples sealed inside a 1 L container for about 5 h. The CO₂ concentration was determined by a CO₂ analyzer (Agri-Datalog, Italy). Chip color was determined on ten potato slices (2 mm thick) per replication. After being cut, potato slices were fried in hydrogenated vegetable oil at 185ºC until ceasing oil bubbling. Chip color was then measured with a portable colorimeter (Minolta, CR310) through a CIE L*a*b* system, twice each replication. The values were expressed as L*, ranging from the black to white (higher values = lighter color). Reducing sugars were analyzed as described by Ross (1959). Briefly, 1 g of powdered dry tuber tissue was vortexed during 45 s with 5 mL of distilled water and centrifuged at 2000 x g for 10 min. An aliquot of 2 mL of the supernatant was added to 0.5 mL of 2,4-dinitrophenol solution (0.038 M). The samples were incubated at 65 to 70ºC for 6 min and then cooled under running water. Reducing sugar concentrations were estimated at 600 nm spectrophotometrically. The total polyphenols concentration was determined using a 0.2 mL aliquot from the supernatant extracted for the reducing sugar quantification, which was added to 1 mL of Folin (10%) with 0.8 mL of Na₂CO₃ (7.5%), according to Singleton & Rossi (1965). For starch quantification, 250 mg of powdered dry tuber samples were autoclaved for hydrolyzation in a solution of 10 mL of distilled water and 0.5 mL of concentrated HCl during 20 min at 120ºC (1 kgf/cm²). After cooling, the samples were then neutralized with NaOH (50%) to pH=7.0. Then, 1 mL of the neutralized solution was added to 3 mL of distilled water and 0.5 mL of 2,4-dinitrophenol solution (0.038 M). After vortexing, starch concentrations were estimated based on the glucose concentration measured at 600 nm spectrophotometrically (Ross, 1959). Data were submitted to analysis of variance and treatment means compared by Tukey's test (p<0.05) or presented as mean ± standard error.

RESULTS AND DISCUSSION

The individual and combined effects of clone, growing season, and storage temperature were statistically significant (p-value <0.001) for all parameters evaluated, with the exception of the clone versus storage temperature interaction for the variable total polyphenols (p-value = 0.354). Tubers stored at 25ºC had about 40% decay and all tubers were senescent by 120 days of storage. Therefore, data from tubers stored at 25ºC for 150 and 180 days were not considered in the analysis.

Tubers grown in spring had shorter dormancy than tubers grown in autumn when stored at 8 and 12ºC (Figure 1). Tubers grown in spring and autumn and stored at 4ºC, as well as tubers grown in autumn and stored at 8ºC had no sprouting after six months of storage. In both growing seasons, higher storage temperatures resulted in shorter tuber dormancy. Among clones grown in autumn, the longest dormancy period during storage at 12ºC was observed in the clone SMINIA793101-3, and at 25ºC was in Asterix. In potato tubers grown in spring, the longest dormancy period observed in storage at 8ºC was in SMINIA793101-3 and Missaukee, at 12ºC was in SMINIA793101-3, and at 25ºC was in Asterix and Missaukee. These results are in agreement with previous studies showing that lower storage temperatures can extend dormancy, which is also determined by the growing conditions (Wiltshire & Cobb, 1996; Bisognin et al., 2008a). In that case, determining the best storage temperature according to the growing season and processing time is important for extending tuber dormancy and obtaining the highest potato processing quality.

Asterix and SMINIA793101-3 tubers showed the highest dry matter content in spring and the lowest in autumn (Table 1). Missaukee had similar dry matter content in both growing seasons. All three potato clones grown in spring had higher starch content than potato clones growing in autumn (Table 1). SMINIA793101-3 showed higher starch content than the other clones in both growing seasons. These results could be explained by the fact that increasing photoperiod, temperature and irradiance with plant development during spring can enhance carbohydrate accumulation in the tubers (Bisognin et al., 2008b). The clone Missaukee showed similar dry matter content in both growing seasons, possibly due to a lack of adaptation to higher temperatures during tuber growth and maturation under spring conditions. Moreover, the concentration of starch content observed in spring might not have been greatly affected the dry matter, since the tuber starch content of Missaukee was only 43% of the dry matter content. Therefore, similar values of dry matter content observed in the clone Missaukee in both growing seasons might be a result of the reduction in other dry matter components such as sugars, nitrogen compounds, lipids, organic acids, phenols, minerals, and non-starch polysaccharides (Kita, 2002). Considering that dry matter content is determined on tuber fresh weight (dry weight + water) basis, it is also possible that the similar dry matter content observed in the clone Missaukee in both growing seasons might be a result of different tuber water content between growing seasons.

The dry matter and starch contents decreased during storage for all potato clones produced in both growing seasons (Figure 2). Lower storage temperatures (4 and 8ºC) tended to be more effective in maintaining dry matter and starch contents of the potato tubers than higher temperatures (12ºC and 25ºC). It is known that higher temperatures increase metabolism, respiration and physiological aging of potato tubers, resulting in the observed earlier sprouting and higher carbohydrate consumption (Wiltshire & Cobb, 1996). The direct relationship between dry matter and starch contents is expected, since starch usually represents most of the dry matter in potato tubers (Liu et al., 2003). In this work, the average starch content after 180 days of storage varied from 34.8% of the dry matter in the tubers of the clone Missaukee to 36.2% in the clone SMINIA793101-3 produced in autumn. However, when produced during the spring season they had starch average values of 42.9% and 52.7% of the dry matter, respectively. Previous works have shown that starch comprises 40 to 80% of the dry weight of tubers, depending on genetic and environmental factors (Kadam et al., 1991). All three clones produced in spring had shorter dormancy, which accelerated the reduction of tuber dry matter during storage at higher temperatures. When potato clones were produced in autumn, the dormancy period was longer, and the effect of increasing storage temperatures on dry matter consumption was lower. The clone Asterix showed similar trend in the reduction of dry matter content during storage in both growing seasons, which can be explained by the fact that different growing conditions had lower effect on Asterix tuber dormancy, compared to the other clones.

Tubers of all three clones grown in spring had higher respiration rates than tubers grown in autumn (Table 1). In both growing seasons, the lowest respiration rates were observed in Asterix tubers. However, in autumn, the respiration rates were similar between Asterix and SMINIA793101-3 tubers. Lower storage temperatures of 4 and 8ºC resulted in lower respiration rates in both growing seasons (Figures 3A and 3B). Respiration rates increased markedly during storage of the tubers at 25ºC (Figures 3A and 3B). Tubers produced in spring had higher increase in respiration rates during storage in response to higher temperatures than tubers produced in autumn (Figures 3A and 3B). These results may be explained by the fact that the faster physiological aging of the tubers produced during spring required higher metabolic activity for carbohydrate metabolism and energy production to support sprout growth (Wiltshire & Cobb, 1996; Geigenberger & Fernie, 2007), which was accelerated by higher storage temperatures. It is well known that storage temperature is an important factor to minimize postharvest losses during storage of potato tubers. Respiration rates of potato tubers have been reported to have a minimum at about 4ºC and increase at higher temperatures (Wustman & Struik, 2007). However, temperatures lower than 6ºC may result in lower processing quality of potato tubers due to accumulation of reducing sugars (Coelho et al., 1999; Knowles et al., 2009). In addition, different genotypes have different regulation of reducing sugar accumulation associated with tuber sprouting and senescence in response to high temperatures (Wiltshire & Cobb, 1996). Therefore, to keep high processing quality of potato tubers during storage, it is necessary to find a suitable temperature that minimizes losses and reducing sugars accumulation.

 

All potato clones grown in spring had lower reducing sugars and polyphenol contents than potato clones grown in autumn (Table 1). The lightest chip color and the lowest reducing sugar content were observed in the clone Missaukee in both growing seasons (Table 1). Clones Missaukee and SMINIA793101-3 did not differ statistically in reducing sugar content in both growing seasons. Clones SMINIA793101-3 and Missaukee had lighter chip color when produced in autumn and Asterix showed lighter chip color when produced in spring (Table 1). Clones SMINIA793101-3 and Missaukee showed the lowest polyphenol content in both growing seasons. Chip color results from spring and autumn growing seasons showed that darkening was associated with higher reducing sugars content in the tubers, which is in agreement with previous studies (Rodriguez-Saona & Wrolstad, 1997; De Freitas et al., 2006). Reducing sugars are substrates for the Maillard reaction, which produces pigments known as melanoidins with brown color and bitter taste (Wiltshire & Cobb, 1996). The darkest chip color obtained in Asterix tubers produced in autumn is possibly the result of a combination between the highest reducing sugars content and the lowest respiration rate observed in this clone. When Asterix was produced in spring, the tubers showed higher respiration rates, consuming more reducing sugars and resulting in a lighter chip color. Total polyphenol content in Asterix was also lower in tubers produced in spring, which probably enhanced the effect of low reducing sugars on lighter chip color. However, clones SMINIA793101-3 and Missaukee did not follow the same pattern of high reducing sugars and polyphenol content and darker chip color comparing potatoes produced in spring and autumn. The final chip color does not only depend upon the reducing sugars and polyphenol content, but also on the concentration of amino acids (lysine, glycine, glutamine and arginine), and protein in potato tubers (Saona & Wrolstad, 1997), as well as the amount of acrylamide formation in fried chips (Pedreschi et al., 2005). In addition, previous studies presenting levels of reducing sugars similar to the ones observed in clones SMINIA793101-3 and Missaukee have shown that a high correlation between potato chip color and reducing sugar content can only be obtained if the concentration of sucrose is also considered (Shallenberger et al., 1959). Therefore, potato tubers of SMINIA793101-3 and Missaukee produced in spring possibly accumulated substances other than reducing sugars and polyphenols that contributed to the darkening of chip color. The involvement of these other substances in chip color development can also explain previous works showing different potato clones with different chip color and similar reducing sugar contents (Rodriguez-Saona & Wrolstad, 1997; De Freitas et al., 2006).

Potato chip darkness increased during storage for all clones and the darkest values were obtained at 4ºC (Figures 3C and 3D). All three potato clones increased reducing sugar content during storage in both growing seasons (Figures 4A and 4B). In spring, higher reducing sugar content was observed in tubers stored at 4ºC, except for Asterix and Missaukee that also accumulated high reducing sugar content during storage at 25ºC (Figure 4A). In autumn, the clone Missaukee had similar reducing sugar content in all storage temperatures (Figure 4B). At 180 days of storage, Asterix showed the highest reducing sugar accumulation at 12ºC (Figure 4B). Tubers of SMINIA793101-3 produced in autumn accumulated higher reducing sugars during storage at 4ºC and 25ºC (Figure 4B). Storage temperatures lower than 6ºC have been shown to decrease respiration rates, and increase the activity of enzymes responsible for the conversion of starch into reducing sugars in potato tuber, which results in high reducing sugar accumulation and dark color of potato chips (Coelho et al., 1999; Sowokinos, 2001; Chapper et al., 2004). However, the rate of starch depletion and soluble sugar accumulation depends upon storage temperature and cultivar, possibly owing to variation in enzyme activities (Kazami et al., 2000). This could explain the different clone responses to growing conditions and storage temperatures in relation to reducing sugar accumulation and chip color observed in our study. All three clones increased the reducing sugar content and the darkness of chip color during storage. This result is possibly the consequence of tuber aging process, in which amyloplast membranes increase permeability, resulting in higher starch degradation and reducing sugar accumulation (O'Donoghue et al., 1995).

 

Polyphenol content in potato tubers tented to increase during storage (Figures 4C and 4D). Tubers of all three clones produced during spring and stored at 4ºC had the highest polyphenol content (Figure 4C). Asterix produced in autumn tended to accumulate higher polyphenols at low storage temperatures of 4 and 8ºC (Figure 4D). Polyphenols are responsible for an enzymatic darkening, in which a ferrous-chlorogenic acid complex is formed during cooking and later oxidizes to a bluish-gray compound called ferri-dichlorogenic acid (Wang-Pruski & Nowak, 2004). In agreement with previous works, our results show that variations in polyphenol concentration in potato tubers are not only genetically controlled, but also influenced by environmental conditions (Percival & Baird, 2000; Wang-Pruski & Nowak, 2004).

The possibility to grow potato tubers in contrasting conditions of temperature, irradiance, and photoperiod in the southern region of Brazil was fundamental to exploit great differences in processing quality of potato clones growing in such conditions and stored at different temperatures. High processing quality of potato tubers is certainly not only determined by genetic makeup, but also by growing season conditions and proper storage temperatures. The clone Missaukee has the best processing quality with high and stable dry matter content and the lightest chip color in both seasons. Potato tubers produced in spring have higher dry matter and starch contents, but have shorter dormancy and store better for a shorter period of time, than potato tubers produced in autumn. Spring produced tubers requiring lower storage temperatures to extend the dormancy and maintain higher processing quality for a longer time than autumn produced tubers. Low storage temperature (4ºC) maintains low tuber respiration rates, which results in higher reducing sugar content and darker chip color. Higher storage temperature (25ºC) maintains lighter chip color, but enhances dry matter reduction in the tubers. The best storage temperature for processing potato tubers is in the range between 8 and 12ºC. To fine tune the best storage temperature, it is necessary to know tuber physiological age based upon cultivar and growing season conditions.

ACKNOWLEDGEMENTS

This work was financially supported by the National Council of Technological and Scientific Development (CNPq). We thank Dr. Adel Kader, professor at the University of California-Davis, who contributed with comments on an earlier version of this manuscript. We also thank all members of the Potato Breeding Program at the Federal University of Santa Maria for their collaborative efforts.

(Received on April 26, 2011; accepted on February 14, 2012)

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Publication Dates

• Publication in this collection
  19 Apr 2012
• Date of issue
  Mar 2012

History

• Received
  26 Apr 2011
• Accepted
  14 Feb 2012