Effects of cooking methods on starch and sugar composition of sweetpotato storage roots

Sweetpotato has rich nutrition, good ecological adaptability and high yield. There is a lack of knowledge about the effects of cooking methods on starch and sugar components in elite Chinese cultivars. In this study, sweetpotato storage roots from four cultivars “Xinxiang”, “Jinyu”, “Zimei” and “Yuzishu 263” were treated by baking, boiling and steaming and subsequently analyzed for starch content, amylase activity and sugar contents including glucose, fructose, sucrose and maltose. Results indicated that cooking reduced starch content and final amylase activity and increased reducing sugar content especially maltose content, but did not have significant influence on non-reducing sugar content. These effects were different among the four cultivars and three cooking methods. Baking led to the least starch reduction. Storage roots of “Jinyu” contained the highest amount of sugar content and thus sweetest. Sugar composition analysis suggested that cultivars “Xinxiang” and “Jinyu” belong to high-maltose cultivars. This study may provide useful information for evaluating the cooking quality of sweetpotato cultivars.


Introduction
Sweetpotato (Ipomoea batatas), a dicotyledonous plant, belongs to the Convolvulaceae family. It is a root vegetable due to its large, starchy, sweet-tasting storage roots. Sweetpotato is native to the tropical regions in America. It is distantly related to common potato (Solanum tuberosum).
One of the most important edible quality parameters of sweetpotatoes is sweetness [14,15]. Sweetness of raw sweetpotatoes has been considered as an index for cultivar evaluation PLOS ONE | https://doi.org/10.1371/journal.pone.0182604 August 21, 2017 1 / 10 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 [16,17]. However, some studies have shown that sweetness has little correlation with soluble sugar content of raw sweetpotatoes, but largely related to sugar content of cooked sweetpotatoes [18,19]. The sugar compositions in raw and cooked sweetpotatoes were reported using varieties from the Ghana [11], US [10], Philippine [12], and Tanzania [8]. However, it was less known about the effects of cooking methods on starch and sugar components among the elite cultivars in China. The objective of this study was to provide information on the effects of cooking methods on carbohydrate composition and its relation to cultivar appraisal of sweetpotatoes. Here we reported the determination of sugar composition with HPLC and analysis of the difference of starch and sugar components among four sweetpotato cultivars under three cooking treatments.

Materials
Sweetpotato cultivars selected in the study were representatives of the yellow flesh cultivars ("Xinxiang" and "Jinyu") and the purple flesh cultivars ("Zimei" and "Yuzishu263"). "Xin-Xiang" and "Jinyu" were created by Agricultural Academic Science Institution of Zhejiang Province. "XinXiang" storage root is red skin and oval shape. "Jinyu" storage root is red skin and long. "Yuzishu 263" was bred by Sweetpotato Institution of Southwest University in Chongqing City, and the storage root is purple skin and looks long. "Zimei" was introduced from Vietnam, and the storage root is purple skin and oval shape.
All storage roots were freshly harvested within one week before being used in the experiments. The same sizes of storage roots (about 250 g) were used in the study. Chemicals (such as methanol, acetonitrile, HPLC grade) were purchased from East China Medicine Company.

Methods
Cooking methods. Boiling: sweetpotato storage roots were washed with water, soaked into water in a pan followed by heating them using electric cooker. It took 28 min for boiling in boiled water. Steaming: sweetpotato storage roots were put on steaming box above the water line in the pan. It also took 28 min for steaming since water boiled. Baking: sweetpotato storage roots were settled in a baking box, set temperature at 200˚C. It took 45 min for baking at 200˚C. We did not peel the storage roots and performed any cutting before cooking. After cooking, samples were cooled to normal temperature, and cut into pieces (1.0 cm Ã 1.0 cm Ã 0.2 cm). Some of them were freezing drought under -45˚C for 48 hr before being smashed; others were baked at 80˚C until the weight kept consistent. Raw sweetpotatoes (as control treatment) were also cut into pieces (1.0 cm Ã 1.0 cm Ã 0.2 cm) before drought as above.
Sugar composition analysis. Soluble sugars were extracted from the sweetpotato samples based on the published method [10,14] with minor modifications: 5 g sample of each sweetpotato mash was added to a sealed test tube. 5 mL of 80% ethanol was added to each sample and mixed. The test tube was placed in a water bath at 80˚C for 15 min. Subsequently, 2.5 mL of 80% ethanol was added at 15 and 30 min. Fresh 80% ethanol was then added to bring the total volume to 10 mL. The mixture was centrifuged under 8000 rpm for 15 min. The supernatant was collected, filtered through a 0.45-μm membrane filter and injected into a high performance liquid chromatography (HPLC, Agilent 1200) with a refractive index detector. HPLC employed reverse-phase C-18 column (XBridgeTM Amide 3.5 μm, 4.6×250 mm); sample  [20] using the following formula (where x is soluble sugar and y is sweetness): Raw sweetpotato: y = 0.0048x +0.0328, r = 0.962. Cooked sweetpotato: y = 0.0170x +0.0766, r = 0.977.
Starch content determination. Starch was prepared and detected based on the method of Cao [21]. It was degraded into sugar by 9 mol/L perchloric acid (AR, supplied by East China Medicine Company) in 100˚C for 30 min. Starch degradation rate = [untreated starch content (g)-treated starch content (g)] / untreated starch content (g) Ã 100%.
Amylase activity determination. α-amylase activities were determined by the starch azure assay with minor modification [22]. The sample (0.5 g) was homogenized in a Mortar in 5 mL extraction buffer containing 100 mM potassium phosphate buffer (pH 7.0), 5mM ethylenediaminetetraacetic acid (EDTA) and 1mM dithiothreitol (DDT). Following filtration and centrifugation at 8000rpm for 15 min, the supernatant was used to determine amylase activities.
Statistical analysis. All the analyses were done in triplicates. The data were subjected to statistical analysis using the statistical software SPSS13.1 and Microsoft Excel. Variance analysis had been applied, and Duncan' test was also used.

Effect of cooking methods on dry weight of sweetpotatoes
There was no statistical difference in dry weight between freeze-dried and bake-dried sweetpotatoes under the same cooking method, regardless of some variation (Table 1). Take "Zimei" as an example, under raw condition, the ratio of Dw/Fw (dry weight to fresh weight) between freeze-dried and bake-dried "Zimei" were 36.5% and 34.9%, while it was 38.2% and 38.5% under baked condition, 27.3% and 27.6% in boiling, and 33.3% and 35.3% in steaming. Every pair of data appeared approximately the same without statistical difference between the freezedried and bake-dried sweetpotatoes. Similarly, no significant difference on Dw/Fw was observed between freeze-dried and bake-dried sweetpotatoes for other cultivars under the same cooking method (Table 1). Boiling resulted in the lowest dry weight of storage roots among the three cooked and the untreated sweetpotatoes from all cultivars except "Xinxiang" ( Table 1).

Effect of cooking methods on starch content of sweetpotatoes
Among raw materials, "Zimei" and "Yuzishu263" had the highest starch content (189.4 and 175.4 mg/g.Dw, respectively), whereas "Jinyu" had the lowest (124.5 mg/g.Dw) ( Table 2). In general, starch content decreased after cooking. Baking degraded starch by up to 20% and performed similar trend to raw sweetpotatoes. Steaming also reduced starch content of all cultivars especially "Jinyu" by more than one third, and "Xinxiang" by steaming had the lowest amount of starch. As to boiling, "Jinyu" and "Zimei" reduced starch content by 40-60%; "Xinxiang" and "Yuzishu263" had the highest amount of starch (Table 2).

HPLC separation of soluble sugars from sweetpotatoes
HPLC was used to analyze the composition of soluble sugars in sweetpotato storage roots. Raw sweetpotato storage roots contained three soluble sugars: fructose (peak a), glucose (peak b) and sucrose (peak c) (Fig 1A). Cooked sweetpotato storage roots contained the same three sugars, as shown in the cultivar "Xinxiang" (peaks a-c) and generated extra maltose (peak d) (Fig 1B).

Effect of cooking methods on sugar composition of sweetpotatoes
Cooking dramatically increased the content of soluble sugars, although it varied among cultivars (Fig 2). The highest content of soluble sugars in cooked sweetpotatoes was from "Jinyu". The total sugar content in sweetpotatoes from "Jinyu" under baking, boiling and steaming were 378, 238 and 250 mg/g of dry weight, respectively. Most of the increased sugars were mainly due to maltose accumulation (Fig 2). Baking changed the order of sugar content in the sweetpotatoes from four cultivars as "Jinyu" > "Yuzishu263" > "Xinxiang" > "Zimei". Maltose content in baked sweetpotatoes from "Jinyu" was increased the most by 240.2 mg/g of dry weight (Fig 2). Boiling also altered the profiles of sugar content and resulted in an order of "Jinyu" > "Xinxiang" > "Yuzishu 263" > "Zimei". After boiling, maltose also increased significantly. Steaming resulted in a similar profile of sugars as boiling. After steaming, maltose was markedly detected in the treated samples. For example, maltose content in steamed sweetpotatoes from "Xinxiang" and "Jinyu" were increased respectively by 135 and 176.7 mg/g.Dw (Fig 2).

Effect of cooking methods on sweetness
One important edible quality index of sweetpotatoes is sweetness, which is closely related to soluble sugars [23]. Therefore, we analyzed sugar concentration to calculate sweetness in order to facilitate quality analysis and cultivar appreciation ( Table 3). Sweetness of cooked sweetpotatoes had increased significantly compared to raw sweetpotatoes, and there were also significant differences among the cultivars. Sweetness of "Jinyu" was the highest either in raw   Table 3). Sweetness of boiled "Zimei" and steamed "Xinxiang" were also high (index more than 4) but boiled "Xinxiang" turned to be the least (index 1.75) ( Table 3). By baking, boiling and steaming, sweetness of "Xinxiang" increased respectively by 4.19-, 2.82-and 6.50-fold (Table 3). Meanwhile, under the above three treatments, sweetness of "Yuzishu263" increased by 6.74-, 5.51-and 8.37-fold (Table 3), while "Zimei" increased by 3.60-, 8.24-and 5.48-fold, respectively ( Table 3).

Effect of cooking methods on α-amylase activity in sweetpotatoes
Raw sweetpotato storage roots had the highest α-amylase activity while those in other treatments approached to be zero (Table 4), which indicated that cooking methods influenced amylase activity significantly. There were differences among cultivars (Table 4). Raw sweetpotatoes "Zimei" had the least amylase activity, and "Jinyu" and "Yuzishu263" had the highest.

Discussion
Our study showed that sugar composition variation was related to the color of sweetpotato flesh. In general, sweetpotato storage roots from the yellow flesh cultivars ("Jinyu" and "Xinxiang") contained more sugars than those from the purple flesh cultivars ("Zimei" and "Yuzishu263"). Cooking increased total sugar content especially reducing sugars. This effect was different among cultivars and cooking methods. Cooking methods did not have any significant effects on non-reducing sugar content, which is in agreement with the results of Bian and Damir [24,25]. We did not cut the storage root into pieces before boiling while Lyimo et al. (2010) sliced the storage root into pieces of 3-5 cm thick then boiled (moist heat) in clean water until ready to eat (about 30-45 minutes). We considered that pieces in hot moist may lose some  [7,25,26]. In our study, cooking decreased starch content, increased sugar content especially reducing sugars and therefore increased sweetness. This is expected since starch is degraded by enzymes and transformed into soluble sugars during cooking processes [27][28][29]. It was also said that αamylase activity increased during the initial period due to the gradual increase of cooking temperature but is then significantly reduced due to the high temperature. This was demonstrated by previous study showing that decreases of reducing sugars in sweetpotatoes was due to loss of more amylase activity at 100˚C by boiling and steaming than those at 80˚C by baking or 40˚C under sunlight [30]. However, the decrease of starch content and increase of soluble sugar content in this report is in conflict with a previous report in which boiling, roasting and sun drying did not have any significant effect on the carbohydrate, protein, fat, ash, calcium, iron and magnesium contents but decreased sugar content [8]. The different results from these two studies might be due to the differences of cooking methods, analytic methods, cultivars, etc.
Our report showed that the sweetpotatoes still had some residual amylase activities after 28 min of cooking ( Table 4). The residual amount of amylase activity suggests that the sweetpotatoes were not thoroughly cooked during the initial cooking process. There may be several alternative reasons for explaining the residue activity. First, there may be extremely high temperature tolerance enzymes existing in sweetpotatoes. Second, starch degradation does not thoroughly rely on enzymes which are sensitive to temperature variation, for example, pyrolysis. Third, the final degraded products, sugars, do not completely come from starch degradation which relies on amylases, such as lipids Our results showed that cooking decreased starch content and increased soluble sugar content, which support the well-known phenomena that cooked sweetpotato usually tastes sweeter than fresh ones. Our experiment demonstrated that different cooking methods produced different sweetness and that steaming produced the highest sweetness. Soluble sugars of cooked sweetpotatoes had higher correlation with sweetness than those of raw sweetpotatoes, suggesting that soluble sugars in cooked sweetpotatoes are more suitable than those in raw sweetpotatoes for evaluating cultivar quality during sweetpotato breeding [20].

Conclusion
We determined the effects of cooking methods on starch, amylase and sugar components in four elite Chinese sweetpotato cultivars. Cooking reduced starch content and amylase activity and increased reducing sugar content especially maltose content as well as sweetness, but did not have significant changes in non-reducing sugar content. These effects were different among the four varieties and three cooking methods. This study provides useful information for quality analysis and cultivar appraisal of sweetpotatoes.