Determination of citric acid pretreatment effect on nutrient content, bioactive components, and total antioxidant capacity of dried sweet potato flour

Abstract Orange flashed sweet potatoes are rich and inexpensive source of diet and antioxidants. The purpose of this study was to evaluate the effects of CA pretreatments and convective hot air drying temperature on proximate composition, bioactive components, and total antioxidant capacity of flour of five orange flashed sweet potato varieties. Moisture, protein, ether extract, ash, carbohydrate, fiber, β‐carotene, total phenolic compounds, and total antioxidant capacity in the dried flour samples were evaluated and reported in the range of 4.1–7.4%, 2.4–4.2%, 1.2–1.1.8%, 2.2–3.2%, 82.7–87.1%, 1.3–1.8%, 35.5–91.6 mg/100 g, 49.8–107.9 mg GAE/100 g, and 27.3–85.4%, respectively. The interaction effects of varieties, drying temperature, and CA concentration were significant (p ˂ 0.05) except for fiber. Kulto and SPK006/6/6 performed better for most of the parameters studied followed by SPK00/06. For almost all varieties, samples dried at 55°C and after treated in 3% CA solution had the highest percentage in terms of proximate composition, bioactive components, and total antioxidant capacities.

White fleshed sweet potato (WFSP) variety is the staple food for 13 million people in the Southern Regional State of Ethiopia (Kurabachew, 2015). In contrast, the orange fleshed sweet potato (OFSP), known to be a good source of β-carotene and energy (293 to 460 kJ/100 g), is easy to cultivate and fairly drought-tolerant (Hagenimana et al., 2001). These characteristics make OFSP an excellent food and nutrition security crop to the region, but in terms of their nutrient content, bioactive components and antioxidant capacity are not characterized. Furthermore, despite its increasing importance as a valuable crop for food security, so far value addition attempts have not been conducted in terms of production of dehydrated product and minimization of the associated after-harvest losses (Tiruneh, 2017).
The use of tuber crop in Ethiopia is limited, and it is consumed as an alternative carbohydrate source. This is generally performed with fresh tuber as postharvest storage or processing technology is not yet well developed. Dehydration could be an inexpensive technology that can be easily adapted to reduce the losses and improve its utilization in food formulations. Proper drying of OFSP can result in a stable product with better quality (Utomo, Man, Yaakob, Rahman, & Saad, 2008) when assisted with predrying treatments. Singh, Raina, Bawa, and Saxena (2003) used potassium metabisulfite and sodium chloride to improve the quality of chips from sweet potato. Ahmed, Akter, and Eun (2010) also used sodium hydrogen sulfite to improve the flour quality of OFSP. It has been shown that losses of scavenging ability, total phenolic contents, and degree of oxidation increase with increasing processing temperature and decrease when tuber is soaked in citric acid solution (Shih, Kuo, & Chiang, 2009). On the contrary, short heating reduces the activity of endogenous polyphenol oxidase which is responsible for oxidation of bioactive compounds (Ahmed et al., 2010). Therefore, this study aimed at to evaluate the effects of citric acid (CA) pretreatment and drying temperature on nutrient content, bioactive components, and antioxidant capacity of OFSP flours produced from different varieties.
The roots were washed in tap water, and only those with uniform overall appearance, size, and shape were selected for the study.
Tubers were then sliced into 1 mm size and dried for 8 hr (after preliminary work) after CA treatment (Ahmed, Akter, & Eun, 2011).

| Proximate composition analysis
For evaluating the effect of treatment on the nutritional quality of the flour, all components of proximate composition were determined using standard analytical methods of AOAC (2005)

| Total polyphenol content
The total polyphenol contents were determined according to Blainski, Lopes, and De Mello (2013) which involved the reduction of Folin-Ciocalteu reagent by phenolic compounds. Absorbances of prepared samples were measured at 765 nm using UV-Vis spectrophotometer (T80 Jiangsu, China). Gallic acid was used as the standard, and the total phenolic contents were expressed as mg of gallic acid equivalent (GAE) per g of sample (mg GAE/g sample).

| β-Carotene determination
Extraction and determination of total β-carotene were based on the method described in Park (1987). After extraction, absorbance was read at 450 nm using UV-Vis spectrophotometer (T80 Jiangsu, China) and estimated against with concentration of β-carotene standard curve (Sigma-Aldrich).

| Determination of total antioxidant capacity and IC 50 value
Antioxidant capacity was determined according to the method of Lu and Foo (2000) which involved DPPH (2,2-diphenyl-1-picryl-hydraz yl) free radical scavenging assay. Briefly, 10 g of sweet potato flour was mixed with 100 ml methanol and the mixture was homogenized for 1 min in a homogenizer (POLYTRON ® 2500E, Switzerland) and kept in a water bath at 20°C for 60 min. The samples were then centrifuged at 748 g for 15 min, and the supernatant was taken for analysis. The solvent extract of the sample was taken in 200, 400, 600, 800, and 1,000 μl concentrations in a test tube, and the volume was made up to 1 ml with the solvent and 2 ml of 0.1 mM DPPH was added to each tube. The mixture was shaken well and incubated at room temperature in the dark for 30 min. The decrease in absorbance of the resulting solution was then measured using UV-Vis spectrophotometer (T80 Jiangsu, China) at 517 nm. Scavenging activity was calculated from absorbance values of samples and control sample using the following equation: where RSA: radical scavenging activity; A c : absorbance of control; A t : absorbance of test solution; and A s : absorbance of standard solution.
The IC 50 value, defined as the amount of the sample to scavenge 50% of the DPPH radicals, was calculated from percentage of radical scavenging activity results by plotting the graph of DPPH free radical scavenging activity versus concentration of the sample.

| Statistical analysis
Statistical analysis was carried out using Minitab version 16 and analysis of variance (ANOVA) to determine the significance differences in nutritional, bioactive, and antioxidant contents of samples. Diagnostic tools like normal plot of residuals were tested prior to data analysis and indicated that the residuals of all parameters were normally distributed. Differences between the sample means were conducted using Tukey's test at α = 0.05 level for parameters showed significant difference. CA and dried at 65°C, and SPK004/6/6 treated with 1% CA and dried at 65°C. There were also significant differences among the varieties with respect to moisture contents, and hence, the differences might also be associated with tissue morphological, structural, and chemical composition difference that would influence the migration of moisture. Results in this study showed maximum moisture content of 7.4%, which could result in a stable product for long-term storage with reduced impact on product quality and stability.

| Protein content
The highest mean value of protein (3.9-4.2%) was recorded from Kulto, SPK004/6/6, and Guntute varieties treated with 3% CA and dried at 55°C. The lowest (2.4-2.6%) was from SPK00/06, SPK004/6/6, Guntute, and Bucteca varieties treated in 1% CA and dried at 65°C. The variation observed in terms of protein contents may be due to varietal difference or nonenzymatic browning due to condensation of reducing sugars with amino groups due to Maillard reaction (Utomo et al., 2008). In terms of crude protein, the observed values are comparable with other root crops, such as cassava and yam (Grabowski, Truong, & Daubert, 2008). Although sweet potato is regarded as a high-energy and low-protein food, its protein in both fresh and flour forms has been reported to be of high biological value and could serve as a fairly important protein source among low-income consumers (Van Hal, 2000).

| Dietary fiber
The two-way interaction effect of variety and drying temperature on fiber is shown in Figure 1, whereas Figure 2 shows the twoway interaction effects of CA and drying temperature.

| Gross energy
The results of the present study indicated that the interaction effect of varieties, CA concentration, and drying temperature showed the significant differences (p < 0.05). The calorific value was highest (373.6 kcal/100 g) in SPK004/6/6 variety treated with 3% CA and dried at 65°C. However, it is not statistically different from SPK00/06, Guntute, and Bucteca varieties treated with 3% CA and dried at 65°C, and SPK00/06, Guntute, Bucteca, and Kulto varieties treated with 1% CA and dried at 65°C.

| Total β-carotene
The β-carotene content in sweet potato can vary depending on the cultivars, harvesting conditions, maturity, and processing conditions (Van Hal, 2000). The β-carotene content of flour samples in this study is within the range of 8.3-156.6 mg/100 g (Table 2) as reported for sweet potato flour (Dansby & Bovell-Benjamin, 2003;Shih et al., 2009). The highest total β-carotene contents (91.6 and 87.7 mg/100 g) were recorded from SPK004/6/6 and Kulto varieties, respectively, which were treated with 3% CA and dried at 55°C ( Table 2). The lowest (33.5-38 mg/100 g) were from SPK00/06 and Guntute treated with 1% and dried at 65°C, and Kulto and Bucteca varieties treated with 3% CA but dried at 65°C. The recorded highest β-carotene from SPK004/6/6 and Kulto varieties dried at 55°C and treated with 3% CA could be due to the fact that oxidation of β-carotene at 55°C is minimal as compared to 65°C. The probable reasons for higher β-carotene content from samples treated with 3% CA could be low activities of enzyme (peroxidases and lipoxygenases) at lower pH which have a capacity to degrade β-carotene (Ahmed et al., 2011). Sharma, Kaur, Sarkar, Singh, and Singh (2009) also reported that β-carotene loss was decreased at lower pH for carrot juice.

| Antioxidant activity potential and IC 50 capacity
The ranges of values for percentage of antioxidant and IC 50 of flour are shown in Table 3 (Table 3).
The recorded highest values from SPK004/6/6 and Kulto varieties treated with 3% CA and dried at 55°C could be because of low oxidation of bioactive compounds at this temperature or because some of the antioxidants in flour were relatively not heat-stable when the temperature increased to 65°C. This observation is in agreement with the work of Laine, Kylli, Heinonen, and Jouppila (2008), who reported that drying sweet tuber slices at a lower temperature results in greater antioxidant activity, but increasing drying temperature could degrade polyphenol compounds of the product and result in the loss of antioxidant activity. Chan, Lee, Yap, Aida, and Ho (2009) also stated that the loss in antioxidant capacities of plant product at high temperature is likely due to the degradation of polyphenols which were previously mobilized at low temperature. Similarly, Shahidi and Naczk (2003) reported that some polyphenols decomposed rapidly under high temperature and thus caused a reduction in the antioxidant capacity of plant sample. However, Lavelli et al. (1999) reported contradictory idea with the present study by stating that temperature releases more bound polyphenol compounds due to breakdown of cell wall phenolic compounds.
Data on the effects of drying temperature on polyphenol contents and antioxidant activity of vegetables are conflicting due to several factors, such as the drying method, type of extraction TA B L E 4 Correlation values of total antioxidant capacity with total polyphenols, total β-carotene, and vitamin C content solvent, antioxidant assays used, and interactions of several antioxidant reactions. Another probable reason for the observed difference in terms of percentage of free radical inhibitions might be due to inactivation of polyphenol oxidase in acidic condition, which is the primary cause of the reduction in polyphenol contents. Retention of polyphenols results in an increase in the percentage of free radical scavenging ability. The treatment that has the highest antioxidant activity has also the highest concentration of bioactive compounds as indicated in Table 4 with a strong positive correlation. Higher positive correlation value with antioxidant capacity was observed for polyphenols as compared to β-carotene and vitamin C contents.
This might be based on the ability of the DPPH radical to react with hydrogen donor species, mainly polyphenols. Islam et al. (2002) and Kalaivani and Mathew (2010) also reported a strong positive correlation between antioxidant activity and polyphenol content. That is why polyphenols are very important plant constituents because of their scavenging ability on free radicals due to their hydroxyl groups (Tosun et al., 2009).
The IC 50 value, defined as the concentration of antioxidant required for 50% scavenging of DPPH radicals, is indicated in Table 3.
The ranges of values for IC 50 of the flour (0.8-17.4 mg/g) are comparable with the range of 1.7-17.4 mg/g reported for sweet potato flour (Teow et al., 2007). The highest values of IC 50 (17.4 and 15.1 mg/g) were recorded from SPK00/06 and Guntute varieties, respectively, which were treated with 1% CA and dried at 65°C (Table 3). This could be due to loss of polyphenols during the drying process with increasing drying temperature and low concentration of CA applied. Islam et al. (2002) also stated that the temperature during drying affects the stability of polyphenols due to chemical and enzymatic degradation which cause a reduction in polyphenol contents. Many studies reported that high polyphenol content contributes to high radical scavenging activity (Islam et al., 2002;Teow et al., 2007).

| CON CLUS ION
Production of sweet potato is seasonal, whereas consumption is all year round. As a perishable product, the tuber cannot be stored for long period of time and experience high after-harvest loss. In line with good postharvest handling, value addition through change of form of the product is a strategy to reduce losses, diversify the product, and to store it for reasonable time under proper storage conditions. OFSP evaluated in this study showed different responses in terms of nutrient contents, bioactive components, and total antioxidant capacity. Treatment of OFSP slices in 3% CA solution after drying at 55°C significantly preserved better nutrients and bioactive components with better antioxidant capacities as compared with 1% CA solution and drying temperature of 65°C. The lower the temperature with 3% CA concentration contributes for better preservation effect. Among the evaluated varieties, at selected drying temperatures and CA solution concentrations, Kulto and SPK006/6/6 performed better for most of the parameters studied, followed by SPK00/06. This implies that these three varieties are more tolerant of drying medium heat and low pH effect. Therefore, under these selected conditions, value-added dried OFSP can be produced for use as an ingredient to make diverse foods for different social groups besides its contribution to the avoidance of after-harvest losses.

ACK N OWLED G M ENTS
The authors acknowledge Jimma University College of Agriculture and Veterinary Medicine for financial support to conduct this research.

E TH I C A L S TATEM ENT
This study did not involve any human or animal testing.

CO N FLI C T O F I NTE R E S T
All authors declare no conflict of interest.

AUTH O R S' CO NTR I B UTI O N S
The first author was responsible for the accomplishment of most of the laboratory experiments with the help of the second author.
All the authors listed in this manuscript contributed equally to the preparation of the manuscript and approved it for publication.