2.1. Reagents Analytical-grade acetone, petroleum ether, n-hexane, dichloromethane, Folin-Ciocalteu reagent and acetic acid were purchased from Merck (Darmstadt, Germany). Gallic acid (PubChem CID: 370) was purchased from TIC (Japan).
2.2. Sample Collection and Preparation The present study was conducted on nine varieties of OFSP grown in Bangladesh to estimate the proximate composition (moisture, ash, protein, fat, carbohydrate and fiber), TPC and total carotenoids content (TCC). The experimental samples were collected from Tuber Crops Research Centre (TCRC), Bangladesh Agricultural Research Institute (BARI), Gazipur, Bangladesh. All of the varieties were harvested in the same experimental field and received the same fertilizer and irrigation practice, whereas no information with regard to the growing conditions in the TCRC was available. The potatoes were harvested when they became mature at about four to five months; the average weight of each potato was 200 to 220 g. Six to ten freshly-produced samples of each potato variety were collected from the production field of TCRC of BARI. All of the samples were collected as fresh as possible and processed at the Institute of Nutrition and Food Science (INFS) laboratory for analysis. Samples were gently washed with tap water immediately after collection to remove sand and other extraneous material before being washed with distilled water and then air dried. The samples were then cut into small pieces, placed in an auto seal bag and kept in a desiccator to inhibit moisture gain or loss. They were then ready for the determination of their proximate composition, TCC and TPC.
2.3. Proximate Composition The proximate composition of the samples was carried out according to the AOAC official method for nutrient analysis. Moisture content was determined by the oven method; ash content was determined by the dry ash method in a muffle furnace; protein content was determined by the Kjeldahl method (nitrogen content multiplied by 6.25); fat content was determined by using chloroform-methanol extraction; and crude fiber was estimated by the gravimetric method. Carbohydrate content was calculated using the following formula described by Rand et al. Carbohydrate content = 100 – {moisture (g%) + crude protein (g%) + total fat (g%) + crude fiber (g%) + total ash (g%)}; where (g%) = grams per 100 grams OFSP.
2.4. Analysis of Total Carotenoids Content TCC in the sample was estimated by acetone-petroleum ether extraction followed by spectrophotometric measurement according to the modified Rodrigues-Amaya and Kimura method of total carotenoids analysis. Extraction of carotenoid was performed by grinding of processed potato sample in a mortar and pestle, filtration through a sintered glass filter under vacuum and separation from acetone to petroleum ether. The petroleum eluent adjusted to a specific volume was read at 450 nm in a spectrophotometer (UV-1601, UV-Visible, Shimadzu, Tokyo, Japan) for the concentration of total carotenoids. Results were expressed as milligrams per 100 g of fresh weight (FW) (mg/100 FW).
2.5. Analysis of Total Polyphenol Content The TPC in sweet potato extracts was estimated by the Folin-Ciocalteu colorimetric method according to Blainski et al. with slight modification. Briefly, to appropriately-diluted sample extract (0.15 mL) was added 0.225 mL of 2-fold diluted Folin-Ciocalteu reagent, and this was kept for 5 min at room temperature. Then, 1.125 mL of 2% Na2CO3 solution were added, mixed well and kept for 15 min at room temperature. Finally, the absorbance was measured at 750 nm by a UV-VIS spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) and used to calculate TPC using a standard curve based on gallic acid. Results were expressed as milligrams of gallic acid equivalent (GAE) per 100 g FW (mg GAE/100 g FW).
2.6. Data Quality The precision and accuracy were maintained by carrying out inter-laboratory analysis of TCC in the raw BARI SP2 between the INFS laboratory, the University of Dhaka and Nutritional the Biochemistry laboratory of the ICDDR, B (International Centre for Diarrheal Disease and Research, Bangladesh). No significant difference (p = 0.893) was found between the results (6.21 ± 1.35 mg per 100 g FW vs. 6.58 ± 1.12 mg per 100 g FW).
2.7. Statistical Analysis The experiments were performed with three replicates for each sample of OFSP. Descriptive statistics were performed for all variables. One-way analysis of variance (ANOVA) was performed to test the differences among varieties for their proximate, total polyphenol and carotenoids contents. All analyses were performed with R software Version 3.2.2. The 5% level of least significance was used to determine any differences in the mean values between different sweet potato cultivars. Differences at p < 0.05 were considered to be statistically significant. Experimental results were expressed as the mean ± standard deviation (SD).