Freeze-thawing and osmotic dehydration pretreatments on physicochemical properties and quality of orange-fleshed sweet potato slice during hot air drying

Abstract

• We report here the superiority of freeze-thawing over other pre-drying techniques. • Freeze-thawed samples had the highest Vit C, TPC and antioxidants contents. • Freeze-thawed samples dried faster and consumed less energy. • Freeze-thawed samples scored the highest in all parameters for sensory analysis. • Drying kinetics of the freeze-thawed samples followed the Newton model. The study investigated the effects of pretreatments of freeze-thawing at - 20°C (FT), osmotic dehydration at 20% (w/v) of sucrose (OD), and water only (WO) on vitamin C content, total phenolic content (TPC), antioxidation (1,1,-diphenyl-2-picrylhydrazyl (DPPH), 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid (ABTS), and moisture content (MC). The study further assessed the energy Consumption Rate (ECR), drying Rate (DR), drying kinetics and Sensorial properties of Orange-Fleshed Sweet Potato (OFSP) slices dried using hot air drying (HAD). FT pretreatment improved the preservation of Vitamin C (12.73 ± 0.26 mg.100 g -1 dm), TPC (26.20 ±0.65 mg GAE/g db.), ABTS (37.12%), and DPPH (44.77%) than the OD and WO. Again, the FT recorded the lowest moisture content (6.5%) after drying, and lower drying time (6 hours), than the OD (7.5 hours), WO (9.5 hours), and Control (11hours). However, the ECR of both FT and OD exhibited the same ECR (15 kWh/kg). The OD recorded higher scores for Colour (7.0), and Appearance (7.0) per the sensory evaluation conducted compared for FT, WO, and Control. However, the FT recorded higher scores in Aroma (8.6), Flavour (8.6), and General Acceptance (8.5) than the OD, WO, and Control. Among the three pretreatments, FT was found to enhance the drying kinetics comparative to the WO and OD pretreated sample. Newton's model fitted well to the experimental results (R 2 > 0.999) than the first order and Weibull models.