The Impact of Thermal and Hydration Pre-treatments on Percentage Yield, Moisture Content, and Functional Properties of Dioscorea hirtiflora Yam Flour

Abstract

The objective of this study was to investigate the effects of thermal (boiling) and hydration (soaking) treatments on the yield, moisture content, and selected functional properties of flour from Dioscorea hirtiflora yams. The mature tubers were cleaned, peeled, sliced, boiled at 100°C for 10-40 minutes or soaked in water (ambient temperature for 20-80 minutes). Both the processed and control (raw) samples were oven-dried, milled, and sieved to obtain flour. The results showed that both boiling and soaking significantly reduced the flour yield compared to the raw sample (25.88%). Boiling for 40 minutes reduced the yield to 18.67%, while soaking for 80 minutes reduced it further to 15.87% (p < 0.05 for both). Moisture content (dry basis) decreased to 7.26% after 10 minutes of boiling, but increased to 11.62% with extended soaking (p < 0.05), while still within acceptable limits. Water absorption capacity was significantly enhanced, reaching 293.33% (20 minutes boiling) and 283.00% (80 minutes soaking), while wettability time decreased from 574.33 s (raw) to 9.33 s (boiled) and 387.33 s (soaked) (p < 0.05). Bulk density significantly decreased from 0.97 to 0.61 g/mL (p < 0.05), with dispersibility remaining above 50% (p > 0.05). Compared to the raw sample, gelatinization temperature decreased by up to 8°C depending on the pre-treatment duration (p < 0.05), and the lowest gelation concentration remained at 4% (p > 0.05). Soaking increased the foaming capacity from 15.2% to 52%, whereas flour boiled for 40 minutes showed reduced foaming stability compared to raw flour. In contrast, soaking for 20 minutes-maintained foam stability at 71%. The swelling power peaked at 60°C (maximum 4.62 g/g at 40 minutes boiling), without significant changes from the pre-treatments (p > 0.05), and oil absorption, particularly for palm oil, increased to 213.33% (p < 0.05). In general, short boiling (10-20 minutes) or moderate soaking (20-40 minutes) effectively balanced the yield and functional enhancements, improving water absorption, gelatinization, and oil uptake. These results suggest that D. hirtiflora flour could be ideal for baked and gluten-free products that require controlled moisture and oil levels, enabling the development of innovative food products. Short boiling or
moderate soaking is recommended to enhance flour quality; further studies should assess the nutritional and sensory qualities of the resulting products.