The combined effect of oil-in-water emulsion soaking and modified pressure parboiling on starch digestibility of whole rice grains

The present work explores the enhancement strategy of resistant starch (RS) and slowly digestible starch (SDS) content in whole rice through oil-in-water emulsion (OIWE) soaking following modified pressure parboiling forming RS5 or lipid-amylose complex and RS3 or B-type crystals. Paddy was soaked for 8 h at 65 °C in OIWE prepared from 5 ml oil (coconut, sunflower, palm, and soybean oil) in 100 ml OIWE and emulsifier (Tween-20 or soy lecithin) through sonication. Soaked paddy was pressure parboiled (PP) and further treated with subsequent modified pressure-parboiling steps, i.e., pressure-parboiling-steam-refrigeration (PPR), pressure parboiling-refrigeration-microwave (PPRM), and pressure-parboiling-refrigeration-microwave-refrigeration (PPRMR). A stepwise increment in RS (significant) and SDS content was observed in all water-soaked samples than in the raw sample when exposed to PP and each step of modified pressure parboiling. All OIWE-soaked samples with different treatments showed an escalation in RS and SDS content than raw and water-soaked samples. Among all samples, the coconut oil-in-water emulsion (COIWE) soaked PPRMR treated sample (oil: Tween 20 and oil: soy lecithin = 1:1, average droplet sizes of 190.9 and 274.5 nm and ζ-potentials of −18.6 and −39.5 mV, respectively) exhibited the highest enhancement in RS level (12.93 ± 0.41 g/100 g and 10.75 ± 0.32 g/100 g for Tween-20 and soy lecithin stabilized COIWE).