Changes in physicochemical properties of rice starch during steeping in the parboiling process

• Steeping at 60–75 °C altered properties of isolated starch and starch in paddy and milled rice.
• Gelatinization temperature of isolated starch increased more than that of starch in kernels during steeping.
• Isolated rice starch was partially gelatinized at 75 °C and 67% water.
• Starch granules in milled rice and paddy rice maintained their granule integrity at 75 °C.

Isolated rice starch, milled rice, and paddy rice kernels of the same variety (18% amylose) were examined after steeping at temperatures (60–75 °C) below and above the onset of rice starch gelatinization temperature for different durations in a 2:1 (w/w) ratio of water to kernel or starch. Changes in gelatinization temperatures were greater for resultant isolated starch >> milled rice > paddy rice. Annealing above its original To caused partial gelatinization, loss of crystallinity, and birefringence of the isolated starch as determined by X-ray diffraction, differential scanning calorimetry, and light microscopy. However, starch granules in milled rice and paddy rice, which are surrounded by non-starch components, maintained their granule integrity. Scanning electron microscopy revealed morphological differences between starch granules within native and steeped rice kernels. Rice starch granules and kernel characteristics were significantly altered during steeping and changes in isolated starch differed from those inside the rice kernels.