Impact of boiling-drying regimes on milling properties, digestion characteristics, and multi-scale starch structure of parboiled rice

Parboiled rice undergoes hydrothermal processing, which comprises soaking, steaming, and drying. Compared with the traditional "soaking, steaming and drying at 45 °C" (S_OT₄₅) process, this study evaluated alternative regimens: Soaking, boiling and drying at 45 °C (S_WT₄₅); Soaking, boiling and drying at 25 °C (S_WT₂₅); Soaking, boiling and drying at 4 °C for 12 or 24 h (S_W12T₄, S_W24T₄). The effects of these processing technologies on milling performance and in vitro digestibility were compared. The S_W24T₄ process produced the highest values: brown rice yield of 77.39 %, white rice yield of 64.79 %, and head rice yield of 61.72 %. Furthermore, compared with the S_OT₄₅ process, S_W24T₄ parboiled rice exhibited slower digestion kinetics, as evidenced by a 16.35 % reduction in rapidly digestible starch and a 141.1 % increase in slowly digestible starch. Crystallinity analysis revealed a transition from an A-type to a B + V-type pattern, with V-type crystallinity increasing from 0.98 % in raw material polished white rice (RWR) to 3.40 % in S_W24T₄. These findings collectively present a novel processing strategy to enhance the milling yield and slow-digestion properties of parboiled rice.