Physicochemical properties of rice and effect of milling method on the properties of flour from different rice Oryza sativa L. cultivars in Thailand

Abstract

This research investigated physicochemical properties of rice and flour from Thailand grown rice cultivars, which are RD 41, RD 45, RD 47, Shaw Lung 97, Prachin Buri 1, Prachin Buri 2, Plai Ngahm Prachin Buri, Khao Dawk Mali 105 and Ayutthaya 1. The research consisted of two parts. The first part involved the investigation of physicochemical properties and cooking quality of rice. Rice grains contained 11.1-12.7% (wb) moisture, 0.3-1.9% (db) fat, 6.3-10.7% (db) protein, 0.2-0.8% (db) ash, 0.1-0.5% (db) fiber and 78.6-86.4% (db) carbohydrates. Cooking time of rice ranged from 18 to 30 min. Hardness values of cooked rice ranged from 227.6 to 602.2 gf. The second part involved the study on the effect of milling methods; wet- and dry-milling, on physicochemical properties of flour. The flours contained 8.2-12.0% (wb) moisture, 0.2-1.0% (db) fat, 5.3-9.2% (db) protein, 0.2-0.9% (db) ash and 87.7-92.5% (db) carbohydrates. The flour samples could be classified into low amylose flour that contained 13.1-18.7% amylose (RD 45 and Khao Dawk Mali 105) and high amylose flour that contained 29.4-36.5% amylose (Ayutthaya 1, Plai Ngahm Prachin Buri, Prachin Buri 1, Prachin Buri 2, RD 41, RD 47 and Shaw Lung 97 cultivars). Wet milling process resulted in flour with significantly lower (P ≤ 0.05) protein content. In addition, dry-milled rice flour contained granules with significantly larger average size (P ≤ 0.05) compared to wet-milled samples. Swelling power at 60oC and solubility of wet-milled samples were significantly lower (P ≤ 0.05) than those of dry-milled flours. Dry milling process caused the destruction of the crystalline and yielded a lower crystallinity degree (P ≤ 0.05) compared to wet-milling process, which resulted in significantly lower gelatinization enthalpy (∆H). Dry-milled flours showed significantly (P ≤ 0.05) lower peak viscosity comparing to wet-milled flours. From the frequency sweep test, 12% (w/w) rice flours gel gave higher G' than G' indicating the typical characteristic of weak gel.