RICE QUALITY AND PROCESSING Effect of Long-Grain Rice Degree of Milling, Moisture Uptake, and Solids Leach During Cooking on Rice Textural Properties

Two long-grain rice cultivars/hybrids (‘Wells’ and ‘CL 161’) harvested from locations in Arkansas were used in this study. Rice samples were milled for 20, 30, 40, and 50 seconds. Milled whole-rice samples were cooked in excess water for 16, 18, 20, and 22 minutes and in a 2:1 water-to-rice ratio for 20 minutes (WR20). Cooked rice texture and solid leach were determined. Results indicated that longer cooking duration resulted in softer rice as a result of fuller hydration of rice kernels. Increased milling duration also resulted in significant (P<0.05) decrease in cooked rice hardness. Milling and cooking duration significantly (P<0.05) affected total solid leached out during cooking. INTRODUCTION Rice texture is a key indicator of rice quality as it affects cooked rice acceptance by consumers (Lyon et al., 2000). Unlike other cereals, rice is consumed largely as cooked whole kernels, which is produced after de-hulling and milling processes. Although cooking methods for rice vary widely worldwide, rice-cooking methods are mostly subtle variations of two basic cooking techniques: namely (1) Excess or American method, where rice is usually cooked in a large amount of water then drained and (2) the Oriental method, where rinsed rice is usually cooked in a measured amount of water (Crowhurst and Creed, 2001). Optimum cooking degree of rice is usually determined when rice reaches an end cooking point where it either absorbed a maximum amount of water or until the core of the cooked rice kernels gelatinized (Kasai et al., 2005). Rice hydration upon cooking has, therefore, a great impact on rice textural properties. AAES Research Series 560 252 Rice-kernel surface area and chemical composition also affect cooked-rice moisture uptake during cooking (Bhattacharya and Sowbhagya, 1971; Bergman et al., 2004). For example, high-amylose cultivars have been reported to turn out firmer and less sticky rice when cooked than low-amylose cultivars (Juliano and Perez, 1983). In addition, slender rice varieties have been found to uptake greater amounts of water during cooking than short and rounder varieties (Bergman et al., 2004). Although several researchers have studied the effect of water-to-rice ratio for rice cooking and resultant textural properties (Srisawas and Jindal, 2007; Bett-Garber et al., 2007), a universal way of selecting a water-to-rice ratio to obtain optimal cooked rice texture does not exist. In addition, there is a lack of information on the effect of cooking rice in an excess amount of water on cooked-rice textural characteristics. Therefore, this study was initiated to investigate the roles moisture uptake, solid leaching, and degree of milling play in determining cooked-rice instrumental textural properties. PROCEDURES Rice Sampling Wells and CL 161 rice cultivar/hybrid were harvested from Keiser, Ark., at moisture contents (MC) of 16.0 and 21.2% (wet bases (wb)), respectively. Rice samples were brought to the University of Arkansas Rice Processing Program laboratories where the rice was cleaned and air dried at ambient temperature to a MC of ~12.5% (wb). Dried rough-rice samples were stored in air-tight plastic storage containers at 22 ±3°C for two months before milling. Rice Milling and Preparations Duplicate 150-g rough-rice samples were de-hulled using a de-husker (THU-35, Satake, Hiroshima, Japan) and milled for 20, 30, 40, and 50 seconds using a laboratory mill (McGill No. 2, RAPSCO, Brookshire, Texas). A double-tray sizing device was used to separate whole from broken kernels and head rice yield (HRY) was calculated. Only full rice kernels were used in this study. Rice surface lipid content (SLC) was determined using a Soxtec system (Avanti 2055, Foss North America, Eden Prairie, Minn.) according to Matsler and Siebenmorgen (2005). Protein content in milled rice flour was determined in duplicate for each treatment using the Kjeldahl procedure, AACC method 46-11A (AACC, 2000). Rice Cooking and Instrumental Texture Measurements Rice was cooked using a miniature rice cooker, consisting of a glass-cooking vessel with a glass top and a heating mantle, the temperature of which was controlled by a temperature regulator (89000-10, Eutech Instruments, Pte Ltd, Singapore). For excess water cooking, 100 ml water was brought to a boil before 20 gram of milled rice