Processing and quality of glutinous rice

Abstract

The loss of stickiness of aged glutinous rice is one of the quality issues. This thesis examined the various factors that could affect the loss of stickiness of glutinous rice and potential pre-treatments and storage conditions that could be helpful to maintain the stickiness during storage. In this work, the effect of modified atmospheric packaging and various pre-processing treatments such as alkali washing, starch modification, and parboiling were assessed.Initially, the effect of different rehydration temperatures (30 to 50oC) and cooking times (2.7 to10.7 min) at 95oC on the pasting properties of flour of three glutinous varieties (TDK11, TDK8, and Hom Mali Niaw) was investigated using the RVA. Increased soaking temperature and time resulted in reduced pasting temperature for all varieties. Extended holding at a cooking temperature (95oC) had a more significant (P<0.05) effect on final gel viscosity.Rice is usually consumed in the form of whole grains. Therefore, it is important to study the cooking kinetics of whole rice grain. To study the water uptake rate and cooking kinetics of TDK8 (fresh and aged) and TDK11, a novel in situ method using Thermal Mechanical Compression Test (TMCT) attached to a texture analyzer was developed. The TMCT cooking method was found valid for in situ analysis of rice cooking by using sample sizes as small as 0.50 g.A new method (X-ray photoelectron spectroscopy) employed to quantify the surface composition of raw and cooked rice grains showed that the surface of uncooked grains of glutinous (TDK11) rice had a higher content of protein and lipids (49.7 and 36.2 %, respectively) to starch (13.4 %) compared the bulk composition (protein ~ 6.6 %, lipids ~ 0.8 %, and starch ~ 92.6 %). Protein was hypothesized to contribute to the loss of stickiness. An alkali washing with different concentrations of NaOH (0 to 0.2 %) of milled grains of TDK8 and Doongara resulted in a significant (P<0.05) increase in the stickiness and hardness of cooked rice grains, and final viscosity of rice flour, and a decrease in the amount of retrograded starch. In another study, the acetylation of starch in the whole grain of TDK8 and Doongara was achieved using various acetic anhydride concentrations (1-7 g per 100 g of milled grains in 225 mL of water). Results showed that acetylation reduced the crystallinity of starch with a significant (P<0.05) reduction in peak and final viscosities, and reduced thermal transition temperatures and enthalpy. Furthermore, the texture of cooked grains was softer and more adhesive. A significant reduction in the glycemic index (GI) of acetylated samples was also observed using the in vitro digestion method.Parboiling of TDK8 and TDK11 was undertaken using various soaking mediums water (control), 3 % NaCl solution and 0.2 % acetic acid solution. These saline and acetic acid soakings improved the milling efficiency (39 to 41 % HRY for TDK8 and 48 to 53 % HRY for TDK11) when compared to the control (26 % HRY for TDK8 and 29 % HRY TDK11). Saline and acetic acid soaking resulted in reduced crystallinity and thermal endotherms. When compared to the control, saline soaking improved water absorption, resulting in a higher peak (~10 % increase) and final viscosity (~5 % increase), whereas, the acetic acid soaking restricted swelling, resulting in a reduced peak (~10 % decrease) and final viscosity (~15 % decrease). Furthermore, parboiling increased hardness (2.6 to 5 N for TDK8 and 2.3 to 3.5 N for TDK11) and adhesiveness (-0.2 to -0.5 N.s for TDK8 and -0.5 to -0.7 N.s for TDK11) of glutinous rice in saline and acetic acid soaking as compared to the control (-0.3 to -0.5 N.s for TDK8 and -0.6 to -0.7 N.s for TDK11). Also, parboiling improved the nutritional quality of glutinous rice by reducing the GI from 116.5 to 100.4 for TDK8 and 94.8 to 72.2 for TDK11. Overall, pre-process treatments of glutinous rice with alkali washing, acetylation of intact starch and parboiling showed improvement in the grain quality and which can have commercial potential.The effect of modified atmospheric packaging (MAP) (control, vacuum, CO₂ and N₂) on the aging-induced changes in the physicochemical properties of TDK8 and TDK11 was also assessed. N₂ and CO₂ induced an increase in pasting temperature (72.6oC ~ control, 73.7oC ~ N₂ and 74.2oC ~ CO₂) and a significant (P<0.05) reduction in final viscosity (2276 mPa-s ~ control, 2157 mPa-s ~ N₂ and 2216 mPa-s ~ CO₂) after 12 months of storage. The in situ TMCT cooking and texture analysis revealed that MAP slightly slowed the aging-induced changes in the cooking quality and stickiness of glutinous rice. Overall, among all the storage conditions used, the vacuum was considered the best to maintain the quality of the glutinous rice.