Rice Quality Training Manual 08

Rice Quality Training Manual
4.0 Quality characteristics of milled rice
The quality characteristics of milled rice are classified both physically and chemically.
Physical characteristics
Milling degree. The degree of milling or percent brown rice removed as bran affects the level of recovery and influences consumer acceptance. Apart from the amount of white rice recovered, milling degree influences the color and also the cooking behavior of rice Unmilled brown rice absorbs water poorly and does not cook well. The water absorption rate improves progressively up to about 25% milling degree after which, there is very little effect.
The flow (frictional property) and packing (bulk density) behaviors of rice are also depend on milling Likewise, the nutrient content of rice is also strongly influenced since most micro-nutrient located largely in the peripheral layers of brown rice are removed with high milling degree.
Head rice percentage. The head rice percentage is the volume or weight of head grain or whole kernel in the rice lot. Head rice normally includes broken kernels that are 75-80% of the whole kernel. High head rice yield is one of the most important criteria for measuring milled rice quality. Broken grain has normally only half of the value of head rice. To a large extent, the characteristics of the paddy determine the potential head rice yield although the milling process is responsible for some losses and damage to the grain.
Whiteness.
This characteristic is a combination of varietal physical characteristics and the degree of milling. In milling, the whitening and polishing greatly affect the whiteness of the grain. During whitening, the silver skin and the bran layer of the brown rice is removed. Polishing is undertaken after whitening to improve the appearance of the white rice. During polishing some of the bran particles stick to the surface of the rice which polishes and gives a shinier appearance.
Chalkiness. Grain appearance is largely determined by the endosperm opacity and this is commonly classified as the amount of chalkiness. Opaqueness has an overall chalky texture caused by interruption of final filling of the grain. Though chalkiness disappears upon cooking and has no direct effect on cooking and eating qualities, excessive chalkiness downgrades the quality and reduces milling recovery.
Chemical characteristics
Gelatinization temperature. The time required for cooking is determined by gelatinization temperature. Environmental conditions, such as temperature during ripening, influence gelatinization temperature. A high ambient temperature during development results in starch with a higher temperature.
Gelatinization temperature is estimated by the extent of alkali spreading and clearing of milled rice soaked in 1.7% KOH at room temperature or at 39oC for 23 hours (Little et al, 1958). The degree of spreading is measured using a seven-point scale as follows:
1. grain not affected
2. grain swollen,
3. grain swollen, collar incomplete and narrow,
4. grain swollen, collar complete and wide,
5. grain split or segmented, collar complete and wide,
6. grain dispersed, merging with collar; and
7. grain completely dispersed and intermingled.

Alkali spreading value corresponds to gelatinization temperature as follows:
• 1-2 high (74.5-80oc),
• 3, high intermediate,
• 4-5, intermediate (70-74oC), and
• 6-7, low (<70oc).>2% amylose),
• very low (2-9% amylose),
• intermediate (20-25% amylose) and
• high (25-33% amylose).
The colorimetric iodine assay indexes the amylose content of milled rice.
Gel consistency Gel consistency measures the tendency of the cooked rice to harden on cooling. Gel consistency is determined by heating a small quantity of rice in a dilute alkali. This test differentiates the consistency of cold 5.0% milled rice paste. Within the same amylose group, varieties with a softer gel consistency are preferred, and the cooked rice has a higher degree of tenderness.
Harder gel consistency is associated with harder cooked rice and this feature is particularly evident in high-amylose rice. Hard cooked rice also tend to be less sticky.