Quality characteristics of milled rice

The quality characteristics of milled rice are classified both physically, and chemically.

Review the following terms before reading about physical and chemical characteristics of milled rice:

• Paddy or rough rice = similar term for paddy, or rice retaining its husk after threshing
• Brown rice or husked rice = paddy from which the husk has been removed
• Milled rice = rice after milling which includes removing all or part of the bran and germ from the husked rice
• Head rice = milled rice with length greater or equal to three quarters of the average length of the whole kernel
• Large brokens = milled rice with length less than three quarters but more than one quarter of the average length of the whole kernel
• Small brokens or "brewers rice" = milled rice with length less than one quarter of the average length of the whole kernel
• Whole kernel = milled rice grain without any broken parts
• Milling recovery = percentage of milled rice (including brokens) obtained from a sample of paddy.
• Head rice recovery = percentage of head rice (excluding brokens) obtained from a sample of paddy.

Physical characteristics

Milling degree

The degree of milling is a measure of  the percent bran removed from the brown rice kernel.  Milling degree affects milling 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 as quickly as milled rice. The water absorption rate improves progressively up to about 25% milling degree after which, there is very little effect.

Head rice

“Head rice” or head rice percentage is the weight of head grain or whole kernels 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.  The actual head rice percentage in a sample of milled rice will depend on both varietal characteristics (i.e. the potential head rice yield), production factors, and harvesting, drying and milling process. In general harvesting, drying, and milling can be responsible for some losses and damage to the grain.

Whiteness

Whiteness 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 after whitening is carried out 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

If part of the milled rice kernel is opaque rather than translucent, it is often characterized as “chalky”.  Chalkiness disappears upon cooking and has no effect on taste or aroma, however it downgrades milled rice. Excessive chalkiness is caused by interruption during the final stages of grain filling. 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 milled rice is determined by gelatinization temperature or GT. Environmental conditions, such as temperature during ripening, influence GT.  A high ambient temperature during development results in starch with a higher GT. GT of milled rice is evaluate by determinining the Alkali spreading value. In many rice-growing countries, there is a distinct preference for rice with intermediate gelatinization temperature.

Amylose content

Starch makes up about 90% of the dry matter content of milled rice.  Starch is a polymer of glucose and amylose is a linear polymer of glucose.  The amylose content of starches usually ranges from 15 to 35%. High amylose content rice shows high volume expansion (not necessarily elongation) and high degree of flakiness.  High amylose grains cook dry, are less tender, and become hard upon cooling.  In contrast, low-amylose rice cooks moist and sticky. Intermediate amylose rice are preferred in most rice-growing areas of the world, except where low-amylose japonicas are grown.

Based on amylose content, milled rice is classified in “amylose groups”, as follows:

• waxy (1-2% amylose),
• very low amylose content (2-9% amylose),
• low amylose content (10-20% amylose),
• intermediate amylose content (20-25% amylose) and
• high amylose content (25-33% amylose).

Amylose content of milled rice is determined by using the colorimetric iodine assay index method.

Gel consistency

Gel consistency measures the tendency of the cooked rice to harden after cooling. 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 tends to be less sticky. Gel consistency is determined by heating a small quantity of rice in a dilute alkali.