Rice Post Production

Harvesting the crop on time is very important to maximize yields and grain quality. Crops harvested too early will have many unfilled and immature grains. Immature grains break easily when milled and will not germinate when used as seed. If crops are harvested late, heavy losses occur through shattering and bird attack. Quality also decreases due to grain weathering, resulting in breakage and downgrading due to undesirable grain colour.

• Grain moisture is between 20–25%, which is normally about 30 days after flowering or 80–85% of the grains are straw-coloured.
• Grains in the lower part of the panicle are hard, not soft, and
• Grains are firm but not easily broken when squeezed between the teeth.

• Minimizing time the harvested panicles remain in large bundles in the field — thresh within 24 hours of cutting.
• Avoid direct contact of panicles with soil for better quality.
• Drying the grains as soon as possible after threshing.
• Turning or stirring the grains at least once every 30 minutes to achieve uniform drying.
• Sun-drying on tarpaulins or clean drying pads.
• Keeping the thickness of the grain layer at 3–5 cm.
• Covering the grain during mid-day on hot days to prevent over-heating and covering immediately, if it starts raining.
• Cleaning the grain by repeated winnowing after drying; and
• Storing the rice in a cool, dry and clean area preferably in hermetic / sealed containers for seed.

Importance of drying the paddy correctly

Rice is usually harvested at grain moisture content (MC) between 20 and 25% (wet basis). Any delay in drying, incomplete drying or uneven drying results in qualitative and quantitative losses, such as:

• Yellowing or discoloration caused by mold development and heat build-up from respiration.
• Reduced milling yields caused by high temperatures and re-wetting of grains.
• Loss of germination and vigour from grain respiration, mold and insect activities or exposure of grains to higher temperatures.
• Damage caused by insects which are more active at higher MC levels.

Rice is best stored as paddy because the husk provides some protection against insects and prevent grain quality deterioration. A safe or hermetic storage system prevents the grain from getting wet after drying and also gives protection from insects, rodents, and birds.

Rice can be stored for longer periods if:

• Moisture content is maintained at less than 14% for grain and 12% for seed;
• Grain is protected from insects, rodents, and birds; and
• Grain is protected from re-entering of moisture by rain or from the surrounding air.

A rule of thumb for seed is that the life of the seed is halved for every 1% increase in moisture content or a 5°C increase in storage temperature above recommended levels.

Milling rice paddy removes the husk and bran layer to produce white rice. Rice is best milled at 13–15% moisture content. Best results are attained when the process is completed in a number of stages. Grain temperatures should not exceed 45°C during the process. An efficient mill removes the husk (20%), the bran or meal (8–10%), and leaves 70% as white rice. Rice grown in irrigated systems should attain 60% white rice as head rice (unbroken, white kernels), and in rainfed systems, 40–50% as head rice. Rice is milled in several ways-

After harvesting, the rice paddy undergoes postharvest processes including drying, storage and milling to ensure good eating-quality and marketability.

Physical characteristics

Milling degree or colour: The degree of milling or amount of the brown rice removed, affects the colour of white rice, and often the price. Under-milled rice absorbs water poorly, does not cook well, and is normally cheaper.

Head rice or broken percentage: Milled rice, having 75-80 % head rice (whole kernels), also includes broken kernels. High head rice yield is one of the most important criteria for measuring milled rice quality. High-quality rice normally has less than 5% broken grains.

Whiteness or translucency:This characteristic is a combination of varietal physical characteristics and the degree of milling. During milling, the whitening and polishing process greatly affects the whiteness of the grain and its transparency.

Chalkiness:Grain appearance is affected by the amount of chalkiness or white belly. Chalkiness is caused by interruption in the final grain-filling. Though chalkiness disappears upon cooking (and has no direct effect on cooking and eating qualities), excessive chalkiness often downgrades the quality and reduces milling recovery.

Gelatinization temperature or cooking time: Environmental conditions such as temperature during ripening influence gelatinization temperature. There is normally a preference for rice with intermediate gelatinization temperature.

Amylose content or stickiness: The amylose content of rice usually ranges from 15–35%. High-amylose rice has high volume-expansion, the grains cook dry, are less tender, and become hard upon cooling. Low-amylose rice cooks moist and sticky. Intermediate-amylose rice (21–24%) is preferred in most rice-growing areas of the world.

Gel consistency: Gel consistency measures the tendency of the cooked rice to harden on cooling. Varieties with a softer gel consistency are preferred if rice is to be consumed after cooling, or if cooked rice with higher degree of tenderness is desired.