RICE BEING one of the leading crops plays a vital role in Pakistan’s economy. In addition to meeting the food requirements, it is a also source of earning foreign exchange.
The quality of grain is poor at the farmer’s field as compared to the grain produced at research stations. Higher prices could be achieved by improving the quality through modern technology. Presently, the country is producing surplus than its national needs. Improvement in quality provides assurance that the surplus will find a rewarding market.
The genetic make-up of grain is the major factor influencing the quality of rice. Modern programmers continually strive to refine and improve the genetic characteristics that influences quality in the most desirable product. Another factor is the environment under which the plant is grown, such as the light rainfall and the temperature. Similarly, the soil and the management practices affect the grain quality. Careful harvesting and post harvest handling may maintain or even improve the rice quality.
Quality characteristics in rice may be categorized into 3 broad areas (1): physical characteristics include moisture content, shape, size, whiteness, translucency, chalkiness, head rice, broken rice, brewers, green kernels and yellow kernels. (2): The analysis of physico-chemical characteristics of rice include amylose content, protein content, gel consistency, volume of expansion of cooked rice, water absorption, and cooking time. (3): The organoleptic of cooked rice include colour, aroma, hardness, stickiness, and consistency.
PHYSICAL CHARACTERISTICS: Moisture content affects rice quality in several ways. Sound dry rice can be kept for years but wet rice takes few days to rot. The moisture content of 13 per cent is accepted for less than 6 months storage, where as 12 per cent or less is recommended for long-term storage. To gain and maintain the optimum milling quality, rice must be harvested at the proper moisture content i.e. 24 per cent, dried carefully up to 14 per cent. The excessive level may adversely affect various cooking and processing quality characteristics in rice.
GRAIN DIMENSIONS: Uniformity in shape and size is considered as the first quality characteristic. In the international market rice is marketed according to 3 grain sizes and shapes known as long having the length of 6.7-7 mm, medium 5.5-5.8 mm and short with 5.2-5.4 mm. The Pakistani Basmati rice has an average length of 6.7 mm, while its width is measured as 1.8 mm. The grains dimension is a genetically-controlled character and not much affected by the management practices.
GRAIN WEIGHT: This test is particularly useful as a comparative indication of total milled rice yields. It provides relative measure of dockage or foreign material present, and the proportion of unfilled, shrivelled and immature kernels. Although, this is a genetically-controlled parameter but may be affected by the environment in which it is growing, e.g. injudicious use of nitrogen fertilizer and also of the light conditions for photosynthesis.
GENERAL APPEARANCE: The overall appearance of the processed kernel is extremely important for judging the quality of rice, whiteness, translucency, chalkiness. The coloured, damaged and the imperfect kernels are equally important and are judged by the human eye. Clear, vitreous, translucent kernel are demanded by all the segments of rice industry, while chalkiness is an undesirable trait for particularly, all forms. The white powdery portion of a rice grain is known as the chalky part and usually results in lower milling yields, because the chalky kernel tends to be weak and breaks up more during the milling. The chalkiness is often referred to as the “white belly”, “white core”, “white back,” “germ tip” or immature, depending on its location on or within the endosperm. Adverse weather condition and cultural practices also influence the incidence of chalkiness in rice and also by neck blast and drought stress during ripening.
MILLING YIELDS: The whole grains and grains having 4/5th of the length are termed as head rice. The rest are taken to be broken (screaming and brewers). It is sufficient to say that no rice variety would make commercially viable unless it possessed the high whole kernel (head) and the total milled rice turnout. The total milled rice yields include the whole kernel (head) and all sizes of broken kernels.
The objectives of rice milling are the removal of hulls, bran and germ with a minimum breakage of the endorsperm. The milling quality in rice is based on the yield of whole kernel (head) rice. The yields of whole kernel (head) rice vary widely, depending on the variety, grain type, chalkiness, and cultural practices. Thus to obtain the maximum whole grains all these factors must be considered right from the sowing to the milling process.
PHYSICO-CHEMICAL AND ORGANOLEPTIC CHARACTERISTICS: It is possible to translate the consumer preference for cooking quality into measurable chemical properties. The cooking quality depends upon the number of chemical characteristics.
Starch, a polymer of glucose, is a major constituent of the milled rice, constituting about 90 per cent of its weight. It occurs in the form of compounds poly hydral granules. It is composed of a major branched fraction, amilopactin, and a linear fraction, amylose. Amylose content is the most important criterion of grain quality of milled rice and is an indicator of amylose/amylopactin ratio. It is usually expressed as a percentage of milled rice and weight rather than a Starch ratio. Based on amylose content, the milled rice is classified as waxy (1-2 per cent) or non waxy (>2 per cent), very low (2-9 per cent), low (9-20 per cent), intermediate (20-25 per cent) and high (25-33 per cent). The tenderness and stickiness of cooked rice inversely correlate with the amylose content. When cooked, the rice with high amylose content is relatively dry, separate, and less tender. The rice also become hard upon cooling. The low amylose content rice (10-20 per cent) tend to be moist and sticky when cooked. Basmati rice of Pakistan tends to have higher amylase content i.e., more than 25 per cent. It is preferred in the international market.
The temperature during grain ripening has been shown to affect the amylose content. It generally decreases as the mean temperature increases, depending on the variety and inherent level of amylose content. It decreases slightly with nitrogen fertilization but not affected by the stage of application. The variation in amylose content between plants was much less (2 per cent) than that between panicles within plants (3-7 per cent). The use of bulk samples for its determination has been recommended.
Rice with similar amylose content can be differentiated according to the tenderness, increased by the gel consistency. Given the same amylose group, cooked rice with softer gel consistency is more tender. According to rice classification: hard (26-40mm), medium (41-60mm) and soft (61-100mm).
The gelatinization temperature (GT) is the temperature at which the starch granule begins to swell irreversibly in hot water with a simultaneous loss of crystallite. This value ranges roughly form 55 to 70C in rice varieties. It is a fact that the GT has an influence on the cooking quality of rice. For rice with higher GT seem to require a little longer time to cook. The GT may have little relation to the palatability characteristics of the table rice.
The water absorption index is a measure of the quantity of water absorbed during cooking. The greater water absorption index makes cooked rice more expandable and heavier. It was assumed that a high water uptake in boiling water is an indication of good cooking quality of rice. Cooking time depends on the chemical composition, rice variety and the length of the storage before the rice is cooked. The average cooking time is 24 minutes with a range of 17-40 minutes. Kernel expansion is determined from the ratio of the cooked volume of rice to that of the uncooked. It ranges form 2 to 4.35 times depending upon the amylose content. The linear kernel elongation after cooking is compared with the original length of kernel before cooking. Basmati rice shows extreme elongation when pre-soaked grain is cooked.
The nutritional value depends on the total quantity and quality of protein. Rice is an important source of protein and supplies more than 50 per cent of the total protein consumed, in some countries. Even a modest increase in protein levels would provide a significant nutritional boost to the rice-based diets. However, the protein content of the milled rice is relatively low (mean of 9 per cent). The milled rice proteins consist of at least 80 per cent or more of glutalin, 10 per cent globulin, 5 per cent prolamin.
The protein content tends to be low when high solar radiations occur during grain development, thus it is generally low in the dry season than in the wet season. The temperature during grain ripening is also reported to affect the protein content. Our rice is not consistently affected by the temperature. The management and cultural practices have been shown to greatly affect the protein content. The protein content is higher with wider plants spacing, where more nitrogen is available to the plants. It also increases with better water management and better weed control, probably because of higher efficiency in nitrogen utilization. It is also reported that the protein content of rice grown in the puddle soil was higher than that grown in the unpuddle soil, is affected by the time of harvest and is generally low at early harvest as compared to late.
Aroma: The scent or the natural fragrance in kernel is a much valued quality factor. Because of this feature, in trade and commerce, the scented varieties are given a premium price, irrespective of the variation in size. The chemical creating the flavour is identified as 2-acety1-1 pyrroline, which is found in the volatile oil component of the cooked rice. This is a climatically-controlled character, which develops in the kernels while growing in that particular climate. Pakistani Basmati rice grown at Kalar tract are aromatic. This special feature is not present in any country’s environment and hence the Pakistani Basmati enjoys monopoly in the international market. As the grain quality is complex, the rice research can improve the quality in several ways. the chemists can identify varietals characters and their links to cooking quality. The economists can measure the consumer preference, cost of processing and handling, and the market efficiency. The agronomists and the engineers can identify production and processing techniques that improve the quality or lower the cost of maintaining quality. Finally, the plant breeder can incorporate quality characteristics into new varieties.
Dear visitor, the comments section is undergoing an overhaul and will return soon.