Rice contributes to 43 per cent of total food grain production and 46 per cent of total cereal production.
The consumption of rice in India is around 84 million tonnes in 2002-03. Rice is a staple diet for more than half of the world’s population. It is principally consumed in Asia.
It is influenced by genetic and environmental factors. The germ, the pericarp and aleurone layers which are richer than endosperm in nutrients like protein, minerals and vitamins are separated from the grain during milling alongwith the husk.
The major carbohydrate of rice is starch which is 72-75 per cent. The amylose content of starch varies according to the grain type.
The longer grain and superior types containing upto 17.5 per cent amylose while some coarse types are completely devoid of it.
Glutinous rice consists almost entirely of amylopectin. Rice also contains some free sugars like glucose, sucrose, dextrin, fructose and raffinose. The fibre of rice is the hemicellulose made up of pentoses, arabinose and xylose.
The protein content of rice is 7 per cent. It is much lower than that of the wheat. Glutelin which is also known as oryzenin is the principal protein of rice.
Rice also contains small quantities of albumin, globulin and prolamines. The proteins of polished rice have a lower biological value but a higher digestibility than those of rice bran and rice polishing.
Parboiling has no effect on the biological value or digestibility of the proteins. The nutritive value of rice protein is of high order being superior to that of the wheat and other cereal products.
The rice proteins are more rich in arginine compared to other cereal proteins. Rice is deficient in lysine and threonine. The biological value of rice protein is 80 whereas wheat protein has 66 and maize protein has 50.
In its mineral content, rice resembles other cereals. Most of the minerals present in the rice are located in the pericarp and germ.
Polished rice is poor in calcium and iron. Coloured types of rice contain more iron than the white rice.
The phosphorus content is high, about 4 per cent of which is present as phyticacid. Rice also contains some trace elements.
Rice contains amylases, proteases, lipases, oxidases, peroxidases and phenolases. On storage the oxidese activity of rice remains constant but the amylase, lipase, peroxidase activities decrease.
The activity of the alpha amylase in fresh rice is probably responsible for its sticky consistency after cooking. This enzyme gets inactivated during storage and cooked grains get separated easily.
Coloured rice contains anthocyanins and carotenoids.
For best milling yield of rice, paddy has to be harvested at an optimum moisture content of 20-24 per cent.
Before milling, paddy is cleaned to remove small and large, heavy impurities. In dehulling and milling coarse outer layer of bran and germ are removed.
Paddy on milling yields hulls 20 per cent, bran 8 per cent, polishing 2 per cent and rice 70 per cent.
Paddy is milled in India either by home pounding or in mechanized rice mills. In home pounding there are more broken rice.
Storage life is short as the fat in the bran develops rancidity. The process of milling involves the following steps:
(i) Rice is passed through two stone or rubber discs rotating at different speeds and by shearing action on the grain, the hull is pulled away. The whole kernel from which the hulls have been removed is known as brown rice.
(ii) This is then milled in a machine called pearler to remove coarse outer layers of bran and germ by a process of rubbing, resulting in unpolished milled rice. Some amount of breakage of rice occurs in this milling.
(iii) Unpolished rice is liable to develop rancidity and so it is next polished in a brush machine which removes the aleurone layer and yields “polished rice”.
(iv) Sometimes the polished rice is further treated in a device known as trumbol to give a coating of sugar and talc to produce a brighter shine on the grains.
(v) Rice is separated from the broken kernels. Large kernels are called as second heads, medium ones are called screenings, smallest ones are called the brewers rice.
Most varieties of coarse rice are not highly polished. As the Indian Government regulations the extent of polishing should not exceed 5 per cent. If rice is milled beyond 10 per cent then most of the thiamin is lost.
The percentage of losses of different nutrients during milling are protein 15 per cent, fat 82 per cent, thiamin 85 per cent, riboflavin 70 per cent, pyridoxine (vitamin B6) 50 per cent.
The degree of milling determines the amounts of nutrients removed. Losses during milling can be compensated by the following processes.
(i) By under-milling or unpolishing rice, the loss of nutrients can be reduced. Rice that are under- milled do not have the customary white lustre and they are more subject to insect infestation and flavour deterioration than white rice is.
(ii) A second method is that of increasing vitamin retention by processing the rough rice prior to milling. This is done by parboiling which is commercially known as converted rice.
(iii) Another means of remedying the losses occurring in the milling of rice is the artificial enrichment of the grain.
A premix has been developed in which the rice is wetted with a solution of thiamin and niacin, then dried.
A second coating of iron pyro phosphate is distributed on the rice. The rice premix is highly resistant to washing, cooking and storage losses.
It is originated in India. Half of India’s rice crop is parboiled. Parboiling is particularly good in the case of coarse and medium rice of soft structure because such rices suffer excessive breakage when milled raw.
Parboiling involves soaking paddy in water for a short time followed by heating once or twice in steam and drying before milling.
This consists of the following steps:
1. Steeping paddy in cold water for 2 or 3 days in large cement tanks.
2. Steaming of the soaked paddy for 5-10 minutes and
3. Drying in the sun.
During soaking of paddy in cold water for 2 or 3 days, fermentation sets in and off flavour develops in the grain.
The moulds present in the grain also grow and may produce toxic metabolites. For example aflatoxin may be produced from aspergillus flavus.
Hot soaking process:
The hot soaking process, developed at the Central Food Technological Research Institute, Mysore consists of the following steps:
1. Soaking of paddy in water at 65-70°C for 3-4 hours.
2. Draining of water and steaming of soaked paddy in the same vessel for 5-10 minutes, and
3. Drying of the paddy in the sun or in mechanical driers.
During parboiling, the milled rice becomes slightly shorter and broader.
Advantages of parboiling:
(i) Dehusking of parboiled rice is easy.
(ii) Grain becomes tougher resulting in reduced losses during milling. This benefit is due to healing of all kernel defects like cracks and chalkiness by parboiling. By this, parboiling eliminates breakage completely.
(iii) Milled parboiled rice has greater resistance to insects and fungus.
(iv) Loss of nutrients due to the removal of husk and bran in milling are decreased. During harvesting the vitamin and mineral present in hull (outer covering of the paddy) and bran coat are dissolved
and seeped into the endosperm. Part of the scutellum and germ which are rich in B vitamins get fixed to the grain and hence losses of B vitamins are less.
(v) Losses of water-soluble nutrients due to washing of rice is less in parboiled rice compared to raw rice.
(vi) Parboiling improves digestibility and Protein Efficiency Ratio is higher compared to raw rice.
(vii) Parboiled rice will not turn into glutenous mass when cooked.
(viii) Parboiled rice swells more when cooked to the desired softness.
(ix) Parboiling stabilizes the oil content of the bran. The discrete oil globules in the aleurone layer of the native rice grain are ruptured into a band by parboiling.
Disadvantages of parboiling:
Sometimes, it has an unpleasant smell and change in colour and hence not preferred. Modern methods of parboiling have eliminated this problem. By controlling soaking and steaming, colour and cooking quality can be maintained.