RICE MILLING
INTRODUCTION, SMALL SCALE AND LARGE-SCALE MILLING
Rice Milling
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 is milled in India either by home pounding (small scale) or in mechanised rice mills (large scale). In home pounding there are more broken rice.
. The objective of the rice milling is to remove the husk and bran with minimum possible breakage of endosperm. Paddy is generally harvested at 18 – 25% moisture and then dried to 12 – 13% moisture either on farm or at the mill before processing.
Milling Procedure
Harvested rice generally has a moisture content of about 20% (wet basis) and the grain must be dried immediately to about 12% for storage. Rice is consumed mostly in the form of whole kernels, and accordingly the processing of paddy is designed to give a high yield of unbroken kernel.
The process of milling involves the following steps:
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.
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.
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.
Sometimes the polished rice is further treated in a device known as ‘Trumbo’ to give a coating of sugar and talc to produce a brighter shine on the grains.
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 per 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 thiamine is lost. The percentage of losses of different nutrients during milling is protein- 15 per cent, fat- 82 per cent, thiamine- 85 per cent, and 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:
By under-milling or polishing rice, the loss of nutrients can be reduced. Rice that is under-milled do not have the customary white lustre and they are more subject to insect infestation and flavour deterioration than white rice is.
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”.
Small scale milling
In case of small-scale milling of rice, paddy is placed in a mortar and pounded with pestles either by hands or with the feet. After some time of pounding, the rice is sifted to separate the husk. The pounding process is repeated several times. Rice obtained by this process is called brownrice/rough rice and contain a greater number of vitamins as a proportion of the pericarp, Testaand aleurone layers remain on the rice grain.
Large scale milling
Schematic representation of the rice milling process is depicted in Fig. 6.1
Cleaning
Cleaning of paddy comprises removal of sticks, stones, dust and other foreign materials. This is accomplished by use of various separation methods. The paddy is first passed over a screen to remove larger particles, straws and string. After that it is passed through second screen, which is having smaller perforations than first screen, to remove weed seeds and sand. The paddy then flows in the form of a thin layer into a channel where an air current removes dead grains and other lighter impurities. At the last, paddy are passed through magnetic separator to remove metal particles.
Hulling/Shelling
Cleaned paddy is then passed through machine (disc huller/sheller) comprising emery/ rubber rolls running in opposite directions; aspirated to remove husk and then sieved to separate from the unhusked and broken rice. The rice with the hull removed is commonly known as “brown rice”/“rough rice”.
Scouring/Pearling/Whitening
Gradual removal of germ and bran from the rough rice is known as scouring/pearling/whitening process. The hulled rice is passed through a series of “pearling cones”. In pearling cones rice passes through the narrow annular space left between an inverted cone coated with abrasive revolving in a conical casing made of steel wire cloth. As it passes down, the bran is pushed through the interstices of the wire cloth. By-product of scouring process is known as “rice-bran” which is used as animal feed. It is also used to extract rice bran oil.
Polishing
The rice grain consisting inner layers of bran is passed through polishing machine often referred to as “brush”. In this machine last bran fraction is removed. The grain is now called “polished rice”.
RICE QUALITY AND GRADING STANDARDS
Characteristics of rice known and referred to as grain quality largely determine market price and consumer acceptance. The grain quality has many connotations and is perceived differnently depending upon the end use, field of interest, specialization, ethnic background, etc. quality in rice may be categorized into grain quality indicators and cooking/eating quality indicators.
Grain quality indicators
Grain dimension – size and shape
There exist enormous variation in the size and shape of the grins as they are conditioned by large number of genes. Grain dimension is expressed as length, breadth and thickness, whereas shape is expressed as the ratio between the length and breadth. These parameters are the major bases for the characterization of rice varieties. Short grain varieties vary in their length from 3 -6 mm (length-to-breadth ratio, 2.5-3.0) whereas long grain varieties have a length of 6mm and above (length-to-breadth ratio, more than 3.0). There are three major types of rice –long, medium and short grained, whch are classified by the grain shape (length-to-breadth ratio) and tend to have different properties and hence different uses.
Grain colour and translucency
White and translucent rice is preferred by people in most parts of the world. The colour of the duhusked, unpolished rice is usually pale white, creamy white, brown or red. On polishing, the kernel becomes white, translucent or opaque according to the nature of the pericarp and endosperm colour of the brown rice.
Cooking/eating quality indicators
Cooking and processing characteristics of the rice are the factors of primary importance in rice eating areas of the world. Milling, cooking and processing qualities are the fundamental components of quality that determine and establish economic value of rice. Upon cooking, long grain rice is dry and fluffy with individual grains, whereas medium and short grain types are moist and chewy with grains that tend to stick or clump together. Major cooking quality parameters are discussed hereunder.
Amylose content
Amylose content is considered as the single most important characteristics for predicting rice cooking and processing behavior. In rice it varies roughly from 15-37%. A high amylose content is usually associated with non-sticky cooking characteristics and vice-versa. Glutinous or waxy rice, which has no or very little amylose content, becomes very sticky on cooking.
Gelatinization temperature
The gelatinization temperature of starch is the range of temperature within which the starch starts to swell irreversibly in hot water with a simultaneous loss of crystalinity, and usually varies from 56° to 79°C. It is correlated with the extent of disintegration of milled rice in a dilute alkalisolution (1.7-2.0% KOH) measured in terms of alkali spread value. Gelatinization temperature is also positively correlated with the cooking time but not with the texture of cooked grains.
Gel consistency
The gel consistency test is the index of cooked rice hardness among high amylose rice. Rice is classified on the basis of gel length as soft, medium and hard. Soft to medium gel consistency is preferred to hard gel consistency. Among high amylose rice, intermediate gelatinization temperature and soft gel consistency are preferred by consumers over low gelatinization temperature and hard gel consistency.
Milling quality
Milling quality means the degree to which the endosperm remains intact at the end of milling.
Milling quality, indicated by total milling yield or head rice (milled rice kernels that are three quarters or more of the endosperm length) yield, is expressed as a percentage of rough rice. Breakage during milling process is not desirable.
Nutritional quality
Brown rice contains more nutrients (minerals and vitamins) than milled rice. However, status of nutrients is dependent on genetic variability of rice throughout the world.
Specific quality designations regarding cleanliness, soundness and purity Special grades are provided for the specific qualities or conditions of rice that affect marketability. These special grades:
1 Rough rice: Parboiled rough rice, Smutty rough rice, Weevily rough rice
2 Brown rice: Parboiled brown rice and Smutty brown rice for processing
3 Milled rice: Parboiled milled rice, Undermilled milled rice
In USA rice grade designation follows this order: (1) the letter US (2) no. of grade (3) class (4) applicable special grade (5) milling yield e.g. U.S. No. 3, long grain rough rice, parboiled, milling yield 50 – 70%.
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