Some Quality Factors of Raw and Processed Rice

In this paper, emphasis is laid on the quality of well threshed paddy, free of contaminants (such as stones, peebles and other cereal grains) as received at the rice processing plant. Methods for the quality evaluation of parboiled rice are outlined and the effect of processing on some of these quality factors is also highlighted.

QUALITY EVALUATION OF PADDY RICE

The paddy rice received at the plant could be subjected to some quality tests such as moisture content, length and shape of the grain, amylose content and gelatinization temperature and pasting characteristics of the grain.

Moisture Content
Moisture content is of importance since it affects the keeping properties of rice during storage, milling quality, processing and cooking characteristics. Methods of determination include the use of air of vacuum oven, electrical devices such as the Marconi and Tag-Heppenstall moisture meters. There is also the titration method using the Karl Fischer reagent.

Length and Shape of Grain
Two of the factors contributing to appearance of grain are its length and shape. For this assessment, length and breadth of twenty whole grains, selected at random from raw and parboiled milled samples are measured, and mean value calculated. Table I gives the FAO classification of milled rice based on length and shape. Using this classification, the effect of parboiling on four local rice varieties is given in Table 2.

Studies have indicated that minor changes or adjustments in the length and breadth occur in the grains as a result of parboiling. However, the effect of parboiling on those properties depends on the variety and processing conditions.
Amylose Content: This is considered one of the most important quality factors of rice since it is known that amylose content is a good index of water absorption and textural properties of rice. The texture of cooked rice and its gloss are principally determined by the amylose: amylopectin ratio of the starch. Increasing the amylose content improves the capacity of the starch granule to absorb water and expand in volume without collapsing because of the greater capacity of amylose to hydrogen bond or retrograde. Thus the amylose content is an index of resistance to disintegration during cooking. It has been shown that an increase in amylose content improves the firmness of cooked parboiled rice, the correlation coefficient (r) between amylose content and firmness being 0.62.

Amylose content of milled rice is determined by the colorimetric methods. Another method is the starch iodine blue test which is used for screening amylose content of samples with less than 30 mylose. The most widely used method involves digesting 100mg of rice flour with 1ml, 95thanol and 9ml of 1NNaoH, heating the sample for 10 min in a boiling water bath, and diluting the gelatinized starch with distilled water, 5ml of the starch solution is pipetted into a 100 ml volumetric flask, 1ml of 1N acetic acid and 2ml of iodine solution added, made up to volume and absorbance read at 620nm. Amylose content is determined by reference to a standard curve.

The amylose content of some Nigerian rice varieties has been shown to vary between 19 nd 28nbsp; Various workers have shown that parboiling does not affect the amylose content of rice.

ALKALI SPREADING VALUE, GEL CONSISTENCY AND PASTING CHARACTERISTICS

Gelatinization temperature, a physical property of starch, is the range of temperature within which the starch granules start to swell irreversibly in hot water with simultaneous loss of birefringence (in polarized light) and crystallinity. Determination could be by alkali spreading value test end the Amylography or Viscography. The alkali spreading value measures the extent of disintegration of whole-milled rice in contact with dilute alkali. Classification is as follows: low < 70c; intermediate, 70 - 74c; or high, 74.5 - 80c. Alkali spreading values correspond to gelatinization temperature as follows: 1 - 2, high; 3, high intermidiate; 4 - 5, intermediate; 6 - 7, low.

For gel consistency test, rice flour is treated with 0.2NKOH, 95thanol containing theymol blue (a dye). The length of the gel inside a culture tube is measured as the gel consistency (an index of cold paste viscosity of the sample).
Gel consistency has been highly correlated with setback value and consistency on the Amylograph.

The Brabender amylograph or Viscograph is a torsion viscometer that provides a continuous record of changes in viscosity of the flour slurry at a uniform rate of temperature increase (or decrease) of 1.5C/min under constant stirring. From the curve, amylogram or viscogram, the following parameters are assessed: gelatinization temperature, peak viscosity is important to the user because in most cases he must cook through this stage in order to obtain a useable starch paste. Drop in viscosity is a reflection of the ability of rice grains to retain their conformations when cooked while set-back value is an index of the tendency fro the cooked rice to harden on cooling as a result of amylase retrogradation, both serve as indices of textural properties of cooked rice i.e. whether separated or sticky. Some of the parameters for ten local rice varieties are given in Table 3.

Classification of Rice Samples for Various Food Uses
Typical U.S. long grain varieties for cooking and processing are characterized and classified by a relatively high amylose content (23 to 27 a slight to moderate alkali spreading value (3 to 5); an intermediate gelatinization temperature range (70 to 75C). Amylographic pasting characteristics of the typical long grain types usually show an intermediate peak viscosity (700 - 900 B.U.). While U.S. short and medium grain varieties of the preferred types show a somewhat lower amylose content (15 to 21 a pronounced and extensive alkali-spreading value (6 to 7); a relatively low gelatinization temperature range (60 to 69C). From the results of the alkali spreading test, rice varieties could be classified to high -, intermediate or low gelatinizing types. Low gelatinizing types are preferred in baby foods, specific brewing uses and many dry breakfast cereals. Intermediate gelatinizing types are characteristic of typical cooking long grain varieties. High gelatinizing types are generally considered undesirable for most cooking and processing uses.
Results obtained for ten Nigerian rice varieties showed that most of the varieties were of high amylose content, hard gel consistency and low to intermediate gelatinizing temperature. It would seem that if properly parboiled most of the varieties would be acceptable to rice consumers, especially in Southern Nigeria, that prefer rices that cook flaky (low cohesiveness score) and with high volume expansion. There are indications that the preferred rice in most parts of Northern Nigeria are those rices that remain soft after cooking (high cohesiveness score). However, it will be important to further investigate and correlate these physicochemical properties with consumer acceptability particularly with respect to various rice cooking methods in Nigeria.
QUALITY EVALUATION OF PARBOILED RICE
The quality of parboiled rice could be assessed from three parameters namely: milling quality degree of parboiling and cooking quality.
Milling Quality
This is one of the most important criteria of rice quality and is evaluated by the determination of milling yield and degree of milling.
Milling Yield: The objective in milling rice is to remove the bran layers and germ with a minimum of breakage of the whole kernel. Two values of milling yield commonly used in grading are: whole-kernel rice yield and total milling yield. The whole-kernel yield is the quantity of whole kernels of milled rice that can be obtained from a given quantity of uncleaned rice. Separation of whole and broken grains is done by sieving.
Milling Yield = Whole Kernels X 100 Rough Rice
Total milling yield is the quantity of whole and broken kernels of milled rice that can be obtained from a given quantity of uncleaned rough rice.
Another method is to determine the breakage by weighing 100g of milled sample and manual separation of grains of less than of normal grain size. Breakage is the amount of grains less than size/100g of milled rice. The results of breakage determination for some rice samples are given in Table 4. A high quantity of broken grains is indicative of poor milling quality, whereas a high percentage of whole kernels is indicative high quality. During the parboiling processes, especially the steaming period, gelatinized starch and disrupted protein bodies occupy all the air spaces in the endosperm and provide a better consistency and greater hardness in the kernel. Thus parboiling reduces breakage of rice during milling. The extent of breakage depends on the variety and processing conditions. Sun drying sets up stresses in the grain resulting in a higher percent breakage. This appears to indicate that the method of drying could be one of the factors which contribute to low demand of locally processed rice since most buyers would prefer rice with low breakage.
Degree of Milling: This is a measure or the extent to which the bran layers and germ have been removed from the rice endosperm. Methods proposed for degree of milling could be broadly classified into two groups:
1. Those estimating the quantity of removed bran or residual bran. Residual bran could be estimated by direct visual estimation with naked eye or with the help of a magnifying glass. This method is highly subjective but has been widely used in classifying milled rice into at least three degrees of milling: Fully milled, medium milled and under milled. Visual estimation could also be made of stained grains using dye-staining procedures e.g. congo red and methylene blue or alkaline alcohol and the intensity of the colour of the absorbed dye measured colorimetrically.
2. Those measuring the effects of removal of outer layers of the rice grains (a) the chemical composition, or (b) optical characteristics of the final milled product determined with the aid of a spectrophotometer.
Some of the chemical constituents of rice which progressively decrease with milling include fat, ash, silica and crude fibre contents. A method based on this principle is that of Hogan and Deobald where the amount of fat extracted from whole milled rice was linearly related to the amount of bran removed up to about 6f the original rice by weight. This is a reliable method but it is time consuming and requires considerable chemical skills and a suitable fat extraction apparatus. It is widely used as a research method because of its accuracy.
Degree of Parboiling
This is a measure of the extent of starch gelatinization. Methods for determining the degree of parboiling include soluble starch content determination and pasting characteristics with the Brabender amylograph.
A method for the determination of soluble starch involves extraction of soluble starch from rice flour at 55C filteration, and development of the iodine blue colour with an iodine solution and measurement of the intensity of the blue colour an iodine solution and measurement of the intensity of the blue colour produced. Soluble starch content has been shown to be increased by parboiling.
Degree of Parboiling can also be assessed by the amylograph as drop in viscosity has been shown to decrease with the severity of pretreatment, especially steaming condition. Other tests used in assessing extent of parboiling include water uptake, expanded volume and colour of parboiled rice, decreased solubility of proteins in various solvents total solids in residual cooking water, and visual observation for distinct white or chalky areas due to incomplete gelatinization of the starch.
Cooking Quality
Cooking quality of parboiled rice is assessed from three factors namely: water absorption, swelling capacity and texture of cooked rice.
Water Absorption is the amount of water absorbed by the grains when cooked for any length of time or to eating consistency.Test water absorption is determined by a procedure which involves immersing a beaker containing 5g of rice and 40ml of water into a boiling water bath for 23 minutes. Draining off excess water and weighing the cooked rice. Increase in weight is a measure of water absorption.
Swelling Capacity is the ratio of the final weight or volume of cooked rice to the initial weight or volume. It gives an indication as to how well the grains will swell when cooked.
Texture of Cooked Rice: Various instruments and tests have been developed for assessing texture of cooked rice. These include texturometer, Gebruder Haake consistometer and sieving test, the latter being an index of stickiness of cooked rice.
TASTE PANEL EVALUATION
Most of the physical and chemical tests discussed above may not fully predict consumer acceptability. The best and final test is to conduct a taste panel evaluation of both parboiled and cooked samples. Quality factors to assess are cleanliness, off-flavour, rice flavour, colour and cohesiveness of cooked grains. These attributes have been found useful in assessing acceptability of rice.
CONCLUSION
In this lecture, some quality tests of importance in rice processing industry have been examined. The list is by no means complete but recent reviews on this aspect are more detailed.
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