Striving for paddy, milled rice quality
Aug. 21, 2012
by Susan Reidy
Quality is an important factor at the front and back end of rice production. A quality paddy must go in the beginning of the process if quality milled rice is expected at the end.
Measuring quality provides data that can be used for decision making, optimization and the development of processes and technologies as well as for evaluating the properties, function, quality and reliability of these, according to the International Rice Research Institute (IRRI).
The measured values are usually compared to a standard for assessment. Farmers and rice producers measure many times a day, whether it’s to determine the maturity of the crop, determine yield, find the moisture content of the grain or measure the temperature in a dryer.
The quality parameters for paddy and milled rice, as well as the methods to measure the quality and the resulting standards and grades for milled rice will be examined in this article.
Several interrelated features determine the quality of paddy including: moisture content, purity, varietal purity, cracked grains, immature grains, damaged grains and discolored/fermented grains, IRRI said. The characteristics are determined by the weather conditions during production, crop production practices, soil conditions, harvesting and post-harvest practices.
Moisture content (MC) influences all aspects of paddy and rice quality, making it essential that rice be milled at the proper MC to obtain the highest head rice yield. Paddy is at its optimum milling potential at an MC of 14% wet weight basis, IRRI said. Higher moisture contents are too soft to withstand hulling pressure, which results in breakage and possible pulverization of the grain.
Grain that is too dry is brittle and has greater breakage, IRRI said. MC and drying temperature also is critical, because it determines whether small fissures and/or full cracks occur in the grain structure.
Purity refers to the presence of dockage (materials other than paddy) including chaff, stones, weed seeds, soil, rice, straw, stalks and other debris, IRRI said. The impurities generally come from the field or the drying floor. Unclean paddy means a longer cleaning and processing time for the grain. Foreign matter in the grain reduces milling recoveries and the quality of rice, and it also increases the wear on milling machinery, IRRI said.
Mixing paddy varieties can cause problems during milling, resulting in reduced capacity, excessive breakage, lower milled rice recovery and reduced head rice. Different sized and shaped grains makes it difficult to adjust equipment such as hullers, whiteners and polishers, IRRI said. This results in low initial husking efficiencies, a higher percentage of re-circulated paddy, non-uniform whitening and a lower quantity of milled rice.
Grain size and shape, or the length-width ratio, is different for the varying paddy varieties. Long, slender grains typically have greater breakage than short, bold grains and therefore have a lower milled rice recovery, IRRI said. The dimensions dictate to some degree the type of milling equipment needed.
For example, Japanese-designed equipment may be better suited to short, bold grains whereas Thai-made equipment is more suitable for longer, slender grain types, IRRI said.
Exposing mature paddy to fluctuating temperatures and moisture conditions can lead to the development of fissures and cracks in individual kernels. Cracks in the kernel are the most important factor contributing to rice breakage during milling, IRRI said. This results in reduced milled rice recovery and head rice yields.
The amount of immature paddy grains in a sample greatly impacts the head rice yield and quality. The immature kernels are very slender and chalky, which results in excessive production of bran, broken grains and brewer’s rice.
Grain should be harvested at about 20% to 24% moisture or about 30 days after flowering. If harvest is too late, grains are lost through shattering or dry out and are cracked during threshing, which causes grain breakage during milling, IRRI said.
Grains can be damaged by water, insects and heat exposure. The paddy deteriorates through biochemical changes in the grain, the development of off-odors and changes in physical appearance.
Yellowing is caused by over-exposure to wet environmental conditions before the paddy is dried. The grains contain partly gelatinized starch cells and resist the pressures applied during milling. The fermented grains do not impact milling yields, but do downgrade the quality of the milled rice because of the appearance.
Insect or mold-damaged grains often have black spots around the germ end of the rice kernel. During milling, the black spots are only partly removed, which increases the presence of damaged grains.
Milled rice quality
Milled rice quality is classified physically and chemically, IRRI said. Physical characteristics include milling degree, head rice, whiteness and chalkiness, while chemical characteristics include gelatinization temperature, amylose content and gel consistency.
Milling degree is the measure of the percent of bran removed from the brown rice kernel. The degree affects milling recovery and impacts consumer acceptance due to its influences on color and cooking behavior of the rice, IRRI said.
Unmilled brown rice does not absorb water well and does not cook as quickly as milled rice. The water absorption rate improves to about 25% milling degree, after which there is little effect.
Head rice or head rice percentage is the weight of head grain or whole kernels in the rice lot, IRRI said. It includes broken kernels that are 75% to 80% of the whole kernel. High head rice yield is one of the key criteria for measuring milled rice quality.
The head rice percentage in a sample of milled rice will depend on varietal characteristics, production factors, and the harvesting, drying and milling process.
Whiteness is determined by a combination of varietal physical characteristics and the degree of milling. During whitening, the silver skin and the bran layer is removed. Polishing afterward improves the appearance of the white rice. In this process, some of the bran particles stick to the surface of the rice, which polishes and gives a shinier appearance.
If the milled kernel is opaque instead of translucent, it is characterized as chalky. This chalkiness disappears after the rice is cooked and has no impact on the taste or aroma, but it downgrades the milled rice quality and reduces milling recovery, IRRI said.
Excessive chalkiness comes from interruption during the final stages of grain filling.
For chemical characteristics, the time required for cooking milled rice is determined by the gelatinization temperature (GT). A high ambient temperature during development results in starch with a higher GT. In many rice-growing nations, there is a preference for rice with intermediate gelatinization temperatures, IRRI said.
High amylose content rice has high volume expansion and a high degree of flakiness. Starch, which makes up about 90% of the dry matter content of milled rice, is a polymer of glucose and amylose is a linear polymer of glucose. The amylose content of starches ranges from 15% to 35%, IRRI said.
Rice with high amylose cook dry, are less tender and become hard when cooled. Low amylose rice cooks moist and sticky. Intermediate amylose rice is preferred in most growing areas of the world. Amylose content of milled rice is determined using the colorimetric iodine assay index method.
Milled rice is classified in groups based on the percentage of amylose:
•Waxy, 1% to 2%
•Very low amylose, 2% to 9%
•Low amylose, 10% to 20%
•Intermediate amylose, 20% to 25%
•High amylose, 25% to 33%.
Gel consistency measures the tendency of the cooked rice to harden after cooling, IRRI said. Varieties with a softer gel consistency are preferred, and the cooked rice has a higher degree of tenderness. Harder gel consistency is associated with harder cooked rice, which is particularly evident in high-amylose rice. Hard cooked rice also tends to be less sticky. Gel consistency is determined by heating a small quantity of rice in a dilute alkali, IRRI said.
Several equations and procedures are available to quantitatively measure the quality of paddy and milled rice, according to IRRI.
To detect cracks in paddy, use the Paddy Crack Detector to count the number of cracked grains in a 100-grain sample. Compute the percent of cracked grains by dividing the number of cracked grains by 100 and multiplying by 100.
For grain dimension, use a caliper or photographic enlarger to collect 20 paddy samples at random from each replicate and measure the dimensions to obtain the average length and width of the paddy grains.
Immature grains can be measured by selecting, segregating and weighing the immature grains in a 25-gram sample. Calculate the percentage by dividing the weight of immature grain by the total weight of the sample and multiplying by 100.
To determine dockage, remove light foreign matter, stones, weed and seeds from a 100-gram sample. Obtain the total weight and compute the dockage percentage by dividing the weight of the dockage by the sample’s total weight and multiplying by 100.
In milled rice to determine milling recovery, divide the weight of milled rice recovered by the weight of the paddy sample. Use the length/width (l/w) ratio to determine the shape of the milled rice. The ISO Classification scale classifies rice with a l/w ratio of more than 3 as slender; between 2.1 and 3 as medium; between 1.1 and 2 as bold; and 1 or less as round.
Measuring moisture content is one of the most important quality parameters in both paddy and milled rice. Accurate moisture content testing is important in managing and marketing paddy and rice because depending on the purpose of the rice, it has different ideal moisture contents.
The primary method for measuring moisture content uses weight measurements. Rice with a certain moisture content is weighed, the water is evaporated using heated air until no weight change is observed, and the remaining dry matter is weighed again. The moisture content is then calculated.
Secondary methods are less exact and measure the electrical resistance or di-electrical properties of paddy using electronic instruments. Results are less accurate but are available within seconds or minutes and are usually used in post-harvest management and for trade.
IRRI recommends that a representative sample be taken across the field, and that at least three measurements are taken at each location for a good average value.
Standards are a quantitative way of measuring and comparing certain quality characteristics. This measured comparison of recognizable quality characteristics can be described as grading, IRRI said. There are few universally accepted international standards for paddy and milled rice because there are differences in the grading paddy and milled rice quality among countries.
In general, IRRI said grading factors for paddy include: purity; foreign matter; defectives and moisture content. For milled rice, the characteristics considered for grading include: head rice, brokens and brewers percentages; defectives; foreign matter; presence of paddy; and moisture content.
Establishing standards and grades achieve several objectives including ensuring only edible rice reaches the consumer; improving post-harvest practices so as to eliminate or reduce waste; improving agronomic practices to increase farm yields; improving processing practices for better milling recovering and for market expansion; and protecting consumers from price/quality manipulation.
Characteristics such as moisture content, foreign material, seeds and discolored grains help ensure only edible rice reaches the consumer, IRRI said. Setting standards for the degree of milling, broken rice content, moisture and damaged grains helps improve post-harvest practices.
IRRI said standards that clearly identify to consumers the true value of their purchases will provide the protection required against unfair trading practices.