Chapter 5 Rice post-harvest processing, parboiling and home preparation

Contents - Previous - Next

Considerable variation in moisture content exists among grains in the same panicle, since panicles flower and develop from top to bottom. Grain weights tend to be lower and protein content tends to be higher in the bottom branches of a panicle. Optimum moisture content for harvesting varies with the season but is usually reached about a month after flowering. Uniformity of flowering among panicles affects the percentage of immatures in the harvest crop; photosensitive rices have more synchronous flowering than non-sensitive varieties. Immature grains reduce the head rice yield and are completely chalky.

Rice is still most frequently harvested by cutting the panicle with enough stem to allow threshing by hand. The panicles are sun-dried on the bund prior to threshing by hand, treading by people or animals or processing by mechanical threshers. When threshing is delayed while the cut crop is stored in heaps, "stack burning" often results as a consequence of the anaerobic respiration of microorganisms on the straw (70 to 80 percent moisture) and grain. Yellow or tan grains are formed when the panicle temperature reaches 60°C for a few days (Yap, Perez and Juliano, 1990). The discoloured grains have a better head rice yield and are more translucent than control grains. The mechanism seems to be non-enzymic browning (Reilly, 1990), which results in decreases in the lysine content of the protein (about 0.5 percent) and in the true digestibility to 92 percent and NPU to 61 percent (Eggum et al., 1984).

Delayed harvest in rainy weather frequently leads to grain sprouting on the panicle, particularly for non-dormant japonica rices. Lodging may also cause sprouting in the panicle for non-dormant rices. The incidence of the heavy rains (cyclones) during the harvesting season in India correlates with aflatoxin contamination of the rice crop (Tulpule, Nagarajan and Bhat, 1982; Vasanthi and Bhat, 1990). Aflatoxin is also a problem in the preparation of pinipig, a rice product from the Philippines wherein freshly harvested waxy grains are directly steeped, without drying, prior to roasting and flaking (Food and Nutrition Research Institute, 1987).

Rough rice drying has been reviewed by Kunze and Calderwood (1985) and Mossman (1986). Solar radiation is usually used, particularly in the dry season. Drying capacity is limited in the wet season, when more rice is grown because of water availability. Flash dryers are ideal for the first drying of harvested rough rice, to decrease the moisture content to 18 to 20 percent, but no mechanical dryer has been adopted widely by Asian farmers (Habito, 1987; de Padua, 1988). Grain cracking is minimal above 18 percent moisture (Srinivas and Bhashyam, 1985; IRRI, 1991b). The initial drying will allow safe storage of the grain for up to four to five weeks before final drying. Deformation of the spherosome particles of the aleurone layer is observed during 6 to 12 months storage in grains dried with hot air at 50°C, accompanied by a decrease in triglycerides and phospholipids (Ohta et al., 1990).

Cracking occurs not during drying as sun-cracking denotes, but when the overdried grain absorbs moisture on cooling (Kunze, 1985).

Storage

Storage changes, or ageing, occur particularly during the first three to four months after harvest and are also known as "after-harvest ripening" (Juliano, 1985b). The grain constituents probably equilibrate to their more stable physical form, which results in a harder, creamier-coloured grain (Yap, Perez and Juliano, 1990). After-harvest ripening is accompanied by a higher yield of total and head milled rice. Stored rice expands more in volume and yields a more flaky cooked rice with less dissolved solids in the cooking water than freshly harvested rice. In tropical Asia, aged rice is preferred and is more expensive than freshly harvested rice (Juliano, 1985b).

The exact mechanism of storage changes is not fully understood, but such changes occur in all starchy foods. In rice they occur mainly above 15°C (Juliano, 1985b). In regions where the sticky japonica rices are preferred, such as Japan and Korea, ageing in the spring and summer reduces grain quality.

The rice grain is very hygroscopic because of its starch content and equilibrates with the ambient relative humidity. The safe storage moisture content is generally considered to be 14 percent in the tropics. Storage pests (insects and micro-organisms) and rodents cause losses in both quantity and quality of the grains (Cogburn, 1985). Gross composition is not affected by storage, but vitamin content decreases progressively (Juliano, 1985b).

Rice is stored as rough rice in most of the tropics but as brown rice in Japan. Dehulling with rubber rollers minimizes bruising of the brown rice surface and improves the shelf-life of the dehulled grain. Brown rice, however, is more sensitive to environmental stress in the absence of the insulating enclosing hull and readily fissures in transit.

Parboiling

The traditional parboiling process involves soaking rough rice overnight or longer in water at ambient temperature, followed by boiling or steaming the steeped rice at 100°C to gelatinize the starch, while the grain expands until the hull's lemma and palea start to separate (Gariboldi, 1984; Bhattacharya, 1985; Pillaiyar, 1988). The parboiled rice is then cooled and sun-dried before storage or milling.

Modern methods involve the use of a hot-water soak at 60°C (below the starch gelatinization temperature) for a few hours to reduce the incidence of aflatoxin contamination during the soaking step. Leaching of nutrients during soaking aggravates the contamination, with the practice of recycling the soak water. Soaking sound, rough rice in water inoculated with Aspergillus parasiticus did not result in aflatoxin contamination of parboiled rice (Yap et al., 1987), suggesting that contamination probably has to be present in the grain prior to soaking (Bandara, 1985).

Vacuum infiltration to de-aerate the grain prior to pressure soaking is applied to obtain a good-quality product, as is pressure parboiling. The parboiled product has a cream to yellow colour depending on the intensity of heat treatment. Aged rice may give a grayish parboiled rice, probably because it has a lower pH owing to the presence of free fatty acids.

Parboiling gelatinizes the starch granules and hardens the endosperm, making it translucent. Chalky grains and those with chalky back, belly or core become completely translucent on parboiling; a white core or centre indicates incomplete parboiling of the grain.

Heated-sand drying results in parboiling of the higher-moisture wetseason crop but not of the dry-season crop. Parboiling results in inward diffusion of water-soluble vitamins, in addition to partial degradation of thiamine during heat treatment, except in heated-sand drying (Padua and Juliano, 1974), (Table 39). Riboflavin content is not decreased by parboiling (Grewal and Sangha, 1990). Despite the degradation of thiamine, parboiled milled rice had a higher vitamin content than raw milled rices in all parboiling procedures tested (Padua and Juliano, 1974).

TABLE 39 - Effect of parboiling method on thiamine content and protein

Source: Padua & Juliano, 1974.

Earlier results demonstrated that the water-soluble B vitamins, thiamine, riboflavin and niacin, are higher in milled parboiled rice than in milled raw rice (Kik and Williams, 1945). Oil and protein are reported to diffuse outward during parboiling, based on microscopic observations; they cannot diffuse as readily through cell walls as water-soluble vitamins, but the spherosome structure is destroyed. At similar degrees of milling, parboiled milled rice has lower protein content than raw milled rice (Table 39), but parboiled rice bran has more protein and oil than raw rice bran (Padua and Juliano, 1974). The composition of the milling fractions can be explained by a lower endosperm contamination of the bran in parboiled rice.

Parboiling results in some yellowing of the grain depending on the severity of the heat treatment. In addition, black spots diffuse to form dark brown to black regions or pecks, wherein at least 25 percent of the grain surface is coloured. Although parboiled grains are harder than raw rice, they are also susceptible to fissuring during drying, particularly below 18 percent moisture when free water becomes scarce in the grain.

TABLE 40 - Nutritional properties of two milled rices, raw and parboiled.

^a Parboiling done at 121°C, properties at 14% moisture content,
^b Eggum, Resumcci6n & Juliano, 1977.
^c Eggum et al., 1984.
^d Eggum & Juliano, 1973; Eggum, Alabata & Juliano, 1981.

Freshly parboiled rice may be milled directly with little breakage since the grains are pliable at high moisture content. Because of the damage to the spherosome structure, the bran of parboiled rice tends to agglomerate during milling and clog the sieves. In addition, greater milling pressure is required for parboiled rice because of the hardened endosperm.

Although parboiled rice is claimed to have a better shelf-life than raw rice because of the gelatinized starchy endosperm, its slightly open hull also makes it more exposed to insect attack. In addition, Asian parboiled rice is known to have aflatoxin contamination which is rarely found in raw rice (Tulpule, Nagarajan and Bhat, 1982; Vasanthi and Bhat, 1990). However, most of the aflatoxin is removed by processing.

The pressure parboiling process decreases the true digestibility of rice protein in growing rats (Eggum, Resurrección and Juliano, 1977; Eggum et al., 1984), (Table 40). However, there is a compensatory increase in biological value such that net protein utilization is comparable in raw and parboiled milled rice. Prolonging the pressure parboiling from 20 to 60 minutes did not further reduce the protein digestibility of IR8 rice.

Parboiling also removes cooked rice volatiles including free fatty acids, inactivates enzymes such as lipase and lipoxygenase, kills the embryo and decomposes some antioxidants (Sowbhagya and Bhattacharya, 1976). Hence, cooked parboiled rice lacks the volatiles characteristic of freshly cooked raw rice hydrogen sulphide, acetaldehyde and ammonia (Obata and Tanaka, 1965). The volatiles identified were mainly aldehydes and ketones (Tsugita, 1986).

Parboiled rice takes longer to cook than raw rice and may be presoaked in water to reduce the cooking time to be comparable to that of raw rice. The cooked grains are less sticky, do not clump end are resistant to disintegration; the grains are also harder. They also tend to expand more in girth rather than in length as compared to raw rice.

Most of the varieties parboiled in Bangladesh, Sri Lanka, India and Pakistan are the high-amylose rices that are common in these regions. In Thailand, both intermediate- and high-amylose rices are parboiled for export. Mainly long-grain, intermediate-amylose rice is parboiled in the United States, and intermediate- to low-amylose coarse japonica rices are parboiled in Italy.

Roasting of steeped rice grain at 250°C for 40 to 60 seconds also results in parboiling, but the product has a softer texture because the starch is immediately dried without permitting recrystallization or retrogradation of the starch gel, mainly the amylose fraction. The roasted grain is flattened or flaked with a wooden mortar and pestle, a roller flaker or an edge runner (Shankara et al., 1984) and then winnowed to remove hull and germ.

Processing

Dehulling of rough rice to brown rice can be carried out either manually (hand pounding) or mechanically. Mechanical hullers are of three main types: Engelberg mills, stone dehullers and rubber dehullers. Stone dehullers are still common in tropical Asia, where the surface-bruised brown rice is immediately milled with either an abrasive or friction mill. Rubber rollers are common in Japan, where brown rice is stored instead of rough rice, with a resultant space saving.

High humidity in the atmosphere during milling improves the yield of head rice. Increasing the moisture content of the grain to 14 to 16 percent by steam vapour prior to milling also improves the head rice yield and its taste (Furugori, 1985), since 14 to 16 percent is the critical moisture content range for crack susceptibility for most rice varieties (Srinivas and Bhashyam, 1985). Susceptible varieties readily crack below 16 percent moisture when exposed to higher humidity, but resistant varieties become susceptible at 14 percent moisture. Thus breakage is minimized for all varieties by tempering the grain to 16 percent moisture before milling. However, the milled rice may have to be redried to 14 percent for safe storage.

Rice mills in Asia range from a single-pass Engelberg mill to multipass systems. Manual technology involving hand pounding results in undermilled rice, which is richer in B vitamins than machine-milled rice because of incomplete removal of the bran layers. In the Engelberg or huller-type mill, dehulling and milling are performed in one step with greater grain breakage. The by-product is a coarse flour mixture of hull and bran. Using a dehuller before milling improves both the head and total milled rice yields. Slender grains require less pressure to mill than bold (i.e. thick) grains because of their thinner aleurone layer, but they are more prone to breakage during milling. In modern mills the milling operation involves several steps and bran and polish fractions are collected separately. Milling of 10 percent bran polish from brown rice by abrasive and friction mills removes all of the pericarp, seed-coat and nucellus and virtually all of the aleurone layer and embryo (Figure 2), but removes very little of the non-aleurone endosperm, except from the lateral ridges (Ellis, Villareal and Juliano, 1986).

The abrasive mill can overmill readily, as in obtaining white core rices with low protein and fat content for sake (Japanese rice wine) brewing.

The presence of chalky regions in the endosperm (white belly or white core) contributes to grain breakage during milling. Presumably a heterogeneous endosperm is more susceptible to cracking since a chalky mutant (Srinivas and Bhashyam, 1985) and waxy rice with a uniformly chalky endosperm (Khush and Juliano, 1985) give good milled head rice yield.

The term "polished rice" refers to milled rice that has gone through polishers that remove loose bran adhering to the surface of milled rice and improve its translucency. The polisher has a horizontal or vertical cylinder or cone, covered with leather strips, that gently removes loose bran as it is rotated in a working chamber made of a wire-mesh screen or a steel screen with slotted perforations.

Some rice consumers prefer a very glossy or shiny rice called coated or glazed rice. This rice is prepared by adding dry talc and a glucose solution to well-milled rice in a tumbler. The rotation of the tumbler distributes the mixture over the grain. The talc used to coat rice in Hawaii does not cause a higher incidence of stomach cancer as it is claimed to do in Japan, where talc-coating is banned (Stemmermann and Kolonel, 1978).

Innovations introduced in the Japanese rice industry include microcomputer control of milling based on the desired degree of milling or whiteness of the milled rice (Furugori, 1985; van Ruiten, 1985). Electronic colour sorting is commonly used to remove discoloured pecky grains. High-degree refining of milled rice, introduced in 1977, includes spraying a mist of moisture through the hollow shaft with the high-pressure air during milling, in combination with a uniquely designed metallic roll-type refining machine. Water is evaporated during milling and keeps the grain temperature lower than in regular milling. A germ rice milling machine introduced in 1976 that uses gentle, abrasive roll milling under very low pressure leaves the germ intact for more than 80 percent of the grains. Germ rice is well received by Japanese consumers because it is rich in thiamine, riboflavin, tocopherol, calcium and linoleic acid. Small coin-operated mills are becoming quite popular in Japan to handle the daily requirements of a family and thus minimize fat rancidity during storage.

Aflatoxin is produced mainly in the bran polish fraction of brown rice (flag and Juliano, 1982). Dehulling removes 50 to 70 percent of the aflatoxin of raw rice, and milling further reduces the toxin content to 20 to 35 percent (Vasanthi and Bhat, 1990). Parboiling reduces the toxin content in already infested rice by 33 to 61 percent; dehulling reduces toxins in parboiled rice further to 19 to 31 percent and milling to 7 to 28 percent. However, parboiled rice is a better substrate for aflatoxin production than raw rice, probably because parboiling makes the fat in rice more available for metabolism by Aspergillus parasiticus (Breckenridge end Arseculeratne, 1986).

Shelf-life is usually shortest for milled rice, followed by brown rice and then rough rice, because of fat rancidity. Fat in the surface cells of milled rice undergoes fat hydrolysis by lipase followed by lipoxygenase oxidation of the liberated free unsaturated fatty acids. With brown rice, the dehuller used is the critical factor; a rubber dehuller is preferred over a stone dehuller, to reduce surface bruises on the grain that trigger lipase action on lipids.

Post-harvest losses

Rice losses occur at all stages of the post-harvest chain. Though quantitative losses are usually simple to assess, qualitative ones are more difficult to define and rely more on subjective judgements and cultural perceptions. Accepted figures for quantitative post-harvest losses in rice range from 10 to almost 40 percent, with the following breakdown:

- harvesting, I to 3 percent,
- handling, 2 to 7 percent,
- threshing, 2 to 6 percent,
- drying, I to 5 percent,
- storage, 2 to 6 percent,
- milling, 2 to 10 percent.

These figures, initially collected in Southeast Asia (de Padua, 1979), were later confirmed for other parts of Asia and Africa by field activities of FAO's Prevention of Food Losses (PFL) programme, among others. They have become the standard values for rice losses.

The timing of the rice harvest influences the level of losses. Depending on the variety, delay in harvesting a mature rice crop leads to lower yields because of lodging and shattering and the exposure of the ripe grain in the field to insects, birds and rodents. It also leads to post-harvest losses by lowering milling yields and recovery of head grains.

Traditional threshing techniques are a frequent cause of loss. These include beating the straws against slats through which the grain falls into tubs or buckets, trampling with feet and occasionally using a tractor or tractordrawn roller. Quality is affected since grains might break or stones and soil become mixed with the threshed rice.

Often a considerable amount of grain is scattered around and gets eaten by poultry and household pets. However, while this quantity can be considered lost for human consumption, it becomes productive within the total household economy.

Threshed rough rice is commonly stored either in sacks or in bulk. The sacks or bags provide a means of separating varieties for specific requirements but do not provide protection against insects and rodents. Good store management, proper dunnage and adequate hygienic conditions significantly limit the losses.

On the large scale, bulk storage and controlled-atmosphere storage, if properly organized, are efficient and relatively inexpensive. However, efficient operation requires considerable capital investment and trained labour which often go beyond the single farmer's capability.

Storing rice as rough rice has advantages over storing milled rice, since the hull protects the kernel against insects and fungal attacks. This possibility depends to some extent on the local economic situation and on supply and demand for rough rice and milled rice at different times in the season.

Contents - Previous - Next