LITERATURE REVIEW
2.1 RICE GRAIN STRUCTURE AND CONSTITUENTS
A mature rice grain (or kernel) contains three components; from the outside, they are respectively: husk (or hull), bran (seed coats and germ) and endosperm (Laguë and Jenkins, 1991). The complete grain is so called paddy or rough rice. Brown rice is obtained by removing the hull. Removal of the bran by abrasive milling yields the final product called white, milled or polished rice (Fig 2.1). According to Kunze and Choudhury (1972), Srinivas et al. (1978) and Kunze and Prasad (1978), white rice absorbs moisture faster than brown rice, and brown rice faster than paddy rice. Aguerre
et al. (1982) stated that it is reasonable to think that the moisture adsorption capacity will be different for each of the constituents, but they found that the non-homogeneity of the grain need not be considered in drying kinetic analysis.
Fig 2.1: Paddy, brown rice and milled rice
Figure 2.2: A dissected paddy grain
Because of the outer husk, paddy grain is different from most other grains. The husk, as shown in Fig 2.2, encloses the caryopsis during harvesting, drying and storage (Wongwises and Thongprasert, 2000; Riahi and Ramaswamy, 2003).
The most important components of the grain, according to Juliano and Bechtel (1985), Brooker et al. (1992), Hoseney (1994), Marshall and Wadsworth (1994), Gwinner et al. (1996), Lásztity (1996), Evers and Millar (2002), and Riahi and Ramaswamy (2003), are
The husk or seed coat that is composed of two modified leaves: the palea and
larger lemma, which protect the seed from many damaging influences. A tight husk may provide storage protection to the grain but may make the milling difficult. The husk is about 18 to 20% of the total kernel weight,
The endosperm, which constitutes the nutritional reserves for the embryo. It
consists largely of starch and a little aleurone. It is about 74 to 78% of the total kernel weight. It is the largest morphological component in all cereal grains and is the component with the greatest value, and
The embryo or germ which is the most important grain component for the
survival of the species as it is capable of developing into a plant of the next generation. It is very small and is located on the central side at the base of the grain. It is particularly rich in oil, protein and vitamins.
The brown rice kernel consists of a pericarp (about 2%), seed coat and aleurone (about 5%), germ (2-3%), and endosperm (89-94%). As with other cereals, the aleurone is the outermost layer of the endosperm but is removed with the pericarp and seed coat during milling (Hoseney, 1994).
A rice kernel can be regarded as a composite consisting of several different biopolymers, and a brown rice kernel is primarily a mixture of starch and protein with a small quantity of lipids with moisture as a plasticizer (Sun et al., 2002; Zhang et al., 2003b). Rice and oats are the only two cereals with compound starch granules (i.e. a starch granule made up of many small granules) (Fig 2.3). Little or no matrix protein
the structure of a rice kernel after sun drying, Dong and Zhihuai (2003) found that most stress cracks are not only propagated along the edges of the starch granules, but also tear some starch granules, dividing them into two parts.
Starch makes up about 90% of the dry matter content of milled rice (Juliano, 1993; Juliano, 1998; Inprasit and Noomhorm, 2001; IRRI, 2002d). The individual rice starch granules are small (2-5 µm) and polygonal in shape (Fig 2.4). Many of the granules in tuber and root starches, such as potato and cassava starches, tend to be larger than those of grain starches and are generally less dense and easier to cook. Potato starch granules may be as large as 100 µm along the major axis (Wilkinson, 2000). Within the rice starch granule, amylose and the branching points of amylopectin contribute to the amorphous phase, while the outer chains of amylopectin contribute to the crystalline phase (Hoseney, 1994).
Fig 2.3: Compound starch granules and protein bodies (arrows) near the aleurone layer of a rice kernel. Bar is 10 µm (Source: Hoseney, 1994)
Fig 2.4: Compound starch granules near the centre of a rice kernel, with certain granules broken, showing individual granules (arrows). Bar is 10 µm (Source:
Wilkinson (2000) stated that amylose and amylopectin in starch granules are arranged radially making the granules contain both crystalline and non-crystalline regions in alternating layers, i.e. the starch granule is constructed like an onion with layers of amylose and amylopectin, but the layers cannot be peeled off (Fig 2.5).
Fig 2.5: A schematic model of the structure of a starch granule (Source: Wilkinson,
2000)
Fractions of amylose and amylopectin in starch granules, as shown in Table 2.1, are different for different rice varieties. Patindol et al. (2003) reported that a rice variety “Bengal” has a higher percentage of amylopectin but is lower in intermediate material and amylose content when compared with another rice variety “Cypress”.
Table 2.1: Percentage (± standard deviation) of starch molecule size of two rice varieties (Source: Patindol et al., 2003)
Starch molecular sizes Bengal (medium grain) Cypress (long grain) Amylose 16.07 ± 0.42 26.20 ± 0.33 Amylopectin 77.37 ± 0.64 58.33 ± 0.51 Intermediate material 6.57 ± 0.31 15.47 ± 0.62
Note: Starch fractions were categorised into amylopectin, intermediate material, and amylose base on the retention time because of their differences in molecular size.
Protein is the second most important rice component after carbohydrates. It is unevenly distributed in the grain kernel and acts as a bio-adhesive that binds the discrete cell structures and starch granules (Zhang et al., 2003a). There are greater concentrations in the bran and periphery of the endosperm and smaller quantities towards the centre of the grain. Accordingly, milled, polished rice has a lower protein content than brown rice; about 82% is retained after milling. Chemical interactions between protein and starch may also influence rice quality. Protein bodies remain intact upon cooking (Juliano and
2.2 RICE GRAIN QUALITY
Different rice grains are demanded by different customers and markets, depending on their preferences and the intended end use (Brooker et al, 1992). Most consumers prefer the best quality they can afford (Bakker-Arkema and Salleh, 1985). Long-grain (higher- quality) rice is sold mostly in Europe and the Near East, medium-quality long-grain rice in the deficit countries of Asia, the short-grain rice in various special-demand areas, high-quality parboiled rice in the Near East and Africa, and the lower-quality parboiled rice in special markets in Asia and Africa. Aromatic rice is demanded mostly in the Near East. Waxy rice meets market needs in Laos, while smaller volumes go to other countries (Juliano, 1993).
To the rice farmers, grain quality refers to quality of seed for planting and dry grain for consumption, with minimum moisture, microbial deterioration and spoilage. Millers or traders look for low moisture, variety integrity and high milling and Head Rice Yields (HRYs). Market quality is mainly determined by physical properties and variety name, whereas cooking and eating quality is determined by physico-chemical properties, particularly the amylose content (Juliano, 1993; IRRI, 2002d).
There are few quality-measuring methods in the literature that are specific to rice. Some methods that are now used in the food industry are adapted from other cereal products. Some procedures, such as moisture determinations, are taken directly from standard methods (Kohlwey, 1994). Rice quality in Japan is evaluated using sensory tests and physicochemical measurements. The sensory test, which measures appearance, aroma, hardness, stickiness and overall quality, is the basic evaluation method, although it requires a large number of samples and many panellists. The physico-chemical measurements are an indirect method of estimating eating quality based on chemical composition, cooking quality, and gelatinisation and physical properties of cooked rice (Ohtsubo et al., 1998). Although sensory evaluations by laboratory panels and consumer panels give some indication on important criteria for rice quality, they do not reflect the properties for which consumers will actually pay a price premium in the retail market (Juliano, 1993).
The quality characteristics of rice that are to be maintained during the drying process include HRY, colour, and subsequent cooking qualities (Zhang et al., 2003a).
2.2.1 Quality characteristics
Quality characteristics of rice grain, according to IRRI (2002a), are either subjective or objective. Subjective characteristics are determined by individual preference, whereas objective characteristics are independent of personal opinion. The subjective characteristics can be
Taste Appearance Smell etc.
The objective characteristics are
Physical (texture, colour) and Chemical (nutritional value).
The growing place and the growers of the grain crop can also be classified as objective characteristics.
2.2.1.1 Physical characteristics of paddy grain
Simulation of heat and moisture transfer phenomena during drying and storage of the grain requires physical, thermal and moisture-transport properties of the grain. Accurate knowledge of the true value of the properties is a requirement for good engineering design of the machines, equipment or methods for processing and handling (Wratten et al, 1969; Morita and Singh, 1979).
Many physical characteristics have been described and used for rice grain, including kernel weight, sphericity, roundness, size, volume, shape, surface area, bulk density, kernel density, fractional porosity, static coefficient of friction against different materials, angle of repose and equilibrium moisture content (MCe) etc. These properties
a. Moisture content, MC
MC has a significant influence on all aspects of paddy rice quality. Details of this characteristic are described in Section 2.4.
b. Maturity
Immature rice kernels are very slender and chalky and result in the production of excessive bran, broken grains and brewers’ rice (see its definition in Section 2.2.1.2). c. Varietal purity
A mixture of varieties in a sample or bulk of paddy grain (Fig 2.6) causes difficulties in milling and usually results in reduced milling capacity, excessive breakage, lower milling and HRYs.
Fig 2.6: Paddy rice sample with single variety and mixed varieties (Source: IRRI,
2002c)
d. Dockage
Dockage includes chaff, stones, weed seeds, soil, rice straw, stalks and other foreign matter. These impurities generally come from the field or from the drying floor (Fig 2.7).
Fig 2.7: Clean paddy grain and the grain mixed with dockage (Source: IRRI, 2002c)
e. Discoloured
Water, insects and heat exposure can cause the grain to deteriorate through biochemical changes in the grain which may result in the development of off-odours and changes in physical appearance.
f. Cracks
Mechanical impact and overexposure to fluctuating temperature and moisture conditions may lead to the development of cracks in individual kernels. Cracks lead to easy infestation and development by mould and insects and because of the breaks in the endosperm tissue, the nourishment that the embryo can get is reduced so as to reduce the vitality of the seeds (Dong and Zhihuai, 2003).
2.2.1.2 Physical characteristics of milled rice
The following are six physical characteristics that, according to IRRI (2002d), are used to determine the quality of milled rice:
from a given quantity of clean paddy after complete milling. Broken rice particles that are larger than 3/4 of the kernel are also considered as head rice. The yield is usually expressed as a weight percentage of paddy rice (Siebenmorgen et al., 1992). It can vary from as low as 25% to as high as 65% depending on the quality of the grain itself and of the milling machine (IRRI, 2002d).
Since rice is consumed mostly in the form of whole grains and because of the greater economic value of head rice, increasing the HRY in production is a universal goal (Sharma and Kunze, 1982). Reduction in the yield decreases the grain value since broken kernels are typically worth half the value of head rice (Arora et al., 1973; Webb
et al., 1986; Muthukumarappan et al., 1992; Siebenmorgen et al., 1992; Siebenmorgen, 1994; Cnossen and Siebenmorgen, 2001; Zhang et al, 2003a and Cnossen et al., 2003). Research has found HRY to be especially sensitive to the mode of drying and is usually used in assessing the success or failure of the drying system (Brooker et al., 1992 and Abud-Archila et al., 2000). It is difficult to ascribe reduction in the yield to a single cause. However, it is generally believed that the yield is strongly related to internal cracking or fissuring (Stermer, 1968; Velupillai and Pandey, 1990). Research has indicated that some breakage in the grain occurs because the kernels have previously been weakened by stress cracks (fissures) caused by rapid moisture adsorption or desorption (Kunze, 1977 and Cnossen et al., 2003).
The efforts of rice breeders to develop new varieties, improvements in design of shelling and milling equipment, improvements in drying conditions, and treatments (parboiling, extractive milling) of the grain prior to, or during, milling have resulted in reducing the fissuring and breakage. However, further means for minimizing the damage would benefit rice millers and farmers more (Matthews and Spadaro, 1976). A number of standardizing testing methods have been developed and applied by different groups of researchers to determine the HRY. Depending on the method and instruments used, the yield obtained from the same sample can be significantly different (Reid et al., 1998; Yadav and Jindal, 2001; and Lloyd et al., 2001).
b. Brewers’ rice
Brewers’ rice refers to the small pieces of broken rice that remain in the milled rice after milling. Its extent depends on the magnitude of the grain damage.
c. Damage
Before milling, paddy rice can be deteriorated through natural biochemical changes in the grain or by insect, mould, water, or heat which can create off-odours and changes in physical appearance. The result is damaged grains (Fig 2.8) that are fully or partially darkened.
Fig 2.8: Damaged grains (Source: IRRI, 2002d)
d. Chalkiness
The endosperm chalkiness or opacity, as shown in Fig 2.9, is due to the loose packing of starch granules in the region caused by interruption of final filling of the grain. Excessive chalkiness downgrades the quality and reduces the grain milling and HRYs. Chalkiness, however, disappears upon cooking and has no direct effect on cooking and eating qualities (Juliano and Bechtel, 1985).
Fig 2.9: Chalky grains (Source: IRRI, 2002d)
e. Red/Red streaked
Red and red-streaked grains (Fig 2.10) occur when part of the bran layer remains clinging to the surface of the grain after milling. Rice consumers almost universally desire well-milled rice because of its better appearance. Therefore, the presence of red and red-streaked grains suggests a lower degree of milling, and subsequently, a less desirable appearance.
Fig 2.10: Red and red-streaked grains (Source: IRRI, 2002d)
f. Appearance
Whiteness, translucency, and milling degree influence the appearance of milled rice. Rice that is not attractive to the consumer will have a lower value in the marketplace. In
other words, improving the appearance of the rice grains through proper milling increases their value (Bakker-Arkema and Salleh, 1985; IRRI, 2002d).
Fig 2.11: Discoloured milled rice (Source: IRRI, 2002d)
Discoloured milled rice grains (Fig 2.11), in many cases, referred to as the yellowing problem, often make the grains unattractive (Dillahunty et al., 2001). Chemical and physical transformations, induced by heating and translocation of colour from rice husk and rice bran to endosperm, cause the discolouration (Inprasit and Noomhorm, 2001; Dillahunty et al., 2001). Delayed threshing causes yellowing of the grain in the field; it can be increased during drying and storage from 0 to 5.5% or even 30% (Brook, 1992 andBrooker et al., 1992).
g. Aroma
Aroma of the grain (from paddy through to cooked rice) has become one of the most important factors for grain attractiveness, and drying the grain at high temperature has been reported to lower the concentration of the grain key aroma compound, 2-acetyl-1- pyrroline (Wongpornchai et al., 2004).
2.2.1.3 Chemical characteristics of milled rice
According to Juliano (1971), Bakker-Arkema and Salleh (1985), Ohtsubo et al. (1998), and IRRI (2002d), the following three chemical characteristics are most commonly used
a. Amylose content
This is the characteristic that affects the cooked rice quality or influences the eating quality of rice. When the content is high, the amount of cooking water absorbed by milled rice increases; the cooked rice will show high volume expansion (not necessarily elongation) and a high degree of flakiness (easy to loosen or separate). The rice will also be dry, less tender, and become hard upon cooling. When amylose is low, the cooked grain will be moist and sticky.
b. Gelatinisation temperature
This is the temperature that determines the amount of water and time required for cooking. The grain with a high gelatinisation temperature requires more water and a longer cooking time (Juliano, 1971; Juliano, 1985; Juliano and Perez, 1993). At this temperature, the grain kernels absorb water and starch granules swell irreversibly, with the core of the grain becoming translucent or gelatinised in hot water.
c. Gel consistency
Gel consistency is the chemical characteristic that affects the cooked rice tenderness. It measures the tendency of cooked rice to harden on cooling. When gel consistency is hard, the cooked rice tends to be less sticky. Harder gel consistency is associated with harder cooked rice and this feature is particularly evident in high-amylose rice. In contrast, when gel consistency is soft, the cooked rice has a higher degree of tenderness (softness).
2.2.1.4 Thermal and moisture-transport properties
Thermal and moisture-transport properties affect the rates of heat and moisture transfer during drying and storage of grains. The properties which are mainly considered in the phenomena are specific heat capacity, thermal conductivity, thermal diffusivity, moisture diffusivity and latent heat of vaporisation. While most studies have reported thermal properties of cereal grains as a function of MC, some studies have evaluated the influence of temperature and composition on thermal properties of grains (Wratten et al., 1969; Sablani and Ramaswamy, 2003).
a. Specific heat capacity, cp
The specific heat capacity of a substance or material indicates the amount of energy a body stores for each degree increase in temperature, on a unit mass basis (J/kg.oC).
Table 2.2: Equations and values describing the specific heat as affected by its MCs from 16.28 to 28.21% db (or 14 to 22% wb) Equation Value calculated, J/kg.oC Grain type Source
1000 (1.0509 + 0.03835 MCdb) 1665 - 2125 Kunze and Wratten, 1985 33.5 MCdb + 1189.9 1735 - 2135 ASAE, 2003a,2004a,2005a 39.2 MCdb + 1039.7 1678 - 2145 medium ASAE, 2003a,2004a,2005a 23.6 MCdb + 1372.1 1756 - 2038 short ASAE, 2003a,2004a,2005a Wratten et al (1969) reported the specific heat of paddy grain (cpp) of 2010.85 J/kg.oC while Kunze and Wratten (1985), Oshita (1992), ASAE (2003a), ASAE (2004a) and ASAE (2005a) declared the specific heat changed under the effect of the grain MC (Table 2.2). Rahman (1995) also declared a change with grain composition and temperature. Mohapatra and Bal (2003) reported the specific heat of rice varies from 1230 to 4340 kJ/kg.oC with temperature varying from -10 to 150oC for MC of 13 and 12.4%, respectively.
b. Thermal conductivity, λ
According to Brooker et al. (1992), thermal conductivity of paddy kernel is a measure of the resistance to the conduction of thermal energy (heat) within an individual kernel. The authors reported a value for the conductivity within a rice kernel of 0.106 W/m.oC.
Kunze and Wratten (1985) proposed that the thermal conductivity of a paddy kernel changes linearly with its MC:
p db
λ = 0.0894 +0.000958.MC … (2.1)
db p db 0.0637 +0.0958.MC λ = 0.656 - 0.475.MC … (2.2)
Chakraverty and Singh (2001) claimed that thermal conductivity of bulk paddy grain or the effective thermal conductivity is 3 to 4 times smaller than that of the single grain kernel. The thermal conductivity of a single paddy grain, according to them, varies from 0.35 to 0.70 W/m.oC, whereas the effective thermal conductivity varies from 0.12 to 0.17 W/m.oC, which is due to the presence of air space in it. The thermal conductivity of air is 0.023 W/m.oC.
Yang et al. (2003c) claimed that bulk or effective thermal conductivity increases with increasing MC and temperature. They reported that the conductivity ranged from 0.082 to 0.138 W/m.oC in the temperature range of 6 to 69ºC and moisture range of 9.2 to