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Rice: Eating Quality
Rice: Eating Quality JS Bao, Zhejiang University, Hangzhou, China ã 2016 Elsevier Ltd. All rights reserved.
Topic Highlights
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Acceptance of rice variety by consumers is primarily determined by eating quality characteristics, but eating quality means different things to different people in different regions and with different cultural backgrounds. The criteria or standards for high quality, based on sensory analysis by panels and physicochemical property evaluations facilitate rice processing, breeding, and marketing. The chemical basis of the texture of cooked rice is attributed to amylose content and particularly long B chains of amylopectin; the genetic basis of the texture of cooked rice is attributed to both the waxy and SSIIa genes on chromosome 6. The chemical basis of the popcorn aroma of cooked rice is attributed to the volatile compound 2-acetyl-1-pyrroline (2AP); the genetic basis of the popcorn aroma of cooked rice is attributed to the fragrance gene (fgr) on chromosome 8. Improvement of eating quality may be fulfilled with marker-aided selection of three genes, that is, waxy, SSIIa, and fgr.
The diverse people of different origins and with different cultural backgrounds consume rice, thus making their demands for rice quality diversified. Different countries have different requirements for the quality of rice, and within countries, preferences also vary. However, the eating quality of rice may be the first thing to be considered for consumers to choose rice for consumption. The eating quality depends on the texture and flavor of the cooked rice or rice products. The texture of the cooked rice is a principal consideration in its consumer acceptability and palatability, which is expressed in terms of hardness or firmness (or, its opposite, tenderness) and stickiness or adhesiveness and moistness to touch. The flavor of the cooked rice is expressed in terms of its aromatics, taste, and mouthfeel. The economic value of rice in domestic and international trade is arguably influenced by the eating quality of cooked rice. Understanding the chemical and genetic basis of eating quality will not only promote the circulation of rice in domestic and international business but also facilitate rice quality improvement through breeding to match the needs in the target markets.
Measuring Eating Quality Learning Objectives
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To achieve understanding of the chemical and genetic bases of eating quality and uses of genetic tools to improve the eating quality of rice.
Sensory Evaluation
Introduction Rice is a staple food for half of the world’s population. However, more than 90% of rice is consumed in Asia, where it is a staple for a majority of the population. Rice consumption in Africa and Latin America is increasing faster than that of any other food staple in the past decade, mainly due to urbanization and changes in eating habits. In addition, European, US, and Australian citizens are eating more rice nowadays, mainly due to an increased interest in Asian cuisine. China and India alone account for over 50% of the world’s rice consumption. Since the early 1990s, strong economic growth in many Asian countries, such as China and India, halted the upward trend in global per capita rice consumption as consumers diversified their diet from rice to high-value products such as meat, dairy products, fruits, and vegetables. Thus, the consumption per capita in these two largest countries and many other Asian countries has recently decreased (Table 1). Some West African countries (Guinea, Sierra Leone, and Guinea-Bissau), Madagascar, and Guyana are in the lists of the top 20 per capita rice-consuming countries (Table 1).
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The eating quality can be directly characterized and measured with descriptive sensory analysis and textural instruments or indirectly predicted with a series of physicochemical property evaluations.
Descriptive sensory analysis is an important tool used to characterize and measure traits of the aroma, flavor, and texture of foods by a panel including around ten specialists trained in the principles and concepts of descriptive analysis. The sensory texture profile is composed of 14 attributes that describe the rice texture at different phases of sensory evaluation (Table 2), beginning with the feel of the rice when it is first placed in the mouth and ending with mouthfeel characteristics after the rice is swallowed. During evaluation, each sample will be presented to the panelists twice, following a randomized design in which each session consists of three samples, a standard, and a blind control. The standard presented as the warm-up sample at the beginning of each session is used to calibrate the panel. After the warm-up sample, test samples are presented to panelists individually at 20 min intervals immediately after cooking, holding, and portioning into serving cups. Evaluations are conducted at individual test stations under red masking lights, during which distilled water will be used to cleanse the mouth between samples. The flavor (aromatics, taste, and mouthfeel) of cooked rice is also evaluated by the sensory panel (Table 3). The rice
Encyclopedia of Food Grains, Second Edition
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FOOD-QUALITY TESTING | Rice: Eating Quality
Table 1
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Top 20 per capita rice-consuming countries
Country
Consumption (kg/capita/year)
Country
Consumption (kg/capita/year)
Brunei Darussalam Vietnam Laos Bangladesh Myanmar Cambodia The Philippines Indonesia Thailand Madagascar
245 166 163 160 157 152 129 125 103 102
Sri Lanka Guinea Sierra Leone Guinea-Bissau Guyana Nepal Korea, the Democratic People’s Republic of China Malaysia Korea, the Republic of
97 95 92 85 81 78 77 77 77 7
Source: FAOSTAT – http://faostat.fao.org/site/609/DesktopDefault.aspx?PageID¼609#ancor.
Table 2
The lexicon and their definitions used to evaluate cooked rice texture
Phases/attributes
Definition
Phase I Initial starchy coating Slickness Roughness Stickiness to lips Stickiness between grains Phase II Springiness Cohesiveness Hardness Phase III Cohesiveness of mass Chewiness Uniformity of bite Moisture absorption Phase IV Residual loose particles Toothpack
Place 6–7 grains of rice in mouth behind front teeth. Press tongue over surface and evaluate Amount of paste-like thickness perceived on the product before mixing with saliva (three passes) Maximum ease of passing tongue over the rice surface when saliva starts to mix with the sample Amount of irregularities in the surface of the product Degree to which kernels adhere to lips Degree to which the kernels adhere to each other Place 1/2 teaspoon of rice in mouth. Evaluate before or at first bite Degree to which grains return to original shape after partial compression Degree to which the grains deform, rather than crumble, crack, or break when biting with molars Force required to bite through the sample with the molars Evaluate during chew Maximum degree to which the sample holds together in a mass while chewing Amount of work to chew the sample Evenness of force throughout bites to chew Amount of saliva absorbed by sample during chewing Evaluate after swallow Amount of loose particles in mouth Amount of product adhering in/on the teeth
Source: Champagne ET, Bett-Garber KL, Fitzgerald MA, et al. (2010) Important sensory properties differentiating premium rice varieties. Rice 3: 270–281.
Table 3
The lexicon and definitions used to evaluate cooked rice flavor (aromatics, taste, and mouthfeel)
Sewer animal Floral Grain starchy Hay-like musty Popcorn Corn Alfalfa/grassy/ green beans Dairy Sweet aromatic Water-like metallic Sweet taste Sour/silage Astringent
An immediate and distinct pungent aroma in the flavor characterized as sulfur-like and generic animal. The animal aromatic in the flavor can sometimes be identified as ‘piggy’ Aromatics associated with dried flowers, such as lilac and/or lavender. This aromatic is characterized as spicy floral as in an ‘old-fashioned sachet’ A general term used to describe the aromatics in the flavor associated with grains such as corn, oats, and wheat. It is an overall grainy impression characterized as sweet, brown, sometimes dusty, and sometimes generic nutty or starchy A dry, dusty, slightly brown aroma/flavor with a possible trace of musty odor A dry, dusty, slightly toasted, and slightly sweet aroma in the flavor that can be specifically identified as popcorn The sweet aromatics of the combination of corn kernels, corn milk, and corn germ found in canned yellow creamed-style corn A dried, green, slightly earthy, slightly sweet aroma/flavor including grassy and fresh green bean aroma/flavor A general term associated with the aromatics of pasteurized cow’s milk. Most apparent just before swallowing A sweet impression such as cotton candy, caramel, or sweet fruit that may appear in the aroma and or aromatics Aromatics and mouthfeel of the minerals and metals commonly associated with tap water. This excludes any chlorine aromatics that may be perceived Basic sweet taste associated with sugar A sour fermented vegetation aroma/flavor, not decaying vegetation The chemical feeling factor on the tongue, described as puckering/dry and associated with tannins or aluminum
Source: Champagne ET, Bett-Garber KL, Fitzgerald MA, et al. (2010) Important sensory properties differentiating premium rice varieties. Rice 3: 270–281.
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lexicon includes 13 unique flavor attributes that are determined by smelling and evaluation in the mouth. The intensities are scored based on a universal scale for all foods, with the maximum rating for rice flavor attributes generally set at about 5. The advantage of sensory evaluation is that it provides direct description of the eating quality (palatability) of rice by consumers, and it may directly determine consumer acceptability. The disadvantage is that the eating quality evaluated by the panels is constrained to the region where the panels live or in the same populations as the panels. Regional difference and cultural background have a great impact on the sensory quality. It is difficult to compare the sensory parameters across different labs and different regions. Needless to say, the cost of training and maintaining a descriptive panel is always high.
25
Kg
20 H1 Height of first curve 15
10
Area first bite Area second bite
5 A1 0
Instrumental Texture Analysis
D1 (4.9 mm)
A2 A3
D2
Area of negative curve
The texture of cooked rice can be measured by different types of textural instruments. Texture profile analysis (TPA) can be conducted with a texture analyzer, for example, the model TA.XT2 texture analyzer (Texture Technologies Corp., Scarsdale, NY). The cooked warm rice grains are arranged in a single grain layer on the base plate. A two-cycle compression, force versus distance program is used to allow the plate to travel, return, and repeat, which simulates human teeth biting the rice grain. A typical texture profile is shown in Figure 1. It should be noted that the texture and flavor of cooked rice may be affected by the water-to-rice ratio during cooking. The amount of water affected 11 of the 14 texture attributes (Table 2). Of these 11, initial starchy coating, slickness, stickiness between grains, cohesiveness, and uniformity of bite increase in intensity with greater amounts of water at cooking, whereas hardness, stickiness to lips, springiness, and chewiness decrease in intensity. However, the flavor attributes across all cultivars are not significantly affected by the water-to-rice ratio. The advantage of the instrumental analysis of the texture of cooked rice is that it provides a relatively objective and easy measurement tool with the characteristics of being cost- and time-consuming and facilitates comparisons of results derived from different labs, regions, and countries. The disadvantage is that it is only a simulation test, which cannot reflect the real sensory attributes of humans so, in many cases, it cannot predict the real eating quality measured by descriptive sensory evaluation.
Physicochemical Property Evaluation Although descriptive sensory analysis methodology is commonly used for evaluating the intensity of the sensory attributes of cooked rice, it is typically time-consuming and only assesses whether the rice cultivar has quality characteristics deemed desirable by a target population or for a specific application, falling short in measuring subtle differences in sensory quality. Amylose content, gel consistency, gelatinization temperature (GT), and pasting viscosity are regarded as traditional parameters to describe the eating quality of rice. All these parameters reflect the properties of the starch that makes
–5 Figure 1 Typical texture profile analysis (TPA) curve of 1 g rice samples. Attributes on curves: H1, hardness (kg), measurement of force at peak of first curve; A1, area under first curve; A2, area under second curve; A4, measured from first data point to first probe reversal point; A5, measured from first probe reversal point to point where force returns to zero; A2/A1, cohesiveness, ratio of area under curves A2/A1; A3, adhesiveness, area of negative force curve, representing work to separate plunger from sample on upstroke after first curve; D2/D1, springiness, ratio of D2 to D1, where D1 is the total distance traveled by plunger on downstroke and D2 is the distance traveled on downstroke by plunger from point of sample contact to end of downstroke.
up 90% of milled rice. For objective traits, the advantage of using these parameters is that they offer possibilities to compare the physicochemical properties among different rices cultivated in different countries/regions, under different environments. The amylose content is measured with a process using I2–KI solution. However, this method always gives an overestimation, due in part to the value being contributed by amylopectin. Thus, the amylose content of starch as determined by reaction with iodine is termed apparent amylose content (AAC). The AAC of milled rice may be classified as waxy (1–2%), very low (5–12%), low (12–20%), intermediate (20–25%), and high (>25%). The AAC is considered the most important determinant of cooked rice texture, but this constituent falls short as a predictor, because cultivars with similar AAC may still differ in textural properties. The actual value of amylose can be obtained by using the size-exclusion chromatography (SEC) method, which separates polymers based on size. Since the size of amylose and amylopectin molecules differs substantially, they may be separated by SEC. This method does not need a calibration curve, but it is low-throughput and so, it is not appropriate for the routine screening of amylose in breeding programs. The gelatinization temperature (GT) is the critical temperature at which starch granules irreversibly lose their birefringence and crystalline order during heating. The GT can be directly measured under polarized light microscopy with a
FOOD-QUALITY TESTING | Rice: Eating Quality
Score
Grain not affected Grain swollen Grain swollen, collar incomplete, or narrow Grain swollen, collar complete, and wide Grain split or segmented, collar complete, and wide Grain dispersed, merging with collar Grain completely dispersed and disintegrated
1 2 3 4 5 6 7
90 CS
PV BD
75
160 HPV 60 80
0
heating stage to record the temperature when the birefringence is lost. The GT can also be measured by differential scanning calorimetry or indirectly by the alkali spreading value (ASV) test for rice grains. The ASV is the degree of spreading and disintegration values of six milled rice grains after soaking in 10 ml 1.7% KOH for 23 h at 30 C (Table 4). The ASV of 5.5–7.0 is classified as low GT (55–69.5 C); 3.5–5.4, intermediate (70–74 C); and 1–3.5, intermediate-high or high (74–80 C). The ASV test is usually used by rice breeders due to its simplicity and high-throughput characteristics. Gel consistency is used as a parameter to index the tendency of cooked rice to harden on cooling. This method was originally developed to differentiate cooked rice texture among high-AAC rice. It measures the distance traveled by a gel after cooking. According to the gel length, gel consistency can be classified as soft (61–100 mm), medium (41–60 mm), and hard (25–40 mm). Pasting viscosity is another useful parameter to differentiate rice with similar AAC and is popularly measured by a Rapid Visco Analyzer (RVA). The RVA records the viscosity continuously as the temperature is increased, held constant for a time, and then decreased (Figure 2). Briefly, according to the method of The American Association of Cereal Chemists (AACC) 61-02, rice flour slurry is made in the RVA canister consisting of 3.0 g (12% mb) of rice flour and 25 g of distilled water. The heating profile is set as follows: (1) the temperature is held at 50 C for 1 min, (2) the temperature is linearly ramped up to 95 C until 4.8 min, (3) the temperature is held at 95 C until 7.5 min, (4) the temperature is linearly ramped down to 50 C at 11 min, and (5) the temperature is held at 50 C until 12.5 min. A typical RVA profile is shown in Figure 2. It should be noted that the degree of milling affects not only the texture and flavor of cooked rice but also the physicochemical properties. All these measurements for eating quality assume that the optimum degree of rice milling has been applied to obtain the milled rice. Similar to instrumental texture analysis, the advantage of physicochemical property analyses is that they provide a relatively objective measurement tool for the indirect prediction of eating quality. All the analyses are easy and fast and can routinely be conducted in a quality lab. These analyses also facilitate comparisons of results derived from different labs, regions, and countries. However, the disadvantage is that they fall short in predicting the real eating quality measured by descriptive sensory evaluation.
CPV SB
240 Viscosity (RVU)
Description
320 Temperature Profile
Temperature (°C)
Table 4 Description of the ASV for milled rice after soaking in 1.7% KOH for 23 h
169
3
6 9 Time (min)
12
45 15
Figure 2 The RVA pasting viscosity profile of rice starch. The pasting viscosity parameters: PV, peak viscosity; HPV, hot paste viscosity; CPV, cold paste viscosity; PT, pasting temperature. In addition, the secondary parameters of breakdown, setback, and consistency viscosities can be calculated as PV-HPV, CPV-PV, and CPV-HPV, respectively.
In terms of eating quality evaluations, it is suggested to keep some rice with different eating qualities as standard rice and used as controls. These standard rice can be not only used for panel training for sensory evaluation but also used in instrumental texture analysis and physicochemical property evaluations, which may facilitate comparisons of eating quality across different labs in different regions.
Relationship Between Texture Attributes and Physicochemical Properties The AAC has long been considered the most important determinant of cooked rice texture. Cooked rice with a low-AAC cultivar is soft, moist, and sticky in texture, and those with high AAC are dry, firm, and fluffy. The sensory properties of freshly cooked rice measured by panelists using descriptive sensory analysis vary widely, but 11 of 14 parameters correlate well, either positively or negatively, with the AAC, with correlation coefficient values in the range of 0.76–0.92. Particularly, the AAC is positively correlated with hardness (r ¼ 0.90) and negatively correlated with stickiness to lips (r ¼ 0.91), stickiness between kernels (r ¼ 0.91), and cohesiveness of mass (r ¼ 0.92). Grain flavor ranges in intensity from 2.2 to 4.9 and correlates highly and negatively with the AAC (r ¼ 0.88). Some RVA parameters have significant correlations with AAC; it was found that none of the cooked rice textural attributes, whether measured by descriptive analysis or TPA, are linearly modeled with high accuracy by the RVA. The sensory texture attributes cohesiveness of mass, stickiness, and initial starchy coating and TPA attribute adhesiveness have the strongest correlations with RVA measurements. Setback explains most of the variance attributed to models describing these attributes; the strongest correlation is with cohesiveness of mass (r ¼ 0.69). Inclusion of amylose and protein contents in regression analyses do not strengthen models, and exclusion of samples that cook atypically, based on amylose content or GT types, slightly improve the accuracy of RVA measurements for predicting the cooked rice texture.
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Criteria for Eating Quality Consumer preferences for rice vary in different areas of the world. It is difficult to find a rice cultivar that could meet the demands of consumers around the world. However, global demand for high-quality rice is ever increasing both in local production regions and in regions in which rice is imported. For the promotion of rice trade, it is necessary to have a standard of grain quality for the commodity. To facilitate appraising the eating quality of new rice cultivars in breeding programs, it is also important to set up standards for eating quality evaluation. US Standards for Milled Rice (2009) provide a system of marketing rice by grade that takes into consideration those quality factors associated with cleanliness, soundness, and purity. However, the eating quality of milled rice has not been included in the standards. In Thailand, Thai aromatic rice has long been well known in international markets and is always exported using the name Thai Hom Mali rice, jasmine rice, or Thai aromatic rice. For the benefit of Thai exporters, the Ministry of Commerce, with cooperation of the Ministry of Agriculture, cooperatives, and private sector, established a notification on the subject: Thai Hom Mali Rice Standards B.E. 2544. These standards comprise both physical and chemical analyses and have two points regarding eating quality: (1) Thai Hom Mali rice such as Khao Dawk Mali 105 and RD15 has a natural fragrant aroma depending on whether it is new or aged rice. Its cooked rice kernels have a tender texture, and (2) it has an amylose content of 13–18%. In China, the Ministry of Agriculture released a standard for cooking rice variety quality in 1986 (NY20-1986) for facilitating the breeding, registration, and extension of new rice cultivars with good grain quality. With changes in grain production, processing, and cropping system rearrangement, the standard was revised in 2002 (NY/T593-2002) with concomitant release of three related standards for indica-type cooking rice (NY/T594-2002), japonica-type cooking rice (NY/T595-2002), and aromatic rice (NY/T596-2002). A new version of these standards was renewed in 2013. In addition to the milling quality and appearance quality required for different grades of high-quality rice, this standard (NY/T593-2013) provides the grade for eating quality (Table 5). Two methods are used for the evaluation of eating quality; one is the sensory test, which is regarded as an arbitration method, and the other is physicochemical property evaluation with three common parameters, ASV, GC, and AAC. Sensory evaluations refer to another national standard – Method for sensory evaluation of paddy or rice cooking and eating quality (GB/T15682-2008). This test includes five items; aroma flavor, surface structure, palatability, taste, and texture of cold cooked rice, which accounts for a score of 20, 20, 30, 25, and 5, respectively, in a total score of 100. If rice meets the requirements of any grades in the standard, it is called high-quality rice including grade 3. Otherwise, it is not a high-quality rice. The eating quality parameters used in indica-type cooking rice (NY/T594-2002) and japonica-type cooking rice (NY/T595-2002) are the same as those in NY/T593-2013 (Table 5). For facilitating rice cooking for sensory evaluation, the water-to-rice ratio is stipulated based on the AAC, where the water-to-rice ratios are 1.2, 1.3,
Table 5 Quality requirements for different grades and different types of rice stipulated in the Chinese standard for cooking rice variety quality (NY/T593-2013)
Rice type
Grade
Sensory evaluation
ASV
GC (mm)
AAC (%, db)
Indica nonwaxy rice Indica waxy rice
1 2 3 1 2 3 1 2 3 1 2 3
90 80 70 90 80 70 90 80 70 90 80 70
6.0 6.0 5.0 6.0 6.0 5.0 7.0 7.0 6.0 7.0 7.0 6.0
60 60 50 100 100 90 70 70 60 100 100 90
13.0–18.0 13.0–20.0 13.0–22.0 2.0 2.0 2.0 13.0–18.0 13.0–19.0 13.0–20.0 2.0 2.0 2.0
Japonica nonwaxy rice Japonica waxy rice
1.4, and 1.5 for rice with AAC in the range of 15.0, 15.1–20.0, 20.1–25, and >25.0%, respectively. To facilitate paddy rice circulation, marketing, and market management and promote the rearrangement of the cropping system, the General Administration of Quality Supervision, Inspection and Quarantine of China released a national standard for high-quality paddy in 1999 (GB/T17891-1999), which is still in effect now. This standard stipulates the definition, classification, quality requirement, test methods, and requirement for the packaging, transportation, and storage of high-quality paddy commodities. It also provides parameters for eating quality by grade (Table 6). These standards provide the eating quality requirement of rice commodities for breeders, processors, and merchants and also allow both exporters and importers to have the chance to meet the standards of rice quality in the target market.
Chemical Basis of Eating Quality The chemical basis of the texture of cooked rice attracted researchers’ attention in the period between World War II and the mid-1980s, during which the research repeatedly suggested that amylose content is the single largest determinant of its texture. The greater the amylose, the firmer the cooked rice texture is and vice versa. However, it has been realized that amylose is not sufficient to explain why the texture of cooked rice still differs among rice with similar amylose content, especially within high-amylose rice. It was also found that part of the amylose in rice starch is soluble in hot water and that the proportion of the soluble amylose as a fraction of the total amylose content differs widely among different varieties. The insoluble amylose content of rice (total minus soluble amylose) shows a striking correlation with rice texture, particularly in the high-amylose group, which could be divided into three distinct subgroups showing a distinct texture after cooking. Perhaps, it is related to retrogradation, because it is clearly the insoluble amylose that retrogrades and thus provides greater rigidity to the starch granules and hence to the cooked rice grain. The chemical nature of the insoluble amylose was intensively investigated by characterizing the structure of amylopectin. It
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Table 6 Quality requirements for different grades and different types of rice stipulated in China national standard for high-quality paddy (GB/ T17891-1999) Rice
Grade
AAC (%, db)
Taste
GC (mm)
Color and smell
Indica paddy
1 2 3 1 2 3
17.0–22.0 16.0–23.0 15.0–24.0 15.0–18.0 15.0–19.0 15.0–20.0 2.0
9 8 7 9 8 7 7
70 60 50 80 70 60 100
Normal Normal Normal Normal Normal Normal Normal
Japonica paddy
Waxy paddy (indica and japonica)
was found that amylopectin also binds iodine, which is apparently caused by the presence of some very long chains. The chain length distribution of rice amylopectin also differs among different rice accessions, with high-amylose rice having more very long B chains than low-amylose rice and intermediateamylose rice having in between. The amylose content of starch as determined by reaction with iodine is always overestimated with part of the value being contributed by amylopectin. It is found not only that amylopectin in high-insoluble amylose rice has more long B chains but also that these chains seemed to be largely located externally in the molecule and vice versa. Thus, the texture of cooked rice is closely correlated with the relative abundance of long B chains in its amylopectin. The more long B chains, the firmer the cooked rice is and vice versa. Long B chains in amylopectin strengthen the starch granules through intermolecular interaction, leading to a firm texture of cooked rice. The chemical basis of the flavor and aroma of cooked rice has received attention. More than 200 volatile compounds have been identified in cooked rice, but only a few of them may relate to the aroma and flavor of cooked rice. It is difficult to determine which volatile compounds are responsible for the perceived aroma/flavor of rice. Only one compound, 2-acetyl1-pyrroline (2AP; popcorn aroma), has been confirmed to contribute a characteristic aroma. Furthermore, 2AP is the only volatile compound in which the relationship between its concentration in rice and its sensory intensity has been established. The fragrance gene (fgr) regulating the biosynthesis of 2AP has been cloned. It encodes the betaine aldehyde dehydrogenase 2 (BADH2). A deletion of 8 bp in this gene truncates and makes nonfunctional the BADH2 protein, leading to accumulation of 2AP in the rice grain. Other factors may also contribute to the difference in eating quality among different rice cultivars. Protein is the second most abundant component of rice endosperm and constitutes 6–10% of the dry matter in the milled rice. The protein content of milled rice has a negative correlation with the appearance, flavor, and stickiness of cooked rice. High-protein rice has much firmer texture than low-protein rice. The eating quality of cooked rice decreases as the ratio of protein body I to the content of total rice protein increases. Further, the cooked rice becomes harder, and the aroma and taste deteriorate with the addition of prolamin. As rice grain storage protein prolamin is located in protein body I, it is expected that the eating quality of cooked rice decreases as the ratio of prolamin to the content of total rice protein increases. The granule-bound starch synthase I (GBSSI) protein is positively correlated with
amylose content (r ¼ 0.95) but negatively correlated with cooked rice stickiness (r ¼ 0.85). The effect of protein content on the texture of cooked rice may also contribute to its glass transition (Tg) being slightly lower than that of starch, so when cooked with limited water, for example, in a rice cooker, the higher the protein content, the more water bound by proteins would be, leaving less water for the swelling of starch granules and leaching of amylose. The addition of proteins or its components in a viscosity test demonstrates their ability to modify the viscosity curves. The modification may be through binding water, which causes the concentration of the dispersed and viscous phases of gelatinized starch to increase, and through the agency of a network linked by disulfide bonds. The lipid content and fatty acids of milled rice may also affect eating quality. Rice lipids are generally classified as starch and nonstarch lipids. The lipid content of milled rice ranged from 0.2% to 2%. Rice varieties with high lipid content and linoleic acid content exhibits better appearance and flavor of the cooked rice and high palatability. The fat content of milled rice correlates positively with the sensory characteristics hardness, color, intactness of grains, puffed corn flavor, raw rice flavor, wet cardboard flavor, and hay-like flavor intensity and negatively with the sweet taste, degree of agglomeration, adhesiveness, cohesiveness, and cohesiveness of mass. Rice with high activities of lipase and lipoxygenase exhibits a low palatability. During storage, the appearance, aroma, taste, cohesiveness, and hardness of cooked rice deteriorate, and this deterioration is accompanied by changes in the chemical components of rice, particularly lipid content and composition. Decreased nonstarch lipid and linoleic acid and increased oleic acid and palmitic acid contents may be responsible for the deteriorations of eating quality after storage.
Genetic Basis of Eating Quality The eating quality of rice is under genetic control. The chemical constituents, that is, starch (amylose and amylopectin) and its structural features, protein, and lipid that influence the eating quality, are characteristics of the rice. These chemical constituents are synthesized through some metabolic pathways under the control of a series of genes. One of the most important pathways is starch synthesis since starch constitutes 90% of the milled rice, so it plays the most important role in the formation of eating quality.
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Amylose is synthesized mainly by GBSSI, and the amylopectin synthesis process is governed by a combination of multiple isoforms of starch synthase, branching enzyme, and debranching enzyme to produce a uniform number of chains per amylopectin cluster. The waxy (Wx) gene on chromosome 6 encoding GBSSI is mainly responsible for the natural variation in amylose content, gel consistency, and RVA pasting viscosity. Five functional markers in the Wx gene, a (CT)n microsatellite (or simple sequence repeat), a 23 bp insertion/ deletion sequence, and three single-nucleotide polymorphism (SNP) markers are well characterized, with different alleles differing in AAC (Figure 3(a)). Among them, the (CT)n microsatellite in the Wx gene located 55 bp upstream of the putative leader intron 50 splice site has many alleles with n ranging from 8 to 22 in diverse rice germplasm (Figure 3(b)). The G/T SNP at the putative leader intron 50 splice site, and a G to T mutation at this site, reduces the efficiency of Wx pre-mRNA processing and thus results in the lower level of spliced mature mRNA, Wx protein, and AAC (Figure 3(c)). Waxy, low-amylose, and some intermediate-amylose rice have the T SNP allele, while some intermediate- and high-amylose rice have the G allele. Over 75% of the total observed variation in the AAC can be explained by the (CT)n microsatellite and the G/T SNP in the nonwaxy rice accessions. The Wx gene is also responsible for biosynthesis of extralong unit chains of amylopectin in rice. Among the high-AAC rice, the exon 10 SNP of Wx gene is responsible for the difference in the gel consistency, the proportion of soluble to insoluble apparent amylose, gel hardness, and RVA pasting viscosity. The rice with SNP allele C at exon 10 produces soft, viscous gels and has a soft texture when cooked, but with high retrogradation, and the rice with SNP allele T gives a short, firm
(CT)n
G/T
gel and has a firm texture when freshly cooked with little change in texture over storage. The SSIIa is a major gene for GT, thermal properties, and amylopectin structure, which elongates the short A and B1 chains with a degree of polymerization (DP) < 10 to form long B1 chains of amylopectin. The GT was negatively correlated with the amount of DP6-11 and positively correlated with the amount of DP12-24 of amylopectin and is positively correlated with the degree of crystallinity of starch. In waxy rice, in which the Wx gene is dysfunctional, SSIIa is the major gene responsible for the variations in pasting, gelatinization, and retrogradation properties. Four functional SNPs in the SSIIa gene have been revealed by several research groups, in which the third site where valine encoded by GTG is replaced by methionine encoded by ATG at 4198 bp position and the fourth site where glycine–leucine encoded by GGGCTC is replaced by glycine–phenylalanine encoded by GGTTTC at 4229/4330 bp are crucial for SSIIa activity (Figure 4(a)). The SSIIa enzyme is inactive when it is A SNP (coding for methionine) no matter which SNP at 4229/ 4330 bp (GC/TT) is present. The GC/TT is most common and is strongly associated with the GT (Figure 4(b)). This GC/TT polymorphism alone can differentiate rice with high or intermediate GT (possessing the GC allele) from those with low GT (possessing the TT allele), explaining 62.4% of the total variation in pasting temperature (Figure 4(c)). Few rice accessions with the GC allele have a low-GT phenotype, which can be explained by their carrying the A SNP allele in the third SNP. However, it should be mentioned that the A allele of the third SNP (G/A) is quite rare in natural populations. In addition to the genetic effects, the eating quality of rice is also affected by the environment. The texture attributes
Exon 2
Exon 6
Exon 10
InDel
A/C
C/T
(a)
Wx
100
AAC (%)
150
35 30 25 20 15 10 5 0 8
10
11
12
17
18
19
20
21
22
(CT)n
T G
AAC (%)
(b) 30 25 20 15 10 5 0
T
G G/T allele
(c)
Figure 3 Relationship between the Wx alleles and apparent amylose content (AAC). (a) Wx gene functional markers, a (CT)n microsatellites, the G/T SNP in intron 1, the 23 bp duplication in exon 2, a A/C SNP in exon 6, and a C/T SNP in exon 10; (b) genotyping the (CT)n microsatellites (left) and comparison of AAC of each microsatellite class; (c) genotyping the G/T SNP and comparison of the AAC of each allele class.
FOOD-QUALITY TESTING | Rice: Eating Quality
G/C
A/G
173
A/G GC/TT
(a)
F7
F22 GC
82.0
TT R21
80.0
R1
78.0
F7F22/R1 5 6 7 8
F7/R1R21 9 10 11 12 F7F22/R1R2 13 14 15 16
76.0
PT (°C)
F7/R1 1 2 3 4
74.0
TT
GC
72.0 70.0
(b)
TT
68.0
GC
66.0 –50
A
50
150
250
350
450
550
(c)
Figure 4 Relationship between the SSIIa alleles and gelatinization temperature. (a) SSIIa functional SNPs. (b) Strategy for genotyping the fourth GC/TT SNP (up) and gel image showing different PCR products amplified by different combinations of the primers. (c) The relationship between pasting temperature and GC/TT alleles; the blue circle shows some rice with A SNP at the third site.
roughness and hardness and the flavor attributes hay-like, sweet aromatic, sour, and astringent are significantly different between two cropping years, which may be attributed to the variation in the AAC and protein content as affected by the environment. The AAC is increased when rice is grown at a cooler temperature. Wx gene expression is activated reversibly in response to a cool temperature (18 C), and it is the Wx promoter that is responsive to a cool temperature. The G/T SNP at the alternate splicing site is also temperature-dependent. At 25 C and particularly at 32 C, the amount of mature GBSS mRNA in developing seeds is dramatically reduced. This is one of the reasons why low-amylose varieties are particularly temperature-sensitive. The steady-state level of GBSS mRNA is essentially identical over this temperature range for the rice with G SNP allele. Environmental factors also affect the GT, resulting in a difference of up to 10 C between some rice starches. Environmental factors, especially temperature during grain filling, can modify the expression of the SSIIa gene and hence modify the amylopectin chain length. The amount of short chains in the amylopectin increases significantly, whereas the amount of long chains decreases in rice grown at lower temperatures, resulting in a lower GT.
Improvement of Eating Quality The improvement of eating quality is an essential target of current breeding programs designed to meet industry standards or eating characteristics preferred by consumers in domestic or international markets. Three questions need to be answered before breeding for high eating quality is conducted. The first question is what kind of rice is preferred by
people in the region or country? If a new rice is for exporting, the same question is what kind of quality rice is preferred by people of the country or needed in the targeted market. The second question is what kind of traits should be improved. The third question is what kind of tools are available for breeding, what molecular markers could be used at the genomic level at this time? The first question has been answered by the International Network for Quality Rice, which has conducted a survey of consumer preferences for rice quality in different riceconsuming regions. The maps showing regional variation in amylose and GT, consumer’s preferences for texture based on gel consistency, and consumer’s references for aromatic rice are useful for breeders worldwide, particularly for those whose rice is targeted to the international market. The current eating quality status of your rice materials should be surveyed to understand whether the quality matches the consumer preference and to determine which trait needs to be improved. If allele information of the microsatellites of the Wx gene, SNP of SSIIa, and insertion/deletion marker (8 bp) of fgr for eating quality traits are available in your rice materials, it is easy to transfer the allele of these genes from one variety to another by crossbreeding and backcross breeding with the help of molecular marker detection. In F2 lines, the phenotype is matched with its genotype very well when these functional markers are used to improve the eating quality of a highAAC, high-GT, and nonfragrant maintainer rice II32B from another low-AAC, low-GT, and fragrant rice (Figure 5). The breeding practice with marker-aided selection is also called molecular breeding. In addition to use in molecular breeding, these three gene markers can also be used in the diagnosis of quality traits for
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FOOD-QUALITY TESTING | Rice: Eating Quality
M P1 P2 1
2 3
4 5 6
7
8 9 10 11 12 13 14 15 16 17 18 19 20
200bp
100bp (a)
500bp
GC
300bp
TT
(b)
300bp
200bp (c)
Figure 5 Segregation of the marker genotypes at three loci in F2 lines: P1: Yixiang B; P2: II-32B; 1–20 F2 individuals; (a) genotypes of Wx marker; (b) genotypes of SSIIa marker; (c) genotypes of fgr marker. GC represents GC SNPs, and TT represents TT SNPs of SSIIa gene.
rice collecting from markets. Since sensory and physicochemical property evaluations may be influenced by processing, milling, and storage, genotyping with molecular markers can avoid the side effects of these factors and can reduce analysis time, cost, and required sample size.
Exercises for Revision
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What kinds of rice are preferred and what are their physicochemical properties in your region? Does your country have standards for high-quality rice and what are the parameters to evaluate the eating quality? Do you think eating quality affects consumer acceptability? What other factors may affect consumer preference? How do you measure the eating quality of rice and their advantages and disadvantages? What are the chemical and genetic bases of the eating quality of rice? How could you use molecular markers to diagnose the eating quality of rice in the supermarket?
Exercises for Readers to Explore the Topic Further
• • •
How could you exploit new techniques such as electronic nose and electronic tongue to predict the eating quality of rice? What is known, and what still needs to be determined, about the genetic bases for other volatile compounds? What is the genetic basis of sensory parameters in rice? How will new molecular tools for eating quality contribute to our ability to manipulate the sensory quality of rice?
See also: Barley, Rice and Maize Processing: Rice Processing: Beyond the Farm Gate; Breeding of Grains: Rice: Breeding; Genetics of Grains: Rice: Genetics; The Cereal Grains: Rice: Overview; Wildrice, Zizania: Overview.
Further Reading Ayres NM, McClung AM, Larkin PD, et al. (1997) Microsatellites and a single-nucleotide polymorphism differentiate apparent amylose classes in an expanded pedigree of US rice germ plasm. Theoretical and Applied Genetics 94: 773–781. Bao JS (2012) Towards understanding of the genetic and molecular basis of eating and cooking quality of rice. Cereal Foods World 57(4): 148–156. Bao JS, Corke H, and Sun M (2006) Nucleotide diversity in starch synthase IIa and validation of single nucleotide polymorphisms in relation to starch gelatinization temperature and other physicochemical properties in rice (Oryza sativa L.). Theoretical and Applied Genetics 113: 1171–1183. Bao JS, Xiao P, Hiratsuka M, et al. (2009) Granule-bound SSIIa protein content and its relationship with amylopectin structure and gelatinization temperature of rice starch. Starch-Starke 61(8): 431–437. Bao JS, Zheng XW, Xia YW, et al. (2000) QTL mapping for the paste viscosity characteristics in rice (Oryza sativa L.). Theoretical and Applied Genetics 100: 280–284. Bhattacharya KR (2005) The chemical basis of rice end-use quality. In: Toriyama K, Heong KL, and Hardy B (eds.) Rice is Life: Scientific Perspectives for the 21st Century, pp. 246–248. Philippines: IRRI. Bhattacharya KR (2009) Physicochemical basis of eating quality of rice. Cereal Foods World 54(1): 18–28. Calingacion M, Laborte A, Nelson A, et al. (2014) Diversity of global rice markets and the science required for consumer-targeted rice breeding. PLoS One 9(1): e85106. Champagne ET (2008) Rice aroma and flavor: A literature review. Cereal Chemistry 85(4): 445–454. Champagne ET, Bett-Garber KL, Fitzgerald MA, et al. (2010) Important sensory properties differentiating premium rice varieties. Rice 3: 270–281. Champagne ET, Bett-Garber KL, McClung AM, et al. (2004) Sensory characteristics of diverse rice cultivars as influenced by genetic and environmental factors. Cereal Chemistry 81: 237–243.
FOOD-QUALITY TESTING | Rice: Eating Quality
Champagne ET, Bett-Garber KL, Vinyard BT, et al. (1999) Correlation between cooked rice texture and rapid visco analyses measurements. Cereal Chemistry 76: 764–771. Cheaupun K, Wongpiyachon S, and Kongseree N (2005) Improving rice grain quality in Thailand. In: Toriyama K, Heong KL, and Hardy B (eds.) Rice is Life: Scientific Perspectives for the 21st Century, pp. 248–250. Philippines: IRRI. Fitzgerald MA, Bergman CJ, Resurreccion AP, et al. (2009) Addressing the dilemmas of measuring amylose in rice. Cereal Chemistry 86(5): 492–498. Jin L, Lu Y, Shao YF, et al. (2010) Molecular marker assisted selection for improvement of the eating, cooking and sensory quality of rice (Oryza sativa L.). Journal of Cereal Science 51: 159–164. Juliano BO (1998) Varietal impact on rice quality. Cereal Foods World 43: 207–218.
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Tran NA, Daygon VD, Resurreccion AP, et al. (2011) A single nucleotide polymorphism in the Waxy gene explains a significant component of gel consistency. Theoretical and Applied Genetics 123: 519–525. Umemoto T and Aoki N (2005) Single-nucleotide polymorphisms in rice starch synthase IIa that alter starch gelatinization and starch association of the enzyme. Functional Plant Biology 32: 763–768. Umemoto T, Yano M, Satoh H, et al. (2002) Mapping of a gene responsible for the difference in amylopectin structure between japonica-type and indica-type rice varieties. Theoretical and Applied Genetics 104: 1–8. Yoon MR, Rico CW, Koh HJ, et al. (2012) A study of the lipid components of rice in relation to palatability and storage. Journal of the Korean Society for Applied Biological Chemistry 55: 515–521.
Topic Highlights
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Acceptance of rice variety by consumers is primarily determined by eating quality characteristics, but eating quality means different things to different people in different regions and with different cultural backgrounds. The criteria or standards for high quality, based on sensory analysis by panels and physicochemical property evaluations facilitate rice processing, breeding, and marketing. The chemical basis of the texture of cooked rice is attributed to amylose content and particularly long B chains of amylopectin; the genetic basis of the texture of cooked rice is attributed to both the waxy and SSIIa genes on chromosome 6. The chemical basis of the popcorn aroma of cooked rice is attributed to the volatile compound 2-acetyl-1-pyrroline (2AP); the genetic basis of the popcorn aroma of cooked rice is attributed to the fragrance gene (fgr) on chromosome 8. Improvement of eating quality may be fulfilled with marker-aided selection of three genes, that is, waxy, SSIIa, and fgr.
The diverse people of different origins and with different cultural backgrounds consume rice, thus making their demands for rice quality diversified. Different countries have different requirements for the quality of rice, and within countries, preferences also vary. However, the eating quality of rice may be the first thing to be considered for consumers to choose rice for consumption. The eating quality depends on the texture and flavor of the cooked rice or rice products. The texture of the cooked rice is a principal consideration in its consumer acceptability and palatability, which is expressed in terms of hardness or firmness (or, its opposite, tenderness) and stickiness or adhesiveness and moistness to touch. The flavor of the cooked rice is expressed in terms of its aromatics, taste, and mouthfeel. The economic value of rice in domestic and international trade is arguably influenced by the eating quality of cooked rice. Understanding the chemical and genetic basis of eating quality will not only promote the circulation of rice in domestic and international business but also facilitate rice quality improvement through breeding to match the needs in the target markets.
Measuring Eating Quality Learning Objectives
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To achieve understanding of the chemical and genetic bases of eating quality and uses of genetic tools to improve the eating quality of rice.
Sensory Evaluation
Introduction Rice is a staple food for half of the world’s population. However, more than 90% of rice is consumed in Asia, where it is a staple for a majority of the population. Rice consumption in Africa and Latin America is increasing faster than that of any other food staple in the past decade, mainly due to urbanization and changes in eating habits. In addition, European, US, and Australian citizens are eating more rice nowadays, mainly due to an increased interest in Asian cuisine. China and India alone account for over 50% of the world’s rice consumption. Since the early 1990s, strong economic growth in many Asian countries, such as China and India, halted the upward trend in global per capita rice consumption as consumers diversified their diet from rice to high-value products such as meat, dairy products, fruits, and vegetables. Thus, the consumption per capita in these two largest countries and many other Asian countries has recently decreased (Table 1). Some West African countries (Guinea, Sierra Leone, and Guinea-Bissau), Madagascar, and Guyana are in the lists of the top 20 per capita rice-consuming countries (Table 1).
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The eating quality can be directly characterized and measured with descriptive sensory analysis and textural instruments or indirectly predicted with a series of physicochemical property evaluations.
Descriptive sensory analysis is an important tool used to characterize and measure traits of the aroma, flavor, and texture of foods by a panel including around ten specialists trained in the principles and concepts of descriptive analysis. The sensory texture profile is composed of 14 attributes that describe the rice texture at different phases of sensory evaluation (Table 2), beginning with the feel of the rice when it is first placed in the mouth and ending with mouthfeel characteristics after the rice is swallowed. During evaluation, each sample will be presented to the panelists twice, following a randomized design in which each session consists of three samples, a standard, and a blind control. The standard presented as the warm-up sample at the beginning of each session is used to calibrate the panel. After the warm-up sample, test samples are presented to panelists individually at 20 min intervals immediately after cooking, holding, and portioning into serving cups. Evaluations are conducted at individual test stations under red masking lights, during which distilled water will be used to cleanse the mouth between samples. The flavor (aromatics, taste, and mouthfeel) of cooked rice is also evaluated by the sensory panel (Table 3). The rice
Encyclopedia of Food Grains, Second Edition
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FOOD-QUALITY TESTING | Rice: Eating Quality
Table 1
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Top 20 per capita rice-consuming countries
Country
Consumption (kg/capita/year)
Country
Consumption (kg/capita/year)
Brunei Darussalam Vietnam Laos Bangladesh Myanmar Cambodia The Philippines Indonesia Thailand Madagascar
245 166 163 160 157 152 129 125 103 102
Sri Lanka Guinea Sierra Leone Guinea-Bissau Guyana Nepal Korea, the Democratic People’s Republic of China Malaysia Korea, the Republic of
97 95 92 85 81 78 77 77 77 7
Source: FAOSTAT – http://faostat.fao.org/site/609/DesktopDefault.aspx?PageID¼609#ancor.
Table 2
The lexicon and their definitions used to evaluate cooked rice texture
Phases/attributes
Definition
Phase I Initial starchy coating Slickness Roughness Stickiness to lips Stickiness between grains Phase II Springiness Cohesiveness Hardness Phase III Cohesiveness of mass Chewiness Uniformity of bite Moisture absorption Phase IV Residual loose particles Toothpack
Place 6–7 grains of rice in mouth behind front teeth. Press tongue over surface and evaluate Amount of paste-like thickness perceived on the product before mixing with saliva (three passes) Maximum ease of passing tongue over the rice surface when saliva starts to mix with the sample Amount of irregularities in the surface of the product Degree to which kernels adhere to lips Degree to which the kernels adhere to each other Place 1/2 teaspoon of rice in mouth. Evaluate before or at first bite Degree to which grains return to original shape after partial compression Degree to which the grains deform, rather than crumble, crack, or break when biting with molars Force required to bite through the sample with the molars Evaluate during chew Maximum degree to which the sample holds together in a mass while chewing Amount of work to chew the sample Evenness of force throughout bites to chew Amount of saliva absorbed by sample during chewing Evaluate after swallow Amount of loose particles in mouth Amount of product adhering in/on the teeth
Source: Champagne ET, Bett-Garber KL, Fitzgerald MA, et al. (2010) Important sensory properties differentiating premium rice varieties. Rice 3: 270–281.
Table 3
The lexicon and definitions used to evaluate cooked rice flavor (aromatics, taste, and mouthfeel)
Sewer animal Floral Grain starchy Hay-like musty Popcorn Corn Alfalfa/grassy/ green beans Dairy Sweet aromatic Water-like metallic Sweet taste Sour/silage Astringent
An immediate and distinct pungent aroma in the flavor characterized as sulfur-like and generic animal. The animal aromatic in the flavor can sometimes be identified as ‘piggy’ Aromatics associated with dried flowers, such as lilac and/or lavender. This aromatic is characterized as spicy floral as in an ‘old-fashioned sachet’ A general term used to describe the aromatics in the flavor associated with grains such as corn, oats, and wheat. It is an overall grainy impression characterized as sweet, brown, sometimes dusty, and sometimes generic nutty or starchy A dry, dusty, slightly brown aroma/flavor with a possible trace of musty odor A dry, dusty, slightly toasted, and slightly sweet aroma in the flavor that can be specifically identified as popcorn The sweet aromatics of the combination of corn kernels, corn milk, and corn germ found in canned yellow creamed-style corn A dried, green, slightly earthy, slightly sweet aroma/flavor including grassy and fresh green bean aroma/flavor A general term associated with the aromatics of pasteurized cow’s milk. Most apparent just before swallowing A sweet impression such as cotton candy, caramel, or sweet fruit that may appear in the aroma and or aromatics Aromatics and mouthfeel of the minerals and metals commonly associated with tap water. This excludes any chlorine aromatics that may be perceived Basic sweet taste associated with sugar A sour fermented vegetation aroma/flavor, not decaying vegetation The chemical feeling factor on the tongue, described as puckering/dry and associated with tannins or aluminum
Source: Champagne ET, Bett-Garber KL, Fitzgerald MA, et al. (2010) Important sensory properties differentiating premium rice varieties. Rice 3: 270–281.
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FOOD-QUALITY TESTING | Rice: Eating Quality
lexicon includes 13 unique flavor attributes that are determined by smelling and evaluation in the mouth. The intensities are scored based on a universal scale for all foods, with the maximum rating for rice flavor attributes generally set at about 5. The advantage of sensory evaluation is that it provides direct description of the eating quality (palatability) of rice by consumers, and it may directly determine consumer acceptability. The disadvantage is that the eating quality evaluated by the panels is constrained to the region where the panels live or in the same populations as the panels. Regional difference and cultural background have a great impact on the sensory quality. It is difficult to compare the sensory parameters across different labs and different regions. Needless to say, the cost of training and maintaining a descriptive panel is always high.
25
Kg
20 H1 Height of first curve 15
10
Area first bite Area second bite
5 A1 0
Instrumental Texture Analysis
D1 (4.9 mm)
A2 A3
D2
Area of negative curve
The texture of cooked rice can be measured by different types of textural instruments. Texture profile analysis (TPA) can be conducted with a texture analyzer, for example, the model TA.XT2 texture analyzer (Texture Technologies Corp., Scarsdale, NY). The cooked warm rice grains are arranged in a single grain layer on the base plate. A two-cycle compression, force versus distance program is used to allow the plate to travel, return, and repeat, which simulates human teeth biting the rice grain. A typical texture profile is shown in Figure 1. It should be noted that the texture and flavor of cooked rice may be affected by the water-to-rice ratio during cooking. The amount of water affected 11 of the 14 texture attributes (Table 2). Of these 11, initial starchy coating, slickness, stickiness between grains, cohesiveness, and uniformity of bite increase in intensity with greater amounts of water at cooking, whereas hardness, stickiness to lips, springiness, and chewiness decrease in intensity. However, the flavor attributes across all cultivars are not significantly affected by the water-to-rice ratio. The advantage of the instrumental analysis of the texture of cooked rice is that it provides a relatively objective and easy measurement tool with the characteristics of being cost- and time-consuming and facilitates comparisons of results derived from different labs, regions, and countries. The disadvantage is that it is only a simulation test, which cannot reflect the real sensory attributes of humans so, in many cases, it cannot predict the real eating quality measured by descriptive sensory evaluation.
Physicochemical Property Evaluation Although descriptive sensory analysis methodology is commonly used for evaluating the intensity of the sensory attributes of cooked rice, it is typically time-consuming and only assesses whether the rice cultivar has quality characteristics deemed desirable by a target population or for a specific application, falling short in measuring subtle differences in sensory quality. Amylose content, gel consistency, gelatinization temperature (GT), and pasting viscosity are regarded as traditional parameters to describe the eating quality of rice. All these parameters reflect the properties of the starch that makes
–5 Figure 1 Typical texture profile analysis (TPA) curve of 1 g rice samples. Attributes on curves: H1, hardness (kg), measurement of force at peak of first curve; A1, area under first curve; A2, area under second curve; A4, measured from first data point to first probe reversal point; A5, measured from first probe reversal point to point where force returns to zero; A2/A1, cohesiveness, ratio of area under curves A2/A1; A3, adhesiveness, area of negative force curve, representing work to separate plunger from sample on upstroke after first curve; D2/D1, springiness, ratio of D2 to D1, where D1 is the total distance traveled by plunger on downstroke and D2 is the distance traveled on downstroke by plunger from point of sample contact to end of downstroke.
up 90% of milled rice. For objective traits, the advantage of using these parameters is that they offer possibilities to compare the physicochemical properties among different rices cultivated in different countries/regions, under different environments. The amylose content is measured with a process using I2–KI solution. However, this method always gives an overestimation, due in part to the value being contributed by amylopectin. Thus, the amylose content of starch as determined by reaction with iodine is termed apparent amylose content (AAC). The AAC of milled rice may be classified as waxy (1–2%), very low (5–12%), low (12–20%), intermediate (20–25%), and high (>25%). The AAC is considered the most important determinant of cooked rice texture, but this constituent falls short as a predictor, because cultivars with similar AAC may still differ in textural properties. The actual value of amylose can be obtained by using the size-exclusion chromatography (SEC) method, which separates polymers based on size. Since the size of amylose and amylopectin molecules differs substantially, they may be separated by SEC. This method does not need a calibration curve, but it is low-throughput and so, it is not appropriate for the routine screening of amylose in breeding programs. The gelatinization temperature (GT) is the critical temperature at which starch granules irreversibly lose their birefringence and crystalline order during heating. The GT can be directly measured under polarized light microscopy with a
FOOD-QUALITY TESTING | Rice: Eating Quality
Score
Grain not affected Grain swollen Grain swollen, collar incomplete, or narrow Grain swollen, collar complete, and wide Grain split or segmented, collar complete, and wide Grain dispersed, merging with collar Grain completely dispersed and disintegrated
1 2 3 4 5 6 7
90 CS
PV BD
75
160 HPV 60 80
0
heating stage to record the temperature when the birefringence is lost. The GT can also be measured by differential scanning calorimetry or indirectly by the alkali spreading value (ASV) test for rice grains. The ASV is the degree of spreading and disintegration values of six milled rice grains after soaking in 10 ml 1.7% KOH for 23 h at 30 C (Table 4). The ASV of 5.5–7.0 is classified as low GT (55–69.5 C); 3.5–5.4, intermediate (70–74 C); and 1–3.5, intermediate-high or high (74–80 C). The ASV test is usually used by rice breeders due to its simplicity and high-throughput characteristics. Gel consistency is used as a parameter to index the tendency of cooked rice to harden on cooling. This method was originally developed to differentiate cooked rice texture among high-AAC rice. It measures the distance traveled by a gel after cooking. According to the gel length, gel consistency can be classified as soft (61–100 mm), medium (41–60 mm), and hard (25–40 mm). Pasting viscosity is another useful parameter to differentiate rice with similar AAC and is popularly measured by a Rapid Visco Analyzer (RVA). The RVA records the viscosity continuously as the temperature is increased, held constant for a time, and then decreased (Figure 2). Briefly, according to the method of The American Association of Cereal Chemists (AACC) 61-02, rice flour slurry is made in the RVA canister consisting of 3.0 g (12% mb) of rice flour and 25 g of distilled water. The heating profile is set as follows: (1) the temperature is held at 50 C for 1 min, (2) the temperature is linearly ramped up to 95 C until 4.8 min, (3) the temperature is held at 95 C until 7.5 min, (4) the temperature is linearly ramped down to 50 C at 11 min, and (5) the temperature is held at 50 C until 12.5 min. A typical RVA profile is shown in Figure 2. It should be noted that the degree of milling affects not only the texture and flavor of cooked rice but also the physicochemical properties. All these measurements for eating quality assume that the optimum degree of rice milling has been applied to obtain the milled rice. Similar to instrumental texture analysis, the advantage of physicochemical property analyses is that they provide a relatively objective measurement tool for the indirect prediction of eating quality. All the analyses are easy and fast and can routinely be conducted in a quality lab. These analyses also facilitate comparisons of results derived from different labs, regions, and countries. However, the disadvantage is that they fall short in predicting the real eating quality measured by descriptive sensory evaluation.
CPV SB
240 Viscosity (RVU)
Description
320 Temperature Profile
Temperature (°C)
Table 4 Description of the ASV for milled rice after soaking in 1.7% KOH for 23 h
169
3
6 9 Time (min)
12
45 15
Figure 2 The RVA pasting viscosity profile of rice starch. The pasting viscosity parameters: PV, peak viscosity; HPV, hot paste viscosity; CPV, cold paste viscosity; PT, pasting temperature. In addition, the secondary parameters of breakdown, setback, and consistency viscosities can be calculated as PV-HPV, CPV-PV, and CPV-HPV, respectively.
In terms of eating quality evaluations, it is suggested to keep some rice with different eating qualities as standard rice and used as controls. These standard rice can be not only used for panel training for sensory evaluation but also used in instrumental texture analysis and physicochemical property evaluations, which may facilitate comparisons of eating quality across different labs in different regions.
Relationship Between Texture Attributes and Physicochemical Properties The AAC has long been considered the most important determinant of cooked rice texture. Cooked rice with a low-AAC cultivar is soft, moist, and sticky in texture, and those with high AAC are dry, firm, and fluffy. The sensory properties of freshly cooked rice measured by panelists using descriptive sensory analysis vary widely, but 11 of 14 parameters correlate well, either positively or negatively, with the AAC, with correlation coefficient values in the range of 0.76–0.92. Particularly, the AAC is positively correlated with hardness (r ¼ 0.90) and negatively correlated with stickiness to lips (r ¼ 0.91), stickiness between kernels (r ¼ 0.91), and cohesiveness of mass (r ¼ 0.92). Grain flavor ranges in intensity from 2.2 to 4.9 and correlates highly and negatively with the AAC (r ¼ 0.88). Some RVA parameters have significant correlations with AAC; it was found that none of the cooked rice textural attributes, whether measured by descriptive analysis or TPA, are linearly modeled with high accuracy by the RVA. The sensory texture attributes cohesiveness of mass, stickiness, and initial starchy coating and TPA attribute adhesiveness have the strongest correlations with RVA measurements. Setback explains most of the variance attributed to models describing these attributes; the strongest correlation is with cohesiveness of mass (r ¼ 0.69). Inclusion of amylose and protein contents in regression analyses do not strengthen models, and exclusion of samples that cook atypically, based on amylose content or GT types, slightly improve the accuracy of RVA measurements for predicting the cooked rice texture.
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Criteria for Eating Quality Consumer preferences for rice vary in different areas of the world. It is difficult to find a rice cultivar that could meet the demands of consumers around the world. However, global demand for high-quality rice is ever increasing both in local production regions and in regions in which rice is imported. For the promotion of rice trade, it is necessary to have a standard of grain quality for the commodity. To facilitate appraising the eating quality of new rice cultivars in breeding programs, it is also important to set up standards for eating quality evaluation. US Standards for Milled Rice (2009) provide a system of marketing rice by grade that takes into consideration those quality factors associated with cleanliness, soundness, and purity. However, the eating quality of milled rice has not been included in the standards. In Thailand, Thai aromatic rice has long been well known in international markets and is always exported using the name Thai Hom Mali rice, jasmine rice, or Thai aromatic rice. For the benefit of Thai exporters, the Ministry of Commerce, with cooperation of the Ministry of Agriculture, cooperatives, and private sector, established a notification on the subject: Thai Hom Mali Rice Standards B.E. 2544. These standards comprise both physical and chemical analyses and have two points regarding eating quality: (1) Thai Hom Mali rice such as Khao Dawk Mali 105 and RD15 has a natural fragrant aroma depending on whether it is new or aged rice. Its cooked rice kernels have a tender texture, and (2) it has an amylose content of 13–18%. In China, the Ministry of Agriculture released a standard for cooking rice variety quality in 1986 (NY20-1986) for facilitating the breeding, registration, and extension of new rice cultivars with good grain quality. With changes in grain production, processing, and cropping system rearrangement, the standard was revised in 2002 (NY/T593-2002) with concomitant release of three related standards for indica-type cooking rice (NY/T594-2002), japonica-type cooking rice (NY/T595-2002), and aromatic rice (NY/T596-2002). A new version of these standards was renewed in 2013. In addition to the milling quality and appearance quality required for different grades of high-quality rice, this standard (NY/T593-2013) provides the grade for eating quality (Table 5). Two methods are used for the evaluation of eating quality; one is the sensory test, which is regarded as an arbitration method, and the other is physicochemical property evaluation with three common parameters, ASV, GC, and AAC. Sensory evaluations refer to another national standard – Method for sensory evaluation of paddy or rice cooking and eating quality (GB/T15682-2008). This test includes five items; aroma flavor, surface structure, palatability, taste, and texture of cold cooked rice, which accounts for a score of 20, 20, 30, 25, and 5, respectively, in a total score of 100. If rice meets the requirements of any grades in the standard, it is called high-quality rice including grade 3. Otherwise, it is not a high-quality rice. The eating quality parameters used in indica-type cooking rice (NY/T594-2002) and japonica-type cooking rice (NY/T595-2002) are the same as those in NY/T593-2013 (Table 5). For facilitating rice cooking for sensory evaluation, the water-to-rice ratio is stipulated based on the AAC, where the water-to-rice ratios are 1.2, 1.3,
Table 5 Quality requirements for different grades and different types of rice stipulated in the Chinese standard for cooking rice variety quality (NY/T593-2013)
Rice type
Grade
Sensory evaluation
ASV
GC (mm)
AAC (%, db)
Indica nonwaxy rice Indica waxy rice
1 2 3 1 2 3 1 2 3 1 2 3
90 80 70 90 80 70 90 80 70 90 80 70
6.0 6.0 5.0 6.0 6.0 5.0 7.0 7.0 6.0 7.0 7.0 6.0
60 60 50 100 100 90 70 70 60 100 100 90
13.0–18.0 13.0–20.0 13.0–22.0 2.0 2.0 2.0 13.0–18.0 13.0–19.0 13.0–20.0 2.0 2.0 2.0
Japonica nonwaxy rice Japonica waxy rice
1.4, and 1.5 for rice with AAC in the range of 15.0, 15.1–20.0, 20.1–25, and >25.0%, respectively. To facilitate paddy rice circulation, marketing, and market management and promote the rearrangement of the cropping system, the General Administration of Quality Supervision, Inspection and Quarantine of China released a national standard for high-quality paddy in 1999 (GB/T17891-1999), which is still in effect now. This standard stipulates the definition, classification, quality requirement, test methods, and requirement for the packaging, transportation, and storage of high-quality paddy commodities. It also provides parameters for eating quality by grade (Table 6). These standards provide the eating quality requirement of rice commodities for breeders, processors, and merchants and also allow both exporters and importers to have the chance to meet the standards of rice quality in the target market.
Chemical Basis of Eating Quality The chemical basis of the texture of cooked rice attracted researchers’ attention in the period between World War II and the mid-1980s, during which the research repeatedly suggested that amylose content is the single largest determinant of its texture. The greater the amylose, the firmer the cooked rice texture is and vice versa. However, it has been realized that amylose is not sufficient to explain why the texture of cooked rice still differs among rice with similar amylose content, especially within high-amylose rice. It was also found that part of the amylose in rice starch is soluble in hot water and that the proportion of the soluble amylose as a fraction of the total amylose content differs widely among different varieties. The insoluble amylose content of rice (total minus soluble amylose) shows a striking correlation with rice texture, particularly in the high-amylose group, which could be divided into three distinct subgroups showing a distinct texture after cooking. Perhaps, it is related to retrogradation, because it is clearly the insoluble amylose that retrogrades and thus provides greater rigidity to the starch granules and hence to the cooked rice grain. The chemical nature of the insoluble amylose was intensively investigated by characterizing the structure of amylopectin. It
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Table 6 Quality requirements for different grades and different types of rice stipulated in China national standard for high-quality paddy (GB/ T17891-1999) Rice
Grade
AAC (%, db)
Taste
GC (mm)
Color and smell
Indica paddy
1 2 3 1 2 3
17.0–22.0 16.0–23.0 15.0–24.0 15.0–18.0 15.0–19.0 15.0–20.0 2.0
9 8 7 9 8 7 7
70 60 50 80 70 60 100
Normal Normal Normal Normal Normal Normal Normal
Japonica paddy
Waxy paddy (indica and japonica)
was found that amylopectin also binds iodine, which is apparently caused by the presence of some very long chains. The chain length distribution of rice amylopectin also differs among different rice accessions, with high-amylose rice having more very long B chains than low-amylose rice and intermediateamylose rice having in between. The amylose content of starch as determined by reaction with iodine is always overestimated with part of the value being contributed by amylopectin. It is found not only that amylopectin in high-insoluble amylose rice has more long B chains but also that these chains seemed to be largely located externally in the molecule and vice versa. Thus, the texture of cooked rice is closely correlated with the relative abundance of long B chains in its amylopectin. The more long B chains, the firmer the cooked rice is and vice versa. Long B chains in amylopectin strengthen the starch granules through intermolecular interaction, leading to a firm texture of cooked rice. The chemical basis of the flavor and aroma of cooked rice has received attention. More than 200 volatile compounds have been identified in cooked rice, but only a few of them may relate to the aroma and flavor of cooked rice. It is difficult to determine which volatile compounds are responsible for the perceived aroma/flavor of rice. Only one compound, 2-acetyl1-pyrroline (2AP; popcorn aroma), has been confirmed to contribute a characteristic aroma. Furthermore, 2AP is the only volatile compound in which the relationship between its concentration in rice and its sensory intensity has been established. The fragrance gene (fgr) regulating the biosynthesis of 2AP has been cloned. It encodes the betaine aldehyde dehydrogenase 2 (BADH2). A deletion of 8 bp in this gene truncates and makes nonfunctional the BADH2 protein, leading to accumulation of 2AP in the rice grain. Other factors may also contribute to the difference in eating quality among different rice cultivars. Protein is the second most abundant component of rice endosperm and constitutes 6–10% of the dry matter in the milled rice. The protein content of milled rice has a negative correlation with the appearance, flavor, and stickiness of cooked rice. High-protein rice has much firmer texture than low-protein rice. The eating quality of cooked rice decreases as the ratio of protein body I to the content of total rice protein increases. Further, the cooked rice becomes harder, and the aroma and taste deteriorate with the addition of prolamin. As rice grain storage protein prolamin is located in protein body I, it is expected that the eating quality of cooked rice decreases as the ratio of prolamin to the content of total rice protein increases. The granule-bound starch synthase I (GBSSI) protein is positively correlated with
amylose content (r ¼ 0.95) but negatively correlated with cooked rice stickiness (r ¼ 0.85). The effect of protein content on the texture of cooked rice may also contribute to its glass transition (Tg) being slightly lower than that of starch, so when cooked with limited water, for example, in a rice cooker, the higher the protein content, the more water bound by proteins would be, leaving less water for the swelling of starch granules and leaching of amylose. The addition of proteins or its components in a viscosity test demonstrates their ability to modify the viscosity curves. The modification may be through binding water, which causes the concentration of the dispersed and viscous phases of gelatinized starch to increase, and through the agency of a network linked by disulfide bonds. The lipid content and fatty acids of milled rice may also affect eating quality. Rice lipids are generally classified as starch and nonstarch lipids. The lipid content of milled rice ranged from 0.2% to 2%. Rice varieties with high lipid content and linoleic acid content exhibits better appearance and flavor of the cooked rice and high palatability. The fat content of milled rice correlates positively with the sensory characteristics hardness, color, intactness of grains, puffed corn flavor, raw rice flavor, wet cardboard flavor, and hay-like flavor intensity and negatively with the sweet taste, degree of agglomeration, adhesiveness, cohesiveness, and cohesiveness of mass. Rice with high activities of lipase and lipoxygenase exhibits a low palatability. During storage, the appearance, aroma, taste, cohesiveness, and hardness of cooked rice deteriorate, and this deterioration is accompanied by changes in the chemical components of rice, particularly lipid content and composition. Decreased nonstarch lipid and linoleic acid and increased oleic acid and palmitic acid contents may be responsible for the deteriorations of eating quality after storage.
Genetic Basis of Eating Quality The eating quality of rice is under genetic control. The chemical constituents, that is, starch (amylose and amylopectin) and its structural features, protein, and lipid that influence the eating quality, are characteristics of the rice. These chemical constituents are synthesized through some metabolic pathways under the control of a series of genes. One of the most important pathways is starch synthesis since starch constitutes 90% of the milled rice, so it plays the most important role in the formation of eating quality.
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Amylose is synthesized mainly by GBSSI, and the amylopectin synthesis process is governed by a combination of multiple isoforms of starch synthase, branching enzyme, and debranching enzyme to produce a uniform number of chains per amylopectin cluster. The waxy (Wx) gene on chromosome 6 encoding GBSSI is mainly responsible for the natural variation in amylose content, gel consistency, and RVA pasting viscosity. Five functional markers in the Wx gene, a (CT)n microsatellite (or simple sequence repeat), a 23 bp insertion/ deletion sequence, and three single-nucleotide polymorphism (SNP) markers are well characterized, with different alleles differing in AAC (Figure 3(a)). Among them, the (CT)n microsatellite in the Wx gene located 55 bp upstream of the putative leader intron 50 splice site has many alleles with n ranging from 8 to 22 in diverse rice germplasm (Figure 3(b)). The G/T SNP at the putative leader intron 50 splice site, and a G to T mutation at this site, reduces the efficiency of Wx pre-mRNA processing and thus results in the lower level of spliced mature mRNA, Wx protein, and AAC (Figure 3(c)). Waxy, low-amylose, and some intermediate-amylose rice have the T SNP allele, while some intermediate- and high-amylose rice have the G allele. Over 75% of the total observed variation in the AAC can be explained by the (CT)n microsatellite and the G/T SNP in the nonwaxy rice accessions. The Wx gene is also responsible for biosynthesis of extralong unit chains of amylopectin in rice. Among the high-AAC rice, the exon 10 SNP of Wx gene is responsible for the difference in the gel consistency, the proportion of soluble to insoluble apparent amylose, gel hardness, and RVA pasting viscosity. The rice with SNP allele C at exon 10 produces soft, viscous gels and has a soft texture when cooked, but with high retrogradation, and the rice with SNP allele T gives a short, firm
(CT)n
G/T
gel and has a firm texture when freshly cooked with little change in texture over storage. The SSIIa is a major gene for GT, thermal properties, and amylopectin structure, which elongates the short A and B1 chains with a degree of polymerization (DP) < 10 to form long B1 chains of amylopectin. The GT was negatively correlated with the amount of DP6-11 and positively correlated with the amount of DP12-24 of amylopectin and is positively correlated with the degree of crystallinity of starch. In waxy rice, in which the Wx gene is dysfunctional, SSIIa is the major gene responsible for the variations in pasting, gelatinization, and retrogradation properties. Four functional SNPs in the SSIIa gene have been revealed by several research groups, in which the third site where valine encoded by GTG is replaced by methionine encoded by ATG at 4198 bp position and the fourth site where glycine–leucine encoded by GGGCTC is replaced by glycine–phenylalanine encoded by GGTTTC at 4229/4330 bp are crucial for SSIIa activity (Figure 4(a)). The SSIIa enzyme is inactive when it is A SNP (coding for methionine) no matter which SNP at 4229/ 4330 bp (GC/TT) is present. The GC/TT is most common and is strongly associated with the GT (Figure 4(b)). This GC/TT polymorphism alone can differentiate rice with high or intermediate GT (possessing the GC allele) from those with low GT (possessing the TT allele), explaining 62.4% of the total variation in pasting temperature (Figure 4(c)). Few rice accessions with the GC allele have a low-GT phenotype, which can be explained by their carrying the A SNP allele in the third SNP. However, it should be mentioned that the A allele of the third SNP (G/A) is quite rare in natural populations. In addition to the genetic effects, the eating quality of rice is also affected by the environment. The texture attributes
Exon 2
Exon 6
Exon 10
InDel
A/C
C/T
(a)
Wx
100
AAC (%)
150
35 30 25 20 15 10 5 0 8
10
11
12
17
18
19
20
21
22
(CT)n
T G
AAC (%)
(b) 30 25 20 15 10 5 0
T
G G/T allele
(c)
Figure 3 Relationship between the Wx alleles and apparent amylose content (AAC). (a) Wx gene functional markers, a (CT)n microsatellites, the G/T SNP in intron 1, the 23 bp duplication in exon 2, a A/C SNP in exon 6, and a C/T SNP in exon 10; (b) genotyping the (CT)n microsatellites (left) and comparison of AAC of each microsatellite class; (c) genotyping the G/T SNP and comparison of the AAC of each allele class.
FOOD-QUALITY TESTING | Rice: Eating Quality
G/C
A/G
173
A/G GC/TT
(a)
F7
F22 GC
82.0
TT R21
80.0
R1
78.0
F7F22/R1 5 6 7 8
F7/R1R21 9 10 11 12 F7F22/R1R2 13 14 15 16
76.0
PT (°C)
F7/R1 1 2 3 4
74.0
TT
GC
72.0 70.0
(b)
TT
68.0
GC
66.0 –50
A
50
150
250
350
450
550
(c)
Figure 4 Relationship between the SSIIa alleles and gelatinization temperature. (a) SSIIa functional SNPs. (b) Strategy for genotyping the fourth GC/TT SNP (up) and gel image showing different PCR products amplified by different combinations of the primers. (c) The relationship between pasting temperature and GC/TT alleles; the blue circle shows some rice with A SNP at the third site.
roughness and hardness and the flavor attributes hay-like, sweet aromatic, sour, and astringent are significantly different between two cropping years, which may be attributed to the variation in the AAC and protein content as affected by the environment. The AAC is increased when rice is grown at a cooler temperature. Wx gene expression is activated reversibly in response to a cool temperature (18 C), and it is the Wx promoter that is responsive to a cool temperature. The G/T SNP at the alternate splicing site is also temperature-dependent. At 25 C and particularly at 32 C, the amount of mature GBSS mRNA in developing seeds is dramatically reduced. This is one of the reasons why low-amylose varieties are particularly temperature-sensitive. The steady-state level of GBSS mRNA is essentially identical over this temperature range for the rice with G SNP allele. Environmental factors also affect the GT, resulting in a difference of up to 10 C between some rice starches. Environmental factors, especially temperature during grain filling, can modify the expression of the SSIIa gene and hence modify the amylopectin chain length. The amount of short chains in the amylopectin increases significantly, whereas the amount of long chains decreases in rice grown at lower temperatures, resulting in a lower GT.
Improvement of Eating Quality The improvement of eating quality is an essential target of current breeding programs designed to meet industry standards or eating characteristics preferred by consumers in domestic or international markets. Three questions need to be answered before breeding for high eating quality is conducted. The first question is what kind of rice is preferred by
people in the region or country? If a new rice is for exporting, the same question is what kind of quality rice is preferred by people of the country or needed in the targeted market. The second question is what kind of traits should be improved. The third question is what kind of tools are available for breeding, what molecular markers could be used at the genomic level at this time? The first question has been answered by the International Network for Quality Rice, which has conducted a survey of consumer preferences for rice quality in different riceconsuming regions. The maps showing regional variation in amylose and GT, consumer’s preferences for texture based on gel consistency, and consumer’s references for aromatic rice are useful for breeders worldwide, particularly for those whose rice is targeted to the international market. The current eating quality status of your rice materials should be surveyed to understand whether the quality matches the consumer preference and to determine which trait needs to be improved. If allele information of the microsatellites of the Wx gene, SNP of SSIIa, and insertion/deletion marker (8 bp) of fgr for eating quality traits are available in your rice materials, it is easy to transfer the allele of these genes from one variety to another by crossbreeding and backcross breeding with the help of molecular marker detection. In F2 lines, the phenotype is matched with its genotype very well when these functional markers are used to improve the eating quality of a highAAC, high-GT, and nonfragrant maintainer rice II32B from another low-AAC, low-GT, and fragrant rice (Figure 5). The breeding practice with marker-aided selection is also called molecular breeding. In addition to use in molecular breeding, these three gene markers can also be used in the diagnosis of quality traits for
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FOOD-QUALITY TESTING | Rice: Eating Quality
M P1 P2 1
2 3
4 5 6
7
8 9 10 11 12 13 14 15 16 17 18 19 20
200bp
100bp (a)
500bp
GC
300bp
TT
(b)
300bp
200bp (c)
Figure 5 Segregation of the marker genotypes at three loci in F2 lines: P1: Yixiang B; P2: II-32B; 1–20 F2 individuals; (a) genotypes of Wx marker; (b) genotypes of SSIIa marker; (c) genotypes of fgr marker. GC represents GC SNPs, and TT represents TT SNPs of SSIIa gene.
rice collecting from markets. Since sensory and physicochemical property evaluations may be influenced by processing, milling, and storage, genotyping with molecular markers can avoid the side effects of these factors and can reduce analysis time, cost, and required sample size.
Exercises for Revision
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• • • •
What kinds of rice are preferred and what are their physicochemical properties in your region? Does your country have standards for high-quality rice and what are the parameters to evaluate the eating quality? Do you think eating quality affects consumer acceptability? What other factors may affect consumer preference? How do you measure the eating quality of rice and their advantages and disadvantages? What are the chemical and genetic bases of the eating quality of rice? How could you use molecular markers to diagnose the eating quality of rice in the supermarket?
Exercises for Readers to Explore the Topic Further
• • •
How could you exploit new techniques such as electronic nose and electronic tongue to predict the eating quality of rice? What is known, and what still needs to be determined, about the genetic bases for other volatile compounds? What is the genetic basis of sensory parameters in rice? How will new molecular tools for eating quality contribute to our ability to manipulate the sensory quality of rice?
See also: Barley, Rice and Maize Processing: Rice Processing: Beyond the Farm Gate; Breeding of Grains: Rice: Breeding; Genetics of Grains: Rice: Genetics; The Cereal Grains: Rice: Overview; Wildrice, Zizania: Overview.
Further Reading Ayres NM, McClung AM, Larkin PD, et al. (1997) Microsatellites and a single-nucleotide polymorphism differentiate apparent amylose classes in an expanded pedigree of US rice germ plasm. Theoretical and Applied Genetics 94: 773–781. Bao JS (2012) Towards understanding of the genetic and molecular basis of eating and cooking quality of rice. Cereal Foods World 57(4): 148–156. Bao JS, Corke H, and Sun M (2006) Nucleotide diversity in starch synthase IIa and validation of single nucleotide polymorphisms in relation to starch gelatinization temperature and other physicochemical properties in rice (Oryza sativa L.). Theoretical and Applied Genetics 113: 1171–1183. Bao JS, Xiao P, Hiratsuka M, et al. (2009) Granule-bound SSIIa protein content and its relationship with amylopectin structure and gelatinization temperature of rice starch. Starch-Starke 61(8): 431–437. Bao JS, Zheng XW, Xia YW, et al. (2000) QTL mapping for the paste viscosity characteristics in rice (Oryza sativa L.). Theoretical and Applied Genetics 100: 280–284. Bhattacharya KR (2005) The chemical basis of rice end-use quality. In: Toriyama K, Heong KL, and Hardy B (eds.) Rice is Life: Scientific Perspectives for the 21st Century, pp. 246–248. Philippines: IRRI. Bhattacharya KR (2009) Physicochemical basis of eating quality of rice. Cereal Foods World 54(1): 18–28. Calingacion M, Laborte A, Nelson A, et al. (2014) Diversity of global rice markets and the science required for consumer-targeted rice breeding. PLoS One 9(1): e85106. Champagne ET (2008) Rice aroma and flavor: A literature review. Cereal Chemistry 85(4): 445–454. Champagne ET, Bett-Garber KL, Fitzgerald MA, et al. (2010) Important sensory properties differentiating premium rice varieties. Rice 3: 270–281. Champagne ET, Bett-Garber KL, McClung AM, et al. (2004) Sensory characteristics of diverse rice cultivars as influenced by genetic and environmental factors. Cereal Chemistry 81: 237–243.
FOOD-QUALITY TESTING | Rice: Eating Quality
Champagne ET, Bett-Garber KL, Vinyard BT, et al. (1999) Correlation between cooked rice texture and rapid visco analyses measurements. Cereal Chemistry 76: 764–771. Cheaupun K, Wongpiyachon S, and Kongseree N (2005) Improving rice grain quality in Thailand. In: Toriyama K, Heong KL, and Hardy B (eds.) Rice is Life: Scientific Perspectives for the 21st Century, pp. 248–250. Philippines: IRRI. Fitzgerald MA, Bergman CJ, Resurreccion AP, et al. (2009) Addressing the dilemmas of measuring amylose in rice. Cereal Chemistry 86(5): 492–498. Jin L, Lu Y, Shao YF, et al. (2010) Molecular marker assisted selection for improvement of the eating, cooking and sensory quality of rice (Oryza sativa L.). Journal of Cereal Science 51: 159–164. Juliano BO (1998) Varietal impact on rice quality. Cereal Foods World 43: 207–218.
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Tran NA, Daygon VD, Resurreccion AP, et al. (2011) A single nucleotide polymorphism in the Waxy gene explains a significant component of gel consistency. Theoretical and Applied Genetics 123: 519–525. Umemoto T and Aoki N (2005) Single-nucleotide polymorphisms in rice starch synthase IIa that alter starch gelatinization and starch association of the enzyme. Functional Plant Biology 32: 763–768. Umemoto T, Yano M, Satoh H, et al. (2002) Mapping of a gene responsible for the difference in amylopectin structure between japonica-type and indica-type rice varieties. Theoretical and Applied Genetics 104: 1–8. Yoon MR, Rico CW, Koh HJ, et al. (2012) A study of the lipid components of rice in relation to palatability and storage. Journal of the Korean Society for Applied Biological Chemistry 55: 515–521.