Grain quality

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Consumers' criteria

When more rice becomes available in the market, consumers' demand for superior quality rice is increased. Although sensory evaluations by laboratory panels and consumer panels give some indication on important criteria for rice quality, they do not reflect the properties for which consumers will actually pay a price premium in the retail market. By clearly identifying the quality characteristics valued by consumers, plant breeders can target attributes that are economically significant in breeding improvement research. The results could provide social scientists with an agenda for public policy research in rice marketing, technology assessment and research prioritization.

Rice grain quality denotes different properties to various groups in the postharvest system (Juliano and Duff, 1989). Although variety is the principal factor contributing to grain quality, good post-harvest handling can maintain or even improve it (Table 19). Moisture content is the most important quality criterion for rough rice. To the farmer, grain quality refers to quality of seed for planting material and dry grain for consumption, with minimum moisture, microbial deterioration and spoilage. The miller or trader looks for low moisture, variety integrity and high total and head milled rice yield. Market quality is mainly determined by physical properties and variety name, whereas cooking and eating quality is determined by physico-chemical properties, particularly apparent amylose content. In countries with marked variability in temperatures during the ripening periods, significant differences in grain quality have been reported within a variety. In tropical Asia, grain physico-chemical properties are relatively constant. Nutritional value is mainly determined by the milled rice protein content.

The major findings of research on the economics of grain quality from 1987 to 1989 by IRRI and national rice research programmes in Indonesia, Bangladesh, Malaysia, the Philippines and Thailand are that rice grain quality and quality preferences vary across countries and regions but some quality preferences are widely shared (IRRI and IDRC, 1992). Consumers in all the countries studied prefer higher head rice yield and more translucent grain. High-income consumers pay higher premiums for a larger number of quality characteristics than low-income consumers, reflecting their ability to pay. Preferences do not vary much across income levels, with one exception: lowerincome consumers prefer rice that is more filling. Laboratory analysis showed that Philippine rice labelled with a traditional variety name is usually a modern variety with shape or cooking characteristics similar to those of traditional varieties (Juliano et al., 1989b). Thus, the "traditional" label signals consumers that these rices have some desirable characteristics.

TABLE 19 - Effects of environment, processing and variety on rice grain properties influencing quality at different steps of the post-harvest system

^a+, quality affected; O. no effect.

Source: Juliano & Duff, 1989.

Quality incentives appear to be transmitted from wholesale rice prices through to rough rice prices in Indonesia and the Philippines (IRRI and IDRC, 1992). However, this transmission is not perfect. The Philippine studies show that barriers to entry in milling influence pricing efficiency. The studies reveal the complexity of the transmission of information about quality from consumers to producers.

Given the importance of quality characteristics for creating and stimulating demand, especially among the higher-income urban sector, transmission of price and market signals and a greater degree of integration of the farm wholesale and retail market will be necessary to improve the farmgate price and to provide incentive to farmers to produce better-quality rice. Moreover, improvements in grain quality that do not lower yields will generally benefit all rice consumers by lowering the cost of better-quality rice (Unnevehr et al., 1985). If higher-quality varieties are widely adopted, producers will benefit by retaining better-quality rice for home consumption and by having a wider domestic market for their products. In addition, countries exporting rice would benefit from quality improvements that would expand their potential export market.

Grain quality indicators

Physical properties such as length, width, translucency, degree of milling, colour and age of milled rice are grain quality indicators. The amylose content of the rice starch is the major eating quality factor. It correlates directly with volume expansion and water absorption during cooking and with hardness, whiteness and dullness of cooked rice (Juliano, 1985b). Genetic studies showed that the nonwaxy trait is dominant over the waxy trait (Kumar, Khush and Juliano, 1987). Among non-waxy parents, high amylose is completely dominant over low or intermediate amylose, and intermediate is dominant over low (Kumar and Khush, 1987).

Final gelatinization temperature (GT) of starch granules refers to the water temperature at which at least 90 percent of the starch granules have gelatinized or lost birefringence (Maltese cross) or swollen irreversibly in hot water. GT is classified for rice starch granules as low (55 to 69.5°C), intermediate (70 to 74°C) and high (74.5 to 80°C). GT is indexed in the breeding programme by the alkali spreading value based on the degree of dispersion of six grains of milled rice in 10 ml of 1.7 percent potassium hydroxide after 23 hours soaking at 30°C (Little, Hilder and Dawson, 1958).

A high GT value is uncommon, particularly in high amylose rices. A low ambient temperature during ripening may increase amylose content and independently reduce GT (Nikuni et al., 1969; Resurrección et al., 1977; Dien et al., 1987). The GT affects the degree of cooking of rice because of the cooking gradient from the surface to the core of the grain. Because GT correlates directly with cooking time, a low GT favours fuel conservation, provided eating quality is not adversely affected. GT also affects the molecular properties of amylopectin.

The gel consistency test was developed to index cooked rice hardness among high-amylose rices (Cagampang, Perez and Juliano, 1973). Rices are classified based on gel length as soft (61 to 100 mm), medium (41 to 60 mm) and hard (27 to 40 mm), (Table 18). Soft to medium gel consistency is preferred to hard gel consistency in both non-waxy and waxy rices. High protein content contributes to harder gel consistency. Amylopectin contributes more than amylose to starch gel consistency and viscosity.

Among rices of the same apparent amylose type, alkali spreading value and gel consistency may be used as quality indices. Among high-amylose rices, intermediate GT and soft gel consistency are preferred by consumers over low GT and hard gel consistency (Juliano, 1985b). Among intermediate-amylose rices derived from C4-63G, those with an intermediate GT value are preferred to those with a low GT value, as the cooked rice is softer. Gel consistency values are similar among these intermediate-amylose rices. Among low-amylose and waxy rices, a low-GT type is preferred to a type with a high GT value. In terms of rice improvement breeding, hard gel consistency is dominant over medium and soft gel, and medium gel consistency is dominant over soft (Tang, Khush and Juliano, 1989).

TABLE 20 - Relative importance of rice quality indicators in rice breeding programmes

^a Amylose content, alkali spreading value (gelatinization temperature), gel consistency.

^B Deterrnined by sensory evaluation or instrument-Instron, Texturometer, Tensipresser, Viscoelastograph, etc.

Source: Juliano & Duff, 1991.

As many countries achieve rice self-sufficiency, grain quality becomes an important breeding objective (Juliano and Duff, 1991). In traditional breeding programmes, both parents are of known quality so that the quality of the breeding lines is predictable by indicators based on physical properties, namely apparent amylose content, alkali spreading value and gel consistency (Table 20). With modern or semi-dwarf varieties, derived from parents of contrasting grain qualities, evaluation of starch properties complements physical methods in indexing quality of breeding lines. Breeding for grain quality involves discrimination among lines with similar starch properties, as in the United States, Japan and the Republic of Korea and at IRRI, where cooked rice texture is the key indicator.

Heritability of protein content is very low. A six-percentage-point range is observed for each variety (Coffman and Juliano, 1987). Environmental factors contribute significantly to protein content. High-protein rice translocates straw N to the developing grain more efficiently, which results in a higher N harvest index (panicle N/panicle N + straw N), (Perez et al., 1973).

Quality characteristics of world rices - country samples

In Asian countries high-amylose rices predominate (Table 21). This is the principal rice type in Bangladesh, Sri Lanka, Thailand and Viet Nam.

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