Global rice production has been increased due to introduction of high yielding varieties and improved agricultural practices. According to the FAO, global milled rice production was estimated to increase from 491.1 million tons in 2012 to 493.9 million tons in 2013 which was by around 0.6% (https://oryza.com/news/rice-news/fao-estimates-2014-global-rice-trade-383-million-tons-3-last-year).Appearance, eating, cooking, milling and nutritional qualities are the primary components of rice grain quality. The values of each of these components are determined by the physiochemical properties and other socio-cultural factors such as the history and traditions of the localities where rice is grown. According to Unnevehr et al., (1992) and Juliano and Villareal (1993) the appearance quality of rice is determined by grain dimensions, specifically by grain length, width, width-length ratio, and the shape and translucency of the endosperm. Rice can be classified into different grain types based on grain dimensions including short, medium, and long types. McKenzie and Rutger 1983 has reported that medium and short grain cultivars tend to be low amylose rice and exhibit high gelatinization temperatures and moist, chewing cooking properties. In contrast, long grain rice generally contains high amylose and remains separated after cooking with a dry, fluffy consistency. Please see section 1.6 for details on amylose.
The physical properties of starch in the endosperm influence both eating and cooking qualities of rice. Starch is composed of a linear chain, amylose and a branched copolymer, amylopectin. Previous studies have reported the amylose content (AC) of starch as one of the most important determinants of eating and cooking qualities of rice (Bao et al., 2002; Juliano 1985; Webb 1980; Zhou et al., 2003). Indica rices which are popular in Malaysia usually contain between 20-25% amylose. Rices with higher amylose content are usually having lower GI (Fitzgerald et al 2011). GI is a concept developed by Jenkins et al in 1981 which is used to quantify the glycemic response to carbohydrates in different foods. It has been shown that AC is controlled by the waxy (Wx) gene and is mapped to chromosome 6 (Tan et al., 1999; Zhou et al., 2003). Other studies have identified that though a single major gene exerts major influence on AC of rice but a large number of minor genes also act as modifiers (Bollich and Webb 1973; IRRI 1976; McKenzie and Rutgers 1983; Okuno et al., 1983; Kumar and Khush 1988). Higher amylose contents of rice have been found to be associated with a slower rate of digestion and lower glucose and insulin responses. However, during processing, mainly cooking, the original cellular structures of starch get disrupted, leads to gelatinization and exert large impact on starch digestibility which suggested that amylose alone is not a good predictor to determine starch digestion rate (Panlaigui et al).
Starch occurs naturally as granules. The germplasm from which the starch is isolated has been reported to affect the granule size and shape of starch. It has also been reported that starch granule size affect the composition, gelatinization, pasting properties, enzyme susceptibility, crystallinity, swelling, and solubility. However there are several other factors which are also considered influential. These include amylose/amylopectin ratio and molecular weight granules and granule fine structure (Lindeboom and others 2004).Starch is a copolymer of linear amylose chain and branched biopolymer- amylopectin. These two functional components primarily influence the functional properties of rice starches.
Amylose is mostly composed of α-1, 4-linked D-glucsopyranosyl units although it also contains a few ((0.3% to 0.5%) α-1,6- linked D-glucopyranose branches. (Whistler and BeMiller 1997). The amylose content of starch varies according to the botanical source of the starch and is also affected by the climatic and soil conditions during grain development (Morrison and others 1984; Yano and others 1985). 15% to 25% are the typical levels of amylose in starches (Manner 1979). Based on amylose content, rice starch may be classified into different categories which include waxy (0% to 2% amylose), very low (5% to 12% amylose) and low (12% to 20% amylose), intermediate (20% to 25% amylose), or high (25% to 33% amylose) starch (Juliano and others 1981; Yu and others 2012).
Cultivar differences, growing zone and environment influence amylose content. It has been observed that rice starches isolated from the same cultivar might have different composition depending on the growing conditions (Peisong and others; 2004). The difference might be due to different growing zones. Amylose content of rice starch is considered to be the major factor controlling almost all physiological properties such as pasting, freeze-thaw stability, turbidity, syneresis (during which the aligned chains form double-stranded crystallites that are resistant to amylases) gelatinization, and retrogradation (Wickramasinghe and Noda 2008). A positive correlation of gelatinization with amylose content has also been reported by Varavinit and others (2003).
Amylopectin consists of α-1, 4-linked D-glucsopyranosyl chains with α-1,6-bonds. Amylopectin molecules are highly branched and constitute the skeleton of the starch granules (Kossmann and Llyod 2000). The basic structure of amylopectin was defined by Peat and others (1956) based on linear A, B, and C chains. A, the outer chains, are attached through their potential reducing end to B chains. However, B chains are linked in the same way and carry one or more A chains. In contrast, the C chains are single reducing group of the amylopectin molecule and carry other chains. The cluster model has been refined on the basis of the A-, B-, C- chain terminology of Peat and others (1956) and Hizukuri (1986). (B-chains that are present within a single cluster as designated are B1 chains, whereas, long B-chains that interconnect clusters are referred to as B2, B3, and B4, depends on the number of clusters interconnected).