Effects of Cultivar and Processing Condition on Physicochemical Properties and Starch Fractions in Parboiled Rice

Cereal Chem. 88(4):414–420 Starch can be classified into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) according to its resistance to amylolytic enzymes. This study investigated the effects of cultivar and feedstock under varying parboiling conditions on the physicochemical properties and starch fractions of parboiled rice. Rice (rough or brown) was soaked, steamed under pressure, dried immediately or stored at room temperature for 24 hr prior to drying, and then treated with or without a repeated steam cycle prior to milling. The storage treatment significantly increased the retrograded amylopectin enthalpy and amylose-lipid complex melting temperature of parboiled rice. Parboiled rice samples prepared from brown rice feedstock had higher peak melting temperatures but lower enthalpy values of retrograded amylopectin than samples prepared from rough rice after the storage treatment. The pasting viscosity of parboiled rice was most affected by the repeated autoclaving treatment and cultivar. Starch fractions in parboiled rice were significantly affected by cultivar and storage and by the interactions of cultivar and parboiling conditions. The storage treatment significantly increased SDS and generally decreased RDS in parboiled rice. Parboiled rice with different SDS and RS contents can be produced by varying rice cultivar and parboiling conditions. There is a growing awareness among consumers of the importance of the glycemic index (GI) of foods because health problems associated with obesity are becoming a major health concern in industrialized countries. GI is a measure of the impact of carbohydrates on blood sugar level (Jenkins et al 1981) and is calculated using the area under the blood glucose response curve after the consumption of carbohydrates from a test food relative to a control food of either glucose or white bread. Foods with a GI of 55 or lower, 56–69, and 70 and above are classified as low, medium, and high GI, respectively (FAO/WHO 1998). Research has shown that consumption of low-GI food can be used in the prevention and management of type II diabetes (Fontvieille et al 1988; Jarvi et al 1999). Many factors affect the digestion and absorption of carbohydrates, such as food composition, botanical source of starch, and starch chemical composition and structure. Behall et al (1998) reported that the ratio of amylose to amylopectin in starch has an impact on the glycemic response to starch-based foods. Engylst et al (1992) classified starch into three fractions according to its resistance to amylolytic enzymes: rapidly digestible starch (RDS), which is hydrolyzed within 20 min; slowly digestible starch (SDS), which is hydrolyzed between 20 and 120 min; and resistant starch (RS), which is not hydrolyzed within 120 min. RDS releases glucose quickly into the bloodstream, causing a higher blood glucose response. SDS causes a slow increase of blood glucose levels and is considered to have a low-to-medium GI (Englyst et al 2003; Lehmann and Robin 2007). RS, considered a dietary fiber, has many positive effects on digestive health when fermented in the gut, such as the production of beneficial short-chain fatty acids that have been shown to improve colon health (Bird et al 2000). High-fiber foods have been shown to produce low glucose responses in normal and diabetic people (Potter et al 1981; Salmeron et al 1997). Rice is one of the most important crops in the world and provides a major source of carbohydrates for many countries. The GI of cooked rice has been reported to range from 54 to 121 when a reference of 100 is used (Jenkins et al 1981, 1988; Brand et al 1985). Parboiling is a hydrothermal treatment to improve the milling, nutritional, and organoleptic properties of rice (Raghavendra Rao and Juliano 1970; Luh and Mickus 1980; Bhattacharya 1985). Traditionally, the parboiling process consists of soaking rough rice at room temperature, steaming or boiling at 100°C, and then sun drying (Bhattacharya 1985; Kar et al 1999). It has been reported that parboiled rice has a relatively low GI of 54 and 65 by Jenkins et al (1988) and Granfeldt et al (1992), respectively, and that it has higher RS than nonparboiled rice (Eggum et al 1993; Marsono and Topping 1993; Tetens et al 1997). Rashmi and Urooj (2003) reported that rice steamed for 20–40 min had decreased RDS levels but increased SDS levels. Storage of cooked rice has also been shown to increase SDS content. Niba (2003) reported that autoclaved rice stored at ambient temperature for 10 days had higher SDS content than that stored at freezing temperature (–20°C). Storage allows starch to retrograde, which renders starch more resistant to enzymatic degradation (Eerlingen et al 1994; Fredriksson et al 2000; Frei et al 2003). Traditionally, rough rice is used as the feedstock for parboiling rice. More recently, brown rice is also used for parboiled rice production because of faster hydration (Luh and Mickus 1980; Kar et al 1999). It has been reported that the parboiling conditions can be altered to promote the formation of ordered starch structures such as crystalline amylose and amylopectin and amylose-lipid (AML) complex, thus decreasing starch digestibility and GI (Hoover and Vasanthan 1993; Hoover and Manuel 1996; Chung et al 2009). The objectives of this study were to investigate how cultivar and feedstock under different parboiling, storage, and milling conditions affected the physicochemical properties and starch fractions (RDS, SDS, and RS) in the resultant parboiled rice. MATERIALS AND METHODS Materials Two long-grain cultivars, Wells and XL723, that were grown in Arkansas in 2008 were provided by the University of Arkansas Rice Processing Program. Wells is the most widely grown longgrain rice in Arkansas, whereas hybrid rice cultivars such as XL723 are increasing in acreage. Rough rice samples were dried to approximately 10–12% moisture content (MC) at ambient temperature and stored in sealed plastic containers at ambient temperature for six months prior to further treatment. The apparent 1 Department of Food Science, University of Arkansas, Fayetteville, AR 72704. 2 Corresponding author. Phone: +1-479-575-3871. Fax: +1-479-575-6936. E-mail: [email protected] 3 Agricultural Statistics Laboratory, University of Arkansas, Fayetteville, AR