Effect of polishing on glycemic index and antioxidant properties of red and white basmati rice
Introduction
Foods having a low glycemic index (GI) and high antioxidant content are gaining increased attention as they can potentially reduce the risk of diseases related to impaired glucose metabolism. Natural antioxidants such as plant polyphenols and flavonoids reduce the incidence of degenerative diseases such as diabetes, cancer, cardiovascular disease and aging (Srinivasan, Sudheer, & Menon, 2007).
Rice (Oryza sativa L.) is the principal dietary source of carbohydrates consumed by almost one-half of the population throughout the world. More than 5000 varieties of rice are available in the world having different physical, biochemical, milling and cooking qualities (Bhattacharjee et al., 2002, Yang et al., 2008). Recently, aromatic rice varieties, collectively known as “Basmati rice”, have obtained a huge demand in Asian and European countries because aroma is rated as the highest preferred characteristic of rice followed by taste and elongation after cooking (Kale et al., 2015, Yang et al., 2008). Basmati rice has unique quality characteristics such as extra-long supreme slender grains, pleasant aroma, sweet taste, soft texture, length-wise elongation with least breadth-wise swelling on cooking and tenderness of cooked rice (Bhattacharjee et al., 2002, Kale et al., 2015). As a result of all these organoleptic properties, basmati rice varieties fetch a premium price in the world market, especially in Europe and USA. It is consumed as whole kernel after milling of rough basmati grain into different degrees by removing the rice bran layer (Babu, Subhasree, Bhakyaraj, & Vidhyalakshmi, 2009).
Widely consumed type of rice in the world is white rice, which is non-pigmented. In addition, there are two other types of pigmented rice consumed in the world known as dark and light pigmented rice. In dark pigmented rice, flavonoid group of anthocyanin pigments are accumulated in different concentrations of the rice bran (pericarp, seed coat and aleurone), therefore raw rice kernels (unpolished) appear in different colours such as black, purple, dark-brown or dark-red (Sompong, Siebenhandl-Ehn, Linsberger-Martin, & Berghofer, 2011). Sri Lanka has the second largest resource of pigmented rice in the world next to China (Sompong et al., 2011). Approximately 30–40% of total rice consumption in Sri Lanka is pigmented rice, generally known as “red-rice” in the market regardless of their bran colour intensity. Previous studies have shown that in most of Sri Lankan “red-rice” pigmentation is due to anthocyanin content (Perera and Jansz, 2000, Sompong et al., 2011). There is a high consumer demand due to its appealing darker colour of the red rice varieties in the market. Red rice varieties such as pigmented dark-red basmati usually undergo low level of polishing to preserve the bran layer than white rice. It has been reported that red rice have higher antioxidant activities than white rice (Jun et al., 2012, Payakapol et al., 2011, Sompong et al., 2011, Sutharut and Sudarat, 2012). However, dark-red rice or less polished rice contains characteristic flavors that consumers find unacceptable due to its characteristic roughness.
The pigmented dark-red rice has potential health benefits due to its high dietary fiber content which could help reduce the GI, thereby reducing the risk of type II diabetes (Babu et al., 2009). According to Miller, Pang, and Bramall (1992), GI of rice can vary within a wide range of 64–93%. Foster-Powell, Holt, and Brand-Miller (2002) reported that GI values of Bangladeshi rice ranged from 37–38%. In Sri Lanka, some of the commercial white rice varieties have shown GI values ranging from 67% to 72% (Darandakumbura, Wijesinghe, & Prasantha, 2013) with majority of them classified as medium-high GI rice. The variation in the GI of rice results from inherent varietal differences of rice grown in different countries (Foster-Powell et al., 2002, Panlasigui and Thompson, 2006, Panlasigui et al., 1991). Therefore, Miller et al. (1992) highlighted the importance of maintaining their own GI testing, mainly with staple agricultural products like rice. In addition, rice bran contains a variety of health-promoting antioxidant compounds such as phenolic acid, flavonoids, anthocyanins, proanthocyanidins, tocopherol, vitamin E, γ-oryzanol and phytic acids which are highly abundant in the pigmented rice (Goufo and Trindade, 2014, Jun et al., 2012). Since most of these healthy phytochemicals are concentrated in the bran layer of rice kernel, they can be substantially removed during bran removal by rice polishing.
Limited research had been conducted on the relationship among GI, rice bran colour and antioxidant potential of basmati rice subject to different degrees of milling. Such information is valuable for consumers, rice millers and nutritionists in the world to formulate rice-based foods. Hence, the objective of this study is to investigate the effect of varietal differences and degree of polishing on the GI and antioxidant properties of pigmented and non-pigmented basmati rice varieties.
Section snippets
Rice samples
Four different pigmented dark-red (red) and non-pigmented white (white) basmati rice varieties namely CIC-red (CIC-RB), CIC-red fragrance (CIC-RF), CIC-white (CIC-WB) and white basmati (At 405) were used in the study. Of these four varieties, CIC-RF was an improved variety from CIC-RB rice. Paddy (rough rice) samples were collected from CIC rice breeding laboratory (Palwehara, Sri Lanka) after two seasons of cultivation, having a moisture content of 10–11% (dry basis). Prior to the experiment,
Effect of variety on nutrient composition of rice
The chemical composition of 10% and 100% polished, rice varieties is presented in Table 1. The crude fat and ash were found in higher concentrations in CIC-RF than other basmati varieties. The highest ash content of 1.44 ± 0.01% was found in 10% polished CIC-RF rice while the lowest of 0.72 ± 0.02% was recorded for 100% polished At 405. Irrespective of the rice variety, significantly higher (P < 0.05) ash content was observed in 10% compared to the 100% polished rice. The average difference of the
Effect of variety on nutrient composition of rice
Rice bran layers consist of non-starchy polysaccharides, fat and dietary fiber. The aleurone layer and germ contain protein, fat and considerable amount of antioxidants, vitamins and minerals (Babu et al., 2009). However most of these nutrients are removed with rice bran during rice milling (Babu et al., 2009, Shobana et al., 2011) leaving behind a higher amount of starchy endosperm (Payakapol et al., 2011). It is well known that the ash, fiber, protein and fat contents of raw rice
Conclusions
This study demonstrated that the nutritional composition of basmati rice was affected by the variety and the degree of polishing. Although the 10% or 100% rice polishing levels had no significant effect on the GI of rice, crude fiber content showed a significant negative correlation with the GI. Red basmati varieties had higher nutritive and low GI values than white basmati varieties. Compared to the white basmati, red basmati contained a higher amount of TPC, TAC and AOA, which are also
Conflict of interest
The authors declare no conflict of interest.
Ethical approval
This manuscript contains studies with human participants. Therefore, ethical clearance was obtained from the ethical review committee of the Faculty of Medical Sciences, University of Peradeniya, Sri Lanka.
Acknowledgements
The financial and technical assistance provided by the CIC Food and Nutrition Research Centre, Sri Lanka is gratefully acknowledged. Our sincere gratitude is due to the volunteers who participated in the study and Mr. K.A.K.L. Chandrasiri, Department of Food Science and Technology for his technical support throughout the study.
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