Nutritional aspects of rice starch composition: analysis of varieties consumed in Italy

Rice is a large consuming food in world human population. As for other cereals, rice provides a wide range of micro- and macronutrients and represents one of the main dietary sources of carbohydrates, especially starch, worldwide. The nutritional properties of rice has been well-documented [1]. However, recently some concerns have arisen regarding the relationship between rice consuming and the effects on human health. Specifically, a large intake of refined white rice have been associated to an increased risk of some chronic diseases as type-2 diabetes, cardiovascular conditions and obesity [2]. On the contrary, the replacement of white rice with brown rice in the dietary habits is associated with a lower risk of these diseases. In the case of type-2 diabetes, the negative effect of rice assumption on health is related to its high-impact on the post-prandial glycaemic responses [3]. Rice, in fact, is generally considered a highly digestible source of starch, despite some differences among rice varieties, the preparation techniques and the cooking methods have been highlighted, resulting in different glycaemic responses in vivo. These differences were found mainly related to the amylose content of rice. Amylose is the linear fraction of starch composing of α-1,4 linked glucose chains. Differently to the branched starch (amylopectin), amylose does not gelatinized during cooking and retains its crystalline structure, being thereby less accessible for the enzymatic degradation into glucose in the small intestine. This results in a slower digestion of starch and, consequently, a prolonged absorption of free glucose in small intestine and lower post-prandial sugar concentration in blood [4]. The aim of the present work was to define the composition of the carbohydrates (in terms of total starch and amylose content) in rice, focusing on varieties predominantly consumed in Italy. This information is useful for a better understanding of the glycaemic index (GI) of rices. For this purpose, eight different varieties of rice were investigated. Rice grains were grinded into powder and resuspended in 0.1 M NaOH solution. Spectrophotometrically analyses were performed to determine the total carbohydrate content (Anthrone test) and amylose content (Iodine test). Results showed some differences in the starch content and composition of the analysed varieties of rice. Regarding the total carbohydrate content, Carnaroli has the largest mean amount of carbohydrates (87.95 gr of carbohydrates/100gr of rice), while Vialone Nano rice has the lowest one (62.94 gr of carbohydrates/100 gr of rice). Anyway, when statistically compared non-significant differences were observed. As regards the amylose content, Fragrance rice has the higher one (37.10 gr of amylose/ 100 gr of rice) while Parboiled Ribe rice the lowest one. Statistical comparison, in this case, revealed some differences in rice varieties, which can be grouped in two statistically different categories named as “low-amylose content rice” including Ribe, Arborio, Paraboiled Ribe and Venere and “high-amylose content rice” including Vialone Nano, Carnaroli, Basmati and Fragrance. All together, our results suggest that the “low amylose content” varieties, especially Fragrance rice, which contains the largest amount of amylose, represent the optimal nutritional choice. On the contrary, the high-amylose varieties, especially Parboiled Ribe, showing a high carbohydrate content and the lowest amount of amylose, is not recommended to consume.