Physico-chemical Properties and Digestibility of Pulse Starch after Four Different Treatments

Abstract

Soybean and peanut are pulses that are made up mainly of lipids, and most of these are edible oils. These pulses also contain a high level of protein. In contrast, Adzuki bean, kidney bean, and pea contain mainly starch, and they are utilized for salads, stews, and an (bean paste) products. Starchy pulses also are an excellent source of dietary fiber. 1,2) Dietary fiber has been reported to have a protective effect in many adult diseases, such as arterio-sclerosis, 3) hypertension, 47) large intestine cancer 8,9) and diabetes mellitus. 10,11) Based on their structural features, three groups of dietary fibers can be identified: nonstarch polysaccharides and lignin, partially digestible starch and related chemical compounds, and other partially digestible materials. 1218) The dietary fiber content of starches increases upon heating, 1,2,19,20) a process thought to be due to the formation of resistant starch (RS). There appear to be several types of RS that differ in physicochemical properties. 2123) Fuwa et al. 24,25) reported that the RS level changes following heatmoisture treatment of starch granules such as those found in high amylose corn starch. RS has been defined as the portion of starch in dietary fiber and related chemicals that is only partially hydrolyzed in the lumen of the small intestine. 2427) Heatmoisture treatment of high amylose corn starch prevents its digestion in the small intestine. As a result, it passes into the large intestine where it can be utilized for shortchain fatty acid production by enteric microbes. 28,29) Cooking is known to increase the dietary fiber content of dried pulses, but the reason for this has not been determined. In this study, we investigated this change in pulse fiber content by examining starch physicochemical qualities following four different treatments. MATERIALS AND METHODS Plant materials. Adzuki beans (Erimo var.), white kidney bean (tebou var.) and red kidney bean (kintoki var.) were harvested in the Tokachi region in 1997. Preparation of cooked beans. The three kinds of bean were left in water overnight. Each type of soaked bean was boiled in three times the volume of water for 1 h, resulting in the cooked beans. Beans were powdered by mill, after having been freezedried. Preparation of starch. The protocols were performed essentially as described previously. 30,31) The seed coat was removed by passing the ground dry pulse through a screen. Next, five volumes of 0.2% sodium hydroxide were added, and the mixture was agitated for 3 h. The mixture was then centrifuged at 3000 × !for 10 min, and the supernatant removed. This operation was repeated until protein was not detected in the supernatant. The dried precipitate was then washed until the pH was 7.0. Defatting of starch. The protocols were performed Abstract: In these studies, we found that the average starch particle size of Adzuki beans, and tebou var. and kintoki var. kidney beans was approximately 3040 µm. In addition, these particles were globular with a C-type crystal structure. Heating enhanced the solubility and swelling power of the three kinds of isolated bean starch. No differences were noticed in the solubilities of the starches, and the swelling power of the Adzuki bean starch was higher than that of tebou var. and kintoki var. starch. The swelling power was also substantially altered by the various treatments. We found that the three different defatted starches had an A-type X-ray diffraction pattern and a higher solubility compared to the respective untreated starches. All three types of hot water-treated starch gave a similar X-ray diffraction pattern, but their solubility and swelling power were low compared to the untreated starch. Heat-moisture-treated starch gave an approximate A-type X-ray diffraction pattern, and the swelling power was even lower. In contrast, the starch that underwent the freeze-thawing treatment gave an amorphous X-ray diffraction pattern and had a much higher solubility at 50 C than any of the other samples. The resistant starch (RS) content for isolated starch from the three kinds of beans was approximately 1%. Although the RS content of defatted or hot water-treated starch did not differ from that of untreated starch, the RS content of the heat-moisture-treated starch increased to between 2.5 and 4.2%. Furthermore, the RS content of the starch exposed to the freeze-thawing treatment was the highest: approximately 4% for Adzuki bean, 9% for tebou var. and 8% for kintoki var. starch.