Characteristic And Formation Mechanism Of Slowly Digestible Starch

Slowly digestible starch (SDS) is slowly digested throughout the small intestine resulting in a slow and prolonged release of glucose into the blood stream, coupled to a low glycemic response. Based on the rate and extent of digestibility using the in vitro Englyst assay, SDS is also the starch fraction digested between 20 and 120 min. As a new functional ingredient, SDS is becoming the research focus in food science and modern nutriology for its unique low glycemic index property. Through lessening the stress on regulatory systems related to glucose homeostasis, this starch type may be helpful in controlling and preventing dietary management of metabolic disorders, including diabetes, prediabetes, glycogen storage disease, cardiovascular disease, and obesity. Currently, there are only some reports about preparation method and physiological effect of SDS in occident countries, such as America and Switzerland. Also, SDS-based product is not commercially available in the current food marke. Thus, elucidating the characteristic and formation mechanism of SDS is important for food industry to utilize the abundant starch resource, make tailor-made carbohydrate foods with rich-SDS, and improve the nutritional property of food.In this study, the method of in vitro analysis was firstly discussed and has unified standardization, which was important for underatanding the digestibility, molecular structure feature and formation mechanism of SDS. The aim of this project was to investigate the slow digestion property, structure basis by using the Rapid viscosity analyzer, High performance size-exclusion chromatography, X-ray diffraction, Differential scanning calorimeter, Scanning electron microscopy, etc., and to probe into the effect partially debraching and recrystallization or controlled gelatinization on SDS in waxy maize starch, which may provide the knowledge for research and development of starchy food with low glycemic index (GI). The main work was listed as follows:The three in vitro methods of Englyst, Guraya and Shin were used to determine and compare the content of SDS of normal maize starch, waxy maize starch and potato starch with the value of in vivo test. The results showed that the method of Englyst with an optimal modification by one factor and orthogonal experiments was the most suitable one for the determination of SDS. The starch samples (200 mg) were added to the acetate buffer (15 mL, pH 5.2) and mixed thoroughly. The enzyme solution (10 mL, porcine pancreaticα-amylase 290 U/mL and amyloglucosidase 15 U/mL) was then added to the substrate, followed by incubation in a water bath (37 oC) with agitation.Based on the modified Englyst test to measure the nutritionally important starch fractions, there is higher SDS (about 50%) in starch form common maize, waxy maize, wheat, sticky rice or rice compared with potato starch (16.9%). Therefore, cereal starches containing a large portion of SDS are considerd as ideal SDS materials while potato starch is the typical resistant starch (RS). When the native cereal starch was cooked in the boiling water bath, the slow digestion property was lost with a huge increase of rapidly digeatible starch (RDS). The inheret layer structure of crystalline and amorphous regions is likely the fundamental structure basis for slowly digestion property of starch. The relationship between slow digestion property and enzymatic hydrolysis rate of starches was investigated by establishment of in vitro hydrolysis kinetics. The starch digestion rate calculated using the exponential curve equation C=C∞×(l-e-kt) was less than 1 h-1 for cereal starches. Starch digestibility was reflected exactly by digestion rate of hydrolysis kinetics other than in vitro Englyst method. The glycemic response profile (the shape of the glycemic response curve) of cereal starch was significantly different from glucose powder with a delayed blood glucose peak and a prolonged and moderate elevation of glucose after the peak. The GI value of cereal starch was also more than 90% and belonged to high GI food, but extended GI (EGI) representing the slow glucose release property was positive and might be used as an in vivo indicator of the amount and/or quality of the SDS in foods. RVA was used to investigate viscosity profiles of different starch and the correlation between starch digestibility and RVA profile characteristics. The breakdown of different varieties starch was negatively correlated with SDS (r=-0.89, p<0.05). The RVA method potentially could be used as a screening tool for starch digestion properties. Six starches from different varieties were used as materials and the chromatogram of debranched amylopection were analyzed by SEC system. The correlation between starch digestibility and fine structure of amylopectin was also investigated. SDS fraction was positively correlated with FrⅠ(DP>30) and FrⅡ(13 40%) was kept prior to the visible morphological and structural changes (at 60°C). Swelling factor began to increase slightly at 50-60°C and continued to maximum value at 80°C. A large decrease in the melting enthalpy, crystallinity, and ratio of 1047/1022 cm-1 attributed to partially dissociation of crystalline clusters and double helices occurred at 65-80°C. These changes showed that controlled gelatinized starch with slow digestion property occurred in the molecular rearrangement process before granule breakdown and SDS mainly consists of amorphous regions and a small portion of less perfect crystallites.