Effect Of Heat Treatment On Structure And Properties Of Highland Barley Starch

Highland barley, as a characteristic species of Qinghai-Tibet Plateau and the most distinctive highland crops, exhibits the characteristics of distinctive composition and high nutritional value, and has been attracted much attention in the field of nutritional and health food in recent years. Starch is the main component of highland barley, occupying 75% to 80% of the endosperm and accounting for the application of highland barley. However, the fundamental and application on highland barley starch is far less than other grain starch. Its multi-scale structure and characteristics is less understood and its modification especially thermal physical modification has been seldom involved, thus limiting the application of highland barley. In fact, the development and research on the structure and properties of highland barley is of great scientific significance and practical values. It not only enriches the scientific theory of highland barley starch and promotes the high-value utilization of highland barley, but also contributes to better understand the properties of highland barley and accurately regulate its nutritional functions, and accelerate the development of highland barley food and related highland barley industry.Knowing the current research status on highland barley and highland barley starch, we extracted starches from highland barley originated from Qinghai Plateua. Multi-scale structure and properties of highland barley starch were systematically investigated. Highland barley starch was modified by moist heat and dry heat treatment, and the effects of heat treatment on the aggregate structure, chain structure, paste, rheological and digestive properties were explored. The relationship of "modification conditions- multi-scale structure- functional properties" was established. Moreover, the effect of addition of highland barley powder or heat-treated highland barley powder on the farinograph and extensograph parameters of low-gluten wheat flour was studied. The results are as follows:The systematic study on multi-scale structure and properties of the highland barley starch showed that highland barley starch granules exhibited a flat oval shape with relatively smooth surface. The highland barley starch granules were composed of A- and B- type granules with different sizes. The bigger A-type granules had the average diameter of 20.12 μm, accounting for 92% of the total granules, while the smaller B-type granules had the average diameter of 3.02 μm, about 8% of the total granules. The thickness of granular lamellar structure of highland barley starch was about 10.1nm-10.8nm, the crystals of highland barley starch were A-type spherulites with the degree of crystallinity of 25.3%. Their amylose content was 23.07%, the molecular weight was over 4 × 107 g/mol with an average weight of 8.796×107 g/mol. Gelatinization temperature of highland barley starch was 67 ?C, exhibiting a higher paste gel and retrogradation and a non-Newtonian pseudoplastic fluid with shear-thinning characteristics. The content of rapid digestible starch, slowly digestible starch and resistant starch was 96.19%, 1.54% and 2.27%, respectively.The modification of highland barley starch by moist heating was investigated, the moisture content was determined as the most important influencing factor after single factor tests. When the moisture content was lower than 30%, the morphology, size and size distribution of highland barley starch granule did not change significantly, however, with the increase of moisture content, though the lamellar structure of particle was destroyed gradually, it was still A-type crystal structure, and the crystallinity and the ordering degree increased. When the moisture was increased to 30%, the lamellar structure and crystal structure were damaged seriously. The weight-average molecular weight of highland barley starch after moist heat treatment became small, about 4.110 × 107 g/mol, mainly distributed in the range of 2×107 g/mol-6×107 g/mol, and their amylose content increased slightly. The pasting temperature was increased, while the viscosity, gel property and coagulation property were decreased, remaining pseudoplastic fluid properties with smaller shear-thinning characteristics. Their digestion resistant performance improved, the total content of slowly digestible starch and resistant starch was up to 18%. The heat moisture treatment had obvious influence on the multi-scale structure and properties of highland barley starch.The dry heat treatment can also effectively influence the multi-scale structure and properties of the highland barley starch. The change trends were similar to those by moist heat treatment, while the changing degree was not same. The average particle size, thickness of layered structure and amylose content of highland barley starch after dry heat treatment were increased, while the decreasing degree of its weight-average molecular weight, paste viscosity, gel property, coagulation performance and shear thinning was higher than that by moist heat treatment. The increasing degree of crystallinity, the ordering degree, gelatinization temperature and digestion resistant performance was higher than that by moist heat treatment. Especially, the slow digestion of starch and resistant starch content was up to 22%. Therefore, the moist heat and dry heat treatment are promising in physical starch modifications, especially in the regulation of starch digestion and nutrition functions.The results showed that the highland barley powder contained as high as 46.52% and 42.06% respectively after moist heat and dry heat treatment. The highland barley powder and heat-treated highland barley powder were added to low-gluten flour with different ratios. The farinograph and extensograph parameters of the mixed flour exhibited difference changes by Farinograph and texture analyzer. The highland barley powder and heat-treated highland barley powder can increase the strength and toughness of the mixed flour, decreasing the dough ductility, plasticity and expansion. These results provide the basic data for the preparation of highland barley nutritional and health foods.