Preparation And Properties Of Pressure-heat Cooling Cycle-enzymatic Hydrolysis Sweet Potato Flour And Its Application In Vermicelli

Sweet potato production ranks fourth in China's crops,with comprehensive nutrition and high utilization value.However,fresh sweet potatoes are not easy to store and rot because of their high moisture content.Sweet potato is currently mainly used to produce starch,and its deep-processing products are relatively single.Sweet potato flour made from fresh potatoes,on the one hand,can be preserved for a long time,on the other hand,can be used as raw materials for deep processing products of sweet potatoes.Compared with sweet potato starch,sweet potato flour has the advantages of comprehensive nutrition.Compared with natural starch,resistant starch has better antienzymatic activity.The application of resistant starch in functional food is a research hotspot at present.Vermicelli is very popular in the south of China,so the market is very big.Making sweet potato vermicelli from sweet potatoes is an important direction for deep processing of sweet potatoes.Whether it can further improve the quality of sweet potato vermicelli is the forefront of current research.The main idea of this paper is to use certain methods to improve the content of resistant starch in sweet potato flour and apply it to the production of vermicelli.In this way,it can not only ensure the nutritional comprehensiveness of vermicelli,but also enable them to have certain physiological functions.In this paper,the technology of increasing resistant starch content in sweet potato flour by pressing-heating method,pullulanase enzymatic hydrolysis and pressing-cooling cycle combined with enzymatic method was studied.The physicochemical properties of treated sweet potato flour and natural sweet potato flour were compared.Preliminary study on the processing technology of sweet potato vermicelli with treated sweet potato flour was carried out.The results are as follows:(1)In the experiment of preparing sweet potato resistant starch by pressing-heating method.Using sweet potato flour as raw material,the effects of sweet potato flour concentration,pH,thermal-press temperature,thermal-press time and refrigerating time on the yield of resistant starch in sweet potato flour were studied.The effect of thermal-press temperature on the yield of resistant starch was relatively insignificant in the range of 115-125?.The yield of resistant starch did not change significantly when the refrigerating time exceeded 24 h.Therefore,the sweet potato flour concentration,pH and thermal-press time were selected for theresponse surface test.The optimal conditions of preparing resistant starch of sweet potato with response surface methodology were as followings: sweet potato flour concentration 25.50%,pH 7.30,thermal-press temperature 120 ?,thermal-press time 31.20 min,refrigerating time 24 h.Under these conditions the yield of resistant starch of sweet potato was 9.41%.Compared with 2.92% resistant starch in natural sweet potato flour,it has a significant improvement.(2)In the experiment of preparing resistant starch from sweet potato by enzymatic hydrolysis.Using sweet potato flour as raw material,the effects of pullulanase addition,enzymatic hydrolysis time,enzymolysis temperature,pH on the yield of resistant starch in sweet potato flour were studied.The optimal conditions of preparing resistant starch of sweet potato with response surface methodology were as followings: the amount of pullulanase added was 4.4 U/g,the time of enzymatic hydrolysis was 13 h,the temperature of enzymatic hydrolysis was 55?,and the pH was 4.9.Compared with 2.92% resistant starch in natural sweet potato flour,it has a significant improvement.(3)The sweet potato flour was treated by pressure-heat cooling cycle-enzymatic hydrolysis method.The physicochemical properties of the treated and natural sweet potato flour were determined.The best cooling cycle number is 2 times.Too many or too few cycles are not conducive to the formation of resistant starch.Natural sweet potato flour has high solubility,strong retrogradation and weak resistance to enzymatic hydrolysis.Natural sweet potato flour particles are mostly spherical,partly concave,with adhesives on the surface,with an average particle size of 17.68?m.Its polarized cross is obvious and its crystalline structure is similar to A-type starch,with a crystallinity of 9.25%,low gelatinization temperature and low gelatinization enthalpy.Comparing with the natural sweet potato flour,the sweet potato flour treated by pressure-heat cooling cycle-enzymatic hydrolysis has lower solubility,weaker condensation and stronger resistance to enzymatic hydrolysis;its particle structure is destroyed,and it is cracked into irregular flakes or clusters with larger average particle size;most of its polarized cross disappears,but the crystal structure basically remains unchanged,the relative crystallinity increases to 10.48%,the gelatinization temperature increases,and the enthalpy of gelatinization increases.(4)In the process of making sweet potato vermicelli by pressing-heat cooling cycle-enzymatic hydrolysis,the vermicelli made from sweet potato flour only have high broken rate,low transmittance of paste soup,high cooking loss rate,low hardness,small elasticity,high viscosity and poor vermicelli quality.When the sweetpotato flour: sweet potato starch = 1:4,the breaking rate decreases,the transmittance of paste soup increases,the cooking loss decreases,the hardness increases,the elasticity increases,the viscosity decreases,and the quality of the vermicelli is better.Therefore,when the treated sweet potato flour is used to make the vermicelli,when the addition of sweet potato starch is 4 times of the addition of the sweet potato flour,the quality of the vermicelli is better,which not only guarantees the better quality of the sweet potato vermicelli,but also improves the nutritional comprehensiveness of the vermicelli.