Study On Thermal And Acidic Or Alcali Depolymerization Of Konjac Glucomannan

Compared with the starch, cellulose, polysaccharide, konjac glucomannan (Konjac Glucomanan, KGM) is subject to degradation When used in practical production because of its complex polysaccharide structure and soft molecular chain structure, thus limiting its application in food with high water content such as beverage. the rheology and structure analysis method was introduced in this paper to explore hydrosol acid degradation and thermal degradation kinetics of KGM, product structure, so as to understand the change of KGM in the course of processing, thereby guiding the recipe formulation and the technology optimization in the konjac application. and provide a basis to realize the controllable degradation of konjac glucomannan, and prepare products with a target molecular weight at the same time.The main research contents and results are as follows:1. The acid degradation and heat degradation kinetics of the KGM was studied under different temperature and pH conditions. The change of intrinsic viscosity and viscosity average molecular weight of KGM with time in different conditions was Determined. the results show that the hydrolysis reaction under acidic conditions was consistent with a first-order kinetic model, the backbone of polysaccharide randomly fractured in the process of acid hydrolysis. the reaction rate constant K was measured under different conditions, and the acid hydrolysis kinetics equation of KGM was established. a small amount of alkali will inhibit the degradation of KGM while the excess alkali accelerates the degradation of KGM.At the temperature of65℃and80℃, alkaline hydrolysis kinetics equations of KGM deviated from first-order kinetics model, and the alkali hydrolysis of KGM may be divided into two steps. the relative viscosity change of the product solution was detected at the same time.the quantitative relation between KGM viscosity and reaction time was preliminaryly built, which was suitable for monitoring the preparation process of products with target molecular weight.In addition, the content of reducing sugar generated in the acid degradation of KGM is higher than that in the alkaline condition. With the prolonging of the hydrolysis time, the reducing sugar content increased gradually. but the yield of the reducing sugar is low on the whole.According to the results of the periodate oxidation experiment, the bond of KGM molecules of broke different in different pH conditions.2.The results of steady state rate scanning test show that the alkaline hydrolysed KGM is a typical pseudoplastic fluid even when it was heated for2H at high temperature (such as100℃), the zero shear viscosity of KGM solution decreased compared with the original KGM solution.But the zero shear viscosity can not be detected in KGM hydrolysed by acid at high temperature (or its value is very small), showing a typical Newton fluid characteristics. Dynamic frequency scanning test results show that the storage modulus (G’) and loss modulus (G")of KGM hydrolysis product increased with the increasing of the scanning frequency, under different temperature and pH conditions. at lower frequencies G ">G’, and then tended to be stable and the intersection point emerged, showing rheological characteristics of a weak gel or concentrated polymer solution. The crossover frequency of G’ and G" of the sample hydrolysed with the addition of Alkali at different temperatures did not change a lot, while the static scan rate of KGM hydrolysed with the addition of acid at high temperature exhibited Newton fluid characteristics, the cross point is not detectable by dynamic frequency scanning. the temperature scans result of KGM solution under Different pH indicated that a certain amount of OH-inhibited the KGM molecular degradation, while excessive OH-accelerate KGM degradation.The higher is the H+content, the more significant is the degradation3. Laser particle size analysis showed that the particle size of KGM hydrolysate varied with the hydrolysis temperature and the hydrolysis pH. the particle size range of hydrolysis product under Acidic condition is wide, and the degree of dispersion is high, the particle size did not change when it was stocked for7days.But when KGM was hydrolysed without adjusting the pH, its size in the storage process increased, which was similar to the starch retrogradation phenomenon, the particle size range of KGM hydrolysed with the addition alkali narrowed in the storage process.KGM and its hydrolysis products presented an amorphous structure containing a small amount of crystallization, and the crystallization degree varied with the hydrolysis temperature and hydrolysis pH. the crystal structure of the product changed with the addition of Acid and alkali. With the increase of pH, the decomposition temperature of KGM hydrolysed products shifts to a higher temperature, circular dichroism(CD) spectra of KGM and its hydrolysed products showed that in pH2.8, the optical activity strengthened with the increasing of hydrolysis temperature, in PH5.6, the positive cotton effect of hydrolysed products is strong, while the negative cotton effect is weak, in PH9.2,the negative cotton effect weakened with the increasing of the hydrolysis temperature.