| Humectants have been widely used in food,medicine,cosmetic and tobacco industries due to their water stabilization capacity during storage,transportation and sales of products.Common humectants,such as glycerin and propylene glycol have poor moisture retention capacity in the later stage of storage,and are liable to decrease the sensory quality of the products.In order to improve the water stability of the product,it is of great importance to develop a humectant with excellent and long-term moisture retention properties and explore its moisture retention mechanism.In this research,the moisture retention properties of different polysaccharides were investigated,humectants with better moisture retention were screened and their moisture retention properties were further improved through the degradation method.The mechanism of moisture retention was studied and the screened humectants were applied to tobacco.This research provides basic research data and reference for improving the product quality.The research contents are as follows:The humectants were added to the filter paper,which was selected as a substrate model.The equilibrium moisture content of the filter paper with fenugreek polysaccharide,chitosan,chlorella polysaccharide,polygonatum polysaccharide and ganoderma lucidum polysaccharide were evaluated.Chitosan and polygonatum polysaccharide had better moisture retention properties than other polysaccharides.To improve moisture retention ability of polysaccharide,chitosan and polygonatum polysaccharide with better moisture retention were subjected to controlled degradation.The relationship between molecular weight?MW?of polysaccharide hydrolysates and moisture retention was studied.Results showed that metal complexed-enzymatic hydrolyzation method could be used to prepare chitosan with relatively uniform MW.The results from Fourier transform infrared?FT-IR?,nuclear magnetic resonance?NMR?and X-ray diffraction?XRD?indicated that the decrease of MW did not cause a significant change in chemical structure of the chitosan residues but the crystal structure was transformed significantly.The moisture retention increased firstly and then decreased with the decrease of MW over a range 3.66×103?1.97×105.Chitosan with MW of 5.665×103 was superior to chitosan with other MW and propylene glycol in terms of the moisture retention.The degradation conditions of the chitosan were as follows:the molar ratio of Cu2+to glucose residues was 1:8,the coordination pH was 4.8,the coordination time was 10 h,the papain dosage was 2%,the enzymatic hydrolysis time was 5 h,and 50?L 30%hydrogen peroxide was added 0.5 h before the end of the enzymatic hydrolysis.PLSR was used to further analyze the relationship between MW of polygonatum polysaccharide and moisture retention.The results showed that when the MW of polygonatum polysaccharide was in the range of 10,000?50,000,there was a significant positive correlation between MW and moisture retention.When MW<5000 or MW>50000,there was a significant negative correlation between MW and moisture retention.When the MW ranged from 5000 to10000,the effect of MW on moisture retention was not significant.The optimal conditions were as followed:addition amount of the enzyme 0.1%(wEnzyme/wPolysaccharide),pH 5.0,temperature50°C and time 2 h.Under these conditions,the proportion of the fragments with a MW of 10,000?50,000 could reach 92.69%,while the amount of fragment with MW>50000 was only 7.31%,and thus polygonatum polysaccharide enzymatic product presented the best moisture retention properties.Furthermore,moisture desorption isotherms and low field nuclear magnetic resonance?LF-NMR?were applied to further analyze the moisture retention mechanism of polysaccharide.Desorption isotherms indicated that the enzymatic products of chitosan and polygonatum polysaccharide were able to hold water in filter-papers and retain a lower aW and thereby presented a better moisture retention and high net isosteric heat of desorption.LF-NMR results showed that the relative content of free water of the filter paper with different humectants were almost the same,but an increase in contents of bound water and a decrease in contents of immobilized water were observed in the enzymatic products of chitosan and polygonatum polysaccharide.Therefore,the increase of the bound water in filter-papers with the enzymatic products of chitosan and polygonatum polysaccharide was probably due to the moderate enzymatic hydrolysis of polysaccharides,which lead to an increasing number of newly produced hydrophilic groups per unit weight.The increased groups could form more hydrogen bonds with water molecules,and more immobilized water migrated to a bound-state,which reduced water activity of the filter paper,increased the net isosteric heat of desorption,and improved the water stability,so as to achieve a long-term moisture retention effect.The enzymatic products of chitosan and polygonatum polysaccharide were applied to tobacco.The moisture content,moisture desorption rate,water state and the structure of tobacco surface were assessed.It was found that the enzymatic products of chitosan and polygonatum polysaccharide significantly increased the equilibrium moisture content and the bound water of tobacco,and yet reduced the desorption rate.The effects of humectants on the surface microstructure of tobacco were studied by environmental scanning electron microscopy.Tobacco with enzymatic products of chitosan and polygonatum polysaccharide had the highest equilibrium moisture content and the lowest desorption rate.Therefore,the surface morphology was maintained well,the surface tissue was plump and smooth,and the dent was also relatively low. |
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