Mechanism And Control Of Quality Deterioration Of Fresh Yuba During Cold Storage

Yuba is rich in nutrition,unique in taste,deep in consumer groups and in great market demand in our country.Traditional dried yuba was not accepted by the majority of consumers because it took a long time to rehydrate before cooking.The taste of fresh yuba was better,and it was easier to eat,so it had a good market potential.However,because of its high water content,microorganisms were easy to grow,propagate and rot.There were some quality problems such as serious browning and poor chewiness of high-pressure sterilized fresh yuba in the market,and there were some problems in frozen storage,such as hard texture,loss of juice,weakening of bean flavor and so on,which seriously affect the market development and consumption of fresh yuba.Therefore,based on the increasingly perfect cold chain transportation and facilities in Chinese cities,it was of great significance to study the quality change,deterioration mechanism and control measures of fresh yuba in the process of cold storage,so as to improve the edible quality of yuba during cold storage.The main results of this thesis were as follows:(1)The quality changes of fresh yuba during cold storage at 4 ℃ were studied,including the changes of physical and chemical composition and index,sensory quality and color,total colony count,texture properties,cooking characteristics and so on.The results showed that during cold storage,the weight loss rate,protein content(nitrogen content)and dry matter loss rate of fresh yuba increased;water content and total fat content decreased;total sugar content decreased at first and then increased;p H,acidity and peroxide value were in dynamic change.In the first8 days,p H increased at first and then decreased,acidity and peroxide value decreased at first and then increased.During cold storage,the sensory score and L*of fresh rotten bamboo decreased,and the total number of colonies of fresh yuba increased significantly(p<0.05).The total number of colonies was 2.06×104 CFU/g,after 4 days of storage and more than 1.0 ×105CFU/g on the 6th day.During cold storage,the hardness,elasticity,stickiness,chewiness,tensile strength and elongation at break of fresh yuba decreased significantly(p<0.05),while the cooking loss rate increased.In the process of cold storage,the physical and chemical composition and index,sensory quality and color,total bacterial count,texture and cooking characteristics of fresh yuba changed significantly(p<0.05),resulting in the deterioration of the quality of fresh yuba.(2)The protein existence form and structure,water distribution,fat existence form,protein film forming force and microstructure of fresh yuba during cold storage were studied.the results showed that with the extension of storage time,the content of soluble peptide increased,the content of soluble protein decreased,the particle size of protein decreased,and the content of TVB-N and free amino acid increased.During cold storage,proteins were decomposed into soluble peptides,amino acids,volatile ammonia,amines and other basic nitrogen-containing substances.Protein nitrogen reacts with reducing sugar in Maillard reaction,which darkened the color of yuba.During storage,the contents of free sulfhydryl and total sulfhydryl groups in the protein structure decreased,while the disulfide bond content increased.The hydrophobicity of the protein surface increased,the α-helix and β-fold structure in the protein secondary structure decreased,and the β-turning angle and irregular crimp structure increased,which made the protein network structure more loose.In the protein composition,the α’ and α subunits of 7s protein and the basic subunit of 11 s protein were first degraded by microorganisms,followed by the β subunit of 7s protein,and the acidic subunit of 11 s protein was the most difficult to be degraded.In the process of cold storage,the protein was degraded into small peptides by microorganisms.at the same time,the degraded protein oxidizes and aggregates during storage to form insoluble or soluble aggregates.During storage,the existing state of water in yuba changed.After 0-4days,the bound water and adsorbed water in Yuba were partially transformed into free water,but the binding of bound water,adsorbed water and Yuba was closer.After 4-8 days,the binding ability of protein in fresh Yuba to bound water and adsorbed water became weaker.In the process of cold storage,the free fat content of fresh yuba began to increase after 2 days of cold storage,the hydrophobic interaction between protein and fat was destroyed,and the force of protein film formation changed.The hydrophobicity and hydrogen bond and disulfide bond force in protein film forming force increased at 0 day and 4th day,decreased at 4th day and decreased at 4th day,and decreased at 4th day.The structure of fresh yuba was dense and uniform,the surface is smooth,and there are only a few small holes.With the extension of cold storage time,the surface of yuba became rough,and a large number of deep holes appeared,and finally the integrity of the film was destroyed.There was a significant positive correlation between the TVB-N index and the total number of colonies.The content of protein sulfhydryl group was positively correlated with hardness,elasticity,stickiness,chewiness,tensile strength and elongation at break.The surface hydrophobicity of protein was significantly correlated with hardness,chewiness and elongation at break.The content of soluble protein was positively correlated with elasticity and stickiness of yuba,and negatively correlated with soluble peptide and disulfide bond.To sum up,the protein was degraded and oxidized during cold storage,the spatial structure of protein and the force of protein film forming changed,and the force between protein and fat was destroyed.these changes lead to the structure of yuba from dense to loose,the surface from smooth to rough,and a large number of holes,resulting in the decline of the texture properties of yuba.In addition,the Maillard reaction between protein nitrogen and reducing sugar resulted in the decrease of water content,which made the color and brightness of yuba darker.(3)The cold storage fresh-keeping technology of yuba was studied.the results showed that after reducing the water content,the hardness of yuba increased,while the elasticity,chewiness and stickiness decreased during cold storage.Reducing moisture can significantly increase the hardness,elasticity,chewiness and stickiness of yuba;the initial water content affects the rehydration of yuba,and the rehydration of yuba was the best when the initial water content was 45%.Reducing the initial water content of yuba can increase the total score of sensory score and reduce the content of soluble peptide in fresh yuba during cold storage.Physical sterilization had a great influence on the sensory quality of yuba,which made the texture of yuba hardened and browned seriously.It was more suitable to dip the fresh yuba in the fresh-keeping agent for a short time.The better fresh-keeping method was as follows: the fresh yuba was dipped in 0.3% tea polyphenols fresh-keeping solution,5minutes,taken out and drained,its moisture content was45% through low temperature oven treatment,and then vacuum packaging method was adopted,which could effectively prolong the storage life of fresh yuba and maintain good edible quality within 7 days.