Study On The Mechanism And Control Of Quality Deterioration Of Wheat Starch-based Prepared Food During Freeze-drying

Wheat starch-based food is an important part of daily diet.However,traditional wheat starch-based food can’t meet the current fast-paced life needs because of its long preparation time and complicated operation.The prepared food which can be eaten directly or simply processed by prefabrication provides an effective way to solve the above contradictions.Compared with products with high moisture content,dehydration can improve the convenience of storage,transportation and eating of prepared food.Among the common dehydration methods,freeze-drying is the best way to prepare high-quality dehydrated food.However,in actual production,it is found that the product quality of wheat starch-based prepared food after freeze-drying and rehydration is far from the high-quality state before dehydration.At present,there is a lack of systematic research on the mechanism and regulation of quality deterioration of wheat starch-based prepared food during freeze-drying.Therefore,this topic takes the wheat starch-based prepared food as the research object,and explores the mechanism of quality deterioration in the freeze-drying process.Then,taking the actual products with increasingly complex system components as carriers,and according to the obtained quality deterioration mechanism,the causes of quality deterioration were analyzed,and the quality deterioration control of freeze-dried wheat starch-based prepared food was studied in combination with the characteristics of each product.The specific work are as follows:1)The food model systems of single starch system,starch-lipid mixture,starch-protein mixture and starch-lipid/protein ternary mixture were constructed to investigate the mechanism of wheat starch-based food quality deterioration and the key points of product quality control during the freeze-drying process.The results showed that the retrogradation of starch in the process of dehydration,the weakening of the binding capacity between the substrate and water of the freeze-dried product and the weakening of the interaction strength between the components of the product were the key mechanisms leading to the quality deterioration of wheat starch-based prepared foods during freeze-drying.And the addition of lipid and protein can inhibit starch retrogradation during freeze-drying,but the inhibition effect of lipid is better.However,the intervention of lipid will cause dehydrated starch to re-adsorb the melted oil/water mixture during the freeze-drying process,resulting in the deterioration of product quality.The intervention of protein will improve the binding ability of freeze-dried wheat starch-based product matrix to water.In the ternary system,the intervention of protein will inhibit the absorption of dehydrated starch to the melted oil/water mixture in the freeze-drying process to a certain extent,but at the same time,it will weaken the inhibition effect of lipid on retrogradation of starch in the freeze-drying process.In addition,the lower drying temperature will lead to the decrease of the interaction strength between the components of ternary mixed system.Avoiding the melting of frozen materials during drying is the key control point to improve the quality of freeze-dried wheat starch-based products.2)Developed a new intelligent judgment technology of sublimation/desorption drying transition point and drying end point in the freeze-drying process of wheat starch-based prepared food based on low-field nuclear magnetic resonance(LF-NMR)and near infrared spectroscopy(NIR),and also discussed the influence of sublimation/desorption drying transition point on quality deterioration control of freeze-dried starch-based prepared food,so as to make a preliminary exploration of high-quality intelligent freeze-drying of wheat starchbased prepared food based on avoiding freezing materials from melting in the freeze-drying process.Model fitting results showed that BP-ANN models based on LF-NMR spectrum and NIR spectrum could accurately predict the moisture content(MC)of wheat starch-based food.BP-ANN model of LF-NMR-MC couldn’t capture the signal of free water during the early stage of FD(about 2 h),while BP-ANN model of NIR-MC had moisture content output values of wheat starch-based food during the whole FD process.The TPs-a and drying endpoint of wheat starch-based food during FD process could be accurately determined by comparative analysis of FD wheat starch-based food samples moisture content output values obtained from two models.3)Taking noodles as one of the typical examples of traditional wheat starch-based food,according to the quality deterioration mechanism of freeze-dried wheat starch-based prepared food,it is analyzed that the quality deterioration of freeze-dried products of prepared noodles is mainly caused by retrogradation of gelatinized starch during dehydration and the weakening of the binding capacity of freeze-dried products with water;At the same time,combined with the characteristics of product rehydration difficulty which due to the tight texture formed during noodle pressing,the quality deterioration control of freeze-dried prepared noodles based on laser micropore teratment combined with compound leavening agent was studied.Results demonstrate that the intervention of laser puncturing treatment and the addition of leavening agent mixture are all positive on reducing drying time and energy consumption of noodles,but the drying enhancement effect of puncturing treatment is better.Combining laser puncturing treatment with leavening agent mixture can further reduce drying time of noodles by up to34.15% and energy consumption by up to 33.17%.Although all single treatment methods can improve the quality of products including soaking time,rehydration rate and hardness,the effect is unsatisfactory.Hybrid strategies are found to have a synergistic effect on the production of high-quality freeze-dried instant noodles and according to the sensory evaluation results,the optimum conditions are as follows: 0.1 mm perforation diameter,10 mm spacing between perforations,2 mm depth and 3% leavening agent mixture(on flour basis).4)Taking jiaozi as another typical example of traditional wheat starch-based food,based on the complex cross-linking of dumpling skin starch components and dumpling stuffing liquid lipid components during the preparation and freeze-drying process of jiaozi,and according to the quality deterioration mechanism of the freeze-dried wheat starch-based food system,it was found that the main reason for its quality deterioration was the absorption of dehydrated starch by melted oil/water mixture.At the same time,considering the unsynchronized drying of jiaozi’s skin/stuffing caused by its complex spatial structure,different cooking methods combined with chitosan coating were studied to regulate the quality deterioration of freezedried conditioned jiaozi.Results showed that,the drying time of steamed dumplings was the shortest(720 min),but its whiteness and rehydration performance were unacceptable to consumers.The whiteness of boiled dumplings was the highest,but its drying time was 90 min longer than that of steamed dumplings.The freeze-dried dumplings pretreated by microwave cooking had the best rehydration performance(rehydration rate,cracking rate of rehydrated dumplings and rehydration loss rate)and hardness.Chitosan coating treatment of cooked dumplings was being found to have a satisfactory positive effect on inhibiting the cracking rate of freeze-dried dumplings,the cracking rate of rehydrated dumplings and the rehydration loss rate of freeze-dried dumplings.Microwave cooking combined with chitosan coating was considered to be the most suitable hybrid strategy for the production of high-quality freezedried dumplings.5)Taking cream mushroom soup as another typical example of traditional wheat starchbased food,according to the quality deterioration mechanism of freeze-dried wheat starchbased conditioned food,it is found that the main reason for its quality deterioration is that the addition of protein weakens the inhibition effect of lipid on starch retrogradation and the weakening of the binding capacity of freeze-dried products with water;At the same time,combined with the demand characteristics of high viscosity and rich flavor of soup products,a novel three-stage combined drying strategy of cream mushroom soup with microwave(MD)+freeze-drying(FD)+ microwave(MD)based on starch retrogradation control was constructed.Results showed that,the drying uniformity of cream mushroom soup was not affected by complex food components.In order to avoid the occurrence of starch reincarnation,the time when the material is in the temperature and moisture content interval where starch is easy to regenerate should be reduced as much as possible.As a result,a hybrid drying strategy that MD+FD+MD was proposed for cream mushroom soup dehydration to reduce the drying time and avoid viscosity decrease of products on the premise of keeping product quality.6)In order to simplify the dehydration process,improve the drying efficiency and meet the demand of industrial modular production,the research on the drying efficiency and quality deterioration of cream mushroom soup based on microwave pre-gelatinization(MPG)and infrared freeze-drying(IRFD)was carried out.Compared to FD,IRFD reduced the drying time and energy consumption by 23.81% and 28.43%,respectively.The microwave pregelatinization of wheat flour and subsequent use in IRFD,while in comparing with single infrared freeze drying,did not affect drying time and energy consumption;however,it significantly improved the flavor and viscosity of soup upon rehydration,and reduced the tendency to retrogradation.The microwave pre-gelatinization time of 120 s at 8 W/g energy level was found to be the most suitable condition for microwave pre-gelatinization of wheat flour.