Textural Formation Mechanism And Control Technology Of Freeze-dried Restructured Fruit And Vegetable Cubes

As one of the most important categories of green,natural,nutritious,and healthy high-end snack foods in the recent food industry,freeze-dried fruit and vegetable products are mainly circulated in the food market in the form of intact FD products and restructured FD products.Texture characteristics are important quality of FD fruit and vegetable products,which directly affect the packaging,storage,transportation,sales,and taste of the products.Due to different processing processes,these two FD fruit and vegetable products exhibit different textural characteristics.However,the texture formation mechanism and control technology of the restructured FD fruit and vegetable products are still an open problem.Therefore,this study selected the dominant influencing factors that determine the texture of freeze-dried fruits and vegetables through a comprehensive characterization of the physicochemical properties of fresh or freeze-dried samples,as well as further correlation analysis.Subsequently,the core reasons for the textural differences between intact FD products and restructured FD products are further explored.In addition,an effective textural control technology will be developed by establishing a pectin simulation system.Finally,the control methods developed in this study were verified in a real restructured apple system to determine its effectiveness and feasibility,providing the theoretical basis and technical guidance for revealing the texture formation mechanism of natural and restructured FD products and the textural improvement of restructured FD products.The specific research content and main results are as follows:(1)In this chapter,12 kinds of fresh fruit and vegetable materials were selected,and the chemical composition and tissue morphology of these 12 fresh/freeze-dried samples were systematically characterized.Correlation analysis was further conducted to determine the texture-influencing factors of traditional freeze-dried fruit and vegetable products.The results exhibited that the cell wall polysaccharides and mono-/disaccharides present in the fruit and vegetable matrix play as the main determinants of the texture quality for FD fruit and vegetable cubes.Specifically,cell wall polysaccharides play as a porous network structure scaffold to support the freeze-dried cubes.Starch granules serve as fillers distributed in the pore space,inhibiting structural collapse and volume shrinkage.Mono-/disaccharides were responsible for the hardness of freeze-dried cubes,and the excessive presence of mono-/disaccharides may promote structural shrinkage and the loss of pores during freeze-drying.(2)Apples,strawberries,and mangoes were obtained as examples to prepare the natural FD samples and restructured FD samples,determining the core reason for the difference between the textural properties of these two types of FD products.The results show that the core reason for the difference between the two types of FD products is the existence of intact cell structure.The intact cell structure in natural fruits could hinder the release of water vapor from the inside of the material,aggravate structural collapse,reduce the freeze-drying rate,and increase the water activity of the final product.Intact FD products generally displayed a freeze-drying time of 20 h,while restructured FD products were extended to 30 h.The pore characteristics of the restructured freeze-dried cubes were mainly affected by the growth behavior of ice crystals,forming an open porous structure to inhibit structural collapse,accelerate the water loss rate,and reduce the water activity of the product.In addition,restructured FD products showed better chewiness,crispness,and overall acceptability scores than natural FD products in sensory evaluations.However,restructuring technology also reduced the structure hardness of the final product.For example,the hardness of restructured mango is about 3.5 N lower than that of natural mango,and the hardness of restructured strawberry is about 6 N lower than that of natural strawberry.(3)A pectin-xyloglucan(Xy G)restructured freeze-drying simulation system was established based on pectin,and samples with or without the gel network were prepared to explore the impact of the remanufacturing of the gel network scaffold on the textural properties of the restructured FD system.The results show that the reconstructed gel network scaffold could significantly increase the crispness of the FD simulated cubes from 1-1.83 to 16.25-32.88.Furthermore,the gel scaffold reconstruction also could enhance the smoothness and continuity of the pore walls of the FD restructured system,and exhibit a microstructure that is highly similar to the porous structure of natural FD products.Xy G could cross-link with pectin chains via hydrogen bond interactions,and participate in the formation of the pore walls within the restructured simulation system.For example,the addition of 0.8%Xy G increased the hardness of simulation systems with gel network from 1.52 N to 3.97 N,while simultaneously increasing the hardness of simulation systems without gel network from 1.65 N to 4.65 N.Furthermore,the addition of high-content Xy G also inhibited the occurrence of structural collapse and volume shrinkage of the restructured simulation system during the FD process.(4)Based on the pectin-restructured gel simulation system,starch with different degrees of gelatinization was added to explore the possibility of starch addition becoming a directional control method for the texture quality of restructured freeze-dried products.Two gelatinization methods were considered,namely starch gelatinization occurring before or after pectin gelatinization.The results show that highly gelatinized starch reduced the pore size of the pectin-starch restructured gel simulated cubes,increased the number of pores in the microstructure of cubes,and expanded the specific surface area of the simulated cubes from 125.13 m~2/g to 259.33 m~2/g.Ungelatinized starch granules act as fillers within the pore space of the simulated gel cubes,reducing the crispness properties.When starch gelatinization occurred before pectin gelatinization,starch(3.0%w/v)with a gelatinization degree greater than 73.8±4%increased the hardness of the simulated system by 8-11 N.The gelatinized starch chains would cross-link with pectin chains to form composite pore walls,thereby increasing the hardness and compression resistance of the porous structure.When starch gelatinization occurred after pectin gelatinization,the pectin gel network would delay the starch gelatinization process and weaken the texture optimization ability of gelatinized starch.(5)Taking apples as an example,the gel network scaffold was remanufactured and starch was added in a real apple restructured system to determine the effectiveness and feasibility of the texture control method developed in this study.The results show that the reconstruction of the gel network could significantly improve the hardness,structural integrity,pore wall continuity,and external force resistance of the restructured apple cubes.For example,FD apple cubes with a gel network show 1.5-2 times higher hardness and 3.5-4 times higher crispness than FD apple cubes without a gel network.Meanwhile,by remanufacturing the gel network scaffold and adding gelatinized starch,the restructured FD apple cubes not only obtained higher hardness,crispness,and acceptability scores in the sensory evaluation,verifying the effectiveness and feasibility of the control technology developed in this study.