Study On Mechanism Of Crust Formation In Simultaneous Catalytic Infrared Blanching And Drying Of Carrot Slices

In fruit and vegetable processing,the traditional hot water blanching has some limitations,such as long blanching time,low heat transfer efficiency,as well as causing loss of water-soluble nutrients,water consumption and potential environmental problems.Catalytic infrared technology（CIR）has gained ever-insreasing attention and application in fruit and vegetable blanching due to its advantages of high efficiency,energy-saving and environmental protection,and the changes in fruit and vegetable texture caused by infrared blanching are worthy of further study.In this paper,the optimal blanching conditions for different thicknesses of carrot slices were screened using a single-plate catalytic infrared device.The crust formation on the carrot surface induced by CIR blanching was visually demonstrated.Furthermore,the effect of infrared blanching on the cell structure of carrots was investigated at the cellular level to reveal the mechanism of this method on the textural properties of carrots.The main contents and conclusions of the study are as follows:（1）Study on the dynamic changes of surface crust formation and quality of carrot slices by CIR blanching and simultaneous drying.In this study,carrot slices of different thicknesses（3,5 and 7 mm）were blanched and blanched at different distances（10,15 and 20 cm）using a CIR device to evaluate the quality of the blanched carrot slices.Besides,the dynamics of the shell formation phenomenon was characterised using scanning electron microscopy and texture analyzer.The results showed that CIR blanching had a good inactivation effect on carrot peroxidase.The optimum blanching distance and treatment time for different thicknesses of carrot slices was determined by 90%inactivation of peroxidase for reference.Scanning electron microscopy images showed that CIR treatment caused the surface of the carrot slices to collapse,with the degree and extent of collapse gradually increasing as the blanching progressed.The results of the texture analyzer showed that the internal hardness of the carrot slices decreased after blanching,and the surface crusted as the blanching progressed,with the hardness of the crust increasing as the blanching progressed.The surface hardness at the end of the blanching process was 25.29-38.2%of the hardness of fresh sample.（2）Study of the effects of CIR blanching on the electrical impedance characteristics,water distribution and cell morphology of carrots.In order to investigate the causes of the crusting of carrot surface by CIR blanching,changes in carrot during blanching were investigated using electrical impedance spectroscopy（EIS）,low-field nuclear magnetic resonance（LF-NMR）,paraffin sectioning and image processing.EIS showed that carrot cells were gradually destroyed during blanching,and the extracellular fluid resistance and cell membrane capacitance were significantly reduced.The cells were more intensely damaged in the early stages of blanching,and the crust on the surface was initially formed after 7 min of blanching and then became increasingly dense.At the same time,the rate of electrolyte leakage tended to increase as the blanching progressed,reflecting the disruption of the permeability and integrity of the carrot cell membrane.LF-NMR results showed that water loss was dominated by free water,and the relative content of semi-bound water increased significantly after blanching;the relaxation times of free and semi-bound water in carrot cells increased slightly at the beginning of the CIR treatment,and then decreased for all three types of water,indicating an increase in the rate of proton exchange between water molecules and the environment.Paraffin sections and image processing quantified the crumpling and deformation of cells during blanching as numerical values,showing a decrease in cell area and an increase in roundness values and lengthening factors,indicating that carrot cells gradually became elongated and irregular to varying degrees from the near-round shape of untreated carrot tissue.Among them,the cell area of the most surface layer of carrot slices decreased to 19%of fresh cells,perimeter decreased to 74%of the original,lengthening factor and roundness increased by 61%and 183%,respectively.（3）Study of the effect of CIR blanching on the cell wall composition of carrot.The effect of CIR blanching on the cell wall composition of carrot was investigated by dividing the treated carrots into crust and non-crust fractions.The polysaccharide content,monosaccharide composition,molecular weight and nanostructure of pectin were determined.The results showed that the alcohol insoluble residue（AIR）of the crust fraction of the cell wall showed better thermal stability and increased hemicellulose content.After blanching,the proportion of chelator-soluble pectin（CSP）in the pectin polysaccharides increased by 7.55-8.48%and the relative content of Na₂CO₃-soluble pectin（NSP）decreased in the crust fraction,both of which play an important role in texture maintenance.Structural analysis of the pectin fractions showed that the CIR blanching treatment increased the degree of branched pectin and cross-linking,leading to the formation of dense crust on the surface of the carrot slices,and that the hardness of the crust increased as the treatment progressed.In summary,the simultaneous CIR blanching and drying treatment caused the permeability and integrity of the carrot cell membranes to be disrupted and the cell expansion pressure to gradually disappear;the cells underwent shrinkage and deformation,the extent of which increased with blanching time and CIR radiation intensity,resulting in overall shrinkage and crust formation on the surface of the carrot slices.The crust fraction produced by blanching has a higher thermal stability of AIR,a higher relative content of hemicellulose and chelating pectin,and an increased degree of branched pectin fraction.The initial clarification of the crust formation mechanism of carrot slices during simultaneous CIR blanching and drying provides a theoretical and scientific basis for the regulation of fruit and vegetable quality during IR blanching.