Change And Mechanism Of Asparagus Lettuce Texture Under High Pressure Processing

High pressure processing (HPP) is a novel non-thermal processing technology. It can prolong the shelf life of food while the original nutritional and sensory properties are maintained due to its limited effects on covalent bonds. However, there is an adverse effect on the texture of fresh-cut vegetables by HPP. The morphological changes of cell, the mechanical and oxidative damage of cell membrane and the physicochemical properties of pectin by HPP were investigated. Based on the results, we proposed the mechanism of high pressure processing in influencing the texture of fresh-cut asparagus lettuce, so as to provide the theoretical basis for the application of HPP in fresh-cut fruit and vegetable industruy. The main results and conclusions were summarized as follows:(1) The change of the asparagus lettuce texture by HPP was studied by the different evaluation methods. The results showed that the hardness of asparagus lettuce was decreased significantly after HPP (p<0.05), and mainly at300MPa in comparison with the untreated samples. However, the cutting force and cutting-force displacement were significantly increased after HPP (p<0.05). Maximum sound pressure level (SPL) was not significantly different for the treatment at100MPa, but for300MPa and500MPa, the significant difference was observed. The correlation analysis indicated that acoustic detection was an effective evaluation method for the crispness of asparagus lettuce by HPP. In conclusion, the texture of asparagus lettuce was kept well at100MPa,500MPa came second and damaged at300MPa.(2) The changes of cellular morphology and membrane structure of asparagus lettuce by HPP were studied. Compared with the untreated samples, the cell area and elongation indices were decreased and the shape factor was increased at100MPa and300MPa with the increase of pressure. However, the cell morphology recovery occurred at500MPa, indicating the hardness of asparagus was kept at500MPa. Neutral red and trypan blue staining showed that HPP induced the change in the integrity of cell membrane. Intact cells membrane was observed at100MPa. Membrane rupture was clearly identified at300MPa or above. Compared with thermal processing, HPP induced the cellular oxidative stress. The deep DAB staining, the high MDA content, hydrogen peroxide content and peroxidase activity were observed at300MPa treated samples. These results indicated that the cell membrane oxidative damage might occur at300MPa. Therefore, the oxidative stress is also an important factor in HPP influencing the asparagus lettuce texture.(3) The biochemical properties of pectin and pectin fractions and the change of cell wall structure were studied. After100MPa and300MPa treatments, the content of galacturonic acid and PME activity were not significantly changed. While after500MPa treatment, the content of galacturonic acid and PME activity were increased and the DE value was decreased. With the increase of pressure, the content of WSP and NSP increased by accompany with the decreasing CSP content. The molecular weight distribution of WSP revealed the proportions of low MW fragments increased after HPP. FTIR showed that group composition and its relative content of pectin were unchanged. Transmission electron microscopy (SEM) image showed that the tricellular junctions of adjacent cells, middle lamella and cell wall structure were integrated after HPP. The plasmodesmata disappeared with the increasing pressure. However, the middle lamella and cell wall structure were damaged after the thermal treatment. In conclusion, HHP preserved the texture of asparagus lettuce by lowering the estified degree of pectin, while the texture of asparagus lettuce was damaged by the pectin degradation in thermal processing.(4) Anti-homogalacturonan antibodies were used to perform ex situ (immuno-dot assays) and in situ (microscopy) analyses on asparagus lettuce pectin which resulted in information concerning the localisation of defined pectic domains in asparagus lettuce cell walls and pectin’s structure. Immune-dot results showed that WSP contained unbranched, moderate esterified pectin and some pectin polymers with abundant unesterified side chains. In terms of CSP fraction, the results showed that there was a wide range of DE in CSP fraction. The average value of DE and the pectin content with different DE were low. CSP mainly contained low esterified pectin with high branch structure. Immune fluorescence results showed that the immune fluorescence intensity with JIM5and LM19was increased, indicating the content of pectin with low estified degree was increased. The fluorescence with LM20mainly focused on the tricellular junctions of adjacent cells after100MPa and300MPa treatments, but not at500MPa. The fluorescence with2F4indicated that the content of calcium crosslinked pectin was increased at500MPa. These results further proved that the texture of asparagus lettuce was kept well at500MPa.(5) The texture change of asparagus lettuce by extreme high pressure pressure (800-1200MPa) was caused by a combiantaion of pressure and temperature. Significant decrease occurred in the hardness, the cutting force, the cutting force displacement and the maximum SPL of asparagus lettuce (p<0.05). However, there were no significant difference among800MPa,1000MPa and1200MPa. The immune fluorescence showed that pectin was dissolved into the tricellular junctions of adjacent cells and middle lamella by the high pressure and temperature. SEM image showed that cell wall dissolved and distorted with the increase of pressure.