Fresh-cut Bamboo Shoots During Cold Storage: Lignification Mechanism And Preservation Coated By Chitosan Films

Fresh-cut bamboo shoot (Phyllostachys praecox f. prevelnalis) was used to investigatethe lignification mechanism during cold-storage, and shelf-life extension coated with achitosan/whey protein edible film (CPI). The activities of phenylalanine ammonialyase(PAL), peroxidase (POD) and polyphenol oxidase (PPO) and the contents ofmalondialdehyde (MDA), polyphenol and lignin in fresh-cut bamboo shoots stored at4℃were measured and the lignification mechanism was deduced. In this presented study, theCPI film was prepared by mixing chitosan (CTS) dissolved in0.1N acetic acid and wheyprotein isolate (WPI) dissolved in deionized water under continuous stirring．In order tooptimize the conditions preparing CPI film, the effect of pH value, the amount of glycerol,temperature, and WPI concentration on the CPI film were evaluated, the tensile strength(TS), breaking elongation rate (E%), water vapor permeability, and light transmittancewere determined, and the physico-chemical properties of CPI film were characterized byDSC, SEM, and FTIR. Moreover, the influences of molecular weight (MW) anddeacetylation (DD) of chitosan on the mechanical properties, water absorption and lighttransmittance of CPI films were also discussed, and the quality of fresh-cut bamboo shootswas evaluated according to the endogenous enzyme activity, polyphenol, malondialdehyde,lignin content, and total number of colonies changes. The first objective of the presentedresearch was to investigate the lignification mechanism of fresh-cut bamboo shoot duringcold-storage. Secondly, the conditions preparing CPI film were optimized and thephysico-chemical properties were also characterized. Thirdly, the influences of CPI film onlignification and quality of fresh-cut bamboo shoot during cold-storage were evaluated.The presented results were to offer theoretical basis for the transport and storage ofbamboo shoot in the future. The main results were showed as following:(1) The activities of PAL and POD, MDA content tended to increase and thendecrease with increasing storage time. PPO activity slowly ascended during the first6days,and began to descend on the15th day. However, polyphenol content sharply declined during the first6days, then slow rose, and began to decline again on the12th day. Lignincontent showed a continuous increase, which was5%～26%in the bottom,3%～24%inthe middle and2%～22%in the top. These data suggest that the activities of PAL and PODconsiderably increase during cold storage, which are the key enzymes in the lignificationof fresh-cut bamboo shoots. Moreover, phenolic compounds are involved in the synthesisof lignin by providing precursors. Meanwhile, MDA may also be involved in thelignification of fresh-cut bamboo shoots, which needs further identification. Lignin contentgradually decreases from the bottom to the top of bamboo shoots, implying thelignification process proceeds from the bottom to the top.(2) The CPI film, which has9.74MPa of the tensile strength (TS),1.19%of breakingelongation rate (E%),0.351mg·cm-2·d-1of the permeability, and83.1%of the lighttransmittance was prepared at the optimal conditions including90%of degree ofdeacetylation and30,0000of molecular weight for chitosan,50%of WPI concentrations,1.5%of additive weight for glycerol, pH3for the CPI solutions, and60℃of mixingtemperature. The mechanical property and water permeability of CPI film were affected bythe degree of deacetylation and molecular weight of chitosan. At the same degree ofdeacetylation, the higher molecular weight forchitosan has the greater the tensile strength.The permeability first increased and then decreased, while the light transmittance declinedafter the first increase with the increase of molecular weight. If chitosan has the samemolecular weight, the change of tensile strength is the same as the effect of molecularweight. However, with the increase of the degree of deacetylation for chitosan, thepermeability of CPI film increased and light transmittance was slightly down.The results determined by using DSC showed that melting temperature and thermalenthalpy change of CPI films was much lower than CTS film. Scanning electronmicroscopy results found that the microstructures of CPI film were uniform and compact,and the appearance was semi-translucent. FITR results suggested that CTS and WPI wasnot a simple mixed in CPI films but the strong interaction between or within CTS and WPI molecules. The results characterized by using FTIR, SEM, and DSC indicated thatthere are good compatibilities between WPI and CTS, and proved that the mechanicalproperty and water permeability of CPI film were more improved than CTS film throughtheir microstructure and secondary structure.(3) Fresh-cut bamboo shoots were soaked in CPI solution (1,2,3times), CTSsolution (1,2,3times), deionized water, and acetic acid solution and stored at4℃for15days. The lignin contents were17.2%,16.8%,15.4%,8.1%,16.6%,15.7%,23.8%and21.4%, respectively. Total number of colonies were4.8×107CFU/g,7.2×106CFU/g,4.7×106CFU/g,4.1×107CFU/g,8.9×105CFU/g,1.3×105CFU/g,1.2×108CFU/g and9.1×107CFU/g.The highest activities of PAL and POD were1.557U/h·g,1.03U/h·g,0.888U/h·g,1.46U/h·g,1.57U/h·g,1.05U/h·g,1.37U/h·g,1.55U/h·g and141.0U/(min g),121.5U/(min g),101.4U/(min g),122.7U/(min g),125.6U/(min g),118.4U/(min g),119.3U/(min g),118.5U/(min g), respectively. The highest MDA contents were4.06μmol/g,2.13μmol/g,2.53μmol/g,3.813μmol/g,2.883μmol/g,2.95μmol/g,4.12μmol/g,3.79μmol/g, respectively. The results indicated that CPI and CTS film coated on the fresh-cutbamboo shoots can significantly reduce the total number of colonies and the contents ofMDA and lignin, inhibit PAL and POD activities, and extend the shelf life of fresh-cutbamboo shoots. CPI film coated on the fresh-cut bamboo shoots has much greater shelf-lifeextention than CTS. Moreover, their shelf-life was also extended with increase of coatingtimes.