Study On The Key Technology About Controlling Secondary Precipitation And Browning Of Litchi Beverage Fermented By Lactic Acid Bacteria

Secondary sedimentation and browning are the common technological problems in processing and storing the fermented litchi beverage.The sediments of litchi juice have more insoluble proteins and dietary fibers,which the main protein is the glutelin.The glutelin was only soluble in dilute alkaline rather than water,it may cause the instability of the litchi juice.In order to understand the reasons why the litchi juice is unstable better,we studied the structures and characteristics of the glutelin from litchi juice.Based on the litchi juice fermented by Lactobacillus casei will be more viscous,we explored whether the viscous fermented litchi juice can suspend the sediments individually,then we selected one stabilizer that can be suspended the sediments effectively.In addition,in order to control the browning of the litchi juice,we confirmed the main changed factors firstly by analyzing the changes of quality parameters when the fermented litchi juice got brown,then studied whether the dilution and adding inhibitor can inhibit the browning of litchi beverage.The main researched conclusions as follows:(1)The glutelin from litchi juice was purified using the osborne sequential protein extraction method,the distributions of molecular weight,thermal stability and secondary structures were analyzed using sodium dodecyl sulfate polyacrylamide gel electrophoresis,differential scanning calorimetry and infrared spectroscopy,respectively.The results showed that its molecular weight mainly concentrated in 50 kDa and more,and there were 10 bands in total so that its components were complicated;After reduced,some larger molecular weight bands disappeared,smaller molecular weight bands appeared.The surface hydrophobicity is represented by the capacity of combining sodium dodecyl sulfate,the results showed that 1mg litchi glutelin sample combined 74.25±5.26 ?g SDS;the content of free sulphydryl group was 29.83±1.72 ?mol/g,the total of sulphydryl was 40.59±2.04 ?mol/g,disulfide bond was 5.38±0.18 ?mol/g;The denatured peak was 105.24 ?.The main secondary structure was ?-sheet and ?-turn,occupied 33.92% and 64.7%,respectively.Hence,we inferred preliminarily the composition of glutelin in litchi was complex,the molecular weight was larger,and it had stronger surface polyacrylamide,fewer disulfide bond,and was stable to heat.(2)We researched preliminarily the fermented characters of the litchi juice fermented by Lactobacillus casei,the results showed that the viscosity of the litchi juice can be effected obviously by the sugar degree,pH value,fermented time and diluted ratio.The viscosity of the fermented litchi juice was 0.06 Pa·s under appropriate conditions.Unfortunately,the fermented litchi juice occurred the sediments obviously.It may caused when the viscosity was low.Then we studied the ability of the fermented centrifuged litchi juice suspended the sediments.The results showed the fermented litchi juice can not suspend the sediments.Comparing the pectin,CMC-Na and xanthan,CMC-Na also can not suspend the sediments,but the xanthan and B pectin can suspend some sediments.After researched further,the ability to suspend the sediments of the B pectin was better than the xanthan,0.15%(m/v)B pectin can suspend different diluted ratio(30%,50%,100%)fermented litchi juice with 5%(m/v)sediments.(3)The non-enzymatic browning reaction will be occurred during the litchi lactobacillus fermented beverage stored at room temperature,the maillard reaction and the polyphenol oxidative condensation reaction are the main reactions.When the litchi lactobacillus fermented beverage got brown,the physiochemical characteristics(total soluble solids,pH,titratable acidity),antioxidant capacity(ORAC),free phenols(rutin,narcissoside,quercetin,kaempferol-rutinose-rhamnoside and isorhamnetin-rutinose-rhamnoside)and partial free amino acids had no significant changes(p>0.05).5-HMF,total polyphenols,8 kinds of free amino acids(Ser,Glu,Ala,Val,?-ABA,Orn,1Mehis,a-AAA)and reducing sugars were the main changing factors when the litchi lactobacillus fermented beverage got brown.Dilution can not relieve the browning effectively.0.2%(m/v)sodium polyphosphate and 0.1%(m/v)sodium erythorbate can not control the browning of the litchi lactobacillus fermented beverage,but 0.05%(m/v)sodium sulfite can do it successfully,comparing of using alone and combinely,the former one was better.