| Saccharomyces cerevisiae contains large amounts of nutrients such as reducing sugars,nucleotides,amino acids and vitamins,especially the protein content is over 50% of the dry weight.However,the low bioavailability of single-cell proteins has greatly limited the application of yeast.Therefore,it is necessary that yeast protein is degraded into amino acids that can be directly digested and absorbed by human and animals.Autolysis is an effective method to improve the biological titer of yeast protein.However,the natural autolysis has some defects such as long-term autolysis process,incomplete cell wall breakage and low product yield.In recent years,in order to accelerate yeast autolysis,many methods for inducing autolysis have been continuously developed and utilized,such as adding chemicals(inorganic salts),acid hydrolysis,adding exogenous enzymes and applying high-pressure homogenization.However,the feasibility of these approaches is limited because of health hazards,inhibition of amino acid production,high cost and environmental pollution.With the development of pulsed electric field(PEF)technology in non-thermal sterilization of liquid food,there are studies using pulsed electric field to extract proteins and nucleic acids from yeast cells.However,few studies on promoting yeast autolysis by PEF are published and the mechanism is still unclear.In order to investigate the feasibility of PEF in inducing yeast autolysis,the lethal effect of pulsed electric field technology on the S.cerevisiae cells were observed at a microscopic level by using scanning electron microscope and transmission electron microscope,and then the treated cells were autolyzed at a specific p H and temperature.The dissolution of nucleic acids,proteins and proteases during autolysis was monitored.Finally,the nutritional characteristics(such as amino acid content)of the autolysates obtained after autolysis and the sensory characteristics(such as flavor)of the yeast extract after Maillard reaction were compared with those produced by other autolysis methods.The specific research content and results are as follows.1.The injury extent of S.cerevisiae cells of PEF was strengthened with increasing electric field strength.At electric field strength of 3 k V/cm,only a small number of cells were disrupted with slight cell wall destruction.When the field strength was 7 k V / cm,the cell lethality reached 99.43%,corresponding to a higher cell disintegration index Z,which indicated that electroporation of S.cerevisiae cells at this time.2.The electroporation effect of PEF on cells was beneficial to the release of intracellular macromolecular substances.The activity of extracellular proteases in the autolyzed supernatants of PEF-treated cells was significantly higher(p < 0.05)than that of the untreated samples.Furthermore,the yield of amino nitrogen increased by 7.11% after exposure to 7 k V/cm.PEF destroyed the cell structure and promoted the release of intracellular proteases,leading to the degradation of proteins into small molecules such as peptides and amino acids.3.PEF treatment of 7 k V/cm and papain could increase the total amount of amino acids in autolysates by 1.49 and 1.65 times respectively,compared to natural autolysates.The content of glutathione in autolysates produced by PEF was 4.9 times higher than papain group,and the reducing sugar content was only 66.7% of the papain group.The autolysates of PEF exhibited good nutritional properties as dietary supplements.Papain can increase the content of glutamic acid by 66.2% on the basis of the PEF group,and the obtained autolysates can be used as a precursor substance for the production of flavor enhancers.4.The autolysates obtained by the three different autolysis methods were submitted to Maillard reaction,the PEF-induced autolysates presented the darkest color and the best antioxidant activity,which indicated the potential of PEF in producing yeast extracts.These results indicated that PEF could accelerate the autolysis of S.cerevisiae,shorten the autolysis duration,and increased amino nitrogen yield of autolysates.The higher amino acid content made the autolysates exhibit good nutritional and taste characteristics,which could be further produced into flavored yeast extracts or nutritional supplements. |
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