Preliminary Study On Formation Mechanism Of Sulfur Dioxide And Acetaldehyde In Brewer's Yeast

Beer staling is one of the key issues,which urgently needs solving in the beer industry.Brewer's yeast is essential for brewing and produces a range of flavor compounds during fermentation.SO₂ is the most effective antioxidant and flavor stabilizer while acetaldehyde with the highest component of aldehydes is directly related to beer staling.Besides,high level of acetaldehyde usually accompanies by the increase of other aging aldehydes.Studies have shown that the relationship between SO₂ and acetaldehyde is strongly correlated but the reason is still unknown.Strains produce more acetaldehyde after the improvement of the capability of sulfite production,which restricts the application in the beer industry.SO₂ is the intermediate metabolite of the sulfate assimilation pathway?SAP?and acetaldehyde plays important roles in alcohol metabolism,both of which are closely linked to energy metabolism in the cell.This study focused on the mechanism and correlation of SO₂ and acetaldehyde production to provide the theoretical basis for improving the anti-staling ability of brewer's yeast.Firstly,the relationship between SO₂and acetaldehyde production was verified.Transcriptome analysis showed that serine and MET5gene had a certain influence on SO₂ and acetaldehyde production.Main results are as follows:?1?Correlation between SO₂ and acetaldehyde production:Lager yeast M₄¹?high sulfite and acetaldehyde production?and its mutant D-A-14?comparably low sulfite and acetaldehyde production?were chosen for the further study.Three amino acids,including methionine,cysteine and threonine,were respectively added into the wort to analyze the changes in SO₂ and acetaldehyde production.Although the addition of three amino acids had different effects on the production of both compounds,Pearson correlation analysis showed the significantly positive correlation between SO₂ and acetaldehyde production?r=0.502?.?2?Transcriptome analysis between strains M₄¹ and D-A-14:Metabolic pathways of two strains differed in sugar metabolism?glycolysis and gluconeogenesis?,protein,and amino acid metabolism?sulfur amino acid metabolism?.The result displayed that SAP was significantly down-regulated in high sulfite-producing strain M₄¹.Meanwhile,the down-regulation of MET5gene?0.35-fold?could lead to low sulfite reductase activity,causing that less SO₂ was reduced to biosynthesize sulfur amino acids in strain M₄¹ than D-A-14.Over 80%of differentially expressed genes in glycolysis and gluconeogenesis were up-regulated,indicating the increased supply for acetaldehyde.Besides,the metabolic flux of serine could influence carbon and sulfur metabolism,thereby affecting SO₂ and acetaldehyde production.?3?Effects of serine on SO₂ and acetaldehyde production in lager yeast:Serine could enhance SO₂ and acetaldehyde production,but the regulation levels were different in both strains.Strain M₄¹which initially had a low flux in SAP produced 1.90-fold SO₂,as SAP was significantly induced with 200 mg·L^-1 serine treatment.Netherless,acetaldehyde production did not change much,which improved the anti-staling ability of beer.PDC1?2.43-fold?and PDC5?2.41-fold?genes were up-expressed after serine treatment in strain D-A-14 which had a high flux in sulfate assimilation,probably causing a rise in acetaldehyde and ethanol production.However,serine addition had little effect on SAP of strain D-A-14.?4?Effects of MET5 gene on SO₂ and acetaldehyde production in lager yeast:SO₂ production increased by 52.7%(5.42 mg·L^-1)and acetaldehyde production correspondingly increased after the disruption MET5 gene in strain D-A-14.MET5 gene was down-regulated to 0.31 and SAP was significantly down-regulated in strain D7 by qRT-PCR.The up-regulation of PDC5 gene?4.20-fold?could promote acetaldehyde biosynthesis.Furthermore,amino acid sequences of MET5 gene between strains M₄¹ and D-A-14 could also affect sulfite reductase activity.