The Influence On Wheat Beer Flavor Of Top-fermenting Yeasts Recovery Methods

Wheat beer was brewed with wheat?more than 40%of the malt?and water as the main ingredients.With the improvement of people's living standards and the change of consumption concept,wheat beer was popular with consumers from all over the world due to its unique ester aroma?ethyl acetate and isoamyl acetate?and phenol aroma?4-vinylguaiacol and 4-vinylphenol?.Wheat beer was considered as a very important top-fermented specialty beer,particularly in Germany.Serial repitching of brewing yeast was an important operation in the brewing industry,as their propagation was time consuming and can damage economic resource.Therefore,adopting an effective recovery regime was of great significance to the brewery.In this article,we investigated the effects of two different yeast recovery methods on the yeast cell morphology and final wheat beer quality,including recovering yeast from the sampling valve during the main fermentation period and recovering yeast from the cone bottom of the fermentation tank after cooling.The experiments were performed on a small scale with three 100 L cylindro-conical fermenter?CCF?to conduct three sets of parallel tests.Sampling at three different time points:?1?the yeast cells had just been repitched to the fermenter;?2?the fermenter had just cooled down to 0?;?3?the yeast cells being kept in cold storage for 5 days,the morphology of top-fermenting yeasts were observed by scanning electron microscope?SEM?.The yeast cells settled down at the bottom of the cone were affected by stress factors such as low temperature,alcohol,fermenter pressure,osmotic pressure and hydraulic pressure.Comparing the yeast recovered by the sampling valve?the 0th generation yeast is denoted as G0?and the yeast recovered from the bottom of the conical tank?1st generation yeast is denoted as G1 and 2nd generation yeast is denoted as G2?,the cell wall folds were more obvious and the degree of depression was increased.As the number of yeast serial repitching increased,the number of bud scars on the cell wall surface increases,the degree of cell wrinkles and cells sag inward deepens,and the cell morphology becomes irregular.The whole experimental process was 15 days contains fermentation and maturation.A detailed follow-up study was conducted on changes in the physical characteristics of the yeast cells?average cell diameter,cell aggregate rate,and cell viability?,the apparent gravity of fermentation liquor,alcohol and diacetyl content during the 15 days sampling process.The results showed that the wheat beer brewed by yeast was recovered from the bottom of the CCF,the number of cells suspended in the fermentation broth during fermentation was reduced,the fermentation time was increased,and the sugar reduction rate of the fermentation broth was slowed.The yeast cells recovered by the sampling valve produced the least amount of alcohol and the fermentation broth first reached the peak of diacetyl.The physical and chemical indexes of finished beer,including total yeast concentration,cell viability,cell aggregate rate,cell diameter,alcohol,lactic acid,bitterness,color,fermentable sugar,CO₂ content,total acid,turbidity.The number of suspended cells in the final wheat beer brewed from the yeast recovered from the cone bottom was higher,and the contents of lactic acid,total acid,turbidity,fermentable sugar and alcohol were higher.Detect the characteristic flavor 4-vinylguaiacol?4VG?in wheat beer by high-performance liquid chromatography?HPLC?,and analyze the volatile flavor substances in the final wheat beer by headspace solid phase microextraction gas chromatography?HS-SPME-GC?,which include isoamyl alcohol,n-propanol,isobutanol,ethyl formate,ethyl acetate,isobutyl acetate,ethyl formate,isoamyl acetate,ethyl caproate,ethyl caprylate,acetaldehyde,DMS.The results showed that the content of acetaldehyde,isobutanol and isoamyl alcohol in final beer brewed with yeast from different recovery methods had significant differences?P<0.05?.The content of acetaldehyde,isobutanol and isoamyl alcohol in the final beer brewed by the yeasts recoverd from the sampling valve were 2.07 mg/L,79.63 mg/L and 44.36 mg/L.The content of acetaldehyde,isobutanol and isoamyl alcohol in the final beer brewed by the yeasts recovered from the bottom of the CCF were 1.02 mg/L,98.54 mg/L and 48.39mg/L,all values were within the normal content of light beer.Sensory evaluation of the final beer,it was found that the acidity of the finished wheat beer brewed by recovering the yeast from the bottom of the conical fermentation tank for re-inoculation was obvious,and higher levels of higher alcohol cause significant headache.In addition,continuous repitching had exacerbated these effects.