Research On Dominant Flavor Compounds In Cloudy Wheat Beer

Cloudy wheat beer,as a high-end beer with unique charm,has been favored by consumers in recent years.In order to study dominant flavor compounds,physicochemical and flavor substances of domestic cloudy wheat beer were analyzed.In addition,the saccharification process and wort components were studied in order to reveal the influences on 4-vinylguaiacol（4-VG）,isoamyl acetate（IAA）in cloudy wheat beer.Furthermore,the contents of 4-VG,IAA and off-flavor substance dimethyl sulfur（DMS）in brewing process were monitored.The main results are as follows.（1）The key flavor substances in cloudy wheat beer were 4-VG and IAA.DMS was an important parameter for domestic brewers to pay more attention on brewing cloudy wheat beer.Thirty-one volatiles were separated from cloudy wheat beer by gas chromatography-ion mobility spectrometry（GC-IMS）.Twenty-seven volatiles were qualitatively identified,among which the substances with high abundance were isoamyl alcohol,isobutanol,EA,IAA,ethanol,acetaldehyde,etc.The substances with the greatest difference were mainly esters:ethyl caproate,isobutyl acetate,ethyl octanate,ethyl butyrate,etc.The key flavor substances in the cloudy wheat beer were 4-VG and IAA.The order of odor activity values（OAV）was IAA>4-VG>ethyl acetate.According to the sensory evaluation results,the main flavor substances and their concentration in 8.2～12.4 ^oP cloudy wheat beer with typical clove and banana aromas were recommended:4-VG ranged from 2.27to 4.00 mg/L,IAA was 1.51 to 5.66 mg/L and its OAV value was 1.26 to 4.71,and the OAV ratio of IAA to 4-VG was 0.86 to 2.00.There was significant correlation between the content of DMS and beer type（p<0.05）.The value of DMS in 25%of Lager beer exceeded the threshold of 60μg/L,with a mean value of 45.66μg/L.However,the value of DMS in 71%of wheat beer exceeded 60μg/L,with a mean of 71.61μg/L.Some samples were tasted by sweet corn taste of dimethyl sulfur.Therefore,DMS was an important parameter for domestic brewers to pay more attention on brewing the cloudy wheat beer.（2）The resting time at 43,50 and 63℃ was the key control point for modulating 4-VG content in the cloudy wheat beer while the time at 63℃ was the key control point for IAA.The results of orthogonal experiments showed that the resting time of 43,50 and 63℃ had a significant influence on the production of 4-VG（p<0.05）,and the order was resting time at 43℃>resting time at 50℃>resting time at 63℃.The 4-VG was positively correlated with free amino nitrogen of coefficient 0.51（p<0.05）and total nitrogen of coefficient 0.83（p<0.05）.The resting time of ferulic acid（FA）at 43℃ was the key control point for regulating the content of 4-VG in beer.In addition,the resting time at 50℃ and 63℃ can also be used to regulate the content of 4-VG in beer.Resting at 63℃ had the most significant effect on IAA,ethyl acetate and total ester（p<0.05）.The correlation coefficient of IAA content with wort glucose and maltotriose contents was more than 0.9（p<0.01）and maltose content was 0.71（p<0.05）.The content of major oligosaccharides and glucose in wort was significant positively correlated with the content of IAA（p<0.05）in fermentation broth.Therefore,the saccharification time at 63℃ was the key control point for IAA content in cloudy wheat beer.（3）DMS and DMSP contents in malt,wort boiling time and the time from the end of wort boiling to the end of wort cooling were the key points to control DMS content in wort.In commercially malt samples the content of S-methyl methionine（SMM）and dimethyl sulfoxide（DMSO）were significantly correlated with the malt types.There were significant differences in DMS and DMSP contents among domestic wheat malts（p<0.05）.The DMS and SMM contents in malt were key points to control DMS content in wort.Wheat malts with free DMS content<2.0 mg/kg and SMM content<5.0 mg/kg should be selected.The amount of DMS in the boiling stage（100℃）from 0 to 40 min decreased rapidly when the p H value was 5.6,and DMS reached to 30μg/L by boiling for 40 min.The half-life of SMM was 31min,and the first-order kinetic reaction index function was C^t=440.07e^-0.023t（R2=0.9907）.In order to reduce the content of DMS in wort effectively,the boiling time of wort should be no less than 40 min.DMS content in cooled wort could be reduced by shortening the high temperature period from the end of wort boiling to wort cooling.（4）During the 2^nd～5^rd days,4-VG and IAA increased rapidly.The content of DMS rose rapidly in the first two days.DMS was firstly increased and then declined rapidly.During fermentation,the formation of 4-VG and IAA was relatively slow because the beer yeast multiplicated on the first day.During the 2^nd～3^rd days,fermentable sugars reduced rapidly because yeast began to anaerobic fermentation.Rapid catabolism of FA was from the2^nd to 5^th days,while 4-VG and IAA increased rapidly.The content of 4-VG and IAA tended to be stable after 5 days fermentation.DMS content was elevated rapidly in the first two days,and the highest value 73μg/L was obtained on the second day.After that,DMS was volatilized and decreased rapidly with releasing of CO₂,and the lowest value 43μg/L was reached on the third day which was only59%of peak value.The content of DMS tended to stable with the sugar decrease became slowly by the yeast after 7.5 days fermentation while increased during the storage period.