Yeast in unfiltered beer (UFB) was inactivated by a two-stage system that was additionally pressurised at an ambient temperature after carbon dioxide microbubbles (MB-CO2) were mixed with UFB at a low temperature and pressure (two-stage MB-CO2). The quality of the treated beer was compared with UFB and heat-treated beer (HTB). Five-log reductions could be achieved by two-stage MB-CO2 treatment with a heating coil at 50 A degrees C for 5 min, while yeast in UFB was not completely inactivated by heat treatment at 80 A degrees C for 5 min. In sensory evaluation of these beers, there were no significant differences on the score of flavour, although bitterness of two-stage MB-CO2-treated beer (MBB) was significantly low on the score of taste. In analysis of volatile compounds, dimethyl sulphide, dimethyl disulphide, beta-myrcene and styrene in MBB were less than those in UFB and HTB. Glucose and maltose contents in MBB and UFB were almost the same, although those in HTB significantly decreased. Organic acids, except for acetic acid in MBB, were nearly identical with those in UFB and HTB. Most of the free amino acids in MBB were lower than those in UFB and HTB. Bitter units of beer increased in the order of MBB, HTB and UFB. These results suggested that two-stage MB-CO2 shows promise as a practical technique for inactivating yeast in UFB. The inactivation efficiency of two-stage MB-CO2 had less influence of materials in beer than that of heat treatment, and two-stage MB-CO2 has little negative impact on the beer quality.
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