WORT ALDEHYDE REDUCTION POTENTIAL IN FREE AND IMMOBILIZED YEAST SYSTEMS

Previous works have shown that a wide range of carbonyl compounds are reduced during fermentation. However, recent studies indicated that al dehyde-reducing systems may be more complex than has been previously a ssumed. The present work investigated process parameters that may affe ct the removal of wort carbonyls in free and immobilized yeast systems . In vivo measurements have shown that the relationship between the ca rbonyl structure and the reduction rate was the same in free and immob ilized yeast. Nevertheless, experiments with permeabilized yeast cells suggested that membrane permeability is not a major limiting factor i n achieving subthreshold carbonyl levels. The comparison of in vivo al dehyde reduction rates in a haploid adh-0 mutant and in an adh-0 strai n subsequently transformed with an ADH1, ADH2, or ADH3 gene showed tha t ADH1 is only one of the enzymes involved in the reduction of carbony l compounds. Data collected in vitro also showed a considerable hetero geneity in the enzymatic systems involved in the reduction of carbonyl compounds in yeast. Results further indicate that an immobilized yeas t fermentation process, at low temperature and high residence time, se ems to offer the best compromise between low alcohol content and subth reshold carbonyl levels.