Identification of bacteria isolated from malt, with the emphasis on lactic acid bacteria and their influence on brewer's yeast

[摘要] ENGLISH ABSTRACT:Changes in the bacterial population throughout the malting process of two barley cultivars, i.e.Clipper (local cultivar) and Prisma (imported cultivar), malted at Southern Associated Maltsters(SAM), Caledon, South Africa, were studied. Samples were taken from four individual runs of eachcultivar at ten different stages, i.e. dry barley before steep, water from the first steep water-stand,barley after draining the first steep, water from the second steep water-stand, barley from the secondsteep water-stand, barley after draining of the second steep, barley from the first, second and thirddays of germination in the germination vessels (GV), and malt after kilning. Emphasis was placedon the taxonomy and composition of the lactic acid bacteria (LAB) isolated from the ten differentphases.The LAB were identified to species level by using numerical analysis of total soluble cell proteinpatterns, RAPD-PCR banding patterns and 16S rRNA sequencing. The Gram-negative bacteria wereidentified to genus level by using the API 20E system and included Citrobacter spp., Enterobacterspp., Pantoea spp., Proteus spp., Seratia spp., Kluyvera spp., Klebsiella spp., Vibrio spp. andEscherichia coli. The number of viable bacteria throughout the malting process of the two cultivarsdid not differ significantly, although the LAB counts in the barley before steep and on the kilnedmalt were higher in Prisma than in Clipper. Leuconostoc argentinum, Leuconostoc laetis andWeissella confusa were the most predominant in both cultivars. A few strains of Weissellaparamesenteroides, Lactobacillus casei, Lactococcus laetis and Lactobacillus rhamnosus were alsoisolated. Lb. casei and Lb. rhamnosus were not isolated from the Prisma cultivar, whilst Wparamesenteroides and Le. laetis were absent in the Clipper cultivar. Kilned malt of the Clippercultivar contained predominantly Le. argentinum, whereas the Prisma cultivar contained mainly Le.lactis.The effect of these bacteria on the fermenting ability of the brewer's yeast Saccharomycescerevisiae SAB 05, was also studied. Fermentations were conducted in wort prepared from Clipperand Prisma malt. Yeast in combination with the different bacteria were used in the fermentationstudies. Wort with only yeast was used as control. Emphasis was placed on the effect the bacteriahas on the gravity, pH, yeast- and bacterial- counts and the different volatile aroma compounds produced throughout the fermentations.The presence of LAB and Gram-negative bacteria had no effect on the yeast to reduce the gravity ofthe fermenting wort, whilst the LAB caused a decrease in the pH of the fermentations in bothClipper and Prisma wort. The cell numbers of the Gram-negative bacteria decreased throughout thefermentations, whilst the LAB cell numbers remained constant. Comparisons could be drawnbetween the volatile aroma compounds produced in the control fermentation and fermentations withyeast and Gram-negative bacteria, yeast and Lactobacillus spp. and yeast and Weissella spp.Leuconostoc spp. had a much greater influence on the aromatic composition of fermented malt, withmuch more clear variations between Prisma and Clipper. No major differences were recorded in thearoma profiles of Prisma and Clipper malt fermented in the presence and absence of Lactococcusspp. The Gram-negative bacteria had no significant effect on the volatile aroma compoundsproduced by the yeast, whilst the LAB had a definite effect on aroma composition in both cultivars.The levels of four of the five principle aroma compounds, present in beer, were in the acceptableconcentration range on the fmal day of fermentation. The compounds with the highestconcentrations were iso-amyl alcohol, acetic acid and acetoin, with acetic acid being present in thehighest concentration in all the fermentations.

[发布机构] Stellenbosch University