Masters Degrees (Institute for Wine Biotechnology)

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Browsing Masters Degrees (Institute for Wine Biotechnology) by Subject "Brewing -- Microbiology"

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    Identification of bacteria isolated from malt, with the emphasis on lactic acid bacteria and their influence on brewer's yeast
    (Stellenbosch : Stellenbosch University, 2001-12) Booysen, Clifford; Dicks, Leon Milner Theodore; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    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 each cultivar 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 second steep water-stand, barley after draining of the second steep, barley from the first, second and third days of germination in the germination vessels (GV), and malt after kilning. Emphasis was placed on the taxonomy and composition of the lactic acid bacteria (LAB) isolated from the ten different phases. The LAB were identified to species level by using numerical analysis of total soluble cell protein patterns, RAPD-PCR banding patterns and 16S rRNA sequencing. The Gram-negative bacteria were identified to genus level by using the API 20E system and included Citrobacter spp., Enterobacter spp., Pantoea spp., Proteus spp., Seratia spp., Kluyvera spp., Klebsiella spp., Vibrio spp. and Escherichia coli. The number of viable bacteria throughout the malting process of the two cultivars did not differ significantly, although the LAB counts in the barley before steep and on the kilned malt were higher in Prisma than in Clipper. Leuconostoc argentinum, Leuconostoc laetis and Weissella confusa were the most predominant in both cultivars. A few strains of Weissella paramesenteroides, Lactobacillus casei, Lactococcus laetis and Lactobacillus rhamnosus were also isolated. Lb. casei and Lb. rhamnosus were not isolated from the Prisma cultivar, whilst W paramesenteroides and Le. laetis were absent in the Clipper cultivar. Kilned malt of the Clipper cultivar 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 Saccharomyces cerevisiae SAB 05, was also studied. Fermentations were conducted in wort prepared from Clipper and Prisma malt. Yeast in combination with the different bacteria were used in the fermentation studies. Wort with only yeast was used as control. Emphasis was placed on the effect the bacteria has 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 of the fermenting wort, whilst the LAB caused a decrease in the pH of the fermentations in both Clipper and Prisma wort. The cell numbers of the Gram-negative bacteria decreased throughout the fermentations, whilst the LAB cell numbers remained constant. Comparisons could be drawn between the volatile aroma compounds produced in the control fermentation and fermentations with yeast 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, with much more clear variations between Prisma and Clipper. No major differences were recorded in the aroma profiles of Prisma and Clipper malt fermented in the presence and absence of Lactococcus spp. The Gram-negative bacteria had no significant effect on the volatile aroma compounds produced 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 acceptable concentration range on the fmal day of fermentation. The compounds with the highest concentrations were iso-amyl alcohol, acetic acid and acetoin, with acetic acid being present in the highest concentration in all the fermentations.