| Summary: | Thesis (MScAgric.)--University of Stellenbosch, 2001. === 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. === AFRIKAANSE OPSOMMING: Veranderinge in die bakteriese populasie van die gars kultivars, Clipper (plaaslik) en Prisma
(ingevoer), vermout by Southern Associated Maltsters (SAM), Caledon, Suid Afrika, is ondersoek.
Monsters is van vier individuele lopies van elke kultivar en tydens tien verskillende fases van die
vermoutingsproses geneem. Die tien verskillende stadia het die volgende ingesluit: Droë gars voor
benatting, water van die eerste benattingsfase, gars nadat water van die eerste benattingsfase
gedreineer is, water van die tweede benattingsfase, gars van die tweede benattingsfase, gars na die
dreinering van water in die tweede benattings fase, gars na die eerste, tweede en derde dag van
ontkieming binne die ontkiemingstenke, en mout na droging. Klem is geplaas op die taksonomie en
samestelling van melksuurbakterieë (MSB) wat tydens die tien verskillende fases geïsoleer is.
Die MSB is tot spesievlak geïdentifiseer deur gebruik te maak van numeriese analise van totale
oplosbare selproteïen bandpatrone, RAPD-PKR bandpatrone en 16S rRNA volgorde-bepaling.
Gram-negatiewe bakterieë is tot op genusvlak geïdentifiseer deur gebruik te maak van die API 20E
toetssisteem. Spesies van die genera Citrobacter, Enterobacter, Pantoea, Proteus, Seratia,
Kluyvera, Klebsiella, Vibrio asook Escherichia coli is geïdentifiseer. Tydens die vermoutingsproses
van die twee kultivars is geen beduidende verskille in die lewensvatbare bakterietellings gevind nie,
alhoewel die MSB-tellings in die gars voor benatting en mout na droging in Prisma hoër was as in
Clipper. Leuconostoc argentinum, Leuconostoc laetis en Weissella confusa het die meeste
voorgekom in beide kultivars. Kleiner hoeveelhede van Weissella paramesenteroides, Lactobacillus
casei, Lactococcus laetis en Lactobacillus rhamnosus is ook geïsoleer. Lb. casei en Lb. rhamnosus
het nie in die Prisma-kultivar voorgekom nie, terwyl W paramesenteroides en Le. laetis nie in die
Clipper-kultivar teenwoordig was nie. Le. argentinum het meestal in die gedroogde mout van die
Clipper-kultivar voorgekom, terwyl Le. laetis meestal in die Prisma-kultivar waargeneem is.
Die effek van hierdie bakterieë op die fermentasievermoë van die brouersgis Saccharomyces
cerevisiae SAB 05 is ook bestudeer. Die fermentasies is in Clipper- en Prisma- wort gedoen. Vir die
fermentasiestudies is gis in kombinasie met verskillende bakterieë gebruik. Wort met slegs gis het as
kontrole gedien. Klem is geplaas op die effek van die bakterieë op die digtheid, pH, gis- en bakterietellings
en die verskillende vlugtige komponente wat tydens die fermentasies geproduseer is. Die
teenwoordigheid van MSB en Gram-negatiewe bakterieë het geen effek gehad op die vermoë van die gis om die digtheid van die gefermenteerde wort te verlaag nie. Die MSB het wel 'n verlaging
van die pH in beide Clipper- en Prisma- wort teweeggebring. Tydens die fermentasie het die Gramnegatiewe
bakterietellings verminder, terwyl die MSB-tellings konstant gebly het. 'n Verband is
gevind tussen vlugtige komponente geproduseer in die kontrole-fermentasie en fermentasies met gis
en Gram-negatiewe bakterieë, gis en Lactobacillus spp. en gis en Weissella spp. Leuconostoc spp.
het groter veskille in die samestelling van die gefermenteerde wort teweeg gebring met duidelike
verskille tussen Clipper en Prisma. Die teenwoordigheid van Lactococcus spp. het nie groot
verskille in die samestelling van die gefermenteerde wort getoon nie. Op die laaste dag van die
fermentasies was die vlakke van vier uit die vyfbelangrikste vlugtige aroma komponente wat in bier
voorkom in die kontrole fermentasies in aanvaarbare konsentrasies teenwoordig. Die Gramnegatiewe
bakterieë het geen beduidende invloed gehad op die vlugtige aroma komponente wat deur
die gis geproduseer is nie, terwyl die MSB 'n besliste effek in die aroma-samestelling van beide die
kultivars gehad het. Die komponente met die hoogste konsentrasies was, isoamiel-alkohol, asynsuur
en asetoin. Asynsuur was in al die fermentasies in die hoogste konsentrasie teenwoordig.
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