| Summary: | Thesis (PhD)--Stellenbosch University, 2005. === ENGLISH ABSTRACT: A number of relevant brewing industry issues associated with malt quality were examined.
These included beer foam quality, premature flocculation of yeast during fermentation and
antimicrobial factors in malt.
The cause of poor foam at a brewery relative to other similar breweries was identified as being
related to the boiling temperature during wort preparation and the associated conformational
changes of the abundant foam protein lipid transfer protein 1 (LTPl). The temperature range
of 96 to 102°C was revealed to be critical. At the higher temperature the denaturation of LTP 1
was more extensive and its effectiveness as a foam protein was reduced. In addition, it was
shown that the prominent role of LTPI with respect to foam was as a lipid binding protein,
forming a lipid sink and protecting foam from lipid damage.
The occurrence of malt associated premature yeast flocculation (PYF) during fermentation
was induced in malt by the addition of extra-cellular fungal enzymes to the malt husk or by
micro-malting barley in the presence of fungi. In addition, treating malt husk with commercial
xylanase or adding commercial arabinoxylan to the fermentation also impacted on yeast
flocculation. It was proposed that a range of molecular weight arabinoxylans formed by the
enzymatic breakdown of the major barley husk component (arabinoxylan) resulted in PYF.
Antimicrobial activity against brewing yeast (Saccharomyces cerevisiae), other fungi and
bacteria was found in barley, malt and malt derived wort trub. Wort trub is the non-specific
precipitate of protein, polyphenols and lipids formed during wort boiling and which is, to
some extend, carried over in the wort to the fermentation. Antimicrobial activity appeared to
increase during malting. The growth of brewery collected yeast was inhibited in the presence
of brewery production wort when compared to the same wort filtered to remove the trub.
Brewery yeast was found to be more sensitive to inhibition than laboratory propagated yeast
of the same strain. Different strains of S. cerevisiae were also found to differ in their
sensitivity to inhibition. Investigation revealed that the activity originated from the inside of
the barley grain and impacted on yeast sugar uptake. However, there was no direct correlation
detected between levels of antimicrobial activity in malt and fermentation performance. At high concentrations the factors were microcidal causing cell lysis. Partial characterisation of
an antimicrobial extract from malt revealed the presence of a factor between 5 and 14 kDa,
containing a cationic peptide component. The optimum pH stability was ±5 when it was also
most cationic. The factor easily and irreversibly lost activity at extreme pH and when exposed
to certain reagents but was heat resistant in accordance with its survival in wort trub.
Preliminary results showed the presence of LTP1 associated with other peptides in the active
cationic fraction from the one malt tested.
The occurrence of malt related PYF and malt antimicrobial factors are associated with
microbial contamination of the grain. The fungi generating the PYF factors from the barley
husk while the barley's defence mechanism generates antimicrobial factors to cope with the
pathogenic effect of the fungi. In addition there is a potential link between the foam protein
LTP 1 and malt antimicrobial activity as LTP 1 or LTP 1 in association with another
component(s) is potentially antimicrobial. === AFRIKAANSE OPSOMMING: 'n Aantal problematiese areas in die broubedryf, wat met mout geassosieer word, is
ondersoek, naamlik bierskuimkwaliteit, voortydige flokkulering van gis tydens fermentasie
en die invloed van antimikrobiese faktore in mout.
Die oorsaak van swak bierskuim by 'n spesifieke brouery relatief tot ander soortgelyke
brouerye was geidentifiseer as die moutekstrakkookpunt tydens moutekstrakbereiding. Tydens
hierdie proses ondergaan dieskuimprotein, lipiedoordrag proteien 1 (lipid transfer protein 1,
LTPI), 'n konformasieverandering. Die temperature tussen 96 to 102°C was kritiek t.o.v.
ideale konformasieverandering vir skuimaktiwiteit. Denaturering van LTPI het by hoër
temperature plaasgevind wat die skuimproteien se aktiwitiet verminder het. Daar is ook
bewys dat LTPI 'n verdere rol in bierskuim speel aangesien dit 'n lipiedbindingsproteien is
wat die skuimnegatiewe lipiede verwyder.
Die voorkoms van moutgeassosieerde voortydige flokkulering van gis (PYF) tydens
fermentasie is op twee maniere in mout geinduseer, naamlik:
• deur die toevoeging van ekstrasellulêre swamensieme tot die moutdop
• deur mikrovermouting van gars in die teenwoordigheid van swamme.
Die behandeling van die moutdop met kommersiele xilanase of die toevoeging van
kommersiele arabinoxilaan by fermentasies het ook die flokkulering van gis beinvloed. Die
hipotese was dat PYF veroorsaak is deur 'n reeks arabinoxilane met verskillende molekulêre
massas wat gevorm het tydens die ensimatiese afbraakproses van die primere
moutdopkomponent (arabinoxilaan).
Antimikrobiese aktiwiteit teenoor brouersgis (Saccharomyces cerevisiae), ander swamme en
bakterie was teenwoordig in gars, mout en moutekstrakpresipitaat. Die presipitaat bestaan uit
nie-spesifieke presipitate van proteien, polifenole en lipiede wat gedeeltelik in die gekookte
moutekstrak agterbly. Daar is gevind dat antimikrobiese aktiwiteit tydens vermouting
toe geneem het. Die groeiproses van brouersgis, gekollekteer by 'n brouery, was geinhibeer
deur die teenwoordigheid van brouery-geproduseerde moutekstrak in vergelyking met
dieselfde moutekstrak wat gefiltreer was om die presipitaat te verwyder. Die brouersgis was meer sensitief heens inhibisie in vergeleke met dieselfde gisstam wat opgegroei is in die
laboratorium. Verskillende S. cerevisiae stamme het ook verskille in sensitiwiteit getoon
t.o.v. the antimikrobiese komponente in die moutekstrakte. 'n Verdere ondersoek het getoon
dat die oorprong van die inhiberende aktiwiteit die interne dele van die gars is, asook dat dit
die gissuikeropname beinvloed. Daar was egter geen direkte verband tussen antimikrobiese
aktiwiteit in mout en fermentasie effektiwiteit, soos gemeet onder laboratorium toestande, nie.
Hoë konsentrasies van die faktore het egter gelei tot seldood weens sellise. 'n Kationiese
peptiedbevattende fraksie tussen 5 en 14 kDa en 'n optimale pH stabliliteit van 5 is gevind
deur gedeeltelike karakterisering van 'n antimikrobiese moutekstrak. Die aktiewe fraksie se
aktiwiteit is onomkeerbaar vernietig by ekstreme pH en blootstelling aan sekere reagense.
Die aktiewe verbinding(s) is egter hittebestand en resultate het getoon dat hierdie aktiwiteit
die brouproses oorleef as deel van die moutektrakpresipitaat. Voorlopige resultate van die een
mout wat getoets is het die teenwoordigheid van LTP 1 getoon, asook die moontlike
assosiasie met ander peptiede of kleiner komponente in die aktiewe kationiese fraksie.
Die voorkoms van moutgeassosieerde PYF en antimikrobiese faktore in mout word met die
mikrobiologiese kontaminasie van gars verbind. Swamme produseer die PYF faktore vanuit
die moutdopkomponente, terwyl die plant weer antimikrobiese faktore produseer as deel van
'n beskermingsmeganisme teen die patogene effek van die swamme. Daar is ook 'n
potensieele verwantskap tussen bierskuimproteien LTP 1 en antimikrobiese faktore in mout,
aangesien LTPI ofLTPl tesame met 'n ander verbinding(s) moontlik antimikrobies is.
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