Thesis (MScAgric)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: The production of quality wine is influenced by numerous factors of which grape
quality is one of the most important factors. The production of quality wine, however,
is not possible without good winemaking techniques and effective quality control.
Critical control points (CCP) during the winemaking process must be identified to
ensure optimum wine quality. Grape must is a complex medium that contains
different micro-organisms which can be either beneficial or negative to wine quality,
depending on the physical and chemical conditions that prevail in the must. Yeasts
are responsible for alcoholic fermentation, lactic acid bacteria (LAB) for malolactic
fermentation (MLF) and acetic acid bacteria (AAB) for the production acetic acid from
ethanol. Yeasts and certain LAB can also produce acetic acid and thereby increasing
the volatile acidity (VA) of wine. These micro-organisms can influence each other in
complex fashions by competing for growth nutrients and by producing inhibitory
substances.
Most winemakers nowadays use commercial yeast strains to inoculate wine
fermentations. This, however, does not assure a problem-free fermentation and
cases of stuck and sluggish fermentations are annually reported worldwide. In these
or most cases fermentation takes longer than 21 days to complete and the wine
contains a residual sugar concentration of more than 4 g/L, which can be utilised by
wine spoilage micro-organisms such as certain bacteria and other wild yeasts. Stuck
and sluggish fermentations also increase the chances of oxidation due to the
absence of the protective CO2 layer on the surface of the wine, which is formed
during alcoholic fermentation. Another effect of stuck and sluggish fermentations is
that valuable tank space is wasted due to the unexpected time consumption of these
fermentation problems. Many factors during the winemaking process can be
responsible for stuck and sluggish fermentations. In this thesis the different factors is
discussed with the emphasis on the effect of the yeast strain. The way that certain
yeast strains influence AAB and LAB numbers during fermentation and MLF through
the production of inhibiting by-products such as medium chain fatty acids has not
been investigated in detail in the past.
Certain fungicides and pesticides that are used in vineyards to control pests (e.g.
mildew) contain copper which can be inhibiting to yeast growth and alcoholic
fermentation. Legal limits and withholding periods on these sprays are not always
strictly obeyed and can lead to stuck and sluggish fermentations. This motivated us
to evaluate the growth and fermentation activities of a selection of commercial wine
yeasts in the presence of copper levels in the range of maximum legal limits. The
effect of these commercial strains on the LAB and AAB numbers during alcoholic
fermentation and MLF were also investigated.
Our results showed that there was no significant difference on numbers of the
AAB obtained from fermentations inoculated with different commercial wine yeast strains. However, with regards to the LAB numbers, one of the strains produced
significantly more sulphur dioxide (SO2), which led to the inhibition of MLF in that
wine. Our results further indicated which commercial yeast strains were capable of
effectively fermenting high sugar musts and which strains were less effective. From
the strains tested VIN13, N96 & L2056 were able to utilize fructose more effectively
than NT50, RJ11 & D80. We could further distinguish between yeast strains that
produced the lowest (VIN13 & RJ11) and the highest (WE372, NT50 & L2056) VA
concentrations in must containing high sugar levels. Strains that were more tolerant
against high copper levels were also identified. We tested six yeast strains in must
with added copper (0.25 mM cu2+) in the form of CuSO4
.H2O. Three Cu2+-tolerant
(D80, Collection Cepage Cabernet & NT50) yeast strains were distinguished from
three less Cu2+-tolerant yeast strains (VIN13, NT112 & RJ11).
This study made a valuable contribution in knowledge gained about commercially
available wine yeast strains that can ferment effectively under certain stress
conditions. Research such as this, where wine yeasts are evaluated to ferment more
effectively during strenuous winemaking conditions, will be very beneficial to
winemakers. / AFRIKAANSE OPSOMMING: Die produksie van gehalte wyn word deur verskillende faktore beïnvloed waarvan
druifkwaliteit seker die belangrikste is. Die produksie van gehalte wyn is egter nie
moontlik sonder goeie wynmaaktegnieke en effektiewe kwaliteitsbeheer nie. Kritieke
kontrole punte (KKP) tydens die wynmaakproses moet dus geïdentifiseer word om
sodoende ‘n verlaging in wynkwaliteit te vermy. Druiwemos het ‘n komplekse
mikrobiologiese samestelling en bestaan uit verskillende mikroörganismes wat vooren
nadelig vir wynkwaliteit kan wees, afhangende van die fisiese en chemiese
toestande wat in die mos bestaan. Giste is verantwoordelik vir alkoholiese
fermentasie, melksuurbakterieë (MSB) vir appelmelksuurgisting (AMG) en
asynsuurbakterieë (ASB) vir die produksie van asynsuur vanaf etanol. Asynsuur word
egter ook deur giste en MSB geproduseer en dra so by tot die vlugtige suurheid (VS)
van ‘n wyn. Hierdie mikroörganismes kan mekaar op komplekse wyses beïnvloed
deur o.a. te kompeteer vir voedingstowwe asook deur die produksie van inhiberende
verbindings.
Die meeste wynmakers maak gebruik van kommersiële gisrasse om alkoholiese
fermentasies mee uit te voer. Gevalle van sogenaamde slepende en gestaakte
alkoholiese fermentasies, waar suiker nie volledig na etanol en CO2 omgeskakel
word nie, kom egter nog gereeld in die wynbedryf voor. In sulke gevalle neem die
fermentasie gewoonlik langer as 21 dae om te voltooi met ‘n suiker konsentrasie van
meer as 4 g/L wat in die wyn oorbly. Dit is nadelig vir wynkwaliteit aangesien dit nie
net die kanse vir bederf deur bakterieë en giste verhoog nie, maar ook die kanse vir
oksidasie verhoog a.g.v. die verlies van die beskermende CO2 lagie bo-oor die wyn.
Hoe sekere gisrasse, ASB en MSB getalle gedurende fermentasie en AMG
beïnvloed deur die produksie van inhiberende verbindings soos medium ketting
vetsure en SO2, is ook nie baie in die verlede ondersoek nie.
Sommige spuitstowwe wat gebruik word in die bekamping van swamsiektes
bevat koper wat inhiberend kan wees vir gisgroei en alkoholiese fermentasie. Wetlike
maksimum limiete en onthoudingsperiodes op spuitstofresidue word egter nie altyd
gehoorsaam nie en kan lei tot slepende en gestaakte fermentasies. Dit het ons
gemotiveer om ‘n seleksie van kommersiële gisrasse te evalueer in terme van
gisgroei en fermentasie in die teenwoordigheid van kopervlakke naby die maksimum
limiet.
Ons resultate het gewys dat daar nie noemenswaardige verskille in AAB getalle
tydens alkoholiese fermentasie tussen behandelings met verskillende kommersiële
gisrasse was nie. Een van die gisrasse het wel noemenswaardig meer SO2
geproduseer wat gelei het tot inhibering van AMG in hierdie wyn. Ons het verder
uitgewys watter kommersiële gisrasse instaat is om meer effektief in hoër suiker mos
te fermenteer en watter van die rasse minder suksesvol was. Ons het ook rasse
geïdentifiseer wat meer weerstandbiedend is teen hoë kopervlakke in mos en
sodoende groter kans op ‘n suksesvolle fermentasie sal hê in mos wat koperresidue bevat wat afkomstig is van sekere spuitstowwe. Die effek van die ASB en MSB
getalle gedurende fermentasie en AMG is ook ondersoek. Ons resultate het verder
gewys watter kommersiële gisrasse instaat was om mos met hoë suikervlakke meer
effektief te fermenteer. Vam die gisrasse wat getoets was het VIN13, N96 & L2056
fruktose meer effektief benut as NT50, RJ11 & D80. Ons kon verder onderskei
tussen gisrasse wat die laagste (VIN13 & RJ11) en die hoogste (WE372, NT50 &
L2056) vlakke van VS produseer in mos met hoë inisiële suikervlakke. Gisrasse wat
meer tolerant was teen koperresidue in mos is ook geidentifiseer. Ons het ses
gisrasse getoets in mos met bygevoegde koper (0.25 mM Cu2+) in die vorm van
CuSO4
.5H2O. Daar is onderskei tussen drie Cu2+-tolerante (D80, Collection Cepage
Cabernet & NT50) en drie minder Cu2+-tolerante gisrasse (VIN13, NT112 & RJ11).
Hierdie studie lewer ‘n waardevolle bydrae in die invordering van kennis oor
kommersieel beskikbare wyngisrasse wat meer effektief sal fermenteer onder sekere
streskondisies wat in mos voorkom. Inligting soos hierdie is belangrik om die
wynmaker se keuse uit die reeks bestaande kommersiële gisrasse te vergemaklik.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16322 |
| Date | 12 1900 |
| Creators | Ferreira, Jacques |
| Contributors | Du Toit, W.J., Du Toit, M., University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | iv, 74 leaves : ill. |
| Rights | University of Stellenbosch |
Page generated in 0.002 seconds