Factors influencing the fermentation performance of commercial wine yeasts

Ferreira, Jacques (Stellenbosch : University of Stellenbosch, 2004-12)

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/16322
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