Effect of yeasts and oenological parameters on acetaldehyde production during alcoholic fermentation of South African grape musts

October, Francois Marc (2020-03)

Thesis (MSc)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Acetaldehyde plays a role in the rate of fermentation and the quality of wine. High levels of acetaldehyde in fermenting juice may result in sluggish/stuck fermentations, and in wine, it may impart undesirable aromas usually associated with oxidative aromas. Depending on its levels, acetaldehyde has an effect on yeast metabolism and can therefore impact alcoholic fermentation. The overall aim of this project was to investigate the effect of yeasts and oenological parameters on acetaldehyde production, to better understand the impact of acetaldehyde on alcoholic fermentation and wine sensorial composition. Ten commercial Saccharomyces cerevisiae strains and 10 non-commercial non-Saccharomyces yeasts were evaluated. These yeasts were screened in a laboratory trial for their acetaldehydeproducing ability during alcoholic fermentation, and resulted in the selection of a high-, medium- and low-acetaldehyde producing yeasts. The selected yeasts were the S. cerevisiae yeasts NT50 (high), NT116 (medium) and VIN13 (low); and the non- Saccharomyces yeasts Torulaspora delbrueckii (high), Candida guilliermondii (medium) and Candida valida (low). The above-mentioned selection of Saccharomyces yeasts was used individually for vinification of grape must, as well as in all possible permutations with the non- Saccharomyces yeasts, and resultant wines analysed chemically and evaluated sensorially. The initial sensory results showed noticeable differences between treatments, in terms of aroma and sweetness. Statistical evaluation of the data from the screening and cellar trials showed that yeast strain and time of fermentation have an impact on levels of acetaldehyde. The ability of the yeast strains to produce acetaldehyde was affected differently by fermentation temperature during the screening trial. Wines coinoculated with non-Saccharomyces cerevisiae yeasts have lower levels of acetaldehyde than wines only inoculated with Saccharomyces cerevisiae yeasts as observed in the cellar trial. Sulphur dioxide (SO2) has a very high affinity for acetaldehyde, therefore the impact of various concentrations of SO2 on the levels of acetaldehyde in fermenting must was monitored in a second cellar trial. The resulting effects on fermentation and final wine quality were monitored. Although it is known that SO2 impacts wine quality, it was also found that the varying levels of SO2 have a direct effect on the acetaldehyde levels produced during fermentation. During a separate fermentation trial (laboratory-scale), using three Saccharomyces cerevisiae yeast strains, the total enzyme activity of alcohol dehydrogenase (ADH) was monitored. The ADH activity showed a similar trend to acetaldehyde concentration, where high enzyme activity of the Saccharomyces cerevisiae yeasts correlated with high acetaldehyde levels. In summary, there were significant differences in acetaldehyde levels between yeast strains tested in this study and the levels were within acceptable ranges normally found in wines. Higher acetaldehyde levels were found in wines inoculated with S. cerevisiae, exposed to high SO2 levels, and fermented at higher temperatures. There was a direct correlation between total ADH activity and total acetaldehyde production of Saccharomyces cerevisiae yeasts. To ensure lower levels of acetaldehyde in wine, winemakers should preferably coinoculate with low ADH activity Saccharomyces cerevisiae and non-Saccharomyces yeast strains, at low fermentation temperatures, while ensuring low levels of SO2 before fermentation.

AFRIKAANSE OPSOMMING: Asetaldehied speel ‘n rol by die fermentasietempo en wynkwaliteit. Hoë vlakke van asetaldehied in gistende druiwesap kan lei tot slepende/steekgistings, en in wyn kan dit lei tot wangeure wat gewoonlik met oksidatiewe aromas geassosieer kan word. Asetaldehiedvlakke het ‘n effek op gismetabolisme en kan dus fermentasiekinetika beïnvloed. Die oorhoofse doel van hierdie studie was om spesifiek te kyk na die bydrae van verskillende gisrasse en wynkyndige parameters op asetaldehiedproduksie, om sodoende die impak van asetaldehied op alkoholiese fermentasie, asook op die sensoriese aspek van wyn, beter te kan verstaan. Tien kommersiële Saccharomyces cerevisiae rasse en 10 nie-Saccharomyces gisrasse was ondersoek. Hierdie giste was in laboratorium skaal proewe geëvalueer vir hul vermoë om asetaldehied te produseer tydens alkoholiese fermentasie en ‘n seleksie van hoog, -medium en lae asetaldehied produserende giste is gemaak. Die geselekteerde giste was S. cerevisiae yeasts NT50 (hoog), NT116 (medium) en VIN13 (laag), en die nie-Saccharomyces giste was Torulaspora delbrueckii (hoog), Candida guilliermondii (medium) en Candida valida (laag). Die bogenoemde Saccharomyces giste was gebruik op hul eie en in alle moontlike kombinasies met die nie-Saccharomyces giste gedurende die eerste wynmaakproef in die kelder. Die wyne was ook chemies en sensories geëvalueer. Die aanvanklike sensoriese resultate het gewys dat die behandelings van mekaar verskil ten opsigte aroma en soetheid. Statistiese analise van die data het gewys dat gisras en fermentasie tyd ‘n impak op asetaldehiedvlakke gehad het. Die vermoë van die gisrasse om asetaldehied te produseer, was deur fermentasie temperatuur beïnvloed. Wyne wat met nie-Saccharomyces giste in kombinasie met Saccharomyces giste geproduseer was, het laer asetaldehiedvlakke gehad as wyne wat slegs met Saccharomyces giste geïnokuleer was. Swaweldioksied (SO2) het ‘n baie hoë affiniteit vir asetaldehied, daarom is die impak van verskeie SO2 vlakke op asetaldehiedproduksie ondersoek in ‘n tweede kelderproef. Die resultante effekte van voorgenoemde interaksies op fermentasie en wynkwaliteit was ook gemonitor. SO2 het ‘n impak op wynkwaliteit en die variasie in SO2 vlakke het ‘n direkte effek gehad op die asetaldehied vlakke gedurende fermentasie asook in die finale wyne. Tydens ‘n onafhanklike fermentasieproef (laboratorium skaal) is die totale alkohol dehidrogenase (ADH) ensiemaktiwiteit van drie Saccharomyces cerevisiae gisrasse gemonitor. Die ADH aktiwiteit het ‘n soortgelyke tendens getoon as die asetaldehiedvlakke, waar hoë ensiemaktiwiteit van Saccharomyces cerevisiae giste gekorreleer het met hoë asetaldehiedvlakke. Die asetaldehiedvlakke het betekenisvol verskil tussen die gisrasse wat getoets was en die vlakke was binne aanvaarbare perke wat normaalweg in wyne aangetref word. Hoër asetaldehiedvlakke was aangetref in wyne wat met Saccharomyces cerevisiae geïnokuleer was, aan hoë SO2 vlakke blootgestel was en teen hoë temperature gegis was. Daar was ‘n direkte korrelasie tussen totale ADH aktiwiteit en totale asetaldehiedproduksie van Saccharomyces cerevisiae giste. Om lae asetaldehiedvlakke in wyne te verseker, word wynmakers aangeraai om lae ADH aktiwiteit Saccharomyces cerevisiae en nie-Saccharomyces gisrasse te gebruik teen lae fermentasie temperature en ook om lae SO2 vlakke voor gisting te handhaaf.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108105
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