The evaluation of malolactic fermentation starter cultures under South African winemaking conditions

[摘要] ENGLISH ABSTRACT: With ever increasing pressure on wine producers to lower the financial costs involved inwinemaking to be able to compete in the market, all while maintaining a high level of winequality, the focus on maintaining control over all aspects of the winemaking process aregreatly emphasized.Malolactic fermentation (MLF) is one of the important processes in red wine production.The advantages of this process, when performed successfully, is widely known andaccepted. One way to gain control over MLF is the use of MLF starter cultures. Startercultures usually consist of Oenococcus oeni that has been isolated from grapes or winesand is in most cases available in a freeze-dried form ready for direct inoculation into thewine when MLF is desired. Starter cultures are induced into wine and usually ensure theimmediate onset as well as a fast and clean execution of the process. Starter culturesused in South Africa are in most cases isolated from cooler viticultural regions in theNorthern hemisphere. The constitution of wines from cooler viticultural regions, differ fromthose in South Africa, which has a warm climate. The most important difference is the acidcontent of the wines which is lower in South African must/wines and results into a higherpH. The three most important changes that develop in wine during MLF are a decrease inacidity due to the conversion of malic acid to the less harsh lactic acid, enhanced flavourand aroma of wine and an increase in the microbiological stability of wine. The decreasein acidity is very important for wines produced for grapes grown in cool viticulture regions.In South Africa though, the climate is warm and higher pH's are present in the musts andwines and the de-acidification due to MLF is not the main aim but rather themicrobiological stabilisation. One of the compounds that could be produced by lactic acidbacteria (LAB) is biogenic amines (BA's). These compounds can be hazardous to humanhealth. This thesis focussed on the performance of MLF starter cultures in high pH SouthAfrican red wines.The first objective of the study was to stretch MLF starter cultures in high pH red winesof South Africa. Stretching means to use less than the prescribed dosage or the re-use ofstarter cultures. The difference in MLF rate, the influence of the natural occurring LAB andthe levels of biogenic amines formed during MLF were determined for the differentstretching treatments. The results showed that different rates in malic acid degradationwere experienced between the treatments, but in all cases MLF fermentation was completed. Biogenic amines were formed at various levels and the influence of the naturaloccurring LAB also played a role.The second objective of the study was the evaluation of the effect of a wine isolatedLAB (Lactobacillus) and an acetic acid bacteria (AAB), inoculated with a MLF starterculture had on MLF at different wine pH's. It was found that especially in the case wherethe Lactobacillus was inoculated in combination with the MLF starter culture a possiblestimulatory effect was experienced with regards to malic acid degradation rate. Biogenicamine concentration was measured at the end of MLF and it was found that no histamineand tyramine were formed in any of the treatments, while the putrescine and cadaverinelevels were found to be at approximately similar levels for the different treatments.The third objective was to evaluate the possible influence of commercial tanninadditions and a pectolytic enzyme on rate of MLF and phenolic composition of high pH redwine. The commercial tannins had possible inhibitory as well as stimulatory effects on therate of malic acid degradation especially during the initial stages of MLF, with the highestdosage having the significant effect. The BA results showed difference in the levelsproduced due to tannin additions as well as strain differences could exist. The phenoliccontent showed a decrease in colour density, total red pigments, total phenolics andanthocyanins between AF and MLF.The fourth objective was to evaluate inoculation time of MLF starter cultures. Theresults showed that the fastest AF/MLF time was with simultaneous inoculation of theyeast and MLF starter cultures. It was also for this treatment where no histamine ortyramine was detected at the end of MLF compared to the other inoculation strategies(before the end of AF and after AF).This study generated a large amount of novel data which made a valuable contributionwith regards to MLF in high pH red wines of South Africa.

[发布机构] Stellenbosch University