by Heinrich du Plessis & Neil Jolly. 

This study follows similar studies done by us on Chenin blanc and Pinotage. The aim of this study was therefore to determine the effect eight different non-Saccharomyces yeast strains had on MLF and Chardonnay flavour.

Introduction

Wine production includes two important fermentation processes, i.e. alcoholic fermentation conducted by yeast, and malolactic fermentation (MLF) conducted by lactic acid bacteria (LAB)1. The yeasts drive alcoholic fermentation by converting sugar to alcohol, carbon dioxide and other secondary compounds that affect the aroma and taste of wine.1,2Malolactic fermentation contributes to further flavour complexity and microbiological stability of the wine, as well as the reduction of total acidity. During MLF, l-malic acid is decarboxylated to l-lactic acid and CO2. White wines do not usually undergo MLF, but it is desired in the production of certain full-bodied white wines.3,4

At the start of alcoholic fermentation, a large number of non-Saccharomyces species may be naturally present in the grape must, but the final stage of fermentation is usually dominated by alcohol-tolerant Saccharomyces cerevisiae strains.1,5 Non-Saccharomyces yeasts have different oenological characteristics to S. cerevisiae and they have been shown to enhance aroma and improve complexity of wines.5,6 During alcoholic fermentation, both Saccharomyces and non-Saccharomyces yeasts deplete the nutrients found in wine. These deficiencies, combined with toxic metabolites produced by the yeasts, can inhibit the growth of LAB.1,7,8 Despite considerable research, MLF remains a difficult process to initiate and control.9 The interaction between non-Saccharomyces yeasts and LAB is another factor that needs investigation.

Materials and methods

Eight non-Saccharomyces yeast strains (one Candida zemplinina, two Lachancea thermotolerans, one Metschnikowia pulcherrima, one Hanseniaspora uvarum and three Torulaspora delbrueckii) were used in mixed fermentations with one S. cerevisiae wine strain. Yeast strains were commercially available cultures or were obtained from the ARC Infruitec-Nietvoorbij microorganism culture collection. Yeast treatments without malolactic fermentation (MLF) and in combination with simultaneous MLF were investigated. A commercial lactic acid bacterium culture was used to induce simultaneous MLF. In total 18 treatments were evaluated in triplicate, and the Chardonnay wines produced with S. cerevisiae with or without MLF, served as the reference treatments. A standardised small-scale winemaking protocol was followed at an ambient temperature of 15°C. After completion of MLF, wines were bottled and subjected to descriptive sensory evaluations four months later.

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