The relationship between grape juice composition and the progress of alcoholic and malolactic fermentation
Difficulties with alcoholic and malolactic fermentation are routinely reported, and can be attributed to a diverse range of causes. Poor fermentation progress can occur even in juices and wines that otherwise satisfy the usual criteria indicative of appropriate fermentation progress (e.g. YAN, Baumé, and SO2). Uncontrolled growth of non-target microorganisms has been reported to be inhibitory to alcoholic fermentation, either through consumption of nutrients, or through the production of secondary metabolites.
Sulfur dioxide additions to bins and crushers are used to control pre-fermentation microbial activity; however, even moderate levels of total SO2 can negatively affect the progress of malolactic fermentation. In addition, some yeasts produce large amounts of SO2 which is inhibitory to malolactic fermentation. This is a particular concern as simultaneous alcoholic and malolactic fermentations are increasingly being used to more efficiently manage scheduling issues associated with conducting malolactic fermentation as a separate process, after alcoholic-fermentation.
Clearly the areas of yeast and bacterial fermentation performance are inter-related, and understanding the risks and capturing opportunities of yeast/bacterial interactions requires an integrated approach as described in this project. Hence this project brings together two previously separate research areas, yeast and bacterial fermentation, in order to realise an integrated approach to the study of alcoholic and malolactic fermentation performance.
The proposed fermentation performance program will study the following:
- yeast/environment interactions, using the barcoded yeast collection to determine strain fitness and implantation efficiency, together with a survey of juice composition across multiple vintages, taking account of transport conditions and other harvest variables to determine their impact on composition (collaboration with Project 3.3.1)
- bacterial/environment interactions, by using model fermentations to identify factors that stimulate or inhibit malolactic fermentation, and through developing a transformation system for Oenococcus oeni to study genetic elements (inter-strain variable regions) and their effects on malic acid utilisation
- pilot and industry trials to evaluate the suitability of uniquely Australian regional isolates of malolactic bacteria, and to determine the robustness of co-inoculated fermentations using a range of winemaking interventions.
Overcoming MLF sensitivity to SO2
In an effort to improve MLF reliability, the MLF performance of Oenococcus oeni strains in model wines and in white wines with limiting conditions was investigated. The intrinsic sensitivity of O. oeni to SO2 was confirmed, a characteristic which can restrict its ability to conduct MLF in wines with moderate to high SO2 content. Further, investigation of the MLF performance of two commercial malolactic starter cultures highlighted the critical importance of starter culture acclimatisation on the success or failure of MLF induction in difficult wine conditions. After direct inoculation of non-acclimatised starter cultures into 2018 vintage Chardonnay wine, both strains rapidly died off and failed to induce MLF. However, one strain was capable of MLF induction following acclimatisation to the wine conditions. Investigations are continuing to identify acclimatisation steps that enhance the performance of malolactic starter cultures in challenging white wine conditions.
Production of SO2 by wine yeast
Concentrations of SO2 in musts can vary considerably due to additions in the vineyard and winery, but a second major potential source of SO2 can be the yeast used to undertake alcoholic fermentation. This can be inhibitory to MLF conducted post-alcoholic fermentation but also to O. oeni when simultaneous MLF is initiated. The yeast-derived SO2 works additively with earlier additions. To better facilitate SO2 management, the SO2 contribution of 96 different S. cerevisiae wine yeasts to final total SO2 concentration was evaluated following fermentation of Chardonnay juice containing 40 mg/L total SO2. The yeasts’ capacity to produce SO2 varied dramatically, with most strains contributing only incrementally to total SO2. However, four strains were found to double the initial SO2 concentration, and in the most extreme case yeast-derived SO2 contributed an additional 150 mg/L above the initial must concentration. The use of such yeasts would essentially make MLF impossible in the wines produced using them.
Interspecies microbial interactions during fermentation
With yeast species that require co-inoculation with S. cerevisiae becoming available on the market, there is the potential for antagonistic or beneficial interactions to occur. Yeasts such as Metschnikowia pulcherrima and Torulaspora delbrueckii can be added prior to addition of S. cerevisiae. There is evidence that nutrient competition can exist between Saccharomyces and non-Saccharomyces yeasts. Co-inoculation of O. oeni generates an added level of complexity. In work initiated this year the project team has begun to explore the extent of these interactions, whether they are strain-dependent and what are the underlying genetic factors contributing to successful co-fermentations.