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.