Overcoming inefficient utilisation of fructose as a cause of problem fermentations (in collaboration with The Australian Wine Research Institute – AWR 05/06)

The glucophilic character of the yeast Saccharomyces cerevisiae has been reported in the literature (Berthels et al., 2004; Guillaume et al., 2007). By this attribute glucose and fructose (the two main sugars present in grape juice) are depleted at different rates during the alcoholic fermentation, with fructose predominating in the latter and more difficult stages. At this time when nutrients are low and alcohol concentration is high, the greater ratio of fructose compared to glucose can make attempts to reinvigorate sluggish fermentations or restart stuck fermentations very challenging. Lower ratios of glucose compare to fructose (e.g. ~1:10) have themselves been reported as a cause of arrested fermentation (Schutz and Gafner, 1993).

Problem fermentations delay grape processing, tie up valuable and limited fermentation space, demand greater resources to manage and rescue, and frequently result in a suboptimal product due to oxidation and spoilage. Ironically, the frequency of such fermentations means that they are widely accepted as an unavoidable downside of winemaking. Improved strains, however, are likely to offer a solution.

In seeking to produce strains which are better able to complete fermentation and specifically exhibit a greater ability to utilise fructose, a range of improvement strategies can be used. These can be broadly classed as either recombinant or nonrecombinant. Both have disadvantages: on one hand in terms of the degree of knowledge of the genetic basis of the sought property and the acceptability of recombinant products to consumers, and on the other, the limited extent of improvementthat is possible or likely.