Antifungal activity of non-conventional yeasts against Botrytis cinerea and non-Botrytis grape bunch rot fungi.

Grapes harbour a plethora of non-conventional yeast species. Over the past two decades, several of the species have been extensively characterised and their contribution to wine quality is better understood. Beyond fermentation, some of the species have been investigated for their potential as alternative biological tools to reduce grape and wine spoilage. However, such studies remain limited to a few genera. This work aimed to evaluate the antagonistic activity of grape must-derived non-conventional yeasts against Botrytis cinerea and non-Botrytis bunch-rotting moulds and to further elucidate mechanisms conferring antifungal activity. A total of 31 yeast strains representing 21 species were screened on different agar media using a dual culture technique and liquid mixed cultures, respectively. Pichia kudriavzevii was the most potent with a minimum inhibitory concentration of 102 cells/mL against B. cinerea but it had a narrow activity spectrum. Twelve of the yeast strains displayed broad antagonistic activity, inhibiting three strains of B. cinerea (B05. 10, IWBT FF1 and IWBT FF2), a strain of Aspergillus niger and Alternaria alternata. Production of chitinases and glucanases in the presence of B. cinerea was a common feature in most of the antagonists. Volatile and non-volatile compounds produced by antagonistic yeast strains in the presence of B. cinerea were analysed and identified using gas and liquid chromatography mass spectrometry, respectively. The volatile compounds identified belonged mainly to higher alcohols, esters, organosulfur compounds and monoterpenes while the non-volatile compounds were cyclic peptides and diketopiperazine. To our knowledge, this is the first report to demonstrate inhibitory effect of the non-volatile compounds produced by various yeast species.

The Use of Candida pyralidae and Pichia kluyveri to Control Spoilage Microorganisms of Raw Fruits Used for Beverage Production.

Undesired fermentation of fruit-derived beverages by fungal, yeast and bacterial spoilage organisms are among the major contributors of product losses in the food industry. As an alternative to chemical preservatives, the use of Candida pyralidae and Pichia kluyveri was assessed for antimicrobial activity against several yeasts (Dekkera bruxellensis, Dekkera anomala, Zygosaccharomyces bailii) and fungi (Botrytis cinerea, Colletotrichum acutatum and Rhizopus stolonifer) associated with spoilage of fruit and fruit-derived beverages. The antagonistic properties of C. pyralidae and P. kluyveri were evaluated on cheap solidified medium (grape pomace extract) as well as on fruits (grapes and apples). Volatile organic compounds (VOCs) from C. pyralidae and P. kluyveri deemed to have antimicrobial activity were identified by gas chromatography-mass spectrometry (GC-MS). A cell suspension of C. pyralidae and P. kluyveri showed growth inhibition activity against all spoilage microorganisms studied. Direct contact and extracellular VOCs were two of the mechanisms of inhibition. Twenty-five VOCs belonging to the categories of alcohols, organic acids and esters were identified as potential sources for the biocontrol activity observed in this study. This study reports, for the first time, the ability of C. pyralidae to inhibit fungal growth and also for P. kluyveri to show growth inhibition activity against spoilage organisms (n = 6) in a single study.

Characterisation of hybrid yeasts for the production of varietal Sauvignon blanc wine - A review.

The wine yeast Saccharomyces cerevisiae forms an integral part of wine production by converting relatively 'neutral' flavoured grape must into varietal aromatic wines. Additionally, non-Saccharomyces strains can be used with S. cerevisiae for the production of wines with more complexity. Yeast strains, to varying extents, produce and/or mediate the release of a whole range of key metabolites, which in turn contribute to enhanced aroma and flavour of the final wine, especially Sauvignon blanc. These metabolites viz. thiols are dependent on yeast-expressed enzymes during fermentation. Inoculation with an appropriate yeast will, therefore, lead to more commercial wine sales due to resultant wines with sought-after aroma and flavour. Likewise, inoculation with the incorrect yeast will have a negative effect on sales. It is also important to have quality control measures in place to ensure that the inoculated yeast strain quickly dominate, and is present throughout the fermentation process. Traditionally, the laborious contour clamped homogeneous electric field (CHEF) DNA karyotyping technique was shown to be reliable in this regard, however cutting-edge matrix-assisted laser desorption/ionisation-time of flight mass spectrometry (MALDI-TOF MS) biotyping is proving to be a faster alternative. As both methods have advantages and disadvantages, they should be used in complementary as opposed to competitively. Standard chemical and descriptive sensory analyses of wine also serve as evaluation and/or characterisation tools of yeast starter cultures. Additionally, metabolomic and proteomic profiling using gas chromatography (GC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) of final wines was shown to be instrumental yeast evaluation tools. Therefore, this review highlights the importance and practicality of a more inclusive approach to evaluate and characterise novel yeast strains used for winemaking by deploying traditional and modern chemical and organoleptic evaluation techniques of wines in conjunction with cutting-edge omics approaches towards enhancing white wine varietal aroma.