Spoilage-yeast metabolism, reduced-sugar formulations and implications for food preservation

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Fungi (yeasts and moulds) spoil 5-10% of all food and drinks produced globally, costing many billions of pounds each year. This contributes significantly to food insecurity. A variety of strategies is used to preserve foods but these are not always effective. In the soft drinks industry, formulations have been developed over a number of years that help to limit the spoilage problem to current levels. The drinks are usually acidic and usually inhibit bacteria. However, yeasts can grow in these conditions and they commonly dominate in soft drinks spoilage. A principal preservative used in soft drinks is the weak acid sorbic acid. This inhibits most yeasts, but a number can still grow at the permitted levels and some can degrade the sorbic acid to products that alter flavour. The resultant chronic level of yeast spoilage in soft drinks manufacture could be set to worsen, as recent market pressures have been prompting major re-formulation of soft drinks products. The 'Soft Drinks Industry Levy' implemented by the UK government in 2018 aligned with similar moves made by certain other governments. As a result, many manufacturers have decreased the sugar content of drinks formulations from more than 10% to less than 5%. However, there is little understanding of how these re-formulations may impact preservative efficacy and spoilage. Reports from the industry indicate a rise in incidence of certain spoilage yeasts since introduction of the reduced-sugar products. Our preliminary studies have illustrated how metabolism of spoilage yeasts is markedly altered by changes in sugar content below 5%. The yeasts shift from a type of metabolism termed fermentation to another, respiration, as sugar level is decreased. Importantly, this shift coincides with marked changes in the yeasts' abilities to resist preservative. Furthermore, these effects are not the same for all individual yeast cells in a population, a phenomenon known as 'heteroresistance', which can be a particular problem for spoilage control. Besides reduced sugar, there is also growing market pressure to use natural products in place of chemical preservatives, for cleaner label drinks products. This project focuses on understanding the impacts of these changes in drinks formulations for yeast metabolism and preservative resistance, and for spoilage control. We will investigate this by testing yeasts in low sugar conditions, which can be precisely controlled using a technology known as microfluidics. This technology also allows us to examine single yeast cells, as some individual cells can be highly resistant to preservative. We will apply the latest genetic technology to single cells to find out what makes them resistant and we will then exploit that information to find alternative agents that could give better inhibition at different sugar levels. Importantly, this will encompass tests of candidate natural-product activities, including some promising candidates from our industry partner supporting this project. The proposed project could offer solutions to give more-complete inhibition of spoilage yeasts in new formulations, an area of particular interest to the industry partner. While soft drinks and resistance to preservatives (including natural products) at different sugar levels provide the exemplar for this work, the knowledge generated will help develop strategies for preventing yeast food spoilage more broadly.