WINETECH Technical Yearbook 2021

OENOLOGY RESEARCH | DECEMBER 2021

INTRODUCTION Healthy grapes have a complex microbial ecology including filamentous fungi, yeasts and bacteria. The variety and population of microorganisms present on the grape surface will depend on many factors. However, high sugar concentrations on the berry surface of damaged grapes will especially favour the increase of undesirable microorganisms, such as acetic acid bacteria and wine spoilage yeasts. These microbes are transferred to the must during processing and can have a profound influence on wine composition, flavour and quality. Populations can increase with extended duration of pre-fermentation operations, such as grape transportation, cold soaking, cold storage of juices or grape dehydration. Winemaking strategies to reduce the population of unwanted microbes usually include the use of sulphur dioxide. With the increasing demand for low sulphur dioxide wines, alternative strategies such as bioprotection are explored. WHAT IS BIOPROTECTION? Bioprotection is a natural method to inhibit undesirable indigenous microorganisms present on the grapes or in the must. By adding living organisms specifically chosen for certain properties, the matrix can be colonised in a controlled manner and

limit the predominance of the unwanted microflora. BIOPROTECTING AGENTS The chosen bioprotector needs to be able to protect the grapes or must from unwanted microbial growth without altering the matrix composition in an undesirable way. The organism needs to be a 1) high-quality strain of 2) oenological origin and 3) have low fermentation activity at the inoculated dose. Other than that, the product needs to be robust to non-rehydration. Several studies have identified two oe- nologically derived non- Saccharomyces yeast strains as effective bioprotectors: Torulaspora delbrueckii and Metschniko­ wia pulcherrima. These strains are ideal for oenological bioprotection due to the following properties: • M. pulcherrima produces a naturally occurring antimicrobial compound, pulcherrimin, which inhibits the growth of various undesirable microorganisms, such as Brettanomyces bruxellensis and Botrytis cinerea (Sipiczki, 2006; Morata, et al ., 2019). • M. pulcherrima scavenge iron which is an essential growth factor for other microorganisms (Sipiczki, 2006).

Bioprotection – a microbial alternative to SO 2

MORNÉ KEMP: Laffort, Paarl KEYWORDS: Bioprotection, microbial control.

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