WINETECH Technical Yearbook 2021

OENOLOGY RESEARCH | APRIL 2021

Can malolactic fermentation enhance volatile phenol production by Brettanomyces ?

DIEMERSDAL WINES

KARIEN O’KENNEDY: Winetech, Paarl KEYWORDS: Malolactic fermentation, Brettanomyces spoilage, Pinot noir.

THE SHORT ANSWER Yes, it can. Researchers from Oregon State University found that the choice of malolactic fermentation (MLF) starter culture can influence Brettanomyces spoilage in Pinot noir wines (Du Bois et al. , J., 2021). The reason? Certain lactic acid bacteria (LAB) display enzymatic activities that can generate more substrate for Brettanomyces to convert to volatile phenols. PRACTICAL ADVICE In cellars with a risk of Brettanomyces : • If commercial MLF starter cultures are used, avoid using cinnamoyl esterase positive (CE+) strains in Pinot noir. • Av o i d s pon t ane ou s ma l o l a c t i c fermentation (MLF) in Pinot noir since the bacteria conducting the MLF can potentially be CE+.

THE LONG ANSWER

Brettanomyces forms 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG) in a two-step process from p- coumaric acid and ferulic acid, respectively. These hydroxycinnamic acids are naturally present in grape juice, but mostly bound to tartaric acid to form the esters: p- coutaric and fetaric acids. Brettanomyces cannot metabolise the tartaric acid esters; it can only metabolise the “free” form of the hydroxycinnamic acids to form the dreaded, foul smelling ethyl phenols. That means, for Brett to really flourish, it needs to get by with a little help from its (microbial) “friends”, in this case: lactic acid bacteria. Certain strains of lactic acid bacteria (LAB) possess cinnamoyl esterase (CE) enzymatic activity that can hydrolyse the tartaric acid esters, to release the free forms of the hydroxycinnamic acids, i.e. Brett substrate.

WINETECH TECHNICAL YEARBOOK 2021 | 67