INOCULATION OF THE SELECTED METSCHNIKOWIA PULCHERRIMA MP1 AS A BIOPROTECTIVE ALTERNATIVE TO SULFITES TO PREVENT BROWNING OF WHITE GRAPE MUST

Abstract

Enzymatic browning (BE) of must is caused by polyphenol oxidases (PPOs), tyrosinase and laccase. Both PPOs can oxidize diphenols such as hydroxycinnamic acids (HA) to quinones, which can later polymerize to form melanins [1], which are responsible of BE in white wines and of oxidasic haze in red wines. SO₂ is the main tool used to protect must from BE thanks to its capacity to inhibit PPOs [2]. However, the current trend in winemaking is to reduce and even eliminate this unfriendly additive. Among the different possible alternatives for protecting must against BE, the inoculation with a selected Metschnikowia pulcherrima MP1 is without any doubt one of the most promising ones.
For that purpose, white grapes were harvested, pressed and diluted 5 times with a model grape must synthetic buffer at pH = 3.50 and supplemented or not with 20 mg/L of SO₂, 2 UA/mL of laccase activity and 250 mg/L of the selected M. pulcherrima MP1 (Level2 Initia™, Lallemand Inc, Montreal, Canada). Immediately, the samples were saturated with O₂ and its concentration was noninvasively monitored overtime by luminescence (Nomasense TM O₂ Trace Oxygen Analyzer by Nomacorc S.A., Thimister Clermont, Belgium) [3]. Once oxygen consumption attained an asymptotic behavior the samples were used for color analysis [4] and for HPLC analysis of HA [5].
As expected, in the absence of SO₂, the must actively consumed O₂ and HA, and it turned intensely brown whereas in the presence of SO₂, the O₂ consumption rate (OCR) was significant lower, the HA concentra- tion was maintained at significant higher levels and the yellow color intensity remained at low values. In presence of laccase, OCR and browning intensity were even higher than in control conditions and the supplementation with SO₂ reduced both parameters but not as much as in the control must. Inoculation with the selected M. pulcherrima MP1 increased significantly OCR and protected the must from BE since the final yellow color was significantly lower and the HA concentration significantly higher than in control conditions although this protection was not so effective as that of SO₂. It seems the- refore that selected M. pulcherrima MP1 consumes O₂ very effectively making that some of the initially dissolved O₂ is not consumed by PPOs. In the presence of laccase, the supplementation with MP also protected the must from browning but not so efficiently.
This data confirms that the use of the selected M. pulcherrima MP 1 can be an interesting tool for redu- cing the dose of SO₂ without affecting seriously its final color quality.
Acknowledges: This research was funded by CICYT (project RTI 2018-095658-B-C33).