The aim of the study to was to investigate the efficacy of commercial tannins to protect white and red wines against oxidative browning as a result of laccase enzyme from Botrytis cinerea infection.
- Five commercial tannins, representing the main botanical origins, were evaluate for their inhibitory effect towards laccase enzyme. They included three condensed tannins from grape skins, grape seeds and quebracho, and two hydrolysable tannins from nut galls (gallotannin) and oak (ellagitannin).
- Muscat d’Alexandrie (hanepoot) grapes harvested in 2017 from an experimental vineyard in Spain were used in the study, with half the grapes healthy and the other half infected with Botrytis spores and incubated for three weeks to allow the fungus to grow.
- Juice was obtained from both types of grapes, healthy and infected, and laccase activity was confirmed in the infected grape juice.
- For red wine trials the white juice was supplemented with 50 mg/l of malvidin-3-O-glucoside (anthocyanin) in order to test the specific effect of the commercial tannins only and not the effect of naturally occurring grape tannins as well.
- Two different doses of tannins were added to the clean and infected juice: 20 g/hl and 40 g/hl. Time to inhibit laccase activity was measured only in the 40 g/hl dose juice.
- The juice was fermented and effect of laccase activity and the tannin intervention was measured on the wines.
- The supplementation with all the tannins caused a reduction in laccase activity in the juice.
- The higher dosage caused a greater reduction.
- The highest reduction was 55% residual activity compared to the control infected juice (100% laccase activity). This was obtained with 40 g/hl grape seed tannin.
- The lowest reduction was 81% residual activity and was obtained with 20 g/hl oak tannin (ellagitannin). However, all tannins exhibited a similar inhibitory effect at the lower dosage.
- However, used at the higher dosage the oak tannins were the second most effective after grape seed tannins with 58% residual activity.
- All the tannins, with the exception of the grape skin tannins, took three minutes to inhibit laccase activity. Grape skin tannins took four minutes.
- The A420 absorbance value for juice with laccase was higher (wine more yellow) than clean wine.
- The wines that received tannins were less yellow than the control laccase wine without tannins, proving that the tannin addition protected the wine against oxidation. The maximum effect was already obtained at the lower 20 g/hl tannin dose.
- In the red wine with laccase the red intensity (A520) was almost half of that of the control wine without laccase, indicating the detrimental effect this enzyme has on red wine colour.
- The actual anthocyanin concentration in laccase containing wines decreased to less than a third of the control (without laccase) wine.
- All tannins had a protective effect on the red wine colour with the condensed tannins being the most effective. In this case the 40 g/hl dose had the maximum effect for all tannins.
Significance of the study
This study demonstrated that the specific tannins tested were all able to protect both white and red wines against oxidative browning to some extent. In the case of the red wines the protective effect of the tannins allowed for no detectable difference in wine colour visible to the human eye between supplemented laccase infected wines and control wines without laccase. The limitation of the study is that the tannins investigated were all from one producer only and results can therefore not necessarily be extrapolated to all commercially available tannins.
Link to article: https://oeno-one.eu/article/view/2361
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