Quantitative analysis of the killer activity of some oenological yeasts. Effect of some additives

<p style="text-align: justify;">The Killer factor was discovered in 1963. Since this time it has been widely studied and nowadays a lot is known about genetics of the factor, the biochemistry of the toxin and also about the way of action of the toxin on sensitive yeasts. The yeast strains are classified in three groups : killer strains, sensitive strains and neutral strains. The killer strain is able to kill sensitive strains while neutral strains are unable to kill any strain and remain unaffected by the toxin. Certainly this classification depends on the couple of strains (killer and sensitive) taken as references. It has been clearly established that the toxin acts on the sensitive cells by inducing important losses of ATP : due to holes created by the toxin in the cell membrane, ATP leaves the cell.</p><p style="text-align: justify;">In the field of Enology, it’s generally accepted that a killer yeast has more probabilities to have a good implantation in a non sterile medium (as the must is) than a neutral or a sensitive strain. Nevertheless, it is difficult to have a precise idea of the sensitivity of a strain as well as of its toxicity, as most of the methods are only able to give a qualitative information.</p><p style="text-align: justify;">In this paper, a new method of evaluation of the killer effect is presented. Also some results dealing with its application to the classification of some enological yeast strains are given and discussed.</p><p style="text-align: justify;">This method is based on the measurement of the ATP released by the sensitive strain under the action of the killer toxin. The criterion that we define is the initial rate of ATP release, that means the quantity of ATP lost in the two first hours by the sensitive cells in contact with the toxin. In a first step, it is shown that this method is reliable and also that it is in a good agreement with the method using the flow cytometry. ATP leak could be correlated with the amount of affected cells (dead and damaged cells). So, using this method, it becomes possible to classify easily yeast strains with respect to their sensitivity or toxicity. Several killer yeast strains were tested against a sensitive strain and different sensitive strains were submitted to the action of a killer toxin produced by a killer strain. It was shown that the losses of ATP were quite different according to the sensitivity of the strain. The initial rate of ATP release was found to be in a range of 0.00 to 0.12 micromole/L/h. The second part of the work deals with the study of the possible effect of some products used for winemaking on the efficiency of the killer effect. The products we have studied were bentonite, tannic acid and enological tannins. It is shown that these products may interact with the killer effect. So, bentonite, for example is able to inhibit completely the efficiency of the killer toxin, as soon as its concentration is about 10 grams/hL.</p>