It is the alcoholic fermentation which is responsible for the transformation of grape juice into wine. This spontaneous phenomenon had been mastered empirically well before it was scientifically explained by Louis Pasteur in the 19th century.

The bio-chemical transformations brought into play by fermentation are numerous and complex. What was the art of wine-making, and has now also become the science of wine-making, still comes down to the successful completion of those transformations in the best possible conditions. This particularly involves a strict control of temperature, to prevent the heat generated by the fermentation from eventually killing the yeasts which are themselves making the juice ferment. Also pumping over, which consists in pumping the must from the bottom of the vat to the top, helping to dissolve the components of the grapes, as well as bringing small doses of oxygen which are necessary for the metabolism of the yeasts. The recent use of selected yeast strains has enabled wine-makers in certain difficult vintages to get round the problem of fermentations which go on too long, and which end up badly...

Once the alcoholic fermentation has finished, the maceration continues until the wine is run off the skins. The wine is then almost «finished», but in order to be completely stable, a last bio-chemical transformation has to be performed : the fermentation by bacteria of the malic acid in the wine into lactic acid. It is spontaneous, but does not always start easily. The wine needs to be maintained at a temperature of at least 20°C (68°F). Before its discovery at the end of the 1950s, it was often noticed that during the first warm spells in spring, the wine suddenly began to « work ». People thought that the flowering of the vine could have been the origin of this mysterious arousal ! It was simply the malo-lactic fermentation which was triggered off by the rise in temperature. All this progress may not have made the great vintages any more remarkable -they are first of all the result of favoured natural conditions, but it partly explains why the lesser vintages are that much better.