During a recent overseas trip, a colleague of mine once again lamented the joys of travelling. This trip kicked off with a baggage issue that ruined her dinner. Other jolly events included fishing her phone out of a toilet, missing a train, almost being run over by an expressionless Parisian, being kicked by a drunken teenager in Lille and finally a screaming taxi driver in Montpellier. Friendly people… the French. But the inside of a fermenting tank can also be a chaotic and even deadly place for yeast.
Modern winemaking can be very stressful and winemakers are putting increasing pressure on their minuscule friends. No, I’m serous! Pressure can indeed be a limiting factor; especially where low pH and high ethanol is concerned. Trapped carbon dioxide gas not only creates turbulence in a tank, but also contributes to a gradual increase in pressure. Pressures upward of 600 kPa (6 atm) typically stop yeast growth (think secondary fermentation of sparkling wine), but not necessarily alcoholic fermentation. In the book “Wine Science: Principles and Applications” by R.S. Jackson, it is stated that a pressure of 3000 kPa (30 atm) and upwards will completely inhibit alcoholic fermentation! What hardy fellows they are, these yeasts!
The use of pressure to control or stop yeast growth is not uncommon in German wineries, but high pressures can cause other problems. Spoilage organisms such as Lactobacillus, Torulopsis and Kloeckera are less sensitive to pressure and can cause a myriad of problems. The latter micro-organism is particularly pesky, as it is quite sulphur dioxide resistant, ferments at temperatures as low as 10°C and can produce high levels of ethyl acetate and amyl acetate.
Some beer brewers postulate that higher pressures have a positive aromatic effect on their ferments, but clear guidelines during vinification have not been established. At least there are many other ways to boost aroma, so don’t be depressed.
Bernard Mocke is a technical consultant for Oenobrands