Wednesday, 14 October 2009

Stuck Ferment!!

Sometimes when making wines the fermentation can stop. The reasons for this spontaneous halt are endless and we winemakers are initially left baffled! But we rack our brains and always find the solution.

The main factors that cause the fermentation to stop are level of nutrients (most importantly nitrogen) which are obtained by the grape from the soil type and vintage conditions in the vineyard. If these nutrients are low we will add a little bit extra so the yeast are happy.

The conditions of the cellar are also important as yeasts don’t like the cold and will die! As the yeasts start to die off, the population dwindles and the remaining active yeast cannot combat the rising alcohol level and finish the remaining sugar. The weeks that follow the grape harvests in Europe are the start of winter and the natural air temperature falls significantly. Fortunately our cellar is well equipped with a heating and cooling machine that can keep the fermentations at ideal temperatures.

Oxygen is also very important and often the wine must be splashed (winemaking term for carefully aerating wine) to give a small dose of oxygen to the remaining yeast and try to kick start the fermentation. If all else fails the last step is to introduce new yeast or a ‘rescue culture’ – ‘killer yeast’ as we winemakers call it. We call it killer yeast because once let loose into the wine it will overcome and fight the weaker yeast for the last sugar.

Because it is more adaptable to the conditions (temperature, alcohol etc) it builds a population – or in this case an army! – finally resulting in the extinction of the struggling yeast and finishing off of the sugar. This is done very carefully by starting a small fermentation in a tiny tank keeping it warm and at a constant temperature – we even wrap a nice warm blanket around the bottom of the vat!!

The sugar level is monitored very closely and when it starts to drop, this means the yeast is starting kick and become stronger. It is when – and only when – the yeast has significantly eaten a large amount of sugar that more wine from the mother tank is added. If this is done to early or too late, it could result in a big shock for the yeast and even the killer yeast will die. The right time is never during normal working hours so we will take shifts to watch the progress all night!


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