Tuesday, 4 February 2014

The Secret Life of Yeast

Part 1

With the onset of Week 3 of vintage, the pace in the yeast area has accelerated to breaking point. We are managing on average 6 simultaneous yeast cultures each 12 hour shift.  

There is a fairly standard methodology for preparing a yeast culture for white wine, with a few tweaks here and there.  The variations will depend upon the grape type, and therefore type of yeast used, as well as the desired style of the wine.

rehydrating dried yeast

In general, the yeast comes freeze dried in 500g sachets and looks no different to normal bread yeast. There are noticeable differences in smell between the strains - I get hints of wet dirt in some and compost in others - but the overarching aroma is very similar to that when baking bread.

To rehydrate the yeast, a 'bath' of water at 35-40 degrees Celsius must first be prepared before the required amount of yeast is added.  In regards to quantities, we add approximately 4kg of yeast to a 2,000L tank for the culture, which then is used to ferment a 40,000L tank.

From here on in, the process is all about acclimatisation.  The yeast is initially weak as it comes out of hibernation, so it needs to sit in the bath for at least 10 minutes before being gently stirred. 

Then the fun begins. To break up any clumps of dry yeast hands hands are required to give it a good mix, whilst enjoying the gooey sensation and fresh bread aromas!

mixing the yeast
Again, another 10 minutes of waiting and then the yeast is ready for some food.  A small amount (1 bucket full) of the grape juice that is about to be inoculated is introduced into the yeast bath to acclimatise the little organisms to sugar (similarly to how you don't give a child too many lollies all at once!). 

After another wait, greater quantities of juice can be added to achieve the final goal: the yeast bath and juice tank must be within 5 degrees of each other before the yeast is introduced.  This is to ensure the yeast is not 'shocked' by the temperature differential, thus giving the best possible chance to survive and thrive.  As more juice is added, an increase in activity is quickly visible.

yeast enjoying a feast of grape juice