Thursday, 6 February 2014

The Secret Life of Yeast

Part 2

In Part 1 of this overview of culturing yeast, I focused on how the yeast is acclimatised to temperature and sugar.  At the same time, the juice which is to be inoculated must be under conditions favourable to yeast growth.

To ensure this is the case, the juice in each culture tank is warmed to a balmy 23 degrees Celcius and some vitamins and nitrogen are introduced, which will assist the yeast with healthy cell growth.

Once the yeast in the bath and the juice in the tank are within 5 degrees of each other, the inoculation can proceed. And so the defining chemical reaction of wine making begins:

Sugar + Yeast = Ethanol + Carbon Dioxide (+ Heat)

Or, in more scientific terms:

C6H12O6 + Zymase → 2 C2H5OH + 2 CO2

The reaction begins very slowly as the yeast continue to acclimatise and build cell mass and structure. This is called the lag phase of yeast growth.

It is not until at least 12 hours later that yeast reaches its exponential phase of growth.  At this point, the sugar content of the juice will begin to decline noticeably hour by hour and the yeast will multiply rapidly.  Their activity makes the top of the juice looks like a soft drink that had just been opened.

The Phases of Yeast Growth
Once inoculated, regular sampling and measurement of the juice must be conducted to monitor when the sugar content starts to decline rapidly.  When exponential phase is identified, cell counts are taken every hour under the microscope to determine how well the yeast population is growing and what proportion of the cells are budding.  

When the desired cell count (as determined by the winemakers) has been reached, the culture is ready for transfer to inoculate a full tank of grape juice (45,000L+).  This is the point at which  the amount of activity generated by the tiny yeast cells really hits home for me:  upon opening the door of the culture tank as it is drained, a surge of carbon dioxide and foam emerges!

Carbon Dioxide pouring from the Culture Tank
Fermentation Foam