Yeast and Fermentation

This post is dedicated to the organism that makes the magic happen, written chiefly in the context of Country Wines, but there is some good general information for the wine-or-beer-kit junkies amongst us!

Yeast is a living organism that can be found naturally occurring on the skins of fruit, however these natural amounts generally aren’t sufficient to ferment your fruit all by itself. As such, most kits or recipes will use a certain type of yeast bought specifically for the job. We’ll profile some later, but first, some technical details….

Aerobic and Anaerobic Fermentation

Aerobic and Anaerobic literally mean “in the presence of air” and “lacking the presence of air” respectively. Yeast converts the sugars in the must to alcohol (producing carbon dioxide as a by-product), and it uses aerobic and anaerobic processes to turn sugary fruit syrup in to a fine country wine.

Aerobic Fermentation

Generally speaking when fermentation starts in a country wine, the aerobic fermentation happens first, so in some recipes the initial fermentation will take place in the fermentation bin, or in a demijohn plugged only with cotton wool. Both of these methods allow the yeast access to oxygen in the air.

In the aerobic process, the yeast reproduces – hence even a surprisingly small amount of yeast will happily multiply into a culture capable of fermenting your large, many-gallon vat of must! The best temparature for this stage of fermentation is around 20-22°C. Anything warmer risks killing the yeast, much cooler than this and the yeast will lie dormant.

Anaerobic Fermentation

The second stage of the fermentation concerns the newly multiplied yeast population settling down to the task of converting those sugars to alcohol. Less reproduction of the yeast culture takes place at this stage.

Generally speaking this stage of fermentation will take place in a demijohn with the airlock fitted, to seal away the oxygen. The air above the liquid in the demijohn is gradually replaced by carbon dioxide from the fermenting sugars, so those recipes where the initial ferment takes place in the bin then are transferred to the demijohn, and airlocked ready for this anaerobic stage. Recipes using the demijohn for the initial ferment will wait for the initial, vigorous, ferment to slow down before fitting the airlock ready for the anaerobic stage.

This stage can take place at a lower temperature, as little as 15-18°C. Again too high a temperature will kill the yeast, and too low will inhibit the fermentation.

The “Stuck Ferment” Nightmare…

At some time, even the most discerning of wine brewers will experience the stuck ferment – that is the premature ceasing of the yeast in converting the sugars to alcohol. This leaves a very sickly sweet wine which is of too high an SG reading (see SG and ABV post).

Fortunately, there are a few tricks you can pull to restart a stuck ferment.

• Is the temperature to low? Move brew to a warmer place
• Is the temperature to high? The yeast could have been killed, activate and add some more
• Too much alcohol already? The yeast could have been killed, this is tougher to solve – blending with another brew is an option

First port of call is to add a little more yeast and nutrient, and leave for a couple of days… If this doesn’t solve then advanced factors such as acidity may come into play.

Readily Available Yeasts

We have plenty of types of yeasts available. Here are some examples that are tried and tested by us at Brew2bottle

• Gervin No 8 – A good full bodied red-wine yeast for e.g. Damson, Elderberry.
• Burgundy Yeast – An alternative to the above for Red country wines.
• Sauterne’s Yeast – a yeast best suited to white wines. We generally use this in Apple wine, or real white grape wine
• All Purpose Yeast – e.g. Gervin no. 1 – Jack of all and master of none? We generally go for this as an alternative to the Sauterne’s
• Re-start Yeast – a specialist strain to help overcome the stuck ferment scenario descibed above.

Check out the yeasts section, particularly the Muntons Gervin Yeasts which have great detail on the specific usage of each strain.

Other Fermentation Terms You May Have Come Across

Further to the aerobic and anaerobic fermentation processes described above you might have heard about the following:

Malo-Lactic Fermentation

This is an ancilliary fermentation which can happen after a wine has been bottled, or certainly after the main fermentation is complete. Strictly speaking it’s a undertaken by a bactieria rather than a yeast – but in layman terms this is a process where certain acids (e.g. malic) are converted to certain other acids (e.g. lactic) of a lower strength. It brings about a a slight reduction in the acid “tang” of your wine, and it’s by-products are generally considered beneficial – increasing the maturity and well-rounded taste of your wine.

Carbonation / Sparkling Fermentation / Priming

This is an important one for beers, and sparkling wines (country or kit!) – essentially this is a yeast-driven anaerobic fermentation which takes place in an enviroment capable of holding some pressure. And in English: some leftover yeast will ferment a little added sugar to create a sparkle of dissolved CO₂ as longs as you’re sealed in a pressure barrel, or in a wired glass bottle, swingtop, or even a plastic pop bottle!

This creates the fizz in a beer,cider or champers and in all cases requires priming – a relatively small amount of sugar (compared to the initial amount involved in the ferment) is consumed by any remaining yeast – and it’s important to ensure some yeast remains so when doing a sparkling brew it is important to draw a little yeast through to ensure this can happen. Don’t over-do it with yeast, sugar or temperature or there’ll be an explosion. Great results and well worth the effort.

I’m sure we’ll write in more detail about priming and sparkling later, and when we do we’ll link to it from here.

Until then….

C₆H₁₂O₆ ->  2CO₂ + 2C₂H₅OH