If you're going to ferment reliably, you have to know quite a bit about yeast. The first thing that you need to know is that it's a living, breathing, organism, and it's as sensitive to its environment as a puppy. Too much heat, and it dies. Too little heat and it stops working. Too much food, or too much waste (think CO2 and ethanol) and it will sicken and die, leaving nasty tastes in your wash that the still can't remove completely. Too much acid, or too little, and it will fall into a coma and stop working.

If that sounds daunting, it is, kinda, but if you keep your yeast warm and happy, carefully fed, and with good nutrition, in an environment not too acidic or caustic, your yeast will produce clean, great-tasting washes, which will in turn give you clean, great-tasting spirits.

On the other hand, if you mistreat your yeast, you'll drink crap.

Why Even Experienced Brewers and Winemakers Need to Know More about Yeast.

As overwhelming good luck (at least for brewers and winemakers) would have it, both grapes and mashed barley malt have just exactly what yeast needs and likes, in terms of sugar concentration, nutrients, and acidity, so it's hard to go wrong making grape wine or barley malt beer.

If, on the other hand, you are designing and building a rum or sugar-vodka wash, or a thin whiskey mash, it's very easy to add too little, or way too much sugar to your wash, or to ignore the yeast's nutrition, to get the acidity wrong, or even to pick the wrong yeast strain.

To the distiller, any of those mistakes can ruin a wash.

The Life Cycle of Yeast - Hydration

If you want to predict and optimize how your yeast will work, you need to understand the steps in the yeast's life. The first step is hydration.

When you first sprinkle dry yeast on juice, or wort, the yeast begins to hydrate itself, absorbing water, repairing damaged cell walls, and starting to figure out how to ferment whatever liquid it finds itself in. Its biggest requirements at this point are comfortably warm water (85F or 29C), plenty of dissolved oxygen, reasonable sugar concentrations, hydration-specific nutrients, and least important to the distiller (assuming you haven't done something dumb), "comfortable" acidity (pH 4.0-5.5).

What can go wrong during hydration? High sugar concentrations can explode yeast cells, hot water can kill yeast, poor nutrition and low oxygen can give a slow start, and perhaps a long bad-tasting, illness and maybe death. A slow start can also make your fermentation vulnerable to infection.

The Life Cycle of Yeast - Aerobic Multiplication

After hydration, if dissolved oxygen is present, the yeast cells will start replicating, and the number of yeast cells will increase dramatically. During this process, the yeast consumes sugar for energy, nitrogen-containing nutrients to make new proteins, and oxygen to build new cell walls. In this phase, no ethanol is produced. CO2 and water are the only products.

What can go wrong during aerobic multiplication? Low oxygen levels will inhibit yeast replication, and the subsequent low yeast numbers will cause a slow or weak fermentation.

The Life Cycle of Yeast - Anaerobic Fermentation

After the yeast has consumed all the dissolved oxygen, it enters the anaerobic fermentation phase. In this phase, the yeast still eats the sugar and and the nutrients, but it does not reproduce. In this phase, CO2 and ethanol are the primary products. This is the phase where the alcohol is produced. During this phase, the wash will become more acid (pH goes down).

What can go wrong in this phase? Accumulating alcohol can poison and kill the yeast. Low temperatures can stop yeast activity, and high temps can kill it.

The Life Cycle of Yeast - Flocculation and Dormancy

In the very best case, the case we hope for, the otherwise healthy yeast simply runs out of sugar to eat, flocculates (clumps together), and settles to the bottom, still alive but in a dormant state. The successful fermentation is complete.