Antifoam in whisky fermentation

Why wash foams in the first place

Foam forms when carbon dioxide bubbles rise through a liquid that contains proteins and other surface active compounds. Malt wort has plenty of both. In the most vigorous phase of a whisky fermentation, usually the first 12 to 36 hours, the yeast produces CO2 fast enough to build a thick head that can climb out of the vessel.

Distilling wort foams more than many beer worts because it is unboiled. The proteins that a brewer's boil would coagulate and drop out stay in solution. Grains such as rye and some barley varieties make it worse. Our guide to whisky wash fermentation covers the stages of fermentation in full.

What foam costs you

A washback that foams over loses wash. Every litre of wash at around 8% ABV is about 80 ml of pure alcohol you will never distil. It also makes a mess that bacteria love: sticky deposits on lids, walkways and drains that are hard to clean.

The bigger cost is often hidden. To avoid foam-overs, many distilleries fill washbacks well below their capacity. If a washback that could hold 25,000 litres is filled to only 20,000 to leave headspace, a fifth of your fermentation capacity is going to foam room. Over a year that is a large share of output, which is why getting foam under control can raise capacity without buying a single new vessel. On a plant running a fixed number of fermentations a week, every extra 1,000 litres per washback is about 80 more litres of pure alcohol per batch.

Mechanical foam control comes first

Before reaching for chemicals, look at the vessel. Traditional malt distilleries fit a rotating arm, often called a switcher, that turns across the top of the washback and knocks the foam back down. It needs nothing added to the wash and works every batch.

Other mechanical options:

• Enough headspace, usually planned at the design stage.
• A foam sensor that warns the operator before the wash reaches the lid.
• Controlled set temperature, because a warmer fermentation runs faster and foams harder.
• Pitch rates that give a steady fermentation rather than a violent one.

How antifoam works and how to dose it

Antifoams are surface active agents, usually silicone based or based on vegetable oils and fatty acid esters, that break the films around bubbles so foam collapses. They work at very small doses. More is not better; overdosing can affect yeast and leave residues in vessels.

Follow the supplier's recommended dose and start at the low end. Add antifoam at filling, where it is most effective, rather than throwing it on top when a washback is already spilling. Keep a record of the dose for every batch so you can see whether foam problems are getting worse, which is a signal in itself. Use only food grade products approved for your purpose and check that your local spirit standard permits them. Rules differ between categories and countries.

What it does in the wash still

Foam control matters again at distillation. A wash still can foam as it comes to the boil, pushing liquid up the neck and over the lyne arm into the low wines. Distillers call this boiling over or puking. It contaminates the low wines with wash and may force a clean-out of the condenser.

Some antifoam carries over from the washback and helps calm the wash still too. Even so, the main defence there is careful heating: bring the charge up slowly, watch the sight glass and ease off when the foam rises. Our guide to what the wash still and spirit still each do covers the still side. A pot still with a broad shoulder and a tall neck copes with foam better than a squat one.

When rising foam is telling you something

If foam gets worse over weeks, look for a cause before increasing the dose. A new malt delivery with a different protein content, a change in mill gap, warmer set temperatures in summer or a shift in the yeast can all raise foaming. An infected washback can foam unusually too. Our guide to yeast nutrition explains how stressed yeast changes the fermentation profile.

Your first step: measure how much headspace each washback actually has at your current fill and note the highest foam level each batch for two weeks. If foam never comes within a hand's width of the lid, you may be able to fill more. If it reaches the lid every time, fix temperature and pitch rate before buying a bigger drum of antifoam.