Sour mash and backset

What sour mash means

In a sour mash process, a portion of the liquid left after distillation is added to the next mash or fermenter. That liquid is called backset, also known as setback or stillage. It is acidic, so it lowers the pH of the fresh mash. The whiskey made this way does not taste sour.

The opposite is a sweet mash, where every mash starts with fresh water and no backset. Most American bourbon and Tennessee whiskey is made by the sour mash process. It is so common that a label saying sour mash tells you little about how one whiskey differs from another.

Sour mash is easily confused with deliberately souring a mash with lactic bacteria, as some sour beers are made. They are different. In the sour mash process, fresh yeast is still pitched into each fermentation. The backset has already been boiled in the still, so it carries acid and nutrients rather than a live culture.

Some distilleries also carry yeast forward from batch to batch, or keep a house yeast culture. That is a separate practice from backset.

Where the backset comes from

American whiskey is usually distilled on the grain in a column beer still. What drains from the bottom is a hot slurry of water, spent grain and yeast. The thin liquid part, screened or separated from the coarse solids, is the backset. It contains lactic acid from the fermentation plus some yeast nutrients and dissolved solids.

On a craft distillery with a pot still, the residue from a stripping run on the grain can be used in the same way, as long as it is screened and handled hot and clean.

Why distillers use it

• Lower pH. A maize mash made with plain water can sit at a pH that suits bacteria. Backset brings it down into a range that favours yeast.
• Consistency. Each mash starts from a similar chemistry, batch after batch, which helps fermentations behave the same way.
• Nutrients. Backset carries yeast nutrients and some dead yeast, which helps the next fermentation.
• Water saving. Recycled backset replaces part of the fresh water each mash needs.

Bacterial control matters most in long, warm fermentations, which American whiskey fermentations often are.

How much backset to use

Proportions vary by distillery and recipe. Backset is often used for something like a quarter of the liquid in the mash or fermenter, though some recipes use more or less. The aim is a target pH at the start of fermentation, not a fixed volume. Measure the pH of each mash after backset goes in and adjust the share until you hit your target consistently.

Too much backset brings in excess acidity and dissolved solids, which can stress yeast. Too little leaves pH higher and fermentation more exposed to bacteria. Our guide to bacteria in long whisky fermentations covers the risks.

Handling backset safely

Backset leaves the still near 100 °C. It is hot, acidic and easy to scald yourself with. Pipe it in closed lines, never in open buckets. Insulate the lines. Store it hot and use it quickly. Cooled backset left sitting in a tank becomes a home for spoilage organisms and smells within hours in Indian heat.

Solids are the other issue. Screen it well. Grain solids in backset build up in pumps and heat exchangers and make cleaning harder.

Sour mash on an Indian craft site

Most Indian malt distilleries follow the Scottish model of lautering and fermenting a clear wort, then distilling in pot stills, so they do not use backset. A craft distillery making a corn or bourbon style spirit and fermenting on the grain may find the sour mash process useful, especially in hot months when bacteria multiply faster.

Whatever spirit you make, check how the effluent rules in your state treat recycled stillage. They vary and change, so ask your pollution control adviser before you design the system. Our guide to corn whisky mash bills covers the grain side.

Tennessee whiskey uses it too

Tennessee whiskey is made in much the same way as bourbon, including the sour mash process, with one extra step. The new spirit is filtered through a thick bed of sugar maple charcoal before it goes into the barrel. That is known as the Lincoln County Process. It is the main thing that sets Tennessee whiskey apart. Our guide to Tennessee whiskey and the Lincoln County Process explains how it works.

For a craft distiller, the lesson is that sour mash is a process tool, not a style. It sits underneath many different whiskeys.

A first step

Before your first sour mash run, measure the pH of a sweet mash made with your water and grain. Then make small test mashes with backset at three different shares, measure pH and ferment each in a clean container. Compare the speed of fermentation and the final gravity. Those numbers set your starting recipe better than any rule of thumb.

Common questions

Does sour mash whiskey taste sour?

No. Sour mash refers to a process where acidic backset from a previous distillation is added to the new mash. It does not make the whiskey taste sour.

What is backset?

Backset, also called setback or stillage, is the thin acidic liquid left after distillation. A portion is added to the next mash or fermenter to lower pH and add nutrients.

Why do distillers use sour mash?

It lowers mash pH to a range that favours yeast over bacteria, helps each batch behave consistently and saves fresh water.

Is sour mash the same as a sour beer?

No. Sour beers are deliberately soured with live bacteria. In sour mash whiskey, fresh yeast is pitched and the backset has already been boiled in the still.