Stopping Fermentation

Halting a fermentation is certainly possible and some brewers do indeed use this technique to produce beer with residual sweetness. I don’t think you are looking for residual sweetness, but the topic all relates to the same thing. When fermentation is stopped by cooling, pasteurizing, or adding sulfites, residual sugars are almost always left behind because the basic idea is to arrest progress at a point that is known to be higher than the terminal gravity. An easy way to know the terminal gravity is to perform a forced or accelerated fermentation of a small sample.

If you want to make a malty lager, perhaps with some diacetyl notes, a sweet cider, or a sweeter fruit beer, stopping fermentation may be the ticket. Commercial operations often use a combination of methods to insure the fermentation does not start back up after packaging. The first step is often to chill the fermentation to halt activity or, in the case of cider and wine, sulfites are used to stop the fermentation. Yeast is then removed by filtration or a combination of centrifugation and filtration and pasteurization or another dose of sulfite is used to preserve this state. These products almost always have a detectable sweet note that is desired.

All brewing yeast strains can ferment glucose and maltose and a healthy, normal fermentation ends with no residual glucose or maltose. However, maltotriose, the second most abundant fermentable sugar in wort next to maltose, is often times not completely fermented by brewing yeast and the variability of attenuation rate by yeast strain is largely due to how well different strains ferment maltotriose. For instance, although some saison strains have irritating tendencies to stall during fermentation, most saison strains are able to produce very dry beers, which is one of the key traits of the style. A general answer to your question is to select yeast strains that have normal or below normal attenuation rates if you do not want to end up with very dry beer. Some yeast, most notably certain Brettanomyces strains, are so-called super attenuators because they produce debranching enzymes that allow for the metabolism of limit dextrins that are not fermentable by typical brewing yeast strains; limit dextrins are the primary source of residual extract in beer.

For as long as I have brewed I have been fascinated with mashing and how malt enzymes are used to convert the malt, and other, starches into a blend of fermentable sugars. The fact that beta and alpha amylase have temperature optima that are sufficiently different gives brewers a fair degree of control over the mashing process and the resultant wort. When I think about designing a beer with either a relatively high or relatively low final gravity, mash profile is always at the top of my mind. The malt bill is closely related to this thought process because some malts, most notably crystal and higher kilned malts, contain carbohydrates that are not fermentable and are not converted to fermentable sugars during mashing.

There are four very important and distinctly different things that happen during mashing that relate to wort fermentability. The first is starch gelatinization; not so related to fermentability when brewing all-malt beers, but very relevant when brewing with adjuncts that require high temperature gelatinization, usually by boiling. Starch crystals must be gelatinized, or hydrated and swollen to the point of bursting, to permit enzymatic activity. The handy thing about malted barley, malted wheat, and raw wheat starch is that these materials all contain starches that gelatinize in the temperature range of beta and alpha amylase activity. Rice and corn/maize starches have higher gelatinization temperatures and are almost always boiled before mashing. Flaked rice and flaked corn/maize contain gelatinized starch and can be added to the mash without boiling.

When hot water and gelatinized starch are mixed together in the 140 °F–158 °F (60 °C –70 °C) range, two changes can be detected using one’s eyes and a spoon. The appearance of the liquid changes from milky to clear and the thickness changes from a heavy porridge to soupy. These changes are generally called conversion and can be crudely monitored using the reaction between iodine and starch. Conversion is just one factor related to mashing and simply knowing that conversion is complete does not reveal much about wort fermentability. Alpha amylase is the enzyme that is responsible for conversion and is most active in the 158 °F–162 °F (70 °C–72 °C) range.

Fermentability is primarily a function of beta amylase activity and this enzyme is most active in the 144 °F–149 °F (62 °C–65 °C) range. The practical problem with alpha and beta amylase is that beta amylase can only completely do its job if alpha amylase does its job, and beta amylase works better at cooler temperatures. This is where multi-temperature mashing methods give brewers a lot of control over mash enzymes. Decoction mashing and double mashing techniques both expose a significant portion of mash starches to alpha amylase activity before bringing beta amylase to the party. Step mashing can also be used to control the interplay between beta and alpha amylase. Depending on the brewing objective, these methods can be used to increase fermentability or to limit fermentability. The latter can be achieved by rapidly adding hot mash or hot water with the bulk mash to very quickly raise mash temperature from the lower end of beta amylase activity (140 °F/60 °C) to the upper end of alpha amylase activity (167 °F/75 °C). If you want an Oktoberfest lager with a nice dose of malty richness and full body, this is one of those methods that may be very helpful.

The fourth important thing that oftentimes happens during mashing is increasing mash temperature to about 168 °F/76 °C for mash-off (also called mash out). This step effectively halts enzymatic activity (although alpha amylase retains some activity up to 176 °F/80 °C and this temperature allows unconverted starch to convert during wort collection) and decreases wort viscosity. The former is useful because it allows the brewer to control mash time and temperature and then hit the stop button and the latter is handy because lower wort viscosity during wort separation improves yield and allows for faster wort collection. Modern brewing malts are quite different from brewing malts used as recently as 20 years ago, tending toward greater degrees of modification and enzymatic power. Adding a mash-off step is an easy way of preventing enzyme activity to continue during wort collection and wort heating prior to the boil. The easiest way for infusion mash brewers to mash-off is simply by adding an aliquot of very hot water to the mash while stirring to increase the temperature to 168 °F (76 °C).

Mashing is clearly a very relevant topic to your question. But don’t forget malt selection! Malt is more than a source of starch and enzymes. Color and flavor compounds come from malt, and these are especially important for brewing styles like Oktoberfest. I am a big fan of the nutty, toasty, and bready notes found in higher-kilned, enzymatic malts like Munich and Vienna. The same compounds responsible for these wonderful colors and flavors also provide a boost to residual extract in your finished beer.

I am a modern brewer, but often times ask myself how brewers managed without our modern tools and understanding of brewing science. Your question about limiting wort fermentability is really a perfect example of such a dilemma. And the things that were done by brewers of yesteryear are the same sorts of things that are still relevant today; malt selection, mashing technique, and yeast selection. You can certainly use refrigeration and pasteurization, but consider some of the basic tools in your brewer’s toolbox before adding to the complexity of your process.

Response by Ashton Lewis.