Dry Yeast Advancements, Alternate Decoction Mash Purpose, and Heating a Fermenter

Q I read the article in the May-June 2023 issue of BYO on advancements for dry yeast with much interest, but I’m still not sure if rehydrating is necessary. What is your take on it? In his book on yeast, Chris White explains why it is necessary, and how to do it. He states that pitching yeast without rehydrating reduces yeast cell viability. It seems that when I rehydrate the lag time before fermentation starts is shorter and fermentation time is shorter, which would agree with higher cell count.

Byron Hovey
Carolina Beach, North Carolina

sprinkling a pack of dry yeast into water to rehydrate
Rehydrating brewer’s yeast was once highly encouraged, but thanks to manufacturing improvements that advice has changed. 

A This is a great example of an advancement in the brewing world that has a real effect on the way home and commercial brewers go about brewing. When I was a young and eager student in Dr. Michael Lewis’ brewing lab at UC-Davis in the early 1990s, we brewed beer using both high-quality, dried yeast and liquid yeast we propagated in the lab. During this time I learned that dried yeast generally had a bad reputation among craft brewers because some commercially available dried yeast products were contaminated with a wide array of unwanted microflora. Indeed, plating dried yeast from dubious sources was one of the things we did in the brewing lab class. When I began brewing beer for commercial sale in 1997, dried yeast was not even on my brewing radar, and I set up an account with White Labs. I’ll get back to Chris White in a bit.

Fast forward the clock 20 years and yours truly begins working in the world of brewing raw materials and starts learning about the changes in the world of dried yeast from the time in the early 90s when most brewers agreed that liquid was king. The first shocker was the realization that many well-respected craft brewers used dried yeast on the regular. I was like, “what rock have I been living beneath?” And the next shocker was comparing my go-to liquid ale strain to a dried version of a very similar strain in a 15-BBL batch of one our top sellers; the shocker was not noting any differences in fermentation rate, final gravity, or beer flavor. I was totally biased at the time and was certain that I would come up with something to gripe about! Suffice to say, I quickly got up-to-speed on things and adjusted my strong opinions.

Back to your question about what Chris White states in Yeast: The Practical Guide to Beer Fermentation he and Jamil Zainasheff co-authored about rehydration practices. I haven’t contacted Chris and don’t know what references he used to shape his views about rehydration. I do know, however, that their book was published in 2010 and there were many yeast suppliers advising brewers to rehydrate yeast at the time. The primary reason for a special rehydration process, versus just sprinkling the yeast directly into wort (aka dry pitching), is decreasing osmotic stress to the yeast cell during hydration. After the yeast cell wall and internal membranes are hydrated, they are better equipped to regulate the flow of water and wort solutes into the cell. When Chris’ book was published, this was the accepted best practice among yeast suppliers.

The thing about yeast hydration is that it takes care to perform correctly. This is not much of a challenge at home, but in commercial brewing operations hydrating yeast with 95 °F (35 °C) to 104 °F (40 °C) water for 30 minutes before use is easier said than done. Back in the early days of craft brewing, some brewers questioned the need to rehydrate because they knew that simply sprinkling yeast on top of wort usually worked fine when they homebrewed and decided to skip the step and dry pitch.

Yeast manufacturers at the time knew about the “debate” surrounding rehydration and concerns about purity. The larger companies had already been developing processes to improve brewing yeast purity, something of lesser importance to most bakers who mainly use yeast for quick production of CO2 from the massive stores of glycogen accumulated using specialized propagation techniques. And some companies advised their customers to up the pitching rate when skipping rehydration while others were working on proprietary drying methods to reduce loss of cell viability with dry pitching.

Today, dried yeast enjoys an excellent reputation around the world and is key to the production of a varied range of beers, ciders, wines, seltzers, hybrid beverages, and distilled spirits. And liquid yeast suppliers are looking at ways to sell into this market. White Labs is now selling some of their best-selling yeast strains in a dried form and promoting the benefits of dry pitching: “No rehydration or wort aeration is necessary due to the high sterol content and simple measurements of pitch rate by weight for hassle-free fermentations.” This list of advantages has been promoted by Fermentis for several years and it’s great to see more yeast experts throwing support behind a product that has morphed from possible menace to trusted tool over the last 30 years of brewing history.

Q I’m a bit confused, and I am hoping you can help. I read an article on how Sam Adams uses decoction mashing to make their light beer. The article stated the idea is that enzymes are destroyed during the decoction process to lower the concentration of fermentable sugars, which in turn lowers both ABV and calories. I also read that decoction mashing helps to destroy the cell walls and makes the starches more accessible to the malt enzymes. To me, more starch would result in more fermentable sugars and thus increase the ABV. This seems to contradict what is said above. What am I missing?

David Corr
Centerville, Ohio

A Thanks for the fun question, David! I want to begin with a bit of housekeeping. I was able to find an article on the Food & Wine website from June 22, 2017 ( that has quotes from Jim Koch from Boston Beer explaining the Sam Adams Light as you have described.

The other thing I want to quickly review is the enzyme content of malt and the differences between dextrinizing units (DU) and diastatic power (DP). DU is a measure of alpha amylase activity, the enzyme that liquefies starches and increases wort extract; some fermentable sugars are produced by alpha amylase, but not much. DP represents both alpha and beta amylase activity, but the DP value is heavily influenced by beta amylase (the enzyme in malt that yields most of the fermentable sugars in wort). Finally, beta amylase is denatured at lower temperatures than alpha amylase, and increasing the concentration of starch in wort does not give more fermentables without beta
amylase activity.

Okay, so Jim Koch is stating that Sam Adams Light uses the decoction mash method as a technique to reduce wort fermentability via enzyme denaturation. What he doesn’t explain is what type of decoction mash method is used to accomplish their goal. But knowing their method is not required to explain how this can work. In classic triple decoction mashing, the first mash rest starts at around 95 °F (35 °C). A thick portion of mash is removed, heated to boiling with a brief rest around 158 °F (70 °C) for alpha amylase to chop up starches and reduce mash viscosity, boiled for some time, then pumped back to the rest mash. This results in an increase in the blended mash temperature to about 122 °F (50 °C). The process is repeated (minus the pause at 158 °F/70 °C) two more times for a mash rest at 158 °F (70 °C) and mash-off at 168 °F (76 °C). The norm in decoction mashing is to boil portions of the thick mash because the enzyme concentration is not as high as in the liquid phase above the mash.

If the goal is to really limit wort fermentability, the heating rate up the mash ladder is usually too slow because mash mixers and decoction kettles can only heat so quickly.

The decoction method is pretty darn intense, typically results in high material efficiency, and can also produce highly fermentable wort depending on malt DU and DP and how quickly the mash is pumped from the decoction kettle back to the rest mash (that’s the portion hanging out reading the paper in the mash tun). How is Boston Beer using decoctions to selectively beat up on enzymes? My guess is some variant of a jump mash using boiling water as part of the decoction process.

When any mash is heated from about 140 °F (60 °C) to 154 °F (68 °C) there is interplay between beta and alpha amylases. And with slower heating rates comes higher wort fermentability. This important ramp is part of step mashing, decoction mashing, and double mashing (primarily used for rice and/or maize adjunct brews). The ramp’s heating rate is affected by how quickly the mash can be heated on the way up in temperature for all of these methods and is also affected by how quickly the boiling mash is returned to the rest mash for decoction and double mash methods. If the goal is to really limit wort fermentability, the heating rate up the mash ladder is usually too slow because mash mixers and decoction kettles can only heat so quickly (about 1.8 °F/1 °C per minute is typical). What’s a brewer to do?

One option is to mash-in at a higher temperature to limit beta activity. Another is to reduce enzyme activity through malt choice, dilution with adjuncts, or heat in mashing. Koch states that they are using a combination of malt choice and heat. My bet is on the jump mash. Here’s one approach. Mash-in all grains at 122 °F (50 °C). Pump about half of the thick mash to the decoction kettle, very quickly “drop” in sufficient boiling water to jump the mash temperature from 122 °F (50 °C) to 158 °F (70 °C). This essentially deletes the ramp and allows virtually no beta-amylase activity. It also makes for one big decoction; a surefire way to destroy a big chunk of enzymes. Boil the decoction and then quickly pump back to the rest mash to jump the temperature from 122 °F (50 °C) to 158 °F (70 °C). The combined mash now has about 40% less enzymes than before the decoction. There is sufficient alpha to convert the starches, less beta at a temperature that is going to finish off what was present before the heating step, and not much of a ramp involved.

Why go to the trouble? In the Food & Wine article, Koch states “It [Sam Adams Light] had to be flavorful and not a pale, fizzy, flavorless brew. When it came to brewing a beer with more flavor, we knew we needed to play around with both the recipe and the brewing process.” Chances are they found the flavor profile they wanted by using decoction mashing and were also able to offer a light beer that fit into the Sam Adams brand identity, as that beer is also decoction brewed. Hope this sheds some light on this heavy topic!

Q Living in southeast Brazil (São Paulo State) I enjoy nice temperatures during the entire year. It ranges from 10 °C (50 °F) in the night during severe winters to 35 °C (95 °F) in the hottest summer. Being a homebrewer brewing 20-L (5.25-gal.) batches, I took over an old chest freezer and adapted it to accommodate my plastic conical fermenter (21-L/5.5-gal. capacity). The fermenter has an appropriate port for a temperature probe, and I use this information to control my chest freezer temperature. This normally poses no problems with lagers. However, when brewing ales in the wintertime, it is sometimes difficult to stabilize the temperature during the night; for example, required fermentation temperature is 20 °C (68 °F) and at night the temperature goes down to 12 °C (54 °F). The chest refrigerator´s insulation helps a little and the temperature only drops down to ~16 °C (61 °F). What should I do?

Luiz-Roberto Reboucas
São Paulo, Brazil

A It’s always exciting to see questions coming in from around the world and realizing how homebrewing communication has truly become global. I assume that you have your chest freezer located somewhere without heating, explaining your concern about the cool temperatures at night. The easiest thing to do may be to simply locate your ale fermentations inside your house during the cooler parts of the year. If your fermenting beer does not drop below about 63 °F (17 °C) during the coolest part of the day, most ale strains will ferment to completion without a problem.

Another, and less approximate solution, is to use a secondary controller that can be used either for heating control or cooling control. When you want to prevent your fermentations from becoming too warm, the controller can be used to turn your freezer on and off, like you are currently doing. A two-channel controller will switch on a heat source when set for use as a heater. There are an assortment of heating blankets, pads, and lights that can be used in this application. Whatever you choose, make sure you carefully consider the safety of your plan. And if you use a heat lamp, make sure to shield your fermenter from the light to prevent damage from UV light.

Increasing the insulation thickness of your chest freezer is yet another option. Depending on the size and shape of the chest, it may be easy to add some insulation panels to the sides and top, either internally or externally. Controlling fermentation temperature is one of the best ways to improve homebrewed beer quality and you have a few options to improve what you have. Cheers!

Issue: September 2023