Mash Stirring

This is a pretty weighty topic because stirring the mash does a few things to the mash. To avoid a geeky treatise I will cover this from a treetop level. So why are mashes stirred? Like all brewing subjects the answers to questions like this almost always involve commercial practices because brewing technology was developed for commercial brewing, not homebrewing. Mashes are stirred in commercial breweries because that is the method that allows brewers the most flexibility with ingredient selection and with mash temperatures.

The most flexible brewhouse configuration in my view of brewing includes a mash mixer and a lauter tun. While I do not believe there are many tangible reasons to use decoction mashing using today’s malts, there were certainly some real benefits to this mashing method when poorly modified malts with less than stellar enzymatic strengths were the norm. Decoction mashing, even the revered triple decoction, can be mastered if the transfer piping between the mash mixer and lauter tun is designed to permit mash to be pumped from the lauter tun to the mash mixer.

As malt quality improved over last century through the successes of barley breeding programs, advances in farming practices, and a better understanding of the biochemistry of malting, the need for intensive mashing methods waned. Exceptional beers are brewed today using step mashing or infusion mashing methods. Indeed, many of the great European lager breweries no longer use decoction mashes, and have replaced these methods with step mashing techniques. Step mashing gives brewers more control over the enzymatic reactions that occur during mashing in comparison to infusion mashing. And if a brewer wants to use step mashing in a commercial-sized brewery, a stirred mash is required.

Another reason to stir during the mashing process is to improve extract yield. Stirring has a pronounced effect on yield and for this reason many breweries using single temperature holds for conversion use mash mixers. What is essentially an infusion mash is further improved when conducted in a mash mixer because the mash temperature can be increased following conversion. This “mash-out” or “mash-off” step has the primary benefit of reducing wort viscosity and improving extract recovery during wort collection. Another benefit to the mash-off step is the ability to stop enzymatic conversion.

When you build a system based on external mash heating with a heat exchanger, there are a few key changes to how the mash behaves during mashing in comparison to stirred mashes. The most significant difference is how the heat is transferred into the mash. With a HERMS system you are pumping wort from the mash tun, through a heater and back into the mash tun. In a mash mixer, the heat is applied to the mash from the exterior surface of the vessel. Since the goal of step mashing is to exert control over the conversion of starch into fermentable and unfermentable sugars, it is vital to uniformly heat the mash, not just a part of the mash. I believe stirring the mash is really quite important for these systems to properly function and have the intended effects on the finished beer.

I understand why you are considering not stirring the entire mash, but my concern is that if you only stir the upper half of the mash that the lower portion will not have a uniform temperature and this will result in inconsistency between batches. While consistency between batches is not important to all homebrewers, I think it is an important consideration for those who want to build more complex mashing systems. Consistency with this process allows brewers who want to perform mashing experiments the ability to evaluate their data with confidence.

Regarding hot side aeration … my answer is no. I do not believe that stirred mashes contribute to hot side aeration. My primary justification for this belief is that the design of most mash mixers allows for mashing without splashing.

 

Response by Ashton Lewis.