The Bloated Homebrew Pack

Yeast off-gasses CO2 as a natural part of its active biological process. Keeping it at low temperatures slows it down, but some strains can get to CO-2-tin’ anyway, which is what causes the pressure build-up in a homebrew pack. 

You might be someone who avoids a bloated pack, but once it’s discounted, maybe it starts to look pretty good. Should you go for it? Why are some of us even suspicious of a swollen pack in the first place?

THAT’S JUST SWELL

Meeting the demand for a greater variety of styles, like those with higher ABVs and/or lower fermentation temperatures, coupled with many homebrewers’ desire to avoid the extra step of a starter, required providing more yeast for some companies, so, larger homebrew packaging. For other yeast companies, it was packaging that appropriately allowed for that natural pressure build up. ​“Flexible pouches are just more practical for the normal concerns with liquid yeast,” says Lance Shaner, yeast and brewing scientist, Omega Yeast co-founder and Head of Omega Yeast’s Chicago plant. ​“Package swell is par for the course with liquid yeast, though it is definitely more pronounced for certain strains.”

Regardless of whether it is part of yeast’s normal biology, it can be perceived negatively by homebrewers when it happens in a homebrew pack — even when what’s going on inside is neutral to fermentation outcomes. That’s why liquid yeast companies not only optimize processes for yeast health, but also adapt propagation techniques for cosmetic and practical reasons, too, to avoid package swell. 

Combating future package swell from the lab-side of things could include washing the yeast in order to remove any possible residual sugars, restricting food at the right interval before packaging, and controlling temperature throughout pitch life.

“At the lab, yeast are at first fed an optimally nutrient-rich diet and given an environment that promotes rapid, healthy growth, but as yeast cells fortify, replicate and bud off to create larger and larger populations of yeast — towards the end of the propagation cycle, when fermentable sugars and other resources have been used up and new resources are not replenished — yeast cells begin to fast,” Lance said. ​“When fasting, they enter a resilience period that they can tolerate for a while, but not permanently. Lowered temperatures help elongate that period by slowing the demand for resources and the creation of byproducts like CO2; nevertheless, CO2 production can still happen, and happens more often with some strains.” Yeast has a lot of variation in all kinds of its characteristics. It’s not surprising that it varies in this one, too. 

Some strains naturally tend to be more productive of CO2 or to be more active for longer in the same conditions than others. The popular strains known as British V and West Coast Ale I are two of them. ​“Those two in particular are chronic offenders for lingering off-gassing, industrywide” Lance said. 

Earlier on in their tenure, the lab crew at Omega Yeast might have thought these strains might have been leaving behind a little bit of maltotriose or maltose during propagation and just kind of slowly consuming it in package to cause this persistent effect, but the more they looked into it, Lance told us, the more they believe it’s possible that they are consuming their carbohydrate stores, like glycogen instead. That means that homebrew shop customers are just more likely to see swelling in packs containing these two strains especially.

If off-gassing is normal, why are we suspicious of a swollen pack?

The practical factors that set the timeline for yeast in a homebrew pack are initial yeast health before packaging, cell count/biomass at packaging, temperature, and then time: 

“The second you put yeast in the pack and they’re not eating and reproducing, that pitch’s viability is starting to wane,” Lance said. ​“So, the ​‘good by’ date is really just where we calculate the longest range that there will reliably be an acceptable amount of live, healthy cells remaining for the brewer to have a good experience making a batch of beer of a certain size at a certain fermentation temperature and/or ABV.” 

“Our position is always that the freshest yeast is best, that’s why we do it the way we do it for professional brewers. Since we can’t custom make a homebrew pitch for every homebrewer in the same way, packaged homebrewer’s yeast has to balance being vital and giving the shop enough time for their brewing customers to get to it,” Lance explained.

A very important part of that calculation assumes that a relatively low temperature will be somewhat constant for the lifetime of the pack. 

If temperatures were not controlled, in higher temperatures, yeast start to be more active, which creates competitive demand for very limited stores and resources, which eventually can speed up the anticipated timeline, and shorten the pack’s ​“good by” window. 

As a result of heightened activity, yeast would also start off-gassing significantly, thus they’d cause the pack to bloat. That means the fear of the swollen pack is likely a fear that temperatures could have increased in transit or in storage while yeast is supposed to be chilling out. The gamble of the swollen pack from the homebrewer’s perspective, then, is mostly a fear of low viability.

So, how do we know if it’s just kind of a gassy strain, or if it could have been a break in the cold chain?

COLD COMFORT

If you’re looking at a bloated pack in a homebrew shop fridge, and you trust your yeast brand of choice is competent, and so is your chosen homebrew shop — and you accept that off-gassing happens from normal biological processes — for some strains, even when raised right and held at low temps, then the probability of a swollen pack being swollen for any nefarious reason is pretty, pretty, preeeetty low. That’s a good start. 

The shop would have noticed, for example, if a few packs had fallen out of a box in transit when they received the package, or if their whole shipment felt warm when they received it (in which case, all packs might be swollen).

Consider also that shops have relationships and programs with their yeast suppliers that are supportive of discarding products that they suspect experienced negative conditions on the way from the lab to the shop’s fridge.

But if you’re weighing your options in front of a swollen pack, here are a few additional things that might help you verify:

A Checklist

• Just know some strains are kind of gassy. There are a couple of particularly gas-prone strains.

• Does it smell like autolysis when you open it? It all can get a little Lord of the Flies inside a homebrew pack eventually if viability is extremely low. When existing enzymes within the cell (ones that in good times made important things happen in service to the cell) get out of their functional areas — as cells collapse after cell death — vacuoles open, and enzymes actually keep working. They just move on to digest elements of the cell itself. Fortunately, opening a pack and taking a whiff will tell you if that’s happening. It smells bad. Real bad. Think dog poo umami. If you’re feeling ambitious, you could try a starter to see if you can salvage any live cells that are there and start anew.

• Starters are great. If there’s plenty of time left on the homebrew pack, but you’re just worried something could have fast-forwarded the timeline — here’s where a starter is your best friend. It can help revive a pitch from a surprisingly small number of cells. Basically, watch that the starter turns a sort of creamy color, and you’re set.

• Check the seams. In general, make sure you’re eyeballing packs for punctures, tears and spillage. Do not use it if the packaging is compromised in this way. Open-air contamination could invite all kinds of seriously untasty things. It could also result in some of the unsafe conditions that NAB LAB brews are subject to, for example.

So what do you think? 

Are you more likely to give that swollen homebrew pack a chance?

ON THE SUBJECT OF WATER (OR LACK THEREOF)

It’s kind of amazing that yeast can survive drying at all. Most organisms can’t.