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Ask A Scientist: What Are Off Flavors?


Here at Bissell Brothers, we (along with probably every other brewery in the world) strive to provide a well-rounded education in our beers for all of our team members: whether hailing from production, tasting room, distribution or the office, none can evade the all-seeing eye of Learning.

Thanks to science and those who practice it, we are lucky to enjoy a rigorous sensory training program at the brewery. During these weekly lessons, staff members are encouraged to refine their palates and become more well-versed in the vernacular of our beers. Not only are we trained on off flavors and how to identify them, but we spend a lot of time discussing the experience of tasting our beers at their best. For example, Substance has been around for five years and counting, but we are still discovering new flavors within this surprisingly complex brew.

The goal is to know our beers inside and out; if we are familiar with what they taste like in the absolute best and worst conditions, we will be better able to serve, guide, and troubleshoot any problems that may arise with our beers. The three off-flavors that we are most concerned about are:

Diacetyl – Diacetyl is a naturally occurring byproduct of the fermentation process. Noticeable levels in a beer are caused by short boiling, low temperatures during fermentation, mutated yeast, or racking too soon. It can also be formed by bacteria contamination. In contaminated beer, it presents with a slick, buttered popcorn flavor. Not exactly thirst-quenching.

Acetaldehyde – Also a naturally occurring byproduct, Acetaldehyde is produced by yeast during fermentation as a precursor to ethanol. It can also be caused by oxidation when, if too much oxygen exists in packaged beer, it can change ethanols back into acetaldehyde, which results in a green apple, latex paint, sour kind of taste.

Oxidation – Oxidation is directly caused by aging. The more oxygen a beer is in contact with, the faster and more severe the oxidation. Cooler storage temperatures slow the process. If you are drinking a beer that has oxidized, it will taste papery, musty, old-booky, and stale. In hoppy beers, it tends to kill all the hops and make the beer taste super flat and muted.

These are the off flavors we are on the lookout for, and local scientist and Bissell employee Matt Nelson is the man responsible for slipping them into our drinks on a weekly basis. What a fun job, right! Read on below for more from Matt.

What inspired you to get the sensory program up and running?

Matt: It was something that came up from a conversation between myself, Eric and Noah. We really needed to be doing it, because you can get all your ducks in a row, and all your ingredients can be dead on, and brew it exactly right, but it comes down to – is it good, and is it consistent. Sensory is the best way to figure that out.

Can off-flavors be more apparent in IPAS?

Matt: Aggressive hops work really similarly to distortion on guitar, where if you throw a lot of distortion on your guitar, it’s gonna hide a lot of the little sins that are going on there. So in a beer like Reciprocal or Nothing Gold, there’s so much going on in there that if there is a noticeable off flavor, it’s going to be a huge amount of that off flavor.

What’s the most common off flavor out in the world, that people might have experienced but not known what it was?

Matt: Almost certainly oxidation.

Since you’ve been working in beer, are you more on alert for off flavors?

Matt: Not intentionally. I’m not thinking about it on purpose, but when I drink beer now I am really kind of subconsciously attuned to trying to figure it out. It’s not ruined my ability to drink beer, but I’m definitely focused on it more than I was before I started doing this.

What other sensory programs are you working on?

Matt: There’s two things going on right now. We are taking cans that are canned normally off the canning line and throwing them into the worst possible condition we have in the brewery. We are keeping them for about as long as we would assume they’d be out in the world. Primarily it’s to keep them in case we do get a customer emailing us, saying hey I got this Subby, the date on the bottom is this, I’m getting this weird taste. We can go up there, grab a can, pop it open, and see if this is actually an issue. We have not had that happen yet, but it’s a good safety net to have up there.

The other thing going on up there is part of sensory, where every now and then we’ll have the production team intentionally skip the purge on some beer, and low fill the cans in order to cause oxidation to happen. Because there’s a lot that happens when beer oxidizes, it’s not just one flavor. The only way to really teach people what oxidation does to each beer is to oxidize the beer.

In terms of the sensory program, what would you like to see in the future?

Matt: The big ones I have in my mind are one – eventually doing a sensory focused on our Milo beers, once we start getting more and more Milo beers down here. There’s an entirely new set of off flavors that are possible in beers brewed in that way. Another direction we’re going to be going in is teaching flavors, but not necessarily off flavors, like what does it taste like if there’s too much of this hop, or what if we change this hop, or what do these hops actually taste like. The big thing with getting the lab is having a dedicated space where we can do small sensory sessions any time, in an isolated environment. Sensory should really be done somewhere where you are absent of all other smells and tastes so obviously in this brewery that’s very difficult.

Thank you, Matt! We are all excited for the opportunity to taste more bad beer in the name of making good beer.