20 Jan The Good, The Bad, and The Smelly: The Similar Effects of Oxygen on Beer and People
Oxygen – it supports life for animals and plants, and it helps to make beer. But at the same time, oxygen can do humans irreparable damage, and hurts beer almost as much if it comes along at the wrong time. Oxygen is a double-edged sword in both biology and brewing, and shows us just how much of a link there is between human life and beer. You think I’m kidding? Stick around and get learned.
Oxygen and Energy. If you asked a hundred first year medical school students, only about 25% of them could actually tell you why humans need to breathe. Stop breathing and you stop living – but why? It turns out to be a fairly unglamorous reason. Oxygen basically acts as a trash can in the making of energy for the human body. When you convert sugars into energy, there is a fairly intricate system of stealing energy from the sugar and using it to build the currency of biology – a molecule called ATP.
In the ATP producing process (called oxidative phosphorylation during aerobic respiration), something has to be the carrier of the energy – in this case it‘s electrons. At the end of the energy making process, you have a leftover electron with no place to go. Something needs to accept that extra electron to keep an even scoreboard. For most animals and plants the electron acceptor is oxygen. It’s that simple; O2 is nothing more than convenient, because it has the capacity to accept that electron without asking for anything in return. There – you’ve learned something today.
The same process isn’t necessarily true for yeast when making beer. Yeast can use oxygen to make lots of ATP, but it can also make some ATP without using oxygen. When yeast makes energy (ATP) without using oxygen as an electron acceptor, it’s called fermentation, with different products. There’s less ATP, but an interesting little molecule called ethanol is built as a by-product. Yeast can ferment in the presence or absence of oxygen, but just as oxygen helps people, it also helps yeast to grow faster and to make little yeast cells. It isn’t that fermenting beer has to have oxygen, but the process is made easier with a little oxygen early in process.
Yeast can make more energy with oxygen thanvia fermentation, but many forms of yeast (including brewers yeast) have been bred to ferment to alcohol even if oxygen is present (aerobic fermentation). In fact, most brewers’ yeasts will only respire (make ATP using oxygen) when the concentration of sugars is very low and the amount of oxygen is very high. In this case, yeast may use some of the ethanol they have already fermented as a carbon source for respiration, and will then produce acetic acid, souring a beer. So, depending on the beer you want to make, having high oxygen during fermentation could be bad.
Other uses for Oxygen. On the other hand, energy production isn’t the only thing oxygen is used for, it is also a component of thousands of large biomolecules, including those that make it possible to make many little yeast molecules from a parent yeast or even little people. In fact, both yeast and animals use way more oxygen for biosynthetic reactions rather than for energy production. Yeast for beer will use up most of the oxygen in wort for building unsaturated fatty acid and sterols (like ergosterol) to be used in its membrane. Building membrane is one of the crucial parts of being able to bud off baby yeast cells.
In beer, the ability of oxygen to help make more yeast cells from parent yeast is important in speeding up fermentation (because you have more cells) and can help a beer be produced in a timely period. The literature is a bit deceiving, much of it saying that yeast has to have oxygen to begin to ferment and to bud, but it isn’t so. The key is to have some oxygen around to build membranes and keep strong cells (weakened cells from fatty acid or sterol starved membranes can die and result in a stuck fermentation).
The truth is, while people without oxygen die in minutes, yeast without oxygen will succumb in a generation or three, unless you supply them with another source of sterols and other biomolecules – sometimes this is done by using fresh yeast instead of re-pitching, or by supplying yeast with nutrients via the cold trub. This has practical applications – does a brewer need to aerate wort?
Usually transferring the wort to the fermenter does enough to add oxygen, and new pitches of yeast usually have more than enough oxygen for membrane biosynthesis – although home brewers can be helped by a bit of aeration. However, a brewer doesn’t want to over aerate, as that can produce problems of its own, and a brewer should take style and yeast strain into account when deciding on whether to aerate. High gravity worts and high yeast pitches for barleywines and wee heavys are usually going to benefit from a little more oxygen in the wort.
Oxygen as enemy. So now we know how oxygen can help us, either in making beer or permitting us to live but what about the other side of the coin? Too much of a good thing can be bad, and such is the case with oxygen and people or oxygen and beer. From a specieist point of view, let’s talk about people first.
Oxygen likes to react with many molecules, it is chemically promiscuous and even more so when it has picked up or given up an electron. Because of this, it can act on many different substrates. While good in some cases, reacting with just about anything that comes along in your body (creating reactive oxygen species) and leading to bad outcomes. All those molecules that end up reacting with oxygen had a job to do before they met oxygen, and afterward they can’t do their job and they may go off and react with other molecules. Luckily the body has many different anti-oxidant mechanisms to lower the chance that oxygen will mess with your biochemistry.
Oxygen and Aging. Trans-2-nonenal (C9H16O) is a degradative oxidation of omega-7 unsaturated fatty acids to an unsaturated aldehyde (actually a family of unsaturated aldehydes). Around age 40, people start to produce more fatty acids on their skin, while at just about this same time, the antioxidant protective mechanisms in the skin start to work less well.
Many antioxidants are produced by our bodies throughout life (SOD, catalase, glutathiones), while other antioxidants are dietary in origin (vitamin A and C, resveratrol, carotenoids) to battle the super-reactive compounds that can be formed when oxygen goes around pulling electrons and atoms off other compounds . The problem is that as we age, these antioxidant system start to work less efficiently and hormonal changes during menopause or “manopause” can exacerbate the process by causing more fatty acid production in the skin – just another example of how getting old sucks, it’s worse than everything other than the alternative.
On the good side, beer contains antioxidants. Dark beers have nearly as much resveratrol as red wines, and there are additional antioxidants from the barley and hops, like xanthohumol and other prenylated flavonoids. So, if you wanted to go a ways out on that limb, you might say that beer helps slow aging. Although you might need to bathe in beer to help your skin – people do it I guess…. mostly Europeans.
When fatty acids are converted to trans-2-nonenal, they pick up one added chemical characteristic – an odor. Nonenal can be described as musty or greasy, or more often like wet paper or cardboard – some people even say it smells like cucumbers. You often pick up on it when entering elder care facilities or the homes of the elderly. The typical “old people smell” is not automatically a function of poor hygiene, it’s better described as inevitable.
Trans-2-nonenal is not water soluble, so casual cleaning of bodies, clothes, and bedding won’t do much to get rid of it – even if the cleaning isn’t that casual. Fortunately, following a good diet and getting plenty of rest and exercise can help to minimize the aging process and the conversion of fatty acids to nonenal, just as it can do for conventional body odor. Most commercial products for nonenal don’t really do much, but the Japanese do have a product they say works; it contains persimmon. Somehow the tannins get rid of the nonenal; is there anything persimmons can’t do? We recently wrote about how persimmons make very bright beer, and are used in Japan to clarify sake.
No matter what we do, we’re going to start picking up some nonenal odor as we age. The amazing parallel is that beer is going to do much the same thing – unless we can keep oxygen away from the finished product. Even more amazing, the stale smell from oxidized beer comes about mostly from the same reaction that causes the odor on humans. Fatty acids with double bonds in the finished beer are oxidized to trans-2-nonenal, just as with people. I’m sure at some point you’ve has a bottled, canned, or even kegged beer that had a wet cardboard odor, and even more of a stale taste – that’s trans-2-nonenal. In the case of beer, we tend to taste it more than we smell it, but the odor is there as well.
Not all stale beer flavor and smell may come from nonenal, but a lot of it does. There are cases where stale beer is found to have little trans-2-nonenal, and others where beer with significant levels of this chemical tastes and smells just fine. Nevertheless, most times you find a stale beer, trans-2-nonenal is going to be the culprit, especially if the beer was aged on the warm side. Warm means more energy/molecule, and more energy means more chance of oxygen moving around in the beer and reacting with beer molecules.
So, that’s why old people smell like old beer and vice versa. In people it’s inevitable – in beer it’s a technical flaw that can be avoided in most cases either by consuming the beer fresh or by flushing systems to remove most of the oxygen when a beer is fermented and packaged. Personally, I think that people could smell like worse things than old beer. Aging is a natural process, so just go with it. Drink good beer and stop worrying about the things you can’t change. Of course, there are things that we can treat – some diseases can also make you smell a bit like beer. Maybe we’ll talk about those sometime in the future.
banner image credit: Warner Bros. Pictures