A good comparison to the role of the brewer is that of the conductor, it’s imperative that at all stages each process variable is understood and controlled with a specification. At any point a conductor can isolate the role of, say second violin and know exactly what it should sound like, so too the brewer…at least in an ideal world. Really this is just an artistic way of describing a science experiment, where all variables are rigorously controlled.
Unfortunately, some of these variables are difficult to control. As instruments lose their tuning so malt specifications move with seasonality, hops with freshness and yeast with each successive generation. Some of those variables which can be controlled, time, temperature, pressure etc… Have an affordable means of measurement and control, but several key variables are less easily controlled. Bitterness can be a challenge, Vicinal Diketone levels are not easily measured, and a critical variable which underpins flavour and haze stability, dissolved oxygen (DO) has historically been a difficult metric to control. Oxygen is like a pair of clash cymbals, largely absent except for one carefully managed interval. Fortunately for the modern brewer huge advancements in oxygen measurement mean it is easier, faster and cheaper than ever to control this fulcrum of beer quality.
Oxygen is a double-edged sword in the brewery, highly desirable at the start of fermentation (to produce sterols and unsaturated fatty acids which yeasts need to facilitate cell growth) but deleterious to beer flavour and shelf-life stability either side of this point.
Hot side aeration
In the brewhouse oxygen ingress will increase the amount of oxidised lipids (most prominent among them trans-2-nonenal), which can cause papery/cardboard aromas and oxidised polyphenols which lead to packaged beer with lower reducing potential.
Normal DO levels in the brewhouse are around 40ppm and can be controlled by reducing splashing, gentle wort re-circulation & transfers and filling tanks from the bottom. In extreme cases the use of deaerated liquor (DAL) for mashing or even purging of the milled grist can be performed to significantly reduce hot-side aeration…Though debate remains around the value of such steps.
Cold side DO control
Upon cooling wort needs to be aerated to ensure good yeast cell growth, this is critical to fermentation rate and control of esters, higher alcohols and other yeast metabolites. Typical dissolved oxygen (DO) targets are around 6-8ppm for a 1.045 ale wort up to 20ppm or higher for stronger beers, especially lagers.
After fermentation DO levels will be very low, around 5ppb or less. This is due to the yeast having mopped up all this oxygen to facilitate cell growth. When making dry hop additions to FV this yeast activity can be an effective in the removal any introduced oxygen.
The transit of beer into final package ‘is beset on all sides’ by the risk of oxygen pick-up, an understanding of the most sensitive areas, and best practice to avoid this is very useful.
Firstly, tank transfers. When moving across to a different vessel it’s imperative that this tank be properly purged. Unfortunately, without a dissolved oxygen meter this can be difficult to manage but a good approach would be; to fill the tank from the bottom up to around 5psi/.3bar and dump this through the top quite slowly. Do this twice, then trickle C02 into the bottom of the tank very, very slowly whilst allowing it to vent from the top, due to the greater density of C02 any air will be removed. Once the tank itself is purged look to do the same with the hose, either with C02 or ideally with deaerated liquor (DAL). Always move beer gently to avoid foaming and ensure there are no leaks, that seals are replaced routinely, and pumps are appropriately sized to avoid cavitation.
Through filtration C02 should be used to purge DE filter housings and bodyfeed liquor, some brewers choose to use sulphite ions or ascorbic acid here to reduce the impact of any oxygen pick up. C02 can also be applied to the bowl during centrifugation.
During packaging oxygen control reaches a crescendo, but again good controls can mitigate any damage. Purging or ideally vacuum pre-evacuation will greatly reduce dissolved oxygen levels on filling, care must also be taken to reduce headspace oxygen, and this is done by closing the can or bottle onto foam which displaces air from the headspace. In concert these measures reduce total in pack oxygen (TIPO), which is the sum of both dissolved and headspace oxygens, to below 50ppb. Bottle conditioned beer has a significant advantage here as active yeast is capable of rapidly metabolising dissolved oxygen, though it is good practise to maintain strict control anyway.
Once in package the rate of oxidation will be determined by several factors, the most significant of which is temperature, as many of the flavour changes in beer are driven enzymically it’s reasonable to assume that the rate of staling doubles for each additional 10°c. Also, the presence of compounds capable of transforming mostly non-reactive oxygen to much more impactful Reactive Oxygen Species (ROS) such as Iron and other heavy metal ions. These reactions will then drive a rise in cardboard/paper aromas, flavour active aldehydes (from alcohols), ‘catty’ or ribes aromas and a reduction in hop aroma and bitterness. Oxygen will also promote the formation of protein-polyphenol haze compounds, cumulatively diminishing the way the beer looks, smells and tastes.
Some beer styles are more vulnerable to high DO than others, certainly a barrel aged sour, or a big stout would be more tolerant than a light lager or a New England IPA. In fact, NEIPA’s are particularly sensitive to oxygen and this is theorised to be due to a particular compound in oat hulls which darkens readily (perhaps polyphenol material) but also as their prominent hop aroma deteriorates quickly.
Putting a number on it
‘If you can’t measure something you can’t control it’ is a popular mantra, and for good reason. Fortunately, dissolved oxygen meters have become more robust, much simpler to use and importantly, more affordable.
A great option for brewers is the Hamilton Beverly, which Murphy and Son are now distributing. This handheld at-line unit uses modern optical sensor technology to deliver accurate dissolved oxygen measurements across a wide range (4ppb-25ppm) allowing for control of wort aeration and cold side DO, and with excellent accuracy (+/-7ppb at 40ppb and +/-100ppb at 25ppm). Ongoing maintenance support is offered by Hamilton but due to intelligent design this is very rarely an onerous expense.
The Beverley is a robust, hand-held unit which can be moved around the brewhouse allowing for the measurement of oxygen on hot side (due to integrated temperature compensation), for wort aeration, validating tank purges and dialling in DO levels during tank transfers and into final pack.
Avoid hitting any bum notes in this highly competitive beer market by controlling dissolved oxygen levels in concert with the Beverly Hamilton.