One of the nice things about craft beer has been how the drive for novelty has ironically lead to brewers revisiting old beer styles and practices that went out of favour when the big breweries decided to focus on super light lager.
I’ve been reading old brewing textbooks recently (because they’re out of copyright and therefore affordable) and it’s fascinating to read about some of the practices they used.
Hot Wort Oxidation (or hot side aeration) is mentioned in both A Textbook of the Science of Brewing (Edward R Moritz and George H Morris, 1891) and A Textbook on Brewing (Jean De Clerck, 1958) as a process step in its own right.
The benefits of hot wort oxidation according to de Clerck are:
- increasing the colour of the wort
- reduced bitterness
- promotes clarification
Moritz and Morris explain the action of oxygen on hot wort as follows.
Oxygen taken up by hot wort plays a different part. It is not mechanically dissolved, but is chemically fixed, entering (as Pasteur shows) into some form of combination with the hop resins. It is this form of aeration that plays so important a part in the natural clarification of beer, or in its ready clarification by isinglass finings. When resins are modified by chemically fixed oxygen, they conglomerate into particles of greater density: these sink easily to the bottom of the storing vessel, forming a compact sediment, and leaving the supernatant beer bright. When, however, for some reason or other, the aeration of the hot wort is incomplete, the resinous substances, instead of conglomerating and acquiring the density necessary for their rapid deposition, remain suspended in the finished beer in a very fine state, and in that condition they are equally unready to deposit naturally or to yield to the action of finings. (E. Moritz and G. Morris, A Textbook of the Science of brewing, 1891, p. 272)
Neither textbook speaks of hot wort oxidation as a replacement for or alternative to cold side aeration (mechanically dissolved oxygen), but a complementary process step to promote clarity of the finished beer.
Hot side aeration happens naturally in coolships during the first minutes after filling the vessel owing to the large surface area. However it takes place only while the wort is close to boiling temperature. After that point the slow cool down to yeast pitching temperature increases the chance of infection so the authors recommend switching to plate heat exchangers before the temperatures favourable to bacteria are reached (150˚-130˚F Moritz and Morris, p 269; 70˚-60˚C de Clerck p.334).
Given that deliberately aerating wort to encourage oxidation goes against what we’ve been taught for ages, I don’t see anyone ordering a coolship to try it out (unless they hope to get spontaneous fermentations as well). But in many breweries it would be easy and inexpensive to modify the pipework slightly to accommodate to create a venturi-effect just before the whirlpool inlet to suck in a steady stream of ambient air.
I’m not ready to do even that much, but I might try it on my pilot brews for the next while to see if I notice an improvement in beer clarity (or any problems with oxidisation off-flavours!) It might be a technique that is better suited to English and Belgian styles.