Hot side aeration

[steve_flack]

I have heard of using KMS (Potassium metabisulfite) in the mash to try and reduce oxidation.

It is now realized that oxidation during mashing has several unwanted effects. Wort gets stale. Proteins containing free-Sulphur Hydrogen groups are oxidized and sulphur-sulphur bonds then formed between them can cause these proteins to form a coating on starch and malt endosperm cell wall fragments. As a result proteolysis, amylolysis and Beta-glucan breakdown are partially inhibited, causing a decrease in the amount of soluble extract obtained, and slows down mash separation. Oxidation of the mash also results in the oxidation of polyphenols. This causes increased colour and astringent bitterness. Even worse, lipid oxidizing enzymes oxidize unsaturated fatty acids and form products that accelerate stale flavours in the finished beer. When using dry milling and dumping malt in through the top of the mashtun air is trapped in the husks again increasing oxidizing potential. Our tiny mashtuns have loads of surface area with air exposure per volume, compared to enclosed mashtuns and bottom filling of big brewers. Good quality sweet wort has a fresh flavour and sparkling quality. This freshness is greatly diminished with long mashing times. Wort tastes dull and bland after a few hours and is irreversible damaged due to oxidation processes. The impact on the final beer is a lack of certain positive flavours – less maltiness, greater astringency and overall dull flavour. Other severe forms of staling (cardboard, aldehyde) may result. Fortunately all this is preventable with the simple technique of adding 20-30 ppm KMS to the mash. There is even now a special kind of anti-oxidant malt that is produced in Europe.

[Aleman]

I have heard of using KMS (Potassium metabisulfite) in the mash to try and reduce oxidation.

Interesting article Steve . . . I hae in the past used several Campden tablets in the mash to this effect . . . In fact with very pale worts (Pilsner type) I still do . . . I certainly see less wort darkening (Via oxidative browning) during the boil when I do this.

[Neal]

Cheers Aleman/Steve,
I understand now. It's not the free oxygen getting carried forward through processing, but chemically bound oxygen in mash compounds.
Very interesting.

[Aleman]

Lets make one thing clear. it is very unlikely that we will experience this this is purely esoteric discussion

<Geek Alert>

The issue is not with aerating hot wort, after all that is done in a lot of commercial breweries with internal calandrias in the kettle, and also in aerating hot wort on the way to the plate chiller. The issue specifically appears to have applied to aeration of the mash, with excessive stirring and splashing about . . . The mash does absorb massive amounts of oxygen over the 90 minutes which 'gets stuck' in the mash as oxidised compounds . . . as conditions change in the beer with storage, these oxidised compounds release electrons (Becoming reduced) and other compounds accept those electrons (becoming oxidised). Now if a malt has high levels of (IIRC) S Methyl Methionine (SMM) then these will oxidise to (eventually via an oxidative pathway . . . there is also an non oxidative pathway as well ) trans-2-nonenal which has a taste threshold in the order of parts per billion . . . so it doesn't take a lot of extra SMM to have an effect.

The other thing to consider is that during the boil conditions change in the wort quite dramatically pH falls for one thing . . . and as others have noticed the potential for oxygen to dissolve into the wort is very much reduced (I should add that it is already pretty low at mash temperatures . . . it is the chemical absorption in the mash that is the problem), which makes the chemical absorption of the oxygen much less of a problem. So by only exposing a small surface area of mash to the air, not stirring once doughed in, and covering the mash tun we are already reducing the absorption of oxygen.

</Geek Alert>

Someone has suggested that I revise my ramblings on this, (Cheers Neil ) First off I should always go back to my sources to check things before posting. SMM is of course the precursor to DMS (DiMethylSulphide), and has nothing to do with t2n or HSA. Secondly (and a minor technical point ) when a compound gains an electron it is reduced (Not oxidised as I said) and when it looses one it is oxidised not reduced Arghhh! Something I still get wrong . . . of course if you are talking protons its the other way round .

So what is the mechanism? We don't know . . . but we do know that it is related to the oxidation of lipids, which are normally present at around 2% in malts, US 6 row does have a slightly higher amount, and if you couple this with using flaked maize or standard corn meal (removing the 'germ' removes a lot of the oil) which has a high level of oil/lipids will have a higher level of t2n precursors . . . and much more susceptibility to oxidative effects in the mash

As I mentioned earlier, there are two routes to the production of t2n one is enzymatic, and it is likely that kilning the malts to darker colours (typical in English maltings) destroys the enzymes that are responsible for this route.

Still it is a moot point as in normal ale brewing production using good quality British ingredients it is very unlikely that we will encounter it . . . especially as most of us seem genetically incapable of allowing a beer to survive much past maturation before we drink it all