Alpha Acid Redox Chemistry Question

[Rosenhops]

I've been stumped on this section of brewing chemistry and can't seem to find answers. If anyone can help me figure this one out I'd be quite grateful. There are several parts of these alpha acid reactions which leave me confused, so I apologize for all the question.

Iso-alpha acids can reduce previously oxidized melanoidin according to Dr Fix. As far as I know, to reduce one thing another needs to be oxidized. Does this mean that the iso-alpha acid is being oxidized in the process? It seems that this would be an undesirable reaction although many pros talk about it like it is a good thing (reducing melanoidin is great of course, but oxidized iso-alpha acids would give off a "goaty flavor) leaving me confused.

Second set of confusion: the acyl side chain, often denoted as R in the books, can get cleaved by light or oxygen to release a fatty acid. Does this also means the fatty acid is oxidized? Is the alpha acid oxidized in the process? Or is it cleavage alone? And finally, is the R side chain always an alpha acid?

Note: since not all people use the term iso-alpha acid, I use it to mean a previously isomerised alpha-acid, so alpha acids which have survived the boil (no dry hopping included).

 

[ajdelange]

Iso-alpha acids can reduce previously oxidized melanoidin according to Dr Fix. As far as I know, to reduce one thing another needs to be oxidized. Does this mean that the iso-alpha acid is being oxidized in the process?

If it is indeed the iso alpha acids that are doing the reducing then, yes, they are being oxidized.

It seems that this would be an undesirable reaction although many pros talk about it like it is a good thing (reducing melanoidin is great of course, but oxidized iso-alpha acids would give off a "goaty flavor) leaving me confused.

When alpha acids are oxidized during storage they generally transform into hard resins (as opposed to alpha and beta acids which are soft resins) and I think the result is increased bitterness but not 'goatiness'. This is AFAIK. I am no expert in this, that's for sure.

Second set of confusion: the acyl side chain, often denoted as R in the books, can get cleaved by light or oxygen to release a fatty acid. Does this also means the fatty acid is oxidized?

If it gets cleaved (cloven?) by oxygen then the reaction is indeed oxidative and this is responsible for the 'cheezy' aroma of old hops (could this be what you mean by 'goaty'?. If it is knocked off by a photon it forms a radical which has an affinity for sulfur (obtained from a protein) and this is responsible for the 'skunked' smell of light struck beers.

Is the alpha acid oxidized in the process? Or is it cleavage alone?

The oxygen will go to either the side chain or the parent (and I think it's the side chain). That is the part that gets oxidized.

the 'skunked' smell of light struck beers.

And finally, is the R side chain always an alpha acid?

No, its an acyl group such as isovaleryl (humulone) or isobutyryl (cohumulone).

This is a complicated subject. 'The Chemistry of Hop Constituents', Chapter 8, in Brewing Science and Practice by Briggs et. al. seems to be a pretty good review.

 

[Rosenhops]

Thank you Ajdelange! I was going to ask what book you got this from, so I will be ordering that book asap, my current book has left too many unanswered questions.

Seems obvious but a hard resin would precipitate out and leave no off flavors correct?

PS: I chose "goaty" because Dr Fix does in his book, but he does seem to exchange goaty and cheesy with each other. So I guess it would be the same thing.