Benefits of 'resting' home-roasted grains

Question

In "Radical Brewing", Mosher endorses home-roasting grains, but suggests that after roasting, you store the grains for 2 weeks or so to let some of the acrid flavors subside.

Specifically, I roasted/toasted 1lb of flaked oats to make Jamils McQuaker Oatmeal stout.

What compounds are forming that taste acrid? Can you let them 'rest' for less than two weeks for them to subside?

Answer 1

I don't know the specific compounds produced that are acrid, but I do know that maltsters rest their dark malts for 3-4 weeks, less for light malt.

The compounds produced are from Maillard reactions - the name for a whole series of reactions between amino acids and reducing sugars. Many of the reactions are still not fully understood - this page shows the initial reactions that can lead to the complex mixtures that form melanoidins, and this page shows one of the proposed pathways for melanoidin production.

Many other compounds are also produced in Maillard reactions

Other byproducts include aldehydes (furans, thiophenes, polysulfides, thiols) which can contribute to the desired smells (furfural which is an aromatic aldehyde has an almost almond smell), Diacetyl (buttery flavor…also produced and consumed in the fermentation process), acrylamide which is shown to be harmful, and many more. Some of the byproducts, harmful and beneficial are volatile and will evaporate during the process or upon the weeks of rest the grains should undergo after roasting. One amino acid reacted with Glucose or Fructose can be shown to produce over 20 different aroma-active compounds and countless other volatile and non-volatile compounds, malted barley has many hundreds of possible amino acids and though maltose is the main reducing sugar there are many more available.

• Grains, Malting and Roasting - Toxic Brew Co.

Acrylamide is formed by the Maillard reactions above 180C, 350F and is present in dark grains. It is carcinogenic in animals, and according to the article above, volatile, so a rest is necessary to reduce the levels of this compound in the grain.