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It is well known that ester formation in beer is mainly a factor of temperature. Higher fermentation temperatures produce more esters. (Other factors play a role as well, such as the yeast's genetic proclivity to ester formation, pitching rates and dissolved oxygen (DO) levels, but if these are considered equal, temperature is the primary controlling factor.)

Lately I have been brewing some Belgian ales with POF+ Belgian yeasts, which causes me to wonder: is the formation of spicy phenols also temperature-related?

(Once again other factors in POF formation play a role here, the most well-known one being the presence of ferulic acid in Weizen worts, which is decarboxylated into 4-Vinyl guaiacol during fermentation. However, this phenol is only a minor one among the desirable volatile spicy phenols typically produced by Belgian yeasts.)

I understand that phenols are produced early in the fermentation cycle, whereas esters are produced later from fatty acids and higher alcohols. However, Belgian fermentations typically start cool in order to keep the higher alcohols in check and prevent a "hot" or harsh alcohol character, and are then ramped up. Pitching warm and starting the fermentation warm can create an excess of sharp and "hot" fusels but I haven't noticed an increase in spicy phenols in these cases. Which suggests that the formation of these spicy notes in POV+ yeasts is not significantly temperature related.

Is this correct?

If so, what factors govern the amount of volatile spicy phenols in Belgian ales and how might they be manipulated to vary the amounts of volatile spicy phenols produced?

  • Yeast are capable of doing all kinds of weird things, like mimicking the flavor of hops npr.org/sections/thesalt/2018/04/04/599147983/… so I doesn't surprise me that yeast are capable of giving you spicy notes at low temperatures. – farmersteve Aug 16 '19 at 13:09
  • Certain yeast strains can indeed create spicy phenols at low temps but what I'm wondering about is whether or not temperature is a factor in the levels of volatile phenols produced, like it is with esters. – Frank van Wensveen Aug 19 '19 at 12:02
  • Frank I'm afraid you are into serious research territory. Have you been looking at brewing text books yet? – farmersteve Aug 19 '19 at 15:09
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    farmersteve: Yes, I have looked at several textbooks and various research papers, but most research in this area has focused on lager brewing (that being the main industrial application of the art) and on other yeasts than saccharomyces. – Frank van Wensveen Aug 20 '19 at 11:14
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tl;dr: this is a complex issue and only a few papers on the subject have been published, but the long and short of it is that wort composition and yeast genetics, not fermentation temperature, are the key factor in the level at which volatile spicy phenols are formed.

Details: After consulting with a brewing scientist (yes, PhD and everything) and reading my way through a bunch of research papers he gave me, I have learned that phenol formation is not significantly dependent on fermentation temperature, but can be dependent on mash temperatures. The ferulic acid rest used in Weizen brewing is the most common example, but similar rests that enhance the levels of other precursors to different phenols exist. These precursors are generally decarboxylated in the boil, after which the yeast genetics are the deciding factor in whether or not volatile phenols are actually formed. Meaning: a proper boil is also essential to spicy phenol formation in Belgian beers, although you need one anyway in order to brew a decent beer, of course.

Classic Belgian yeast strains have a marked proclivity for the formation of the volatile phenols that are so characteristic for the style, and require little help in this department. A single infusion mash at normal mashing temperatures is enough to provide them with everything they need in the phenol formation department. (Noting that most Belgian ales are mashed low because a high fermentability wort is vital for these big beers.)

The perceived temperature dependence in phenol formation stems from the fact that higher fermentation temperatures, especially with classic Belgian strains, may lead to the formation of significant levels of higher alcohols which are partially or entirely converted into fruity esters, all of which masks the phenols to some degree. Fermenting cool inhibits the formation of other flavors that may help mask the phenols so they come through more prominently on the palate. However, the levels of spicy volatile phenols is pretty much the same throughout the entire fermentation temperature range appropriate for the yeast.

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