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Ask the Experts: John Palmer

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Nathaniel from California asks:
I recently read that the sugar composition of wort (i.e. percentage maltose, sucrose, etc.) has an effect on the fermentation profile of the resulting beer. The extent of it was that more esters are produced from glucose metabolism than are from maltose metabolism. What I'm wondering is, how do the other sugars present in wort affect the fermentation profile? Is it all simply related to ester formation, or are other compounds at play? And are there any practical methods of controlling the concentrations of different sugars present in the raw wort?

Palmer answers:
I think I read the same article, by Graham Stewart? Studies on the "Uptake and Metabolism of Wort Sugars During Brewing Fermentations", MBAA TQ, Vol 43, no. 4, 2006? Your question on what other sugars or compounds play a significant role in ester formation and beer flavor is a good one, but a bit beyond my expertise. I will have to wait for the next issue of the Technical Quarterly and see what’s what. But on the other hand, I can lay out a few basic facts which can place some boundaries around the unknown answer. First, brewer’s yeast only ferment a few sugars – glucose, fructose, galactose, and mannose (all monosaccharides), maltose and sucrose (disaccharides) and maltotriose (trisaccharide). There is one other, a tetrasaccharide called maltotetraose, but not much is known about the fermentation of that sugar and it occurs at low concentrations. As you have learned, worts high in sucrose, glucose or fructose tend to produce a lot of esters, versus worts of the same gravity having a higher proportion of maltose. I have not read anything on worts with a higher proportion of maltotriose than maltose. My point is that there are not a lot of mystery sugars waiting in the wings that yeast can secretly utilize to create new flavors.

Second, both ale and lager yeast strains ferment glucose, fructose, sucrose, and maltose equally well, but lager yeast strains are generally better at fermenting maltotriose than ale strains.

Third, a typical wort consists of mostly maltose, then maltotriose, then sucrose, glucose and fructose in descending order. Yeast that have been grown from maltose-rich worts have higher viability and vitality than yeast raised on glucose.

Fourth, while there are several theories on why yeast create esters, a leading theory is that yeast create more esters when they are under stress. Yeast do not have to expend energy to metabolize glucose and fructose, but they do have to work to take in maltose and maltotriose. Therefore I am conjecturing that glucose and fructose represent junk food to the yeast, whereas maltose and maltotriose are meat and potatoes. The addition of some junk food to the fermentation can make for a non-typical result and some interesting esters. Too much junk food to the yeast ends up not tasting like beer.

Finally, beer flavor comes from more than just esters, and one primary example are melanoidins that come from the malts and boil. Melanoidins are a chemical combination of a protein and a sugar. And this is where other sugars can play a big role in beer flavor. These Maillard reaction compounds can produce a wide variety of flavors and aromas that can smell like fruity esters, or vanilla, chocolate, or coffee.


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