Saturday, 1 October 2016

Ester synthesis during brewery fermentation

Of late I've been reading Brewing Yeast Fermentation Performace by Katherine Smart*. It's full of fascinating facts and I found it a surprisingly easy read. This may have been helped by skipping the bits where it got into too much detail about genetics. Even I have my limits.

On chapter I particularly enjoyed was Control of Ester Synthesis During Brewery Fermenation. Just look at the start to the introduction:
"The synthesis of volatile aliphatic esters by yeast is of major industrial interest because the presence of these compounts determines the fruity aroma of fermented beverages. Esters represent the largest group of flavour compounds in alcoholic beverages. In beer, the major esters are ethyl acetate, isobutyl acetate, isoamyl acetate, phenylethyl acetate and the C6-C10 medium chain-fatty acid (MCFA) ethyl esters"
Isn't it gripping? And it keeps getting better. One important thing is the concentration of individual esters, rather than the total ester level. Of the esters mentioned only one of them is above the flavour threshold in most lager beers. Now that didn't come as much of a surprise, as I knew the cold fermentations of lagers produce less esters than warm ale fermentations. But who'd have thought that the ester that can be tasted in lagers in isoamyl acetate? That's right, the one that tastes of banana and is particularly known for its high levels in German wheat beer. I'm almost tempted to go an buy a lager just to see if I can notice it.  Almost, I said.

Esters ares synthesised from two substrates: an alcohol and a carboxylic acid, and a number of factors can influence this. They can be divided into: yeast characteristics, medium composition and fermentation parameters.

Selection of yeast strain is important as each yeast strain produces a specific ester profile. Though higher levels of esters are found in ales than lagers this is due to higher fermentation temperature and "there is no conclusive evidence to support the idea of higher production of esters by ale yeast, all other conditions being kept constant". Who'd have thought it?

Contradictory results have been found in studies on pitching rate and yeast performances are variable and can change through successive fermentations.

Here's a handy table about the wort composition and fermentation conditions:

Long chain unsaturated fatty acids (such as oleic acid), which mainly come from trub, decrease ester synthesis. As you'd expect from that aeration/oxygenation also decreases ester levels. Zinc stimulates yeast growth and the production of higher alcohols and their corresponding esters.

Stirring stimulates yeast growth as it increases access to nutrients and decreases carbon dioxide supersaturation. This increases higher alcohol production but decreases ester production. Increased CO2 pressure also increases higher alcohols but decreases esters.

Fermenter size and shape has a large influence on higher alcohol and ester levels. The greater liquid height in large fermenters increases dissolved CO2 with increasing hydrostatic pressure, and the shape of cyclindro-conical vessels encourages stirring. These factors decrease ester levels. Successive additions of wort during fermentation (drauflassen) can increase ester production, particularly if the added wort is not or only minimally aerated.

Increasing fermentation temperature not only increases ester levels, but changes the ester profile. Acetate esters are increased but medium chain fatty acid esters are not. Finally yeast esterases could play a significant role in the final ester profile, particularly with bottle conditioned beers (and in fact even more so if they're conditioned with Brettanomcyes but that's another story).

* The wife of one of my many bosses so I'll have to say nice things about it. But to be honest I have really enjoyed it.

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