Tuesday, 29 December 2009

Flavour Compound of the Week - H2S




This week we are looking at H2S or hydrogen sulphide. This is my least favourite natural flavour compound in beer. Its aroma is the disgusting smell you will remember from the playground after someone let off a stink bomb or when your grandmother has been on the spouts at Christmas. At lower concentrations it has a sweet eggy smell like your lunchbox when you didn’t get around to eating your egg sandwiches.

I can’t understand how at any level, flatus and rotten eggs is a welcome component of beer flavour but some drinkers welcome it as a quirk of the brew. It would be interesting to see if they missed it if it was removed.

H2S is produced during fermentation by yeast from various sources. The most significant source is through yeast metabolism of amino acids. The amount of H2S is influenced by sulphate levels in water and the availability of sulphur containing amino acids so the brewer has some control over levels by ensuring that these are correct. The most significant factor in the production of H2S is the yeast strain. Some yeasts pump the stuff out like a fart machine while others need extreme conditions to produce any. H2S is also produced prodigiously by certain bacteria and wild yeast so may indicate an infection especially in bottled beer.

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