OK… So just catching up on my commenting. I’m a bit behind. Things would be so much easier if I dodn;t have to work for a living…
Andy – yes, your reasoning for why the instructions say to ferment hot is current, but your reasoning on the mechanism isn’t. It does make fermentation go faster. The reason is that yeast metabolism goes up as the temp goes up (to a point obviously.. Then it dies) but also becomes less efficient. They get lazy and don’t complete the full pathway and you get off flavours developing because they are releasing compounds that would normally be metabolised. Higher temps also stresses yeast and can caused increased rates of autolysis (cell death and breakup). Vegemite/Marmite flavours are normally autolysis flavours – the taste of dead, broken up yeast cells. In small qualities autolysis gives a bready taste to some wines like champagne (they leave the wine on the dead yeast for a while to develop the flavour). In large quantities it makes your beer taste like Marmite.
Cameron – This is an enzyme driven reaction but remember that the enzymes are inside the yeast. This is cell metabolism. The cell brings in glucose through the cell menmbrane and metabolises it into energy through a long pathway with many enzymes involved. The waste products are co2 and alcohol. The limiting factor of the reaction rate isn’t enzyme activity it’s the rate at which the glucose can move through the cell membrane. That’s by osmosis and that is limited by the concentration gradient. If you make the sugars too concentrated outside the cell though you can cause the cells to move so much sugar inside that they will literally explode. This is called osmotic stress. Biology is complicated.
Bulk T-bag (I should call you catering pack) – since Andrew is too chicken to read out what I wrote originally, you might want to check outthe brewery wh last year did actually brew a beer with yeast cultured from the brewer’s beard. I think it was dogfish head brewery.