In article , Dan Smithers writes:
| gavin wrote:
|
| Stick to your 'directly'. Yeast can also use starch, but the mechanism
| involves quite a lot more complexity in converting it to sugar.
|
| That depends on the strain of yeast. Years ago one of the home winemaking
| supply producers used to market a yeast which would ferment a proportion of
| starch and so was great for rice wine and similar.
|
| The usual technique with starch is to use an enzyme to break it down
| into sugars before fermentation. It's called brewing and is what is done
| for beer.

Yes, but that is a gross over-simplification; look up the acid hydrolysis
of starch for another mechanism, which is also one of the ones used by
yeast.

| The problem with starches is that they take a _really_ long time to
| ferment. I also seem to remember that they are insoluble in water and
| tend to settle out.

That is misleading. It is the conversion that takes the time. In
brewing, it is hastened and takes only an hour or so; acid hydrolysis
is typically slower at the near-neutral pH in most fermentations.

Yes, they are insoluble. That is essentially the only difference
between a sugar and a starch.


Regards,
Nick Maclaren.