Ok Revvy, thanks for posting that. Here's another point
From "Brewing" by Michael J. Lewis (Dept. of Food Science
and Technology, UCal) and Tom W. Young (School of
Biochemistry, University of Birmingham UK)pp149-150:
"To obtain the large amount of yeast needed for fermentation,
cultures on maintenance media or in their preserved
state are first grown up in the laboratory before transfer
to specialized equipment....This equipment is designed
to hygienically produce the large amount of yeast needed
for fermentation. This guarantees a regular supply of
high-quality, healthy yeast, free from other microbes
and, by reducing variation, assists in assuring both
consistent fermentation and beer quality. Recourse to
the laboratory stocks occurs at regular intervals
(perhaps twice yearly) but fresh yeast is drawn from
the propagator at much more frequent intervals. Typically
yeast will be replaced after five to ten successive
fermentations. Most fermentations are therefore
conducted with yeast drawn from a previous one and not
from a propagator. Brewers involved in small-scale
operations often use mixed cultures (more than one
strain) and have very limited (if any) laboratory
facilities. In case of problems with yeast, they
obtain slurry from other brewers or sometimes re-isolate
the strains from their mixture.
Increasingly, microbrewers are using dried
brewer's yeast as their primary source. This is highly
viable and supplied in larg (kg) amounts. The yeast
may be cultured by the producer under conditions far
removed from those found in a brewery and may contain
contaminating microbes (particularly lactic acid bacteria).
Both these factors may influence the quality of the first
and especially subsequent fermentations conducted with
(This is why making a starter with dried yeast is not
recommended. Because if the dried yeast is contaminated,
it will propagate with the yeast in your starter.)
A few pages later, they describe how a typical brewing
operation creates an actively fermenting culture that
is 1/10 the size of the batch of beer they are making.
When that batch is done, 1/10 is removed and used for
the next batch. They also describe how some brewers
take a yeast slurry from a batch and press it into
a cake that is about 25% by weight yeast, and use
it for the next batch, using 0.3kg per hectoliter.
By my calculation that would be 14 grams or about
half an ounce of dry yeast per 5 gallon batch.
The fact that some microbreweries save money by using
dried yeast doesn't mean that their beer wouldn't be
better if they used an actively fermenting culture.
So I think I'll stick to making starters because
even if the dried yeast isn't contaminated, the lag
time will still allow any microbes from the air/dust
to get a foothold.