I had heard for years that bock beer was made from the dregs that were left when the vats were cleaned around Easter time.
This just isn't true. For health reasons, the brew tuns are cleaned and sterilized after EVERY batch. The only thing the dregs are used for is the yeast for succeeding batches. There are breweries today in some parts of the world that are using yeast cultures that were started in batches over 100 years ago.
I learned this when I got into brewing beers in 1973.
Bock beer is a style of lager that was first made in Germany. Traditionally brewed around the end of the growing season when barley and hops were at their peak, it was allowed to age all winter in a process called lagering. It was then served in the spring (around Easter?).
When you're able to find real bock beer, you're in for a real treat as they are usually a very rich beer. In Europe, bock beer is available in the traditional dark color which is called dunkles, and also the pale varieties (helles). They even have what they call doppel bock, that is to say, double bock, that has a higher alcohol content than regular beer.
These wonderful beers are definitly not made from dregs.
Much of this information was acquired from:
http://www.faqs.org/faqs/beer-faq/part1/section-7.html
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