IF YOU'VE NEVER taken a tour of Anchor Brewing's plant on Potrero Hill in San Francisco, you have a treat in store.

Personally, the most interesting part is a peek at the shallow, open fermenters. These are large rectangular, stainless steel containers, each with fermenting beer bubbling away, creating lively chunks of churning foam.

Across the narrow hall, one can see hops in bags and get a whiff of citrusy Cascades, a key hop in many Anchor beers. This brings us to Anchor Bock , the company's spring beer and our Beer of the Week. When I pop the cap, I find myself back on the Anchor tour, inhaling the hoppy aromas and the fermenting beer.

Anchor Bock was first brewed last year. Assistant Brewer Mark Carpenter said the company wasn't sure if it would become an Anchor regular or not.

This year's version is 5.5 percent alcohol by volume, a blend of pale barley malt and wheat. Anchor says little about recipes, but the hops may be spicy Saaz. It's made with Anchor's ale yeast, fermented in those open top fermenters and aged at near-freezing.

It's dark brown, with a lively, creamy head with cocoa notes and perhaps vanilla. This year's version is slightly sweeter than I recall, but the finish is dry with lots of fruit — raisins and plums. An unusual, distinctive beer. It's $7.99 for six 12-ouncers, $2.99 for 22-ounce singles.

Anchor Bock is a hybrid, made with ale yeast and not as strong as a German bock — a strong, dark beer that originated in Einbeck in northern Germany. The style became popular in Bavaria in the south. Catholic monks made a strong, dark Double Bock for consumption during Lent, when they fasted, but could drink beer. It was so strong and malty, it was like liquid bread.

The best double bock example is long Paulaner Dubbel Bock , but today, it's much lighter, more like a copper, Oktoberfest beer. A better, usually available German version is Ayinger Celebrator Doppel Bock .

- Want to tour Anchor? Call (415) 863-8350 for a reservation.

Beer ratings are based on a star system. — world classic; — outstanding; — excellent; — good; — average.