Wednesday, March 7, 2007

Brewing chemistry: Part 1- Gibberellic acid

Gibberellic acid is a plant hormone that induces the formation of a number of key enzymes, and in a sense, it gets things started in the brewing process. So, I thought I would start with gibberellic acid.

Beer is made from the sugars in malted barley (along with a few other key ingredients). In brief, the malting process involves soaking the barley in water to induce germination. During germination cell walls are broken down and starch is released within the grain [there is a bunch of plant anatomy that I could get into, but I'm not really interested in that]. Enzymes are formed that will eventually be used by the brewer to break down the starch into maltose. At a certain point germination is stopped and the grain is kiln dried and/or roasted. This is now malted barley, and it is ready to be mashed. I will deal with mashing in another post.

I want to zero in on what causes the starch-hydrolyzing enzymes to form. As the barley is germinating, any free carbohydrates are consumed during respiration (i.e converted to pyruvate and then CO2). Once the carbohydrates are depleted, the starving barley grain turns to its starch reserves. Since starch doesn't just fall apart into glucose, enzymes are needed. When the free glucose gets low, a signal is sent to start forming enzymes (amylase) to break up starch. The "signal" that triggers the formation of these enzymes is the plant hormone, gibberellic acid (GA3).

The structure of GA3 is:



This is a very interesting molecule. It was first synthesized by a fellow by the name of Elias James Corey in 1978. I'd love to write a bit about his synthesis, but thanks to very limited (and embarrassingly so) library access at my institution, I can't easily get the papers. Thanks to Peter J. Stang, I can at least see the first page of the 1978 communications [JACS, 1978, v.100, p.8031 and p. 8034]

GA3 gets the germinating plant to form mRNA that codes for the formation of things like amylase and other starch hydrolyzing enzymes. How any of the gibberellins work is not well understood. What is known is that the cells in the aleurone layer of the barley seed contain a membrane-bound receptor for GA3. When GA3 binds, a Myb transcription regulator is produced. This Myb protein induces transcription of the amylase gene. The amylase is sent on a tour of duty to destroy starch, but a well trained maltster will stop the malting before this takes place. The amylase is going to be used during the mashing process by the brewer.

To be continued...

2 comments:

Anonymous said...

good stuff, you've helped me out with some practical work :)

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