Monday, February 12, 2007

Hop chemistry

Sure, we all love a "40" of "O.E." every now and then. But, that bad boy is missing something that makes beer unique. Hops. Most malt liquor is unhopped or only slightly hopped.

Hops are a critical ingredient in beer. These flowers from the female plant botanically classified as Humulus lupulus provide beer with the resins and essential oils that give beer its bitterness, aroma and to some extent flavor. There are a ton of resins and terpenes found in hops. Each variety of the 50+ types of hops, offers a different chemical profile. The possibilities are endless.

Hops have typically 3 uses in making beer. 1) Bittering 2) Flavor 3) Aroma. How a hop is used, depends on how long it is boiled during the brewing process.

Bittering hops are boiled in the wort for at least 60 minutes. A critical isomerization occurs during this time (see below). Flavor hops are added during the last 15 minutes of a boil, and aroma hops are added during the last 1-5 minutes of the boil. The role is determined by how oxidized the hop chemicals get during the boil.

I want to just consider the bittering aspect right now. Hops contain varying amounts of what are known as alpha acids. Many hops range from 4-15% alpha acids. Humulone (as seen in the figure below) is one example of an alpha acid. There are of course many others, but they are all phenolic compounds that are only slightly soluble in water. The alpha acids get isomerized to iso-alpha acids during the boil in the slightly acidic wort. The iso-alpha acids are more soluble in water and contribute the bitterness of beer. A good 60 minute boil is necessary to extract and isomerize the alpha acids. The % alpha acids is a good measure of how bitter a hop will make the beer.

Below is my propose mechanism for the isomerization of alpha acids to iso-alpha acids.


MJenks said...

Because no one is here yet this morning except me, and I can't start the laborious effort of removing all the solvent from my fractions from Friday afternoon, I sat here and stared at your mechanism for the isomerization. I couldn't come up with anything better nor could I refute anything you've proposed.

The only question (comment) that I have is that it seems as though the alpha-acids could also serve as a minor anti-oxidant role that we hear about so often when it comes to the values of food from a nutrition standpoint, as it looks like humulone could easily withstand an extra electron being pumped into it. I didn't draw it out, but I would expect a similar rearrangement would take place. Now, my question: Have you ever heard about anti-oxidant powers of hops? I remember a guy at Wisconsin doing a study a couple of years ago, but I don't recall the specifics and if it was an antioxidant study or not.

Chemgeek said...

I do know that these compounds have been studied for their anti-oxidant properties. However, every reputable journal article I want to read on the matter is not part of our library's offerings. As you can imagine, most information freely available is dubious at best.

annalou said...

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Trap the ketene with a nucleophile and you have a chemistry paper.

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Anonymous said...

I'm not sure if anyone reads this but how bout the top alcohol drops electrons in, the bottom left carbonyl pulls electrons up than some intramolecular proton transfer action as the first step to get an alcohol at the bottom left and a carbonyl at the top?

Anonymous said...

It is possible that if the enol-intermediate form shown here forms, that it would undergo tautomerization to a ketone, ruining the product. However, my knowledge on this subject is very limited.

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