Horseradish: Kick ass chemistry

I like horseradish. It is used in cocktail sauce, spicy mustards, wasabi sauces and Arby's horsey sauce.

Several years ago I planted horseradish in my backyard. Horseradish is essentially a weed and grows with little attention. During October, the roots can be harvested and ground up into a most delightful condiment. I recently did that.

Here's the dirty roots:

Here they are after cleaning. One of them (top left) has been peeled:

I do everything outside so I don't die from the strong fumes. Using a food processor, I grind up the roots and add enough water to make a grindable paste. This is when the odor gets strong. To stop the enzymatic action (see below) I add a few tablespoon of a 5% aqueous acetic acid solution (vinegar) and bottle it. Here is the prepared horseradish:

Yes, I made a quart and a half. That's a lot.

So, what about the chemistry? Glad you asked.

Horseradish is not pungent until you start destroying cell walls. When that happens, enzymes are released and they go into action and start breaking down glucosinolates. The result is a number of compounds in the isothiocyanate class such as allylisothiocyanate and 2-phenyl ethyl isothiocyanate as shown:

These compounds are noxious and irritating.

But, they also have a special place in my heart. I used isothiocyanates regularly in my graduate work. I spent some time functionalizing amino terminated dendrimers with isothiocyanates. What is great about the isothiocyanate group is that it reacts exclusively with amino groups in the presence of alcohols. The isothiocyanate is tuned just right to react with the more nucleophilic N instead of the less nucleophilic O. Cool (see above and ignore the misspelled isothiocyanate).