Mad Food Science

Cream of tartar

Wine.
It’s delicious, sophisticated and makes everyone more attractive.
And it’s where cream of tartar comes from. Seriously.
Is there anything wine can’t do? 

Cream of tartar is a chemical called potassium bitartrate. It’s a carboxylic acid, which is a kind of organic acid. An inorganic acid, by comparison, would be something like sulphuric acid which doesn’t have carbon atoms. You’re more familiar with carboxylic acids than you might think: acetic acid is vinegar; citric acid makes lemons sour; lactic acid makes your muscles burn after running a marathon.

Look at you go, you marathon-running chemist!

Tartaric acid (the base molecule of cream of tartar) is found in the greatest concentration in grapes, where it’s the chemical that makes them tart. Hence, tartaric. It’s also found in bananas (which don’t seem particularly tart to me) and tamarinds, but in lesser amounts.

It was first isolated from the inside of wine barrels by a Persian alchemist way back in the year 800. When grape juice ferments, the tartaric acid “falls” out of the liquid and crystalizes on any available surface in a process called “precipitation.” Kind of like rain. But with acid crystals. Like it’s the 60s all over again.

In the kitchen, cream of tartar has a bunch of roles to play. It’s most commonly used as a stabilizer – it gives airy mixtures like whipped cream and mirangue a firmer texture that helps them hold on to the air trapped inside and keep their volume up. Its stabilization properties are also critical to the icing when you’re making a gingerbread house. Without cream of tartar, it won’t have enough structure to glue the walls and roof together. I decorated the occasional gingerbread plank in my childhood as a result of skipping the cream of tartar.

Because it’s an acid, cream of tartar can be reacted with baking soda to create carbon dioxide gas that acts as a leavening agent. In fact, many brands of baking powder use cream of tartar as the acid ingredient. I don’t want to spill the beans on baking powder yet (I’ll save it for a future post), so I’ll just leave it at that.

Finally, cream of tartar is used as a catalyst (something that helps a chemical reaction happen) for inverting sugar. Regular sugar is a crystal made up of two parts: glucose and fructose. Left to its own devices when it’s dissolved in a liquid, it will eventually recrystalize and give whatever you’re making a crunchy, gritty texture. Not pleasant. But by boiling sugar with a bit of cream of tartar, you can split the sugar into its two separate components. The resulting “inverted” sugar syrup is resistant to recrystalization because you’ve broken the molecules apart, and will stay silky smooth for longer.

Only one mystery remains for me, then: what’s with the “cream of” part of the name? It’s a crystal that gets ground into a powder! It’s not creamy at all! It doesn’t come from cream; it comes from wine! But I suppose an ingredient that’s been around for over 1200 years deserves to have a few secrets, so I’ll have to let it slide.

• 4 April 2011
• 17