Tartaric Acid---Wine Diamonds

Recently, I took a look at one of the red wine corks that we have and saw this shiny, glinting object that I knew was a crystal of tartaric acid. Doing some further online research into tartaric acid took me on a little trip down memory lane. (In a former professional incarnation, I was a small and macromolecular crystallographer, thus my affinity for crystals!)
Grapes, the baobab tree and the tamarind fruit are the only plants that contain tartaric acid.¹ The chemical structure of tartaric acid looks like this:

Tartaric acid is also used in cooking, where it is known as cream of tartar. When it occurs in wine, tartaric acid can exist as the potassium salt (potassium bitartrate), and when it crystallizes out of solution, are known as wine diamonds. In fact, Louis Pasteur was studying wine sediments when he noticed that there were two types of crystals. He tediously separated out the two forms of tartaric acid crystals that he saw under his microscope, and recrystallized the separated piles. He then found that the two forms of crystals rotated plane polarized light in different directions. What Pasteur had done was to separate out the mixture of D- and L-tartaric acid. When the two forms of tartaric acid are found together, they do not rotate plane polarized light and this mixture is called a racemic mixture. I found out that the word racemic comes from the Latin racemus, meaning ‘a bunch of grapes’).³ This elegant(tedious) experiment that Pasteur conducted lead to the discovery of molecular chirality!

Wouldn't you agree that this little cutie looks like a diamond!
References:
1. Patrick McGovern, Ancient Wine: The Search for the Origins of Viniculture.
2. The chemical structure of tartaric acid was drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.
3. Crystallography 365, Pasteurized Crystals – Tartaric acid, December 27, 2014.