From Grapes to Diamonds: The Fascinating World of Wine Crystals 從葡萄到鑽石：葡萄酒結晶的奇妙世界 By Roshni Printer Wine Crystals Explained At the bottom of some wine bottles or on the cork, you may notice small, clear crystals that winemakers call “wine crystals” or “wine diamonds (Figure 1).” Though they can resemble shards of glass, which sometimes surprise consumers, these crystals are completely harmless. They are a natural salt that forms from tartaric acid, an acid naturally present in grapes. In fact, crystals like these once helped unlock one of the most profound ideas in modern science: chirality. The Process of Crystallization: From Grapes to Diamonds Let’s first examine how these crystals form. Grapes contain several organic acids, but tartaric acid is the signature acid of grapes and wine [2]. During winemaking, depending on the wine’s pH, tartaric acid can lose a hydrogen ion (H+) and become bitartrate, also known as hydrogen tartrate. Meanwhile, wine contains potassium ions (K+) derived from grapes as well. When potassium ions meet bitartrate, they combine to form a salt called potassium hydrogen tartrate (KHT). Due to its relatively low solubility in water, KHT can “drop out” as crystals under certain conditions [3, 4]. Firstly, its solubility decreases with temperature, so chilling wine in a cold room, fridge, or cool climate encourages crystallization. Also, its solubility in aqueous ethanol drops as the ethanol content rises during alcoholic fermentation, causing KHT to precipitate during the winemaking process. To avoid consumer’s concerns about food safety, wineries use different ways to prevent post-bottling crystal formation [3]. A common technique is “cold stabilization,” where the wine is chilled on purpose so that the salt crystallizes in storage tanks and can be filtered out before bottling. Other methods include the removal of compounds that involve KHT precipitation, and the introduction of additives to inhibit or decelerate the crystallization process. Louis Pasteur and the Discovery of Chirality in Tartaric Acid Although we now have extensive knowledge about tartaric acid and its salts, in the 1800s, tartaric acid presented an intriguing puzzle to scientists. To understand their early experiments, it helps to Figure 1 Wine crystals on a cork. Photo credit: Francesco Santini [1]
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