Sometimes, there is nothing better in this world than a glass of Californian Chardonnay. Cool but not too cold – you want to make sure it’s warm enough to release the delicious volatiles that make up its ‘bouquet’. And the taste – like buttered toast, sprinkled with vanilla. Now don’t worry, this hasn’t suddenly become a wine tasting blog, we’re getting to the science… Because that butteriness, plus the toasty and vanilla aromas in wines like Californian Chardonnay are courtesy of just a few chemicals.
The buttery aroma/flavour comes from a compound called diacetyl. This is also found in cultured butter, hence the similarity. The process that produces the diacetyl in wine is known as malolactic fermentation. This is a second bacterial fermentation of the wine using Oenococcus* bacteria that break down malic acid from the fruit into lactic acid and aroma compounds like diacetyl.
The toasty vanilla aromas come from the oak barrels the wine is fermented in. Vanillin is present in the wood while it is growing, along with tannins and other aroma compounds like eugenol (which smells of cloves) to stop boring insects munching their way into the wood. The toastiness comes from when the insides of barrels are ‘toasted’ to add flavour to the wines stored in them. The heating breaks down structural molecules in the wood (like cellulose and lignin) into chemicals like acetaldehyde (that smells of green apple), guiacols (that give that toasty, smoky smell) and furans (that smell sweet and bready). When the wine is fermenting in the barrels, it reacts with the wood and absorbs many of these flavour and aroma compounds, giving the characteristic bouquet.
* – Thanks to Tony Milanowski who pointed out that the Leuconostoc bacteria involved in malolactic fermentation had in fact been reclassified as Oenococcus.