Infographic on the chemistry of port. There are several different types of port: ruby port is aged for a short time in oak vats; tawny port is aged in oak barrels for longer periods; rosé port is a newer style of port made with minimal contact with grape skins; and white port is made from white grapes. The colour of port is caused by anthocyanins found in grape skins. As ports age, anthocyanins form pyranoanthocyanins and polymeric pigments, leading to a change towards an orange hue. Sotolon is a key odour molecule in aged port, and its concentration correlates with the length of aging. Barrel-aged ports gain some compounds from barrel wood which also contribute to favour and aroma.
Click to enlarge

Port’s often relegated to the festive period, and forgotten about for the rest of the year (at least, in the UK). This is a shame as if you look beyond the standard ruby port, there are several other delicious types, particularly the varieties that are aged for longer periods of time. There’s also some interesting chemistry behind these varieties and how they transform during ageing.

There are a few different ways of classifying the varieties of ports. Sugar content varies between different varieties, but more commonly they’re grouped into classifications denoting how they’ve been aged. Broadly, they can be split into four categories: ruby port, tawny port, rosé port and white port. There’s been plenty written on these different varieties by people who know a lot more about them, so we’re going to jump straight to the chemistry here.

Ruby ports are the most familiar type of port and take their colour from the red anthocyanin compounds that come from the grape skins. The main anthocyanin that contributes to the red colouration is oenin (malividin-3-O-glucoside). White ports don’t have this colouration since they’re made from white grapes that don’t contain these anthocyanins.

As ports age, the anthocyanin concentration decreases as they react to form other compounds. Pyranoanthocyanins are one such group of compounds, and their gradual creation is the partial cause of a shift towards orange hues. The anthocyanins can also react with each other to form larger, polymeric pigments over time. These shift the colour further towards orange and brown. The tawny ports, aged for long periods in oak barrels, lose their deep red colour and take on the colouration that lends them their name.

Ageing also affects the flavours of port wines. This is immediately detectable when comparing the tawny ports to the ruby ports; ageing gives them flavours that veer more towards nutty and caramel. These flavours intensify with age, so they are more apparent in a 30-year-old tawny port compared to one that’s only a 10-year-old.

A molecule that’s been identified as being particularly important to the aroma and flavour of aged ports is sotolon. Sotolon imparts nutty and spice-like aromas, and its concentration in port wine correlates with the length of ageing. In taste tests, samples of ports which had sotolon added to them were consistently ranked as older, proving its importance to the character of aged ports.

Ageing also develops other aroma compounds. Ports aged in bottles can develop some floral notes, related to compounds such as beta-damascenone and beta-ionone. On the other hand, those aged in barrels take on some compounds from the wood which contribute to their flavour. Lactones, including whisky lactone (also found, as the name suggests, in whisky), give spicy, woody aromas.

Sulfur compounds can also affect the aroma and flavour of ports; these are present in larger quantities in younger ports, with their concentrations dropping as ports age.

As with many other types of alcoholic drinks, this is just a selection of the many chemical compounds found in a glass of port. Even those found in small quantities can affect its flavour in some cases, and the balance of compounds varies in different varieties, giving surprisingly different characteristics.

Port’s something of a disappointment for me, not because I dislike it (quite the opposite!), but because I hadn’t explored beyond the traditional ruby port until a trip to Porto last year. For me, at least, the fruity, nutty flavours of a golden tawny port more than justify opening a bottle more than once a year at Christmas!

The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’s content usage guidelines.

Enjoyed this post? Support Compound Interest’s graphics on Patreon!

References & Further Reading


1 CommentClose Comments