Food Chemistry - Chemistry of a LemonWith it being Pancake Day tomorrow, it seemed like a good time to look at the chemistry of the humble lemon, and the compounds that give it its sour taste. Of course, citric acid is already well known – it even has its own E number (E330). However, a couple of other acid compounds are also contributors towards the chemical make up of a lemon.

One of these is malic acid, a compound that also has its own E number (E296). Citric acid is present in much higher quantities than malic acid and is the main contributor to the lemon’s sour taste; however, malic acid is present in around 5% of the concentration of citric acid. It is also found in apples and cherries, and responsible for aspects of their flavour.

Another acid present in lemons, and one with which citric acid is occasionally confused, is ascorbic acid, or vitamin C. The vitamin C levels in a lemon, at around 50 milligrams per 100 grams, are on a par with those of an orange, and significantly higher than those in a lime (~29mg/100g). This last fact in particular is one that the British Navy discovered belatedly to their detriment in the early 1900s.

Vitamin C is required by the body to produce collagen, the main protein of connective tissues in animals. Scurvy is a disease caused by a lack of vitamin C, the symptoms of which include spots, bleeding gums, loss of teeth, jaundice, fever, and eventual death. The disease was a major problem for seafarers, who would spend months at sea, and without a supply of fresh citrus fruit to supplement their vitamin C levels often succumbed to scurvy. By the mid-1700s, physicians had, however, discovered that citrus fruits were an efficient cure for the disease, and in the late 1700s all Royal Navy ships were required to serve lemon juice in rations.

Despite this recommendation, a lack of awareness of vitamin C, and the differing vitamin C content of lemons and limes, meant scurvy again became an issue in the early 1900s. When the Royal Navy began to start substituting lime juice for lemon juice, as they could source these from within the British colonies, they did so under the assumption that the acidity of lemons was what warded off scurvy, and as limes were more acidic, it followed that they would be equally effective. This had dire consequences, with several arctic expeditions succumbing to scurvy due to the failure of lime juice to provide enough vitamin C.

The confusion this caused was not fully resolved until the eventual isolation and discovery of vitamin C by the Hungarian Albert Szent-Györgyi in 1932. Vitamin C was actually named after its scurvy-preventing abilities – the name, ‘ascorbic acid’, comes from ‘antiscorbutic’, a term used to refer to substances preventing scurvy.



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