Chemistry · Ch 16 — Chemistry in Everyday Life
Browning of Cut Fruits and Vegetables
Browning of Cut Fruits and Vegetables
One of the most familiar examples of food-quality change is the browning of a cut fruit or vegetable -- a banana, an apple, a potato, a bottle gourd -- within minutes of being peeled or sliced. The mechanism starts at the moment of cutting: the cutting action physically damages plant cells, and that damage releases chemicals, including polyphenol compounds, that were previously kept separate inside intact cell compartments. Once released and exposed to the pH conditions naturally present in the fruit or vegetable, these polyphenols are oxidised by atmospheric oxygen, a reaction driven by an enzyme that is likewise released by the cell damage. The direct product of this enzymatic oxidation is a quinone: in general terms, a polyphenol (a benzene ring bearing two -OH groups) reacts with 1/2 O2 under the enzyme's action to give the corresponding quinone (the same ring with both -OH groups converted to =O groups) plus water. The quinone itself is not the brown colour seen on the cut surface -- quinones are reactive and go on to undergo further reactions, including polymerisation, and it is this polymerisation that produces the actual brown-coloured products, called tannins. Because the whole sequence depends on an active enzyme and on atmospheric oxygen reaching the cut surface, browning can be slowed in either of two chemical ways: by adding a reducing agent, such as SO2 or ascorbic acid (vitamin C), which intercepts the oxidation before quinones can form, or by lowering the local pH with an edible acid such as citric acid (lemon juice) or acetic acid (vinegar), which suppresses the enzyme's activity. A worked problem in the text extends the same reasoning to food storage generally: because oxygen and airborne microorganisms are the two main causes of quality loss during storage, exposure to air is minimised by using an airtight container, by evacuating the co …
Worked out. A worked problem asking how the chemical reactions of stored food with its environment are kept in check. The book's solution identifies oxygen and airborne microorganisms as the two main causes of spoilage, so exposure to the atmosphere is minimised by storing food in an airtight container, by evacuating the container's air, or by flushing/filling it with nitrogen gas instead of air. Because the rate of a chemical reaction falls as temperature falls, refrigeration is a second useful control, slowing down the unwanted reactions between stored food and its surroundings. A third control targets the enzymes that catalyse many of these spoilage reactions (including the browning reaction covered in this section): boiling denatures the enzyme p …
What this figure shows. An in-text reaction scheme (printed without its own figure number, unlike the numbered Fig. 16.1/16.2 later in the chapter) showing the chemical step behind fruit/vegetable browning. A polyphenol -- drawn as a benzene ring carrying two -OH groups on adjacent (ortho) carbons -- reacts with half a mole of molecular oxygen (O2) in the presence of an enzyme released when the plant cell is damaged; the product is the corresponding quinone (the same ring, but with both -OH groups converted to =O groups) plus one molecule of water. The quinone formed is not the final brown colour itself: quinones go on to undergo further reactions, including polymerisation, and it is this polymerisation that produces the actual brown-coloured products, which are called tannins. The book notes the browning reaction can be slowed by adding a reducing agent such as SO2 or ascorbic acid (vitamin C), or by lowering the pH with an edible acid such as lemon juice (citric acid …