Browning of cut fruit and vegetables

Why

In intact cells of apple, pear, banana, avocado, artichoke, celeriac or potato, phenolic compounds and the enzyme polyphenol oxidase are kept apart; a knife or grater ruptures the cells, the enzyme meets the phenols and, with oxygen from the air, converts them into compounds that link up into brown pigments. Acid, such as lemon or lime juice, slows the enzyme, vitamin C (ascorbic acid) also turns the forming compounds back, water limits access to air, cold slows the reaction and a brief boil or blanch destroys the enzyme entirely. According to Penn State Extension, a vitamin C solution protects apples and fruit for freezing better than lemon juice, but in the kitchen the acidic ingredient the recipe already contains is usually enough, if the fruit goes into it straight after cutting. With avocado only real separation from air helps: in an ATK test, neither vitamin C nor the stone, plastic wrap, oil or a rub of lemon saved halved avocados overnight, whereas vacuum sealing or storing cut side down in lemon water worked. Browning is harmless to health and only spoils appearance and a little flavour; greying of grated potato is the same process (Potato pancakes and rösti card).

How to do it

• Cut and grate just before serving or the next step, and toss cut fruit straight away with the acidic ingredient of the recipe, lemon or lime juice or the dressing.
• Put apples or pears that have to wait into cold water with lemon juice, about 120 ml per 2 l of water; peeled potatoes only need cold water.
• Rub an artichoke with lemon after every cut and put it straight into acidulated water.
• Store a halved avocado cut side down in a bowl of water and lemon juice in the fridge; in the ATK test it stayed green for 48 hours, only slightly softer and tart.
• Make guacamole as close to serving as possible; according to ATK salted avocado browns even faster, so add the lime together with the other ingredients.
• Fruit and vegetables you will cook or freeze can be scalded or blanched briefly; the heat destroys the enzyme.

If it goes wrong

Scrape off the browned layer of guacamole or avocado, the flesh underneath is usually green, and stir the rest with a little fresh lime juice. Browned apple or pear can be eaten, or used for baking, compote or a cake where the colour does not matter. Use grey grated potato straight away; the pancakes will only be darker. Next time cut just before serving and add the acid at once.