Few things destroy the presentation of a dish faster than fruit and vegetables that turn brown shortly after you cut them up. Slices of apples or avocados can brown quickly, making your fresh salad appear old and unappetizing. Shredded or sliced potatoes can darken almost immediately, before you have a chance to fry them. If you don't want cut produce to turn brown, put lemon juice on it. It's simple, it's cheap and it works.
An enzyme called polyphenol oxidase exists in most produce. All enzymes require a specific catalyst to be activated. This occurs in polyphenol oxidase when it comes into contact with the oxygen in the air, producing molecules responsible for the brownish color, diminished nutritional value and unappetizing flavor changes we associate with fruit and vegetables past their peak freshness. Ployphenol oxidase's enzymatic reaction is the reason some fruit rots so fast once it is cut open, but its presence doesn't always mean deterioration; it also causes the dark coloration of variety of foods and beverages, including coffee, tea and raisins.
The Acid Test
Lemon juice contains large amounts of ascorbic acid, more commonly known as Vitamin C. Because oxygen reacts more readily with ascorbic acid than it does with polyphenol oxidase, when applied to cut produce, ascorbic acid creates an effective barrier between the air and the enzyme responsible for browning. Placing cut produce into a solution of six Vitamin C tablets crushed into a gallon of water for 15 minutes provides a the same benefit as lemon juice without imparting any additional flavor. The produce does not need to be rinsed prior to use.
Lower pH, Slower Browning
The acidity of a substance is expressed as its pH level. The lower the pH level of a substance, the longer it will prevent browning when applied to cut fruits and vegetables. Polyphenol oxidase is rendered inactive at a pH level below 3.0, so an application of any substance with a pH level below that will forestall the browning process. Lemon and lime juice have a pH level of 2.0 and cranberry juice has a pH level of 2.3, providing different flavor and color options while also acting as anti-browning agents. Orange juice, with a pH level of 3.3, is not acidic enough to stop browning.
Different apple cultivars brown at different rates, with Red Delicious browning the fastest and Arangeh the slowest. Reducing the amount of air in contact with cut fruit, using a physical barrier like a water bath or vacuum sealing, delays browning but only by 15 to 20 minutes. Contact with iron or copper will accelerate browning. Raising the temperature of produce to 170 degrees Fahrenheit by blanching it for five minutes stops browning by deactivating polyphenol oxidase. Refrigeration slows the progress of the enzymatic reaction and freezing halts it until produce is thawed.