Keeping apples or avocados from browning after being cut is impossible; within minutes of being exposed to air, these fruits (yes, avocado is a fruit) begin to brown. No matter what you try—adding lemon juice, keeping the pits in place, immediately sealing the produce in an airtight bag—brown discoloration always occurs. However, air is not the only reason that foods like apples, avocados, and lettuce brown: it's also due to the knife you're using. Why Does Browning Occur? Apples and other produce contain both polyphenol oxidase (PPO), an enzyme, and polyphenols, a chemical. Normally, these two are separated and do...