What causes the browning of fruits and vegetables, and how can I prevent it?

The browning of fruits and vegetables is a common phenomenon that occurs when the inside of the produce is exposed to oxygen. As you've mentioned, the enzyme polyphenol oxidase (PPO) plays a significant role in this process. When PPO reacts with oxygen, it turns the phenolic compounds in the fruit or vegetable into brown pigments, resulting in the characteristic browning effect.

To prevent browning, you can try a few different methods. One approach is to use an acid, like lemon juice, to denature the PPO enzyme and prevent it from reacting with oxygen. However, as you've found, lemon juice isn't always effective. You can also try using other types of juice, like orange or grapefruit, but the acidity level may vary, and the results might not be consistent. For example, you can use 1 tablespoon of lemon juice per cup of water as a starting point and adjust to taste.

Another technique to prevent browning is to use an anti-browning agent, such as ascorbic acid (Vitamin C) or sodium metabisulfite. These agents can be found in some commercial products, like fruit preservatives, or you can purchase them in powder form and mix them with water to create a solution. For instance, you can mix 1 teaspoon of ascorbic acid powder with 1 cup of water to create a solution that can be brushed onto the cut fruit or vegetable.

In addition to using acids or anti-browning agents, you can also try to minimize the exposure of the fruit or vegetable to oxygen. This can be achieved by storing the cut produce in an airtight container, as you've mentioned, or by using a vacuum sealer to remove the air from the container. You can also try to cut the produce just before consuming it, or use a technique called "cutting under water" to