Why do avocados turn brown when cut open?

The browning of avocados, also known as enzymatic browning, is a natural process caused by the exposure of the fruit's flesh to oxygen. Here's a step-by-step explanation of what happens when an avocado is cut open and why it turns brown:

1. Exposure to Oxygen: When you cut open an avocado, the flesh is exposed to oxygen from the air. This initiates a series of chemical reactions that lead to the production of melanin.

2. Enzymes and Substrate: Inside the avocado, there are enzymes called polyphenol oxidases (PPOs) that act as catalysts in the browning process. These enzymes are present in the chloroplasts, which are organelles responsible for photosynthesis. When the avocado is cut, the PPOs come into contact with a substrate called polyphenols, which are also present in the fruit.

3. Reaction with Oxygen: In the presence of oxygen, the PPOs catalyze the oxidation of polyphenols, converting them into brown pigments called melanin. Melanin is the same pigment that gives skin, hair, and eyes their color.

4. Chain Reaction: The oxidation process is a chain reaction, meaning that each oxidized polyphenol molecule can further oxidize other polyphenol molecules, leading to a rapid spread of browning throughout the exposed avocado flesh.

5. Factors Influencing Browning: The rate at which an avocado turns brown depends on several factors, including temperature, light exposure, and the specific avocado variety. Higher temperatures and exposure to light can accelerate the browning process. Some avocado varieties, such as Hass avocados, have higher levels of PPOs and polyphenols, making them more prone to browning.

To slow down the browning of cut avocados, various methods can be used. One common technique is to store the avocado in an airtight container or wrap it tightly with plastic wrap to minimize its exposure to oxygen. Another trick is to add an acidic substance like lemon juice or vinegar to the avocado, as the acidity can inhibit the activity of PPOs.

Understanding the enzymatic browning process in avocados can help you appreciate the natural reactions that occur when this fruit is exposed to oxygen. By employing simple storage and preparation techniques, you can preserve the freshness and color of your avocados for a longer period.