Apples turn brown shortly after being cut or bruised due to a natural process. This discoloration, while unappealing, does not necessarily indicate spoilage. Understanding the underlying science and factors influencing this change can help in managing and preventing it.
The Science Behind Apple Browning
Apple browning is an enzymatic oxidation process. When an apple’s cells are damaged, three key components mix: polyphenol oxidase (PPO) enzymes, phenolic compounds, and oxygen from the air. Under normal conditions, these components are separated within different compartments of the apple’s cells.
Polyphenol oxidase (PPO) is an enzyme naturally present in apples, located primarily in the plastids, while phenolic compounds (also called polyphenols) are stored in vacuoles. The PPO enzyme acts as a catalyst, initiating a reaction where it oxidizes the phenolic compounds into new chemicals called o-quinones.
These o-quinones are highly reactive. They polymerize, meaning they link together, and can also react with amino acids present in the apple. This chain of reactions ultimately leads to the formation of melanin, a brown-colored pigment. Melanin is the same pigment responsible for skin and hair color in humans.
Factors Influencing Browning Speed
The specific apple variety plays a significant role, as different cultivars possess varying levels of PPO activity and phenolic compound content. For instance, varieties like ‘Liberty’, ‘McIntosh’, and ‘Fuji’ hybrids tend to brown more quickly, while ‘AutumnCrisp’ and ‘Granny Smith’ often show minimal browning. Apples with higher concentrations of phenolic compounds generally exhibit a greater potential for browning.
Temperature is another factor, with warmer conditions accelerating the enzymatic browning reaction. Conversely, colder temperatures can slow down the enzyme’s activity, delaying the browning process.
The pH level of the apple’s flesh also affects browning speed. PPO enzymes function best within a specific pH range, typically between 5 and 9. Altering the pH outside this optimal range, particularly to a more acidic environment, can reduce or inhibit the enzyme’s activity. Finally, the extent of cellular damage directly correlates with browning; more damage exposes a greater quantity of PPO and phenolic compounds to oxygen, resulting in faster and more widespread discoloration.
Preventing Apple Browning
One effective strategy is to reduce the apple’s exposure to oxygen. Submerging apple slices in water or placing them in an airtight container can limit oxygen availability, thereby slowing the reaction. Covering slices tightly with plastic wrap also creates a barrier against air.
Altering the pH of the apple’s surface can significantly inhibit PPO activity. Acids, such as those found in lemon juice or citric acid, lower the pH, which can denature the PPO enzyme, rendering it less active or inactive. A solution of lemon juice diluted with water (e.g., three tablespoons per quart of water) can be effective without imparting an overpowering flavor.
Using natural antioxidants, particularly ascorbic acid (Vitamin C), is another common method. Ascorbic acid preferentially reacts with oxygen before the PPO enzyme does, effectively “sacrificing” itself to prevent the browning reaction. It can be applied by dipping apple slices in solutions containing ascorbic acid powder or crushed Vitamin C tablets. A salt solution, typically half a teaspoon of kosher salt per cup of water, can also interfere with oxidation and is a recommended method for preventing browning. Refrigeration further slows the enzymatic reactions, providing additional protection against browning.