It is commonly know that freshly cut fruits turn a brownish color when exposed to the air for a number of minutes (see below). This is due to the action of the enzyme polyphenol oxidase. It is also well known that the formation of the brown color can be prevented or slowed by chilling the fruit, boiling it, or rubbing it with lemon juice (which contains ascorbic acid). Explain why each of these three methods work to slow down the browning process.

These methods works by denaturing the enzymes. The first and the second method is to change the temperature and the third method is to change the pH.

Explanation:

The browning of the fruits or vegetables takes place due to the action of the enzymes. The enzymes inside the body of the organism works under specific conditions.

These conditions are temperature and pH. Change the optimum pH and temperature of the environment of enzymes could denature it.

In these case the enzymes would not work efficiently and the browning of fruits will not take place.

By cooling or boiling the fruit changes the temperature of the enzyme and in this temperature the enzymes do not work effectively.

In case of ascorbic acid the pH is acidic and enzymes do not work at lower pH.

So, by changing the pH and temperature, the enzymes do not work.

your body is trying to warm itself up. or more precisely, sensors in your skin send messages to your brain to generate heat to your body.