Cut surfaces of fruit and vegetables often becomes dark because

Options

A. dirty kinife makes it dark
B. oxidation of tannic acid in the presence of trce of iron from the knife makes it dark
C. dust of the air make it dark
D. none of these

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks why the cut surfaces of fruits and vegetables often become dark after being cut.

2. **Identifying the Options**: The options provided are:
- A) Dirty knife makes it dark
- B) Oxidation of tannic acid in the presence of trace of iron from the knife makes it dark
- C) Dust of the air makes it dark
- D) None of these

3. **Analyzing the Options**:
- **Option A**: A dirty knife could introduce bacteria or contaminants, but it does not directly cause the darkening of cut surfaces.
- **Option C**: Dust in the air does not chemically react with the fruit or vegetable to cause darkening.
- **Option D**: This option suggests that none of the previous options are correct, which we will evaluate after considering Option B.

4. **Focusing on Option B**: The darkening of cut surfaces is primarily due to the enzymatic reaction involving polyphenols. When fruits and vegetables are cut, polyphenol oxidase enzymes are activated, leading to the oxidation of tannic acid (a type of polyphenol). This reaction is often accelerated by the presence of iron, which can come from the knife used to cut the produce.

5. **Conclusion**: The correct answer is **Option B**: "Oxidation of tannic acid in the presence of trace of iron from the knife makes it dark."

6. **Additional Information**: To prevent this enzymatic browning, methods such as cooking (which denatures the enzymes) or applying acidic substances like lemon juice or vinegar can be used.

---