In fats, the number of OH groups can be expressed as

The correct option is Acetyl number.

In the chemical analysis of fats and oils, different analytical values (numbers) are determined to characterize their composition and properties:

1. Acetyl value (or Acetyl number): This is a measure of the number of free hydroxyl (-OH) groups present in a fat, oil, or wax. It is defined as the number of milligrams of potassium hydroxide (KOH) required to neutralize the acetic acid liberated by the saponification of 1 gram of acetylated fat or oil. The reaction involves acetylating the free hydroxyl groups with acetic anhydride, and then measuring the amount of acetic acid released upon hydrolysis. Therefore, the acetyl number directly expresses the quantity of free OH groups in the substance.

2. Reichert-Meissl number: This measures the amount of water-soluble volatile fatty acids (such as butyric and caproic acids) present in a fat.

3. Polenske number: This measures the amount of water-insoluble volatile fatty acids (such as caprylic, capric, and lauric acids) present in a fat.

4. Iodine number: This measures the degree of unsaturation (number of double bonds) in a fat or oil, based on the amount of iodine absorbed by 100 grams of the substance.

Thus, the parameter that specifically expresses the number of OH groups in fats is the Acetyl number.