In food chemistry, the main cause of rancidity in fatty foods is the gradual ______ of fats and oils when exposed to air, light, or heat. Which process correctly completes the sentence?

Introduction / Context:
This question concerns the concept of rancidity in food chemistry. Rancidity is the unpleasant smell and taste that develop in fats and oils when they spoil. Understanding the main chemical process behind rancidity helps in learning about food preservation and storage conditions as well as the role of antioxidants in packaging and formulation.

Given Data / Assumptions:

• The focus is on rancidity of foods containing fats and oils.
• The sentence states that rancidity is due to some chemical change of fats and oils.
• Options include hydrolysis, clarification, reduction, oxidation, and crystallisation.

Concept / Approach:
Rancidity, especially oxidative rancidity, occurs when unsaturated fats react with oxygen in the air. This oxidation produces aldehydes, ketones, and other compounds with strong off odors and flavors. Although hydrolysis can also contribute to certain types of rancidity, the main cause commonly discussed at basic level is oxidation. Thus, the correct word to complete the sentence is oxidation.

Step-by-Step Solution:

Verification / Alternative check:
Food science references define oxidative rancidity as the main mechanism by which edible oils spoil. Antioxidants are added to packaged foods to slow this oxidative process, confirming that oxidation is at the center of the problem. Sensory changes such as off odors and off flavors correlate with increased levels of oxidation products. While hydrolytic rancidity can occur when fats react with water and enzymes, the general exam level statement usually points to oxidation as the main cause.

Why Other Options Are Wrong:

• Hydrolysis: This process breaks fats into glycerol and fatty acids and can play a role in some spoilage, but the typical textbook definition of rancidity focuses on oxidation.
• Clarification: Refers to making a liquid clear by removing impurities, not a spoilage process.
• Reduction: The opposite of oxidation and does not describe the main cause of rancidity.
• Crystallisation: Refers to formation of solid crystals from a solution or melt, not the chemical spoilage of fats.

Common Pitfalls:
Students sometimes choose hydrolysis because they associate it with splitting fats, or they may simply guess from unfamiliar terms. Remembering that rancidity is closely linked with oxygen and that antioxidants are used to prevent it is a helpful clue that oxidation is involved. In exam questions, words like air, oxygen, and exposure often point directly to oxidation as the underlying process.

Final Answer:
The main cause of rancidity in foods is the oxidation of fats and oils.