Aspartame is most successful artificial sweetener but is only limited to cold food and cold drinks because:

Unstable at cooking temperature

The correct answer is option 4. Unstable at cooking temperature.

Aspartame is a widely used artificial sweetener known for its sweetness and low-calorie content. However, it is indeed limited to use in cold foods and drinks, primarily due to its stability concerns at high temperatures.

When exposed to high temperatures, such as those encountered during cooking or baking processes, the molecular structure of aspartame can become unstable. Specifically, the methyl ester bond present in aspartame can break down under heat, leading to the degradation of the sweetener. This degradation not only diminishes the sweetness of the product but can also result in the formation of undesirable tastes and odors.

Additionally, aspartame's breakdown at high temperatures can compromise its safety and suitability for consumption. The degradation products that form under heat might pose health risks or have adverse effects on the sensory qualities of the food or beverage.

As a result of these stability concerns, food manufacturers typically avoid using aspartame in products that require exposure to high temperatures, such as cooked or baked goods. Instead, it is primarily used in cold foods and drinks, where its stability is not compromised, and its sweetening properties can be effectively utilized without adverse effects.

Therefore, while aspartame is a successful artificial sweetener due to its sweetness and low-calorie content, its application is limited to cold foods and drinks to ensure product quality and safety.