Aspartame is not used in hot drinks as a sweetener because:

It is unstable at high temperature

The correct answer is option 2. It is unstable at high temperature.

Let us look into why aspartame is not used in hot drinks, focusing on its instability at high temperatures.

Aspartame’s Composition and Stability:

Chemical Structure: Aspartame is a dipeptide ester composed of two amino acids: aspartic acid and phenylalanine, along with methanol. Its chemical formula is \( C_{14}H_{18}N_{2}O_{5} \).

Sweetness: Aspartame is about 200 times sweeter than sucrose (table sugar), so only a small amount is needed to achieve the desired level of sweetness.

Stability Issues with Heat:

Decomposition:

Heat Sensitivity: Aspartame is sensitive to heat. When exposed to high temperatures, such as those found in hot beverages, it undergoes hydrolysis. This means that the ester bond in aspartame breaks down, leading to the formation of its constituent components: aspartic acid, phenylalanine, and methanol

Products of Decomposition:

Aspartic Acid: An amino acid that is less sweet than aspartame.

Phenylalanine: An amino acid that is also less sweet and, in excessive amounts, can be harmful to individuals with phenylketonuria (PKU), a genetic disorder.

Methanol: A type of alcohol that can be toxic in high amounts. While the amounts produced in aspartame breakdown are small, the breakdown affects the sweetness of the sweetener.

Impact on Sweetness:

Loss of Sweetness: The breakdown of aspartame results in a loss of its sweetening power. When aspartame decomposes, the resulting products do not have the same sweetening effect as the original compound.

Flavor Changes: The decomposition process can also lead to undesirable changes in flavor, affecting the overall taste of the beverage.

Practical Considerations:

Use in Hot Drinks:

Temperature Limitations: Due to its instability, aspartame is typically avoided in applications where it will be exposed to high temperatures, such as in hot drinks like coffee or tea.

Alternative Sweeteners: For hot beverages, other sweeteners that are more stable at high temperatures, such as saccharin, sucralose, or steviol glycosides, are preferred.

Other Options:

Sucralose: Known for its stability at high temperatures and is often used in cooking and baking.

Steviol Glycosides: Derived from the stevia plant and also stable at high temperatures.

Conclusion:
The primary reason aspartame is not used in hot drinks is its instability at high temperatures. When exposed to heat, aspartame decomposes into its constituent amino acids and methanol, resulting in a loss of sweetness and potential changes in flavor. This makes it unsuitable for use in beverages that are served hot.