Final answer:
Aspartame is the artificial sweetener composed of two amino acids that is not heat stable and breaks down upon heating, hence it is not used for baking.
Step-by-step explanation:
The artificial sweetener composed of two amino acids that are not heat stable is aspartame. Aspartame is approximately 180 times sweeter than sucrose and is a methyl ester of a dipeptide, which consists of aspartic acid and phenylalanine.
However, aspartame is not used in baked goods or exposed to heat for prolonged periods because it breaks down, which leads to a loss of sweetness and the production of methanol, and further metabolization can lead to formaldehyde. People with phenylketonuria (PKU) must also avoid aspartame, as they lack the enzyme necessary to metabolize phenylalanine.