Final answer:
Tooth enamel demineralization begins when the enamel surface's pH drops below 5.5, leading to dental caries or cavities.
Step-by-step explanation:
Demineralization of tooth enamel begins when the pH of the enamel surface drops below 5.5. This occurs when bacteria in the mouth metabolize sugars to produce acid, leading to an acidic environment that can dissolve the calcium phosphate mineral content of the enamel. Below this critical pH, the enamel is vulnerable to tooth decay, which manifests as dental caries or cavities.