Which response did food naturally produce in Ivan Pavlov’s dogs?

In Ivan Pavlov’s experiments, food naturally produced salivation in his dogs.1

Pavlov identified food as an unconditional stimulus because it invariably elicits this response, which he termed an unconditional response. While Pavlov initially viewed these "psychic secretions" as an interference with his research on the digestive system, he eventually recognized them as a fundamental tool for studying associative learning and conditioning.1