Receptors in your stomach that determine when food enters because the ph changes would be classified as

Chemoreceptors

Explanation: Chemoreceptors is a sense organ, or one of its cells (such as those for the sense of taste or smell), that can respond to a chemical stimulus; a chemosensor. Chemosensors converts chemical signals and generates biological signals. In this case, a taste receptor is used which is the tongue.

The two main classes of receptors are direct chemoreceptor(taste) and distance chemoreceptor(olfactory).

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