As with other speech forums and topics, persons are generally allowed to share or publish their opinions and truthful statements of fact about restaurants without legal liability—whether the publisher is a professional food critic or a customer at a neighborhood restaurant.
A person can publish critical statements such as "I wouldn't serve their food to my dog" or "it was the worst meal I have ever eaten" without fear of legal liability. But if a person publishes a statement of fact—a statement that can be proven to be true or false, such as whether a certain ingredient was included in a food item or whether there was a bug or worm in the food—the statement must be truthful or it will expose the person who published it to others (by speaking or writing it) to legal liability for defamation and damage to the restaurant's reputation.
Defamation is a false statement of fact that (1) is published; (2) damages or injures the reputation of another party; and (3) is made without legal excuse (defense or privilege).
Defamation may be in the form of a written statement (libel) or a spoken statement (slander) and is a tort or legal wrong that may create civil liability (for money damages) for a person who makes a defamatory statement that is not privileged (e.g., made during a judicial proceeding) and for which there is no defense (such as truth or retraction).
To be actionable and subject the person making the statement to liability the statement must be published to a third party. In other words, a written or spoken statement made from one person to another that is not read or heard by a third person (or could not have been reasonably expected by the person making the statement to be read or heard by a third person) is not published and is not defamation.
But a statement spoken to another person, or sent in an email or text message to another person, or published online meets the requirement of publication for a potential defamation claim.