Hola Bernadette

In this case, in the phrase "con olor a bergamota" we need "a" because we always use it when we talk about what sort of smell something has, the same way we also use it when talking about taste "con sabor a...", for example:

con olor a bergamota

con olor a fresas

con olor a chocolate

con sabor a limón

con sabor a piña

etc

Saludos cordiales