This duo sentence sounds very strange. Sure this might be grammatical but I'd say to 99% you encounter "Im Essen sind keine Zitronen." and "Es sind keine Zitronen im Essen".
The preposition in requires dative case when it refers to a location (and accusative case when it refers to a direction). Here, we are talking about lemons already within the food, rather than going into the food, so we need dative case. The neuter noun das Essen becomes dem Essen in dative case, which when combined with in leads to the contraction im Essen rather than in dem Essen (which would only be used if you wanted to add emphasis).