It's probably a lot of work to build these relationships up front.

It's probably easier to allow the user to make their own relationships. In other words, when I search for "lemon", give me the ability to easily roll up "lemon juice" and "lemon peel" and "lemon zest". Make this a one time activity and have it save to a profile (also, easy incentive for users to create an account).

You can monitor all of your users' custom relationships and curate the good ones. Automate this for common relationships, i.e., if 50 people build the same lemon relationship, push it out to all users. You could make it opt out as an advanced setting for the purists I suppose.