Resolving nested data in express GraphQL
Only resolvers for queries and mutations can be defined through root, and even then this is bad practice. I'm guessing you're building your schema using buildSchema, which is generally a bad idea since the generated schema will only use default resolvers.
The only way to define resolvers for a field like ingredients when using plain GraphQL.js is to not use buildSchema. Instead of generating your schema from a string, you would define it programatically (i.e. defining a GraphQLSchema and all the types it uses).
Doing the above is a huge pain, especially if you already have your schema defined in a string or document. So the alternative option is to use graphql-tools' makeExecutableSchema, which lets you inject those resolvers into your type definitions like you're trying to do. makeExecutableSchema returns a GraphQLSchema object, so you can use it with your existing code (you don't have to change your middleware to apollo-server if you don't want to).