I enjoyed this exercise and have similar questions

The difference between à and de when it comes to food is something like "made with" and "made from". "Made with" an ingredient is an ingredient that lends something its flavor. "Made from" an ingredient means that this ingredient is the main ingredient, i.e., the stuff the dish is really made from.

un sandwich au jambon -- a sandwich made with ham, i.e., there are other ingredients that are important but ham gives it its taste.
un café au lait -- a coffee "made with" milk, i.e., there's still the coffee that's the main thing, not the milk.
une glace au café -- ice cream "made with" coffee, i.e., there's the taste that comes from the coffee but the ice cream isn't made from coffee primarily.

Un jus de pommes -- a juice "made from" apples, i,e., apple juice. Clearly, the juice itself is from apples, not just "made with" apples.
Une barre de céréales -- a bar "made from" granola, i.e., a granola bar.

Un verre de vin -- a glass of wine. This is a different topic, because here, un verre is a quantity. After a quantity you don't use the partitive article.

Hey Chris, 

Thanks for your clear explanation. A follow-up question about a remaining detail: why is it un jus de pommes and une barre de céréales, not un jus des pommes and une barre des céléales? After all, the plurality can also be found in une tarte aux framboises. Consequently, does that also mean that du and de la are never used in the center of a food description? 

Many thanks.