Symbolism, Imagery, Allegory
There's a lot of growing, making, and sharing food going on in the book. Jack's mom and Miss Volker both provide meals that nourish people and also help maintain the community's bonds. Food and nourishment, then, are strongly associated with the female realm in the book. (Boys get cars; chicks get food—right down the Girl Scouts and their tasty, seasonal morsels. Gee, this is verging into the realm of the stereotype, here.)
Also, remember how Jack equates heaven with "a great loaf of warm bread" in one of the few extended descriptive paragraphs in the book (14.90)? There's also a spiritual aspect to food. Any time bread is mentioned in a church setting, you can be sure that we're supposed to think of the Christian tradition of Communion, even though Dead End in Norvelt isn't exactly heavy on religion.
On the other hand, there's a dark side to these positive associations with food. Mr. Spizz poisons the casseroles that are given to the sick and elderly, which does the exact opposite of what the casseroles are supposed to do—it kills them. It also disrupts the bonds that hold the community together, since now neighbors all suspect each other of the terrible deed, and community trust has been ruined.
So, while food is a symbol of all that's good and neighborly about communities, it also seems to represent something about how fragile those bonds can be—and how one bad-tempered guy can ruin the whole town.