Allowing for individuality usually has its own rewards. If the students work on a treat for the class they should be rewarded for this. Cost of food doesn't count. And to preserve individual feelings, every student should at least taste each food brought in. And these do need to be only tastes, we are not supplying lunch.
Evaluation is flexible.
They apply enough energy to this project to deserve it. Most students receive grades near 100. The most often missed area is the nutrition definition. Please be clear to the students about what is expected here. I do not allow them to make up their own definition.