Yes!

Does this world have magic?

No?

Are there persistent, measurable phenomena that cannot be explained within the framework of physics?

I don't think so but I'm purple, they don't teach me physics.

I cannot think of any mechanism by which red people touching food poisons it. 

They're filthy.

Food grows in the ground. It's covered in dirt. Worms and bugs live in the ground. Washing it fixes that. Why doesn't washing it fix having been handled by red people?

They touch corpses and sewage and garbage and each other. Food doesn't stay in the ground that long but they've been unclean forever, it's in their blood now.

Your concept of cleanliness does not seem to capture any actual material property of the world, so I kind of wouldn't expect it to be able to do things with material consequences, like get people sick.

They're sick anyway. They shouldn't let the reds handle food.

Given that letting reds handle food wouldn't cause sickness, I think maybe the cause of the sickness is something else.

It's definitely that, the fancy stuff they still aren't allowed to touch isn't making anybody sick.

Were other food handling procedures changed when red people were first allowed to handle the food? 

I don't know, I'm purple but retail purple not farm purple.

I think they must have been, and that that, not the handling by red people, is what is causing the illnesses, because handling by red people causing illnesses doesn't make any sense.

Yes it does, you're an alien so maybe you don't have an unclean caste on your planet but they're inherently disgusting and of course people will throw up if they try to eat things they've touched.

Has anyone checked what happens if people are told that perfectly good food was touched by a red person, or fed food that was touched by a red person while not being told that it was?

Um, no, nobody has tried sneaking unclean food to someone or purposefully trying to put them off their appetite while they're on rations.

If they did, what do you expect would happen? Would they get sick if they thought they were eating red-touched food, even if the food was fine?

They wouldn't eat it if they had any other choice.

The thing I'm interested in is whether people are making themselves sick by expecting to be sick.

I don't think that makes sense.

A lot of things about this don't make sense. If I assume that people who eat red-touched things would get sick whether they knew about it or not, then the red-touched things have to be somehow physically different than non-red-touched things, but I have a hard time believing no one checked whether new substances were being introduced into the foods, or wondered if a change in food handling procedures was causing trouble. If I assume that the food is not physically different, except for the fact a red person touched it, then either your people can notice things that are not physically measurable except in the effects they have on you or you're making yourselves sick with anticipation. 

They don't teach me physics, I told you.