There was a Purina manufacturing site in the area where I grew up.
We knew people that would take them up on the panel test offers.
A free big bag of food & the pets were treated like royalty.
As I recall it, they'd give small amount of the food to a pet, then another, then another.
Then, they'd put all 3 letting the pet smell it, before they lifted the lids to the bowls.
They would do it with dozens of pets & use the tendencies to decide on the "tastiest" formula.
BTW: that same site made the food for laboratory animals. They called it "Rodent Chow" but, the workers referred to is as Rat Chow! 
Boy, do I have stories to tell on taste panels for toothpaste & vitamins! Let's just say I find it hard to understand why anyone volunteers to do that
Edit history
= new reply since forum marked as read