would it be a chillul hashem for one to refer to junk food as garbage in the presence of other people?
Answer:
While in the Gemara and Shulchan Aruch we find various halachos of treating food with respect, this refers to not physically disgracing the food and not to speaking disparagingly about food. Nevertheless someone who is seen as a religious person may cause a chillul Hashem by speaking in a course, unrefined manner. So it would depend on the circumstances, and a person should always be careful what he is saying and in front of whom.