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I too get sick when I eat at Salad bars. A couple of culprits come to
mind.
Culprit #1: Vinegar. Some of the items like peppers, pickled beets and
salad dressings have White distilled vinegar.
Culprit #2: Cross contamination: Maybe that ladle or tongs brushed up
against croutons, or vinegar, salad dressing, or any gluten in the area.
Culprit #3: Mayonnaise. Mayonnaise pasta salad, or potato salads likely
contain mono & diglycerides & vinegar in the mayo.
Culprit #4: Spices can have wheat or gluten as a free flowing agent.

I was really disappointed when I went to the soccer team party @ the
pizza place & just ate salad from the salad bar & became sick with a
reaction.