sulfite. a salt of sulfurous acid. Sulfites are used as preservatives for salad, fresh fruit and vegetables, wine, beer, and dried fruit. In susceptible individuals, especially those with asthma, they can cause a severe reaction; because of this their use has been curtailed, and foods that contain them must be labeled.ulfite. a salt of sulfurous acid. Sulfites are used as preservatives for salad, fresh fruit and vegetables, wine, beer, and dried fruit. In susceptible individuals, especially those with asthma, they can cause a severe reaction; because of this their use has been curtailed, and foods that contain them must be labeled.