All of the other responses are good and I agree. I just wanted to add some. My family has a corn allergy which is in just about everything but not listed as "corn". We didn't know this for 3 years with my 2nd son. My two oldest sons react differently and then I react differently. My oldest gets sores around his mouth and headaches and sometimes stomach aches, he also won't complain of any discomforts, but has behavioral issues. My second one gets stomach aches and diahrea. I get migraines, sores in and around my mouth, diahrea, fatigue, brain fog, stomach aches. All three of us will violently through up and have diahrea if we overdose on it. Corn has MANY different derivatives and reacts differently in your body depending on the derivative. Although we react very quickly once ingesting, it can last days. I will also have anaphalactic reactions to alcohol. One more thing, you can have a reaction if not actually allergic to a specific food, but if are allergic to latex. I am allergic to latex and get the itchy mouth, inside of ears and throat when I eat avacados, melons (almost any kind), bananas, strawberries, raw potatoes and several more.
If you are concerned about food allergies, I would have a blood test done instead of the scratch test. Blood test for food allergies is much more accurate and less invasive. The reaction to food can be very different on the skin rather than being digested where there is a chemical reaction happening.
Good luck. I hope this helps.