Do Onions Absorb Bacteria That Cause Illness?

02/18/2013

Have you heard the theory that placing an onion next to your bed will keep you from getting the flu? Are you curious if onions absorb bacteria? Will an onion turn black after attracting all of the bacteria? Is it possible that onions have powerful antibacterial, antiseptic properties?

It is a widely circulated report such as this Facebook post about onions curing the flu, and we wondered many of the same questions. We reached out to Ruth MacDonald, PhD, RD, Chair and Professor of the Department of Food Science & Human Nutrition at Iowa State University.

Do onions absorb bacteria and cure illness?

Dr. MacDonald: “No, onions do not absorb bacteria. The idea that a vegetable would attract and suck into itself bacteria from the air is not even logical. The onion may turn black because it would eventually rot from both cell breakdown events and bacterial contamination if you left it out, not because it absorbs germs. Onions and garlic are slightly acidic, which could have antibacterial effects if you rubbed the juice on things, but these are much less effective than bleach or chemical antibiotics. Eating these vegetables provides antioxidants that can have health benefits, but they are unlikely to prevent or cure disease.”

Onions do contain sulfur compounds and were used in some cultures for medical purposes, but the effectiveness of these treatments is not significant.

The post also mentioned that you don’t need to refrigerate mayonnaise. Is this true?

Dr. MacDonald: “Mayonnaise that is made from eggs needs to be refrigerated. Products made from oils only may not require refrigeration – however, once opened, they will grow bacteria just as any food might, due to exposure from utensils or hands during use. It is best to refrigerate any mayonnaise after it has been opened.

“The possible sources of foodborne illness are many, especially in salads, and it is usually not possible to trace it back to the original source; however, modern technology is making that more possible. All foods can carry pathogens, and any food left at room temperature or above for more than 30 minutes can become contaminated to the point of causing illness. Pathogenic bacteria in those foods could grow at that temperature. The increased amount of the pathogen (or a toxin produced by the bacteria) in the food causes the illness when the food is consumed.”

Is it bad to feed onions to dogs, as mentioned in the post?

Dr. MacDonald: “I don’t know if dogs can eat onions – but I do know that stomachs do not ‘metabolize’ onions. The stomach of all mammals provides acid and enzymes that break down food prior to entering the small intestine.”

Previously, we asked Dr. MacDonald if it is OK to use leftover onions. Click here to learn more about the food safety rules concerning leftover onions.

What do you think about onions?