eli5: There are labels on certain products saying “x product was produced in the same factory as y product” where y product contains an allergen. Why does it matter that both products were made in the same factory?
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It’s very difficult to guarantee that cross-contamination doesn’t occur. Sometimes just having a little dust of an allergen present is a problem for people who are allergic to it.
Some people’s food allergies are extremely sensitive. Dust or microscopic particles can move through the air from one machine to another. If you can walk into the factory and smell what they are making, then particles are in the air.
If a manufacturer labels their food as “peanut-free” but the next line over is making peanut butter, there are probably peanut parts/dust getting into the “peanut-free” product.
Even with the best of cleaning and air filtration there’s a chance an allergen can make it form one production line to the next. Also not all production lines need to be cleaned to within an inch of their life between every run so it’s possible some trace amounts of allergen will remain in the production line even if that specific product doesn’t have it as an ingredient.
It’s one of those better to be safe than sorry kinds of things.
It is extremely hard to get rid of all possible material for one food item you produce so nothing ends up in the next produce. Most of the time that is irrelevant but in the case of allergens it can be a huge problem. It can remain on machines, utensils, surfaces, containers or even just spread as dust in the air. A very small amount of allergen can be very problematic for someone that eats it if they are allergic to it. So what they are saying is they can’t guarantee there is no cross-contamination.
The are keeping people with severe allergies safer because they do not guarantee there is not residue from the allergen. For example made peanut butter then sesame butter.