1) Metal grinder. Regular mill has two metal burrs. One is fixed and the other is rotating at very high speed. Friction created between two burrs causes wear. Inspect it while cleaning. Think where the worn pieces goes!
2) Cooking acidic foods in steel and aluminum cookwares. Muriatic acid reacts to steel and other metal objects. The same is true for acidic foods like fruits and vinegar flavored dishes. Remember how wine is contaminated with lead!
3) Defective lacquer of canned foods. If acidic foods are not compatible with metal containers, then how about canned meat, sardines, mixed fruits, juices and sodas. The inner side of can is coated with appropriate lacquer/enamel to separate food components and metal. Function is null and void of the lacquer is defective.
Common enamels are oleoresins, vinyl, acrylic, phenolic and epoxy-phenolic.
4) Repeated contacts. When it comes to repeated contact, nothing beats the steel mill. However, there are activity that may cause wear of metal parts. The best example is aluminum ladle and frying pan. Food mixing involves contact of both metal tools resulting to slow wear over long period.
5) Rusty water pipe. Water has a big role of carrying contaminants to food if not given immediate attention. Galvanized water pipes are prone to rust which is eventually carried by water to food.
6) Fragments and loosened parts. This happened to me several times. A metal nut loosened and fell in food. I spent hours of searching in over ten kg milled food. The other one. While cleaning, I noticed a broken screw. Sadly, I cannot determine where the fragment fell – to food or somewhere else.
Contaminations cause by number 1 to 5 are often too small to cause human harm. Number 6 is a bit dangerous. I might cause choking and internal organ wound.