Avoiding milk because of a milk allergy can be frustrating---there's an omnipresence of milk and milk derivatives in today's prepackaged foods. Even foods that do not have milk ingredients may still be "cross-contaminated" with milk during processing. However difficult, it's important to be careful to avoid these ingredients since ingesting even trace amounts of milk protein can cause an allergic reaction in a milk-allergic person.
Milk, Butter, Butterfat, Cheese, Yogurt, Sour Cream
These are all milk products. The fat and lactose content is irrelevant if you have a milk allergy, since the allergy is actually to the proteins found in milk. Thus all milk products are off limits if you have a milk allergy.
Casein/Whey
These are the main proteins in milk and are common ingredients in many foods, including most margarine, some deli meats and even tuna. Ask the deli to show you the ingredients before you buy cold cuts, and make sure they are not sliced on the same machine that is used to slice the cheeses.
Lactic Acid Starter Culture
Although not definite, this ingredient could be derived from milk. If the package does not indicate that it is not milk derived, assume it is and avoid it. This ingredient is commonly found in processed and cured meats such as hot dogs, bacon and deli meats.
Skim Milk Powder/Milk Solids
Even dry milk ingredients are milk.
Kosher "D" or Dairy
Even if you find that an item does not contain any milk ingredients, look on the front of the package for a kosher certification, which will be a "U" with a circle or various graphics with a "K." If it is kosher and also has the letter "D" or says "Dairy" it is best to err on the side of caution and avoid these products. This symbol indicates that an item may have come in contact with some dairy product. If the item is kosher "Parve" or "Pareve," this would help ensure that the item is not cross-contaminated with milk.
"May Contain" Statements
These statements are OPTIONAL and worded in different ways. "Product manufactured on equipment which also processes milk" or "Made in a factory which also processes milk" are commonly seen statements. Due to the unpredictability of the presence of milk in these products, you should avoid them---even if the statement is very strong, such as, "Best practices used to avoid contamination in a factory which also handles milk ingredients," Because they are optional on a food label, if a product does not offer a "may contain" statement you cannot be sure what allergens it may contain.
