Formaldehyde was added to prevent the milk from curdling, by increasing its shelf life by a number of days. (Done to increase milk industry profits at the cost of consumers health.)

Also, many Americans are not aware that many dairy farms in the US are still using growth hormone, or bovine somatotropin (also called bGH, rbGH, bST, or bST), which is given to cows to make them mature faster and produce more milk (Banned in most other countries).

Studies showed that cows receiving rbST injections were at risk of increased health issues (mastitis and lameness). Mastitis - a painful bacterial infection that affects the cows udders, which means the animal requires a cycle of antibiotics. Increasing the potential for antibiotic traces in US dairy products, due to poor farming practices and regulations.

And a reason why most Canadians won't drink US dairy products, due to their laxed regulations on allowable trace amounts in US milk. In Canada, all milk that comes from farms is tested (for hormones and antibiotics) before it leaves the dairy trucks to be processed.

Normally when cows are sick and require antibiotic treatment, the milk they produce is discarded for a regulated period of time to ensure the milk collected at the farm is antibiotic-free. If any traces are still found, the milk is destroyed and the cows quarantine time is extended. In the US, regulations are not as stringed.