Decontaminate or contaminate

Solution 1:

The use of decontaminate instead of contaminate is almost certainly a mistake.

Purely in terms of grammar, the use of decontaminate could make sense if the instructions were meant to accomplish something more than just decontamination.

In other words:

Make sure all the tops are rinsed beforehand and ensure they do not have rubber or metal attached to the tops as these will only decontaminate the mix. [The mix should be both decontaminated and visible from above.]

In this possible context, the use of rubber or metal attached to the milk bottle tops will prevent anybody from seeing the contents. So, this version of the sentence is saying be saying that while rinsing ensures decontamination, the use of rubber or metal tops doesn't allow for the further requirement of transparency.

However, while this is possible, it's much more likely that what's being expressed is that having rubber or metal attached to the milk bottle tops will cause the milk to become contaminated when it comes into contact with that material. So, in short, the opposite word should have been used.