Which noun should be used here?
Some of the grain ____________to be contaminated.

This type of sentence can be confusing most of the times but it is important to remember the subject-verb agreement. Since the subject here is the word grain is in singular form, we must follow the rule of adding s to the verb.

So to complete the sentence correctly, it should be added with appears. If the sentence used the word grains for example, then the word appear must be used. The word some is not considered as a collective noun and even it usually connotes more than one, it is still not the subject in the sentence but the singular grain.