Is it known to add dust cover to food and beverage?

As we all know, food and drink are added with dust-proof covers. At present, in the practice of patent examination, whether in the actual examination stage or the re-examination stage, when judging whether a certain technical means is common knowledge, there are usually great differences and disputes between the examiner and the applicant or the re-examination requester, and a tug-of-war often occurs because a certain feature is common knowledge.