It seems that the comments have already conveyed all the information you need, but in the interest of having the answer present in the "answers" section:
One of the meanings of "cake" is as a verb referring to a fluid which hardens to become encrusted, particularly as a result of drying or if it covers another object. Examples of the phenomenon are wet sand drying or cement curing. "Caked" here is simply the past tense of the verb. Some other examples of the word are "His boots were caked with mud" (covered in the dried mud) or "due to imperfect seals, moisture was able to enter the cans and cause the powder therein to cake."