Masculine nouns (ο χυμός) change to 'τον χυμό' when they become the object of a sentence, that is the accusative case. Notice the 'ς' has been removed.
You must keep the 'ν' for masculine accusative if the letter after 'τον' begins with any of the following :κ, π, τ, ξ, ψ, γκ, μπ, ντ or any vowels. In this case, 'το χυμό' is correct.
Το χυμό is wrong speaking strictly grammatically. The Masculine article τον, έναν, always keeps the -ν in order not to be confused with the neutrum το, ενα. The rule above (of the first letter of the following word) applies only to feminine article την., as well as to the negatives δεν and μην. Verbally you will hear το χυμό more often than τον χυμό, but in written Greek the -ν must be kept.