Learn Russian: Я прочитал всю этикетку, чтобы понять пищевую ценность. - I read the entire label to understand its nutritional value.

Both mean "whole" or "entire," but весь/вся/всю acts as a pronoun meaning "all of it" (the entirety of this specific label). The word целый emphasizes that something is physically unbroken or massive. For reading a text or label from start to finish, всю is the natural choice.

If пищевую ценность is the direct object, why doesn't the word ценность change its ending?

Ценность is a feminine noun ending in a soft sign (-ь), which makes it a 3rd declension noun. For these nouns, the Accusative case form is exactly the same as the Nominative case form. However, the adjective пищевую takes the standard feminine Accusative ending (-ую), which proves the phrase is acting as the direct object.

Why is there a comma before чтобы?

In Russian grammar, dependent (subordinate) clauses must always be separated from the main sentence by commas. Because чтобы понять пищевую ценность is a dependent clause explaining the purpose of reading the label, a comma before чтобы is strictly required.

Why are the verbs прочитал and понять both in the perfective aspect?