A word for milk that has not "gone off"

When milk has not gone bad (become spoiled), the word I invariably hear is "good," as in "the milk's still good," or simply "the milk's good." (People also say, "It hasn't gone bad," but that doesn't utilize the word you're looking for.)

The same statement, the milk is good, can also be used as a statement of the milk's quality, and people know which one you mean simply by context and circumstances. (Another way of distinguishing this other use of good is that when we say the milk is of high quality, we have a tendency to phrase it differently, thus: that (instead of the) milk is good or that's good milk.)

Edit because I forgot to answer your second question: I don't consider "on" or "viable" to be viable options in the least. :)

Potable is the formal general term. Drinkable is the standard everyday synonym.

Milk that is not off in any way is fresh. If the taste or smell is off, then it's no longer fresh.

However, thanks to pasteurization and refrigeration, we are able to enjoy milk that is virtually fresh even though has been in storage for many days.

Though milk that has been refrigerated for some days may still be hard to distinguish by taste from its condition when it was just bought, the word fresh does not apply.

A pattern for expressing the condition of perishable goods is [number-]{day|week|month}- old.

• Bread that was baked yesterday is day-old bread.
• Milk in the fridge that still tastes good might be three-day-old milk or week-old milk, and so forth.

"This week-old milk is still perfectly drinkable; there isn't even a hint of any off taste to indicate the onset of spoilage."

Whether or not such a designation refers to something that is spoiled depends on the quantity of time and our understanding of how sensitive that commodity is to spoilage. If someone speaks of three-week-old milk, the understanding is that the milk is bad, but there is no such assumption about three-day-old milk.

Unspoilt. A bit poetic maybe, but the least ambiguous.