The clearest sign to look for when choosing string cheese is an ivory-white colour and a surface that looks slightly moist but not sticky. A string cheese that looks yellowed or dry has either been sitting around too long or was stored the wrong way.
The second thing to check is whether the cheese is pliable enough to pull apart into thin strands; that flexibility is a sign it was kneaded and stretched at the right temperature during production.
What do colour and surface tell you?
A fresh string cheese has the same even white colour all the way through when cut; yellowish rings inside mean it's been sitting for a while. A thin layer of salt crystals on the surface is normal, but a thick, brittle crust of salt is a sign the cheese is starting to dry out.
The outer layer should give slightly when pressed with a finger and spring straight back; a hard, brittle surface means the cheese has aged past its best.
Brined or vacuum-packed — which is better?
String cheese sold in brine usually stays fresh longer, since the liquid keeps it from drying out; vacuum-packed cheese has a shorter shelf life because it sits without any liquid, but it tends to taste less salty.
Alongside the production date on the pack, check whether the brine itself looks clear; cloudy or foamy brine is an early sign the cheese is starting to spoil.
How do you pull it into strands?
Slice the cheese crosswise into strips about 1 centimetre thick, then hold each strip with both hands and gently pull along the cheese's natural grain to separate it into thin strands. Pulling against the grain tears the cheese into ragged pieces instead of clean strands.
Pulled straight from the fridge while still cold, the cheese snaps instead of stretching; letting it sit at room temperature for about 10 minutes first makes it easier to pull into even strands.
How do you test freshness?
A slice should bend without snapping when flexed gently; string cheese that cracks and crumbles has lost its freshness. It should smell only faintly sour and milky — a sharp ammonia smell is a sign it has spoiled.
For cheese sold in brine, the colour of the liquid itself is a clue too: clear, whitish brine means it's fresh, while yellowish, cloudy brine means it's been sitting for a while.
Common mistakes
The most common mistake is trying to pull the cheese into strands straight out of the fridge; cold cheese is brittle and won't pull evenly. The second is leaving opened cheese without its brine uncovered in the fridge, which dries and toughens it quickly.
Leftover cheese should be stored back in its original brine, or in a lightly salted water you make yourself; left in a dry container, it hardens and loses flavour within days. A third common mistake is trying to use all the pulled strands at once; strands you don't use right away keep their texture for a few more days if stored separately in brine. A fourth is trying to freeze it; once string cheese has been frozen and thawed, its fibrous structure breaks down and it never pulls into clean strands again, so it's better to buy only the amount you'll actually use within a few days rather than freezing the extra for later.