Originally Posted by froggythefrog
Oils seem to preserve foods well.
This is true to a point, in foods that would otherwise support the growth of bacteria (high protein, neutral to slightly acidic, moist) oil limits oxygen. That will prevent the growth of aerobic bacteria, but allow anaerobic bacteria to grow in places they normally wouldn't. Anaerobic bacteria are less common than aerobic bacteria but they tend to make you sicker, so while you're less likely to get sick, it's more likely to be serious if you do.
Tahini, etc. doesn't go bad because if you removed the oil, which won't support bacterial growth, it would be very dry.