Residual Moisture, and Why the Last Percent Is Expensive

1–4% for most products, and lower is not automatically better

The target is a range for your product, not a universal number.

Most freeze-dried food lands between 1% and 4% residual moisture, which puts water activity well below the roughly 0.6 threshold where microbial growth stops. Cultures and pharmaceuticals often target tighter, but over-drying biological material damages what you spent the cycle protecting.

Lower is not free either: going from 2% to 1% can add hours of secondary drying for a very small amount of water.

Bound water is not frozen water

Different physics, different cost.

During primary drying, ice sublimes. In secondary, what remains is water adsorbed onto the product’s own surfaces, held by surface forces. Releasing it means overcoming those forces — more heat, more time, far less water per hour. That is the whole reason the tail of a cycle looks so unproductive.

Karl Fischer, loss on drying, and water activity

Three methods answering slightly different questions.

Karl Fischer titration is the reference method — specific to water, accurate at low levels. Loss on drying is simpler and cheaper but measures everything volatile, not only water. Water activity measures availability rather than quantity, and is arguably the more meaningful number for shelf stability.

Whichever you use, use it consistently, and establish the target on your product rather than borrowing someone else’s.