Dry in a blend,
back to shape in a soup.

A freeze-dried ingredient is used in one of two ways. In a dry application, such as a seasoning, a rub, a topping or a drink powder, it stays dry until the consumer uses it, and the job is to keep it dry, crisp and free-flowing. In a wet application, such as a soup, a sauce, a dip or a ready meal, it takes up water and the job is to bring the piece back to shape, color and aroma at the right moment.

Both rely on the same structure. Freeze drying removes the water as ice, so the piece keeps an open, porous frame. That frame takes up water fast when you want it to, and takes up moisture from the air when you do not. Most of what follows is managing that one property. For how freeze drying compares with hot-air drying on this point, see freeze dried vs air dried.

In a dry blend, a freeze-dried herb needs no rehydration. Its low water activity keeps it crisp and keeps microbes from growing. The risk is moisture moving into it from the components around it. Water moves from the component with the higher water activity to the one with the lower, not from the higher moisture to the lower, so compare water activities, not moisture percentages.

In practice: blend freeze-dried pieces with other dry ingredients of similar water activity, add them at the end of mixing to limit breakage, and pack the blend in a moisture barrier. In a moist or fatty matrix, such as a bar, a chocolate or a yogurt cup, plan a barrier coating or match the water activities, or the pieces will soften over shelf life.

Work in a dry room where you can, and close pouches between doses. A freeze-dried ingredient left open in humid air takes up moisture fast and starts to clump.

Freeze-dried pieces are porous, so water gets in quickly. In published trials on carrots, water entered freeze-dried pieces about 100 times faster than air-dried ones, and the more open the structure, the more water the piece took back. In hot water, rings and flakes come back to shape in minutes. Cold water works too, more slowly; freeze-dried fruit pieces have been shown to reconstitute quickly in cold milk.

How much water a piece takes back, and how fast, depends on the crop, the cut, the piece size and the water temperature, so there is no single rehydration ratio. Measure it for your cut in your product.

Add the ingredient late where aroma and color matter: after the main heat in a sauce or a ready meal, or in the dry sachet of an instant product. Long cooking takes aroma and color out of any herb, however it was dried.

Once rehydrated, treat the ingredient as a perishable one. Drying does not kill microbes, and a rehydrated piece above a water activity of about 0.85 can support the growth of any that survived. Use it, or chill it, as you would a fresh ingredient.