Freeze-drying

Freeze-drying (lyophilisation) is a sophisticated dehydration method: the food is frozen, then its water is removed under vacuum, so the ice goes straight from solid to gas (sublimation) without passing through a liquid state. The final product is dry, light and porous, and regains its shape when rehydrated.

Dehydration that preserves everything: unlike conventional drying (with heat, which degrades), freeze-drying works in the cold, which remarkably preserves:

1. Heat-sensitive vitamins (C, some B vitamins), far better than hot drying or sterilisation
2. Taste, aroma and colour (close to fresh)
3. Cellular structure (hence the faithful rehydration)
4. Probiotics! Freeze-drying is precisely what is used to keep probiotic bacteria alive in powder form (they “survive” in the dehydrated state and wake up on rehydration)

It is the preservation method that best preserves nutritional value, hence its higher cost.

Fascinating applications:

1. Space food: astronauts eat freeze-dried food (light, long-lasting, rehydratable), the emblematic image
2. Hiking, survival and military: freeze-dried meals (light, energy-dense, long shelf life)
3. Quality instant coffee, fruit powders and gourmet dishes
4. Probiotics in capsules and powder
5. Medicines and vaccines (preserving fragile molecules)

Caveats:

1. High cost (a slow, energy-hungry process), so reserved for specific uses
2. Despite excellent preservation, it is still a processed food, and commercial freeze-dried products may contain added salt, sugar or additives (read the labels)
3. The texture changes (crunchy and porous), which is appreciated for snacks (crunchy freeze-dried fruit) but less for other uses

In practice: freeze-drying is a “noble” preservation process that keeps most of the nutrients. Plain freeze-dried fruit makes an interesting snack (watch out for sweetened versions). It is also why good probiotics are often freeze-dried.