FOOD PROCESSING AND PRESERVATION: DRYING
Freeze drying and spray drying: two ways to a dry powder
Freeze drying
Food frozen, then ice turns straight to vapour under vacuum
versus
Spray drying
Liquid sprayed as fine droplets into hot air, dry in seconds
Figure 1: Freeze drying removes ice without melting, while spray drying removes water from droplets in hot air. The two boxes are alternatives, not steps in order.
Introduction
Freeze drying and spray drying both turn food into a dry, shelf-stable product, but they start from opposite ends. Freeze drying starts with a solid food, freezes it, then pulls the ice out as vapour under vacuum, so the food never gets hot. Spray drying starts with a liquid, sprays it as fine droplets into hot air, and the droplets dry in seconds into a powder. Freeze drying gives the best quality and costs the most; spray drying is fast and cheap, and suits liquids such as milk.
Quick comparison
Table 1: Freeze drying compared with spray drying
| Aspect | Freeze drying | Spray drying |
|---|---|---|
| Starting material | Solid food, frozen first | Liquid, such as milk or fruit juice |
| How water leaves | Sublimation: ice turns straight into vapour under vacuum, without melting | Evaporation: droplets dry in flight in hot air |
| Heat | Food never gets hot | Droplets are hot for a few seconds only |
| Shape and colour | Shape, colour, and flavour kept best of all drying methods | Powder form; flavour damage is small because exposure is short |
| Rehydration | Best of all: ice leaves open pores, so water rushes back in | Powder dissolves fast in water, but the product is a powder, not the original shape |
| Speed | Slow: freezing plus vacuum takes hours | Very fast: droplets dry in seconds |
| Cost | Most expensive drying method | Cheap per kilogram for liquids |
| Typical foods | Instant coffee, herbs, dried fruit for snacks, ready meals | Milk powder, instant coffee, egg powder, fruit juice powder |
Scientific explanation
Definition
Freeze drying is drying frozen food under vacuum so its ice sublimes away, keeping shape, colour, and flavour better than other methods. Spray drying is drying a liquid by spraying it as fine droplets into hot air, so the droplets dry in seconds into a powder.
Working principle
In freeze drying, the food is frozen, then put under vacuum so the ice turns straight into vapour, a change called sublimation, without melting. Because the food never gets hot and never shrinks much, it keeps its shape, colour, and flavour, and it rehydrates fast, because the ice left open pores where the water was, so water rushes back in. In spray drying, a liquid such as milk or fruit juice is sprayed as fine droplets into hot air, and the droplets dry in seconds into a powder. The droplets are hot for such a short time that the flavour damage is small, and the powder has very low aw, so it keeps for months in a sealed pack. Spray drying suits liquids; freeze drying suits solid pieces and high-value foods.
Apparatus
A freeze dryer is a vacuum chamber with cold shelves that freeze the food and a condenser that traps the vapour leaving it. A spray dryer is a tall chamber of hot air with an atomiser at the top that sprays the liquid as fine droplets; the dry powder falls to the bottom and is collected, while the air carries the vapour away. Before spray drying, milk is usually concentrated in an evaporator first, because evaporation removes most of the water cheaply in a closed vessel, using far less energy per kilogram than a spray dryer, and the dryer then only has to remove the last part of the water.
Measurement procedure
Both methods are judged by the same numbers: the water activity of the finished product, and its moisture content. A fully dried food aims below aw 0.60, where no microbe grows. The powder must be cooled before packing, or it will cake, and the pack must block moisture, because a dry powder such as milk powder is hygroscopic and goes lumpy in a humid kitchen if the pack is left open.
Factors affecting readings
For spray drying, the feed must be concentrated first, the droplets must be fine and even, and the powder must be cooled before packing. For freeze drying, the freezing step decides the pore size: fast freezing makes small crystals and fine pores, while the vacuum level decides how fast the ice sublimes. Freeze drying is slow and costly, because freezing and vacuum both cost energy, so it is used for high-value foods such as instant coffee, herbs, and dried fruit for snacks. Spray drying is the common factory method for milk powder because it is fast and cheap at scale.
Formulas and calculations
Neither freeze drying nor spray drying has a standard formula given in the books for this comparison, so this section states that plainly instead of inventing one. The numbers that do govern both methods are the water activity limits from the drying unit: most bacteria stop below aw 0.91, most yeasts below about 0.88, most moulds below about 0.80, and nothing grows below about aw 0.60. The freezing time of the food going into a freeze dryer can be checked with Plank’s equation, given in the article on freezing and chilling.
Industrial applications
- Freeze drying: instant coffee, herbs, dried fruit for snacks, ready meals, and high-value ingredients that must keep their shape and rehydrate fast.
- Spray drying: milk powder, instant coffee, egg powder, fruit juice powder, and infant formula, where a liquid must become a free-flowing powder at scale.
Advantages and limitations
Freeze drying, advantages: best shape, colour, flavour, and nutrient keeping of any drying method; rehydrates fastest; no heat damage. Limitations: slowest and most expensive drying method; needs freezing and vacuum equipment; only suits high-value foods.
Spray drying, advantages: turns liquids into powder in seconds; cheap at scale; short hot exposure keeps flavour and solubility good; powder keeps for months sealed. Limitations: only suits liquids; feed must be concentrated first; powder must be cooled before packing or it cakes; the original shape of the food is lost.
FAQ
Which drying method keeps flavour and shape best? Freeze drying, because the food never gets hot and ice sublimes away without shrinking. It is slow and costly, so it is used only for high-value foods.
Why is milk evaporated before spray drying? Evaporation removes most of the water cheaply in a closed vessel, using far less energy per kilogram than a spray dryer, so the dryer only has to remove the last part of the water.
What is sublimation? The change of ice straight into water vapour without melting. In freeze drying, ice sublimes away from frozen food under vacuum, leaving the shape intact.
Why does freeze dried food rehydrate so fast? Because the ice left open pores where the water was, so new water rushes back in, while hot-air drying can collapse cell walls and seal the surface.
Can any liquid be spray dried? In practice, liquids that can be pumped and atomised, such as milk, juice, and egg, are spray dried. The feed must be concentrated first and the droplets must be fine and even.
References and further reading
FTN subjects: Principles of Food Preservation, Unit 4 (Drying and Water Activity); Principles of Food Processing, Unit 3 (Mass Transfer, Drying, Evaporation). Books used: Fellows, P. J., Food Processing Technology: Principles and Practice; Rahman, M. S., Handbook of Food Preservation; Chakraverty, A., Mujumdar, A. S., Raghavan, G. S. V., and Ramaswamy, H. S., Handbook of Postharvest Technology: Cereals, Fruits, Vegetables, Tea and Spices. Google Books page read: Spray-freeze-drying of Foods and Bioproducts: Theory, Applications, and … by S. Padma Ishwarya (books.google.com).
