FOOD PROCESSING AND PRESERVATION: DRYING
Diagram
Spray into cold and freeze dry, or spray into hot air
Spray-freeze-drying
The liquid is sprayed into a cold space, the drops freeze, and then the ice is removed by freeze drying. The powder is light and porous.
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Spray drying
The liquid is sprayed into hot air, and the water evaporates fast. The powder is made quickly and cheaply, but the food meets heat.
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The key point
One joins spraying with freezing and freeze drying; the other dries by hot air alone. This pair is distinct from the built article on freeze drying versus spray drying, which compares the two parent methods.
Figure 1: Spray-freeze-drying sprays, freezes and then freeze dries; spray drying sprays into hot air. Drawn in our own style from S. Padma Ishwarya, Spray-Freeze-Drying of Foods and Bioproducts. This pair is distinct from the built article on freeze drying versus spray drying.
Introduction
Spray-freeze-drying and spray drying both dry liquids into powders, but they use different routes. Spray-freeze-drying sprays the liquid into a cold space, so the drops freeze, and then the frozen drops are freeze dried, so the ice leaves by sublimation. Spray drying sprays the liquid into hot air, and the water evaporates fast. The first gives a light, porous powder and protects heat sensitive foods, while the second is fast and cheap but exposes the food to heat.
Ishwarya states that spray-freeze-drying takes the merits of both spray drying and freeze drying, while overcoming the limitations of those two methods. This article follows that book.
Quick comparison
Table 1: Spray-freeze-drying compared with spray drying
| Aspect | Spray-freeze-drying | Spray drying |
|---|---|---|
| Steps | Spray, freeze, then freeze dry | Spray into hot air and dry in one step |
| Temperature | Cold freezing, then low temperature drying | Hot air, so the food meets heat |
| How water leaves | Ice leaves by sublimation in freeze drying | Water evaporates into the hot air |
| Powder structure | Light and porous, with a large surface | Denser, often spherical particles |
| Heat damage | Very little, because drying is cold | Some, because the food meets hot air |
| Heat sensitive foods | Suited to proteins, enzymes, probiotics and flavours | Less suited, because heat can damage them |
| Speed | Slower, because freeze drying takes time | Fast, a continuous and rapid process |
| Cost | More costly, because freezing and freeze drying need energy and time | Cheaper, a standard industrial drying method |
| Solubility | Dissolves fast, because the powder is porous | Dissolves well, but less porous than the spray freeze dried powder |
| Typical foods | Probiotics, enzymes, flavours, heat sensitive powders | Milk powder, coffee, soups, flavours, infant formula |
Scientific explanation
Definition
Spray-freeze-drying is the drying of a liquid by spraying it into a cold space so the drops freeze, and then freeze drying the frozen drops, so the ice is removed by sublimation. Spray drying is the drying of a liquid by spraying it into hot air, so the water evaporates quickly and a powder is left. Ishwarya describes spray-freeze-drying as taking the merits of both spray drying and freeze drying, while overcoming the limitations of those two methods.
Working principle
In spray-freeze-drying, the liquid is atomised into fine drops that freeze at once in the cold space, so each drop keeps its shape as a frozen particle. The frozen particles then go into a freeze dryer, where the pressure is lowered and the ice turns straight into vapour, leaving a light, porous particle with the food structure largely intact. In spray drying, the liquid is atomised into hot air, and the water evaporates from the surface of each drop in seconds, leaving a dry particle. The heat is brief, but it is still heat, so heat sensitive parts can be damaged.
Apparatus
The books used here name the atomiser and the freeze dryer for spray-freeze-drying, and the atomiser and the hot air drying chamber for spray drying. No laboratory apparatus for telling the two apart is named, because the distinction is in the process, not in a test.
Measurement procedure
The distinction is made from the process record: freezing temperature and freeze drying conditions for spray-freeze-drying, and inlet air temperature and feed rate for spray drying. The books used here give no worked calculation with numbers for this comparison, so none is given.
Factors affecting readings
For spray-freeze-drying, the drop size, freezing rate and freeze drying time change the porosity of the powder. For spray drying, the inlet air temperature, feed rate and drop size change the moisture and the particle size. In both cases, the composition of the liquid decides how well it dries and how the powder flows.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Spray-freeze-drying is used for probiotics, enzymes and flavours that heat would damage.
- Spray drying is used for milk powder, instant coffee, soups and infant formula.
- Spray-freeze-drying is used where a fast dissolving, porous powder is wanted.
- Spray drying is used where a large volume of powder must be made quickly and cheaply.
- Both are used to turn liquid foods into stable powders.
Advantages and limitations
- Spray-freeze-drying: advantage: protects heat sensitive foods and gives a light, porous, fast dissolving powder. Limitation: slower and more costly than spray drying.
- Spray drying: advantage: fast, continuous and cheap, and widely used in industry. Limitation: heat can damage sensitive foods and change flavour.
- Spray-freeze-drying: advantage: takes the merits of spray drying and freeze drying together, as Ishwarya states. Limitation: needs freezing and freeze drying equipment, so the cost is higher.
FAQ
Is spray-freeze-drying the same as freeze drying?
No. It sprays the liquid first, freezes the drops, and then freeze dries them. Freeze drying alone freezes a bulk food, not sprayed drops.
Is this the same as freeze drying versus spray drying?
No. That built article compares the two parent methods. This article compares spray-freeze-drying, which joins them, with spray drying alone.
Why is spray-freeze-dried powder porous?
Because the ice leaves by sublimation and leaves empty space behind, so the particle is light and full of pores.
Which method is cheaper?
Spray drying is cheaper, because it is a single fast hot air step, while spray-freeze-drying adds freezing and freeze drying.
References and further reading
- S. Padma Ishwarya, Spray-Freeze-Drying of Foods and Bioproducts: Theory, Applications and Advances, Springer (Google Books page read).
- M. Shafiur Rahman (ed.), Handbook of Food Preservation, 2nd ed., CRC Press, 2007 (Google Books page read)
