Dehydration

Dehydration refers to the nearly complete removal of water from foods to a level of less than 5%. Although there are many types of driers, spray driers are the most widely used type of air convection drier. It turns out more tonnage of dehydrated products than all other types of driers combined. It is limited to food that can be atomized, i.e. liquids, low viscosity pastes, and purees. Drying takes place within a matter of seconds at temperatures approximately 200° C. Evaporative cooling maintains low product temperatures, however, prompt removal of the product is still necessary.

Spray Drying - Process Summary

The liquid food is generally preconcentrated by evaporation to economically reduce the water content. The concentrate is then introduced as a fine spray or mist into a tower or chamber with heated air. As the small droplets make intimate contact with the heated air, they flash off their moisture, become small particles, and drop to the bottom of the tower and are removed. The advantages of spray drying include a low heat and short time combination which leads to a better quality product.

Diagram of a 2 stage dryer

Diagram of a spray dryer

Principal components include: 

  • a high pressure pump for introducing liquid into the tower
  • a device for atomizing the feed stream
  • a heated air source with blower
  • a secondary collection vessel for removing the dried food from the airstream
  • means for exhausting the moist air
  • usually includes a preconcentration step i.e. MVR evaporation

Atomizing devices are the distinguishing characteristic of spray drying. They provide a large surface area for exposure to drying forces:

1 litre = 12 billion particles = >300 ft2 (30m2)

The exit air temperature is an important parameter to monitor because it responds readily to changes in the process and reflects the quality of the product. Generally, we want it high enough to yield desired moisture without heat damage. There are two controls that may be used to adjust the exit air temperature:

  • altering feed flow rate
  • altering inlet temperature

If heat damage occurs before the product is dried, the particle size must be reduced; smaller particle dries faster, therefore, less heat damage. This can be accomplished in three ways: 

  • smaller orifice
  • increase atomizing pressure
  • reduce viscosity - by increasing feed temperature or reducing solids

Powder Recovery

It is essential for both economic and environmental reasons that as much powder as possible be recovered from the air stream. Three systems are available, however wet scrubbers usually act as a secondary collection system following a cyclone.

Bag Filters

Bag filters are very efficient (99.9%), but not as popular due to labor costs, sanitation, and possible heat damage because of the long residence time. They are not recommended in the case of handling high moisture loads or hygroscopic particles.

Cyclone Separators

Cyclones are not as efficient (99.5%) as bag filters but several can be placed in series. Air enters at tangent at high velocity into a cylinder or cone which has a much larger cross section. Air velocity is decreased in the cone permitting settling of solids by gravity. Centrifugal force is important in removing particles from the air stream. High air velocity is needed to separate small diameter and light materials from air; velocities may approach 100 ft/sec (70 MPH). Higher centrifugal force can be obtained by using small diameter cyclones, several of which may be placed in parallel; losses may range from 0.5-2%. A rotary airlock is used to remove powder from the cyclone. (An example of a rotary airlock is a revolving door at a hotel lobby which is intended to break the outside and inside environments).

Wet Scrubbers

 Wet scrubbers are the most economical outlet air cleaner. The principle of a wet scrubber is to dissolve any dust powder left in the airstream into either water or the feed stream by spraying the wash stream through the air. This also recovers heat from the exiting air and evaporates some of the water in the feed stream (if used as the wash water).

Diagram of wet scrubbers

Wet scrubbers not only recover most of what would be lost product, but also recover approximately 90% of the potential drying energy normally lost in exit air. The exit air picks up moisture which increases evaporative capacity by 8% (concentration of feed). Cyclone separators are probably the best primary powder separator system because they are hygienic, easy to operate, and versatile, however, high losses may occur. Wet scrubbers are designed for a secondary air cleaning system in conjunction with the cyclone. Either feed stream or water can be used as scrubbing liquor. Also, there are heat recovery systems available.

Two and Three Stage Spray Drying With a Fluidized Bed

Principles of Fluid Beds

Air is blown up through a wire mesh belt on porous plate that supports and conveys the product. A slight vibration motion is imparted to the food particles. When the air velocity is increased to the point where it just exceeds the velocity of free fall (gravity) of the particles, fluidization occurs. The dancing/boiling motion subdivides the product and provides intimate contact of each particle with the air, but keeps clusters from forming.

Diagram of a fluid bed

With products that are particularly difficult to fluidize, a vibrating motion of the drier itself is used to aid fluidization; it is called vibro-fluidizer which is on springs. The fluidized solid particles then behave in an analogous manner to a liquid., i.e. they can be conveyed. Air velocities will vary with particle size and density, but are in the range of 0.3 - 0.75 m/s. They can be used not only for drying but also for cooling. If the velocity is too high, the particles will be carried away in the gas stream, therefore, gravitational forces need to be only slightly exceeded.

Two- and three-stage drying processes

 In standard, single stage spray drying, the rate of evaporation is particularly high in the first part of the process, and it gradually decreases because of the falling moisture content of the particle surfaces. In order to complete the drying in one stage, a relatively high outlet temperature is required during the final drying phase. Of course the outlet temperature is reflective of the particle temperature and thus heat damage .

Consequently the two stage drying process was introduced which proved to be superior to the traditional single stage drying in terms of product quality and cost of production.

The two stage drier consists of a spray drier with an external vibrating fluid bed placed below the drying chamber. The product can be removed from the drying chamber with a higher moisture content, and the final drying takes place in the external fluid bed where the residence time of the product is longer and the temperature of the drying air lower than in the spray dryer.

This principle forms the basis of the development of the three stage drier. The second stage is a fluid bed built into the cone of the spray drying chamber. Thus it is possible to achieve an even higher moisture content in the first drying stage and a lower outlet air temperature from the spray drier. This fluid bed is called the integrated fluid bed. The inlet air temperature can be raised resulting in a larger temperature difference and improved efficiency in the drying process. The exhaust heat from the chamber is used to preheat the feed stream. The third stage is again the external fluid bed, which can be static or vibrating, for final drying and/or cooling the powder. The results are as follows:

  • higher quality powders with much better rehydrating properties directly from the drier
  • lower energy consumption
  • increased range of products which can be spray dried i.e., non density, non hygroscopic
  • smaller space requirements

Agglomerating and Instantizing

These processes have allowed the manufacturing of milk powders with better reconstitution properties, such as instantized skim milk powder. 

Agglomeration Mechanism: Powder is wetted with water or steam. The surface must be uniformly wetted but not excessively. The powder is held wet over a selected period of time to give moisture stability to the clusters which have formed. The clusters are dried to the desired moisture content and then cooled (e.g., fluid bed). Dried clusters are screened and sized to reduce excessively large particles and remove excessively small ones. The agglomeration process causes an increase in the amount of air incorporated between powder particles. More incorporated air is replaced with more water when the powder is reconstituted, which immediately wet the powder particles. 

Agglomerating Techniques

Rewet Methods:

This method uses powder as feed stock. An example is the ARCS Instantizer. Humidified air moistens powder, which causes it to cluster. It is re-dried and wetted. The clustered powder is then exposed to heated, filtered, high-velocity air. The dried clusters are then exposed to cooled air on a vibrating belt. It is then sized (pelleted to uniform size) and the fines are removed. 

Straight Thru Process:

A multi-stage drying process produces powders with much better solubility characteristics similar to instantized powder. This method uses a low outlet temperature which allows higher moisture in powder as it is taken from spray drier with excess moisture removed in the fluid bed. The powder fines are reintroduced to the atomizing cloud in the drying chamber.

More information on specific evaporated and dehydrated products is located in the Dairy Products section.