Module 10. Freeze and membrane concentration

Lesson 23

23.1 Introduction

Freeze concentration is an excellent alternative to evaporation and reverse osmosis for concentration of many liquid foods. Product quality is generally high since low temperatures are used and no vapor-liquid interface occurs. However, current commercial freeze concentration technology is not economically competitive with the more established alternatives. Freeze concentration is applied where focus is on aroma retention and high quality products. It is specially suited for heat sensitive products. Freeze concentration is used for coffee extracts, fruit juices, vinegar, beer, wine and practically any other aqueous solution .

23.2 Process

  • Freeze concentration has been practiced for centuries. In its earliest form it was as simple as leaving a barrel of liquid outside in the cold winter night. Water would crystallize and grow as a thick layer of ice along the inside walls of the barrel. The next day they would simply cut a hole through the ice shell and drain the now concentrated product. The water (now ice) was simply discarded.
  • Understanding the principles by which ice crystals grow in fluid foods would aid in furthering freeze concentration technology. In particular, if optimal heat balance conditions can be maintained throughout the freeze concentration process, large, easily separated crystals can be grown in short times. Modern freeze concentration processes consist of a crystallization section, where part of the water is converted into solid ice crystals using a refrigeration system. The ice crystals are then separated by filters, centrifuges or using the wash columns.
  • Now, through a process of innovative engineering, process simplification and component standardization, the patented technology has reduced both equipment costs and energy usage significantly making Freeze Concentration a practical option for the constantly growing number of applications throughout the food and drink sector.

23.3 Freeze Drying

Freeze-drying (also known as lyophilization or cryodesiccation) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze-drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to gas. Freeze drying is a superior preservation method for a variety of food products and food ingredients. The plant sizes available ranges from pilot scale to large industrial batches and continuous plants. During the freeze drying process deep-frozen products are dried at temperatures below -18°C in freeze dryer. No thawing of the product takes place, which ensures a high quality product.

23.4 Advantages of Freeze drying

The modern plants provide high-quality products for customers as well as unrivalled financial and operational advantages for the company by eliminating product loss, reduced energy costs and maximizing plant reliability and ease of use. Other benefits are:

  • The plants offer an advanced technology and efficient design to ensure the preservation of excellent quality in a wide range of food products, such as vegetables, temperate and tropical fruits, fish, meat, TV-dinners, coffee, flavour essences and several other products.
  • The original flavour, proteins and vitamins are preserved.The products will retain their original shape, colour and texture.
  • Re-hydration is rapid and complete.
  • The process results in stable products with long shelf life.
  • The products are durable at a wide range of temperatures, eliminating the need for complicated cold chain distribution systems.
  • The low weight and easy handling of freeze dried products reduce shipping costs dramatically.

23.5 Applications of freeze-drying

Freeze-drying is a relatively expensive process. The equipment is about three times as expensive as the equipment used for other separation processes, and the high energy demands lead to high energy costs. Furthermore, freeze-drying also has a long process time, because the addition of too much heat to the material can cause melting or structural deformations.

  • Therefore, freeze-drying is often reserved for materials that are heat-sensitive, such as proteins , enzymes , microorganisms , and blood plasma . The low operating temperature of the process leads to minimal damage of these heat-sensitive products.
  • Freeze-drying is used to preserve food and make it very lightweight.
  • The process has been popularized in the forms of freeze-dried ice cream, an example of astronaut food.
  • It is also popular and convenient for hikers because the reduced weight allows them to carry more food and reconstitute it with available water.
  • Instant coffee is sometimes freeze-dried, despite high costs of freeze-dryers. The coffee is often dried by vaporization in a hot air flow, or by projection on hot metallic plates.
  • Freeze-dried fruit is used in some breakfast cereal.
  • However, the freeze-drying process is used more commonly in the pharmaceutical industry.
  • In bacteriology freeze-drying is used to conservate special strain.

23.6 Freezing

  • The freezing process consists of freezing the material.
  • In a lab, this is often done by placing the material in a freeze-drying flask and rotating the flask in a bath, called a shell freezer, which is cooled by mechanical refrigeration, dry ice and methanol, or liquid nitrogen.
  • On a larger-scale, freezing is usually done using a freeze-drying machine. In this step, it is important to cool the material to the lowest temperature at which the solid and liquid phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following steps
  • Larger crystals are easier to freeze-dry.
  • To produce larger crystals, the product should be frozen slowly or can be cycled up and down in temperature.
  • This cycling process is called annealing.
  • However, in the case of food, or objects with formerly-living cells, large ice crystals will break the cell walls.
  • Usually, the freezing temperatures are between −50°C and −80°C.
  • The freezing phase is the most critical in the whole freeze-drying process, because the product can be spoiled if badly done.
  • Amorphous (glassy) materials do not have a eutectic point, but do have a critical point, below which the product must be maintained to prevent melt-back or collapse during primary and secondary drying.
  • Large objects take a few months to freeze-dry.

23.7 Primary Drying

1. During the primary drying phase, the pressure is lowered (to the range of a few millibars), and enough heat is supplied to the material for the water to sublimate.

2. The amount of heat necessary can be calculated using the sublimating molecules’ latent heat of sublimation.

3. In this initial drying phase, about 95% of the water in the material is sublimated.

4. This phase may be slow (can be several days in the industry), because, if too much heat is added, the material’s structure could be altered.

5. In this phase, pressure is controlled through the application of partial vacuum.

6. The vacuum speeds sublimation, making it useful as a deliberate drying process.

7. Furthermore, a cold condenser chamber and/or condenser plates provide a surface(s) for the water vapour to re-solidify on.

8. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump's performance. Condenser temperatures are typically below −50°C.

9. In this range of pressure, the heat is brought mainly by conduction or radiation; the convection effect can be considered as insignificant.

23.8 Secondary Drying

1. The secondary drying phase aims to remove unfrozen water molecules, since the ice is removed in the primary drying phase.

2. This part of the freeze-drying process is governed by the material’s adsorption isotherms.

3. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0°C, to break any physico-chemical interactions that have formed between the water molecules and the frozen material.

4. Usually the pressure is also lowered in this stage to encourage desorption (typically in the range of microbars, or fractions of a pascal). However, there are products that benefit from increased pressure as well.

5. After the freeze-drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.

6. At the end of the operation, the final residual water content in the product is around 1 to 4%, which is extremely low.

23.9 Properties of Freeze-Dried Products

  • If a freeze-dried substance is sealed to prevent the re-absorption of moisture, the substance may be stored at room temperature without refrigeration, and be protected against spoilage for many years. Preservation is possible because the greatly reduced water content inhibits the action of microorganisms and enzymes that would normally spoil or degrade the substance.
  • Freeze-drying also causes less damage to the substance than other dehydration methods using higher temperatures.
  • Freeze-drying does not usually cause shrinkage or toughening of the material.
  • In addition, flavours and smells generally remain unchanged, making the process popular for preserving food. However, water is not the only chemical capable of sublimation, and the loss of other volatile compounds such as acetic acid (vinegar) and alcohols can yield undesirable results.
Freeze-dried products can be re-hydrated (reconstituted) much more quickly and easily because the process leaves microscopic pores. The pores are created by the ice crystals that sublimate, leaving gaps or pores in their place.
Last modified: Monday, 22 October 2012, 5:37 AM