FST 151FOOD FREEZINGFOOD SCIENCE AND TECHNOLOGY 151

Special topics: Freeze Drying Lecture Notes

Prof. Vinod K. Jindal(Formerly Professor, Asian Institute of Technology)Visiting ProfessorChemical Engineering DepartmentMahidol UniversitySalaya, NakornpathomThailand

Freeze-drying is a kind of preservation technology, by which the material is cooled below its eutectic temperature or glass transition temperature firstly to be solidified completely, then dried in vacuum space at low temperature by sublimation drying and desorption drying till 95-99% of moisture is removed.

The product can be stored at room temperature or 4 0C for long time. Freeze-drying has become a most important technique for the preservation of heat-sensitive pharmaceuticals and foods.

Freeze Drying - Introduction

Sublimation Drying Process

Sublimation drying, also called primary drying, is heating frozen material inside the drying chamber; and the chamber is kept under low pressure by a vacuum pump. During this step, the frozen water inside the material is sublimated directly to water vapor; and the material is dried.

The sublimation starts from the outside surface and passes inward gradually; the porous dried layer is formed in the same way simultaneously. The interface of frozen layer and the dried layer is called the sublimation front or the ice front.

Several typical modes of heat transfer and mass transfer in the drying process

(a) (b) (c)

(d) (e)

plate frozen layer

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G Q G

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(f)

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heat transfer

mass transfer

Mass transfer

During sublimation drying, there is massive vapor to be produced from the material. The vapor must be moved away promptly, otherwise the pressure in the drying chamber will be elevated, the sublimation surface temperature will increase, and finally the frozen layer will melt.

The water vapor escaped from the sublimation surface, passing through the dried layer, flows to the cold trap and forms frost on its surface.

Freeze drying• 1) freeze food in the conventional freezing

equipment • 2) remove water during subsequent drying

• (If the water vapor pressure of a food is below 4.58 Torr (610.05 Pa) and the water is frozen, when the food is heated the solid ice sublimes directly to vapor without melting.

• The water vapor is continuously removed from the food by keeping the pressure in the freeze drier cabinet below the vapor pressure at the surface of the ice, removing vapor with a vacuum pump and condensing it on refrigeration coils.

• As drying proceeds a sublimation front moves into the frozen food leaving partly dried food behind it.

Freeze-drying

• Completely remove water from some material, such as food, while leaving the basic structure and composition of the material intact

• Two reasons– Keeps food from spoiling for a long period of time – Significantly reduces the total weight of the food

• How?– Freeze the material– Lower the pressure (<0.006 atm)– Increase the temperature slightly

Normal (right) and freeze-dried (left) spaghetti

Freeze-drying

• How?– Freeze the material– Lower the pressure– Increase the temperature slightly

Normal (right) and freeze-dried (left) spaghetti

A commercial freeze dryer

Methods for transferring heat to the sublimation front

• 1 Heat transfer through the frozen layer• 2 Heat transfer through the dried layer• 3 Heating by microwave

Mass transfer during freeze drying

• When heat reaches the sublimation front, it raises the temperature and the water vapor pressure of the ice. Vapor then move through the dried food to a region of low vapor pressure in the drying chamber.

• 1 g of ice forms 2 m3 of vapor at 67 Pa.

Factors that control the water vapor gradient:

• The pressure in the drying chamber• The temperature of the vapor condenser.• The temperature of ice at the sublimation

front, which should be as high as possible, without melting

• (the lowest economical chamber pressure is approximately 13 Pa and the lowest condenser temperature of ice should be -35 ºC).

Freeze drying equipment

• Freeze dryer consists of:– Vacuum chamber (with trays to hold the food

during drying)– Heater to supply latent heat of sublimation.– Refrigerator coil for condense the vapors directly

to ice– Vacuum pump removes non-condensable vapors.

Types of freeze dryers

• Conduction type • Irradiation type

(Convection heating is not important in the partial vacuum of the freeze drier cabinet.)

(The surface temperature of the food does not exceed 60ºC.)

• Contact ( or conduction) freeze dryers:• Foods is placed onto trays which rest on heater plates.• Slow drying process ( heat transferred by conduction to only one

side of the food).

• Accelerated freeze dryers:• Food is held between two layers of expanded metal mesh and

subjected to a slight pressure on both sides.• Heating is by conduction, but heat is transferred more rapidly

into food by the mesh than by solid plates, and vapor escapes more easily from the surface of the food.

• Radiation freeze dryers:• Infrared radiation from radiant heater is used to heat shallow

layer of food on flat trays.• Heating is more uniform than in conduction types.

• Microwave and dielectric freeze dryers:• Radio frequency heater have potential use in freeze drying but

not widely used (Difficult to control because water heated more than ice).

Effects of freeze drying on foods

• Advantages:• Very high retention of sensory characteristics and nutritional qualities and

a shelf-life longer than 12 months when correctly packaged.• Aroma retention up to 80 – 100 %• The texture of freeze-dried foods is well maintained; little shrinkage and

no case hardening.• The open porous structure allow a rapid and full re-hydration, but it is

fragile and requires protection from mechanical damage.• Only minor changes to proteins, starches or other carbohydrates.

• Disadvantage:• The open porous structure of the food may allow oxygen to enter and

cause oxidative degradation of lipids (packaging in inert gas necessary).

Storage of the Freeze Drying Products

The main purpose of freeze-drying is to strengthen the stability of products, i.e., to reduce the chemical and biological reactions to lengthen the shelf life of products from hours to days even years. However, even if the products are stable at the conclusion of freeze drying, they may also lose activeness in long-term storage.

The quality changes of products include physical, chemical, microbiological natures protein denaturation, browning ( Maillard reaction), oxidation, hydrolysis and others.

Freeze drying of foods

The freeze drying of fruits and vegetables is emerging and high value-added technology.

Application: Freeze-dried foods are high value-added products, regarded as high-quality food, convenience food, leisure food. exported vegetables and fruits, or in travelling, exploration, navigation and so on.

Problems and difficulties: Freeze-drying process is a time-consuming and energy consuming process; the parameters of the freeze-drying process have decisive impact on the quality of freeze-dried food .

   

Freeze-dried carrot, Freeze-dried garden radish, Freeze-dried parsley

Freeze-dried broccoli Freeze-dried meat Freeze-dried onion

Freeze-dried chestnut Freeze-dried ginger slices

Freeze-dried pumpkin Freeze-dried Potato wedges