Food Science: Courses: FS 532

12. Freeze Drying

Introduction

Freeze drying (lyophilization) is the process of removing water from a product by direct sublimation of ice to water vapor. For food products, this method of drying generally results in the highest quality of product attainable. Two primary factors account for the maintenance of product quality are:

  1. little shrinkage of tissue systems occurs, permitting rapid and nearly complete rehydration.
  2. limited heating of product minimizing degradative reactions.

The primary disadvantage of freeze drying is cost and therefore its application is primarily for expensive food items. In most conventional systems the freeze drying process is accomplished by subjecting the food material to reduced pressure to maintain a water vapor pressure gradient between the ice front in the material and the surrounding environment. The water vapor is then removed by a condensing system. The rate of freeze drying is primarily dependent on the rate of water vapor transfer from the ice surface through the porous layer of dry material and the rate of heat transfer. Other obvious factors are total surface area, pressure within the system and condenser temperature and capacity.

Purpose

To demonstrate the characteristics of the freeze drying process and its effect on food materials.

Material

Carrot dice, frozen shrimp, milk, egg albumin, coffee

Procedure
  1. Carefully examine and record the initial organoleptic properties of the materials. Measure and record the volume of a 25 gm sample of each solid material by using a graduated cylinder.
  2. Obtain 250ml of each liquid sample. Place the liquid sample in the large dehydration flasks and freeze them by rapidly spinning the nearly horizontal flasks in a dry ice-acetone bath. Pack the flasks in dry ice until the freeze drying process is initiated.
  3. Thoroughly freeze the normally solid materials by immersing them in finely crushed dry ice. Place the frozen materials in small, cold dehydration flasks, distributing the particles over a maximum area. Store the flasks in dry ice if there is any delay before the freeze drying process is started.
  4. Attach the dehydration flasks to the freeze drier and place the equipment in operation. Record the pressure after it has stabilized. A pressure of less than 500 microns (0.5mm Hg) should be maintained. Continue the drying operation until the frost on the exterior of the flasks has dissipated and the contents have attained the ambient temperature. Overnight operation will be necessary if air at room temperature is used as a source of heat energy.
  5. Open the air inlet and shut off the vacuum pump. Before removing the sample, clean the necks of the dehydration flasks with a solvent such as ether. Record the weight and yield of each product (dry weight as percent of the original weight). Record the volume of the dehydrated products. Calculate the percent shrinkage.
    %shrinkage = originalvolume - dryvolume x 100
    originalvolume

    Record the characteristics of the dehydrated products.

  6. Rehydrate each of the normally liquid products in an amount of cold water sufficient to restore its original concentration. Vigorously agitate the water while the product is gradually sprinkled on the surface.
  7. To rehydrate the carrots, establish the time in boiling water on the basis of the minimum time needed to restore a nearly normal appearance. Drain for 1 min and reweigh. Rehydrate the other normally solid products in boiling water for a duration which provides optimum quality.
  8. Record all the data and perform all calculations indicated on data sheets.
Questions

1. What is the primary disadvantage of freeze drying as compared to other methods of drying? What are its main advantages?

2. What single improvement would be most effective for accelerating the rate of drying in the equipment used in the laboratory?

3. If a product normally liquid in nature foamed during freeze drying, what would this indicate?

4. What are the possible reasons for a sample thawing during the freeze drying process? How can this be avoided?

Worksheet [PDF]

University of Wisconsin College of Agricultural & Life Sciences
Copyright © 2003, Department of Food Science,
University of Wisconsin-Madison. 		Updated March 25, 2003.
foodsci@facstaff.wisc.edu