Labconco 7949030, 7948020, 7948030, 7949020, 7754030 Data Sheet

A

Guide

To

Freeze

Drying

for the

Laboratory

An Industry Service Publication

2

Foreword

Introduction

This booklet has been developed to serve as a basic
guide to the freeze drying process. The information
presented is generic in nature and is the result of
research and experience by Labconco personnel and
users of freeze drying equipment. It is our intention to
provide a non-biased review of preparation techniques
and freeze drying methods. The purpose of this booklet is
to help you make an informed choice of equipment for
your laboratory applications.

Our Method

We begin our discussion of freeze drying for the
laboratory by examining the three steps in the process:
prefreezing, primary drying and secondary drying. Next,
we examine a typical freeze drying cycle and the methods
available to facilitate the freeze drying process using
equipment designed for use by laboratories. Finally,
suggestions to optimize successful results are discussed,
including determination of end point, contamination,
backfilling of dried samples and product stability.
A glossary of terms used throughout this booklet to
explain the freeze drying process follows the text, along
with a bibliography.

applications for many years, most commonly in the food
and pharmaceutical industries. There are, however, many
other uses for the process including heat-sensitive
sample preparation, plant material research, the
stabilization of living materials such as microbial
cultures, long term storage of HPLC samples,
preservation of whole animal specimens for museum
display, restoration of books and other items damaged by
water, and the concentration and recovery of reaction
products.

conditions conducive to the freeze drying process. This
equipment is currently available and can accommodate
freeze drying of materials from laboratory scale projects
to industrial production.

solvent from a frozen product by a process called
sublimation. Sublimation occurs when a frozen liquid
goes directly to the gaseous state without passing
through the liquid phase. In contrast, drying at ambient
temperatures from the liquid phase usually results in
changes in the product, and may be suitable only for
some materials. However, in freeze drying, the material
does not go through the liquid phase, and it allows the
preparation of a stable product that is easy to use and
aesthetic in appearance.

freeze dried products do not need refrigeration, and
can be stored at ambient temperatures. Because the cost
of the specialized equipment required for freeze drying
can be substantial, the process may appear to be an
expensive undertaking. However, savings realized by
stabilizing an otherwise unstable product at ambient
temperatures, thus eliminating the need for
refrigeration, more than compensate for the investment
in freeze drying equipment.

Freeze drying has been used in a number of

Specialized equipment is required to create the

Freeze drying involves the removal of water or other

The advantages of freeze drying are obvious. Properly

3

Principles of Freeze Drying

Solid

Phase

Liquid

Phase

Va

por

Phase

F

U

S

I

O

N

L

I

N

E

V

A

P

O

R

I

Z

A

T

I

O

N

L

I

N

E

S

U

B

L

I

M

A

T

I

O

N

L

I

N

E

CRITICAL
POINT

TRIPLE
POINT

A

B

C

D

PRESSURE

TEMPERATURE

prefreezing, primary drying, and secondary drying.

from the solid phase to the gaseous phase, material to be
freeze dried must first be adequately prefrozen. The
method of prefreezing and the final temperature of the
frozen product can affect the ability to successfully freeze
dry the material.

preserving structures to be examined microscopically,
but resulting in a product that is more difficult to
freeze dry. Slower cooling results in larger ice crystals
and less restrictive channels in the matrix during the
drying process.

makeup of the product. The majority of products that are
subjected to freeze drying consist primarily of water, the
solvent, and the materials dissolved or suspended in the
water, the solute. Most samples that are to be freeze dried
are eutectics which are a mixture of substances that
freeze at lower temperatures than the surrounding water.
When the aqueous suspension is cooled, changes occur
in the solute concentrations of the product matrix. And
as cooling proceeds, the water is separated from the
solutes as it changes to ice, creating more concentrated
areas of solute. These pockets of concentrated materials
have a lower freezing temperature than the water.
Although a product may appear to be frozen because of
all the ice present, in actuality it is not completely frozen
until all of the solute in the suspension is frozen. The
mixture of various concentration of solutes with the
solvent constitutes the eutectic of the suspension. Only
when all of the eutectic mixture is frozen is the
suspension properly frozen. This is called the eutectic

temperature.

product to below the eutectic temperature before
beginning the freeze drying process. Small pockets of
unfrozen material remaining in the product expand
and compromise the structural stability of the freeze
dried product.

undergoes glass formation during the freezing process.
Instead of forming eutectics, the entire suspension
becomes increasingly viscous as the temperature is
lowered. Finally the product freezes at the glass
transition point forming a vitreous solid. This type of
product is extremely difficult to freeze dry.

to freeze dry a frozen suspension. While these factors can
be discussed independently, it must be remembered that
they interact in a dynamic system, and it is this delicate
balance between these factors that results in a properly
freeze dried product.

established in which ice can be removed from the frozen
product via sublimation, resulting in a dry, structurally
intact product. This requires very careful control of the
two parameters, temperature and pressure, involved in
the freeze drying system. The rate of sublimation of ice
from a frozen product depends upon the difference in

The freeze drying process consists of three stages:

Prefreezing: Since freeze drying is a change in state

Rapid cooling results in small ice crystals, useful in

Products freeze in two ways, depending on the

It is very important in freeze drying to prefreeze the

The second type of frozen product is a suspension that

Primary drying: Several factors can affect the ability

After prefreezing the product, conditions must be

4

vapor pressure of the product compared to the vapor
pressure of the ice collector. Molecules migrate from the
higher pressure sample to a lower pressure area. Since
vapor pressure is related to temperature, it is necessary
that the product temperature is warmer than the cold
trap (ice collector) temperature. It is extremely
important that the temperature at which a product is
freeze dried is balanced between the temperature that
maintains the frozen integrity of the product and the
temperature that maximizes the vapor pressure of the
product. This balance is key to optimum drying. The
typical phase diagram shown in Figure 1 illustrates this
point. Most products are frozen well below their eutectic
or glass transition point (Point A), and then the
temperature is raised to just below this critical
temperature (Point B) and they are subjected to a
reduced pressure. At this point the freeze drying process
is started.

Figure 1

A typical phase diagram.

Some products such as aqueous sucrose solutions can

undergo structural changes during the drying process
resulting in a phenomenon known as collapse. Although
the product is frozen below its eutectic temperature,
warming during the freeze drying process can affect the
structure of the frozen matrix at the boundary of the
drying front. This results in a collapse of the structural
matrix. To prevent collapse of products containing

A vacuum pump is essential to evacuate the
environment around the product to be freeze dried.