Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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product is frozen can be of critical importance.

Table 1 illustrates the influence of temperature on

the amount of water frozen out for various foods,

determined experimentally. Similar values can be cal-

culated by thermodynamic analysis provided the ini-

tial water content and freezing point of the product

are known. (See Water Activity: Principles and Meas-

urement; Effect on Food Stability.)

Drying Phase

Sublimation

0008 In practice, sublimation of ice crystals usually begins

at about 20



C in a vacuum of less than 1.33 mbar,

provided sufficient heat is available from the sur-

roundings (benign heat). How long will a product

take to dry by sublimation? Ideally, the rate of sub-

limation is derived from a classical kinetic model of

sublimation which assumes: (1) a dynamic equilib-

rium between the ice surface and its vapor; (2) the

rate of escape of water molecules depends on the

temperature of the ice; and (3) the tendency of mol-

ecules to return to the ice depends on the temperature

and pressure of the vapor. This model has limited

value in practice. Of greater value in understanding

the rate of freeze drying of a food product, is to take

account of the rate of vapor diffusion as the water

molecules migrate through the vacuum space (con-

taining fewer air molecules than vapor molecules)

between the product and the condenser. The migra-

tion rate is described by the following equation:

RTl

P  p

ð1Þ

where G

is the migration rate, M the molecular

weight of water vapor, D

the effective diffusion coef-

ficient, P the vacuum pressure, R the gas constant, T

the temperature, l the distance in the vacuum space, p

the vapor pressure outside the product surface, and

the vapor pressure outside the condenser surface.

If the freeze drier is considered as a closed system,

then the rates for sublimation (G

(ap

 p

)), conden-

sation (G

a(p

 p

)), and migration (G

) are equal,

i.e., G

¼ G

, where p

is the sublimation vapor

pressure and p

the condensation vapor pressure.

Apart from the separate vapor pressure differences

 p

and p

 p

), the difference in vapor pressure

overall (p

 p

) is usually regarded as the driving

force for sublimation. This model of freeze drying

is illustrated in Figure 1.

Vapor Flow

0009As a piece of food freeze dries, the porous dry product

formed around the subliming ice core can cause a

severe restriction to vapor flow, and may prevent

continuation of the drying process. A useful model

of the above conditions assumes the pores in the dry

material resemble a bundle of circular cylinders or

tubes, with a porosity E expressed as a ratio of pore

volume to the intrinsic volume of the layer of dried

product, and a tortuosity factor (L

/L)

taken as the

square of the ratio of the effective flow path L

to the

thickness of the layer L. It can be shown mathematic-

ally that:

¼ ED= L

=LðÞ

ð2Þ

where D

is the effective diffusion coefficient, and D

is the water vapor diffusion coefficient in air. In the

Partially dry material

To vacuum

pump

fig0001Figure 1 Representation of a common freeze drier. See text

for notation of the mathematical variables illustrated. Repro-

duced from Freeze Drying, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler

MJ (eds), 1993, Academic Press.

tbl0001 Table 1 Amount of water frozen in various foods as a function of temperature

Food Water content (%) Freezablewater (%) frozen at different temperature (



C) Nonfreezable water (kg kg

1

solids)

5 10 15 20 30

Lean beef 74 83 93 97 99 100 0.35

Cod 80.5 85 94 97 98 100 0.39

Egg 74 90 95 98 99 100 0.20

Fruit juice 88 75 87 93 96 100 0.20

White bread 46 50 87 97 99 100 0.30

Peas 78 68 86 92 96 100 0.2–0.3

Data from Kuprianoff J (1964) In: Rey R (ed.) Advances in Freeze-Drying, p. 48. Paris.

2698 FREEZE-DRYING/The Basic Process

porous layer, D

varies inversely with vacuum

pressure P and directly with temperature T to the

1.75 power; therefore:

=LðÞ

1013:3

273



1:75

ð3Þ

0010 The value D

increases with decreasing pressure

until the mean free path of the gas molecules at inter-

mediate pressures becomes comparable with the pore

diameter, so that wall collisions begin to compete

with collisions in the gas stream itself in limiting

flow by diffusion. This condition is known as transi-

tion flow and is the dominating vapor flow in the

freeze drying of food.

Heat Flow

0011 In food freeze driers, heating the product is the rate-

controlling variable when it is applied to the inner

core of ice crystals through an outer layer of dry

product. If heat is applied directly through the ice

layer by conduction, as is common in laboratory

and medical freeze driers, then vapor diffusion is the

rate-controlling variable. Food freeze driers usually

apply heat by radiation to the product surface which

proceeds by conduction through the dry layer. How-

ever, the dry product layer insulates the product and

limits the radiated heat supply.

0012 Various attempts have been made to compensate

for this insulating effect by maximizing heat distribu-

tion to the product. These include: (1) the accelerated

freeze drying (AFD) technique, illustrated in Figure 2,

which employs heat-conducting expanded metal

sheets that are hydraulically pressed against the

frozen product’s surface and serve to conduct heat

to the product while also allowing water vapor to

escape, and (2) the tempomatic or ribbed-tray

method which exploits the conducting properties of

a number of ribs that reach into the bulk of the

product. Both these methods have seen extensive

industrial use. The latter has been used extensively

for drying of coffee, berries, and dairy products.

Refrigeration Condensers

0013It is common to condense the escaping water vapor,

produced by sublimation at the product surface, on a

refrigerated coil, where condensation is the reverse

of sublimation, from vapor to solid without passing

through the liquid phase. As ice builds up on a re-

frigerated coil, it insulates the condenser and renders

it less effective. Heat flow through the ice deposit can

be modeled to give an indication of the build-up of

ice with time, and the resulting temperature deficit

(difference between ice surface temperature and coil

temperature).

0014The available surface area of the condenser deter-

mines the thickness of ice that will be condensed on to

it. The surface area of the condenser is, therefore, an

important consideration in designing a food freeze

drier.

0015Condensers have some special characteristics

which have been generally neglected in considering

the basic physical processes in freeze drying.

0016It is one of the phenomena of freeze drying that the

partial pressure of air near the condenser is always

relatively high, this air having been liberated from the

product during sublimation, and having come from

leaks in the vacuum chamber. The larger the space

available to be screened by the condenser, the lower

will be the vapor pressure of air in the rest of the

Radiation Distance

Mechanical press

ressure

(

kPa

)

Accelerated

0.1

100

Expanded

platens

Heating

metal

Material

Heating platens

Materials

Dry

−25

Temperature (⬚C)

100

Frozen

fig0002 Figure 2 Temperature and vapor pressure gradients (heavy black line) in freeze drying with heating through the dry layers.

Reproduced from Freeze Drying, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler

MJ (eds), 1993, Academic Press.

FREEZE-DRYING/The Basic Process 2699

vacuum chamber, and hence less air pressure will

interfere with the escape of water vapor from the

subliming product.

Desorption (Secondary Drying)

0017 After the disappearance of the ice from the freeze-

dried product, the residual moisture (nonfreezable

water content and, as frequently occurs in practice,

incompletely sublimed ice) can be reduced by a sec-

ondary drying step called desorption. This consists of

heating the product while under a vacuum of between

0.13 and 0.67 mbar. The effect of heating is dimin-

ished by the insulating effect of the vacuum in the

porous dry product.

0018 An endpoint to secondary drying should be deter-

mined to prevent a result in which the product either

retains too much water or becomes overdried and

damaged by secondary heating. The endpoint can be

determined by plotting the ratios of the pressures in

the vacuum line before and after isolating the vacuum

pump from the chamber by means of a valve. The

endpoint is reached when the ratio approaches unity,

though values in the range 1.1–1.2 may be practic-

able. This method assumes the chamber has a low

leak rate.

0019 A more exact endpoint can be determined by trap-

ping a sample of the vapor/air mixture from above the

desorbing product, then condensing the vapor com-

ponent outside the vacuum chamber. As in the previ-

ous method, the ratio of the partial pressure before

and after trapping approaches unity at the endpoint

of drying. Precise calibration can be achieved by ref-

erence to moisture content predicted by a vapor pres-

sure isotherm for the particular product being dried.

Breaking the Vacuum

0020 A dry inert gas, such as nitrogen, is often used to

break the vacuum, and thereby prevent entry of

moisture-laden air (and oxygen) into the dry product.

Supplementary (Conditioning) Phase

0021 A freeze-dried product is dry, light, and porous, and

ideally retains its original shape. It can be stored

for long periods provided its container excludes

moisture and oxygen. Residual moisture should be

evenly distributed and should not exceed 2% (dry-

weight-basis measurement). A conditioning pro-

cedure is often necessary to achieve even distribution

of residual moisture through the entire mass of the

product, since freeze driers with multiple shelves

usually have a temperature gradient between the top

and bottom shelves and, therefore, do not achieve

exactly the same drying conditions on every shelf.

Conditioning is achieved by combining the product

from all shelves and allowing the mixture to stand for

24 h in an airtight drum before packaging into smaller

quantities.

Rehydration

0022Rehydration restores the lyophile (dry product) to its

original physical and organoleptic properties. How-

ever, rehydration by immersion of the product in

water can only proceed with difficulty in some

cases, where pores may have collapsed and protein

chains may have changed positions. Protein can be

partly digested by adding a proteolytic enzyme to the

fluid, and so assist rehydration. The product should

be allowed to stand incompletely immersed in the

fluid during rehydration, thereby minimizing resist-

ance to rehydration by air becoming entrapped in the

porous product.

Technological Improvements

0023Freeze-drying rates and results for certain food prod-

ucts can be improved by variants of the basic process.

Microwave Heating

0024This is best applied to remove residual moisture in the

desorption stage, since it can speed up this final stage

most advantageously. Use of microwave heating in

the primary phase is not recommended, because of

the danger of ionization of the air under vacuum

pressures of less than 1.33 mbar, and possible explo-

sive effects.

Cyclic Pressure

0025If the vacuum pressure is deliberately increased peri-

odically (two-thirds of the cycle), and then reduced

again (one-third of the cycle), a vapor flash occurs at

the product surface, as a result of raising the thermal

conductivity of the gas, and a small increase in ice

interface temperature. The result of this process is a

reduction in overall drying time of up to 34% for

products such as cooked meat, whole egg, and apple.

Adsorbents

0026The adsorption process removes water vapor by

means of a desiccant rather than refrigeration coils.

The desiccant creates a high and well-sustained vapor

pressure driving force, particularly at moderately low

temperatures, since the equilibrium water vapor pres-

sure of the desiccant decreases as the temperature is

lowered.

0027At 20



C and 1.04 mbar vacuum, silica gel

provides a vapor pressure force to ‘drive’ freeze

drying equivalent to refrigeration coils at 40



2700 FREEZE-DRYING/The Basic Process

and similar vacuum pressure. An adsorption system

can, therefore, be set at 20



C and many of the

engineering complexities, involving low-temperature

refrigeration, high vacuum, and problems of drying

the vacuum chamber after defrosting the refrigeration

coils, are eliminated.

0028 In an adsorption system, secondary drying can be

achieved at 20



C instead of þ20



C, with a greatly

reduced risk of overheating the product.

See also: Drying: Theory of Air-drying; Freezing:

Operations; Water Activity: Principles and

Measurement; Effect on Food Stability

Further Reading

Bell GA and Mellor JD (1986) Development of the adsorp-

tion freeze-dryer. CSIRO Food Research Quarterly 46:

56–58.

Dalgleish JMcN (1990) Freeze-Drying for the Food Indus-

tries. London: Elsevier Applied Science.

Goldsmith SA, Rey L and Rothmeyer WW (eds) (1975)

Freeze-Drying and Advanced Food Technology.

London: Academic Press.

King CJ (1970) Freeze-drying of foodstuffs. CRC Critical

Reviews in Food Technology 1: 379.

Mellor JD (1978) Fundamentals of Freeze-Drying. London:

Academic Press.

Structural and Flavor (Flavour)

Changes

M Marin, Unite

Mixte de Recherche de Ge

nie et

Microbiologie des Proce

s Alimentaires (INRA-INA

P-G), Thiverval-Grignon, France

Background

0001 Freeze-drying is a dehydration process especially

suited to the conservation of biological products. In

comparison with other drying processes, freeze-

drying is considered as a reference for manufacturing

high-quality dehydrated product. The direct transi-

tion of water from solid to vapor (sublimation), with-

out a liquid phase, helps to preserve most of the initial

raw material’s properties such as appearance, shape,

taste, color, and flavor. As an important functional

property, the freeze-dried product has a high rehydra-

tion capacity. The main limit to the industrial devel-

opment is its cost due to the low productivity.

Consequently, except the application for biological

active material (bacteria, vaccine), the use of

freeze-drying is restricted in food industry to high

added-value products like coffee, ingredients for

ready-to-eat foods (fruits and vegetables, meat and

fish), and aromatic herbs.

Freezing

0002Freezing, the first step of the process, determines the

structure of the freeze-dried product. The rate of

freezing and the temperature level, depending on the

nature and the composition of the product, define

the number (nucleation) and size (growth) of the ice

crystals (Figure 1).

−50

−40

−30

−20

−10

Time (h)

(c)

(d)

Temperature (⬚C)

(c)

(a) (d)

1 2 3 After freeze-drying

Freezing step

0.00

−50

−40

−30

−20

−10

0.02 0.04

Time (h)

0.06 0.08

(a)

(b)

0.10

Temperature (⬚C)

fig0001Figure 1 Influence of freezing rate on the final structure of

freeze-dried products. (a) Rapid freezing rate; (b) very rapid

freezing rate; (c) slow freezing rate; (d) slow freezing with sub-

cooling. The liquid product is shown schematically with black

points representing ice. Step 1, appearance of the first ice crystal;

step 2, ice nucleation and crystal growth; step 3, end of freezing

step. Porosity of the freeze-dried product is shown in relation to

freezing rate.

FREEZE-DRYING/Structural and Flavor (Flavour) Changes 2701

0003 In the case of rapid freezing (more than several

centimeters per minute), ice crystals remain small,

and the high nucleation rate leads to the creation of

numerous crystals. In food products, water crystal-

lizes simultaneously inside and outside the cell, and

the microcrystals yield small pores after freeze-

drying. This method of freezing preserves the struc-

ture of the material as much as possible, but at the

same time, the absence of a high porous network

tends to decrease the rate of subsequent drying

steps and the rehydration capacity of the freeze-

dried product.

0004 In the case of slow freezing (less than one centi-

meter per hour), ice crystal formation is based on two

mechanisms. If crystallization begins at a temperature

lower than but close to 0



C(1to3



C), the ice

crystal growth is faster than the ice nucleation and

takes place preferentially outside the cell. Owing to

the resulting osmotic effect, water inside the cell tends

to move out, and the large crystals obtained in the

interstitial areas can cause mechanical injury to the

structure. In a few cases, subcooling may occur (sev-

eral degrees under freezing point without ice), and

very rapid crystallization suddenly occurs, yielding

a final structure similar to that obtained by quick

freezing.

0005 In noncellular systems (gel, juices), the result of

slow freezing is a heterogeneous medium with a

large dendritic crystal structure and a highly concen-

trated phase that can form an impermeable layer at

the surface of the product. During subsequent drying,

if the vapor is thus prevented from leaving the prod-

uct, the vapor pressure inside the product will increase

and make the product foam. To a lesser extent, a

nonhomogenous freezing step, based on rapid freez-

ing at the surface (with a refrigerating effect of the

superficial water evaporation in a dry cold atmos-

phere) and a slow freezing inside the product, also

creates a less porous layer on the surface. This layer

constitutes a barrier to the elimination of the water

vapor.

0006 Quenching in liquid nitrogen prevents any crystal-

lization (vitrification) but is not usually carried out

in food processing. Meanwhile, if the temperature

achieved at the end of the freezing process is suffi-

ciently low, often far below 20



C, the frozen

product is in an overall amorphous state. This is a

good condition to maintain the stability of the frozen

structure. Moreover, a lower storage temperature has

to be applied in order to preserve the structure

obtained after a rapid freezing than after a slow

freezing rate.

0007 The freezing step gives the high porosity of the

freeze-dried product but explains the main part of

the texture loss when damaging the cell wall structure.

The volume difference between the fresh and freeze-

dried product is generally considered negligible. A

rapid (homogeneous) freezing will contribute to pre-

serve the structure and overall texture of the material,

while creating the uniform porous network of the

freeze-dried product. Slow freezing is nevertheless

preferred because the interstices and openings are

larger and facilitate mass transfer during drying and

rehydration before eating.

Drying

0008The drying steps (primary – sublimation, and second-

ary – desorption) may also affect the structure of the

product, and the most important factors affecting

the criteria of final product quality are the heating

temperature and the working pressure.

Temperature

0009Large differences between the temperature values

from the frozen part to the dried layer are observed

during freeze-drying.

0010In the frozen core, it is well known that the tem-

perature must remain under the melting point in

order to prevent fusion. Moreover, the structure

resulting from the freezing step is stable if the

temperature also remains under another lower tem-

perature level, which is characteristic of the glass

transition step of the concentrated medium around

the ice crystal (Figure 2). As an example, when the

vitreous region is obtained (area (3) in Figure 2), the

reactions like the recrystallization, also called ‘Ost-

wald maturation,’ are highly reduced. Moreover, if

Temperature (⬚C)

Freezing

temperature

(1)

(2)

5 100

(4)

(3)

⬘

Glass transition

temperature (

)

Solute content (%)

fig0002Figure 2 Phase state diagram for a binary mixture. Curves

showing transition temperatures – glass transition and melting

– are plotted against moisture content. There are four regions: (1)

liquid, (2) ice crystal with concentrated liquid, (3) amorphous

solid, and (4) liquid with solute crystal. In the amorphous solid

region, the viscosity of the matrix is higher than 10

–10

Pa.s.

These diagrams may be experimentally determined for a mater-

ial by differential scanning calorimetry. T

: Glass transition tem-

perature, of the maximally frozen concentrated solute, C

2702 FREEZE-DRYING/Structural and Flavor (Flavour) Changes

the temperature in the frozen core during the primary

drying phase is below the melting point but above the

glass transition temperature (T

), the structural

damage may be significant, and the product may

collapse when the frozen ice crystal is sublimated. A

low temperature (under T

) maintains a high appar-

ent viscosity; shrinkage and, consequently, collapse

are avoided. The product-collapse temperature is

closely related to the glass transition temperature

but sometimes appears higher than T

, especially for

cellular material (biological tissue). For foods with a

high sugar content, the collapse temperature is low

(40 to 60



C) and can be raised by the addition of

high-molecular-weight additives (polysaccharides).

0011 The glass transition temperature decreases strongly

with the water content, due to the plasticizing effect

of water (Figure 2). In the dried layer, a higher

temperature, allowing a higher drying rate, can be

applied without product damage. But over a critical

value of 60



C, another kind of damage occurs in the

biological product, i.e., protein degradation. This

results in a change in the product structure, especially

in protein-based foods (meat, fish), and a reduced

tenderness and hydration capacity.

0012 During the freeze-drying process, the passage of the

product from the frozen state to the dried state is a

critical step in which there is a wide variation in water

content as well as the temperature. The probability of

structural changes (i.e., collapse) is important. As a

general rule, to preserve the original structure, the

product temperature should remain under the glass

transition temperature (T

), which depends strongly

on the water content.

0013 On a practical point of view, the control of tem-

perature is based on the temperature and energy sup-

plied from the heating source. Among the traditional

techniques, conductive heating is more preferable for

use with thermally sensitive products than a radiation

heat source. With microwave heating, it is more diffi-

cult to control a homogeneous supply of energy inside

the frozen core of the product, and this often leads to

a reduction in final product quality.

Pressure

0014 In the atmosphere of the freeze-drier under vacuum,

the total pressure is generally assumed to be close

to the vapor pressure. Following the equilibrium

rule, the vapor pressure is directly related to the tem-

perature at the sublimation front inside the product

where the vapor is created. Thus, a control of the

total pressure is an indirect way to monitor the prod-

uct temperature at the sublimation front. At first, the

total pressure has to be sufficiently low to avoid the

melting of ice. After a rapid abnormal increase in

pressure in the freeze-drier, the food product foams

and shrinks due to an increase in front temperature

above the melting point. Furthermore, during pri-

mary drying, if the pressure is sufficiently low to

avoid the melting point but higher than the glass

transition, shrinkage and collapse mechanisms can

be observed. In order to maintain the quality of

the product, the total pressure should be sufficiently

low in comparison with the sublimation front

temperature.

0015In conclusion, the main modification of the struc-

ture of the product is a consequence of the freezing

step, assuming that the drying steps are well con-

trolled (no recrystallization and no collapse). During

the subsequent drying steps, low values of pressure

and temperature (levels depending on the nature of

the dry matter of the product) tend to preserve the

shape and texture of the frozen product. Knowing that

an increase in temperature is a good way to reduce the

freeze-drying time, an optimum temperature has to be

defined for industrial applications.

Flavor Losses

0016Freeze-drying is often considered the best drying pro-

cess as far as flavor compound retention is concerned,

due in a first approximation to the low temperature

level.

0017Molecules, responsible for food flavor, usually

have a high relative volatility to water and a low

molecular weight (less than 300 g mol

1

). Owing to

the low concentration of aroma compounds in the

food products (p.p.b. to p.p.m.), food flavor is usually

evaluated using two kinds of tools: analytical meas-

urements (gas chromatography and mass spectrom-

etry analysis) and/or sensory panel. From these

properties, it is difficult to imagine removing water

as a vapor phase, without any other volatiles like

flavor compounds. Also, flavor losses, even in low

proportions, are rapidly considered to be significant.

As a matter of fact, the loss of flavor compounds can

be limited during freeze-drying with regard to water

elimination. The freeze-drying operating conditions

that improve the drying rates give an increased reten-

tion of volatiles. Rapid primary drying is expected

to result in higher levels of volatiles. This can be

explained from the principle of selective diffusion.

The diffusion coefficient (mobility) of the volatile

decreases more rapidly with the water content inside

the product than the diffusion coefficient of water

itself (Figure 3). The loss of volatile decreases and

stops as soon as a critical moisture is attained. One

way to decrease the first drying duration is to

increase, in a preliminary step, the dry matter (addi-

tives, osmotic dehydration). Based on the techniques

of encapsulation, additives can also decrease the

FREEZE-DRYING/Structural and Flavor (Flavour) Changes 2703

flavor mobility in the matrix during drying. With this

approach, a decrease in flavor losses has been ob-

served in coffee, fruits, and vegetables when slow

freezing is applied.

0018 Using a constant heating temperature source,

retention of flavor decreases when the total pressure

increases. Shrinkage and collapse phenomena (high

pressure and low apparent viscosity of the medium)

are correlated to a poor aroma retention. Considering

flavor compounds (or lipids) encapsulated in the

amorphous part of the freeze-dried product, recrys-

tallization (either water or solutes) above T

results

in the release of entrapped compounds. Thus,

crystallization is related to a loss of flavor. Moreover,

exposure of lipids to ambient oxygen is enhanced,

which promotes oxidation and creates off-odors.

Low-fat meats like chicken, turkey, and ham are

easier to freeze-dry. During secondary drying, low

heating temperatures are recommended to obtain a

freeze-dried product without a cooked flavor, thus

preventing new and often undesirable aroma com-

pounds.

0019 Lowering the driving force is another way to

reduce the mass transfer of flavor compounds. Redu-

cing the total pressure under vacuum implies that a

vacuum pump continually extracts the volatiles from

the chamber. Freeze-drying under atmospheric pres-

sure has been proposed to remove less volatiles, but it

is necessary to use a highly selective adsorbent that

depletes only the water vapor in the freeze-drying

atmosphere.

0020 The flavor losses remain low during the freeze-

drying process. Under vacuum freeze-drying, the low

total pressure prevents oxidation. The storage condi-

tion, especially the temperature, can also be detrimen-

tal to flavor sensory components.

Packaging and Storage of the

Freeze-dried Product

0021The moisture of the freeze-dried material associated

with the temperature during the storage has to be

finely controlled in order to preserve the high quality

of the freeze-dried product.

0022The freeze-dried product has a very low water

content (less than 5%), and so it is very hygroscopic

and necessitates the use of packaging film that is

impermeable to water. If there is a gain of a few

percent of water, the product can become rubbery at

ambient temperature (temperature above the glass

transition temperature). Its structure becomes sticky,

and the product collapses into lumps after a few

weeks. In high-sugar products, there is also a risk of

solute crystallization. This loss of structure is associ-

ated with an increase in all kinetic reactions inside the

product.

0023Moreover, because of the high porosity of the

freeze-dried product, the ambient oxygen is very effi-

cient, and the oxidation reaction can be accelerated

(flavor losses, browning).

0024Depending on the product, a vacuum seems to be

better than an inert gas like nitrogen, in order to

prevent off-flavors from developing.

0025A good stability of the freeze-dried product is

achieved with a nonpermeable packaging, referring

to water vapor and oxygen. Packaging under vacuum

and at a low storage temperature (above ambient

temperature) is always favorable to increase the time

of preservation of the original flavor to several

months.

Other Quality Criteria: Nutrients, Color,

and Rehydration

0026Freeze-drying is definitively taken as a reference when

quality criteria should be maintained close to the

original product. Superior retention of nutrients and

a better preservation of the color are also obtained

when freeze-drying operating conditions have been

chosen to maintain the structure and flavor of the

product.

0027Fruits and vegetables are major sources of vitamins

and provitamins, with a reported anticancer activity,

which are highly unstable and oxidizable. Freeze-

drying at low heating temperatures and under an

inert gas gives the best preservation of nutrients com-

pared with other processes (drying, canning). High-

activity enzyme extracts from fruits have also been

obtained with freeze-drying processes.

0028In freeze-dried foods, the color is brighter than that

of products dried by other techniques. Browning due

to Maillard reactions, if compared with other drying

−4

−3

−2

−1

10 20 30 40 50 60 70 80 90 100

Water content (%)

Diffusivity of volatiles

Diffusivity of water

fig0003 Figure 3 Influence of water content inside food products on the

ratio between the diffusion coefficients of volatile compounds and

water vapor.

2704 FREEZE-DRYING/Structural and Flavor (Flavour) Changes

processes, is reduced because of the low temperature

used. Nevertheless, a slow freezing rate gives a more

pronounced color and higher luminance than a

high freezing rate but disappears after rehydration.

Referring to the chemical reaction, slow freezing,

associated with the cryoconcentration effect, tends

to increase the intensity of the red color in freeze-

dried products like strawberries and beef meat. This

risk of enzymatic browning reactions in fruits is

higher with slow freezing. The effect of freeze-drying

pressure has been observed after rapid freezing on

freeze-dried coffee, with a darker product at high

pressure. Secondary drying at more than 60



C will

promote browning (Maillard reactions). Eventually,

freeze-dried products are very sensitive to UV light.

This is particularly true for vegetables like garlic and

carrots, which may become virtually white after a few

weeks’ storage under light. A layer of aluminum is

usually introduced in the packaging film to make it

impermeable to UV radiation.

0029 The rehydration capacity is an interesting func-

tional property for industrial use of the product as

well as for the consumer. The freezing step, associated

with negligible shrinkage, leads to the characteristic-

ally high porosity of the freeze-dried product. A low

freezing rate promotes a high rehydration capacity.

This effect on different freeze-dried food products is

not obvious if recrystallization has taken place during

freeze-drying. The percentage of sublimed water,

which often occupies 80–90% of the volume of bio-

logical materials, coincides with the porosity index of

the freeze-dried product. The freeze-dried structure

explains the instantaneous and high rehydration cap-

acity, which may reach up to 90% of the lost water.

Deformation as well as protein denaturation, due to

high temperatures during freeze-drying, can decrease

this ratio. Nevertheless, it will not achieve the

rehydration ratio observed for traditional drying

processes, i.e. not higher than 10–20%. During rehy-

dration in boiling water, hot liquid rapidly reenters

the pores, and freeze-dried products such as beverages

(coffee or tea), vegetables, and all ready-to-eat meal

preparations can be instantaneously heated and

cooked.

See also: Drying: Dielectric and Osmotic Drying; Physical

and Structural Changes; Chemical Changes; Flavor

(Flavour) Compounds: Structures and Characteristics;

Production Methods; Freezing: Storage of Frozen Foods;

Structural and Flavor (Flavour) Changes; Nutritional

Value of Frozen Foods; Oxidation of Food Components;

Water Activity: Effect on Food Stability