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

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Table 1 Typical drying conditions and properties of some spray-dried foods

Product Total solid

content of

feed (%)

Air inlet tem-

perature (



Type of

atomizer

Type of cham-

ber

Air outlet tem-

perature (



Product

moisture

content

(%)

Particle

size of

powder

(mm)

Bulk dens-

ity of

powder

(kgm

3

)

Notes

Skim milk (ordinary) 40–50 180–230 Various 85–100 3.5–4.0 30–50 600–700

Skim milk

(agglomerated

)

40–50 180–230 Various 85–90 3.5–4.0 150–200 450–600 Bulk density may be as low as 250 kg m

3

if rewetted

Whole milk (ordinary) 40–50 180–200 Various 75–95 2.5–3.0 30–50 550–650

Whole milk

(agglomerated)

45–50 175–200 Various 90–100 2.5–3.0 150–200 450–550

Whey (ordinary) 45–50 180–200 Centrifugal

;

Concurrent 90–100 3.0–4.0 30–40 600–700

Whey (precrystallized

and agglomerated)

40–60 180–190 Centrifugal 80–90 2.0–3.0 100–150 550–650 Particle size may be >1000 mm and bulk density

<500 kg m

3

if a second crystallization stage

follows spray drying

Buttermilk 45–50 175–190 Various 75–90 3.5–4.5 30–50 750–830

Eggs (yolk and whole) 25–48 150–200 Centrifugal 50–80 2.0–3.0 450–550

Egg (white) 10–12 145–200 Centrifugal 50–80 2.0–3.0

Instant coffee 35–40 250–300 Nozzle Concurrent 110–115 2.0–3.0 150–300 180–300

Instant tea 35–40 200–250 Nozzle Tall chamber 90–100 2.0–2.5

Tomatoes 25–40 140–150 Centrifugal Concurrent

with cooled

wall

70–80 3.0–3.5

Wet weight basis.

Agglomeration in fluidized bed.

tbl0001

lecithin to improve their wettability when reconsti-

tuted in water or aqueous solutions.

0024 Whey is also spray-dried. A number of drier

designs, usually featuring nozzle atomizers, may be

used. To reduce the hygroscopicity and improve the

handling characteristics of whey powder, the lactose

is crystallized into the stable a-monohydrate form,

prior to drying.

0025 Buttermilk is also spray-dried. Driers similar to

those used for skim milk are used.

0026 Icecream mixes have high sugar contents, up to

30%, which cause problems during drying. One solu-

tion is to keep back some of the sugar and add it to the

dried mix. Two-stage drying yields a good noncaking

powder. (See Ice Cream: Methods of Manufacture.)

0027 Cheese is ground and mixed with water prior to

drying. Some deposition on chamber walls may occur.

This can be overcome using devices to assist in

powder removal (see above) and/or two-stage drying.

Strong odors can escape to the atmosphere during

cheese drying. The exhaust air may need to be

deodorized or a semiclosed drying system used (see

above).

0028 Butter and high-fat powder (60–80% fat) may be

spray-dried. Precautions must be taken to avoid

breakdown of the emulsion structure and the product

must be cooled after drying.

0029 Sodium caseinate after manufacture is relatively

viscous and is fed to the drier at a lower than usual

solids content (20–25%). Centrifugal atomizers and

two-stage drying are usually used.

Eggs

0030 Whole egg, egg yolk and egg white may be spray-

dried. The feeds are pasteurized prior to drying. Glu-

cose, if present in the dry powder, can give rise to the

development of discoloration and off-odor during

storage. Glucose may be removed, prior to drying,

by fermentation using bacteria, yeasts, or enzymes.

This process is mainly applied to egg white. Whole

egg and egg yolk are fed to the drier at 25–27% and

45–48% solids contents, respectively. Centrifugal

atomizers are usually used. Egg white feed usually

contains 10–12% solids, but this may be increased

by reverse osmosis.

Beverages

0031 Instant coffee is another product spray-dried on a

large scale. Ground, roasted coffee beans are ex-

tracted with hot water in semicontinuous or continu-

ous extractors. The extract contains 20–25% solids

which may be fed directly to the spray drier or pre-

concentrated up to 60% solids. Driers featuring tall

cylindrical bodies (Figure 4a) are used, and nozzle

atomizers. Air and spray follow a streamlined path

down the tower. The conical base of the drier may be

jacketed and cool air passed through this to keep the

wall temperature low. Fines may be recycled to the

drying chamber. Coffee powder may be agglomerated

by rewetting. Instant coffee may also be freeze-dried.

Coffee substitute made from mixtures of chicory,

barley, rye, wheat, and coffee may be dried in a simi-

lar manner to the pure coffee. (See Coffee: Instant.)

0032Instant tea may also be produced by spray drying.

Dried leaves are extracted with hot water to give an

extract containing 3–5% solids. This extract is con-

centrated by vacuum evaporator. The volatile aroma

compounds which flash off in the early stages of evap-

oration are condensed and added back to the concen-

trate prior to drying. Driers similar to those used for

coffee extract are used but at lower inlet air tempera-

tures. (See Tea: Types, Production, and Trade.)

Fruits and Vegetables

0033Whole fruits and fruit juices may be spray-dried.

However, with the exception of tomatoes, additives

must be used to facilitate drying.

0034Tomatoes are prepared for drying by the hot-break

or cold-break processes. The former method is

favored as the reconstituted powder made in this

way has less of a tendency to settle than that made

by the cold-break process. The pulped tomatoes are

comminuted and sometimes homogenized and then

concentrated to 26–48% solids by vacuum evapor-

ation. The drier most commonly used has a relatively

short cylindrical body and conical base. The whole of

the drying chamber may be jacketed and cool air

drawn through the jacket to cool the chamber wall.

A centrifugal atomizer is used. The powder leaving

the chamber is cooled on a vibrating fluidized bed.

0035Citrus and other fruits such as bananas, apples,

pears, strawberries, raspberries, peaches, and apricots

need to have fillers added to permit spray drying.

Without such additives the wall of the drying cham-

ber will become coated with sticky powder and the

product will be tacky and hygroscopic, with a strong

tendency to cake. The most common additive used is

liquid glucose with a dextrose equivalent in the range

15–20. This may be added in amounts up to 75% of

the fruit, on a solids basis. Skim milk powder has also

been used as an additive. To insure good atomization

the fruit must be comminuted thoroughly. Both

centrifugal and nozzle atomizers and various designs

of drying chamber, some featuring cooled walls and

aftercoolers, have been used.

0036Whole vegetables and vegetable juices may also be

spray-dried. These include carrots, potatoes, cab-

bage, peas, and asparagus. These are easier to dry

than fruits. Again, efficient comminution is necessary

and additives may be required.

DRYING/Spray Drying 1937

Other Spray-dried Products

0037 Beverage whiteners These are oil-in-water emul-

sions whose ingredients include vegetable fat, sodium

caseinate, corn syrup, and emulsifying agents. They

are spray-dried, usually using pressure nozzles, down

to a moisture content of 2–5%. Often two-stage

drying is used.

0038 Vegetable proteins These may be extracted from

soya beans, peanuts, potatoes, and other sources.

They are spray-dried using concurrent chambers and

pressure jet atomizers. Some dried products may be

tacky and hygroscopic and require cooling during

handling and packaging.

0039 Single-cell protein (SCP) SCP is produced from

methane, methanol, ethanol, and Z-paraffins by

fermentation, concentrated by centrifugation and

spray-dried. The fine powder is often agglomerated.

Relatively high inlet and outlet temperatures are used.

There is a danger of dust explosion and semiclosed,

self-inertizing spray-drying systems may be used.

0040 Meat Pure

es of beef and chicken may be spray-dried

for use in soups and sauces. Concurrent drying cham-

bers and centrifugal atomizers are usually used. Blood

from slaughterhouses is spray-dried to produce a

range of blood, albumin, and plasma powders. Rela-

tively low drying temperatures are used.

0041 Fish byproducts A range of products, including sol-

uble fish powder, fish meal, fish flour, and fish pro-

tein, produced by hydrolysis of fish flesh, is produced

by spray drying. The odor released during drying can

be a problem.

0042 Enzymes A number of enzyme powders are pro-

duced by spray drying. To minimize inactivation,

inorganic salts may be added and relatively low

drying temperatures used. In some cases aseptic

spray drying may be carried out.

0043 Vitamins Vitamins from various sources may be

spray-dried. Vitamins B and C are spray-dried dir-

ectly. Vitamins A and D are usually microencapsu-

lated (See Vitamins: Determination.)

See also: Agglomeration; Ascorbic Acid: Properties and

Determination; Physiology; Cholecalciferol: Properties

and Determination; Physiology; Coffee: Instant; Drying:

Theory of Air-drying; Fluidized-bed Drying; Physical and

Structural Changes; Chemical Changes; Eggs: Use in the

Food Industry; Fish: Miscellaneous Fish Products; Fish

Meal; Ice Cream: Methods of Manufacture;

Instrumentation and Process Control; Powdered Milk:

Milk Powders in the Marketplace; Characteristics of Milk

Powders; Retinol: Properties and Determination;

Physiology; Single-cell Protein: Algae; Yeasts and

Bacteria; Soy (Soya) Beans: Processing for the Food

Industry; Tea: Types, Production, and Trade; Processing;

Vitamins: Determination; Whey and Whey Powders:

Production and Uses

Further Reading

Barbosa-Canovas GV and Vega-Mercado H (1996) Dehy-

dration of Foods. New York: Chapman and Hall.

Brennan JG (1994) Food Dehydration – A Dictionary and

Guide. Oxford: Butterworth-Heinemann.

Brennan JG, Butters JR, Cowell ND and Lilly AEV (1990)

Food Engineering Operations, 3rd edn. London:

Elsevier Applied Science.

Masters K (1991) Spray Drying Handbook, 5th edn. New

York: Longmans.

Mujumdar AS (ed.) (1995) Handbook of Industrial Drying,

2nd edn. New York: Marcel Dekker.

Van Arsdel WB, Copley MJ and Morgan AI Jr. (1973) Food

Dehydration, Drying Methods and Phenomena, vol. 1

Principles, vol. 2 Practices and Applications. Westport,

CT: AVI.

Dielectric and Osmotic Drying

J G Brennan, The University of Reading, Reading, UK

Application of Dielectric Energy to Drying

Principles

0001Here the term dielectric heating is used to represent

the radiofrequency (RF) and microwave (MW) bands

of the electromagnetic spectrum. RF heating is in

the frequency range 1–200 MHz and MW from

300 MHz to 300 GHz. By international agreement,

certain frequencies have been allocated for industrial

use, in order to avoid interference with telecommuni-

cations. These are RF 13.56 and 27.12 MHz and MW

2450 and a band within the range 896–915 MHz.

The advantages of dielectric heat for drying, cooking,

thawing, and melting of foods are that heat gener-

ation is rapid and occurs throughout the body of the

food material. An additional advantage for drying is

that water is heated more quickly than the other

components of the food. In conventional methods of

drying, heat is transferred to the surface of the food

by convection, conduction, or radiation. It has then to

transfer throughout the body of the food by conduc-

tion. Some penetration does occur in the case of

radiant heat but this is limited and often difficult to

1938 DRYING/Dielectric and Osmotic Drying

control. The penetration of electromagnetic waves in

the dielectric range into materials such as foods

depends on frequency and the characteristics of the

material. The lower the frequency, hence the longer

the wavelength, the greater the penetration. Penetra-

tion may be quantified by half-power depth, i.e., the

thickness of the material, which reduces the wave

energy to 50% of the incident level. The half-power

depth for water is about 12 mm at a frequency of

2450 MHz and 75 mm at 100 MHz. Penetration into

foods is less than into water.

0002 In the case of dielectric heating, the heat is trans-

ferred into the body of the material rather than just

to the surface. When a wet material is exposed to

high-frequency electromagnetic radiation molecules,

which are dipolar, such as water, are stressed by the

alternating magnetic field and this results in the gen-

eration of heat. This is the main mechanism which

causes heating on the application of MW energy. At

the lower RF frequencies, ionic conduction plays a

part. Ions present in the material are accelerated by

the electric fields, and this leads to the generation of

heat. Since most foods contain ions in the aqueous

phase, RF heating can be effectively applied to them.

The mechanism of drying using dielectric heating is

different from that which operates when convected or

conducted heat is used. The rapid generation of heat

within the material causes rapid evaporation of water.

This creates a total pressure gradient, which results in

the rapid movement of liquid water and water vapor

to the surface. Thus, shorter drying times and lower

product temperatures are attainable compared to

more conventional methods of heating.

0003 Overheating of the surface can be avoided. Drying

is more uniform as thermal and concentration gradi-

ents are less. There is also less movement of solutes

within the material. Since the water absorbs most of

the heat there is more efficient use of energy com-

pared to drying by the application of convected or

conducted heat. However, too high a heating rate can

lead to scorching or burning of the material. If the

water vapor becomes entrapped within the material,

high pressures can develop, which can lead to the

rupture of solid pieces.

Equipment

0004 A basic RF (platen) applicator consists of two metal

plates between which the food is located or conveyed.

Often the conveyor belt may represent one of the

electrodes. The plates are at different electrical volt-

ages. The whole system is housed in a metal chamber

to prevent leakage of radiation. In a stray field appli-

cator a thin layer of material passes over electrodes

of alternating polarity. In the staggered applicator

rods extend beyond the plates to form staggered

throughfield arrays between which the layer of ma-

terial passes. This last type can cope with thicker

layers Figure 1.

0005In a basic batch MW applicator, MWs, generated

by a magnetron or klystron, are directed into a metal

chamber via a waveguide or coaxial cable. The food is

placed in the chamber. To improve the uniformity of

heating the beam of MWs may be disturbed by a

mode stirrer, resembling a slowly turning fan, which

causes reflective scattering of the waves. Alterna-

tively, the food may be rotated in the chamber. In

continuous systems, the food is conveyed through a

chamber, which has a number of wave injection ports

in its sides, top, and bottom.

Applications

0006In drying of foods, dielectric heating is usually com-

bined with conventional heating. It may be used to

preheat the feed to a hot-air drier. By raising the tem-

perature of the feed quickly and causing moisture to

move to the surface, it can decrease the overall drying

time. Dielectric heating may be applied part-way

through the drying cycle, when the food enters the

falling rate period. (See Drying: Theory of Air-drying.)

(a)

−

(b)

(c)

fig0001Figure 1 Electrode configurations for dielectric heating sysU

tems. (a) Platen type for bulky objects. (b) Stray field type for thin

layers. (c) Staggered type for thicker layers. Adapted from Schiff-

mann RF (1995) Microwave and dielectric drying. In: Mujumdar

AS (ed.) Handbook of Industrial Drying, 2nd edn, vol. 1, pp. 345–372.

New York: Marcel Dekker.

DRYING/Dielectric and Osmotic Drying 1939

This can boost the rate of drying. If dielectric heating is

applied near the end of hot-air drying it can also

shorten the drying time significantly and hence increase

the throughout of the drier. It is more usual to use

dielectric heating in the later stages of drying.

0008 One of the major applications of RF heating is in

the postbaking of biscuits. The objectives in baking

biscuits are to produce a product of the right size,

shape, color, and moisture content. In a conventional

oven, reducing the moisture content to the desired

level can take up a large part of the total baking

time. The application of RF heating can shorten the

baking time. The oven is set to produce biscuits of the

right size, shape, and color, but the RF heating is used

to remove the remaining moisture, without any

further change in these properties of the biscuit. The

capacity of an oven can be increased by more than

50% by the use of RF heating. Postbaking by RF

heating has also been applied to breakfast cereals

and cereal-based baby foods.

0009 MW heating is used, in combination with heated

air of high humidity, to dry pasta products. The humid

air prevents cracking of the product. The drying time

is reduced from 8 h, using heated air only, to 1 h. MW

heating is used in drying onions. The moisture con-

tent is reduced from 80% to 10% by means of heated

air and then down to 5% using MWs. This combined

process results in more uniform drying and a saving in

energy of up to 30%, compared to hot-air drying.

Potato chips (French fries) and crisps are finish-dried

after frying to prevent darkening, if the sugar content

is high.

0010 MW heating has been used in vacuum drying. (See

Drying: Equipment Used in Drying Foods.) Pasta is

dried under vacuum using MW heating. This results

in much shorter drying times, compared to the use of

conducted or radiant heat. There are reports of other

dried products being produced in this way, including

fruit juice, tea and enzyme powders, mushrooms,

asparagus, and soya beans.

0011 MW heating in freeze drying (See Freeze-drying:

The Basic Process; Structural and Flavor (Flavour)

Changes), has been widely studied. The loss factors

of ice and liquid water are much higher than that of

dry tissue. When a frozen food is exposed to MWs,

the ice will absorb energy much faster than the solid.

This should be an advantage in freeze drying. How-

ever, ionization of the rarefied gases can occur be-

cause of the very low pressures used. This can result

in plasma discharge and overheating of the food. The

use of higher-frequency MWs, 2450 MHz, can reduce

this problem. The loss factor of liquid water is much

higher than that of ice. If some melting of the ice

occurs, the water formed will absorb energy quickly

and vaporize. This could cause solid food particles to

explode because of the build-up of pressure internally.

Good control of the freeze-drying process should pre-

vent this happening. The use of MW heating in freeze

drying is technically feasible. However, it has not

been used on a large scale as yet.

Osmotic Drying

Principles

0012This term is applied to the removal of water from fresh

foods, mostly fruits and vegetables, by immersing

pieces of the food in a solution with a higher osmotic

pressure, and hence a lower water activity, than the

food. This solution is sometimes referred to in the

literature as the hypertonic solution. Water will pass

from the food into the solution under the influence of

the osmotic pressure gradient. In this process, the walls

of the cells in the food act as semipermeable mem-

branes. However, the membranes are not completely

selective. Some soluble natural substances, such as or-

ganic acids, sugars, salts, and vitamins, may be lost

with the water, while solutes from the solution may

penetrate into the food. This gain of solutes by the

food can contribute to the reduction in its water

activity, but may affect the taste of the product.

Osmotic Agents

0013The solutes used to make up the hypertonic solution

must be highly osmotically active, nontoxic, edible,

with an acceptable taste and flavor. In the case of

fruits, sugars are the solutes used to make up the

hypertonic solution. Sucrose is the most widely used

sugar, but glucose, fructose, glucose/fructose and glu-

cose/polysaccharide mixtures, high-fructose syrups,

and lactose have been used experimentally, with vary-

ing degrees of success. The addition of small amounts

of sodium chloride (0.5–2.0%) to the sugar solution

can enhance the rate of water loss. Other low-

molecular-weight substances, such as malic acid,

lactic acid, and even hydrochloric acid, can have a

similar effect. There is some evidence that the mole-

cular size of the osmotic solute can affect its penetra-

tion into the food. The smaller the size of the

molecule, the greater the depth and extent of

the penetration. For vegetables, sodium chloride is

the solute most widely used. Glycerol and starch

syrup have also been investigated.

Processing Conditions

0014In osmotic drying, the rate of water loss is high

initially but reduces significantly after 60–120 min.

However, it can take days for the process to reach

equilibrium. A typical processing time to reduce the

weight of the food by 50% is 4–6 h. Some researchers

1940 DRYING/Dielectric and Osmotic Drying

have reported that, while water loss falls off rapidly in

the first 2 h, the gain of solutes from the hypertonic

solution continues for longer. The initial concentra-

tion of solute in the solution can affect the rate of

water loss. Sugar concentrations in the range 40–70%

are used. In general, the higher the initial concentra-

tion of the sugar, the greater the rate and extent of

drying. Sometimes, the upper limit in concentration

may be dictated by the gain of solids, which can affect

the taste of the product, i.e., making it too sweet. In

the case of vegetables, sodium chloride concentra-

tions in the range 5–20% have been used. Again, at

the high concentrations, the taste of the product may

be adversely affected.

0015 The temperature at which the osmosis takes place

can affect the rate of water loss. Generally, increasing

the temperature will increase this rate. However,

there will be an upper limit on temperature above

which the food may be adversely affected. Cell walls

may be damaged resulting in excess loss of soluble

materials, such as vitamins, from the cells. Discolor-

ation of food may also occur. This upper limit on

temperature will be different for different foods. Tem-

peratures above 70



C are seldom used, and for some

heat-sensitive fruits, osmosis may be carried out at

room temperature, i.e., c. 20



0016 The size and shape of the food pieces can also affect

the osmotic process. The smaller the pieces, the

greater the surface area through which water loss

can occur and hence the higher the rate of water

loss. However, the smaller the pieces, the more cell

damage will occur during slicing. This can result in

excessive loss of soluble matter. The shape and size of

the pieces may be dictated by the form of the end

product, e.g., some fruits such as plums may be pro-

cessed whole or in halves. Apples and peaches may be

in slices or segments. Vegetables may be in the form of

slices or cubes. The literature suggests that the upper

limit on thickness is 10 mm.

0017 Movement of the osmotic solution over the surface

of the food pieces would be expected to increase the

rate of water loss. However, vigorous mixing of the

pieces is seldom desirable because it causes damage to

them. The food pieces may be immersed motionless in

the solution. To increase the rate, the solution may be

recirculated through the tank by means of a pump.

The pieces may be suspended in a vibrating basket in

the solution. Another option is to pump the solution

through a bed of the pieces contained within a tall

vessel.

0018 The weight ratio of food to solution can affect the

process, in terms of rate of water loss and solute gain.

In general, the lower this ratio, the higher the water

loss and the solute gain. Ratios of 1:4 to 1:5 are

commonly used.

0019Blanching, either in water or osmotic solution, will

prevent browning. In the case of large fruit pieces,

blanching can sometimes shorten the osmotic pro-

cess, due to relaxation of the structural bonds in the

fruit. For some small fruit pieces and for vegetable

pieces, blanching can adversely affect the osmotic

process, reducing water loss and increasing solids

gain. However, this effect is different for different

foods.

Recycling of Hypertonic Solution

0020Following the osmotic process, the isotonic solution

will be diluted with the water removed from the food.

In order to reuse this solution, its concentration has to

be increased to that of the original solution. This can

be done by adding more solute to the diluted solution.

However, this will result in a surplus of solution

which has to be used for some other purpose. An

alternative approach is to reconcentrate the solution

by vacuum evaporation. There will be a limit to the

number of times the solution can be recycled as some

darkening can occur. It is usually necessary to filter

the solution free of insoluble solids before evapor-

ation. It is necessary to monitor the microbiological

quality of the solution which is being recycled. There

can be a build-up of microorganisms, mainly yeasts,

in sugar solutions, if the processing temperature is

low. Pasteurization of the solution can overcome

this problem.

0021Since there is no change of phase during osmotic

drying, the energy requirements should be less that

in conventional drying methods. Heat is required

to dissolve the solute in preparing the hypertonic

solution, to maintain the temperature of the solution

and to evaporate water from the solution, if it is being

recycled. Additional energy is necessary to mix and

recirculate the solution. It has been reported that the

energy consumption during osmotic drying of fruits is

100–2400 kJ kg

1

of water removed, depending on

the processing temperature and how the surplus

solution is treated, compared to 5000 kJ kg

1

when

drying in heated air.

Further Processing of Osmotically Dried Foods

0022The amount of water removed in osmotic drying,

typically about 50% of the fresh weight of the food,

does not result in a shelf-stable product. The water

activity attainable in fruits and vegetables is usually in

the range 0.90–0.95. Consequently, osmosed prod-

ucts have to be processed further to obtain a product

with an extended shelf-life.

0023Air drying Osmotically dried fruits and vegetables

may be stabilized by further drying in heated air. The

DRYING/Dielectric and Osmotic Drying 1941

rate of drying of such products is usually lower than

that of fresh material and relatively low air tempera-

tures may have to be used to avoid caramelization

and case hardening. The sensory properties of such

products are different to those produced by air-drying

alone. They have a good color and flavor, with re-

duced acidity, and a soft chewy texture. Shrinkage is

usually less than in air-dried products.

0024 Vacuum drying Osmosed products may be stabil-

ized by vacuum drying. The quality of such products

can be superior to those completed by air drying but

the process, which is known as osmovac, is more

expensive.

0025 Freeze drying Predrying by osmosis can shorten the

drying time, and hence reduce, the cost of freeze

drying. The end products are less brittle and less

prone to browning than those freeze-dried without

the osmotic step.

0026 Freezing Freezing of fruits and vegetables is an

energy-intensive operation. By partly drying food

before it is frozen, freezing costs can be reduced.

This process is known as dehydrofreezing. The partial

drying prior to freezing can be achieved by osmosis.

When thawed, such products can have a better

texture, with less drip, than those frozen from fresh.

Unlike conventionally frozen foods, they require

rehydration after thawing.

0027 Solar drying It has been reported that osmotic

drying before solar drying can reduce the load on

the solar driers and enhance product quality. Solar

energy may be used to reconcentrate the solution

before it is recycled.

0028 The range of osmotically dried products available

commercially is limited to date. Some good-quality

fruit products are on the market. These are osmotic-

ally dried, followed by air or vacuum drying. How-

ever, there is considerable interest in this process

worldwide.

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

The Basic Process; Structural and Flavor (Flavour)

Changes; Drying: Equipment Used in Drying Foods

Further Reading

Brennan JG (1994) Food Dehydration – A Dictionary and

Guide. London: Butterworth-Heinemann.

Jones PL (1993) Alternative methods of industrial drying.

In: Encyclopaedia of Food Science, Food Technology

and Nutrition, vol. 3, pp. 1476–1480. London: Aca-

demic Press.

Lewicki PP and Das Gupta DK (1995) Osmotic dehydra-

tion of fruits and vegetables. In: Mujumdar AS (ed.)

Handbook of Industrial Drying, 2nd edn, vol. 1, pp.

691–713. New York: Marcel Dekker.

Schiffmann RF (1995) Microwave and dielectric drying. In:

Mujumdar AS (ed.) Handbook of Industrial Drying, 2nd

edn, vol. 1, pp. 345–372. New York: Marcel Dekker.

Physical and Structural

Changes

W F A Horner, University of Hull, Hull, UK

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001The concentration of solutes as drying proceeds has

consequences on the nature of foodstuffs: some of

these changes are reversible and some are irreversible.

The manner of drying, its conditions, and extent de-

termine this reversibility and, therefore, how closely

the characteristics of the rehydrated food resemble

the material from which it originated.

Physical Changes

0002Changes in color and texture, accompanied, in solid

structured materials, by shrinkage and deformation,

are expected outcomes of drying. As solutes concen-

trate in the milieu, osmotic gradients cause water

molecules to move out of their close association

with hydrophilic macromolecules. The latter tend to

aggregate: some, like starch and cellulose, to such an

extent that they became almost crystalline. Such close

aggregations of molecules are not easily dissociated

on rehydration. This helps to explain the frequent

observation, illustrated in Figure 1, that many foods

with the same water content have a higher water

activity-(a

) (which usually means greater suscepti-

bility to microbiological spoilage) when rehydrating

than when dehydrating. Although dehydration may

also cause changes in the structure of macromol-

ecules, e.g., the denaturation of proteins, experimen-

tal evidence has shown that many such denatured

structures hold more hydration water than undena-

tured structures. This is because the denatured form

often exposes hydrophilic groups not exposed in

the native form. (See Water Activity: Effect on Food

Stability.)

1942 DRYING/Physical and Structural Changes

000 3 It seems likely, therefore, that the major textural

changes in food, characterized by collapse of cell

walls and membranes and the change from the gel-

like to the fibrous, are due more to the purely physical

moving together of macromolecules than to denatur-

ation. Such aggregation may subsequently favor the

cross-linking between macromolecules via hydrogen

bonds and sulfur bridges, further restricting the

extent to which rehydration can occur. If meat, for

example, is overdried, down to the a

0.15–0.40

region, it reaches maximum toughness and elasticity.

Moreover, upon rehydration, muscle tissue dried in

air to this extent has lost its natural succulence. To

chew such material is to chew inert wetted fibers

which very quickly yield their absorbed water to

become an unpleasantly dry ball of fibers in the

mouth.

Texture Changes and Rehydratability

0004 The modulus of elasticity in vegetable tissue decreases

rapidly as, in drying, cells lose their turgor and the

plant wilts. In this way, the trend in texture for vege-

table tissue appears opposite to that in flesh foods as

decreases. It seems, therefore, that the textural

properties of foods, such as crispness, hardness, and

toughness, on drying, depend on their cellular nature.

Pretreatment with a glycerol solution, however, helps

to protect vegetable cell walls from disruption and

fracture during freeze and air-drying and allows the

tissue to return to its original shape on rehydration.

Untreated tissue remains partly shriveled on rehydra-

tion.

000 5 In dehydrated animal tissues, aggregation of the

actomyosin fraction causes the texture to become

tough and woody so that the flesh rehydrates slowly

andincompletely.Theformationofmethylene(-CH

linkages between myosin molecules was thought to be

responsible for the gradual toughening of fish muscle

texture during frozen storage. The plausible initiator

of such cross-linking was formaldehyde released

when trimethylamine oxide, naturally present in

marine and some fresh-water fish species, is enzyma-

tically broken down to dimethylamine (eqn (1)). For-

maldehyde from this reaction could react with certain

protein end-of-side-chain groups like the amino

group and possibly form methylene bridges between

protein molecules (Figure 2). Analytical evidence sug-

gests, however, that such methylene bridges between

protein chains occur rarely, if ever. Spoilage product

molecules, however, being much smaller than the

structural protein molecules, are much more mobile

and tend to react together, excluding water and caus-

ing irreversible texture changes. The muscle protein

myosin, if very fresh, can dry like a gel and can be

easily rehydrated. Myosin which is less fresh or is

dried under harsh conditions tends to dry as diffi-

cult-to-rehydrate fibers.

NðCH

O ! HNðCH

þ HCHO ð1Þ

0006Freezing is a form of dehydration and as such pro-

motes the macromolecule aggregation which hinders

the rehydration process. Nevertheless, it is used to

improve rehydratability in dried vegetables. Ice crys-

tal growth during freezing forces tissues apart and

leaves pores in the material upon drying. Thus,

being more porous, the dried food imbibes water

more quickly upon rehydration. Water-holding

capacity, however, is not improved. The use of cryo-

protectants in freezing reduces the tendency of

macromolecules to aggregate by holding more water

molecules around them. (See Freezing: Structural and

Flavor (Flavour) Changes.)

0007With the exception of freeze-drying, dehydration

processes cause shrinkage which is greatest during

the initial stages. If the rate is too fast, shrinking

HCH

fig0002Figure 2 Formation of a methylene bridge between two protein

molecules. Reproduced from Drying: Physical and Structural

Changes, Encyclopaedia of Food Science, Food Technology and Nu-

trition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Aca-

demic Press.

O content (%)

1.0

fig0001 Figure 1 Typical food sorption isotherm showing hysteresis.

Reproduced from Drying: Physical and Structural Changes, En-

cyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

DRYING/Physical and Structural Changes 1943

capillaries bearing water to the food surface soon

become blocked by solids carried in the flow. The

surface becomes dry and impermeable – a condition

known as ‘case hardening’–halting the drying pro-

cess whilst the interior is still wet. Flesh foods dried

thus decompose from the inside but, with some vege-

table foods, a high initial drying rate is advantageous

because the final volume is fixed early in the drying

process. As drying proceeds further, the tissues split

and rupture internally, forming an open microstruc-

ture. The product, in this case, has a low bulk density

and good rehydration characteristics. Such materials

subjected to low initial drying rates shrink inwards

progressively with process time, yielding a high bulk

density product which is difficult to rehydrate. (See

Freeze-drying: The Basic Process.)

0008 Liquids like milk and hydrocolloid solutions are

frequently spray- or roller-dried. The extremely

short contact time (microseconds for droplets of

10 mm diameter) required to dry milk to a powder in

a spray drier causes very little damage to components,

loss of nutrients, or destruction of microorganisms.

However, the powder produced, composed as it

would be of particles which are mainly very small,

does not easily or quickly reconstitute because surface

tension effects on such tiny particles prevent efficient

dispersal. To improve reconstitutibility, the fine

powder is often rehumidified, causing particles to

stick together, and redried as larger more easily

dispersible agglomerates. This process is referred to

as ‘instantization’.

0009 Roller-dried liquids and slurries are exposed to

high temperatures for a longer period than spray-

dried ones; indeed, the extra contact time may con-

tribute to ‘cooking’ the paste as well as drying it.

Instant or pregelatinized starch powder, which is

used as the main functional component of instant

pudding mixes, is manufactured in this way.

0010 Unlike the corresponding spray-dried version, the

roller-dried product tends to be coarse and more

dispersible. Thermal damage to the nutrients and

sensory characteristics of such products may be

minimized by carrying out the roller-drying process

in a vacuum chamber.

Flavor and Color Retention

0011 In spray drying, where there is a huge temperature

differential between feed material and drying

medium, the consequent large loss of volatiles close

to the point of liquid introduction to the chamber

may be reduced by the addition of an emulsified oil

phase to the feed which alters the droplet size distri-

bution and the pattern of flavor loss. Flavor oils

themselves, moreover, can be emulsified with suitable

hydrocolloids dispersed in water (continuous phase)

so that, on spray drying, the oil droplets are protected

from evaporation by the hydrocolloid ‘skin,’ which

instantly encloses them. This microencapsulation

process yields almost odorless flavor powders which

release their volatiles slowly – an advantage when

baking a cake, for example – or quickly when the

microcapsules are ruptured, as in ‘scratch-and-sniff’

perfume samplers.

0012The retention of the flavor components of a food

material during the drying process in unaltered form

is a major positive quality attribute for some dried

products. Consumers are prepared to pay a premium,

for example, on instant coffee expensively produced

by freeze-drying which, in spite of losses due to the

high vacuum, retains a greater proportion of the

highly volatile characteristic flavors than does spray

drying. (See Coffee: Instant.)

0013The loss of volatiles can be very serious in products

which depend significantly upon their aroma for ac-

ceptability. Even for milk there is evidence that the

loss, during drying, of volatiles such as methyl sul-

fides and the lower fatty acids may be responsible

for the lack of ‘fresh milk’ flavor in the dried product.

(See Sensory Evaluation: Aroma.)

0014Heat-induced reactions such as enzymatic and

nonenzymatic browning may lead to significant

changes in flavor as well as color. Such changes are

not always undesirable; indeed, they may be an essen-

tial product attribute as in the case of black tea com-

pared to green tea production. Enzymatic changes in

dried foods are dealt with in greater detail in the

following section, but control can be exercised by

selection of the variety and stage of maturity of the

material to be processed. In this way it may be

arranged that the starting material is as deficient as

possible in the substrate or enzyme involved in

browning. Blanching vegetables before dehydration

and excluding oxygen from the product, respectively,

destroy the enzyme and prevent the oxidation stage of

the sequence leading to browning. Sulfur dioxide acts

both as an enzyme poison and reducing agent and is

therefore frequently used to preserve the color of dried

fruits and vegetables. Because it is volatile, it is easily

driven off as the food is reconstituted and cooked.

Normally, sulfur dioxide is applied to vegetable matter

prior to drying through dipping them in a solution of

sodium metabisulfite for a period which yields a prod-

uct conforming to the maximum permitted limits for

that chemical in that specific material. (See Browning:

Nonenzymatic; Preservation of Food.)

0015Selection of the raw material can also play a sig-

nificant role in maintaining the desired color quality

of dried foods subject to nonenzymatic browning.

Potato crisps, if made from stored potatoes, where

1944 DRYING/Physical and Structural Changes

the starch has begun to break down to sugars prior to

germination, become too dark before water content

has been reduced by the frying process sufficiently to

give the required characteristic crisp texture.

0016 The Maillard nonenzymatic browning reaction

sequence is initiated when the carbonyl group of

reducing sugar reacts with the amine group of a pro-

tein peptide or amino acid. The dire effects of this on

the visual quality of spray-dried egg have required

that one of the reactants, glucose, be removed by

treatment with glucose oxidase, before the drying

process.

0017 Fish which has spoiled considerably before drying

contains substantially more of the mobile Maillard

reactants than fresh fish and so becomes an unsightly

brown color upon drying. Figure 3 shows how the

rate of browning is affected by the water activity of

the product. (See Oxidation of Food Components.)

Microstructure of Dried Products

0018 Dried foods may be particulate (e.g., potato powder),

glass/rubber-like (e.g., many sugar confections and

pasta), fibrous, or a combination of these.

0019 Particulate dry foods may range from simple crys-

talline chemicals, like salt and sugar, to amorphous

conglomerates of highly complex substances like milk

powder.

0020 In many instances, dried foods are eaten without

rehydration (e.g., dried fruits and sugar confections)

and their chewy, tough, or brittle texture contributes

to the pleasure of eating them. The size range of

simple crystalline substances like sugars is critical in

determining the smoothness of a fondant and the

brittleness/crunchiness of a biscuit.

0021The manner in which lipid material is distributed in

relation to the structural matrix (carbohydrate or

protein) and interstitial components affects what is

referred to as the ‘shortness’ in texture of the product.

The lipid interrupts the potential continuity of such

matrices which would otherwise lead to toughness

and elasticity. This ‘shortening’ facility may be used

to isolate relatively small units of glutenacious mater-

ial so that no continuum is formed, in the baking of

shortbread biscuits, for example, or to isolate layers

of glutenacious matrices from one another, as in

cream crackers and puff pastry.

0022Removal of water from foods inevitably leads to

the precipitation of solutes and the aggregation of

insoluble structural components – with consequent

effects on texture and rehydratability. Some of the

soluble and colloidal components of the food starting

material, gelatinized starch, for example, form a

glassy continuum within the food material as it

dries. This phenomenon is a highly significant feature

in the cook–extrusion of starchy materials. Indeed,

the viscous glassy continuum formed in the extruder

barrel momentarily holds the water, as it suddenly

turns to steam the instant the material emerges from

the exit die, and allows the extrudate to expand and

quickly solidify as the steam escapes. The hygrosco-

picity and continuity of such glassy components favor

very rapid rehydration – an unwelcome property with

respect to long-term storage in humid environments

for many cook-extruded products destined for the

snack-food market. The ‘melt-in-the-mouth’ charac-

teristic imparted by the products’ aeration and hygro-

scopicity is, however, its strongest sensory advantage

– although with some such products rehydration in

the mouth is so rapid that the product tends to cling

to the palate and the teeth. Packaging for these hygro-

scopic, aerated extruded products must provide an

efficient barrier against the ingress of water vapor

and – if, as is usual, the product contains a significant

proportion of fat – oxygen. The porosity of dried

foods greatly assists the process of oxidative rancidity

which is also promoted (Figure 3) by the removal of

the coating of water molecules around reactive sites

at very low a

. In many dehydration processes, an

attempt is made to maximize dried product porosity

(as in puff, foam-mat, and freeze-drying) to improve

dehydratability. Less controlled drying processes

often yield shrunken or case-hardened products

which, due to the capillary constriction, are very dif-

ficult to rehydrate. Porosity and rapid hydratability

are not always desirable properties in dried foods. In

Indonesia, fish may be dried, smoked, and pressed

alternately until a dry horny product (ikan kaju) is

produced. This product, which is grated over soups

as a condiment, resists rehydration from the humid

0.1

Water content (% dry-weight basis) ;

relative reaction rate ,

100

0.2 0.3 0.4 0.5

0.6 0.7 0.8 0.9

fig0003 Figure 3 Dried product storage stability (susceptibility to lipid

oxidation and Maillard browning) related to its water activity.

Squares, sorption isotherm; triangles, lipid oxygenation; circles,

Maillard browning. Modified from Labuza TP, Tannenbaum SR

and Karel M (1970) Water content and stability of low moisture

and intermediate moisture foods. Food Technology 24: 543–550.

DRYING/Physical and Structural Changes 1945