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

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equipment and the processing environment must be

maintained, cleaned, and sanitized to ensure that

microbial build-up and spread are prevented. Staff

must be trained in practices to maintain high stand-

ards of hygiene. A supply of good-quality water and

air for the process is also essential. Many factories

now have ongoing monitoring systems for Salmonella

and Listeria in place. If these bacteria are detected in

the processing environment, special clean-up regimes

and extra product testing are implemented.

0016 The modern approach to ensuring that milk

powder is microbiologically safe involves preventa-

tive management to ensure manufacture under appro-

priate conditions of hygiene. Many factories now

either have in place, or are moving towards, the

hazard analysis critical control point (HACCP)

system. Although end-product testing is still used to

verify compliance and to detect gross process failures,

it cannot be relied upon to ensure the safety of a batch

of product. Testing can be labor-intensive and time-

consuming, taking up to 7 days to obtain final results.

To overcome these problems, samples may be com-

posited and rapid techniques for detection of patho-

gens based on ELISA or DNA methods applied. These

methods have advanced efficiencies in testing, and

product can now be cleared in 24–48 h. (See Hazard

Analysis Critical Control Point; Quality Assurance

and Quality Control.)

Physical Properties

0017 The physical properties of milk powders are governed

by process variables, the type of dryer, and the com-

position of the milk. The physical properties of milk

powders play an important role in their use as food

ingredients. Their ability to be readily incorporated

into products and to perform specific functions in a

food formulation can be influenced by many physical

properties. There are many physical attributes that

must be taken into account when either evaluating

a current product, setting specifications for new

products or designing or modifying a drying system.

Tighter and more demanding specifications have

meant that powders are now often manufactured on

specialist dryers designed specifically to produce the

best possible product of defined specification.

Moisture

0018 The final moisture content is critical for several

reasons and is therefore defined in all powder specifi-

cations. It can affect functionality and microbio-

logical quality, and is an economic consideration in

the manufacture of powders. There are several factors

during manufacture that can influence the moisture

content of powders. These include the characteristics

of the concentrate fed into the dryer, the type of

atomization used, and the operating conditions

during drying.

Insolubility Index

0019The insolubility index of a powder is a measure of the

degree to which it can be readily solubilized in water

prior to use. It is related to the amount of sediment

obtained under defined conditions of mixing milk

powders. The main reason for loss of solubility is

the temperature of the particles during the primary

stage of the drying process where the majority of the

moisture is removed. During this stage, an imperme-

able crust can form on the particle surface that

severely restricts water removal, leading to the pro-

duction of case-hardened particles and subsequent

loss of solubility.

Bulk Density and Particle Density

0020Bulk density is the amount of powder by weight that

is present in a defined volume. It is usually expressed

as g ml

1

and is obtained by measuring the volume of

a fixed weight of powder after it has been tapped for a

defined number of times. A high bulk density is very

important in packaging and transportation, and is

desirable as it can significantly reduce costs. The

bulk density is influenced by a range of factors.

These include the amount of air entrapped in the

powder particles (occluded air), the overall density

of the particle (determined by the composition), the

air between the individual powder particles (intersti-

tial air), the particle size distribution and the particle

shape. The bulk density of powders is influenced by

dryer design and configuration (Tables 2 and 3). Par-

ticle density is the density of the solids (determined by

the composition), which determines the particle dens-

ity, together with the amount of occluded air.

Particle Size Distribution

0021The individual particles produced during drying can

vary greatly in size. The distribution of particle size

then can be further altered by the degree of agglomer-

ation or after grinding. An indication of the range of

particle sizes obtained from different dryer configur-

ations is given in Table 4.

Interstitial Air and Occluded Air

0022Interstitial air is the amount of air that exists between

particles or agglomerates as well as the air inside

porous agglomerates. The sphericity of the particles,

the particle size distribution, and the degree of ag-

glomeration determine the amount of interstitial air.

To obtain minimum interstitial air, the particles need

to be smooth, have a range of particle sizes, and be in

compact agglomerates.

POWDERED MILK/Characteristics of Milk Powders 4705

0023Occluded air is the amount of air entrapped within

the powder particles. It is affected by the preheat

treatment of the original milk, with a higher pretreat-

ment of milk resulting in less occluded air, and the

amount of air incorporated in the concentrate.

Higher total solids generally result in lower occluded

air. Powders atomized by a nozzle contain less air

than rotary atomized powders, despite improvements

to the modern rotary atomizers. Gentle drying also

reduces the level of occluded air, and therefore, the

use of multistage dryers is recommended for the pro-

duction of powders with low occluded air.

Flowability

0024With the ever-increasing diversity of use of milk

powders today, the need for properties such as

tbl0002 Table 2 Ability of various spray dryers to manufacture nonagglomerated and agglomerated skim milk powder with low or high bulk

densities

Typ e o f dr yin g

process

Chamber

configuration

Postprimary treatment Atomizer type Nonagglomerated

skim milk powder

Agglomerated

skim milk powder

Low BD

High BD

Low BD

High BD

Single stage Conventional None Rotary/nozzle Yes Yes No No

Tall form None Nozzle Yes Yes No No

Conventional Cooling bed Rotary/nozzle Yes Yes No Yes

Tall form Cooling bed Nozzle Yes Ideal No Yes

Two stage Conventional External bed Rotary/nozzle Yes Yes No Yes

Tall form External bed Nozzle Yes Ideal No Yes

Compact Integrated bed Rotary/nozzle Ideal Ideal No No

Three stage Compact Integrated bed þ external bed Rotary/nozzle Yes Yes No Yes

Multistage Integrated bed þ external bed Rotary/nozzle Yes Yes Ideal Ideal

Integrated belt Integrated belt Nozzle Ideal No Yes Yes

BD ¼bulk density; nonagglomerated skim milk powder: low bulk density  0.72 g ml

1

; high bulk density  0.72 g ml

1

; agglomerated skim milk powder:

low bulk density 0.30–0.50 g ml

1

; high bulk density 0.45–0.55 g ml

1

Adapted from Pisecky J (1997) Handbook of Milk Powder Manufacture, p. 79. Copenhagen: Niro A/S.

tbl0003 Table 3 Ability of various spray dryers to manufacture nonagglomerated and agglomerated full-cream milk powder with low or high

bulk densities

Typ e o f dr yin g

process

Chamber

configuration

Postprimary treatment Atomizer type Nonagglomerated

full-creammilk powder

Agglomerated

full-creammilk powder

Low BD

High BD

Low BD

High BD

Single stage Conventional None Rotary/nozzle Yes Yes No No

Tall form None Nozzle Yes Yes No No

Conventional Cooling Bed Rotary/nozzle Yes Yes No Yes

Tall form Cooling Bed Nozzle Yes Yes No Yes

Two stage Conventional External bed Rotary/nozzle Yes Yes Yes No

Tall form External bed Nozzle Yes No Ideal No

Compact Integrated bed Rotary/nozzle No Ideal Ideal No

Three stage Compact Integrated bed þ external bed Rotary/nozzle Yes Yes Yes No

Multistage Integrated bed þ external bed Rotary/nozzle Ideal Yes No Ideal

Integrated belt Integrated belt Nozzle Yes No No Yes

BD ¼bulk density; nonagglomerated full-cream milk powder: low bulk density  0.63 g ml

1

; high bulk density 0.63 g ml

1

; agglomerated full-cream

milk powder: low bulk density 0.30–0.50 g ml

1

; high bulk density 0.45–0.55 g ml

1

Adapted from Pisecky J (1997) Handbook of Milk Powder Manufacture, p. 79. Copenhagen: Niro A/S.

tbl0004 Table 4 Mean particle size obtained from dryers of different

configuration

Powder characteristics Dryerconfiguration Particle size

(mm)

Individual particles Concurrent with

pneumatic conveying

20–200

Tall form – tower 30–250

Flakes Roller dryer 200–5000

Loose agglomerate Mixed flow with

integrated fluid bed

100–400

– open structure Concurrent with

integrated fluid bed

100–200

Compact agglomerate Concurrent spray dryer

with integrated belt

300–2000

– porous structure Mixed flow with

integrated fluid bed

100–400

From personal communication (E. Refstrup), Niro A/S, Denmark.

4706 POWDERED MILK/Characteristics of Milk Powders

flowability is increasing. Powders are used in applica-

tions ranging from dispensing machines through to

the large-scale recombining operations that utilize

mechanical handling and dosing. For both agglomer-

ated and nonagglomerated powders, a better flow-

ability can be obtained by producing larger powder

particles with smooth and rounded particle surfaces

within a narrow particle size distribution. Flowability

is also influenced by other factors such as total fat in

the powder and the amount of ‘free fat.’

‘Free Fat’

0025 ‘Free fat’ in powder is defined as the fat fraction that

is extractable by organic solvents under specific con-

ditions of solvent type, time, and temperature of ex-

traction. In most instances, ‘free fat’ is considered a

defect. The exception is where ‘free fat’ is required for

a specific application, e.g., chocolate manufacture.

One of the most critical influences of ‘free fat’ is the

moisture content of the powder. If the moisture is too

low (< 2.5%), ‘free fat’ increases and then decreases

as the moisture content is raised from 2.5 to 4–5%

but increases again if the moisture content is > 6–7%.

Instant Properties

0026 Very fine powder particles are difficult to handle and

have poor reconstitution properties. Agglomeration of

powders allows water to permeate the powder particles

more readily, breaking up the agglomerate and

allowing the individual powder particles to dissolve.

Instant milk powder is highly soluble and designed to

reconstitute completely in water at both hot and cold

temperatures. The other properties required in instant

powders are wettability and dispersibility. The wett-

ability of a powder is measured by determining the

time taken for a given amount of powder to pass

through the surface of water. Wettability may be en-

hanced by lecithination. The dispersibility of a powder

is a measure of how completely a powder dissolves

under controlled conditions. Other tests carried out

on milk powders related to their instant properties

include slowly dispersible particles, coffee test, white

flecks number (minute particles that are seen on the

surface of reconstituted milk), and the sludge test.

Color

0027 The color of a powder is determined by composition,

preheat treatment, drying conditions, and particle

size distribution. Scorched particles can be a visual

defect that will often show up as deposits on the

bottom of mixing vats and in strainers.

Other Properties

0028 Apart from the properties described above, there are

others that influence a powder’s acceptability. These

include the mechanical stability of the powder, which

influences the degree of agglomeration breakdown

during transport and storage, hygroscopicity, which

is related to the degree of water attraction a powder

exhibits, and cakiness, an attribute that is a measure

of the extent to which a powder adheres to itself,

especially under compression. (See Agglomeration;

Drying: Spray Drying; Rheological Properties of

Food Materials.)

Chemical Characteristics

0029The chemical properties of milk powders are deter-

mined by the composition of the milk and the heat

treatment applied during powder manufacture.

Chemical Composition

0030Skim and full-cream milk powders are obtained by

dehydration of skim milk and full-cream milk to

 4% moisture. Full-cream milk is usually standard-

ized to a fat:solids-nonfat ratio of 1:2.67 to meet the

26% legal minimum fat content for this powder. The

protein content of skim milk powders may be stand-

ardized also. Variations in milk composition owing to

factors such as cow breed, feed, stage of lactation,

and season are reflected in the composition of milk

powders. The American Dairy Products Institute

standards for skim and full-cream milk powder com-

positions are as follows: skim milk powder should

have a maximum fat content of 1.25% and a max-

imum water content of 4.0%, whereas full-cream

milk powder should have a minimum fat content

of 26% and a maximum water content of 4.0%.

Control of the moisture content of milk powders to

a maximum of 4% is essential for good shelf-life

stability. Table 5 shows the range of values observed

in milk powder.

tbl0005Table 5 Composition of milk powders

Constituent Skim

milk powder

Full-cream

milk powder

Moisture (g per 100 g) 3–5 2–4

Fat (g per 100 g) 0.7–1.3 25–28

Crude protein (g per 100 g) 35–37 25–27

Lactose (g per 100 g) 49–52 36–38

Citric acid (g per 100 g) 1.8–2.1 1.3–1.4

Ash (g per 100 g) 7.5–8.0 6.0–7.0

Sodium (mg per 100 g) 400–550 370–420

Potassium (mg per 100 g) 1550–1750 1150–1350

Calcium (mg per 100 g) 1200–1300 900–1000

Magnesium (mg per 100 g) 110–140 85–100

Phosphorus (mg per 100 g) 950–1050 700–770

Chloride (mg per 100 g) 1100 750–800

Adapted from Walstra P and Jenness R (1984) Dairy Chemistry and

Physics, pp. 418–419. New York: John Wiley.

POWDERED MILK/Characteristics of Milk Powders 4707

0031 Another important indicator of milk powder qual-

ity is the titratable acidity of the reconstituted

powder. This is an indicator of the microbiological

quality of the milk. The American Dairy Products

Institute sets a maximum of 0.15% for titratable

acidity of skim milk powder.

Heat-treatment Classification

0032 The characteristics of milk powder can be influenced

by the heat treatment received by the milk powder

during manufacture. The time and temperature of the

preheat treatment affects the level of whey-protein

denaturation. The whey-protein nitrogen index,

which is a measure of the undenatured whey-protein

nitrogen in the powder and was developed by the

American Dairy Products Institute, is commonly

used to classify powders into low-heat, medium-heat

and high-heat milk powders. Typical preheat treat-

ments used for the manufacture of these powders are

listed in Table 6. As the composition of milk, includ-

ing the initial level of whey proteins in milk, can vary

with season, the same heat treatment can result in a

different whey-protein nitrogen index. Other

methods for heat classification of milk powders,

such as the heat number, cystine number, and thiol

number also may be used as a measure of the heat

treatment given to the milk during powder manufac-

ture. (See Heat Treatment: Ultra-high Temperature

(UHT) Treatments.)

Functional Properties of Milk Powders

0033 When milk powders are used as ingredients in food

applications, they contribute to the physical attri-

butes of the food. The ability of milk powders to

impart desirable properties to food is related primar-

ily to functional properties of milk-protein compon-

ents in the powders. These functional properties

include solubility, hydration, heat stability, viscosity,

gelling, foaming, and emulsifying. In milk powders,

the functional properties of the milk proteins may be

modulated by heat, ions, and other components. Heat

treatment of milk prior to concentration and drying is

the most common method used to alter the functional

properties of milk powders. Milk powders with the

same composition given different preheat treatments

prior to concentration and drying have different func-

tional attributes when used as ingredients.

Solubility

0034Solubility is a fundamental functional property that is

a prerequisite for most other desired functionalities.

The solubility of milk powders is dependent on pH.

Proteins have a minimum solubility at the isoelectric

pH, and solubility is increased on the acid and alka-

line side of this pH. Caseins, the major proteins in

milk, are least soluble at pH 4.6.

Hydration

0035Hydration is related to the ability of the milk proteins

to bind or entrap water. Caseins hold about 3.3 g of

water per gram, whereas undernatured whey proteins

hold 0.4 g of water per gram. Heat denaturation of

whey proteins increases the water holding to 2.5 g of

water per gram. Milk powder contains other com-

ponents, such as lactose, that bind water in addition

to the protein. Skim milk powders have a water sorp-

tion of 0.96–1.28 g water per gram, depending on the

conditions used during powder manufacture.

Heat Stability

0036Heat stability is an important property in certain

applications such as the manufacture of recombined

evaporated milk. Single-strength milks made from

low-, medium- or high-heat milk powders have a

similar heat stability to fresh milk. They are heat-

stable at the pH of milk (pH 6.7), being able to

withstand coagulation for up to *20 min at 140



However, for adequate heat stability of evaporated

milks under sterilization conditions (e.g., 120



C for

12 min), high-heat milk powders are necessary. Heat

stability is affected by the pH of the milk, mineral

content, and other components in the milk (e.g.,

lecithin, urea).

Viscosity

0037The viscosity of milks reconstituted from milk

powders is dependent on their state of dispersion,

concentration of solids, and temperature. Increasing

the concentration of milk solids increases the viscos-

ity. Decreasing the temperature increases the vis-

cosity, but heating milk to a temperature that results

in denaturation of whey proteins also increases the

viscosity.

tbl0006 Table 6 Heat classification of skim milk powder

Heat class Whey protein nitrogen index

(milligrams of undenatured

whey protein N per gram

of powder)

Preheat treatment

of milk

Low heat Not less than 6 72



C for 15 s

Medium heat 1.51–5.99 75



C for 3 min

High heat Not more than 1.5 90



C for 10 min

120



C for 2 min

From American Dairy Products Institute (1990) Standards for Grades of Dry

Milks including Methods of Analysis, Bulletin 916.

A range of other preheating conditions may be used to achieve a desired

whey protein nitrogen index.

4708 POWDERED MILK/Characteristics of Milk Powders

Gelation

0038 Milks reconstituted from milk powders have the abil-

ity to form gels under similar conditions to those

required for the formation of gels from fresh milk,

i.e., by rennet action for formation of rennet gels and

by acidification of milk under quiescent conditions.

Foaming and Emulsifying

0039 Milk powders can be used in applications where

foaming and emulsifying properties are required.

The ability of milk proteins in the milk powders to

stabilize foams and emulsions may be exploited when

these properties are required. (See Aerated Foods;

Emulsifiers: Uses in Processed Foods; Mixing of

Powders.)

Functional Requirements of Milk Powders

in Major Food Applications

0040 For milk powders to have the desired performance in

food applications, the functional characteristics of the

powders have to be matched to the application. This

requires an understanding of the required functional

properties of the milk powder ingredients in the target

application.

Milk Powders for Recombined Dairy Products

0041 A significant amount of milk powder is used in the

manufacture of reconstituted and recombined dairy

products. In these applications, the milk powders are

combined with water and milkfat to reestablish the

fat:solids-nonfat:water ratio of milk or other dairy

products. Some of the major applications of milk

powders in the recombination industry are for the

preparation of pasteurized fluid milk, UHT milk,

cream, evaporated milk, sweetened condensed milk,

yogurt and cultured dairy products, recombined

cheese, and icecream. Different functionalities of the

milk powder ingredients are needed in these various

recombined dairy products. Table 7 lists the major

functional requirements of milk powders for recom-

bined dairy products.

0042 Pasteurized milks and UHT milks These products

have a similar composition to fresh milk. For pasteur-

ized milks, low-heat or medium-heat powders are

used to obtain a flavor similar to milk and to minim-

ize heat-induced flavors. In the case of UHT milks,

any type of powder can be used, as single-strength

milks made from low-, medium-, or high-heat

powders are stable to UHT conditions.

0043 Evaporated milks It is essential to use high-heat

powders for this application to obtain evaporated

milk with the desired viscosity. A high-preheat treat-

ment improves the heat stability of a recombined milk

concentrate (typically 26% total solids; 18% solids-

nonfat: 8% fat) to in-can sterilization conditions used

in its manufacture. Additionally, high-heat powders

are screened using heat-stability tests to ensure

that they withstand sterilization without excessive

thickening or coagulation.

0044Sweetened condensed milk This is a traditional

dairy product containing 74% total solids (20%

milk solids nonfat: 8% fat: 46% sucrose). The most

important physical attribute of this product is its

viscosity. Low- and medium-heat powders are used

in this application. Milk powders given a high-heat

treatment (e.g., 85



C for 30 min) cannot be easily

processed, because the high viscosity of concentrates

made from these powders also results in rapid age

thickening during storage of the product. There are

a number of viscosity tests that may be used as indi-

cators of suitability of powders for sweetened con-

densed milk manufacture.

tbl0007Table 7 Functional requirements of milk powders in recom-

bined dairy products and selected processed foods

Product Functionalproperties

required inmilk

powder

Heat treatment

of milkpowder

Pasteurized milk Good flavor Low–medium heat

Emulsifying

UHT milk Good flavor Low–medium–high heat

Heat stability

Emulsifying

Cream Good flavor Low–medium heat

Emulsifying

Evaporated milk Heat stability High heat

Viscosity

Sweetened

condensed

milk

Viscosity Low–medium heat

Yogurt Water-binding Low heat

Viscosity

Gelling

Cheese Rennetability Low heat

Icecream Foaming/whipping Low–medium–high heat

Emulsifying

Confectionery Water-binding High heat

Foaming/whipping

Emulsifying

Heat stability

Bakery Water-binding High heat

Foaming/whipping

Emulsifying

Gelling

If a low-heat milk powder is used, the yogurt milk has to be given a high-

heat treatment during yogurt manufacture. Alternatively, a high-heat milk

powder may be used, in which case, the yogurt milk requires only a low-

heat treatment to pasteurize the milk during yogurt manufacture.

POWDERED MILK/Characteristics of Milk Powders 4709

0045 Yogurt Milk powders may be used as a partial or

total replacement for fresh milk in this application.

Viscosity development, gelling, and good water-

binding properties are necessary for the production

of high-quality yogurts. These properties are obtained

in yogurt by preheating the yogurt milk at a tempera-

ture that causes significant denaturation of whey

proteins (e.g., 90



C for 10 min). Low-heat milk

powder may be used if a high-heat treatment is

given during yogurt manufacture. If a high-heat

milk powder is used, the yogurt milk requires only a

low-heat pasteurization treatment during yogurt

manufacture.

0046 Cheese Only low-heat milk powders are suitable for

recombined cheese manufacture. This ensures good

rennetability of the reconstituted milk. With a high-

heat treatment of milk, there is association of the

denatured whey proteins with the casein, which

hinders the reaction of the rennet.

0047 Ice cream Milk powders contribute to the flavor

and texture of ice cream. The milk powder aids

in the emulsification of the ice cream mix and has a

role in the development of an aerated matrix. (See

Condensed Milk; Evaporation: Basic Principles;

Uses in the Food Industry; Recombined and Filled

Milks.)

Milk Powders for Selected Food Applications

0048 Milk powders are used as functional ingredients in a

number of processed foods.

0049 Chocolate and confectionery products Milk powders

contribute to the flavor, color, and texture develop-

ment in chocolate and confectionery applications.

The emulsifying properties of the milk proteins influ-

ence the miscibility of the ingredients used in choc-

olate and confectionery, hence influencing flow

properties and texture. In confectionery products

such as toffee, good water-binding properties of

milk proteins contribute to the texture of these prod-

ucts. The Maillard reaction, which is the reaction of

the amino groups of the proteins with reducing sugars

in the formulation, is responsible for color develop-

ment and for the production of caramelized flavors;

the lactose in milk powders participates in the Mail-

lard browning reactions.

0050 Bakery products High-heat milk powders are useful

in bakery applications. In addition to enhancing the

nutritive value of cereal-based baked goods, milk

powders contribute to the texture and flavor of these

products. Their emulsification and foam-stabilization

properties and their ability to participate in the

Maillard browning reaction are important require-

ments in bakery applications.

0051Other applications The functional properties of

milk powders also make them useful in a number of

other applications, such as processed meat products,

soups, gravies, and dips.

Characteristics of Stored Milk Powders

0052The characteristics of milk powder are dependent on

the quality and composition of the raw milk and the

manufacturing process used during its manufacture.

However, even if milk powders are manufactured to

meet the desired standards and specifications,

changes in the properties of milk powders may

occur during storage and distribution. The compos-

ition of the powder, the type of packaging material

used, and the conditions of handling and storage

influence the shelf-life of the powder.

0053Deterioration of milk powders resulting from

Maillard browning, lactose crystallization, and

oxidation of fat may lead to flavor and physical

defects in the powder. It may also affect the function-

ality of the milk powder when it is used in a

food product. Some of the changes that may occur

during storage include the development of a

brown color, a reduction in pH, reduced solubility,

development of off-flavors, and reduced heat stability

of powders.

See also: Biofilms; Condensed Milk; Emulsifiers: Uses

in Processed Foods; Evaporation: Basic Principles; Uses

in the Food Industry; Heat Treatment: Ultra-high

Temperature (UHT) Treatments; Mixing of Powders;

Pasteurization: Principles; Rheological Properties of

Food Materials; Powdered Milk: Milk Powders in the

Marketplace; Quality Assurance and Quality Control;

Recombined and Filled Milks

Further Reading

American Dairy Products Institute (1990) Standards for

Grades of Dry Milks Including Methods of Analysis,

Bulletin 916, rev ed. Chicago, IL: ADPI.

Early R (1998) Milk concentrates and milk powders. In:

Early R (ed.) The Technology of Dairy Products, 2nd

edn, pp. 228–300. London: Blackie Academic & Profes-

sional.

International Commission on Microbiological Specifica-

tions for Foods (1998) Microorganisms in Foods 6,

Microbial Ecology of Food Commodities. London:

Blackie Academic & Professional.

International Dairy Federation (1984) General Code of

Hygienic Practice for the Dairy Industry and Advisory

Microbiological Criteria for Dried Milk, Edible Rennet

4710 POWDERED MILK/Characteristics of Milk Powders

Casein and Food Grade Whey Powders, Bulletin No.

178. Brussels: International Dairy Federation.

International Dairy Federation (1990) Recombination of

Milk and Milk Products, Special Issue No. 9001. Brus-

sels: International Dairy Federation.

International Dairy Federation (1991) IDF Recommenda-

tions for the Hygienic Manufacture of Spray Dried Milk

Powders, Bulletin No. 267. Brussels: International Dairy

Federation.

International Dairy Federation (1999) 3rd International

Symposium on Recombined Milk and Milk Products,

Special Issue No. 9902. Brussels: International Dairy

Federation.

Knipschildt ME and Andersen GG (1994) Drying of milk

and milk products. In: Robinson KR (ed.) Modern Dairy

Technology, 2nd edn., vol. 1, pp. 159–254. London:

Chapman & Hall.

Masters K (1997) Spray dryers. In: Baker CGJ (ed.) Indus-

trial Drying of Foods, pp. 90–112. London: Blackie

Academic & Professional.

Mettler AE (1994) Present day requirements for effective

pathogen control in spray dried milk powder production.

Journal of the Society of Dairy Technology 47: 95–107.

Pisecky J (1986) Standards, specifications, and test methods

for dry milk products. In: MacCarthy D (ed.) Concen-

tration and Drying of Foods: Proceedings of the Kellogg

Foundation 2nd International Food Research Sympo-

sium (Cork: 1985), pp. 203–220. London: Elsevier

Applied Science.

Pisecky J (1997) Handbook of Milk Powder Manufacture.

Copenhagen: Niro A/S.

Walstra P and Jenness R (1984) Dairy Chemistry and Phys-

ics, pp. 418–419. New York: John Wiley.

POWER SUPPLIES

Use of Electricity in Food

Technology

D Graham, R and D Enterprises, Walnut Creek, CA,

USA

Introduction

0001 The food industry in the USA consumed 17.4 billion

kcal in 1999. Fifty-eight percent of total food industry

energy costs are from electricity. The industry pur-

chases 64.0 billion kWh of electricity annually at a

cost of $3.36 billion. Purchased energy is 1.3% of the

value of product shipments for the food industry

compared with 1.7% for the total US industry.

0002 Major uses of electricity include freezer and re-

frigeration compressors, conveyors, air handling,

pumping, lighting, process controllers and monitors,

and packaging forming and sealing. Natural gas is the

primary source of thermal energy for ovens, fryers,

dryers, evaporators, and boilers, with relative costs

being the main reason. Closely related uses of electri-

city in agricultural food production include crop

irrigation, pest control, produce disinfection, grain

harvesting and storage, weed reduction, livestock

waste management, and fish farming.

0003 Irradiation, ohmic heating, microwave processing,

ozonation, freeze concentration, nonthermal pasteur-

ization, and the use of heat pumps are examples of

relatively new electrotechnologies that the food indus-

try may apply increasingly in the future. Application of

these technologies provides opportunities for food

processors to improve operating efficiencies and

helps insure the quality and safety of processed food

products.

Electricity Use by Food Industry Sectors

0004Food energy costs rose steadily over the past two

decades. In 1996, purchased electricity comprised

over half of the total energy used in food processing.

Electrical consumption by grain milling, meat pro-

cessing, preserved fruits and vegetables accounts for

23, 19, and 13%, respectively, of the total food indus-

try use of electricity. Some food processors operate

cogeneration facilities using surplus heat to produce

electricity, but approximately 92% of total electricity

used by the food industry in the USA is purchased

from utility companies. Approximately 87% of the

electricity is used by motor-driven equipment such

as compressors, pumps, mixers, grinders, fans, etc.

Table 1 compares the cost of electricity with total

energy costs in food processing. Electricity consump-

tion has increased approximately 3% annually from

48.9 billion kWh in 1986 to 69.1 billion kWh in

1999. Table 2 shows the trends in electric energy

cost from 1980 to 1996.

New Electrotechnologies for Food

Processing and Preservation

0005Electron beam irradiation, X-ray, microwave process-

ing, membrane separation technology, ozonation,

ohmic heating, high pressure pasteurization, infrared

POWER SUPPLIES/Use of Electricity in Food Technology 4711

radiation, laser, ultraviolet light, freeze concentration,

and pulsed electric fields are developing technologies

that offer future potential for improvements in food

safety, quality, and more efficient processing.

Irradiation of Food Products

0006 Treatment of a food product with a controlled source

of irradiation can be very effective for improving the

safety of the food product, since the irradiation can

destroy any viable food pathogens that may be pre-

sent. Food products are not made radioactive when

irradiated, and no toxic byproducts are formed.

Irradiation has been used successfully on a variety

of products, including cereals, fruits, vegetables,

prepared foods, dairy products, meat and poultry,

beverages, spices, and seasonings. Products can be

irradiated after being sealed in packaging material,

which reduces the possibility of contamination by

handling after treatment.

0007 Safety of irradiated foods Controversy has sur-

rounded the use of food irradiation since its discovery

in the late 1940s. Much concern has resulted from

fear that the irradiation process might make the food

radioactive. Treatment of food with gamma irradi-

ation does not inject radioactivity into the food.

High-energy electrons pass through the food product

and, if sufficient energy is applied, kill microorgan-

isms that they impact. In approving food irradiation,

the Food and Drug Administration (FDA) reviewed

over 400 toxicity studies, including animal feeding

studies before issuing any approvals. The FDA con-

cluded that food irradiated up to 1 kGy is ‘‘whole-

some and safe for human consumption, even where

the food that is irradiated may constitute a substantial

portion of the diet . . . ’’

0008Electron beam irradiation may provide an effective

way to control harmful E. coli 0157:H7 and other

emerging pathogens in ground beef and other pro-

cessed meat products such as pork sausage and re-

frigerated poultry. Electron beams (gamma rays) from

machine sources up to 10 million electron volts (10

MeV) and X-rays generated from machine sources up

to 5 MeV are permitted commercially. Similar ener-

getic rays also can be produced by decay from sealed

units of radioactive sources such as cobalt-60 or

cesium-137. Gamma rays are used effectively for ster-

ilizing a variety of products including food packaging

materials, syringes, bandages, and other heat-sensi-

tive pharmaceutical products. X-rays also are effect-

ive in some applications. One kilogray is 1 J of energy

absorbed per kilogram of the substance being irradi-

ated. The FDA has approved electron beam radiation

providing absorbed doses of radiation from 0.2 to

7.0 kGy, depending of the particular food product.

Table 3 lists the radiation levels approved by the

FDA for food products.

Microwave Heating

0009Microwave ovens found extensive use in home food

preparation and in commercial food service during

the past two decades. Industrially, microwaves are

used primarily for tempering, thawing large blocks

of frozen meat, and preheating. Studies of microwave

application for sterilization of food products have

shown limited success.

0010Microwaves are electromagnetic waves similar to

TV, radio, light, radar, or infrared waves, with the

main difference being the frequency of the wave

motion. Food products contain molecules such as

water, salts, and proteins that have dipolar properties.

Microwave energy passing through a food product

causes these dipolar molecules to vibrate. The resulting

internal friction produces heat. Thus, the food prod-

uct is heated from within, and heating time is greatly

reduced compared with heating by external applica-

tion of heat. This rapid heating provides improved

food quality and energy savings in many applications.

tbl0002 Table 2 Growth of electricity use in food processing as a

percentage of the total energy cost

1980 1986 1994 1996

Total food 41.0 51.8 57.4 58.0

Meat products 51.4 59.6 69.2 68.5

Dairy products 47.3 58.8 66.1 64.3

Preserved fruit/vegetables 37.3 52.1 55.8 57.1

Grain mill products 44.0 53.0 57.5 59.8

Bakery products 43.3 47.9 55.7 56.6

Sugar/confectionery n/a 35.9 34.9 41.7

Fats and oils 26.8 40.1 42.5 39.4

Beverages 43.1 52.1 61.1 59.1

Miscellaneous 48.0 54.8 53.8 59.1

Source: Annual Survey of Manufactures. US Dept. of Commerce, with

permission.

tbl0001 Table 1 Electricity vs total energy used in food processing

Electricity

($ million)

Total

energy

($ million)

Electricityas

a percentage

of thetotal

Total food industry 3364 5799 58.0

Meat products 614 884 69.5

Dairy products 401 632 62.4

Preserved fruit/vegetables 459 804 57.1

Grain mill products 683 1142 59.8

Bakery products 241 426 56.6

Sugar/confectionery 177 424 41.7

Fats and oils 180 456 30.8

Beverages 345 584 59.1

Miscellaneous 464 447 59.1

Source: 1996 Annual Survey of Manufactures, US Dept. of Commerce, with

permission.

4712 POWER SUPPLIES/Use of Electricity in Food Technology

0011 Microwaves are in the radio frequency portion of

the electromagnetic spectrum between 300 and

300 000 MHz. Microwaves are generated by a mag-

netron that converts electrical energy into an electro-

magnetic field with centers of positive and negative

charges that change direction billions of times a

second. In the USA, the Federal Communication

Commission limits commercial microwave sources

to 915 and 2450 MHz.

0012 Microwave heating is used for cooking or partial

cooking of meat, chicken, other prepared foods, rapid

thawing of frozen meat, poultry, and seafood, drying

of pasta, fruits, and cereal products, freeze drying of

meat and juices, and, pasteurization of milk, yogurt,

beer, and wine. Table 4 shows the penetration of

microwave energy into a variety of food substances

at different temperatures.

Ohmic Heating

0013 Ohmic heating describes an innovative process in

which an electric current is passed directly through a

food product to generate heat by internal resistance

without any need for intermediate heat exchangers.

The electric energy is applied from electrodes inserted

into the product and is converted directly to thermal

energy within the food product with an overall con-

version efficiency of 90%.

0014Food products sterilized by ohmic heating undergo

less damage to flavor and color as compared with

conventional heating methods. Electrical conductiv-

ity of the food product influences the flow of electric

current through the food and can cause uneven heat

distribution. This has been troublesome with some

prepared foods that contain ingredients of varied

composition, size, and shape. Ohmic heating can pro-

cess a wide variety of pumpable products including

stews, dairy sauces, eggs, custards, soups, diced fruits,

pie fillings, etc. Microorganisms present in the food

are killed by heat, just as in conventional heat pro-

cessing. The temperatures required to assure commer-

cial sterility are identical to the temperatures required

in conventional heat processing. The ohmic advan-

tage stems from the rapid temperature increase due

to internally generated heat, making conductance

of heat through the food to the center of the mass

unnecessary, and the 90% conversion of electrical

energy to heat. Conventional heat exchangers achieve

only about 50% transfer of heat to the food product.

Figure 1 shows a typical electrode assembly for ohmic

heating.

0015A typical 75-kW commercial unit capable of

heating 750 kg h

1

uses a 5-cm-diameter electrode

housing. A 300-kW, 3000 kg h

1

unit requires a 10-

mm-diameter housing. A basic ohmic heating system

typically consists of four electrodes in series con-

nected by short spacer tubes. The top and bottom

electrodes are connected to one phase of a three-

phase transformer and are grounded. Each of the two

middle electrodes is connected to one of the other

phases of the transformer. The voltage required for

heating the product is varied by adjusting the primary

side of the transformer.

tbl0003 Table 3 Food irradiation processes approved by the FDA

Yearapproved Food product Dose (kGy) Purpose

1963 Wheat and wheat flour 0.2–0.5 Disinfestation of Insects

1964/5 White potatoes 0.05–0.15 Inhibit sprouting

1983 Spices and seasonings 30 max. Disinfestation of insects

1985 Pork, fresh unheat-processed 0.3–1.0 Control of Trichinella spiralis

1985/6 Dry of dehydrated enzymes 10 max. Control of insects and pathogens

1986 Fruit 1 max. Disinfestation and delay ripening

1986 Fresh vegetables 1 max. Disinfestation of insects

1986 Herbs and spices 30 max. Control of microorganisms

1986 Vegetable seasonings 30 max. Control of microorganisms

1990 Poultry, fresh or frozen 3 max. Control of microorganisms

1995 Meat, frozen and packaged (limited to use by NASA) 44 minimum Sterlization

1995 Animal feed and pet food 2–25 Control of Salmonella

1997 Red meat, uncooked, chilled 4.5 max. Control of microorganisms

1997 Red meat, frozen 7.0 max. Control of microorganisms

tbl0004 Table 4 Microwave energy penetration

Material

temperature (



Half power depth (cm)

915 MHz 2450 MHz

Ice 12 1600 780

Water 2 4 0.6

Water 45 14 2

Water 75 21 3

Water 95 30 5

Beef, lean 51 70 46

Beef, lean 18 10 8

Beef, lean 4 2 2

Beef, lean (freeze-dried) 18 550 190

POWER SUPPLIES/Use of Electricity in Food Technology 4713

Membrane Technology

0016 Membrane technologies separate fluid mixtures in

many different applications. They can separate ma-

terials based on size, electrical nature, physical prop-

erties, and are used widely in the food processing and

chemical industries, as well as in water treatment. All

membrane processes are electrically driven, using

high-velocity/high-pressure pumps.

0017 This technology originated in the early 1930s with

cellulose acetate membranes, and has developed rap-

idly as new and improved media have been created.

The arsenal of membranes now includes a wide range

of polymeric membranes, ceramic, and sintered stain-

less steel. Among many applications are: concentra-

tion of fluid products; sterilization by removal of

bacteria or viruses; removal of constituents such as

proteins, acids, and fats from process streams; desal-

ination; purification; demineralization; refining of

oils; and clarification.

0018 Membrane separation could be considered as a

modern, highly efficient, versatile form of the trad-

itional filtering process. The objective is still the same,

i.e., to remove one component such as small solid

particles from a second component such as a liquid

or to separate a given molecular component from a

solution.

0019The efficiency with which separations can be made

and the wide range of components that can be pro-

cessed with membranes are noteworthy. Separations

can be based on particulate size differences of a less

than 1 mm down to a molecular level. Separation

depends on the passage of specific molecules through

a semipermeable membrane while the membrane

retains other molecules.

0020In traditional simple filtration, the osmotic pres-

sure due to differences in concentration on the two

sides of a membrane, a differential pressure, and/or

gravity were the only driving forces. In membrane

processes, pumps apply high pressure and high

cross-flow velocity to the solution being filtered to

speed up the separation process. High-velocity turbu-

lent flow at the membrane/liquid interface continu-

ously cleans the membrane surfaces and extends their

useful life. In some instances, when strongly highly

ionic systems are being filtered, an electrical potential

will be applied to accelerate the process and increase

selectivity of the separation process.

0021Membrane separations typically are classified into

subtechnologies based on the pore size of the filter

media. Common classifications include microfiltra-

tion (coarsest), ultrafiltration, nanofiltration, and

reverse osmosis (finest). Figure 2 illustrates the

tremendous range and flexibility of the filtration

spectrum. Particle sizes are measured in microns and

alternatively stated as Daltons. (MWCO, molecular

weight cutoff).

0022The ability to concentrate delicately flavored solu-

tions such as fruit juices or dairy products without the

application of heat provides the food processor with

the potential to significantly improve product quality.

The ability to separate components selectively from a

product stream or effluent stream can lead to the

creation of new products, allow recycling or reclaim-

ing of valuable ingredients, or simplify and reduce the

cost of effluent disposal.

0023Membrane filtration has been commercially used

in the USA since about 1930, primarily in the water

and beverage industries. Widespread applications and

use were initially limited due to the lack of practical

membranes. Early membranes tended to clog easily,

were hard to clean, and had short process lives. De-

velopment of new polymeric membranes, or ceramic

membranes in the 1980s provided the food processor

with filtering systems with greater durability, greater

efficiency, far more versatility and lower costs. These

membranes have a higher capacity, are less prone

to clog, are easier to clean, and withstand multiple

Out

Electrode

assembly

Plastic-lined

stainless steel

pipe

Feed in

Electrode

Electrodes

Electrode

fig0001 Figure 1 Typical electrode assembly. Drawing courtesy of

APV.

4714 POWER SUPPLIES/Use of Electricity in Food Technology