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

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1.0014 appearance, structure, size, and handling proper-

ties of the powder;

0015 easiness and completeness of reconstitution;

0016 appearance and lack of defaults of the reconsti-

tuted solution.

0017 Fat-containing powders such as whole milk, spray-

dried fats, some chocolate drinks and soup or gravy

mixes are difficult to reconstitute in cold water even if

they are agglomerated. Such powders are usually

covered by a thin layer of surface fat that makes

them water-repellent. Coating these powders with

small amounts of wetting agents such as lecithin dis-

persed in fat makes them wettable and cold-water-

soluble. This operation has in some instances been

termed ‘instantization.’ In fact, instantization is the

production of an instant powder that fulfills certain

predetermined standards and may be achieved either

by agglomeration or some other form of granulation,

lecithination, or both.

The Agglomeration Process: Molecular

Bonding and Growth

0018 Knowledge of the types of bonds generated between

agglomerating particles and the mechanisms of gran-

ule growth is of paramount importance in under-

standing the agglomeration process. Primarily, the

bonds formed between solid particles are due to elec-

trostatic and intermolecular attraction forces. In the

presence of a liquid, though, the distance between

centers of attraction is reduced and stronger bonds

are formed. Their manifestation is termed ‘liquid

bridges.’

Liquid Bridges

0019 As the name indicates, these are bonds formed be-

tween the liquid and the particles. They are strongly

dependent on the wetting angle, the adsorption char-

acteristics of the powder, and the amount and viscos-

ity of the binding liquid. When the particles are held

together by a low-viscosity liquid, owing to surface

tension forces and depending on the liquid saturation,

four different states are possible:

0020 Pendular state – the liquid is held in discrete

lens-like rings at the point of contact between the

particles. The air forms a continuous phase

throughout the agglomerate.

0021 Funicular state – at higher liquid saturations the

liquid rings coalesce to give a continuous liquid

phase throughout the agglomerate, in which dis-

crete pockets of air are entrapped.

0022 Capillary state – the pore space is completely sat-

urated with liquid but the liquid does not cover the

external agglomerate surfaces. A negative capil-

lary pressure is built up within the agglomerate,

which gives it a degree of tensile strength.

0023In addition to the above, if the liquid covers the

external surfaces of the agglomerate, the particles

become completely encapsulated within the liquid.

0024Use of liquids with a high viscosity usually results

in agglomerates of higher strength than is achieved

using low-viscosity liquids, depending on the fluid

cohesion strength and the fluid–solid adhesion force.

Solid Bridges

0025Liquid bridges hold the particles together during

granule growth. Upon drying, these usually result

in solid bridges through phase transformation, i.e.,

recrystallization of the dissolved material, hardening

of binder liquid or chemical reaction. In this way the

agglomerate comprises particles connected by solid

bridges, in which capillary pores are present. The

tensile strength of solid bridges is of the same order

of magnitude as that of the constituent particles.

Mechanisms of Growth

0026The overall mechanisms of granule growth determine

not only the rate of agglomeration but also the final

properties of the agglomerate. Although these mech-

anisms are very strong functions of the process equip-

ment used, the following steps are generally accepted:

0027nucleation of primary particles by random coales-

cence;

0028transition stage growth;

0029final ‘ball’ growth.

0030In the initial nucleation period, the feed particles

grow by rapid random coalescence. During the tran-

sition period, particles grow by preferred coalescence

between large and small granules (in the case of wide

particle size distributions) or by crushing and layering

(in the case of a narrow particle size distribution). The

growth rate passes through a maximum, which can

be perceived as the rate at which the diameter of the

granule increases to a size such that any further

increase is accompanied by a corresponding increase

in attrition crushing.

0031The principal variable used to control agglomer-

ation is the liquid saturation necessary to achieve the

surface tackiness, which leads to a greater propensity

of particle coalescence. Research has shown that in

many cases this critical moisture content is around

70–90% of that needed to saturate the voidage in the

powder being agglomerated.

0032The choice of equipment and process para-

meters, such as fluidization velocity and speed of

rotation, determine to a large extent the growth

74 AGGLOMERATION

mechanism, which for the same moisture content can

be based on coalescence or crushing and layering, or a

combination of both. This explains the different dens-

ities of agglomerated products obtained from the

same material and the same volume ratio of liquid

to solids but made on different equipment or of prod-

ucts manufactured on the same equipment but at

different agitation motions.

Characteristics of Agglomerated Powders

in Contrast with the Starting Materials

0033 It is the physical properties of fine powders that are

the source of problems in powder handling. Agglom-

eration is essential in order to modify these and

improve the usefulness of fine powders. There follows

a brief description of these properties and how they

are modified by agglomeration.

Shape

0034 The shape and surface roughness of particles dictate

the number of contact points between adjacent par-

ticles and the minimum proximity. The shape has a

pronounced effect on the flowability; for example,

particles that have a fibrous, highly asymmetric

shape can interlock and inhibit free flow. Agglomer-

ation changes the shapes of the particles dramatically

and, in general, creates more symmetrical particles

with fewer ‘cooking points’ and therefore improved

flowability.

Particle Size

0035 Particle size is a contentious property, because most

particulate matter has an irregular shape and its size

differs according to the method used to measure it. It

is usually defined as the ‘dimension of an imaginary

spherical particle having the same value in the phys-

ical property measured as the irregularly shaped

particle.’ Not all of the particles of a powder are the

same size, and a particle size distribution is usually

employed to characterize a powder. The particle size

distribution correlates well with many other material

properties and bulk characteristics.

0036Figure 1(a) and (b) shows two particle size distri-

butions. The first is typical of a powder resulting from

a milling process. It contains a large proportion of

very fine material, causing dust formation in dry

handling and wet lump formation during reconstitu-

tion. Furthermore, the vast range of particle sizes will

lead to acute flowability problems, time consolida-

tion behavior, variable density due to nonuniformity

of packing and, finally, segregation or compaction

during transportation. Thus, the effects of a wide

particle size distribution are spread across the whole

range of uses of powdered material. Agglomeration

changes the particle size distribution in two ways

(Figure 1b). First, it shifts the mean to a larger particle

size, thus eliminating all problems arising from the

presence of a very fine material. Second, it gives a

product with more uniform size, thus narrowing the

width of the particle size distribution curve, which

eliminates all the other problems outlined above.

Density, Porosity, Surface Area, Pore Size

Distribution

0037In the case of particulate matter, there are four types

of density used – true, particle, pouring, and tap

density. True density is the density of the material

when it does not include any interstitial pores; par-

ticle density is the density of the material when it is in

Weight (%)

125 175 250 350 500 750 1000

Size ( mm)

Weight (%)

125 175 250 350 500 750 1000

Size ( mm)

(a)

(b)

fig0001 Figure 1 Particle size distributions of (a) a fine powder showing a bimodal particle size range and (b) an agglomerate with a narrow

range of particle sizes. Reproduced from Agglomeration, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R,

Robinson RK and Sadler MJ (eds), 1993, Academic Press.

AGGLOMERATION 75

particle form and includes interstitial pores; pouring

density is the bulk density of the powder when it is

poured into a container; and tapped density is the

density measured after the container has been shaken

and the powder has consolidated. Both of the latter

include interparticle porosity. The difference between

the poured and tapped densities determines the

degree of consolidation and how much empty space

will be left on top of a jar filled with product at the

factory after it has been shaken through transporta-

tion. Density is also important during reconstitution.

After wetting, the density of particles determines how

fast they sink and whether they have time to dissolve

or be stabilized in suspension.

0038 The knowledge of surface area and pore size distri-

bution provides insight into the structure and adsorp-

tion potential of particulate matter. This is often

calculated from the particle size distribution and by

use of the mercury porosimetry method.

0039 In general, agglomerated granules are of larger size

with a lower particle or bulk density. Such granules

sink slowly into water and therefore can disperse

and enter solution before reaching the bottom of the

container. Agglomeration also increases the porosity

and, particularly, the formation of large numbers of

interconnecting micropores, which have potentially

large capillary pressures during wetting, i.e., liquid

is sucked into them by capillary action. It has been

shown that the total wetting time is a decreasing

function of porosity. Also, the presence of micropores

is instrumental in the spontaneous deagglomeration

process, which is essential for instant powders.

0040 It is of interest to contrast the reconstitution of

agglomerates with that of aggregates of fine powders.

Agglomeration produces an increase in the amount

of air between particles. During reconstitution air is

replaced by water. In fine powders the surface area is

large and the interstitial space small and usually not

interconnected. As a result, water penetrates slowly

and, after a short time, the space between particles is

filled with high-concentration dissolution products,

resulting in a sticky jelly with islands of unwetted

powder and residual air. Furthermore, lumps form,

which are wet and swollen outside, while dry inside.

These hamper further reconstitution (even under

strong agitation) because their structure is impervious

to water. In an agglomerate the large amount of

interstitial air results in the particles being dispersed

in the liquid before the highly viscous solution is

formed.

Cohesiveness

0041 To insure easy and successful handling during pro-

cessing, powders must be sufficiently ‘flowable.’

The factors that influence flowability include surface

properties, shape and size distribution, and the geom-

etry of the processing equipment. The forces that are

involved are gravitational, cohesive (between par-

ticles), and adhesive (between particles and walls).

0042Agglomeration produces granules that show a

marked improvement in the flowability properties,

which is qualified by a significant decrease in their

angle of repose. This occurs because, in general, there

is an increase of the mean particle size coupled with a

narrowing of the width of the particle size distribu-

tion curve.

Hygroscopicity

0043Hygroscopicity is the quantity that expresses the pro-

pensity of the material to pick up moisture, which

occurs when the equilibrium relative humidity of the

product is below atmospheric humidity. Therefore,

when nonagglomerated hygroscopic powders are

used in containers open to the atmosphere, the

powders tend to pick up moisture and form lumps

that are hard on the outside and enclose dry material.

These problems of caking and lumping are often

encountered in the discharge of powders both in

hoppers and in vending machines, particularly since

in the latter the atmospheric humidity is high. Anti-

caking agents are often included in hygroscopic

powders and their function is to act as moisture

scavengers and thus prevent moisture pick-up by the

powder. The equilibrium relative humidity is a func-

tion of product moisture; thus, if water is used as

an agglomerating medium it will at the same time

increase product moisture content and product

equilibrium relative humidity, decreasing the likeli-

hood of moisture pick-up and caking.

Wettability

0044The wettability of a material is a property of the

chemical composition of the surface of the material.

Changes in the surface structure of a powder can be

affected by the adsorption of a surfactant. The term

‘instantizing’ is often loosely used to describe a pro-

cess whereby agglomeration is followed by coating

with a surfactant, resulting in granules that are easily

wettable. Fat-containing powders such as ordinary

spray-dried whole-milk powder, chocolate drink

powder, or dried soup mix have poor wettability

and the object of the instantizing process is to im-

prove it. In the case of powder products, however, the

effective contact angle of the material in powder form

controls the wetting process. The effective contact

angle is a function of particle size, porosity, and mois-

ture content as well as material wettability. The

behavior of complex powders cannot therefore be

predicted by simple static contact angle measure-

ments. By measuring the height of wetted beds of

76 AGGLOMERATION

powders against time, however, it has been shown

that agglomeration decreases the effective contact

angle and the time required for bulk wetting provided

that the agglomerate size lies within certain limits

(0.4–1 mm for some powders).

Sensory Qualities

0045 A final consequence of agglomeration is the change of

the color and appearance of the product. These are

important qualities from the consumer’s point of

view and their change might improve or diminish con-

sumer appeal. For example, the color of an agglomer-

ated product may be denser and richer than that of the

original fine powder. Alternatively, a composite

powder may be agglomerated to give a product of

improved speckled appearance, which will also

have minimum separation problems and enhanced re-

constitution characteristics influenced by the most sol-

uble of its individual components. (See Sensory

Evaluation: Sensory Characteristics of Human Foods.)

Equipment and Processes used for

Agglomeration

0046 There are two main types of process used to agglom-

erate powders – the rewet process and the straight-

through process.

0047 The principle of the rewet process is the addition of

water, or water containing a binding agent, to a dry

powder, and then mixing of the particles in such a

way as to cause interparticle collisions to occur and

agglomerates to form. The particles of the powder

can occur either by addition of the liquid as a spray

(droplet agglomeration) or by condensation of water

from steam or humid air on to the surface of the

powder (surface agglomeration). The liquid may act

on the powder material in several different ways:

0048 It may dissolve part of it or transform it in such a

way as to make the surface of the particles sticky

or tacky and therefore make them adhere to each

other in clusters.

0049 If the fluid contains a binder in solution or in

suspension, this binder may be responsible for

the bonding function between the particles either

on contact with the particles or as the agglomerate

is redried and the binder crystallizes or precipi-

tates. This process is commonly used to agglomer-

ate inert materials.

0050 It may be a material with a relatively high melting

point, used as (1) or (2) above, but which on

cooling solidifies to form the interparticle bond.

0051 The straight-through process uses liquid feed as the

raw material. In the first stage, this is dried into a

powder, usually by spray drying, allowing a moisture

content a few percent higher than that anticipated

for the finished product. The particles thus formed

show an increased tendency to adhere to each other.

In the next stages of the straight-through process, the

particles are agitated in ways similar to the rewet

process.

0052In the next sections some of the types of plant used

in the rewet or straight-through process will be de-

scribed according to the mechanism used to promote

interparticle collisions.

Agitation Tumbling Equipment (

See

Agitation)

0053This category includes plant that provides agitation

by the rotation of a container around its central axis.

It can comprise either batch or continuous units and

these are mainly variations on the inclined dish

agglomerator or the rolling drum.

0054The essential element of the inclined dish agglom-

erator is a dish, which is rotated on an inclined axis.

The feed or recycled particles enter the dish and are

sprayed with water whilst undergoing rolling motion.

The relative positions of the input of feed material

and sprays influence the size and structure of the

agglomerates formed. Other features include a side

or bottom scraper. An important aspect of the dish

agglomerator is the pronounced size segregation that

occurs as a result of the centrifugal forces (Figure 2).

During continuous operation this allows the largest

particles to be discharged first while the smaller

particles have more time to grow. Therefore, a prod-

uct of narrow particle size distribution is obtained

and subsequent screening is unnecessary. Drying the

product evaporates the moisture, leaving a granular

product.

0055The rolling drum comprises a cylinder rotating

around its longitudinal axis, which is slightly inclined

to the horizontal so as to assist the continuous circu-

lation of materials through the drum. It normally

incorporates dam rings at either end in order to pre-

vent spillback and increase the bed depth in the drum.

The geometry of the drum makes it more suitable

than the dish for handling dusty materials with

minimum loss of fines, although most industrial

dishes are covered to prevent fines from escaping in

the atmosphere. The solids are wetted near the inlet

end of the drum and the wet powder cascades on to

the tumbling bed. Build-up of moist material is pre-

vented by incorporating a stationary scraping bar,

knockers that rap the walls, and reciprocating

scrapers, and by roughening the inner surface of the

drum.

0056An important design parameter affecting the per-

formance of the drum is the critical rotational speed

at which the material is carried completely around by

centrifugal action. At low speeds the material forms a

AGGLOMERATION 77

static bed and slides along the bottom with very little

relative motion between the particles. Cascading

takes place at around half the critical speed.

0057 With all agitation and tumbling equipment the

growth rate and final size depend on the residence

time, binder saturation, and bulk loading. The speed

of rotation, the angle of inclination, and the residence

time can be varied to affect the density and strength

of the granules produced. Sometimes mixers are

incorporated inside the dishes to promote further

particle collisions and break clusters in the case of

very sticky materials. Agitation tumbling equipment

has been used extensively in the chemical and min-

eral-processing industries but its use in the food sector

is not so widespread. Maintaining a clean, contamin-

ation-free environment can be a problem as most

available units are open to the atmosphere. Drying

of the granules can be done using rotary drum driers

or fluid beds. The large throughput and low cost of

these units should make their application financially

attractive in the future for food applications provided

that the contamination problems are solved.

Mixers/Agglomerators

0058 In this type of equipment water is sprayed on to a

powder while the latter is in a turbulent state or in the

form of a falling curtain. Shear processing leading

to the formation of granules can be carried out in

batch equipment including planetary mixers, ribbon

blenders, Z-blade units, and high-speed intensive

mixers. They rely on time and energy to effect the

necessary contact between powder and liquid phases.

All have their advantages for specific applications,

although final granule properties may be adversely

affected by excessive compression during prolonged

mixing and the batch-mixed material requires sieving

before drying in order to remove lumps.

0059In the NICA SYSTEM

turbine mixer/granulator,

the powder phase, accurately metered, enters the

inner section of a turbine, which deagglomerates the

material and throws it outwards as primary particles.

A gear pump feeds liquid binder to the underside of

the inner turbine, at the periphery of which it is

atomized into a mist. Mixing under high shear condi-

tions takes place in the outer turbine; the degree of

work and properties of the granule are controlled by

varying the area of the exit port. Contact time is thus

reduced to a fraction of a second. The resulting ag-

glomerate is fed directly into a close-coupled fluidized

bed and drying commences immediately. In this way,

time-dependent problems such as solubility of the

binder or lumping are eliminated.

0060The Schugi mixer/agglomerator (Figure 3) is a con-

tinuous device for blending small amounts of liquid

with solids so that the product remains free-flowing.

The mixing chamber consists of a 10-cm diameter

hollow vertical cylindrical tube with a shaft running

through its center. To the shaft are attached three

sets of six knife-edged blades, the relative positions

of which can be varied throughout the whole length

of the mixing chamber. The shaft belt is driven at

3000 rpm. Liquids are introduced into the mixer

through the jets, which are positioned above the top

set of blades. The Schugi mixer is mounted vertically

and solids are fed into the top. On entering, the solids

are caught into contraflow turbulence created by the

high peripheral velocity of the mixer blades. Liquids

are pumped into the mixer under pressure and enter

the mixing area as continuous threads of liquid. In the

mixer the liquids are atomized by the combined

Atomizing sprays

Agglomerates

Powder

fig0002 Figure 2 Particle size segregation in the dish agglomerator. Reproduced from Agglomeration, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

78 AGGLOMERATION

action of the top set of blades, which act in a similar

fashion to a disk atomizer, and also by the solids

themselves, which because of their high speed have

a high energy potential. The atomized liquid is

dispersed through the powder and the two adhere

together to form agglomerates, which leave by spiral-

ling around the walls of the mixer. The residence time

is very short, of the order of 0.5 s, and that limits the

temperature increase of the product to 2



0061The structure of the product formed depends both

on the relative sizes of the liquid droplets and the

powder particles and on the feed ratio of the two.

Apart from the physical properties of the materials

being mixed, the equipment control variables include

the rotational speed, the position of the blades, the

direction of rotation, and the size of the liquid injec-

tion jets. The agglomerated product is dried in a fluid

bed.

0062A third type of mixer agglomerator is referred to as

the ‘agglomerating tube.’ In this device agitation is

insured by dropping the powder into a high-velocity

stream of humidified air along the length of a tube.

The resulting powder clusters are separated in a

cyclone and then picked up by a hot air stream and

blown through a second tube for drying. A consider-

able amount of impact between particles takes place

inside the agglomerating tube, which makes it suit-

able for ‘difficult’ powders that tend to form sticky

deposits easily.

0063In another process the powder is passed through

a series of vibrating hoppers or conveyor belts and

falls in the form of a thin continuous curtain inside

the agglomerating vessel. Steam or atomized water is

then injected on to the curtain, which has the force

to redirect the traveling plane of the particles. The

induced turbulence makes the particles collide and,

because they are wet, adhere to each other. The

powder clusters formed are carried on to a drying

section.

0064It is evident that some of these units like the ‘falling

curtain agglomerator’ are ‘inhouse’ types of equip-

ment developed and modified to suit the particular

needs of specific products. Recently, they have been

superseded by the fluidized bed agglomerators, which

together with the straight-through process are the

‘state of the art’ agglomerators in use today. It is,

however, worth considering the use of mixing as the

first stage in an agglomerating process, especially in

cases where the nonwetting character of the powder

requires high shear to effect the initial moistening of

the material surface.

Fluid Bed Agglomerators – The Straight-through

Process

0065Fluid bed operations are characterized by:

0066good random mixing;

0067stable, uniform temperature profile leading to

effective interphase contact and controlled high

rates of heat and mass transfer;

0068high thermal efficiency, which allows use of low

exhaust air temperatures and therefore avoids

degradation of thermally sensitive food powder

components.

Powder

Atomizing sprays

Agglomerates

fig0003 Figure 3 The Schugi mixer. Reproduced from Agglomeration,

Encyclopaedia of Food Science, Food Technology and Nutrition,

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

Press.

AGGLOMERATION 79

0069 These exceptional properties of fluidized beds are

the cause of their almost universal use in agglomer-

ation processes – batch, continuous, spouted beds,

vibrofluidizers – or in straight-through combinations.

0070 The continuous rewet agglomerator usually con-

sists of three sections that may or may not be separ-

ated by weirs. Powder is fed from the feed hopper into

the first section of the fluid bed. Here it is rewetted by

means of steam, atomized water, or binder solution

delivered from jets positioned on top of the fluid-bed

layer. The powder particles increase in size either by

surface layering or random coalescence. Surface

layering is more likely to happen with low-viscosity

wetting agents which allow the moisture to distribute

evenly around the particles. ‘Blackberry’-type ag-

glomerates form when particle surfaces are made

just wet enough for collision of particles to lead to

capture. Typical moisture contents of the rewetted

powder in the first section are 6–10%. Too high

wetting will result in deposits in the bed and a

blocked distributor plate. Too low wetting will reduce

the level of agglomeration and produce less stable

agglomerates. Vibration is used to reduce channeling

and break powder clusters. The agglomerated mater-

ial passes into the second section, where it is dried in a

hot air stream. After drying it moves to the third

section for cooling. Feed particle sizes range between

30 and 300 mm. Agglomerates of 0.5–1.5 mm in size

are typically produced.

0071 Straight-through combination systems consist of a

spray drier in which liquid feed material is dried to

produce a powder at a moisture content slightly

higher than would be expected for a final stable

product (6–9%). The partially dried powder is then

fed to a number of static and/or vibrating fluid beds,

where final drying and cooling take place. Fine unag-

glomerated material is removed pneumatically and

taken back to the atomization zone in the spray

drier, where it is mixed with the atomized droplets.

Agglomeration occurs through contact of fine

powder and liquid feed in the atomization zone as

well as through interparticle collisions of the damp

particles in the fluid beds. The remainder of the oper-

ation consists of final drying to 2–5% moisture and

cooling and is similar to that in rewet fluid-bed ag-

glomeration. If a lecithination (instantization) stage

is required to be included to improve the instant

properties of the powder, this is usually carried out

by spraying a lecithin-in-oil solution on the agglom-

erates after the drying stage. Lecithination may be

carried out either by direct spraying on the fluidized

powder or in a powder trap between two fluid beds.

A conditioning fluid-bed stage is then usually added

to the process in order to distribute the lecithin

uniformly on the surface of the particles.

0072A number of such straight-through systems

differing in geometry and number of stages have

been produced by companies such as APV Anhydro

(Invensys), NIRO, and others. Powders produced

from the straight-through process are nondusty,

free-flowing, and with a particle size ranging between

250 and 500 mm. The process has been successfully

applied to a wide range of products where a liquid

raw material needs to be dried and transformed into

an instant powder. Its almost universal application is,

however, offset by the high investment cost.

Microbiological Hazards of Agglomeration

0073Few outbreaks of foodborne diseases have been

reported due to dried powders. Microbiological

hazards are usually associated with the liquid raw

materials and the emphasis of any manufacturer

should be to insure that adequate heat treatment

is given to any liquid product that is going to be

spray-dried and agglomerated. Tests for Salmonella,

Enterobacteriaceae (Escherichia coli, coliforms)

and Staphylococcus aureus should be carried out

routinely. There are, however, occasions where the

product may be contaminated after drying through

contact or handling. (See Food Poisoning: Tracing

Origins and Testing.)

See also: Agitation; Drying: Theory of Air-drying; Spray

Drying; Food Poisoning: Tracing Origins and Testing;

Sensory Evaluation: Sensory Characteristics of Human

Foods

Further Reading

James M, Brault M and Coucoulas L (1991) Lecithination –

Instant Character Dependence and Shelf-life Tests.

LFRA Research Report no. 689. Leatherhead: Leather-

head Food Research Association.

Pisecky J (1980) Instant whole milk powder. Australian

Journal of Dairy Technology 35(3): 95–99.

Pisecky J (1990) 20 years of instant whole milk powder.

Scandinavian Dairy Information 2: 74–78.

Retsina T and Coucoulas L (1988) Agglomeration of

Powders. LFRA Scientific and Technical Survey no.

164. Leatherhead: Leatherhead Food Research Associ-

ation.

Silliker JH, Baird-Parker AC, Bryan FL et al. (eds) (1988)

Application of the hazard analysis critical control ap-

proach (HACCP) to ensure microbiological safety and

quality. In: Microorganisms in Foods, vol. 4, Chapter 2,

pp. 22–43. Oxford: Blackwell Scientific.

80 AGGLOMERATION

AGING – NUTRITIONAL ASPECTS

N P Hays and S B Roberts, The Jean Mayer USDA

Human Nutrition Research Center on Aging at Tufts

University, Boston, MA, USA

Aging

0001 Life expectancy at birth in the USA is now 76.5 years,

compared with about 47 years at the beginning of this

century. Consequently, the proportion of the popula-

tion that is elderly, as well as the mean age of the

population, has increased. A continuation of these

trends is anticipated through the beginning of the

twenty-first century. One of the consequences of this

profound demographic shift is an increased aware-

ness that nutritional influences on health should be

optimized for the older population. This manuscript

is based on an earlier review by Roberts and Hays,

describing the nutritional requirements of older

people.

0002 Old age is a time when maintaining a good nutri-

tional status is a critical determinant of health – but at

the same time is often more challenging than it is

earlier in life. As described below, the body’s need

for some nutrients is actually increased relative to

that in earlier adult life, while the body’s subcon-

scious ability to regulate food and nutrient intakes

diminishes. For these reasons, the elderly are a

group particularly vulnerable to inadequate dietary

intakes. The consequences of excessive nutrient

intakes, particularly of energy, are also more severe

in older adults. Obese elderly persons are much more

likely to suffer disabling comorbidities associated

with obesity such as heart disease, osteoarthritis,

and reduced mobility, which in turn can lead to a

downward spiral of disability and frailty. Thus, it is

important to know the nutritional requirements of

elderly persons, and the food choices that can insure

adequate nutrition.

0003 Determining accurate recommended dietary allow-

ances (RDAs) for any age group is challenging.

Historically, RDAs have been focused towards pre-

venting nutrient deficiencies, whereas there is now

increasing recognition that increased or decreased

intakes of some nutrients may also have protective

functions against late-life chronic diseases such as

coronary heart disease and some cancers. The newest

update of the RDAs, now called dietary reference

intakes (DRIs), does in fact consider these protective

functions as a factor in setting nutrient recommenda-

tions. At the time of preparation of this manuscript,

1997 DRIs are available for calcium, magnesium,

phosphorus, fluoride, and vitamin D, 1998 DRIs are

available for thiamin, riboflavin, niacin, vitamin B

folate, vitamin B

, pantothenic acid, biotin, and

choline, and 2000 DRIs are available for vitamin C,

vitamin E, and selenium. Recommendations for all

other nutrients remain based on the 1989 RDAs

(new DRIs for the remaining nutrients are expected

to be released over the next several years). In some

cases, the increased intakes that appear to decrease

the risk of late-life chronic disease are unfeasible

without resorting to nutrient interventions through

fortification or supplementation. Despite increased

recommendations for several nutrients in the new

DRIs, the question of whether DRIs should recom-

mend levels of nutrients compatible with what can be

consumed through foods, or whether in some cases

pharmacological supplementation may be appropri-

ate, is an ongoing debate. Of particular relevance

to this issue is the fact that, in epidemiological studies

linking diet to disease, it is the consumption of

foods, rather than nutrients, that is actually meas-

ured. Metabolically active ‘phytochemicals’ in such

foods as fruits and vegetables may play important

roles in disease prevention and are not at the pre-

sent time understood well enough to be used as

supplements.

0004The determination of accurate and meaningful

DRIs is particularly difficult in the elderly population.

Whereas the majority of young adults are healthy, the

majority of elderly persons have one or more chronic

diseases or disabilities that affect nutrient uptake

and utilization and at the same time use multiple

medications to treat chronic diseases. If nutritional

recommendations are to be made not only for healthy

elderly persons, but also for sub-groups with different

health limitations, this vastly increases the complex-

ity of formulating accurate recommendations. More-

over, the objective of DRIs may change with adult

age, with increasing focus on maintaining current

health rather than preventing deficiency or future

disease. For example, a recommendation to consume

a low saturated fat diet to prevent later coronary

heart disease may be inappropriate for 85-year-old

men and women, who often find it hard to consume

enough food to maintain weight. Finally, the ‘elderly’

are not one group. Currently, DRIs provide specific

recommendations for individuals aged 51–70 and

> 70 years, which assumes that all individuals over

the age of 70 years are metabolically equivalent with

regard to their nutrient needs. This is certainly not the

case for several nutrients, and individuals in the age

AGING – Nutritional Aspects 81

range 70–80 years should be judged differently from

those aged 80–90 years or older.

0005 With these considerations in mind, this article

focuses on the effects of age on the body’s ability to

use different nutrients. When an increase in the intake

of a particular nutrient is needed to achieve the same

circulating level of the nutrient in the body or the

same level of adequacy, as judged by a particular

enzymatic activity, this is taken as evidence of an

increase in requirements. A more advanced treatment

of nutritional requirements in the elderly must await

the collection of data more sophisticated than is

currently available.

Energy Requirements

0006 Current recommendations on dietary energy intake

define expected average amounts of dietary energy

required for sustaining normal metabolic processes,

together with desirable or expected levels of physical

activity in healthy individuals. In weight-stable adults,

energy requirements are equal to total energy expend-

iture since body energy stores remain approximately

constant. Current dietary guidelines also note that not

more than 30% of dietary energy should be supplied

by fat, and only 10% or less supplied from saturated

fat. This recommendation applies to adults of all ages.

A further general consideration concerning energy

requirements is that body weight should ideally be

maintained within the body mass index (BMI, weight

in kilograms divided by the square of the height in

meters) range of 20–25 kg m

2

0007 Age, body weight, and the level of physical activity

are the primary determinants of energy requirements

for body weight maintenance in adults of all ages.

Concerning the effects of age, the older the person,

the lower are the energy requirements. For body

mass, the larger the body mass, the greater are the

energy needs of the body. Height is an important

determinant of body mass since taller persons tend

to weigh more than shorter persons. Physical activity

is also an important determinant of energy require-

ments. Table 1 shows the recommended energy re-

quirements of adult men and women of typical weight

expressed as ‘PAL’ (physical activity level) units,

which are energy requirements divided by the esti-

mated ‘basal metabolic rate’ (the minimum expend-

iture of the body when lying supine at rest at least

12 h after the last meal). PAL units are given for light,

moderate, and heavy levels of activity, and the basal

metabolic rate can be calculated using the equations

at the bottom of Table 1.

0008 As predicted by the US National Academy of Sci-

ences recommendation, elderly adults typically have

substantially decreased energy requirements relative

to young adults by 1255–2510 kJ per day (300–

600 kcal per day) (Table 2). This is thought to be for

two main reasons: (1) basal metabolism decreases

with age, owing to a reduced amount of lean tissue

(the actively metabolizing component of the body) as

well as a decrease in the amount of metabolic activity

per unit of lean tissue; (2) although not inevitable,

the capacity for physical activity is typically decreased

in older persons in concert with increased body

fat mass.

0009The estimated energy needs given in Tables 1 and 2

have been challenged as somewhat inaccurate. This is

tbl0001Table 1 Estimated energy requirements by physical activity

level

PAL PAL u nit s

Energyneedsat 65 years (kJper day)

Man (75 kg) Woman (60 kg)

Light activity 1.50 10 615 8 025

Moderate activity 1.65 11 678 8 828

Heavy activity 2.00 14 154 10 698

Estimated BMR of men (kJ per day)

Age 18–30 BMR ¼ (64.0  weight in kg) þ 2841

30–60 BMR ¼ (48.5  weight in kg) þ 3678

>60 BMR ¼ (56.5  weight in kg) þ 2038

Estimated BMR of women (kJ per day)

Age 18–30 BMR ¼ (61.5  weight in kg) þ 2075

30–60 BMR ¼ (36.4  weight in kg) þ 3469

>60 BMR ¼ (43.9  weight in kg) þ 2494

1kJ  0.239 kcal.

Physical activity level (PAL) ¼ basal metabolic rate (BMR) multiplied by

activity factor.

Based on values obtained from WHO (1985) Energy and Protein.

Requirements. Report of a joint FAO/WHO/UNU expert consultation.

Technical Report Series 724. Geneva: World Health Organization, with

permission.

tbl0002Table 2 Effects of age on estimated energy requirements

Age group

(years)

Body weight (kg) PAL units

Estimated energy

needs (kJper day)

Men

19–24 72 1.67 12 134

25–50 79 1.60 12 134

51þ 77 1.50 9 623

Women

19–24 58 1.60 9 205

25–50 63 1.55 9 205

51þ 65 1.50 7 950

Based on values obtained from National Research Council,

Subcommittee on the 10th Edition of the RDAs, Food and Nutrition Board,

Commission on Life Sciences (1989) Recommended DietaryAllowances,

10th edn. Washington, DC: National Academy Press, with permission.

Physical activity level (PAL) ¼ basal metabolic rate (BMR) multiplied by

activity factor.

1kJ 0.239 kcal.

82 AGING – Nutritional Aspects

because, based on research using doubly labeled water

to measure energy requirements (published after the

most recent recommendations were released), they

appear to underestimate the usual energy needs of

healthy men and women. The underestimation has

been suggested to be due to an underestimate of the

energy expenditure associated with normal daily

living. In a summary of recent energy requirement

studies in elderly persons, the measured PAL value

of older men and women averaged 1.65, compared

with the suggested recommendation of 1.51. This

change would mean that the energy requirements of

older men and women with typical body weights

would average 10 539 kJ (2519 kcal) and 8828 kJ

(2110 kcal) per day, respectively. Like the recom-

mended dietary allowances, however, the new re-

search confirms a substantial decrease in caloric

requirements with age.

Protein Requirements

0010 The 1985 FAO/WHO Committee on Energy and Pro-

tein requirements defines the protein requirements of

an adult as ‘the lowest level of dietary protein intake

that will balance losses of nitrogen from the body in

persons maintaining energy balance at modest levels

of physical activity’. Nitrogen balance is determined

in metabolic studies as the difference between meas-

ured nitrogen intake (from dietary protein which is

about 16% nitrogen by weight) and nitrogen losses.

The major routes of nitrogen loss – in urine and feces

– are measured, and miscellaneous nitrogen losses,

including those in hair, skin, sweat, and breath am-

monia, are assumed to be equal to 8 mg per kilogram

of body weight. Based on this nitrogen balance meth-

odology, it is estimated that the mean requirement for

good quality protein in adults is 0.6 g per kilogram

of body weight, and the recommended safe protein

intake (to reflect individual variability in require-

ments) is 0.75–0.80 g per kilogram of body weight.

0011 No change in protein requirements with age is

currently recommended, because of the limited and

conflicting information in the literature at the time

the most recent recommendations were developed.

However, it was recognized in the recommendations

that values of 0.75 and 0.80 g per kilogram of body

weight provide for higher intakes of protein per unit

of lean tissue in older individuals, because of the

reduced amount of lean tissue in this group. Thus,

even if elderly persons have an increased requirement

for protein relative to their amount of lean tissue, this

may be accommodated already within the current

RDAs.

0012 Recent studies have raised the question of whether

the current recommendations may underestimate

usual protein needs in elderly persons. In a review

and reanalysis of previous studies, protein require-

ments of elderly persons appeared to be increased to

> 1.0 g per kilogram of body weight per day. However,

this tentative observation was based on potentially

inaccurate assumptions including that miscellaneous

nitrogen losses from the body are the same in young

and elderly subjects (when they are probably lower in

the elderly), and that an equilibration period of only

5 days is sufficient to bring elderly persons into nitro-

gen balance when protein intake is adequate. Another

study found that protein requirements were un-

changed in healthy elderly women, compared with

younger women, and reduced in elderly men. This

controversy over whether the need for dietary protein

is increased in the elderly cannot be resolved without

further investigation.

Vitamin Requirements

0013RDAs and DRIs for vitamins in adults are shown in

Table 3. Different approaches were taken to deter-

mine these estimated vitamin requirements in adults,

including depletion–repletion studies conducted in a

metabolic ward, measurement of intakes required to

maintain constant body stores, and the activity of

enzymes requiring vitamins as cofactors. In studies

used to determine 1989 RDAs, there was a consistent

emphasis on the prevention of clinical or biochemical

deficiency as well as consideration of usual dietary

intakes, rather than long-term prevention of chronic

disease. Although an attempt was made to set the

1997, 1998, and 2000 DRIs at levels designed to

reduce the risk of chronic disease, the DRI commit-

tees judged that, except for calcium and fluoride, the

available scientific evidence was too weak to allow

reduction of disease risk to be used as a criteria for

setting recommendations. The RDA values listed are

derived by adding two standard deviations to the

estimated average requirements. The DRI values for

vitamin D and biotin, however, are based simply on

the observed mean nutrient intakes that appear to

sustain a population’s normal nutritional status.

0014The recommendations given in Table 3 are, for the

most part, the same for young and older adults –

again defined as less than and greater than 51 years

of age. The exceptions are vitamin D requirements,

which are set at a higher level in older persons based

on evidence suggesting that the capacity of human

skin to synthesize vitamin D decreases with age, and

vitamin B

requirements, which are also higher in

older persons based on a small number of studies

suggesting that the level of vitamin B

required to

normalize cofactor-dependent enzyme function and

excretion levels increases with age.

AGING – Nutritional Aspects 83