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

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0004 Lactose may appear in different crystal poly-

morphs, depending on the crystallization conditions.

Each polymorph has its specific properties. As an

example, a-lactose (which crystallizes as a hydrate

containing equimolar amounts of lactose and water)

has very hard crystals and is not hygroscopic, as with

b-lactose. However, a hydrous b-lactose (which crys-

tallizes at temperatures above 93.5



C) forms small

crystals with a solubility up to 10 times higher than

that of a-lactose. Because of this higher solubility,

b-lactose is experienced sweeter than a-lactose.

0005 The solubility of a-lactose is more temperature-

dependent than that of b-lactose, and the solubility

curves intersect at 93.5



C. A solution at 60



Ccon-

tains approximately 59 g of lactose per 100 g of water.

Therefore, if a 50% solution of lactose (approx. 30 g

of b- and 20 g of a-) at 60



C is cooled to 15



C, then

at this temperature, the solution can contain only 7 g

of a-lactose or a total of 18.2 g per 100 g of water at

equilibrium. Therefore, a-lactose will crystallize very

slowly out of solution as irregularly sized crystals,

which give rise to a sandy, gritty texture.

0006 a-andb-lactose differ considerably in solubility

and in the temperature dependence of solubility. If

a-lactose is brought in water, much less dissolves at

the outset than later. This is because of mutarotation:

a-lactose is converted to b, hence the a-concentration

diminishes, and more a can dissolve. If b-lactose is

brought in water, more dissolves at the outset than

later (at least below 70



C): on mutarotation, more

a-lactose forms than can stay dissolved, and a-lactose

starts to crystallize.

0007 The solution of lactose is temperature-dependent,

and solutions are capable of being highly supersatur-

ated before spontaneous crystallization occurs, and

even then, crystallization may be slow. In general,

supersolubility at any temperature equals the satur-

ation (solubility) value at a temperature of 30



higher. The low solubility of lactose, coupled with

its capacity to form supersaturated solutions, is of

considerable practical importance in the manufacture

of concentrated milk products. This is of prime im-

portance in sweetened condensed milk, and crystal

size must be controlled if a product with a desirable

texture is to be produced. (See Carbohydrates: Clas-

sification and Properties.)

0008 Usually, a-lactose crystallizes as a hydrate contain-

ing one molecule water of crystallization. The crystals

are very hard, slightly hygroscopic, often fairly large,

and dissolve slowly. The water of crystallization

is very strongly bound. Above 93.5



C, anhydrous

b-lactose crystallizes from an aqueous solution.

b-lactose is not very hydroscopic, and it dissolves

quickly; its solubility is good. Therefore, dehydrating

a-hydrate is difficult. It may cause problems when

determining the dry matter of milk and milk prod-

ucts; this determination implies evaporation of water

at elevated temperatures. Maintaining the tempera-

ture > 93.5



C during the assay is paramount to pre-

vent the formation of a-lactose hydrate crystals.

0009Amorphous lactose is formed during rapid drying,

as in a spray drier. It is present in the glassy state,

which means that many properties, including hard-

ness, density, and specific heat, are similar to those of

crystalline sugar but that the packing of the molecules

does not show perfect order. Amorphous lactose con-

tains at least a few percent of water and can quickly

dissolve on addition of water. But then, a-lactose

hydrate may start to crystallize. If the water content

of the amorphous lactose is low, say 5%, crystalliza-

tion is postponed. However, the product attracts

water from moist air, and when moisture content

rises about 8%, a-lactose hydrate starts to crystallize

(at room temperature). The postponed crystallization

is an important factor in relation to spray-dried

powders made from skim milk or whey because it

leads to hard lumps in the powder, and eventually,

the whole mass of powder turns into one solid cake.

0010This caking is caused by the crystallization of lac-

tose, as it causes the powder particles, largely consist-

ing of lactose, to grow together (to sinter). Since

water is needed for crystallization of a-lactose, caking

does not occur at a low a

, below 0.4. At a higher

temperature, crystallization can occur far more read-

ily, a

being higher; moreover, the viscosity of the

highly concentrated lactose solution (essentially the

continuous phase of the powder particles) is lower,

leading to a more rapid nucleation, hence crystalliza-

tion. The susceptibility of caking, especially higher in

whey powder, is considerably reduced if most of the

lactose is crystallized before the drying (in the concen-

trate). Such precrystallized powder is usually called

‘nonhygroscopic,’ which may be a misnomer because

the powder concerned does not attract less water (this

is determined by its a

in relation to that of the air),

but the effects differ.

Sensory Properties and Digestibility

0011Lactose is a disaccharide that yields d-glucose and d-

galactose on hydrolysis. Lactose occurs in both a- and

b-forms, which differ in the steric configuration on

the H and OH around the C-1 glucose. Lactose is not

as sweet as sucrose or fructose. For example, 1.08,

2.2, and 4.91% sucrose are of equivalent in sweetness

to 3.75, 7.5, and 15% equilibrium lactose solutions.

b-lactose is 1.05–1.22 times as sweet as a-lactose, but

this small difference in sweetness is of no practical

value because equilibration of the anomers occurs

rapidly in solution.

LACTOSE 3473

0012 The low sweetness of lactose compared with other

saccharides may be due to a weaker binding of the

molecule to the tongue papillae when compared with

fructose, sucrose, or glucose. To take advantage of the

fact that the b-form is sweeter than the a-form,

methods have been developed to convert the mono-

hydrate a-form to the b-form by treatment with

methanol and sodium hydroxide. However, when

the b-form is in solution, mutarotation occurs

quickly, and the solution becomes a mixture of a-

and b-anomers and must be utilized in quick dissolv-

ing applications if only the b form is specified.

0013 The sweetness of lactose syrups is increased with

partial hydrolysis to form a solution of galactose,

glucose, and lactose. The three sugars have a syner-

gistic effect on sweetness when mixed in solution.

0014 Hydrolysis of lactose to glucose/galactose mark-

edly changes the two properties of greatest commer-

cial importance in sugar, i.e., sweetness and solubility.

The hydrolyzed lactose syrup is expected to find

applications in nonalcoholic beverages, ice cream,

yogurt, salad dressings, jams, pectin jellies, toffees,

fudge, and boiled sweets. The technology exists for

the commercial conversion of glucose (from starch

hydrolysis) into fructose via immobilized glucose

isomerase. Thus, using two linked immobilized

enzyme reactors, it is possible to produce sweet

glucose–fructose–galactose syrup from lactose:

Lactose

β-galactosidase

glucose + galactose

glucose isomerase

glucose + fructose + galactose

0015 Low levels of intestinal lactase activity occur in

many otherwise healthy adults and children in vari-

ous population groups. Current evidence indicates

that these low levels are normal for most populations

of the world, notable exceptions being peoples of

northern European extraction: approximately two-

thirds of the world’s adult population are believed to

be lactose malabsorbers. Individuals who have genet-

ically acquired low lactase levels as adults were able

to drink milk without any ill effects as infants.

0016 Clinically identifiable lactose malabsorption

evolves over time and may be classified into three

distinct periods: the initial period is characterized by

a decline in lactase activity through childhood; the

second phase in this continuum is noted when the

decline in lactase activity is sufficient to result in a

level of undigested lactose quantitatively high enough

to cause symptoms associated with the ingestion of a

lactose load; and the third phase occurs when the

level of lactase declines to the point that it is no longer

sufficient to digest even the modest levels of lactose

usually consumed.

0017 Diarrhea, intestinal cramps, and abdominal swell-

ing are the symptoms experienced by individuals with

lactose malabsorption when foods containing milk

products are consumed. The unhydrolyzed lactose

passes to the large intestine and is fermented to lactic

acid by microflora, contributing to diarrhea. People

experiencing lactose intolerance should limit their

consumption of milk and milk products or select

lactose-modified milk. (See Food Intolerance: Lactose

Intolerance.)

0018Lactose-hydrolyzed milk is a viable product of

enzyme technology, but its manufacture must be con-

ducted optimally in terms of adaption to milk pro-

cessing and lactase efficiency. Possibilities for the

widespread production of lactose-modified dairy

products have been opened up by the development

of commercial sources of the enzyme lactase. Lactose-

modified nonfat dry milk may be manufactured for

the use as a food ingredient, and cultured products

such as yogurt may be prepared successfully from

lactase treated milk. Lactose-modified whey may be

condensed to high-solid noncrystallizing syrups for

use in soft drinks, confections, and ice cream.

0019A rare, and possibly fatal, condition of lactose

intolerance is galactosemia, in which an infant is

found to have a deficiency of galactose-1-phosphate

uridyltransferase, which converts galactose to glucose

for absorption. Infants with this condition must be

diagnosed early and given substitute milk products

that have fructose or sucrose added for energy and

sweetening properties.

Isolation and Purification

0020The principal raw material for lactose production is

cheese whey, although casein may be used. With the

application of new membrane processing techniques

in dairy technology, ultrafiltration permeate from the

production of cheese and whey protein is now becom-

ing a major dairy waste in the USA, Canada, Europe,

and New Zealand. In membrane processing oper-

ations, the problem of waste is reduced almost

entirely to milk salts and lactose. Expansion in the

use of ultrafiltration will depend largely on the prof-

itable use of permeate. A general solution to the prob-

lem of utilizing permeates rests with the human food

industry.

0021The whey protein byproduct of lactose serves as a

functional ingredient for other foods. The recovery of

lactose and protein from whey resolves two prob-

lems: (1) an economical source of a functional ingre-

dient and (2) the utilization of a product (whey) in

abundance from cheese production that otherwise is a

disposal problem. (See Whey and Whey Powders:

Protein Concentrates and Fractions.)

0022The manufacture of lactose is similar to that

of sucrose. It is basically a five-step process:

3474 LACTOSE

concentration, coagulation and removal of whey, fur-

ther concentration, crystallization, and recovery of

crystals (usually by centrifugation). The crystals are

usually washed and may be dissolved and recrystal-

lized to yield a product of high purity. The normal

form of commercial lactose is a-monohydrate, but

there is a limited market for b-lactose that is pro-

duced by crystallizing above 93.5



C. Commercial

lactose is available in a number of grades.

0023 The addition of b-galactosidase to milk for cheese

manufacture shortens the time needed to produce

cheese. The hydrolysis of lactose by the enzyme

stimulates the action of the microorganisms by con-

verting the monosaccharide to glucose and galactose,

which are more readily available for fermentation.

However, this limits the suitability of whey for lactose

recovery and poses a problem of disposal of whey that

cannot be used for lactose recovery. (See Cheeses:

Chemistry and Microbiology of Maturation.)

Food Applications

0024 One of the oldest uses of milk sugar is as a constituent

of baby food to adapt cow’s milk to the composition

of human milk. Due to its flavor-enhancing properties

and low sweetening powers, lactose is primarily used

now in numerous foodstuffs. Particularly worth

mentioning here are: baby food, cakes and biscuits,

chocolate and chocolate products, sugar confection-

ery, soups, and sauces.

0025 The presence of lactose can affect a number of prop-

erties of confectionery systems that can have implica-

tions for the processibility of the system and for the

overall product characteristics, such as flavor, mouth

feel, texture, color, and stability. The changes observed

when lactose is used in confectionery formulations

would be expected to be a function of (1) the concen-

tration, type and physical characteristics of the lactose,

(2) the function and physical characteristics of the in-

gredient that is substituted by lactose, and (3) the pro-

cessing parameters used in the products’ manufacture.

0026 Typically, at least 10% lactose can be used in most

systems as a part-substitute for sucrose or skimmed

milk powder without altering the textural acceptabil-

ity of a product to an unsatisfactory degree, whereas

up to 20% substitution is possible in certain cases. The

tendency for lactose to crystallize and grain the con-

fection appears to be a factor limiting the use of higher

levels in many cases, whereas the difference in viscosity

behavior compared with sucrose also inhibits the

preparation of textural attributes similar to products

containing sucrose. These observations, however, sug-

gest that modifications to the manufacturing proced-

ure to account for these properties could enable even

higher levels to be incorporated successfully.

0027Human milk contains *7.0% of lactose (bovine

sim5.0%) and 1.0% of protein (bovine

sim3.5%).

Most baby formulas are essentially ‘humanized’

cow’s milk, i.e., the lactose content is increased and

the protein content reduced. Most of the lactose used

in baby food preparations is added in the form of

demineralized whey powder. Lactose may be used as

an additive in powdered foods as a free-flowing

agent. The hygroscopicity of lactose glass may be

exploited to advantage as a means of absorbing free

moisture from low moisture foods and fixing it as

water of crystallization of lactose. For example,

food particles may be coated in a solution of flavoring

and/or coloring materials and then the coated par-

ticles tumbled in a powdered lactose glass that

absorbs surface moisture and forms a capsule around

the food particles. Lactose glass is used in ‘instantiz-

ing’ or increasing the dispersibility of certain foods.

Products are prepared containing 5–50% lactose,

spray-dried, and then ‘instantized’ by moistening

and redrying. Alternatively, powdered lactose glass

is added to the powdered food, which is then wetted

slightly, causing the lactose to crystallize and form

agglomerates that entrap the other food components.

Such products are free-flowing and capable of disper-

sing rapidly, similar to instant milk powders.

0028Lactose is used extensively in the formulation of

various types of candy, fudges, and caramels. It has

certain advantages in these products, e.g., less sweet-

ness, better color binding and better mouth-feel par-

ticles, but it also has the disadvantage of tending to

crystallize and causing grittiness. Lactose, along with

sucrose, may be used to sugar-coat chocolate buttons

and hazelnuts: the lactose suppresses sucrose crystal-

lization, thereby allowing the coating to be effected at

relatively low temperatures, and reduces the sweet-

ness of the coating.

0029Lactose is difficult to crystallize and can be a dis-

advantage in food applications. a-Lactose has a very

low solubility and cannot be used in partially pre-

pared food products that are rehydrated without

heating such as instant puddings. Large crystals may

form in the manufacture of ice cream when the mix is

cooled to a low temperature. The result is a sandy

texture or crystals large enough to be felt by the

tongue and palate. The problem can be overcome by

inhibiting crystal formation by the addition of gums

or by seeding with additional crystal nuclei to keep

the crystal size small. (See Ice cream: Methods of

Manufacture.)

0030The concentration of lactose in sweetened con-

densed milk is critical in protecting the texture of

the milk product. Condensed milk has 60% of the

water removed and sucrose added to produce a prod-

uct with a total carbohydrate concentration of 56%.

LACTOSE 3475

Too much lactose results in a product that is grainy,

and too little lactose produces a product that is slimy

in texture. (See Condensed Milk.)

0031 Lactose is used in the bakery and confectionary

industries, where it reacts with protein amino groups

to give Maillard browning, for color and flavor.

Lactose enhances the emulsifying and creaming

properties of shortenings, which improves product

quality, facilitates baking operations, gives increased

loaf volume and external appearance score, and

extends the shelf-life freshness by 50–100% com-

pared with the shelf-life of standard formulations.

Lactose may also be used in sugar coating and as a

humectant in bakery products. If proper precautions

are taken, lactose is a very suitable icing sugar for

many purposes: it gives a reduced sweetness and

better body with less chipping and cracking.

0032 The vulnerability of lactose in the Maillard

browning reaction is a problem in the manufacture

and storage of dried milk and dried whey powder.

Both products brown when stored at ambient tem-

perature, making them undesirable in terms of solu-

bility for rehydration and nutritional value. Dried

milk that has browned is difficult to reconstitute,

and browned milk is deficient in available lysine, the

e-amino group of which takes part in the Maillard

reaction. Dried milk and whey solids should be stored

under dry, cool conditions and rotated when in stock

to avoid browning during storage. (See Browning:

Nonenzymatic.)

See also: Browning: Nonenzymatic; Carbohydrates:

Classification and Properties; Cheeses: Chemistry and

Microbiology of Maturation; Condensed Milk; Ice Cream:

Methods of Manufacture; Whey and Whey Powders:

Protein Concentrates and Fractions

Further Reading

Aquilar CA, Hollender R and Ziegler GR (1994) Sensory

characteristics of milk chocolate with lactose from

spray-dried milk powder. Journal of Food Science 59:

1239–1243.

Aquilar CA and Ziegler GR (1995) Viscosity of malted milk

chocolate with lactose from spray-dried whole milk

powders. Journal of Food Science 60: 120–124.

Arvanitoyannis I and Blanshard JMV (1994) Rates of crys-

tallization of dried lactose–sucrose mixtures. Journal of

Food Science 59: 197–205.

Delaney RAM, Donnelly JK and O’Sullivan AC (1973)

Lactose and reverse osmosis. In: Birch GG and Green

LF (eds) Molecular Structure and Function of Food

Carbohydrate, pp. 155–179. New York: Wiley.

Fox PF and McSweeney PLH (1998) Dairy Chemistry and

Biochemistry, pp. 21–66. London: Blackie Academic

and Professional.

Hartel RW and Shastry AV (1991) Sugar crystallization in

food products. Critical Reviews in Food Science and

Nutrition 1: 49–112.

Kedword CJ, MacNaughton W, Blanshard JMV and

Mitchell JR (1998) Crystallization kinetics of

lactose and sucrose based on isothermal differential

scanning calorimeter. Journal of Food Science 63:

192–197.

Morrissey PA (1985) Lactose: chemical and physico-

chemical properties. In: Fox PF (ed.) Developments in

Dairy Chemistry – 3, pp. 1–35. London: Elsevier

Applied Science.

Nickerson TA (1977) Lactose sources and recovery. In:

Birch GG and Shallenberger RS (eds) Developments in

Food Carbohydrate, 1st edn, pp. 77–90. London:

Applied Science.

Thelwall LAW (1980) Lactose. In: Lee CL (ed.) Develop-

ments in Food Carbohydrate, 2nd edn, pp. 275–326.

London: Applied Science.

Walstra P, Guerts TJ, Noomen A, Jellema A and van Boekel

MAJS (1999) Dairy Technology: Principles of Milk

Properties and Processes, pp. 33–36. New York: Marcel

Dekker.

Whitaker R (1993) Some factors influencing the crystal-

lization of lactose in ice cream. Journal of Dairy Science

16: 177.

Yang ST and Silva EM (1995) Novel products and new

technologies for use of a familiar carbohydrate, milk

lactose. Journal of Dairy Science 78: 2541–2562.

Lactose Intolerance See Food Intolerance: Types; Food Allergies; Milk Allergy; Lactose Intolerance;

Elimination Diets

Lager See Beers: History and Types; Raw Materials; Wort Production; Biochemistry of Fermentation;

Chemistry of Brewing; Microbreweries

Lamb See Sheep: Meat; Milk

3476 LACTOSE

LEAD

Contents

Properties and Determination

Toxicology

Properties and Determination

A R Flegal and G M Scelfo, University of California at

Santa Cruz, Santa Cruz, CA, USA

Background

0001 Lead (Pb) is a highly toxic trace element with no

recognized biological requirement in organisms

and no established threshold concentration for sub-

lethal toxicity. Moreover, lead concentrations in the

biosphere have been substantially elevated by indus-

trial releases of lead over the past five millennia.

These contaminant lead emissions have been difficult

to quantify accurately, and many measurements of

lead in the biosphere have been biased by undetected

contamination during sampling, storage, and an-

alysis.

Occurrence in Foods

0002The relative magnitude of environmental lead con-

tamination is unparalleled among the elements.

Approximately 350 million tonnes of lead have

been produced since the advent of cupellation (a

smelting process that separates precious metals

from lead ores) five millennia ago (Figure 1), and

most (> 50%) of it has been dispersed as an environ-

mental contaminant on a global scale. Production

rates of *3.4 million tonnes of lead per year were

estimated to release *1.6 million tonnes of con-

taminant lead into the environment annually during

the preceding decade. These inputs included atmos-

pheric emissions of industrial lead aerosols

(3 10

kg year

1

), which were nearly two orders of

magnitude (10

) greater than the sum of all natural

atmospheric emissions of lead (4.5 10

kg year

1

These industrial emissions increased average lead

concentrations in the biosphere by approximately

5500

5000

Discovery of

cupellation

Rise and fall

of Athens

Roman Republic

and Empire

Introduction

of coinage

Exhaustion

of Roman

lead mines

Silver

production

in Germany

Industrial

Revolution

Spanish production

of silver

in New World

4500 4000 3500 3000

Years before present

Production worldwide (tonne year

−1

)

2500 2000 1500 1000 500 0

fig0001 Figure 1 Industrial production of lead over the past five millennia. From Settle DM and Patterson CC (1980) Lead in albacore: Guide

to lead pollution in Americans. Science 207: 1167–1176, with permission.

LEAD/Properties and Determination 3477

one order of magnitude above natural levels. How-

ever, there have been measurable reductions in envir-

onmental lead concentrations that closely correlate

with reductions in industrial lead emissions over the

past three decades.

0003 One consequence of industrial lead emissions into

the biosphere has been a 500–1000-fold increase in

the lead body burdens of older (> 40 years) adults

of industrialized countries, since most (> 90%) of

that lead is in bone, where its biological residence

time is relatively long (Figure 2). Contemporary

baseline lead concentrations of those individuals are

estimated to be still approximately 40 000 mgofPb

per 70-kg person, which is *1000-fold (10

) higher

than natural levels in prehistoric individuals (40 mg

of Pb per 70-kg person), and only fourfold lower

than levels in individuals with overt, clinical lead

poisoning (160 000 mg Pb per 70-kg person). The

body burdens of younger adults and children are

now estimated to be much lower (*10 000 mgofPb

per 70-kg person), since they have had less exposure

to industrial lead.

0004 Presumably, there have been comparable decreases

in the lead concentrations of other components of the

biosphere, including foods in human diets. Yet, con-

taminated foods are now a principal source of ele-

vated lead concentrations in humans living in regions

where the use of leaded gasolines has sharply declined

(Table 1). The dietary intake of lead by adults in those

areas is estimated to account for approximately

two-thirds of their baseline daily consumption of

lead (*50 mgday

1

0005Background lead concentrations in basic food

crops and meats range from 2 to 45 p.p.b. (ng g

1

)

fresh weight (FW) (Table 2). Most (50 to > 99%) of

that lead is derived from direct and indirect atmos-

pheric inputs of industrial lead aerosols. The rest (< 1

to 50%) is derived from soils (primary minerals,

humic substances, and soil moisture), which have

natural lead concentrations of < 8 to 25 p.p.m.

(pg g

1

). Consequently, contemporary background

lead concentrations in foods are two- to 100-fold

above natural concentrations, which are estimated

to range from 0.1 to 20 ng g

1

(FW).

0006Leafy crops are most susceptible to contamination

from atmospheric deposition of industrial lead

(Table 2). These crops include wheat, rice, spinach,

fig0002 Figure 2 Relative amounts of lead contamination in adult

humans. Each dot represents 40 mg Pb per 70-kg person. The

figure on the left with one dot represents the natural concen-

tration of lead in humans (40 mg per 70 kg). The figure in the

center represents the current average lead concentration of

adults in urban environments (40 000 mg per 70 kg) in previous

decades; recent studies of ambient blood lead concentrations

indicate that average lead concentrations in children and young

adults are now substantially lower (Pirkle et al., 1998). The figure

on the right represents the minimum concentration that will actu-

ate classical lead poisoning in a significant fraction of the popu-

lation (160 000 mg per 70 kg). From Patterson CC (1980) An

alternative perspective – lead pollution in the human environ-

ment: Origin, extent and significance. In: National Research

Council. Lead in the Human Environment. Washington, DC: National

Academy of Sciences.

tbl0001Table 1 Recent trends in total baseline human exposure to

lead (units in mg day

1

)

Age/sex category 1982 1983 1984 1985

2-year-old children

Food and beverage Pb 28.4 21.6 21.7 13.1

Air Pb 2.4 1.8 1.7 1.1

Dust Pb 20.1 20.1 20.1 20.1

Total food, air, and dust Pb 50.9 43.5 43.5 34.3

Teenage females (14^16 years)

Food and beverage Pb 37.6 25.7 27.6 14.7

Air Pb 4.8 3.5 3.4 2.2

Dust Pb 4.5 4.5 4.5 4.5

Total food, air, and dust Pb 46.9 33.7 35.5 21.4

Teenage males (14^16 years)

Food and beverage Pb 53.8 36.1 41.1 20.0

Air Pb 4.8 3.5 3.4 2.2

Dust Pb 4.5 4.5 4.5 4.5

Total food, air, and dust Pb 63.1 44.1 49.0 26.7

Adult females (25^30 years)

Food and beverage Pb 37.9 27.2 28.0 14.5

Air Pb 4.8 3.5 3.4 2.2

Dust Pb 4.5 4.5 4.5 4.5

Total food, air, and dust Pb 47.2 35.2 35.9 21.2

Adult males (25^30 years)

Food and beverage Pb 55.2 36.3 42.5 19.8

Air Pb 4.8 3.5 3.4 2.2

Dust Pb 4.5 4.5 4.5 4.5

Total food, air, and dust Pb 64.5 44.3 50.4 26.5

Senior adult females (60^65 years)

Food and beverage Pb 36.7 28.6 28.3 15.5

Air Pb 4.8 3.5 3.4 2.2

Dust Pb 4.5 4.5 4.5 4.5

Total food, air, and dust Pb 46.0 36.6 36.2 22.2

Senior adult males (60^65 years)

Food and beverage Pb 45.8 34.4 36.1 18.8

Air Pb 4.8 3.5 3.4 2.2

Dust Pb 4.5 4.5 4.5 4.5

Total food, air, and dust Pb 55.1 42.4 44.0 25.5

From Flegal AR et al. (1990) Lead contamination in food. In: Nriagu JO and

Simmons MS (eds) Environmental Food Contamination, Advances in

Environmental Science and Technology. New York: John Wiley.

3478 LEAD/Properties and Determination

and lettuce, whose contemporary baseline lead con-

centrations are five- to 100-fold above natural

concentrations. Contaminant lead concentrations in

foods not directly exposed to atmospheric depos-

itions (e.g., root crops, nuts, beans, and some fruits)

are approximately twofold lower. Atmospheric in-

dustrial leads are also a primary (> 90%) source of

lead contamination of some livestock and poultry.

This occurs initially through the ingestion of contam-

inated forage, feed, and soils, and subsequently

during the processing of meat products.

0007 Current baseline lead concentrations of foods are

several orders of magnitude lower than lead concen-

trations of foods in some industrial areas (Table 3).

For example, lead levels in some plants are at sub-

part-per-million concentrations in remote, rural

environments (0.2 mgg

1

) and part-per-thousand

concentrations in some highly contaminated urban

areas (950 mgg

1

) as a consequence of local emissions

of industrial lead aerosols (e.g., automobile exhausts

of leaded fuels). Lead concentrations of consumer

organisms (herbivores, omnivores, and carnivores)

may also be increased by one to two orders of magni-

tude above baseline concentrations by the ingestion

of contaminated foods and soils.

0008 Numerous other sources of lead contribute to the

lead ingested by humans. For example, lead solder

may contribute approximately 40% of the dietary

lead accumulated by adults through the contamin-

ation of both drinking water and foods. It is estimated

that 42 million people in the USA are exposed to

drinking water with elevated lead levels (> 20 mgl

1

)

due to the lead solder in pipes. Lead solder also ac-

counts for > 99% of the lead in some foods packaged

in lead-soldered cans. Lead-glazed ceramics may

increase lead concentrations in foods to parts-per-

million concentrations, and are a notable source of

acute lead toxicity. Leaded crystal, pewter, potions,

and sealants are other sources of lead contamination

to foods. Finally, dirt may be ingested inadvertently as

a food contaminant or directly through pica. A child

that ingests as little as 1 g of soil and 200 mg of lead

paint chips a day may be consuming 2600 mgof

lead per day and absorbing 550 mg of lead per day.

Chemical Properties

0009Lead has the most stable divalent (þ2) oxidation state

of the group IVB elements, though it also occurs

in a tetravalent (þ4) oxidation state. Coordination

numbers for its divalent compounds range from 2 to

7. Those for its tetravalent compounds range from 4

to 8. The stereochemistry of lead is usually octahedral

or tetrahedral. The divalent oxidation state domin-

ates the inorganic chemistry of lead, while its organic

chemistry is dominated by the tetravalent (þ4) oxida-

tion state.

0010Divalent lead, Pb

2þ

, is a B-type or soft sphere metal

cation. Lead’s classification, which is based on the

number of electrons in its outer shell, indicates that

2þ

ions tend to form covalent bonds, in sharp con-

trast to the mainly ionic binding of hard sphere ions,

such as Ca

2þ

. This is evidenced by the tendency of

2þ

to form strong bonds with groups containing

nitrogen and sulfur (e.g., CN, R-S, -SH, and imidazol)

in biological systems. It is also apparent in mineral

deposits, where galena (PbS) is the most abundant

lead ore and where anglesite (PbSO

) and cerrusite

(PbCO

) are less common minerals.

0011Despite differences in their chemistries, lead is

introduced and cycled in biological systems as an

analog of calcium. Most (70 to > 90%) of the lead

tbl0002 Table 2 Background lead concentrations in basic food crops

and meat (units in mgg

1

fw)

Crop Natural Indirect

atmospheric

Direct

atmospheric

To t a l

Wheat 0.0015 0.0015 0.034 0.037

Potatoes 0.0045 0.0045 0.009

Field corn 0.0015 0.0015 0.019 0.022

Sweet corn 0.0015 0.0015 0.003

Soyabeans 0.021 0.021 0.042

Peanuts 0.005 0.005 0.010

Onions 0.0023 0.0023 0.0046

Rice 0.0015 0.0015 0.004 0.007

Carrots 0.0045 0.0045 0.009

Tomatoes 0.001 0.001 0.002

Spinach 0.0015 0.0015 0.042 0.045

Lettuce 0.0015 0.0015 0.010 0.013

Beef (muscle) 0.0002 0.002 0.02 0.02

Pork (muscle) 0.0002 0.002 0.06 0.06

From US Environmental Protection Agency (1986) Air quality criteria for

lead, vol. II. US EPA Environmental Criteria and Assessment Office,

Research Triangle Park, NC. EPA/600/8-83/0286F.

tbl0003Table 3 Environmental lead concentrations in remote/rural

and urban areas (units in mgg

1

, except those for ‘Air,’ which

are mgm

3

)

Remote/rural Urban

Air 0.05 2.3

Fresh water 1.7 10

5

0.005–0.030

Soil 10–30 150–300

Plants 0.18

950

Herbivores 2.0

Omnivores 1.3

Carnivores 1.4

193

Values can be highly variable, depending on organism and habitat

location.

Fresh-weight basis.

Dry-weight basis.

From Flegal AR et al. (1990) Lead contamination in food. In: Nriagu JO and

Simmons MS (eds) Environmental Food Contamination, Advances in

Environmental Science and Technology. New York: John Wiley.

LEAD/Properties and Determination 3479

in vertebrates is associated with calcareous tissues,

where lead can substitute for calcium in apatite matri-

ces. In addition, lead competes with Ca

2þ

for sites in

cellular and subcellular systems. These include synap-

tosomes, mitochonrdria, membrane vesicles, protein

kinase C, calmodulin and other Ca

2þ

-binding pro-

teins. This ionic substitution is associated with some

of the primary mechanisms of lead toxicity, which are

attributed to the alteration of calcium-mediated cel-

lular processes and the mimicry of Ca

2þ

binding to

regulatory proteins.

0012 Since lead is a biogeochemical analog of calcium,

lead concentrations are normalized to calcium con-

centrations (atomic ratios) to quantify distributions

and fluxes of lead within organisms and food chains.

This normalization indicates a systematic decrease in

the atomic ratio of lead to calcium at higher trophic

levels, a process referred to as biopurification. The

biopurification of calcium relative to lead is attrib-

uted to the selective transport of Ca

2þ

across cell

membranes, the selective retention of Ca

2þ

in meta-

bolic processes, and the selective removal of Pb

2þ

detoxifying processes. These biopurification pro-

cesses may be circumvented by the direct assimilation

of contaminant lead by organisms, which occurs in

addition to the ingestion of lead within food. As a

result, atomic ratios of lead to calcium in humans

may exceed the ratios in their diets.

Isotopic Composition

0013 Lead (atomic number 82) has an atomic weight of

207.19, based on the relative abundance of its four

stable isotopes:

204

Pb (1.5%),

206

Pb (23.6%),

207

(22.6%), and

208

Pb (52.3%). The latter three isotopes

(

206

Pb,

207

Pb, and

208

Pb) are formed by the radio-

active decay of the long-lived natural isotopes

238

235

U, and

232

Th, respectively, while

204

Pb has no

radioactive progenitor. Since each parent isotope has

a different half-life, the relative natural abundance of

stable lead isotopes in a geological deposit varies

systematically with the age and evolution of the for-

mation. Different geological formations may, thus, be

distinguished by their characteristic (but not necessar-

ily unique) stable lead isotopic ratios.

0014 Ratios of stable lead isotopes may also be utilized

to identify and trace different sources of lead in the

environment. Natural differences in stable isotopic

compositions of lead, which are characteristic of dif-

ferent geological formations, persist in the environ-

ment because there is no measurable physical,

chemical, or biological fractionation of lead isotopes

within the biosphere. This allows sources of indus-

trial lead inputs into the atmosphere, water, soils, and

organisms to be distinguished by their isotopic

composition. Additionally, stable lead isotopic tracer

techniques may be utilized to investigate lead expos-

ure and uptake by organisms, as well as the cycling

and therapeutic manipulation of accumulated lead.

0015However, most lead tracer studies, to date, have

used man-made radiolead isotopes. One radioisotope

of lead,

210

Pb (with a half-life (t

) of 22 years), is

commonly used to characterize and date relatively

short (< 100 years) geological events. It is also

employed as a radiotracer in biological systems, as

are three other radioisotopes with shorter half-lives:

211

Pb (t

= 36.1 min),

212

Pb (t

= 10.64 h), and

213

= 26.8 min). The applicability of of those radioiso-

topes is constrained by their half-lives, which often

are not suitable for analyses of many metabolic pro-

cesses, the relative insensitivity of radiometric meas-

urements, and increasing controls on the use of

radioactivity in human health studies. Conversely,

the applicability of stable lead isotopic tracers has

been rapidly increasing with the development of

high-resolution inductively coupled plasma mass

spectrometers.

Analysis – Isolation and Concentration

0016Analyses of lead concentrations in environmental

matrices have been plagued by problems of contamin-

ation during sampling, storage, and analysis. Unrec-

ognized or poorly documented contamination have

invalidated most environmental lead data reported

prior to 1980 and indicated that many subsequent

data are highly questionable. This has been demon-

strated by numerous comparisons of data from

traditional laboratories and those using ultraclean

analytical techniques. Perhaps the most celebrated

of those comparisons were analyses of lead in tuna,

which demonstrated that the lead concentration of

fresh tuna (0.3 mgg

1

FW) was three orders of magni-

tude lower than the concentration (400 mgg

1

FW)

measured with previously accepted techniques.

0017Numerous procedures have been developed to

isolate and concentrate lead for analysis. One of

the most common procedures is liquid/liquid extrac-

tion, which typically involves a water-immiscible

solvent, an aqueous solution, and a complexing

agent that forms a nonionic or neutral chelate with

lead. Another common procedure is an ion-exchange

extraction, which sequesters lead ions on a chelating

functional group immobilized on a solid substrate. A

third is electrodeposition, which concentrates lead on

to an electrode for direct electrochemical analyses or

to preconcentrate it for other instrumental analyses.

0018Total lead concentration measurements usually re-

quire a rigorous acid digestion prior to these isolation

and concentration procedures. They are required for

3480 LEAD/Properties and Determination

most of the following analytical methods, because

they do not have the requisite selectivity, sensitivity,

or freedom from matrix interferences to accurately

and precisely measure trace concentrations of lead in

complex matrices. Measurements of lead concentra-

tions in different fractions or phases typically have the

same constraints.

Colorimetric Methods

0019 Historically, lead concentrations were measured by

colorimetric methods, including spectrometry. The

primary spectrometric method for measuring lead

now is with the reagent dithizone (diphenylthiocar-

bazone). It is a relatively simple and inexpensive tech-

nique, with a relatively low (pg) level of sensitivity.

The accuracy and precision of this method is highly

dependent upon the skills of the analyst and the

utilization of rigorous trace metal clean techniques.

The same qualification applies to the following tech-

niques, which involve much more elaborate instru-

mentation.

Spectrometric Methods

0020 The most common method of measuring lead concen-

trations in a variety of matrices is atomic absorption

spectrometry (AAS). This method measures lead con-

centrations by the absorbance of lead spectra emitted

from a source lamp by vaporized lead atoms. While

flame AAS techniques may still be appropriate for

measurements of relatively high (pg g

1

) concentra-

tions of lead, most AAS analyses of lead are now by

graphite furnace AAS. Evolving AAS methodologies

include the development of alternatives to graphite

furnaces (e.g., heated quartz tubes) and couplings

with chromatographic separations for species com-

position measurements. (See Spectroscopy: Atomic

Emission and Absorption.)

0021 Atomic emission spectrometry (AES) is similar to

AAS, except that lead is measured by the light emitted

by excited ions rather than by the light they absorb.

The primary advantage of AES is that numerous

elemental concentrations may be measured simultan-

eously in a relatively small sample. This methodology

is being improved with the development of induct-

ively coupled plasma (ICP) systems, which increase

sensitivity and decrease analytical interferences. Cur-

rent ICP–AES systems now provide the sensitivity

required for analyses of lead in most biological

matrices.

0022 The most accurate method of measuring lead

concentrations in biological matrices is with isotope

dilution thermal ionization mass spectrometry

(TIMS). This definitive method is yield-independent,

extremely sensitive, and precise. It is, also, the most

accurate and precise method for measuring stable

lead isotopic compositions.

0023However, other rapidly evolving mass-spectromet-

ric methods provide comparable or complementary

measurements of lead concentrations and isotopic

compositions in different matrices. All of these mass

spectrometric methods involve the electromagnetic

and physical separation of lead ions within an instru-

ment flight tube. The most prominent of these is

inductively coupled plasma mass spectrometry (ICP-

MS), which includes both the quadrapole ICP-MS and

the high-resolution magnetic sector ICP-MS. The

latter’s sensitivity, accuracy, and precision now

rival those of TIMS. Additional mass-spectrometric

methods which have evidenced potential for analyses

of lead in biological matrices include secondary ion

mass spectrometry, glow-discharge mass spectro-

metry, and laser microprobe mass analysis.

0024Nuclear magnetic resonance (NMR) techniques are

being developed for diagnostic analyses of lead. These

focus on the resonance frequencies of

207

Pb, which

are influenced by the molecular environment around

the isotope. NMR provides a measure of lead speci-

ation in both environmental and clinical settings. For

example, NMR has been used to simultaneously

measure intracellular free calcium and lead concen-

trations in erythrocytes. (See Spectroscopy: Nuclear

Magnetic Resonance.)

Electrochemistry

0025Electrochemical methods are used to measure both

lead concentrations and its organic complexation in

aqueous systems. These measurements are based on

the current produced as lead is reduced or oxidized by

varying the potential of a working electrode in an

electrochemical cell containing the sample. The sensi-

tivity of total lead concentration measurements,

which are determined after digesting the sample, can

be increased by a preconcentration step. Organic

complexation is determined by titrating the sample

with lead and measuring the current produced after

each lead addition.

0026The two principal electrochemical methods for

determining lead in environmental samples are dif-

ferential pulse polarography and anodic stripping

voltammetry. The first method measures the faradaic

current produced by the reduction of Pb(II) to Pb(0),

while the potential on the working electrode is

scanned to negative potentials. In the second method,

lead is preconcentrated on a mercury electrode by the

reduction of Pb(II) to Pb(0) during a reducing step.

The potential is then scanned positive, and the oxida-

tion current is measured.

LEAD/Properties and Determination 3481

Chromatography

0027 Chromatography separation techniques can detect a

wide range of molecular species containing metals or

metalloids. This method involves isolation of the

species of interest from the sample in a sufficiently

volatile, thermally stable, and physicochemically re-

sistant form for separation by gas–liquid or high-

performance liquid chromatography. These methods

are coupled with elemental detectors, principally

flameless atomic absorption spectrometers, which

provide low (< 50 pg) detection limits. Other detect-

ors include flame atomic absorption spectrometers,

thermal conductivity detectors, flame ionization

detectors, electron-capture detectors, thermionic

specific detectors, and atomic emission spectrometers.

(See Chromatography: High-performance Liquid

Chromatography; Gas Chromatography.)

0028 There have been numerous applications of chroma-

tography methodologies to investigate the speciation

of lead in the environment, because of its widespread

dispersion, diverse chemical forms, and toxicity. Gas

chromatography has been used to detect organolead

compounds, including trialkyllead chlorides, in the

environment. Gas–liquid chromatography was ini-

tially used to measure tetraalkyl derivatives of lead

in the environment, and it has since been used to

investigate the biomethylation of inorganic and

ionic alkyl lead by microorganisms. Additional speci-

ation measurements may be obtained with high-

performance liquid chromatography coupled with

GFAAS, mass spectrometry, and NMR.

X-ray Fluorescence

0029 A rapidly evolving methodology for measuring lead

concentrations in situ is with X-ray fluorescence

(XRF). Sufficient incident radiation from an X-ray

generator or a radioactive source is provided to excite

an inner shell electron, with the resultant emission of

a fluorescent X-ray. The emitted energy is character-

istic of the element that absorbed the original X-ray,

and the amount of energy released is proportional to

the mass of element present.

0030 Two types of XRF are applicable for noninvasive

analyses of lead concentrations. L-line techniques

require 10.5 keV to remove an L-shell electron, and

K-line techniques require 88 keV to remove a K-shell

electron. These XRF techniques are most appropriate

for analyses of bone lead concentrations of individ-

uals. This is significant because bone is the major

reservoir of lead in humans and accounts for > 90%

of their total lead content.

See also: Chromatography: High-performance Liquid

Chromatography; Gas Chromatography; Mass

Spectrometry: Principles and Instrumentation;

Spectroscopy: Atomic Emission and Absorption; Visible

Spectroscopy and Colorimetry