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

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the observed responses to nutritional support. Main-

tenance of an adequate food intake is an essential part

of treatment but more information is needed on its

combination with appropriate anabolic therapies to

maintain normal body composition and functioning.

0054 Improvements in the drug therapy of HIV manage-

ment have had significant impacts on morbidity and

mortality but the emergence of the fat redistribution

syndrome as a side-effect of therapy represents a new

challenge to the clinical and nutritional management

of HIV infection.

See also: Energy: Energy Expenditure and Energy

Balance; Malabsorption Syndrome; Malnutrition: The

Problem of Malnutrition; Viruses

Further Reading

Bhasin S, Storer TW, Javanbakht M et al. (2000) Testoster-

one replacement and resistance exercise in HIV-infected

men with weight loss and low testosterone levels. Jour-

nal of the American Medical Association 283: 763–770.

Corcoran C and Grinspoon S (1999) Treatments for

wasting in patients with the acquired immunodeficiency

syndrome. New England Journal of Medicine 340:

1740–1750.

Dobs AS (1999) Is there a role for androgenic anabolic

steroids in medical practice? Journal of the American

Medical Association 281: 1326–1327.

Dwyer JT, Bye RL, Holt PL et al. (1988) Unproven nutrition

therapies for AIDS: what is the evidence? Nutrition

Today Mar/Apr: 25–33.

Grunfeld C and Feingold KR (1993) Body weight as essen-

tial data in the management of patients with human

immunodeficiency virus infection and the acquired im-

munodeficiency syndrome. American Journal of Clinical

Nutrition 58: 317–318.

Macallan DC, Noble C, Baldwin C et al. (1993) Prospective

analysis of patterns of weight change in stage IV human

immunodeficiency virus infection. American Journal of

Clinical Nutrition 58: 417–424.

Macallan DC, Noble C, Baldwin C et al. (1995) Energy

expenditure and wasting in human immunodeficiency

virus infection. New England Journal of Medicine 333:

83–88.

Miller TL and Gorbach SL (eds) (1999) Nutritional Aspects

of HIV Infection. London: Arnold.

Mulligan K, Grunfeld C, Hellerstein MK et al. (1993) Ana-

bolic effects of recombinant human growth hormone in

patients with wasting associated with human immuno-

deficiency virus infection. Journal of Clinical and Endo-

crinological Metabolism 77: 956–962.

Safrin S and Grunfeld C (1999) Fat distribution and meta-

bolic changes in patients with HIV infection. AIDS 13:

2493–2505.

Strawford A, Barbieri T, Van Loan M et al. (1999) Resist-

ance exercise and supraphysiologic androgen therapy in

eugonadal men with HIV-related weight loss. Journal of

the American Medical Association 281: 1282–1290.

Task Force on Nutrition Support in AIDS (1989) Guidelines

for nutrition support in AIDS. Nutrition 5: 39–46.

Wanke CA (1999) Epidemiological and clinical aspects of

the metabolic complications of HIV infection. The fat

redistribution syndrome. AIDS 13: 1287–1293.

Homocysteine See Coronary Heart Disease: Etiology and Risk Factor; Antioxidant Status; Intervention

Studies; Prevention; Vitamin B

: Properties and Determination; Physiology

HOMOGENIZATION

R A Wilbey, The University of Reading, Reading, UK

Introduction

0001 An emulsion is a stable mixture of two normally

immiscible phases as a continuous and a disperse

phase. Particles of the disperse phase should be

small, ideally 1 mm or less, when Brownian motion

becomes a significant factor. The stability of an emul-

sion also increases with its viscosity. These conditions

are the reverse of those needed for efficient separation,

the movement of particles above 1 mm following

Stokes’ law. There are many examples of emulsions

in foods, e.g., milk, cream, ice cream, butter, margar-

ine, mayonnaise and salad dressings, sauces, cake

batters, and some beverages. (See Colloids and

Emulsions.)

HOMOGENIZATION 3119

0002 The first treatment of milk to give a finer, more

stable, emulsion that would remain homogeneous

on standing has been attributed to Auguste Gaulin,

whose first patent was granted in 1899. High-pres-

sure homogenization is the most widely used method

for stabilizing milk and milk products. It has also

been widely used in other sectors of the food industry

and in the preparation of nonfood emulsions of low

to medium viscosity.

0003 The term ‘homogenization’ has since been applied

to similar emulsification processes, so that the terms

‘emulsification’ and ‘homogenization’ may often be

treated as synonyms rather than homogenization

being regarded as a specialized method of emulsifica-

tion. A wide range of equipment has been developed

for the homogenization of foodstuffs. The simplest

of these methods is the high-shear mixer, which

has been adapted for both batch and in-line process-

ing. The colloid mill has been widely used for the

homogenization of more viscous foodstuffs such

as sauces and mayonnaises. Emulsification may

also be the result of shear forces acting during other

food-preparation processes, such as the cooling of

margarine in scraped-surface heat exchangers, the

working of butter and in the use of pin mills and

disk mills.

0004 The formation of a stable homogeneous foodstuff

will depend on both the nature of the foodstuff and

the process. There is often an optimum viscosity of

the continuous phase for a given homogenizing tech-

nique, while lower viscosities in the disperse phase

will aid homogenization. The presence of surface-

active components may play a critical role in the

formation and maintenance of an emulsion. The

quantity of disperse phase will also affect the particle

size distribution in the emulsion. This article will

concentrate on the processes rather than the food

components.

High-shear Mixers

Batch Mixers

0005 High-shear mixers are commonly used for the batch

mixing of food ingredients and the formation of

emulsions. Most high-shear mixers rely on the devel-

opment of a shear gradient between the rapidly

rotating rotor and a static surface or stator. For

low-viscosity liquids, the rotor is normally a

simple turbine blade, and the stator is perforated.

Figure 1 illustrates a mixer with a vertical-slotted

disintegrating head that may also be used for break-

ing up fibrous tissue; a larger number of small square

or round holes would normally be used in the stator

screen for emulsification alone.

0006The mixer is normally coupled directly to an elec-

tric motor, wound to run at 2850 r.p.m. for small

mixers or at 1425 r.p.m. for large models, giving

rotor tip speeds of approximately 100–150 m s

1

for

pilot-scale applications. The flow of liquid from the

turbine is principally radial, the flow being obstructed

by the stator. Some liquid will pass between the rotor

tip and the stator: the smaller the clearance, the

greater the shear forces that will be developed. The

bulk of the liquid, however, will be forced through

the perforations in the rotor: the smaller the perfor-

ations, the greater the shear exerted on the mixture to

be emulsified.

0007With batch mixing, the mixer may be left running

until the desired homogenization/emulsification has

been achieved. This method is limited by the viscosity

of the foodstuff as adequate flow within the batch

mixing vessel is essential, and more viscous foods and

large batch sizes are better homogenized by other

techniques.

In-line Mixers

0008In-line mixers for homogenization use the same basic

principles as the batch mixers. In the simplest system,

fig0001Figure 1 Batch high-shear mixer (courtesy of Silverson

Machines Ltd).

3120 HOMOGENIZATION

the liquid passes from the turbine through the screen

into an outer chamber and is discharged into the

pipeline without any recirculation (Figure 2). The

turbine has a weak pumping action. For some foods,

there may not be sufficient shear in one pass through

the homogenizing head so that either two or more

mixers need to be placed in series, or the mix recircu-

lated back to the feed tank until a sufficient degree of

emulsification has been achieved.

0009 Where recirculation needs to be carried out, this

may be incorporated into the design of the mixer by

allowing recirculation of part of the discharge from

the shear head back to the inlet. The degree of recir-

culation depends upon the mixer design; the type

shown in Figure 3 uses a high degree of recirculation

to ensure a homogeneous discharge.

Colloid Mill

0010 The colloid mill derives its emulsifying action from

the high shear on the foodstuff as it flows between the

rotor and stator, as illustrated in Figure 4. The rotor

may be coupled directly to a motor or driven via a

variable-speed gearbox. Typical rotor tip speeds are

20–50 ms

1

0011 Normally, the gap between the rotor and stator are

adjustable, typically between 100 and 200 mm, to

enable a range of shear rates and throughputs to be

achieved. The shear on the product will follow the

general relationship:

Shear stress T ¼ 

where m ¼ viscosity, v ¼ peripheral velocity, and h ¼

clearance.

0012Greater disruption may be achieved by using

toothed surfaces on the rotor and stator, e.g., for

peanut butter, though where a grinding action is

needed, a larger clearance may be used. The colloid

mill may be used as an in-line emulsifier and is par-

ticularly effective with viscous foodstuffs, though the

minimum particle size of the disperse phase (1–2 mm)

will not reach the submicron sizes that may be

achieved with the high-pressure homogenizer.

The High-pressure Homogenizer

0013The high-pressure homogenizer may be regarded as

comprising two parts: a high-pressure pump and one

or more homogenizing valves.

The High-pressure Pump

0014Pressure is used as the driving force for the homogen-

ization process, the liquid mixture to be homogenized

being raised to the homogenization pressure by a

positive displacement pump. In all but the smallest

pumps, there are at least three, sometimes five or

fig0002 Figure 2 Schematic section through an in-line high-shear

mixer.

Static Version

Exit pipes

Deflector

Mixing zone

Propulsive jet

fig0003Figure 3 Section through the mixing chamber and shearing

zone of a Burdosa Dynamic Loop Mixer (courtesy of dmt Gmbh,

Fischbach, Germany).

HOMOGENIZATION 3121

seven, pistons. The pistons must be arranged so that

they operate consecutively to maintain as even a feed

pressure as possible. Both the pistons and the pump

block are typically of stainless steel construction, but

the piston seal rings are of a soft composite material

and can be a source of cleaning problems. The layout

of the high-pressure pump is illustrated in Figure 5.

0015 The mixture to be homogenized is fed into the

pump block under a slight positive pressure. A gravity

feed is sufficient to ensure that the inlet valves

are flooded, but continuous operation may be better

served by feeding at a small positive pressure (e.g., up

to 0.1 MPa) from a centrifugal pump via a preheater.

0016 Two types of valve have been used in the pump

block, either ball valves or poppet valves, as illus-

trated in Figure 6. The upper, discharge, valves are

usually spring-loaded to ensure rapid closing.

0017 Ball valves have a smaller contact area with the

valve face, and they exert a greater pressure at the

point of contact. This is an advantage for viscous

liquids and when small particulates may be present

in the feed stock, since valve leakage is the limiting

factor in the high-pressure homogenization of liquids

containing suspended particles. Poppet valves need to

be fitted more carefully to the larger valve face, but,

when properly maintained, they have a better per-

formance with low-viscosity liquids such as milk.

0018 A third, mushroom, design of valve has been intro-

duced recently, designed as an optimum compromise

between the ball and poppet designs. Both inlet and

discharge valves are spring-loaded to ensure good

seating.

0019The mixture is discharged from the pumping cham-

bers of each piston into a high-pressure manifold. The

pressure in this manifold is normally monitored by a

pressure gauge, which must be of hygienic construc-

tion, with a diaphragm between the product and the

sensor mechanism, plus a means of cleaning the space

around the diaphragm. The only exit for product

from this high-pressure manifold is through the

homogenizing valve.

The Homogenizing Valve

0020Many designs of homogenizing valve have been

invented, but for most valves, the same basic prin-

ciples apply. The mixture, which must already be a

homogeneous mix and itself a coarse emulsion, flows

from the high-pressure manifold into the fixed valve

seat. The internal diameter of the valve seat is smaller

than the manifold, so the velocity of the liquid is

increased. To escape from the valve seat, the liquid

must flow radially across the face of the valve. The

gap between the valve face and the seat is controlled

so that a constant pressure drop across the valve is

maintained. The opposing force (operating pressure

times the valve area) may be provided by a heavy duty

spring, a torsion bar, hydraulic pressure or by direct

contact with a screw threaded lever. The basic design

Stator with

axial

adjustment

Inlet

Rotor

Stator

Outlet

Driving shaft

fig0004 Figure 4 Schematic section through a colloid mill (courtesy of Fryma Ltd, Hemel Hempstead, UK).

3122 HOMOGENIZATION

is illustrated in Figure 7. Pneumatic actuators have

been used in some machines with capacities of less

than 10 000 l h

1

, thus reducing the variance in

homogenization pressure. For most dairy products,

the homogenization pressure is in the range of 2.5–

30 MPa with throughputs of up to 50 000 l h

1

for

milk. Machines are available that will produce a feed

pressure of more than 100 MPa, though these are

restricted to lower throughputs by the power required

to drive them.

0021 The gap between the valve and its seat is many

times greater than the diameter of the droplets created

in the emulsion, e.g., in a normal large-scale dairy

application, the gap would be greater than 100 mm,

the size of the gap tending to increase with the

throughput of the machine. Even so, the cross-

sectional area of the flow path within the valve is

much smaller than that in the high-pressure manifold,

so the velocity of the liquid will rise as it enters the

valve. This acceleration can result in distortion of

the particles of the disperse phase (often called the

wire-drawing effect), which will reduce their stability.

Within the valve, there will be a number of effects on

the liquid that may contribute to the disruption of the

fat globules:

0022Shear effects will result from the high velocity

gradients between the liquid and the surfaces of

the homogenizing valve. This effect, in combin-

ation with the wire-drawing effect, will lead to

disruption of globules.

0023Turbulence also results from the high velocity of

the liquid in the valve, causing eddy currents

within the flow. High-velocity impacts between

particles can lead to disruption, and so increases

in velocity are associated with decreased fat

globule size in the case of oil-in-water emulsions.

0024High-frequency vibrations (> 10 kHz) have been

observed in some valves that may be of mechanical

origin or a result of turbulence or cavitation. When

homogenizing at high temperature, the drop in

pressure as the liquid accelerates within the valve

may result in the pressure momentarily falling

below the saturation vapor pressure of an aqueous

continuous phase, leading to the formation of

steam bubbles. Collapse of these bubbles as the

pressure increases again sets up high-pressure

shock waves. Opinions vary about the contribution

of cavitation and other sources of vibration to the

overall disruptive effect during homogenization.

0025Impact can contribute to the homogenizing effect

in some designs of homogenizing valve, where an

impact ring is placed perpendicular to the flow

of liquid from the valve. With valve exit velocities

of 200–300 m s

1

, there can be a significant

disruptive effect as the jet of emulsion impinges

on the ring.

fig0005 Figure 5 Schematic section of a high-pressure pump.

fig0006 Figure 6 Types of valve: (a) ball; (b) poppet.

HOMOGENIZATION 3123

0026 Homogenizing valves have been produced from very

tough corrosion-resisting alloys such as stellite. A

better resistance to erosion may now be achieved

using tungsten carbide and ceramic valves, though

these are more difficult to produce and are more

fragile. A very wide range of homogenizing valve

designs using both single and multiple homogenizing

surfaces have been produced, with the aim of increas-

ing homogenization efficiency and thus reducing the

energy costs. Various claims for up to 30% energy

savings or a 40% reduction in homogenization pres-

sure for an equivalent size reduction with these new

designs have been made, in comparison with the

traditional Gaulin valve design.

Practical Considerations in the Use of

High-pressure Homogenizers

0027 Since homogenizers operate at high pressures, prob-

lems can be encountered with the seals. The pistons

are normally lubricated with potable water to minim-

ize wear of the rear seals, though in aseptic machines,

the pistons are also surrounded by a steam seal to

avoid product contamination. With nonaseptic ma-

chines, the homogenization step is normally carried

out prior to the heat treatment to minimize hygiene

risks. Icecream mix may be homogenized at the heat

treatment temperature (66–80



C). In general, the

efficiency of a high-pressure homogenizer increases

with temperature, but temperature is normally

limited by the product.

0028 Constant homogenizing pressure is necessary

for efficient reproducible emulsification. Rapidly

fluctuating pressure may be due to a faulty gauge,

but the most common cause is poor seating of the

valves. Seating problems can result from wear, from

incorrect placement of valves in the seatings, or from

dirt or foreign bodies on the seats. The presence of air

in the feed can also cause problems, and deaeration

may be needed in some processes prior to homogen-

ization.

0029With many dairy emulsions, the fine particles pro-

duced by homogenization tend to flocculate. These

flocculated globules behave as large particles and

separate from the emulsion. The introduction of a

second homogenization stage or a multistage valve

can overcome this problem. The pressure drop across

the second stage valve should be relatively small,

typically 3–5 MPa, to break up the aggregates. With

two-stage homogenization, it is common practice to

set the second-stage pressure first then to set up the

first stage, remembering that the gauge will indicate

the total pressure. The introduction of a second pres-

sure gauge between the first- and second-stage valves

aids control of the process. Where very find disper-

sions are desired, multiple high-pressure homogeniza-

tion has been used.

Other Emulsifying Systems

0030Ultrasonic homogenization uses a transducer or a

mechanical wedge to generate high-frequency vibra-

tions at over 10 kHz, typically 18–50 kHz, so that the

output is barely audible. These give rise to shock

waves plus possibly cavitation in the mixture.

fig0007 Figure 7 Schematic section through a homogenizing valve assembly.

3124 HOMOGENIZATION

Efficiency declines with increasing frequency, the

upper limit being about 50 kHz. It has been suggested

that the efficiency of the process is increased by intro-

ducing or creating microbubbles or particles that

resonate with the frequencies generated in the appar-

atus. The transducer method has been used on a small

scale for producing emulsions and for disrupting cells,

whereas the wedge method has been used for emulsi-

fication in food products

0031 Emulsification may also be achieved by mixing

high-speed streams of the two phases, the resultant

mixture then being forced through a small orifice to

create a high-velocity impinging jet. This method,

known as microfluidization, is essentially a modifica-

tion of the high-pressure homogenizer principle but

can be employed for very small sample sizes.

0032 As with other methods, there can be considerable

heat production.

0033 Part of this article was first published in the Journal

of the Society of Dairy Technology, Volume 45 (1992)

and was reproduced by the kind permission of the

Society of Dairy Technology.

See also: Colloids and Emulsions; Emulsifiers: Organic

Emulsifiers; Phosphates as Meat Emulsion Stabilizers;

Uses in Processed Foods; Ice Cream: Microbiology;

Separation and Clarification

Further Reading

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

Food Engineering Operations, 3rd edn. London:

Elsevier Applied Science.

Kessler HG (1981) Food Engineering & Dairy Technology.

Freising: Ferlag A. Kessler.

McClements DJ (1999) Food Emulsions; Principles, Prac-

tice and Techniques. Boca Raton, FL: CRC Press.

Phipps LW (1985) The High Pressure Dairy Homogenizer.

Reading, UK: National Institute for Research in Dairy-

ing.

Walstra P (1985) Formation of emulsions. In: Becher P (ed.)

Encyclopaedia of Emulsion Technology, vol. 1, pp. 57–

127. New York: Marcel Dekker.

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

MAJS (1999) Dairy Technology. Marcel Dekker, New

York.

HONEY

L W Doner, US Department of Agriculture, Wyndmoor,

PA, USA

Background

0001 Honey serves to assure the survival of honeybee

colonies and, consequently, the pollination and sur-

vival of many plants. It was one of man’s earliest

foods, as honeybees were producing it long before

man appeared on the earth. Honey remained the

primary sweetener for humans until the late nine-

teenth century, when its consumption was exceeded

by sugar cane-, sugar beet-, and later corn-derived

sugars. Honey remains the only sweetener that can

be stored and used precisely as it is produced by

nature. It is an exceedingly complex substance that

is produced by honeybees from floral nectar and,

less frequently, honeydew. Honeydew consists of

syrups deposited on plants by other insects, such as

aphids. Whereas nectar from the many floral sources

generally contains just the sugars fructose, glucose,

and sucrose, honey contains these and many other

more complex sugars. The additional sugars arise

largely as a result of honeybees adding several

enzymes during the nectar-to-honey ripening pro-

cess. The presence of active enzymes and the acidity

of honey result in its composition being in a con-

stant state of change.

0002Comprehensive surveys of average honey compos-

ition have established that the major components

are fructose (38.4%), glucose (30.3%), and water

(17.2%). In addition to the two major sugars are an

array of more than 20 higher sugars, which are

formed by linking the fructose and glucose in various

combinations. Honey is therefore primarily a carbo-

hydrate material, and sugars comprise over 95%

of its solids content. The principal physical character-

istics of honey are due to its sugars, but subtle

differences in minor constituents from nectar are re-

sponsible for the flavor differences of honey from

different floral sources. These constituents include

flavoring and aroma compounds, pigments, acids,

and minerals. These substances are concentrated

during the ripening process, where most of the

water from nectar is removed. In order to assure

the integrity of honey markets, methods have been

developed to detect the addition of inexpensive sugar

mixtures as adulterants.

HONEY 3125

Honey Production and Processing

0003 To summarize, foraging honeybees ingest nectar and

honeydew and store it in their honey sac. Pollen is

also collected, and this provides the protein portion of

the honeybee diet. Pollen, however, is not integral

to the production of honey. The honey sac weighs

40–70 mg when full and can account for more than

90% of the weight of the bee. Enzymes are secreted

and mixed with the sac contents. Upon returning to

the hive, the contents are passed on to the house bees,

who carry out the ripening process. This consists of

alternately expelling and ingesting the honey sac fluid

for 15–20 min. When the nectar has lost about half of

its moisture, small droplets of this semiripened honey

are deposited in the comb cells. In 1–3 days, numer-

ous enzyme-mediated changes will have occurred and

honey density attained. The cells are then capped over

with wax and the honey stored as food. Compos-

itional changes continue during ripening, the most

prominent result being the production of an increas-

ingly complex array of sugars. This is largely a result

of continuing enzyme activity. An enzymatic process

is also employed to produce hydrogen peroxide, in

order to stabilize the honey against microbial growth.

0004 The primary objectives of commercial honey pro-

cessing include maintaining the desired physical state,

be it liquid or finely granulated, and keeping it free of

fermentation. Methods for accomplishing the object-

ives are well established and have been used for many

years. Beekeepers remove the hive units (supers) from

the colony and then remove the wax cappings by

mechanical slicing. The honey is then centrifugally

extracted from the wax comb. Raw honey contains

pollen, some wax, sugar-tolerant yeasts, and often

some crystals of glucose monohydrate. Flavor and

aroma are optimal just after processing, but further

treatment is required before large-scale marketing.

The process of pressure filtration is used to produce

liquid honey, after controlled heating to destroy

yeasts and to dissolve sugar crystals. The application

of heat to honey requires careful control so that the

color, flavor, and aroma of honey are not impaired.

Honey is sold in the liquid form, in a finely granu-

lated form known as honey spread or finely

granulated honey, and as comb honey. The require-

ments for the stability of these forms are increasingly

stringent, as honey has become a world commodity

available in markets throughout the year.

0005 World honey production has increased steadily

and, in the year 2000, was estimated to be about

1.2 million tonnes. About 25% of this production is

the object of international trade. The three largest

honey-exporting countries are China, Argentina,

and Mexico, and the largest importers are Germany,

Japan, and the USA. The USA is the leading consumer

of honey, and honey consumption in Asia has in-

creased substantially in recent years, especially in

Japan and China.

General Composition

0006Honey samples vary in composition, and this vari-

ability reflects contributions of the plant source of

nectar, beekeeping practices, and climate and envir-

onmental conditions. Honeys representing many

floral sources in the USA have been analyzed, and

the average compositions are listed in Table 1.

Many honeys from other countries have been

analyzed and generally found to provide similar

values. There is wide variability among all the honey

components. Among the major components, how-

ever, fructose levels exceed those of glucose, the only

exceptions being dandelion and blue curls honeys.

Moisture

0007Moisture content is one of the most important char-

acteristics of honey, as it influences the keeping qual-

ity, granulation, and body. The natural moisture of

honey in the comb is that remaining from the nectar

after it is ripened by the bees. The amount of moisture

is a function of factors involved in ripening, such

as environmental conditions, original moisture con-

tent of the nectar, and strength of the honeybee

colony. Normal ripened honey has a moisture content

below 18.6%, and when the moisture content

exceeds that level, honeys are liable to ferment. This

can occur if the count of ubiquitous osmophilic

yeasts is sufficiently high. After extraction of the

honey from the comb, its moisture content may

change, depending on the conditions of storage.

Low moisture levels can result in the crystallization

of glucose, a process termed granulation. Few

tbl0001Table 1 Average composition of honey

Property Average Range

Moisture 17.2 12.2–22.9

Fructose 38.4 30.9–44.3

Glucose 30.3 22.9–40.7

Sucrose 1.3 0.2–7.6

‘Maltose’

7.3 2.7–16.0

Higher sugars 1.4 0.1–3.8

Ash 0.17 0.02–1.03

Nitrogen 0.04 0.0–0.13

pH 3.91 3.4–6.1

Data for 490 samples; all values in percentages except for pH; ‘maltose’

includes all reducing disaccharides.

From White JW, Jr. (1962) Composition of American Honeys. Technical

Bulletin No. 1261. US Department of Agriculture.

3126 HONEY

beekeepers measure the moisture content of honey,

instead relying on guesswork and leaving this meas-

urement to processors.

0008 Several methods are available to measure honey

moisture, including direct drying, viscosity measure-

ment, refractometry, and use of the Karl Fischer titra-

tion method. The last method is the most accurate,

but the hand refractometer is sufficiently accurate

and more convenient to use. This instrument must be

used with appropriate sugar solutions as standards.

Sugars

0009 The simple sugars fructose and glucose account for

over 85% of its solids, so honey is essentially a highly

concentrated sugar solution in water. Another 10% of

the honey solids includes at least 25 other more com-

plex sugars (Table 2), giving it the most complex

sugar profile of any natural product. Some of these

sugars are present in very low levels, and they are

formed by linking fructose and glucose in many dif-

ferent combinations.

0010 Until the middle of the twentieth century, the only

sugars thought to be present in honey were the nectar

sugars fructose, glucose, and sucrose, and small

amounts of an ill-defined, highly complex carbohy-

drate. After the ill-defined material was shown to

consist of many individual sugars, researchers pro-

vided some explanations as to how they could arise

from the nectar raw material. It was revealed that

while the acids of honey may be responsible for gen-

erating some of the sugars, most are generated as a

result of the action of enzymes contributed by the

honeybees during the ripening process. The enzym-

atic formation of these sugars is discussed in the

following section on the honey enzymes.

0011 A variety of methods have been required to charac-

terize the many honey sugars. Traditional colorimet-

ric methods continue to be useful for determining

levels of reducing and nonreducing sugars in honey,

but these methods do little to reveal the high degree of

complexity. Enzymatic methods can also be used,

such as glucose oxidase for measuring glucose levels.

Beginning in about 1950, various chromatographic

methods have been shown to be extremely useful.

Charcoal-celite chromatography continues to be

applied for separating the honey sugars into mono-,

di-, and higher saccharide fractions. These fractions

are then further resolved using paper-, charcoal-celite

column-, thin-layer-, high-performance liquid-,

capillary gas-liquid-, and high-performance anion

exchange chromatography. The latter two chromato-

graphic modes have been especially useful in separat-

ing closely related sugars. After their purification,

physical methods such as infrared and nuclear

magnetic resonance spectroscopy have been applied

to identifying the sugars listed in Table 2 as com-

ponents of honey. Some of these are present at

extremely low levels, providing additional challenges

to the analyst. (See Fructose; Glucose: Properties and

Analysis; Sucrose: Properties and Determination.)

Enzymes

0012Enzymes that have been identified as honey compon-

ents are listed in Table 3, along with the number of

samples analyzed. These are mostly added by the

honeybee, although some traces of plant enzymes

may be present. Honeybees add these enzymes in

order to accomplish the nectar-to-honey ripening pro-

cess, and they are largely responsible for providing

the compositional complexity of honey. The pro-

cesses involved in the conversion of the three nectar

sugars into at least 25 additional sugars of greater

complexity are far from being totally understood. It

is becoming increasingly clear, however, that enzyme-

catalyzed reactions are responsible for the formation

of many of these sugars.

tbl0002 Table 2 Sugars that have been identified as honey constituents

Monosaccharides Disaccharides Trisaccharides Higher saccharides

Fructose Sucrose Melezitose Isomaltotetraose

Glucose Maltose Maltotriose Isomaltopentaose

Maltulose Isomaltotriose

Isomaltose 3-a-Isomaltosyl glucose

Nigerose 1-Kestose

Turanose Panose

Kojibiose Isopanose

Laminaribiose Erlose

a,b-Trehalose Theanderose

Gentiobiose Centose

Palatinose Laminaritriose

Cellobiose

From Doner LW (1977) The sugars of honey – A review. Journal of the Science of Food and Agriculture 28: 443–456.

HONEY 3127

0013 The enzyme a-glucosidase is probably the most sig-

nificant honey enzyme, since it is responsible for many

of the important changes that occur during riping. It is

often referred to as invertase or sucrase in the honey

literature. a-Glucosidase converts the nectar disac-

charide sucrose into its constituent monosaccharides,

fructose and glucose, a process known as inversion.

This reaction is vital because sucrose crystallizes easily

and also because its inversion allows the preparation of

the super-saturated glucose/fructose mixture found in

honey. a-Glucosidase catalyzes additional reactions,

leading to the formation of several addition sugars.

Researchers have found that incubation of the enzyme

with fructose, glucose, and sucrose results in sugars not

from hydrolysis reactions but from transglucosylation

reactions. By this process, higher sugars are produced

from simpler sugars. Sugars from Table 2 that have

been identified are kojibiose, nigerose, maltose, iso-

maltose, turanose, a, b-trehalose, and erlose. The tri-

saccharide erlose was first discovered as a honey

component, and is formed by the reaction of glucose

with the glucose moiety of the disaccharide sucrose.

0014 When the most recently discovered honey enzyme,

b-glucosidase, has been incubated with fructose, glu-

cose, and sucrose, other honey sugars have resulted.

These were the glucose–glucose disaccharides, lami-

naribiose and gentiobiose. Thus, at least nine of the

honey sugars are produced by the action of a-and

b-glucosidase on the nectar sugars. The mode of for-

mation of most of the honey trisaccharides remains to

be elucidated.

0015 Another honey enzyme contributed by the honey-

bee, glucose oxidase, is largely responsible for the anti-

bacterial property of honey. This activity was termed

inhibine until it was identified to be hydrogen perox-

ide. This antibacterial agent is produced by the action

of glucose oxidase on its glucose substrate. In addition

to hydrogen peroxide, gluconic acid is produced by the

enzyme. The acidity of this sugar acid is largely respon-

sible for the low pH of honey. In conjunction with the

high solids content and resulting low water activity, it

helps to stabilize honey against fermentation.

0016The honey enzymes catalase and acid phosphatase

have not been studied in great detail. Catalase was

discovered in honey in 1910 and is responsible for

converting hydrogen peroxide into water and oxygen.

The role of this enzyme appears to be in controlling

the levels of hydrogen peroxide in honey, since there is

an inverse relationship between the levels of enzyme

and substrate.

0017Acid phosphatase was first reported in 1938 when

incubation of b-glycerophosphate with diluted honey

was found to result in formation of phosphoric acid

and glycerol. The source of this enzyme is unclear.

One study suggested that it is only present in slightly

fermented honey samples and that yeast is the prob-

able source. A more recent study suggested a plant

source for acid phosphatases, since activities were

found in pollen and nectar.

0018Diastase enzyme is added to nectar by bees during

ripening, and is a mixture of a- and b-amylase. These

enzymes are known to degrade starch, but since

nectars are not known to contain starch, their role

during ripening is unknown. The activity of honey

diastase is known to be reduced upon heating and is

easily measured. As a result, its measurement has

been used for many years to estimate the extent of

heating to which a honey has been exposed. This

information is used in countries where such heating

is believed to reduce or deplete certain ill-defined

health-promoting properties. A problem with this

approach is that diastase levels in fresh honeys vary

considerably, and extended storage can significantly

deplete its level. (See Enzymes: Functions and Char-

acteristics.)

Minerals

0019The mineral content of honey is reflected in its total

ash content (Table 1). In general, dark honeys have a

higher mineral content than light honeys. Like other

honey components, ash content is highly variable,

ranging from 0.02% to over 1%. The levels of indi-

vidual minerals that comprise the ash are also vari-

able, but potassium accounts for about one-third of

the total amount. Its level exceeds by about 10-fold

that of the other major minerals in honey, sodium,

calcium, and magnesium. Less abundant elements

found in honey are iron, manganese, copper, chlorine,

phosphorus, sulfur, and silicon. Sensitive instrumen-

tal methods such as atomic absorption or plasma

emission spectroscopic analysis can be used to accur-

ately determine levels of even the most trace minerals.

Acids

0020The great sweetness of honey largely masks flavor

contributions of the acids, which generally account

tbl0003 Table 3 Enzymes that have been identified as honey

constituents

Enzyme Number of samples

a-Glucosidase 1468

b-Glucosidase 11

Glucose oxidase 124

Catalase 38

Acid phosphatase 25

a-Amylase 1746

b-Amylase 1746

From Doner LW (1991) The enzymes of honey. In: Fox PF (ed.) Food

Enzymology, pp. 143–161. London: Elsevier Applied Science.

3128 HONEY