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the heated water bath on bars that move at a given

speed to ensure the required scalding time, usually

1–1.5 h.

0034 A recently formed company, which commenced a

commercial clotted cream enterprise in 1998, pro-

duces three clotted cream products, using a combin-

ation of hot-water scalding, a convection oven and

batch heating in a closed, jacketed vat. The company

processes 600–750 kg daily, depending on the season

and makes about 15 tonnes in the 2-week pre-

Christmas period.

0035 The company’s traditional clotted cream is pro-

duced by hot-water scalding in four batches daily.

Container sizes include 113-g, 227-g, 453-g and

906-g tubs, and 1.81-kg and 4.53-kg trays for

catering establishments. A convection oven is used

for small 21-g airline catering portions, because trays

holding large numbers can be put into the oven on

racks, making them easier to handle. The industrial-

grade product is made in a batch pasteurizer using

quite a different process.

Thickened Cream for Confectionery

0036 Confectionery products, such as clotted cream

fudge, toffee, and icecream, require a product that

can be described as clotted cream but do not need

the crust, the flavor, or the texture of true clotted

cream.

0037 Cream at 55% fat is homogenized at 1315–

1754 gsc, heated to 65



C for 45 min, then cooled to

30–35



C when it has a thick blancmange-like con-

sistency. This is poured into block molds and frozen.

It is kept frozen until required by the confectionery

manufacturer. The confectionery demand is increas-

ing, and this outlet helps to even out seasonal require-

ments for the traditional product.

Cooling and Packing

0038 To produce a good traditional clotted cream, which-

ever heating method is used, slow cooling is an im-

portant factor in achieving a firm even texture and the

typical grainy crust. If the cream is cooled too rapidly

by putting it into a cold store, at a temperature down

to 4



C, the cream loses its full flavor, and the texture

becomes hard.

0039 Containers are transferred from the heating area to

a closed room at 12–14



C, where the air is still, or

preferably fitted with fans creating positive pressure,

to avoid contamination at this vulnerable stage.

The room can be fogged, i.e., the air sprayed, using

peracetic acid at 25 p.p.m. to control molds and

yeasts. It takes 4–5 h at this temperature for the fat

to cool sufficiently to avoid crust collapse and loss

of flavor. The containers can then be moved to a

cold store maintained at 4



C for a further 12–24 h

before lidding and subsequent storage. Some com-

mercial operations lid the retail pots after 5 h of

cooling but run the risk of condensation forming

inside the pots.

0040Steps should be taken to avoid contamination

during transportation of cream to and between

cooling areas, particularly from dust, the main source

of mold and yeast infection. Many operators transfer

the trays of cream on trolleys, which must be cleaned

daily along with the rest of the equipment.

0041Farming enterprises and commercial operators

who scald cream in large trays, either sell it in the

returnable trays directly to catering establishments or

pack it at a later stage into retail containers. Farmers

may ladle the desired amount of cream into cartons or

jars for individual customers, or pack it into lined tins

for posting to regular consumers.

0042One commercial enterprise cools the cream briefly

before filling into containers using filling machines

while the cream is still liquid. Care must be taken

with mechanical handling of the cream at this stage,

from the point of view of damaging the texture

and postheating contamination. If this practice is

followed, the cream must be cooled further before

the pots can be sealed. Cream processed in retail

containers is best cooled for 24 h or overnight before

lidding, but some operators seal the containers much

sooner than this.

0043Lids may be of the snap-on variety, which can be

applied by hand in small enterprises, with a hand-held

applicator, or automatically in a lidding machine. The

individual portions for airlines have a foil lid that is

heat-sealed in place. This method would be used for

larger retail tubs also.

Shelf-life and Problems

0044Raw cream of good bacteriological quality and hy-

gienic production methods should give a traditional

product with a shelf-life of at least 14 days. The most

common problem is the appearance of molds on the

surface, caused by postheating contamination during

transfers in the cooling and packing operations. An

entry air lock to avoid direct contact with outside air

or an area with positive air pressure where these parts

of the process can be carried out will do much to

alleviate this problem.

0045The production of clotted cream is governed in the

UK by Cream Regulations. Intending manufacturers

should contact the Ministry of Agriculture, Fisheries,

and Food or their local authority to find out the

relevant details for their operation. If the product is

labeled ‘clotted cream,’ it must contain no ingredients

1696 CREAM/Clotted Cream

other than cream, including flavoring or other added

ingredients, whether or not that ingredient is a con-

stituent of milk. Nisin is the only permitted preserva-

tive in clotted cream, but it is not used widely. If it is

used, it must be declared on the label. (See Cream:

Types of Cream; Heat Treatment: Ultra-high Tem-

perature (UHT) Treatments; Nisin.)

Acknowledgment

0046 Thanks to Torridge Vale Creamery, Torrington,

Devon, who produce Definitely Devon clotted

cream, for supplying information about their com-

mercial process for use in this entry.

See also: Cream: Types of Cream; Heat Treatment: Ultra-

high Temperature (UHT) Treatments; Nisin

Further Reading

MAFF Publications (1971) Leaflet 438 Farmhouse Produc-

tion of Clotted Cream. Alnwick, UK: Lion House.

Martin C (1999) Clotted Cream. Redruth, UK: Tor Mark

Press.

Rothwell J (ed.) (1989) Cream Processing Manual, 2nd edn.

Huntington, UK: The Society of Dairy Technology.

Street L and Singer A (1975) The Backyard Dairy Book.

Dorchester, UK: Prism Press.

Thear K (ed.) (1978) The Home Dairying Book. Saffron

Walden, UK: Broad Leys.

Cream Liqueurs See Liqueurs: Composition; Cream Liqueurs

Crustacea See Shellfish: Characteristics of Crustacea; Commercially Important Crustacea; Characteristics

of Molluscs; Commercially Important Molluscs; Contamination and Spoilage of Molluscs and Crustaceans;

Aquaculture of Commercially Important Molluscs and Crustaceans

Cryogens See Freezing: Principles; Operations; Blast and Plate Freezing; Cryogenic Freezing; Storage of

Frozen Foods; Structural and Flavor (Flavour) Changes; Nutritional Value of Frozen Foods

CRYSTALLIZATION

Basic Principles

L Yu and S M Reutzel-Edens, Lilly Research

Laboratories, Indianapolis, IN, USA

Background

0001 Crystallization is the physical transformation (phase

transition) of a liquid, solution, or gas to a crystal,

which is a solid with an ordered internal arrangement

of molecules, ions, or atoms. Many substances of

scientific, technological, and commercial importance

are crystalline, ranging from large-tonnage commod-

ity materials to high-value specialty chemicals. Crys-

talline substances of importance to food sciences and

nutrition include sugars, sugar alcohols, salts, fats,

fatty acids, artificial sweeteners, etc. Crystallization

is a means to isolate chemical substances in the solid

form for long-term storage and downstream process-

ing. As a purification technique, crystallization relies

on the stringent structural requirement for crystal

formation to exclude impurities. Crystallization per-

formed under different conditions can yield crystals

of different sizes and morphologies, thus providing a

way of modifying particles to desired specifications.

The growth of large single crystals is essential to the

determination of the structures of molecules and crys-

tals by crystallography and the fabrication of optical

and electronic devices. The science of crystallization

has been developed on the foundation of thermo-

dynamics, kinetics, fluid dynamics, crystal structures,

and interfacial sciences.

0002This article is divided into three sections. The first

section, Principles of Crystallization, is concerned

with the thermodynamic conditions of crystallization

and the kinetics of crystal nucleation and growth. The

second section, Molecular Aspects of Crystallization,

discusses how the knowledge of molecular and crystal

CRYSTALLIZATION/Basic Principles 1697

structures provides mechanistic insights into crystal-

lization and how elemental steps of crystallization

can be manipulated at the molecular level to gain

control over competing crystallization pathways.

The third section, Techniques of Crystallization, de-

scribes the various ways in which crystallization can

be performed.

Principles of Crystallization

0003 Crystallization usually occurs through a nucleation

and growth mechanism. Nucleation is the formation

of stable molecular aggregates (nuclei) capable of

growing into macroscopic crystals. Crystal growth is

the actual development of the nuclei into visible di-

mensions. Both nucleation and growth require a

thermodynamic driving force. In the case of solution

crystallization, the thermodynamic driving force is

supersaturation. Supersaturation is generated when

the concentration of a solute exceeds its equilibrium

solubility, the solute concentration in a solution that

is in equilibrium with solute crystals. Supersaturation

can be expressed as the difference between the con-

centration of a saturated solution c

and the equilib-

rium solubility c

* or as the ratio between the two:

c ¼ c

 c

*orS ¼ c

*. The thermodynamic

driving force for crystallization from a melt is gener-

ated by lowering the temperature below the crystal

melting point, or undercooling. For crystallization to

occur from a gas, the vapor pressure of the crystalliz-

ing component must exceed its equilibrium vapor

pressure, i.e., the vapor pressure attained when the

gas is in equilibrium with the crystals.

0004 Although any nonzero supersaturation can in

principle cause crystallization from a solution, crystal-

lization usually does not occur unless the supersatur-

ation exceeds a certain threshold, or metastable limit.

A diagram illustrating this phenomenon is shown in

Figure 1. The solid line depicts the equilibrium solu-

bility of a solute as a function of temperature. The

region below the solid line is undersaturated,

where crystallization is thermodynamically impos-

sible. The region above the solid line is supersat-

urated, where crystallization is thermodynamically

possible, and the solution is metastable. The metasta-

ble limit is indicated by the dashed line, and the

region between the solid and dashed lines is called

the metastable zone. A supersaturated solution free of

crystal seeds can remain in the metastable zone indef-

initely. For a solute to crystallize spontaneously,

the solution concentration must be raised not only

above the equilibrium solubility (solid line), but also

above the metastable limit (dashed line) into the so-

called labile zone. The width of the metastable zone

depends on the nature of the solute and conditions of

crystallization (e.g., stirring, solvent, temperature,

pressure, the presence of impurities, and the surface

characteristics of the crystallization vessel).

0005Crystal nucleation may be classified as primary or

secondary. Primary nucleation refers to the formation

of crystal nuclei from a solution that contained

no preexisting crystals. Primary nucleation occurs

through both homogeneous and heterogeneous mech-

anisms. Homogeneous nucleation is the formation of

nuclei within a homogeneous fluid, and heterogeneous

nucleation is initiated by contact with foreign particles

and surfaces. The effectiveness of surfaces or interfaces

as templates for nucleation frequently makes the het-

erogeneous mechanism the dominant mechanism when

particulate contaminants are present. Secondary nucle-

ation refers to the generation of crystal nuclei from

preexisting crystals, such as those introduced through

seeding.

0006The classical theory of homogeneous nucleation

treats crystal nucleation as a thermally activated pro-

cess whose activation energy is the free energy change

(DG) for forming a microscopic crystal within a liquid.

DG is further divided into a surface term (DG

) and a

volume term (DG

). DG

is the surface free energy

change for creating a crystal–liquid interface, which

is thermodynamically unfavorable (DG

> 0) owing to

the generation of surface tension. DG

is the volume

free energy change for the liquid-to-crystal trans-

formation, which is thermodynamically favorable

(DG

< 0) owing to the greater thermodynamic stabil-

ity of the crystalline product. Thus, the volume and

surface terms represent the driving force and the obs-

tacle of nucleation, respectively. The interplay of the

two terms dictates whether or not a crystal nucleus is

viable, i.e., capable of growing into larger sizes. Since

both terms increase in magnitude with the crystal size

r, but at different rates (DG

proportional to the sur-

face area and DG

proportional to the volume of the

Supersaturated

Labile zone

Metastable

zone

Solubility curve

Undersaturated

Temperature, T

Concentration, c

fig0001Figure 1 Equilibrium solubility, supersaturation, and metasta-

ble limit. An undersaturated solution can be induced to crystallize

by solvent evaporation (path a), temperature change (path b), or

both.

1698 CRYSTALLIZATION/Basic Principles

crystal), DG achieves a maximum at some crystal size

r*, termed the critical nucleus size (see Figure 2). The

critical nucleus size is so named because crystals

smaller than r* will dissolve spontaneously, whereas

crystals larger than r* will grow spontaneously.

0007 The process of crystal growth requires the transfer of

mass to the growth interface and the dissipation of heat

generated by crystallization. The rate of crystal growth

therefore depends on the rates of mass and heat trans-

fer, which in turn depend on the viscosity of the

medium and the degree of agitation. Impurities strongly

absorbed to crystal surfaces can significantly retard

crystal growth and modify crystal morphologies.

0008 Crystallization rate depends on temperature be-

cause temperature affects both the molecular motions

required for nucleation and growth and the degree

of supersaturation. If crystallization occurs from a

single-component melt, supersaturation becomes ir-

relevant, and a characteristic temperature depend-

ence often emerges (Figure 3). The crystal growth

rate, r

, usually increases with temperature until the

arrival of the crystal melting point, above which no

crystallization is possible. Contingent upon the for-

mation of molecular clusters, nucleation requires a

low temperature, but not too low so that molecular

mobility becomes limited. Thus, the nucleation rate,

, plotted against the temperature typically shows a

maximum below the melting point. The combination

of the two temperature effects leads to the fastest

crystallization rate, r

, achieved at an optimal tem-

perature that is intermediate between the preferred

temperatures for nucleation and growth.

0009Crystal growth from solution does not proceed at

the same rate in all directions. As crystals grow, dis-

tinct morphologies or habits emerge as a result of

face-specific growth rates. Slower growth rates trans-

late to larger crystal faces and vice versa, as shown in

Figure 4. The face-specific growth rate depends on

both the internal structure of the crystal and growth

conditions (e.g., supersaturation and impurities),

enabling crystals of the same internal structure to

develop different external habits.

0010A crystallization process may produce crystals of

different sizes. The control of particle size distribu-

tion relies on the control of nucleation and growth,

which can be achieved by adjusting supersaturation,

seeding, agitation, resident time, etc. Crystals allowed

to remain in contact with a saturated solution can

undergo ripening, which causes small crystals to

Radius of a spherical nucleus

Activation energy for nucleation

∆G

fig0002 Figure 2 Classical theory of homogeneous nucleation. The

free-energy change of crystal nucleation, DG, is divided into a

surface term DG

, representing the free energy increase for

creating a crystal–liquid interface, and a volume term, DG

, rep-

resenting the free energy decrease for the liquid-to-crystal phase

transition. The different dependence of DG

and DG

on the

crystal size r leads to a maximum of DG at the critical nucleus

size r*. Nuclei dissolve if they are smaller than r* and grow if they

are larger than r*.

Rate

Temperature

m.p.

fig0003Figure 3 Typical temperature dependence of crystallization

rate from a single-component melt. r

, crystal growth rate; r

crystal nucleation rate; r

, overall crystallization rate. The fastest

crystallization rate is achieved at an intermediate temperature

between the preferred temperatures for nucleation and growth.

fig0004Figure 4 Illustration of the growth of a single crystal. Slower

crystal growth rates lead to larger crystal faces.

CRYSTALLIZATION/Basic Principles 1699

dissolve and large crystals to grow. The driving force

for ripening is the slightly higher solubility of smaller

crystals owing to their higher surface energies.

0011 Polymorphism is the ability of a molecule to

crystallize in multiple crystal forms that differ in mo-

lecular packing and/or conformation. A related phe-

nomenon, pseudopolymorphism, arises if a molecule

can cocrystallize with solvent molecules, forming a

solvate. A solvate may be stoichiometric or nonstoi-

chiometric, depending on whether the amount of

solvent is fixed or variable. Since polymorphs and

solvates have different physical properties (e.g., solu-

bility and crystal morphology), more stringent control

may be necessary for their crystallization. In crystal-

lizing polymorphic compounds, unstable polymorphs

sometimes crystallize first and then transform into

stable forms. This observation is summarized by

Ostwald’s step rule, which states that the first form

to crystallize is the most soluble (the least stable)

form, which then transforms to the next most

soluble form through a process of dissolution and

recrystallization, and so on. This rule, which has not

gone unchallenged, is applicable to crystallization

initiated at sufficiently high supersaturations that

the least stable (most soluble) polymorph is driven

to crystallize. Obtaining metastable polymorphs fre-

quently requires rapid crystallization and immediate

harvest because of potential conversion to more stable

polymorphs.

Molecular Aspects of Crystallization

0012 Crystallization at the most basic level is the assembly

of molecules; thus, an understanding of the molecular

events of crystallization promises the ultimate control

of crystallization. Certain crystallization processes

have been identified as the assembly of molecular

aggregates, rather than single molecules. These mol-

ecular aggregates, called prenucleation aggregates or

growth units, are often in the form of dimers, trimers,

or high multimers of molecules. The growth units of

some carboxylic acids, for example, are found to be

cyclic dimers held together by hydrogen bonds. Thus,

in the molecular theory of crystallization, a multistep

process is envisioned: the association of molecules

into prenucleation aggregates in solution, the assem-

bly of prenucleation aggregates into crystal nuclei,

and the growth of crystal nuclei into macroscopic

crystals (Figure 5).

0013The identification and manipulation of prenuclea-

tion aggregates underlie molecular strategies for

crystallization control that go beyond traditional ap-

proaches based on the nucleation-growth model.

Examples of such strategies include the use of ‘tailor-

made additives’ for polymorph control, morphology

modification, and inhibition of unwanted crystalliza-

tion. Chemical principles of molecular structures and

interactions are used to guide the selection of such

additives. For example, molecular details of a domin-

ant growth surface can be analyzed to determine

suitable impurities that may preferentially bind to

and inhibit the growth of the surface, thereby altering

the crystal habit. The introduction of impurities with

molecular conformations resembling those present in

one polymorph (‘conformational mimicry’) has been

used to prevent its crystallization, leading to the crys-

tallization of another polymorph.

0014Crystal engineering is the design of crystal struc-

tures with desired properties (e.g., nonlinear optical

activity arising from noncentrosymmetric structures)

from molecular structures. The same chemical prin-

ciples guiding the molecular control of crystallization

are employed in such engineering of crystals, as are

reliable structural patterns of intermolecular inter-

actions in crystals (e.g., close packing, hydrogen

bonding and organometallic coordination chemistry).

0015The separation of mirror-related molecules that are

not superimposable with each other (enantiomers)is

called chiral resolution and is an important applica-

tion of crystallization. With some molecules, chiral

resolution occurs spontaneously; that is, a solution

containing opposite enantiomers crystallizes to

yield a physical mixture (conglomerate) of crystals

containing pure enantiomers. A conglomerate can

be separated manually into crystals of pure enantio-

mers, as Louis Pasteur famously demonstrated with

sodium ammonium tartrate. With seeding, one enan-

tiomer can be induced to crystallize from a self-

resolving solution. This process, known as kinetic

resolution, requires that the system does not come

Molecule Prenucleation

aggregate

Crystal

nucleus

Crystal

fig0005 Figure 5 Molecular model of crystallization.

1700 CRYSTALLIZATION/Basic Principles

into equilibrium to precipitate both enantiomers.

For most chiral molecules, spontaneous resolution is

impossible, and opposite enantiomers crystallize to-

gether to form a so-called racemic compound.In

these cases, chiral resolution usually requires the add-

ition of a resolving agent, which is a chiral molecule

that forms a crystal with one enantiomer more readily

than with the other. Preferential crystallization of

one enantiomer may also be accomplished with a

chiral solvent, in which the solubility of the opposite

enantiomers is different.

Techniques of Crystallization

0016 Crystallization techniques differ in the way in which

supersaturation is generated and relieved, and nucle-

ation is initiated. The importance of selecting an ap-

propriate technique lies in the fact that the same

crystallization performed under different conditions

can yield crystals of different properties (size, morph-

ology, chemical purity, polymorphic form, etc.). The

design of a cost-effective crystallization process re-

quires many types of data, including mass and heat

transfer and equilibrium solubility as a function of

solvent, temperature and pH.

0017 To induce crystallization from a solution, the ne-

cessary supersaturation can be generated in several

ways, including solvent evaporation, temperature

change, antisolvent addition, and chemical reaction.

In Figure 1, solvent evaporation at a constant tem-

perature corresponds to a vertical line from the under-

saturated to the supersaturated region. Solvent

removal at controlled temperatures can be performed

either with or without vacuum. Slow evaporation of

solvent is frequently used to obtain large, high-quality

single crystals.

0018 Crystallization induced by temperature change

(temperature gradient) takes advantage of the tem-

perature dependence of solubility. In Figure 1, the

generation of supersaturation by temperature change

without solvent removal corresponds to a horizontal

line from the undersaturated to the supersaturated

region. If solubility increases with temperature, a

high-temperature solution can be cooled to generate

supersaturation. In general, slower cooling rates lead

to nucleation at higher temperatures, producing

fewer and larger crystals. To effect additional control,

it is customary to seed the solution so that crystalliza-

tion is initiated at the desired temperatures. Once

crystallization begins, the suspension can be cooled

to improve the product yield (assuming that solubility

increases with temperature).

0019 Crystallization by antisolvent addition (also called

drowning out) depends on the solvent dependence of

solubility. With this technique, a solution to be

crystallized is mixed with an antisolvent, which is

miscible with the initial solvent and in which the

solute is less soluble. The addition of antisolvent

lowers the solubility and generates supersaturation.

The onset of crystallization is signaled by turbidity,

after which precipitation usually follows.

0020Vapor diffusion is a common technique for grow-

ing high-quality single crystals. With this technique,

an antisolvent is diffused into a solution through the

vapor phase and slowly generates supersaturation.

An analogous technique, liquid diffusion, is carried

out by slow addition of a lower-density antisolvent,

allowing the initial solution to float to the top. As the

antisolvent gradually diffuses into the initial solution,

crystals may grow at the interface.

0021Crystallization may directly follow chemical reac-

tions that produce low-solubility products. For

example, an acidic or basic solute may be crystallized

as a less soluble salt with an appropriate counter ion.

Acid–base titration is a recrystallization technique

used to purify free acids and bases, whereby a soluble

salt is first generated at a high or low pH, and then the

solution is neutralized to the starting pH to precipi-

tate the solids.

0022The condensation of vapors produced by sublim-

ation provides a way to grow crystals from the gas

phase. With this technique, a solid is placed near a

condensation surface and heated to produce sufficient

vapor pressure. The condensation surface supplies a

substrate for the formation of crystals.

0023Freeze-drying, also called lyophilization, is a drying

process in which the solvent (typically water) is first

frozen and then removed by sublimation in vacuum.

Owing to the low temperature used, freeze-drying

is frequently a gentle drying technique, especially

suitable for thermally labile substances (e.g., proteins

and peptides). The low processing temperature often

prevents solute crystallization in freeze-drying; none-

theless, many solutes crystallize predictably (e.g.,

glycine, mannitol and NaCl). (See Freeze-drying:

The Basic Process; Structural and Flavor (Flavour)

Changes.)

0024Spray-drying is a one-step drying process that con-

verts a solution to a powder. In a spray-drier, the feed

solution is first atomized into a spray. The spray is

then dried through contact with heated air streams.

The dried powder is usually separated from the air

stream via a cyclone separator and collected at the

base of the drying chamber. Spray-dried particles are

often roughly spherical and have a narrow size distri-

bution. (See Drying: Spray Drying.)

0025Crystallization with the aid of supercritical

fluids takes advantage of the dissolving power of a

substance (e.g., CO

) existing in the supercritical

state, a state of matter created by high pressure and

CRYSTALLIZATION/Basic Principles 1701

exhibiting gas- and liquid-like properties. A supercrit-

ical fluid rapidly returns to the gaseous state upon

reduction of pressure (e.g., by expansion through a

nozzle). When this happens, solutes dissolved in a

supercritical fluid will precipitate quickly, often

yielding fine, evenly sized, and spherical particles. It

is also possible to use a supercritical fluid to rapidly

extract the solvent from a solution, causing the solute

to precipitate.

0026 Spherical crystallization is a technique of forming

dense and spherical agglomerates of crystals. This

technique is based on adding to a crystallizing

medium a small amount of an immiscible liquid

(bridging agent) that preferentially wets the develop-

ing fine crystals and encourages them to compact into

spherical agglomerates.

0027 Epitaxy refers to the oriented growth of one crystal

on the surface of another crystalline substance, with

the crystallization promoted by the matching period-

icity of the two crystal lattices. The lattice match

lowers the activation energy for nucleation and

growth. Structural studies by crystallography and mi-

croscopy of substrate-promoted crystallization have

identified epitaxial match as the underlying cause for

oriented growth of crystalline overlayers. Epitaxy is

also invoked as an explanation for nucleation induced

by foreign particles.

See also: Freeze-drying: The Basic Process; Structural

and Flavor (Flavour) Changes

Further Reading

Davey R and Garside J (2000) From Molecules to Crystal-

lizers. An Introduction to Crystallization. Oxford:

Oxford University Press.

Garti N and Sato K (eds) (1988) Crystallization and Poly-

morphism of Fats and Fatty Acids. New York: Marcel

Dekker.

Jacques J, Collet A and Wilen SH (1991) Enantiomers,

Racemates, and Resolutions. Malabar, FL: Krieger.

Mullin JW (2001) Crystallization, 4th edn. Oxford: Butter-

worth-Heinemann.

Myerson AS (1993) Handbook of Industrial Crystalliza-

tion. Stoneham, MA: Butterworth.

Cucurbits See Vegetables of Tropical Climates: Commercial and Dietary Importance; Root Crops of

Uplands; Root Crops of Lowlands; Edible Aroids

Cultured Milk Products See Fermented Milks: Types of Fermented Milks; Dietary Importance;

Yogurt: The Product and its Manufacture; Yogurt-based Products; Dietary Importance

CURING

M Shimokomaki and E Youssef Youssef, Londrina

State University, Londrina, PR, Brazil

N N Terra, Santa Maria Federal University, Santa

Maria, RS, Brazil

Introduction

0001 The use of salt and curing is undoubtly the tech-

nological procedure which is most commonly

employed to transform meat into a meat product of

quality. This practice has been applied before Roman

times in order to preserve meat products. Curing

methods consist of the application of salt (sodium

chloride), sodium nitrate, and nitrite, and reducing

agents. There is no curing processing without salt and

probably nitrate was discovered as an impurity of

salt. Sodium nitrite is the most important component

since it plays many roles in meat curing. It is the

chemical agent which develops the typical color

cured meat products through the formation of

nitric oxide compound from the reaction of nitrite-

myoglobin. In contrast, it helps to develop the typical

flavor and increases the stability for oxidative

reactions. Most importantly, nitrite inhibits the devel-

opment of several undesirable pathogenic micro-

organisms, including Clostridium botulinum. These

1702 CURING

multiple functions of nitrite make it difficult to

find an ideal nitrite substitute; nitrite gives rise

to nitrosamine compounds, which are known to be

potent carcinogenic agents. However it has recently

been reported that on average only one third of daily-

ingested nitrite originates in cured meat products.

This article will discuss the events that occur in curing

processing.

Raw Material

0002 Care should be taken in relation to the raw material.

The meat system characteristically presents a weak

acid, pH 5.5–6.0, after completion of rigor mortis.

This final pH value is important for curing reactions

because under these conditions nitrite forms nitrous

acid which also participates in curing reactions. This

pH control is very relevant, for instance, during

cooked ham processing; the pH of the semimembra-

nosus muscle should be above 5.8 and refrigerated

pork meat pieces should not be frozen because freez-

ing would create colour defects. The meat tempera-

ture should not be too cold as the full series of color

reactions cannot take place and the final cured color

formation is impaired.

Ingredients

0003 The ingredients applied for curing processing are salt,

phosphates, nitrite, sugar, reductants, binders and

extenders, and antioxidants.

Salt

0004 Salt (NaCl) is the main ingredient in quantity and has

two major functions. It solubilizes myofibril proteins,

helping to stabilize meat emulsion: the concentration

of 6–8% solution is most effective. In contrast, salt,

being a dehydrating agent, would alter osmotic pres-

sure and thus inhibit bacterial growth and subsequent

spoilage. Years ago, it was common to use a high

concentration of salt and presently this concentration

is around 2–3%, therefore it is necessary to store

derived meat products under refrigeration. Under

higher salt concentrations with subsequent lower

water activity values, these products are able to resist

bacterial spoilage, for instance, copa, salami, charqui

meats in South America, and biltong in South Africa;

these are known as intermediate-moisture meat

products. Their shelf-life can be extended, even at

room temperature, for several months. NaCl is also

an important factor to enhance the flavor of meat

products. This is a beneficial factor and is why it is

included in the preparation of these products but can

cause problems by increasing consumers’ high blood

pressure. However the amount of sodium chloride

can be reduced by mixing with potassium chloride,

up to 50% concentration.

Sodium Nitrite

0005Nitrite in meat products has the following functions:

(1) it stabilizes the color; (2) it contributes to the

flavor of cured meat; (3) it inhibits the development

of spoilage microorganisms, in particular avoiding

the appearance of botulism; and (4) it delays the

development of rancidity. Nitrate is used as a source

of nitrite formed after the action of microorganisms

and it has not been used so frequently, since it has

been substituted by nitrite in meat processing. There

are a few exceptions to this: country cured hams and

Lebanon bologna.

0006The color formation in cured meat is the result of a

series of complex chemical reactions involving sodium

or potassium nitrite (NO

). This complex series of

chemical reactions can be summarized as follows:

2KNO

> 2KNO

2KNO

> 2HNO

2HNO

> H

O þ N

> NO þ NO

The nitric oxide formed combines with myoglobin

heme, a constituent of meat pigment, and this colored

complex pink color (hemocrome) becomes stable

after it has been heated. The other reaction for nitrite

is a direct interaction with myoglobin (Fe

2þ

); as this is

a strong oxidizing compound, it can produce brown

oxidized metmyoglobin (Fe

3þ

). The Fe

3þ

ions formed

have the capacity to be active promoters of oxidative

reactions and there is a need to convert them to

inactive and stable Fe

2þ

forms. This can be achieved

by the action of reducing agents added as ascorbate or

erithorbate. In the final cured meat, it has been found

that residual nitrite found reduced to 10–20% of the

original amount added. Only about 5–10% of added

nitrite reacts with myoglobin, 1–5% combines

with lipid components, and 1–5% forms volatile

components.

Phosphates

0007Sodium phosphates are used in meat processing in

order to increase water-binding capacity because this

helps to solubilize myofibril proteins. In these func-

tions, only alkaline phosphates are efficient for this

role because they raise the pH value, becoming effect-

ive water binders. Also they bind heavy metal, having

in the end a preservative role to inhibit the develop-

ment of microbial growth. Normally, phosphates are

used in most pumped meats, i.e., ham, bacon, roast

CURING 1703

beef, and pastrami, which give the advantage of redu-

cing the cookout, improvement of sliceability, reten-

tion of flavor, and juiciness. There is a limit on the

quantity of phosphates used of a maximum of around

3000 mg kg

1

, because above that level would give a

metallic or soapy taste to the product.

Sugar

0008 The addition of sugar to the curing process is primar-

ily for flavor. The presence of sugar counterbalances

the harsh taste promoted by a high concentration of

salt. In case of fermented products, the addition of

sugar is necessary as an energy source for micro-

organisms to perform fermentation. When heat treat-

ment is used during processing, a browning reaction

results in a burning flavor.

Reductants

0009 The isomers ascorbate and erythorbate are com-

monly used in modern curing practice. Because of

their reducing role, their function is primarily as a

curing accelerator reducing nitrite to nitric oxide and

reducing the meat pigment, thus giving stability to

meat product color. By stimulating nitric oxide for-

mation they suppress nitrosamine formation and

therefore become an important ingredient, in particu-

lar for bacon.

Binders and Extenders

0010 These are used to increase the binding properties of a

meat mixture, giving an economic advantage to the

producers. Thus, they improve emulsion stability,

give better yields and better sliceability, and are per-

mitted in cooked sausage at 3.5%. These compounds

are nonmeat proteins like soy and its hydrolyzed

products, several starches and of course hydro-

colloids like carrageenan.

Antioxidants

0011 There is a group of compounds which function to

prevent rancidity. Butylated hydroxytoluene (BHT),

butylated hydroxyanisole (BHA), tertiary butyl

hydroquinone (TBHQ), and propyl gallate are the

major antioxidants in use. Recently the application

of natural compounds has become popular and the

practice of including for example, vitamin E, in the

dietary ration for daily animal consumption has been

shown to extend the meat product shelf-life greatly.

Processing

0012 In practice, there are at least three methods of curing:

(1) dry curing; (2) wet (pickle) curing; (3) and com-

bined dry and wet curing.

Dry curing

0013This old-time procedure is the easiest method of

curing since all that is needed is to rub the salt alone

or in combination with nitrite and/or nitrate and

sugars into the surface of the meat. This technique is

applied without any water, thus the curing ingredients

are dissolved in the moisture of the original samples

and then they diffuse. Also, fermented dry-cured saus-

ages are prepared by mixing minced meat and ingre-

dients and stuffing them into natural pork or lamb

casings or artificial casings made from reconstituted

collagen. Depending on the type of product, they are

placed in natural or air-conditioned drying chambers

at a specific temperature and time, hence natural

microorganisms develop, constituting the ripening

of the product. In some cases, starter cultures are

added to accelerate the fermentation process and

concomitantly sugars are transformed into lactic

acid, lowering the pH value. This final pH is similar

to the meat isoelectric point lowering the water-

holding capacity that would further help the dryness

of the product. The advantage of this methodology

lies in the higher shelf-life because of the intermediate

water activity values and firmness and the product

presents more flavor. The disadvantages are econom-

ical since more labour is required and there is a waste

of processing laboratory space; the process is time-

consuming, and the final product has a harsh salty

flavor. The list of cured products derived from the

application of this technique includes ham (Spanish

serrano, Italian parma, French bayonne, American

country-style, German westphalia), South American

charqui meats, and Italian copa.

Wet curing

0014Historically this method originally involved the immer-

sion of meat pieces into cold brine containing dissolved

curing salts. Although the diffusion of these ingredients

was quicker than rubbing dry salt into the meat, it was

in fact a time-consuming technology. A technique to

speed up curing is injecting pickle into the entire ham

through artery pumping. The needle is usually inserted

in the femoral artery using the same ingredient as for

dry curing but the brine is dissolved in water to make a

pickle. Thus, the need to speed up even more this curing

technology brought about the multiple-needle stitch

automatic injection that is currently used to prepare

ham and bacon, followed by tumbling to accelerate

brine diffusion and give uniformity of the curing mix-

ture. To finish this process, these products are exposed

to heat and/or smoke.

0015Finally, for the wet curing of minced meat, the

curing mixture is added to the meat and fat to prepare

the meat emulsion. In order to prevent excessive heat,

1704 CURING

ice is included to cool down the process. This helps

emulsion stabilization by avoiding protein denatur-

ation and fat fusion point. Thus the product is stuffed

in cellulose, plastic, or reconstituted collagen casings,

followed by heat treatment, and can be smoked. This

technique is applied to process frankfurters, Bologna

sausages, and mortadella. However, the disadvan-

tages are again the poor utilization of space and the

products have a milder flavor in comparison to dry

curing, although less labor is required.

Combined Dry/Wet Curing

0016 Some cured products are processed by applying a

combination of both techniques. This is the case for

South American charqui meat processing. The condi-

tions for charqui meat preparation are to incorporate

the curing ingredients into the meat to achieve uni-

form distribution, and thus to accelerate curing and

to stabilize the color. Figure 1 shows a flow diagram

for charqui processing. Essentially it is prepared by

using the whole sides of muscles, immersing them in

concentrated brine of approximately 25



B contain-

ing 3–4% sodium or potassium nitrite for hours,

allowing the brine to diffuse through the meat. The

curing procedure can be considerably shortened by

injection of brine into the meat to achieve uniform

distribution. In a more modern technique, multiple

injections are applied automatically, and brine is

injected simultaneously. Thereafter samples are sub-

mitted to dry salting on a concrete floor on which the

meat pieces are stacked into piles separated from each

other by layers of coarse sea salt (approximately

1 mm thick). After about 8 h, the meat is restacked

and the uppermost meat pieces are repositioned at the

bottom of the new piles. The maneuver is repeated

every 24 h (three to five times), and, after washing to

remove excess salt from the meat surface, samples are

subjected to the drying stage directly in the sun on

wooden rails. At night, meat pieces are collected and

piled on the concrete floor and covered with a tar-

paulin. Finally, samples are packed under vacuum in

polyethylene bags of 1–5 kg and commercialized.

Functional properties of cured products

0017 The changes promoted by curing processing in rela-

tion to raw materials are relevant and consequently

color, flavor, texture, and nutritive values are de-

scribed below.

Color formation

0018 Fresh meat color is primarily determined by the

oxidation state of iron and the radical attached to

the heme group of the myoglobin pigment that also

presents the globular protein portion, globin, in its

structure. Myoglobin can be found in three forms:

(1) desoxymyoglobin (Mb) or reduced myoglobin,

which is a purple red color, with the iron state of

2þ

, (2) oxymyoglobin (MbO

), oxygenated myo-

globin, which is a bright red color with Fe

2þ

;and

(3) metmyoglobin (MMb), oxidized myoglobin,

brown color with Fe

3þ

. During the curing process,

because nitrite is a oxidizing agent for Mb, it converts

Mb and MbO

to MMb. Thus, nitric oxide reacts

with MMb to form nytrosylmetmyoglobin (brown

with Fe

3þ

) that is reduced to nitric oxide myoglobin,

nitrosylmyoglobin (MbNO), which is the desired

bright pink color, although unstable, with Fe

2þ

Finally, MbNO when heated is denatured to form

nitrosylhemochrome (stable pink color). This is the

typical cured-meat color with Fe

2þ

, as shown in

Figure 2.

Flavor

0019Cured meat flavor is a complex issue and the chemical

reactions responsible for its formation are not well

understood. This is understandable since it is believed

that there are nearly 1000 volatiles compounds in the

fresh meat. The presence of sodium nitrite decreases

not only rancidity but also the warmed-over flavor

present in cured meat. The reaction of the heme group

with nitric oxide giving a typical color is also con-

nected to the cured meat flavor.

Texture

0020Texture is a subject which is seldom discussed with

reference to cured meat. However, some insights have

already been reported with whole meat such as char-

qui meats. Before consumption charqui meats are

immersed in several changes of water in order to

remove salt. Because of the meat buffering properties,

about 3–4% is the final amount of remaining salt.

These meat products are tougher than control raw

samples. There is five to six times the increase in

texture in charqui meat in comparison to raw mater-

ial and two to three times in relation to desalted

charqui, as measured by a texturemeter. A similar

pattern is observed after cooking these samples. The

water content plays a significant role in the texture of

charqui. The moisture content determined in raw

material is 76% and the measured shear force is

17 N; charqui contains 46.3% moisture and 96.5 N

and desalted charqui 59.0% moisture and 46.5 N

shear force. Cooked samples have a moisture value

of 52.0, 31.7, and 44.8%, in control, charqui, and

desalted charqui, respectively and shear force equiva-

lent to 52, 300 and 86 N for control, charqui, and

desalted charqui, respectively. There is an inverse

relationship between the amount of water and

CURING 1705