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

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prevailing temperature and humidity. Studies of

wood-smoke composition have shown the presence

of over 400 volatile components comprising 48 acids,

22 alcohols, 131 carbonyls, 22 esters, 46 furans, 16

lactones, 75 phenols, and some 50 miscellaneous

compounds. The precise composition of woodsmoke

and the extent to which the food absorbs smoke

constituents depend on several factors.

Factors Affecting the Composition of

Woodsmoke

Type of Wood

0007 Wood generally contains some 40–60% celluloses

(b-glucans), some 20–30% hemicelluloses (heterogly-

cans containing pentose and hexose residues), some

20–30% lignins (complex three-dimensional phenolic

polymers), and a small amount of protein. In add-

ition, there is a quantitatively minor, but chemically

complex, low-molecular-mass fraction in which

phenols and terpenes often dominate, particularly in

softwoods, and variable water content.

0008 Commonly woods are classified as hard or soft,

depending on their botanical origin, the former from

broad-leaved angiosperms, and the latter from

needle-leaved (often evergreen) gymnosperms, but

the terms ‘hard’ and ‘soft’ in this context are not

good guides to physical properties. Compared with

softwoods, hardwoods have larger contents of hemi-

celluloses, those in softwoods having a greater pro-

portion of pentosans. Softwoods have greater lignins

contents, with more guaiacyl (2-methoxyphenyl)

residues relative to syringyl (2, 6-di-methoxyphenyl)

residues than is usual in hardwood lignins.

0009 There is a general belief that smoking with hard-

woods such as oak (Quercus spp.), hickory (Carya

spp.), beech (Fagus spp.) and alder (Alnus spp.) pro-

duces superior smoked foods compared with smoking

with softwoods. Hardwoods generate more syringyl

derivatives relative to guaiacyl derivatives and more

acid smokes (see Table 1), due to structural differ-

ences in the lignins and the greater pentosan content

of hardwood hemicelluloses, respectively. These acids

produce smoked foods with lower pH values and

hence a greater microbiological stability, and this

may explain the traditional preference.

Wood Combustion Temperature

0010The temperature of smoke generation influences

its composition. The total carbonyls concentration

increases in the temperature range 200–600



C, total

phenols concentration in the range 400–600



and polycyclic aromatic hydrocarbons (PAH), while

virtually absent below 400



C, increase rapidly

thereafter with increased generation of methylene

radicals. The PAH have implications for product

safety, and modern methods of smoking are designed

to keep the generation temperature below 400



Cor

otherwise remove the PAH. The balance between

individual compounds within these broad groups is

also affected by combustion temperature, with guaia-

col, acetovanillone, and acetosyringone domina-

ting the phenols fraction at 380



C, but with

acetosyringone and propiosyringone dominating at

560



C. Some further data are shown in Table 2.

Smoke House Humidity

0011It is thought that smoke components enter the food

primarily by absorption into the interstitial water in

the food, at least for low fat foods. Initially, most

foods being smoked will contain about 80% moisture

and have a high water activity, both of which decline

progressively during the smoking process. The ab-

sorption of smoke components is thus more rapid at

the start of smoking and declines as the water content

declines.

0012The rate of water loss from the food is greatly

influenced by the absolute humidity of the air in the

kiln relative to the surface water activity in the food.

More rapid loss of water due to the use of hotter and/

or dryer smoke gives rapid surface drying and a firm

‘skin’ to the food, which may inhibit absorption of

smoke components. The use of wetter smoke slows

dehydration, but if insufficient drying occurs, the

product may be microbiologically unstable due to its

high water activity. Often, wet sawdust is used so as

to keep the smoldering temperature low and prevent

combustion, but excessive damping must be avoided,

tbl0001 Table 1 Effect of wood source on composition of smoke

Source Aroma fraction (mgper100 gof wood)

Total Phenols Carbonyls Acids

Oak (Quercus seriata) 1800 151 117 1140

Oak (Quercus acuta) 1600 225 323 820

Cherry 1490 101 111 660

Pine 1180 166 53 640

tbl0002Table 2 Effect of combustion temperature on smoke compos-

ition

Combustion

temperature (



Content insmoke

(mgper100 g of sawdust)

Phenol Guaiacol Syringol

450 20 60 85

550 30 80 150

650 55 125 260

SMOKED FOODS/Principles 5297

otherwise the very wet atmosphere created will

inhibit drying. Smoke composition is in any case

influenced by the moisture content of the wood that

is used – see Table 3.(See Water Activity: Principles

and Measurement.)

0013 Some woodsmoke phenols have antioxidative and

antimicrobial properties and, together with other

chemicals such as formaldehyde, can increase the

stability of smoked foods relative to unsmoked

foods. However, it is generally accepted that the

main preservative effect of traditional smoking is

through drying, especially in combination with

salting. Modern smoked foods have comparatively

low salt and high water contents (thus increasing the

yield and palatability of the product), but due to the

relatively high water activity that results, the product

shelf-life must be extended by chilling, freezing, or

canning.

Effect of Air Flow Rate

0014 Air flow is essential to keep wood smoldering, to

transfer smoke from the generator to the kiln, and

to minimize nonuniform smoke distribution within

the kiln. As discussed above, air flow rate and humid-

ity influence the rate at which the food surface dehy-

drates and thus the rate of smoke absorption. It has

been demonstrated that the dispersed particles of the

smoke act as a reservoir for the volatile vapor con-

stituents. Diluting the smoke with air at constant

temperature, or raising the smoke temperature, drives

volatiles from the dispersed droplets into the vapor

phase and encourages rapid and extensive transfer of

these volatiles to the food, whereas dilution of smoke

with cold air has the opposite effect.

Time of Smoking

0015 The moisture content in the surface layers of the food

decreases as smoking time increases, and this affects

the rate and extent of smoke absorption. The distri-

bution of fat on the surface of the food may also

change during the process, particularly where high

temperatures are employed. Higher temperatures

will cause greater protein denaturation and shrink-

age, and movement of fat from the interior to the

surface of the food, which will modify smoke absorp-

tion. Since smoke absorption is a first-order process,

the rate declines as surface saturation is approached,

since inward diffusion is slow and limiting. Even after

extensive smoking, the smoke concentration falls

markedly over the first 5 or 10 mm below the surface.

0016It should be noted that factors such as kiln tem-

perature, kiln humidity, air flow rate, and smoke

density are difficult to control in traditional kilns

where these parameters are strongly influenced by

weather conditions. In modern mechanical kilns,

such factors can be controlled within much closer

limits.

Smoke-generation Technology

Conventional Smoke Generators

0017In this type of generator, the smoke is produced by the

slow combustion of sawdust using heating coils or gas

flames. The sawdust is gently agitated continuously,

in the presence of sufficient oxygen. The resulting

smoke is then either drawn or pushed into the actual

smoking chamber and allowed to interact with the

food. The amount of tar and solid particles that enter

the chamber can be controlled by varying the distance

of the smoldering unit from the smoking unit; with a

longer distance, there is more opportunity for tar to

settle out. Various baffles and filters can be placed in

line to remove tar. Smoke production or generation

temperature using the smoldering technique can vary

over a fairly wide range, dependent upon the avail-

able air. If a large quantity of air is present, the actual

smoldering temperature may be in excess of 800



but if the amount of available air is controlled, lower

smoldering temperatures will result. The smoke-

production temperature can be lowered by control-

ling the moisture content of the sawdust. Low

smoldering temperatures also produce smoke gener-

ally lower in polycyclic aromatic hydrocarbons,

which has significant health implications.

Friction Smoke Generators

0018Some processors use the friction method, whereby an

intact wood block is pressed against a rapidly rotating

metal friction wheel. Heat produced by friction is

sufficient to cause wood pyrolysis. Air enters through

the center of the friction wheel, thus producing a

cooling effect on the resulting smoke. The more air

that is present, the lower the resulting smoke tem-

perature. This is in direct contrast to the smoldering

technique. The temperature can be regulated by

adjusting the pressure on the block of wood, the

tbl0003 Table 3 Influence of wood moisture content on smoke

composition

Moisture content

(% asis)

Yield per100 g of sawdust (as is)

Condensate

(g)

Phenols

(mg)

Acids

(mg)

Formaldehyde

(mg)

1.8 28.2 236 3203 122

21.5 41.5 136 3288 81

24.5 43.5 100 3003 78

31.2 32.8 33 890

5298 SMOKED FOODS/Principles

wheel speed (which also governs the amount of air

used), and the shape of the friction edges. The actual

temperature during wood pyrolysis using friction is in

the region of 450–560



C; since rapid cooling occurs,

the actual smoke temperature is relatively low.

Moist Smoke Generator

0019 In the wet smoke or condensate smoke method,

sawdust and superheated steam containing varying

amounts of heated air are blended, causing sawdust

pyrolysis. The resulting smoke has a temperature of

approximately 80



C and is moist because of con-

densed water from steam. This represents another

flameless method, and pyrolysis normally occurs in

the 300–400



C range. Advantages cited for the

process include a short treatment time and improved

smoke yield.

Fluidized-bed Smoke Generator

0020 With this technique, air is heated to between 300 and

400



C by means of an electric heater, and under high

velocity, mixed with sawdust. Because of the resulting

turbulence, the sawdust is ‘suspended’ or fluidized;

pyrolysis occurs at approximately 350



C in around

10 s. A cyclonic separator is used to yield smoke and

charred sawdust.

Two-stage Production of Smoke

0021 The two-stage procedure is a modification and

extension of the fluidized technique. The first stage

involves initial wood pyrolysis by the action of nitro-

gen or carbon dioxide at a temperature of 300–

400



C on sawdust. In the second stage, the reactants

of the first stage are mixed with oxygen or air heated

to 200



C, and further pyrolysis occurs. The second

stage promotes oxidation, condensation and poly-

merization reactions, resulting in a more complex

mixture of flavor compounds in the smoke.

Production of Smoke by Carbonization

0022 In this method, sawdust is compressed in a cylindrical

pipe by a screw, so that most of the air is squeezed

out. An electrical resistance heater situated at the end

of the cylinder heats the sawdust to 3000–4000



and because of a lack of air in the compressed sawdust,

the product gives off smoke during carbonization.

Smoking Techniques

0023 Smoking of food products may be performed in

various ways; the two main methods are cold or hot

smoking, but techniques such as electrostatic smoking

and treatment with liquid smoke condensate are also

in use. The physical processes taking place during the

action of the smoke on the product are adsorption,

adhesion, condensation, diffusion, and absorption,

and the smoking technology uses one or more of

these processes.

Cold Smoking

0024In cold smoking, the temperature is maintained be-

tween 15 and 25



C. The temperature of the smoke is

regulated either by regulation of air or by passing

smoke through a heat exchanger. The duration of

treatment can range from a few hours to several

days. It is suitable for cooked, raw, fermented, or

dried products. Examples of foods that are typically

cold-smoked are raw ham, bacon, salami and dried

sausage.

0025For dried products, the rate of circulation of smoke

must be controlled since too high a flow rate will

cause crust formation on the surface, which will

retard the subsequent drying process.

Hot Smoking

0026Depending on the temperature at which this is

performed, the product undergoes either partial or

complete cooking. Hot smoking is carried out at

temperatures in the range of 55–80



C. The activity

of smoke is considerably enhanced under these condi-

tions, allowing the treatment to be greatly shortened,

relative to cold smoking. Some processors practice

what is called moist smoking, which involves using

a smoke temperature of 24–48



C and relative humid-

ities in excess of 30%. Typical hot-smoked foods

include cooked ham, frankfurters, salami, and cock-

tail sausages.

0027The choice of operating temperature is largely de-

termined by the nature of the product and the desired

organoleptic attributes, as constrained by microbial

safety. Guidelines on safe operation have been pub-

lished by MAFF, USFDA, and FAO, among others.

Electrostatic Smoking

0028In this type of smoking, a continuous tunnel type

arrangement is used, where the product is positioned

between live electrical wires that are charged between

20 and 60 kV. Smoke passing through this system is

charged according to its phase (smoke is a two-phase

system, particulate and vapor), and smoke compon-

ents can then precipitate on the oppositely charged

food surface. This stage is usually followed by infra-

red irradiation, which dries the product surface.

Treatment with Smoke Essences or Condensates

0029This technique relies on obtaining a liquid essence of

the smoke produced in a conventional smoke gener-

ator. Generally, condensed smoke can be obtained

SMOKED FOODS/Principles 5299

dissolved in aqueous solutions (e.g., water or vin-

egar), as an organic solution (e.g., in oil or alcohol)

or as a dry powder (e.g., on a salt, spice, starch, or

gum arabic carrier).

0030 These liquid smokes can be introduced into the

product by various means, for example by addition

to the recipe or by dipping, injection, or atomiza-

tion. For comminuted products such as sausages,

frankfurters, and salami, smoke with a dry carrier

can be added to the mix during the chopping

sequence.

0031 For products manufactured using liquid processes,

smoke essence can be incorporated by dipping. This is

generally for a period of 5–60 s, and, while dipping

imparts a smoked appearance, the smoked taste is

weak. For injected products such as bacon and ham,

smoke essence can be included in the injection brine

at a concentration of 0.25–1%.

0032 There are several advantages in using liquid smoke.

Flavor can be incorporated throughout a product

in a uniform manner instead of having a higher

concentration on the outside than on the inside, as

occurs in traditional smoking. There is also a closer

control on the amount of smoke flavor a product

receives. The liquid smoke condensate can be frac-

tionated to intensify the smoke flavor and to remove

potentially harmful compounds before it is used in

food. It is also cheaper, as wood and smoking equip-

ment are not required as part of a food-smoking

plant.

Effects of Smoking on Food

Changes in Nutritive Value

0033 Smoking, particularly hot smoking and especially as

practiced in the tropics, can lead to destruction of

tryptophan, cysteine, lysine, and other basic amino

acids whether free or protein-bound. The losses

recorded for lysine are variable, reflecting the

variations in the smoking process (and analytical

procedure), but may reach 55%. Such losses are con-

centrated in the outer 5–10 mm where conditions are

harshest. It is clear that these losses are not just a

function of temperature, but are due in part also to

interactions with woodsmoke constituents and ingre-

dients such as nitrate and nitrite, which may be added

during the curing stage. Among the products detected

are imines from carbonyl–amino interactions, some of

which contribute to the characteristic yellow or red-

dish brown surface coloration of traditionally smoked

foods. In the case of certain amino acids, these imines

react further, yielding various b-carboline carboxylic

acids (from tryptophan and woodsmoke carbonyls),

and at least one nitroso-thiazolidine carboxylic acid

(from cysteine, formaldehyde, and nitrite). (See

Amino acids: Properties and Occurrence.)

Sensory Properties of Smoked Food

Texture

0034European cold-smoked products will generally have a

texture that is soft and tender, whereas the equivalent

hot-smoked products will have a firm dry surface

with softer interior, being more succulent, the higher

the fat content. The main determinants of texture are:

0035the extent and rate of water loss, with a greater

loss giving a firmer texture, and a more rapid loss

giving a greater difference in texture between the

surface and interior;

0036the fat content and its distribution, largely a

function of the food, with a food with a higher

fat content being more succulent;

0037the extent of denaturation of structural and con-

nective tissue protein, denaturation being more

extensive for higher temperatures and higher salt

contents, with fish in general being more suscep-

tible than meat, and with greater denaturation

giving a firmer texture;

0038the extent of autolysis, particularly proteolysis,

with ungutted fish being most susceptible, and

greater proteolysis giving softer texture. (See

Sensory Evaluation: Texture.)

Color

0039As referred to above, carbonyl–amino interactions

are important, with model system studies suggesting

that interaction of lysine with glycolic aldehyde,

2,3-butane-dione, pyruvaldehyde, coniferaldehyde,

or sinapaldehyde are particularly important. Colour

may derive from colored substances in the smoke

being deposited directly, or from permitted colorings

(such as the azo food color Brown FK (for kippers) or

natural carotenoids) incorporated during processing.

(See Colorants (Colourants): Properties and Deter-

minants of Synthetic Pigments.)

Flavor

0040The presence of an odoriferous or sapid compound in

woodsmoke or smoked food does not necessarily

equate with organoleptic importance, since the quan-

tity present must be sufficient to exceed the odor or

flavor threshold of the consumer (see Table 4). (See

Sensory Evaluation: Taste.)

0041Many investigators consider the phenolic constitu-

ents of woodsmoke, especially 4-methyl-guaiacol,

guaiacol, and eugenol to be particularly important,

at least in providing those smoky flavor notes that are

5300 SMOKED FOODS/Principles

essentially unique to smoked foods. There is no

doubt, however, that other classes of smoke volatiles

are also very important, and attention has been

directed towards the pyrazines and lactones.

0042 Aqueous extracts of smoke condensates may be

prepared as flavoring ingredients and to avoid the

potentially hazardous PAH fraction. Analysis of com-

mercially available extracts indicates that there can be

considerable variations in composition, depending on

the method of manufacture. Whether friction smoking

or electrostatic smoking duplicates the flavor achieved

by the traditional process is also a matter of some

debate. (See Flavor (Flavour) Compounds: Structures

and Characteristics.)

Assessment of Toxic Hazard

0043 Concern about the possible safety of smoked foods

centers primarily on the presence of PAH. Thirteen

PAHs have been detected in smoked foods, at least

one of which, benzo[a]pyrene, is a known carcinogen.

The greatest levels appear to be associated with prod-

ucts such as charcoal broiled steaks, which are only

incidentally smoked, rather than those products nor-

mally described of as smoked. It has been estimated

that the average daily per-capita intake of benzo[a]-

pyrene in the UK is less than 4 mg, and that smoked

foods do not contribute significantly to this burden in

the UK. (See Carcinogens: Carcinogenicity Tests;

Polycyclic Aromatic Hydrocarbons.)

0044 The safety of smoke condensates was reviewed by

the Joint FAO/WHO Expert Committee on Food

Additives (JECFA) in 1987. Their concern centered

mainly on the hazard that would arise if nitrosamines

and carcinogenic PAH were present. The evidence

available to JECFA indicated that neither class of

substance was detectable in the preparations tested.

More recently, a limit of 1 mgkg

1

has been suggested

as an upper limit for benzo(a)pyrene, although studies

have shown that some foods, particularly ethnic

foods and the outer layers of smoked fish, may con-

tain levels above this.

0045 Concern has also been expressed regarding the

presence of heterocyclic aromatic amines in foods,

particularly in grilled meat and fish, and presumably

also in the corresponding smoked foods. These com-

pounds are pyrolysis products derived from amino

acids, such as tryptophan, glutamic acids, or creati-

nine and products of the Maillard reaction. Some are

potent mutagens in certain in vitro mutagenicity tests.

0046Recently, varying amounts of three b-carboline-3-

carboxylic acids have been detected in some smoked

foods produced in Europe. Although some b-

carbolines are known to be pharmacologically active,

for example inhibiting monoamine oxidase, there has

been no evaluation of the significance of any of those

detected in smoked foods.

See also: Amino Acids: Properties and Occurrence;

Carcinogens: Carcinogenicity Tests; Cellulose;

Colorants (Colourants): Properties and Determinants of

Synthetic Pigments; Flavor (Flavour) Compounds:

Structures and Characteristics; Hemicelluloses; Lignin;

Polycyclic Aromatic Hydrocarbons; Sensory

Evaluation: Texture; Taste; Water Activity: Principles

and Measurement; Effect on Food Stability

Further Reading

Gangoli SD (1986) The toxicology of smoked foods. Pro-

ceedings of the Institute of Food Science and Technology

19: 69–78.

Garcia Falcon MS, Gonzalez Amigo S, Lage Yusty MA and

Simal Lozano J (1999) Determination of benzo(a)pyrene

in some Spanish commercial smoked products by HPLC-

FL. Food Additives and Contaminants 16: 9–14.

Maga JA (1987) The flavour chemistry of woodsmoke.

Food Reviews International 3: 139–183.

Papavergou K and Clifford MN (1986) Chemical inter-

actions between woodsmoke components and foods.

Proceedings of the Institute of Food Science and Tech-

nology 19: 13–17.

Toth L (1982) Chemie der Ra¨ucherung. Weinheim: Verlag

Chemie.

WHO (1987a) Evaluation of Certain Food Additives and

Contaminants. Thirty-first Report of the Joint FAO/

WHO Expert Committee on Food Additives. Technical

Report 759, pp. 21–23. Geneva: WHO.

WHO (1987b) Toxicological Evaluation of Certain Food

Additives. WHO Food Additives Series, vol. 21: pp.

41–57. Cambridge: Cambridge University Press.

tbl0004 Table 4 Threshold values, sensory index values and described odor for some woodsmoke phenols

Compound In water In oil Odor description

Taste threshold Tasteindex Odor threshold Odorindex Odor threshold Odorindex

Guaiacol 0.013 6 400 0.021 4 600 0.07 1 000 Sweet, smoky, and

somewhat pungent

4-Methyl-guaiacol 0.065 90 000 0.09 58 800 0.4 18 000 Sweet, smoky

2,6-Dimethoxyphenol 1.65 1 400 1.85 1 200 0.34 7 000 Smoky

SMOKED FOODS/Principles 5301

Production

M Vaz-Velho, Escola Superior de Tecnologia e

Gesta

o, Viana do Castelo, Portugal

Background

0001 Smoking is an ancient process for preserving and

flavoring food, and is a traditional part of the diet of a

large section of the world population. Smoking pro-

cesses are mainly applied to flesh food (meat and fish),

but other foodstuffs such as cheese can also be smoked.

0002 The original principle of preservation was due to a

combination of lowered water activity of the product

and the uptake by the product of bactericidal and

antioxidant components of smoke. The secondary

objective is to impart sensory characteristics such as

smoke color and smoke flavor to the product. With

the exception of some African and Asian countries,

where a refrigeration chain is not widely established,

nowadays, smoking is used mainly to give the product

a characteristic flavor, the preservative effect being

slight, and many smoked fish products keep in good

condition for very little longer than the fresh product

from which they were made.

0003 Smoking fish is a very traditional and ancient

process in Northern European countries. However,

smoked meat products, mainly from pork meat,

have a huge impact on the economy of Mediterranean

countries, where a great variety of smoked meat

products are industrially or traditionally produced.

0004 In industrialized countries, despite globalization

and standardization of smoking processes and prod-

ucts, the elements that differentiate and identify a

product have an increasing importance for producers

and consumers to whom it is important to link prod-

ucts to the region where they originated and to their

specific means of production. Within each country,

local resources represented by traditional smoked

products, although produced on a small scale, now

have a great economic impact owing to the creation

of organizations and schemes to exploit diversities

and complementarities and consequently allowing

the achievement of considerable profits.

Smoke Production

0005 A smoked product can be defined as a product that

was prepared by subjecting the raw material to the

direct action of smoke from pyrolysis of wood, wood

shavings or chips, and sawdust.

0006 In most parts of the world, wood is still preferred to

sawdust. Wood leads to a hotter fire, and the products

might be charred, rather than smoked. However,

commercial smoked products in industrialized coun-

tries are usually smoked by a sawdust fire where,

unless there is a forced rapid air flow, the sawdust

smolders rather than burns. This lower smoke tem-

perature gives more flavor and preserving substances

to the product. Higher temperatures and more

oxygen oxidize these substances.

0007Smoke is composed of two phases:

0008A particulate or dispersed phase (droplet phase) –

the visible portion of smoke composed of tars,

wood resins, high-boiling phenolic compounds,

and lower boiling compounds that vary with

temperature and smoke concentration.

0009A gaseous or dispersing phase (vapor phase) –

most of the characteristic flavor associated with

smoke is present in this gas phase:

0010Conventional smoke generation methods can be:

0011Simple – smoke is generated from pyrolysis of

sawdust and woodchips in a simple fire box.

These wood chips burn the sawdust, and smolder-

ing starts.

0012Continuous – Modern smoke producers use a con-

tinuous smoldering by feeding the sawdust slowly

on to a very hot surface.

0013In both methods, smoke generation temperatures

are dependent on the rate of burning of the wood to

produce heat.

0014Reducing the temperature to which wood is heated

to minimize the production of undesirable smoke

components such as polycyclic aromatic hydrocar-

bons (PAH) has been an important issue. (See Poly-

cyclic Aromatic Hydrocarbons.)

0015Two methods of smoke generation, in which the

temperature is controlled externally, have been de-

veloped for reducing PAH levels:

0016Fluidized bed generator – sawdust is fluidized by

hot air at 350



C, and smoke with a high propor-

tion of acids is formed;

0017Steam smoke generator – high-pressure steam at

350



C is passed through a bed of sawdust and

smoke with a high proportion of water is formed.

0018Electrostatic precipitation has also been shown to

reduce the concentration of PAH. It can be performed

in a continuous tunnel-type kiln, in which the product

is placed between electrical wires with a charge of

20–60 kV. The smoke moving through this system is

charged according to its phase, and smoke com-

ponents can precipitate on the oppositely charged

product surface. Electrostatically filtered smoke was

found to be lower in acids, phenol, and carbonyls

than untreated smoke, giving a product with less

smoky aroma and color development. However,

5302 SMOKED FOODS/Production

reducing the electrical wires spacing can lead to

similar phenols levels as untreated smoke.

0019 When using traditional smoke generators, by

merely extending the distance between the smoke

generator and the smoking chamber, most of the par-

ticulate phase of the smoke is allowed to settle before

it makes contact with the product, thereby removing

much of the PAH.

Smoking Kilns

0020 As smoking is an ancillary process for increasing the

storage life of food products, an enormous variety of

smoking kilns and smoking processes are used for

that purpose all over the world.

0021 If smoking in industrialized countries is mainly a

pleasurable alternative to the consumption of freshly

cooked food, preservation, nevertheless, is still the

prime objective of smoking in many parts of the

world. In these countries, the smoking process usually

takes place in simple boxes, or products are just

placed over a fire place.

0022 In European Mediterranean countries, traditional

local smoked products can be produced in chimney

structures many centuries old, usually coupled to a

brick oven, and may still used for different purposes –

every day cooking, as a heater in cold days, and for

smoking meat in winter time (Figure 1a). These trad-

itional kilns were built inside the house, the operation

is dependent on the weather conditions, and produc-

tion is seasonal, taking place in winter time. Hams,

sausages, and sides of bacon were suspended and

exposed to smoke from fires of hard wood (usually

Quercus sp.). This produced a hotter fire because

wood was used in preference to sawdust. However,

as the flames were formed under an iron pot, they

were not in direct contact with the products. During

daytime, a mixture of smoke, water vapor and air

reached the products. During the night, as no fire

was needed for cooking, the smoking process con-

tinued by means of a wood brazier kept alight by

the high brick temperature, but the process was

flameless. Of course, as nowadays most of the chim-

neys are no longer used for cooking, the smoking

process is kept flameless as it was during night.

0023 Northern European countries still have chimney-

like kilns hundreds of years old, but most of them

were built even in that time with one single purpose –

to smoke food, mainly fish. These traditional kilns,

built outside the house, can vary in size from 1–2m

(hot smoking) to over 20 m in height (cold-smoking)

(Figure 1b). Production takes place throughout the

year, but in summer usually during the night when

the ambient air is cooler. Traditional smoking

involves the generation of smoke from hard woods

(oak, beech, and hickory in Europe). Whole fish, fish

fillets, sides of bacon, ham, and other meat products

are suspended in the brick kilns and exposed to smoke

from smoldering fires of wood chips or sawdust.

0024In these primitive techniques, smoke generation

and application steps are accomplished in the same

chamber. Hot air is less dense than cold air and moves

upwards, carrying the smoke with it (Figure 1a and b).

As temperatures are higher and smoke more dense

lower down the kiln, the products have to be moved

to different parts of the kiln so that all items receive the

same smoke treatment.

0025These traditional kilns are notoriously difficult to

control and have been replaced by a modern type of

smoker in which the smoke is generated externally and

blown into a metal chamber containing the products to

besmoked(Figure1c–e).Also,inmostofthesemodern

kilns, the air movement is across, rather than up, and

smoke is drawn across the products by means of an

electric fan toensurethat allproducts are dried/smoked

tothesameextent(Figure1dande).Inmodernsmoking

kilns primarily designed for hot-smoking (Figure 1d),

the smoke is introduced by a series of ducts into the

smoking chamber, but smoke distribution is less uni-

form than in the tunnel-type kiln (Figure 1e).

0026In recent commercial developments (Figure 1e), the

direction of the smoke flow can be reversed automat-

ically at frequent intervals, which obviates the need to

move the product. However, there is still little chance

of an even smoke or temperature treatment in all

parts of the kiln because, as the smoke/air mixture

passes from one rack to another, it becomes less

smoky, more humid, and cooler. But, as racks are

placed on a trolley, they can be easily moved into

another part of the smoker.

0027These horizontal kilns – mechanical kilns – have

different sizes for large- and small-scale productions.

They can be used for cold- and hot-smoking by the use

of electric heaters in the smoking chamber. In large

kilns, the heaters can be placed at different points in

the smoke path to keep the temperature more even

throughout the smoking chamber. Humidity can also

be controlled, and the consistency of smoking is high.

Owing to the concern over PAH in smoke, some gener-

ators are equipped with a purification stage that in-

volves the removal of undesirable compounds either by

water sprays or by precipitation.

The Smoking Process

0028The preservative effect of smoking combines the effect

of salting, drying, heating, and smoking. Although the

general operations in all smoked processing plants are

similar, the specific processing procedures vary consi-

derably, owing to differences in equipment, national

SMOKED FOODS/Production 5303

Smoke

Water vapor

Fresh air Fresh air

(a) (b)

(c)

(d)

(e)

Smoke Smoke Smoke

Air damper

Smoke Smoke Smoke

Smoke

producer

Smoke

Discharge fan

Smoke

producer

Smoke

Discharge fan

Smoke

producer

Reversible circulating fan

Smok e dis

rge

Discharge fan

Fresh air

fig0001 Figure 1 (a) Traditional Mediterranean smoker; (b) traditional north European smoker; (c) mechanical smoker (vertical smoke flow);

(d) mechanical smoker (horizontal smoke flow); (e) mechanical smoker (horizontal smoke flow – tunnel type).

5304 SMOKED FOODS/Production

and regional consumer preferences, raw materials,

and tradition.

Salting

0029 Salting can be done by soaking or injecting the prod-

uct with a prepared salt solution or by embedding or

rubbing the products with salt. Excess of salt on the

surface of the product, in these last methods, should

be removed by showering with water. In both

methods, salting nowadays, rather than for preserv-

ing, is used mainly to firm the flesh and impart flavor

to the product.

0030 In terms of safety, for cold-smoked fish, the aim is

to obtain a concentration of 3–3.5% of salt in the

water phase throughout the product to minimize the

growth of Clostridium botulinum. The salt content in

the water phase means the percentage of salt in the

water contained in the fish. Salt in the water phase

links the two conditions of moisture and saltiness.

This should be distinguished from the salt content,

which is the percentage of salt in the whole weight of

fish, that is, water plus protein plus fat, etc.

0031 The uptake of salt by products from brine or dry

salt is a diffusion-controlled process dependent on the

temperature, water content, salt concentration, and

the amount of fat. The time required for a given

product to absorb a given amount of salt is approxi-

mately inversely proportional to the square of the half

thickness of the product. (See Curing.) The salt used

should be of food-grade quality, low in calcium and

magnesium, and essentially free of iron and copper.

0032 In dry-salted products when the surface of the

product dries out too quickly, owing to the small

grain size of the salt, it can inhibit the diffusion of

the water from the bulk of the product to the surface,

which can lead to the decomposition of the center of

thick muscles before it reaches the level of salt con-

centration sufficient to prevent spoilage.

0033 The main objective of the brining method of salting

is limitation of air penetration during processing, thus

limiting the amount of rancidity that can develop.

This is why it is the preferred method for fatty prod-

ucts. It also gives an attractive glossy appearance to

the smoked fish fillets. However, as the water content

of the product is increased, more drying is required

after brining, than after dry salting.

0034 A70–80% saturated brine is commonly used. A

100% saturated brine can leave powdery salt crystals

on the surface of finished product, whereas a 50%

saturated brine would cause the product to swell,

necessitating a longer drying period.

0035 Making and maintaining brine quality involve the

use of good-quality water and ingredients, the control

of brining temperatures, and assuring high standards

of equipment hygiene.

0036When products are brined or brine-injected, curing

agents are added to the salt in the solution, e.g., for

whole pieces of meat. Curing agents can be added to

the ingredients together with salt, e.g., chopped or

minced products. Nitrates, nitrites, ascorbic acid,

sodium glutamate, polyphosphates, and sugar are the

main additives used. Nitrates and nitrites are used in

smoked meats to deliver the characteristic color by

reacting with the heme pigments, contributing to

flavor and, owing to its antimicrobial activity, inhibit-

ing particularly Clostridium botulinum growth and

toxin production. (See Clostridium: Occurrence of

Clostridium botulinum; Curing; Nitrates and

Nitrites.)

Drying

0037In conventional smoking, drying can be performed

before or after the smoke uptake or can be con-

comitant to smoke production. During smoking, the

drying continues. It is important to balance the drying

rates and quality/safety issues. The amount of mois-

ture transfer from the product to the air is dependent

on the relative humidity: the lower the humidity, the

better the moisture transfer. However, if the rate of

water removal is too high in comparison with the

diffusion rate, the surface will form a hard case

(case hardening) creating a barrier to water transfer

and evaporation; consequently, the diffusion of the

bulk water to the surface of the product is blocked.

Thus, spoilage and possibly Clostridium botulinum

growth might occur in the bulk of the product. To

prevent this, the protein must be set or denaturated

with low-temperature drying before applying higher

temperatures. In conclusion, the product must be

dried slowly enough to avoid case hardening, but

fast enough to avoid spoilage caused by bacterial

and enzymatic activities.

0038A large proportion of the drying of flesh food

muscle usually occurs during constant-rate drying –

while the surface of the product being dried remains

moist, water is considered to evaporate as from a free

water surface. The evaporation rate is thus controlled

by the conditions of the drying air, namely speed over

the surface, temperature, and humidity. As evapor-

ation takes place at the product surface, water travels

to the surface by diffusion and by capillaries formed

by structural components. When the movement of

this water through the product can no longer keep

pace with the evaporation rate, the surface becomes

dry, and this point in the drying sequence is referred

as ‘critical moisture content.’ Thereafter, the rate of

drying falls progressively and is dependent on the

permeability of the already dry part of the product.

During this stage, humidity and air velocity do not

significantly affect the process, whereas temperature,

SMOKED FOODS/Production 5305

as it affects diffusion rate, should be adjusted during

this period of falling. (See Drying: Theory of Air-

drying; Water Activity: Principles and Measurement;

Effect on Food Stability.)

0039 Drying for cold- or hot-smoked fish usually takes

place inside the smoker before smoke production

starts – the diffusion distance between the center

and surface of the fish or fish fillets is small, and

water activity is reduced rapidly.

0040 For whole pieces of meat (smoked hams), separate

chambers are needed because the maturing/drying

process can last for several days before products are

exposed to smoke. The temperature and humidity of

those chambers must be carefully controlled and are

set according to the level of dehydration required for

each particular product. It is important to evaluate

the maximum temperature to which the drying air

can be heated in order not to cause the product

surface temperature to rise above the upper limit

for maintaining specific product quality.

0041 Modern equipment is designed to accelerate the

drying process by increasing the evaporation rate or

by using faster air speeds to remove the water vapor

and increase the surface heat transfer rate.

Smoking

0042 The smoking process takes anywhere from a few

hours to a few days. A combined heating/drying/

smoking process entails application of smoke, at tem-

peratures below 30



C for cold-smoked products or

alternatively 60–80



C for a hot-smoked product. In

cold-smoking processes, the product is subjected to

heat for a period of time that does not coagulate

protein. Cold-smoking does not cook the flesh or

coagulate protein, so refrigerated storage and further

cooking are necessary before consumption. Cold-

smoked fish like salmon, salmon-trout, swordfish,

and tuna fish are not cooked before consumption.

The maintenance of temperatures below 30



C avoids

protein coagulation, and the cold smoke is more

dense, thus allowing the deposition of smoke on the

fish surface giving the product the desirable appear-

ance and flavor. Drying should be minimal at the start

but will increase towards the end of the process. The

length of the smoking process for such products is

much greater than for hot-smoked fish, but a ‘safety’

(i.e., pasteurization) temperature is not achieved in

any stage of the process. In a hot-smoking process, the

product is subjected to heat for a sufficient period

of time to coagulate protein throughout. The hot-

smoking process adequately cooks the flesh, and so

the products are ready to eat.

0043 The uptake of wood smoke components from the

particulate and vapor phase of wood smoke is de-

pendent on the nature of the product, temperature,

humidity, and velocity of smoke. The deposition of

volatiles is maximum at the beginning of the smoking

process whilst the surface remains wet. As the surface

becomes dried, the uptake of volatiles decreases, and

the proportion of smoke constituents arising from the

particulate phase increases. The particulate phase acts

as a reservoir of volatile and nonvolatile smoke com-

ponents, releasing more volatiles as they are adsorbed

from the vapor phase. Raising the smoke temperature

or diluting the smoke with air at a high constant

temperature drives the volatiles from the droplets

into the vapor phase, and from here, rapidly to the

food product, whereas dilution of smoke with cold air

has the opposite effect.

0044Cold-smoking involves keeping the smoke tem-

perature below 30



C throughout the smoking

process. A humidity of 70% RH seems to be a good

compromise between maximum smoke adsorption

and minimum case hardening in cold-smoking of

fish. In a mechanical smoker, hot-smoking of fish

usually undergoes a typical three-stage process: ini-

tially, cold-smoking for 1 h, then at 50



C for 30 min,

and finally at 80



C for 2 h.

0045In a mechanical smoker, the temperature regime

most often used for hot-smoked pieces of meat,

smoked directly after curing, follows a two-stage pro-

cess. The first, cold-smoking (low temperature/long

time) involves smoking at 32–38



C for 15–18 h; the

second, hot-smoking (high temperature/short time),

uses a temperature of at least 60



C for 2–4 h).

0046When large pieces of meat are smoked after a long

curing/maturation process, cold-smoking takes place

for a few days.

0047Alternatively, in either cold- or hot-smoking

methods, the product can be smoked by means of

smoke-flavoring processes with liquid smoke, which

imparts to a product the flavor of smoke. The

classical liquid smoke is a natural aqueous conden-

sate of wood smoke that has been aged and filtered to

remove tars and particulate matter, and is dissolved in

water, oil, or organic solvents.

0048For preparing a smoked-flavored product, after

the salting step, the product can be smoked by

adding smoke extracts to the mixture (smoked saus-

ages), injecting (smoked ham) or dipping (smoked

sausages, smoked ham) in a solution of liquid

smoke or applying the liquid smoke by pulverizing

it or using it as an atomized spray within a modern

automatic smoke kiln. According to some recent

research, spraying is preferable because it is easiest

to control flavor and acceptability with this method.

0049In conventional smoking, it is expected that phenol

components are at much lower concentrations to-

wards the center of the product compared with liquid

smoked products, where their levels should be very

5306 SMOKED FOODS/Production