231

Chapter 12

Smoking

Zdzis ł aw E. Sikorski and Edward Ko ł akowski

Introduction

Smoking, drying, and salting of meats belong to the oldest methods of food preservation. Ages ago, meat hung above a fi re was pre-served by the combined action of drying and smoking, which was often preceded by pick-ling in brine. Smoking extended the shelf life and changed the sensory properties of the meats. The procedures of smoking have been gradually improved to suit the requirements of people in different regions of the world in respect to shelf life and sensory properties. The role of the preservative effect of smoking diminished in many countries, while fl avor-ing and safety are of paramount importance for the processor and consumer. Nowadays, various smoking procedures are applied throughout the world in rural households for treating the meat and sausages for domestic use, as well as in large processing plants for the market. It is estimated that as much as 40% – 60% of the total amount of meats and meat products are smoked.

The meats hung in a kiln are exposed to smoke and heat for a time suffi cient to cause the desirable sensory and preservative effect. The smoke generally comes from smoulder-ing wood chips or sawdust, either directly below the hanging meat or in an external generator. Its density, relative humidity, and fl ow rate are controlled in a traditional smoking oven by natural draft and depend on the construction of the kiln, the weather con-ditions, and the actions of the operator. In modern, automatic smokehouses, the draft is

forced by mechanical equipment and shaped according to a computer program adjusted to the kind of smoked goods. The temperature of the smoke affects the sensory properties and the preservative effect, and controls the rate of the process. Cold smoking takes place in the range of 12 – 25 ° C and warm smoking at 23 – 45 ° C. In hot smoking, since thermal denaturation of the meat proteins is required, the smoke temperature during various stages of the process ranges from about 50 ° to 90 ° C.

In the past few decades, various aspects of the process of meat smoking have progressed. In traditional smoking, the most signifi cant developments include:

• control of the composition of the smoke by applying rational procedures of smoke generation;

• use of engineering principles regarding heat and mass transfer to shorten the pro-cessing time and control the weight loss of the product;

• optimization of the process parameters to assure the required sensory properties and safety of the smoked goods

• modernization of smokehouses, which affects smoke generation as well as han-dling of the smoked material and control of the process; and

• treatment of the spent smoke to avoid pol-lution of the environment.

In nontraditional smoking, various new liquid smoke preparations have come into

Handbook of Meat Processing Edited by Fidel Toldrá

© 2010 Blackwell Publishing. ISBN: 978-0-813-82182-5

232 Chapter 12

coalescence of some particles. Due to the action of the gravitation and centrifugal forces, as well as the temperature gradient, some components are deposited on the smoked goods, in the smoke ducts, and on the walls of the smokehouse. This leads to changes in their concentration in the smoke. The surface electrical charge of the particles also contributes to the physicochemical state of the aerosol. The dispersed components absorb and disperse light; thus their concen-tration affects the optical density of the smoke. The optical density is proportional to the number of particles in a unit volume of the smoke. In constant conditions, when the dimensions of the particles do not change, the optical density is correlated to the mass con-centration of the dispersed components in the smoke. Thus photo optical measurements can be used for the determination of smoke density, which refl ects the contents of all components.

Chemical Composition of Smoke

Introduction

The chemical composition of smoke and smoke condensates produced from various kinds of wood was comprehensively reviewed over two decades ago (T ó th and Potthast 1984 ). In numerous later publications, the effect of the conditions of smoke production prevailing in different generators has also been investigated.

Wood smoke contains air, water vapor, CO 2 , CO, and at least several hundred organic compounds in different concentrations. About 400 of them have been unequivocally identifi ed by chromatographic and spectral analytical methods. The composition of the smoke depends on the kind of wood used for smouldering (i.e., mainly on its dryness and the contents of hemicelluloses, cellulose, lignin, and resins), as well as on the tempera-ture and access of air to the zone of oxidation of the volatile products. The content of water

use, along with procedures for their application.

Curing Smoke

Generation and Properties of Wood Smoke

Curing smoke develops as a result of the partial burning of wood with a limited oxygen supply. Generally, hardwood is used, mainly oak and beech. However, for imparting spe-cifi c color or fl avor to some products, wood from other trees that are rich in resins, includ-ing coniferous, as well as heather, may be used. In some areas, other carbohydrate - rich material (e.g., bagasse [sugarcane], beet refuse from sugar making, or coconut husks) is used.

The smoke consists of gaseous products of thermal degradation and subsequent partial oxidation of the wooden material and of the dispersed soot and compounds present in fl uid or particle form. The temperature of thermal degradation in the wood constitu-ents — hemicelluloses, cellulose, and lignin — ranges from 180 ° to 300 ° C, 260 ° to 350 ° C, and 300 ° to 500 ° C, respectively. The tem-perature in the glowing zone may even reach up to 900 ° C.

The numerous components of the smoke differ in chemical and physical properties. The gases and low - boiling compounds con-stitute the gaseous phase, while the higher boiling ones are dispersed in the form of fl uid droplets or solid particles. The mass of the dispersed phase makes up about 90% of the total mass of the smoke. The proportion of different smoke constituents in both phases depends not only on their chemical composi-tion, but also on the conditions of smoke generation and the temperature and turbu-lence in the duct leading from the generator to the smokehouse. Cooling increases the weight proportion of the higher boiling com-pounds, while heating raises the concentra-tion of vapors. Brownian motion leads to

Smoking 233

pyrocatechol, phenol, and their various alkyl derivatives. The highest yield of phenols, especially of guaiacol and syringol and their derivatives, compounds that are essential for the sensory and preservative action of smoking, is at 400 – 600 ° C.

Aldehydes and ketones of smoke form a group of about 110 compounds, which includes also a number of aldehydealcohols, ketoalcohols, and ketoaldehydes. In the smoke from alder and fi r wood, 28 and 34 carbonyl compounds, respectively, were identifi ed (Borys 1978 ). Aliphatic and cyclic carbonyl compounds, as well as furan deriva-tives, are the products of pyrolytic degrada-tion of cellulose and hemicelluloses, while aromatic carbonyls are formed from lignin. The total content of carbonyl compounds ranges from about 25 to 110 mg/m 3 ; thus it is similar to that of phenols. The carbonyls present in the highest concentrations are acet-aldehyde, formaldehyde, and acetone; also, several O - heterocyclic carbonyls have been identifi ed in wood smoke (e.g., furfuralde-hyde and 5 - hydroxymethyl - 2 - furaldehyde).

Wood smoke contains several aliphatic and aromatic alcohols, including methanol, ethanol, allylalcohol, n - amylalcohol, benzyl alcohol, and phenylethylalcohol. Methanol may be the substrate for the generation of formaldehyde and formic acid.

The group of carboxylic acids in wood smoke consists of about 30 various com-pounds. In the aliphatic fraction, the follow-ing acids were identifi ed: acetic, propionic, isobutyric, butyric, crotonic, isocrotonic, valeric, isovaleric, heptanoic, caprylic, and nonanoic (K ł ossowska 1979 ). Among the dicarboxylic acids are oxalic, malonic, fumaric, maleic, and succinic acids. Wood smoke also contains several ketocarboxylic acids. In the esters group, the methyl esters of formic, acetic, butyric, and acrylic acids were identifi ed, as well as the benzoic acid ethyl ester.

One of the important groups of smoke constituents contains aliphatic and aromatic

vapor in the smoke is related to the humidity of the wood and air. The relative humidity of the smoke varies within a broad range and can be controlled by the operator. Although numerous investigations have been carried out on the effect of the parameters of genera-tion on the composition of the curing smoke, it is still not possible to predict precisely the contents of various compounds in the smoke. However, it is known which factors affect the generation of phenols, aldehydes, ketones, alcohols, acids, esters, and hydrocarbons. The concentrations of these fractions in the curing smoke, in mg/m 3 of the aerosol or in mg/100 g of wood, differ considerably as reported by different investigators, since the conditions of smoke production in various experiments were different. The yield and gross chemical composition of smoke depends more on the temperature and oxygen access than on the humidity and kind of wood.

The Main Groups of Compounds

The phenolic fraction of wood smoke con-sists of about 250 components, with 85 of them identifi ed. Phenols are formed primar-ily due to pyrolysis and oxidation of lignin, at comparatively low temperature (200 – 400 ° C), and cellulose at 700 ° C. The total contents of phenols depend on the kind of wood, temperature, and the density of the analyzed smoke. According to different data, it may be from 10 to 200 mg/m 3 ; the yield of phenols from 100 g of wood ranges from 50 to 5000 mg. This fraction includes com-pounds containing one, two, or three hydroxyl groups bound to the benzene ring, besides alkyl or ether derivatives, as well as those containing additional alcohol, aldehyde, acid, and ester groups. Therefore, they differ in water solubility, boiling point, sensitivity to oxidation, chemical reactivity, sensory prop-erties, and antibacterial activity. Among the identifi ed phenols, those present in the highest concentrations are syringol, guaiacol,

234 Chapter 12

phase is more effective than that of the particles and droplets. A rise in the tempera-ture of the smoke also accelerates sorption of these compounds by the meat being smoked.

The quantity of different smoke compo-nents accumulated in the meat depends on the temperature, humidity, agitation, and composition of the smoke; the properties of the components, particularly their volatility and solubility; the characteristics of the surface of the product; and the duration of smoking. Wet surfaces absorb about 20 times more phenols than dry ones. The published data on the total amount of smoke compo-nents absorbed by meat products are incom-plete and vary within a broad range. The large range of values is caused both by the properties of the products and the parameters of smoking, as well as by differences in the analytical procedures applied by various investigators. The use of phenol as a standard in colorimetric determination of total phenols may lead to signifi cant underestimation, since the amount of phenol in smoked meats is rather low. The content of phenols in dif-ferent smoked sausages, according to pub-lished data, ranges from about 0.02 to 300 μ g/g. In smoked pork belly strips and summer sausage, the total recovery of phenols was, according to Lustre and Issenberg (1970) , about 280 μ g/g and 60 μ g/g, respec-tively. The composition of the absorbed fraction depends more on the conditions of smoking, especially the humidity of the surface of the meats, than on the concentra-tion of individual phenols in the smoke. The quantity of formaldehyde in cold smoked goods may be as high as 20 – 40 μ g/g. The amount of formaldehyde in different assort-ments of sausages may reach 2 to 50 μ g/g; the surface layers of some products may contain about fi ve times more than the inner layer.

The rate of deposition of smoke compo-nents can be greatly increased by applying high - voltage electrostatic precipitation, as in

hydrocarbons. In the fraction of about 20 aliphatic hydrocarbons, the compound present in the highest concentration is methane, known also as the product of dry distillation of wood. Much larger is the group of polycyclic aromatic hydrocarbons (PAH) identifi ed in numerous investigations (Obiedzi n ski and Borys 1977 ). Some smoked products contain up to 100 different PAH and their alkylated derivatives, of molecular weights from 116 (indene) to 302 (dibenzo-pyrenes). They are generated at temperatures above 420 ° C and, having high boiling tem-perature, are present mainly in the dispersed phase of smoke. Thus the contents of PAH can be decreased by reducing the wood smouldering temperature and fi ltering the smoke.

Smoke fl avorings produced commercially for the food industry contain only traces of PAH. They are generally smoke extracts fi l-tered and separated from the tars or distillates of pyroligneous liquids. Various fl avorings of different brands are available as aqueous solutions or in free - fl owing, dry form on salt, yeast, or other material.

Wood smoke also contains a number of other chemicals, including NO, NO 2 , and NO 3 , as well as various heterocyclic com-pounds, including the N - heterocyclic pyrrole, pyrazine, and carbazole.

Accumulation and Interactions of Smoke Components in Meats

Deposition on Smoked Surfaces

Due to Brownian motion, the smoke particles and droplets undergo coalescence and settle on the smoked products under the effect of gravitational and centrifugal forces. Their natural, electrostatic charge and absorption in the wet surface layer also contribute some-what to deposition on the meat. Thus, the high humidity of the smoke increases the rate of smoking. On wet surfaces, the deposition of components present in the dispersing

Smoking 235

the deposited compounds, as well as by the thermal conditions of smoking. In electro-static smoking, the rapidly formed thick, sticky coat of smoke components is initially not fi xed to the meat surface and can be easily removed (Fig. 12.1 ). However, during subsequent heating to the desired tempera-ture, rapid penetration of the smoke compo-nents into the mass of the meat takes place. Most phenols accumulate on the skin, on the sausage casing, and under the surface at about a 6 mm deep layer, especially in the fatty tissue. However, in some products, even as much as 60% of the total mass of phenols can diffuse into deeper layers. Carbonyl compounds and acids are rather equally dis-tributed throughout the mass of some smoked meat products.

The smoke compounds accumulated in the meat cannot be 100% recovered by extraction, since many of them interact chemically with the meat components. The proteins and lipids of meat contain various reactive groups, which in appropriate condi-tions, especially at high temperature, can react with phenols, carbonyl compounds, and acids of the smoke. The most reactive are the – SH, – NH 2 , and – NH – C( = NH)NH 2 groups of amino acid residues in proteins and of nonprotein nitrogen compounds, as well as the polyenoic fatty acids and different reac-tive oxygen species and oxidation products like · O 2 − , · OH, RO · , ROO · , and ROOH. Smoke compounds may react with amino acids in meat products, leading to a signifi -cant decrease in the contents of amino acids in acid hydrolysates (Seuss 1986 ). Coniferaldehyde and sinapaldehyde present in the smoke condensates were not recovered from the smoked meats in the experiments of Lustre and Issenberg (1970) , possibly due to their interactions with the meat proteins. A signifi cant decrease in the contents of smoke carbonyl compounds caused by reactions with gelatine was shown by Ziemba (1969) . High loss of available lysine in sausage casings due to smoking was found by Ruiter

an electrostatic fi lter. If the meat is placed on a metal conveyor connected by a conducting wire to the ground, and a corona discharging wire electrode under high voltage is arranged nearby, the smoke particles in the space between the meat and the electrode get ionized and deposited under the effect of the electrostatic fi eld (Fig. 12.1 ). At a proper distance between the product and the elec-trode, a thick, viscous layer of smoke com-ponents can be deposited in a few minutes. Usually, the installation works at about 20 to 60 kV at an electrode distance of 7 to 20 cm. In the conditions for electrostatic smoking, not only deposition of the dispersed, ionized smoke particles is accelerated, but also that of the components of the vapor phase. This happens because the rapid movement of the particles creates an “ electrical wind, ” which increases the motion of the smoke toward the meat surface.

Diffusion and Interactions

The components deposited on the product and dissolved in the humid surface fi lm grad-ually diffuse into the deeper tissues due to the concentration gradient. The rate of diffusion is controlled by the properties of meat and of

–40 kV

Figure 12.1. Artist ’ s view of the principle of electro-static smoking. (Courtesy of Pawe ł Kowalski.)

236 Chapter 12

sheet to smoke in the high - voltage fi eld of an electrostatic apparatus; in two or three minutes, the sheet turns yellow or brown. According to Ziemba (1969) and Ruiter (1979) , a signifi cant contribution to the for-mation of the color of smoked goods comes from the reactions of carbonyl compounds, mainly glycolaldehyde and methylglyoxal present predominantly in the vapor phase of the smoke, with the amino groups of proteins and nonprotein nitrogen compounds. The smoke phenols form stable colors in reac-tions with proteins at weak alkaline conditions.

The intensity J of the color of the smoked products is primarily related to the optical density of the smoke E 0 and the time of smoking τ :

J kE= 0τ

The value of k increases with the rise in smoke temperature and velocity. High tem-perature favors the development of dark color, since it increases the concentration of the components of the dispersing phase of smoke and the rate of the carbonyl - amino reactions and polymerization of various com-ponents. The higher the temperature and water activity of the surface of small - caliber Br ü hwurst sausages, within limits set by other technological requirements, the darker is the color of the sausages. The kind of wood used for smoke generation is also important. Smoking with beech, maple, ash, sycamore, or lime - tree smoke leads to gold - yellow color; yellow - brownish tint comes from oak, nut, and alder smoke, and lemon - like from acacia smoke. Products treated with smoke from coniferous wood have dark coloration.

The Flavor and Taste

The smoke compounds are the dominant factor directly responsible for the smoky fl avor. Smouldering wood smoke generated at 450 – 550 ° C is regarded as the most suitable for imparting the smoky fl avor to smoked

(1979) . In laboratory experiments, the smoke phenols have been shown to reduce signifi -cantly the concentration of thiol and amino groups in solutions of amino acids, peptides, and proteins and in meat. Such interactions may decrease the lysine availability some-what. However, since the concentration of smoke components is the highest on the surface and in the thin outer parts of the smoked meat products, no signifi cant decrease in the nutritional value of the meat proteins should be expected. Furthermore, the phenolic constituents of smoke absorbed by the sausage can be oxidized. The contents of guaiacol and phenol in smoked sausages stored 1 month may decrease by about 35%.

The Sensory Effects

Introduction

The desirable, sensory properties of smoked products result from the concerted action of salting or curing, seasoning, pre - drying, smoking, and heating, and in some cases also dyeing. The smoke compounds induce smoky color and fl avor themselves and by interact-ing with the meat components, which results in the creation of other sensory - active sub-stances. Interactions with the nitrogenous meat constituents may lead to some texture changes. The desirable intensity of sensory changes induced by smoking depends on the kind of meat products; some assortments are expected to acquire only a slight smoky note, while for others, mainly regional products, very heavy smoking must be applied to suit the typical consumer preferences.

The Color

The color developed on the surface of the products is due to the presence of colored smoke components and to the interactions of reactive compounds with those in the meat or sausage casings. The direct coloring role may be easily shown by exposing a plate of tin

Smoking 237

during processing, concentration of salt, and composition and quantity of smoke deposited in the meat. Various products preserved by curing and heavy smoking may have a shelf life of even several months at room tempera-ture, while mild treatment, as applied in manufacturing of some frankfurter - type sau-sages, yields products that can be kept only a few days under refrigeration. By smoking frankfurters 30 minutes at internal tempera-tures 60 – 76 ° C, the total number of aerobic bacteria may be reduced by about two log cycles; higher temperature and longer pro-cessing time is slightly more effective. Smoke components delay the growth of microfl ora in cold - stored frankfurters, whereby the effect increases with the smoking time. Natural smoking can retard the onset of greening of frankfurters caused by Leuconostoc mesenteroides during storage (Anifantaki et al. 2002 ).

Numerous smoke compounds (phenols, carboxylic acids, and formaldehyde) in con-centrations similar to those in heavily smoked goods are effective antimicrobial agents. Their activity against various microorgan-isms at different stages of development is not equal. The phenols prolong the lag phase of bacterial growth proportionally to their concentration in the product. Therefore, the amount of the smoke components depos-ited on the meats during smoking has a sig-nifi cant infl uence on the preservative effect. Generally, hot smoking decreases the number of viable microorganisms in the products by one to two log cycles, whereby the effect increases with the rise in processing time and temperature. Among the most effective antimicrobial agents of wood smoke are: guaiacol and its methyl and propyl deriva-tives, creosol, pyrocatechol, methylpyrocate-chol, 2,6 - dimethoxyphenol, and pyrogallol and its methyl ether. Formaldehyde inhibits Cl. botulinum in concentrations of 40 μ g/cm 3 . Adding to raw minced beef 8% of liquid smoke, containing in 1 cm 3 about 1.4 – 4.0 mg of phenols and 20 – 70 mg of carbonyl com-

meats. The products of thermal decomposi-tion of cellulose and hemicelluloses are the result of carmelization and the source of fruity and fl oral scents, while the phenols generated by decomposition of lignin con-tribute the fl avor associated with smoke, scorch, spices, vanillin, and clover. Various fractions of smoke condensates separated by chromatography reveal different fl avors, including fruity, diacetyl - like, spicy, protein hydrolysate - like, or that of freshly baked bread. The desirable smoky fl avor is associ-ated with the presence of a mixture of syringol and 4 - methylsyringol, although 4 - allylsyringol, guaiacol, 4 - methylguaiacol, and trans - isoeugenol also contribute to the typical sensory sensation. However, the mul-titude of variations of the smoky fl avor is probably due to the contribution of the osmic effect of different carbonyl compounds and their reaction products, furans, esters, short - chain carboxylic acids, pyrazine and its derivatives, terpenes, and other unidentifi ed constituents, as well as various products of interactions of smoke compounds and reac-tive meat constituents.

The smoky taste is a result of the sensory properties of smoke constituents, mainly numerous phenols and carbonyl compounds, as well as various products of the interactions with proteins and lipids. Some results of experiments point to the crucial role of the fraction of smoke condensates containing guaiacol and its four derivatives, eugenol, phenol, 3 cresols, 4 - ethylphenol, 3 xylenols, tyglic acid, and 4 carbonyl compounds.

The Antimicrobial Activity of Smoke Components

The shelf life of smoked meats depends on the time and temperature of heating during the process, on decrease in water activity, and on the antibacterial and antioxidant activity of smoke components. Thus the pre-servative effect is related to the effectiveness of the heat pasteurization, loss of water

238 Chapter 12

similar concentrations. To the most active smoke phenols belong: pyrogallol, 3 - methyl-pyrocatechol, 4 - methylpyrocatechol, pyro-catechol, butylhydroxytoluol, resorcinol, hydroquinone, α - naphthol, 4 - methylguaia-col, 4 - vinylguaiacol, and 4 - trans - propenyl-syringol. The antioxidant properties of the phenolic fraction of wood smoke were already recognized about 50 years ago (Kurko 1963, 1966 ). Liquid smoke in a concentration of 1.5% was shown to effectively retard the lipid oxidation in precooked beef patties during 90 days of storage at − 15 ° C (Estrada - Mu ñ oz et al. 1998 ).

Possible Health Hazards Caused by Smoked Meats

The health hazards associated with the smoking of meat may be caused by carcino-genic components deposited from wood smoke: PAH, N - nitroso compounds, and pos-sibly also heterocyclic aromatic amines.

Most of the PAH contained in wood smoke have a molecular weight below 216 Da, and they are regarded as noncarcino-genic. However, smoke also contains highly carcinogenic or mutagenic PAH (Table 12.1 ). Very mutagenic and carcinogenic is benzo(a)pyrene (BaP). It has a molecular weight of 252 Da and has been chosen as the indicator PAH, representing the other carcinogenic hydrocarbons in wood smoke and smoked products. By applying the principle of toxic equivalency factors (TEF), it is possible to estimate the total equivalent exposure (TEQ) to PAHs in various smoked meat products

pounds, may reduce by two log cycles the number of viable cells of E. coli O157 H7 after 3 days at 4 ° C. This result, however, was shown at a very high concentration of the liquid smoke, 8%, while the recommended percentage is 1.5% (Estrada - Mu ñ oz et al. 1998 ). Several strains of thermotolerant Staphylococcus epidermis do not survive commercial hot smoking on inoculated rainbow trout. In cold - smoked salmon, the growth of Listeria monocytogenes was found to be inhibited proportionally to the smoking time; 12 hours of smoking reduced the number of the population by three log cycles. However, well - adapted strains may persist in the smokehouse environment, so that L. monocytogenes can often be found in vac-uum - packaged cold smoked salmon. The total concentration of smoke components present in lightly smoked vacuum - packed nonrefrigerated foods is not high enough to effectively prevent the formation of Cl. botu-linum toxin.

Generally the vegetative forms of bacteria are most sensitive to smoke. Molds are con-siderably resistant. A large population of molds and yeasts may survive in frankfurters smoked 30 minutes at an internal temperature 67 ° C. Smoking has little effect on the yeast count in the early stages of manufacturing of fermented sausages; however, in stored samples the population of yeast is lower in smoked sausages than in unsmoked controls.

The Antioxidant Properties of Smoke Components

The antioxidant effect of smoking was noticed previously by observing that the lipids in smoked meats and fi sh were resis-tant to oxidation (Watts and Faulkner 1954 ). Among the smoke components that have the highest antioxidant activity are phenols; some of them are more effective than butyl-ated hydroxyanisole (E 320) and butylated hydroxytoluene (E 321) when applied in

Table 12.1. Polycyclic aromatic hydrocarbons regarded as potentially genotoxic and carcino-genic for man

Benz [a]anthracene Dibenzo[al]pyrene Benzo[b]fl uoranthene Dibenz[ah]anthracene Benzo[j]fl uoranthene Indeno[1,2,3 - cd]pyrene Dibenzo[ae]pyrene Benzo[k]fl uoranthene Benzo[ghi]perylene Dibenzo[ai]pyrene Dibenzo[ah]pyrene Chrysene Benzo[a]pyrene 5 - Methylchrysene Cyclopenta[cd]pyrene

Smoking 239

method of smoking, the quality of smoke, and the protection of the edible parts by the skin. The external parts of the fi sh exposed to the smoke, especially the skin of eel, may contain up to fi ve times more BaP than the fl esh. In the edible parts of fi sh smoked in a modern automatic kiln with external smoke generation, the contents of BaP are about 0.1 ng/g.

Some smoked foods may also be contami-nated with nitropolycyclic aromatic hydro-carbons. In smoked sausages, 1 - nitropyrene, 2 - nitronaphtalene, and 2 - nitrofl uorene were found in concentrations of about 4.2, 8.4, and 19.6 ng/g, respectively, while in roasted coffee beans the concentrations were 2.4, 4.0, and 30.1 ng/g.

Cured meats and bacon, as well as smoked cured meat products, contain several N - nitroso compounds, most of which are carcinogenic in laboratory animals. The contents of N - nitrosoproline, N - nitrosohydroxyproline, and N - nitrosodime-thylamine in smoked cured mutton after cooking reached up to 230, 500, and 2.2 ng/g, respectively (Dennis et al. 1984 ). The alde-hydes of smoke can react with cysteamine and cysteine, yielding various thiazolidine precursors, which can be easily nitrosated. Formaldehyde reacting with cysteamine and cysteine yields thiazolidine and thiazolidine - 4 - carboxylic acid, respectively, which, upon nitrosation, turn into N - nitrosothiazolidine and N - nitrosothiazolidine - 4 - carboxylic acid. In traditionally smoked fried bacon, the content of N - nitrosothiazolidine may be about 5 ng/g (Ikins et al. 1986 ). In the presence of glycolaldehyde from smoke, 2 - (hydroxymethyl) - N - nitrosothiazolidine and 2 - (hydroxymethyl) - N - nitrosothiazoli-dine - 4 - carboxylic acid (HMNTCA) may be formed. In various cured smoked products, including smoked ham, sausages, salami, pepperoni, and smoked poultry products, the contents of HMNTCA ranges from about 10 to 260 ng/g (Sen et al. 1993 ). Generally, these compounds occur in higher concentrations in

relative to BaP. The proposed TEF for BaP and dibenzo(a,h)anthracene is 1, for benzo(a)anthracene, benzo(a)fl uoranthene, indeno (1,2,3 - c,d)pyrene, and benzo(k)fl uoranthene it is 0.1, for chrysene and fl uoranthene 0.01. Among PAH isolated from smoked products are mainly compounds of m.w. smaller than 216. In different smoked meat products, their total mass may be from about 30 to 250 times larger, while that of the heavy PAH about 10 times larger than that of BaP. If the specifi c carcinogenicity of various heavy PAH con-tained in smoked foods is taken into consid-eration, the total carcinogenicity of all PAH is about 10 times higher than would result from the content of BaP alone (Scientifi c Committee on Food 2002 ). Although many PAH are regarded as not carcinogenic, some of them may function in living organisms as synergists, increasing the carcinogenicity of other PAH.

German regulations in force since 1973 require that the content of BaP in smoked meat products not exceed 1 ng/g. In about 75% of market samples of meat products in Germany, the contents of BaP were not found to be higher than the limiting value, but in about 1% of investigated samples, even as much as 40 ng/g were present. The actual European limit of BaP in smoked meats and smoked meat products is 5 ng/g wet weight (Commission Regulation (EC) No 208/2005). Hot smoked sausages and smoked beef spreads usually contain below 1 ng/g, but some black smoked products even have as much as 55 ng/g. According to data reviewed by Simko (2002), the content of BaP in 10 different kinds of smoked meat and meat products ranged from 0.03 to 1.2 ng/g and was 17.1 ng/g in dark smoked meat products. In 1 g of fl ame - grilled sausages, 18 to 42 ng BaP were found. This could be compared with the contents of BaP in barbecued pork and beef: 1.5 – 10.5 ng/g, and in charcoal - broiled steaks 5 – 8 ng/g. Traditionally smoked fi sh may contain from about 1 to about 60 ng of BaP/g of product, depending on the

240 Chapter 12

made of wood logs, and the burning logs are covered with a layer of damp sawdust or wood chips to keep the fl ame down and cause smoldering of the woody material. Here the control of the quantity and quality of the smoke depends totally on the kind of sawdust, the experience of the operator, and the weather conditions. Increasing or decreasing the volume of air entering into the fi replace by widening or shutting the inlet openings can adjust the smoldering temperature only coarsely. The smoke produced in such condi-tions may be high in PAHs. This principle is applied also in industrially manufactured generators, in which there are several boxes connected to one smoke duct. Here the uni-formity of smoke output can be better assured by proper, simultaneous controlling of the air inlets to the boxes.

In industrial smoking, generators are most often used, in which the sawdust or wood chips are automatically fed onto a plate or grate heated to a controlled temperature of about 350 ° C. The air needed for smoldering is blown from below the plate. The tempera-ture in the layer of sawdust depends on the temperature of the heated grate and on the volume of supplied air. When small mesh sawdust is used, the fl ow of air may be hin-dered and the temperature tends to be below that of the plate. Such smoke is rich in CO. Adding wood chips to the pile increases the temperature and makes the smoke richer in phenols. However, in the presence of too large chips, the development of smoke may be disturbed by the occurrence of fl ame.

Friction - Type Generators

The principle of action of different construc-tions of these generators depends also on thermal degradation and partial oxidation of woody material. However, the temperature needed to initiate the process results from friction of a log pressed at about 1 kg/cm 2 against a rotating drum or disk (Rasmussen 1956 ). Access of air is possible due to perfo-

meat products smoked in traditional smoking ovens than in meats processed in modern smokehouses. The total amount of various N - nitroso compounds in smoked fried bacon, some of which are still unidentifi ed, has been reported to be 430 – 6800 ng/g.

Heterocyclic aromatic amines, known to be generated due to pyrolysis of amino acids and proteins and in nonenzymatic browning, may be found in very heavy smoked goods in amounts lower than 1 ng/g.

The Equipment for Smoking

Introduction

The implements used for smoking meat and fi sh have been gradually improved during ages of development from very primitive burrows or huts with a fi replace on the ground to modern installations controlled electroni-cally. However, in some parts of the world, the most ancient procedures and equipment may still be in use. In contemporary indus-trial smoking, high attention is paid to the temperature of smoke generation, proper circulation of the smoke and drying air in order to achieve the required degree and uniformity of smoke deposition, water evaporation, and heating, as well as to control all process parameters affecting the quality of the products. In accordance with the requirements of hygiene and environmental protection, the industrial installations also include the necessary gear for effi cient cleaning and for neutralization of the spent smoke. The main components of smoking equipment are the generators of smoke and the smokehouses.

Smoke Generators

Smoldering - Type Generators

In primitive, usually small - scale or artisan processing, the smoke is produced in a fi re-place directly below the meat, which is hung above it on spits or laid on a mesh. A fi re is

Smoking 241

chips of low mesh size is kept below 400 ° C to prevent self - ignition. The composition of the smoke can be controlled by modifying the content of oxygen in the air mixture. Self - ignition of the fl uidized sawdust bed may be prevented even at a temperature as high as 750 ° C if the concentration of oxygen in the gas mixture is reduced to 6% (Balejko and Miler 1988 ). The quality of the smoke pro-duced in a fl uidized bed at such a high tem-perature is equal to that from a smoldering - type generator.

Smokehouses

Introduction

Meats and meat products can be successfully smoked in very simple devices (e. g., in a barrel inverted over a pile of smoldering sawdust, fi tted with supports for spits, rods, or wire mesh, and outlets for the spent smoke). Here, as well as in smoking ovens, but also in kilns or tunnels supplied with smoke from generators, the quality of the products is affected by the duration of the process, the properties of the smoke, and the conditions of heat and mass transfer. The same factors are also important in smoke-houses equipped with atomizers of liquid smoke preparations. Additionally, in smoking and steaming chambers, the parameters of the heating steam have to be considered, as do the effects of the high - voltage electro-static fi eld in electrostatic smoking units.

Smoking Ovens

Smoking ovens, generally built as a cross section of a rectangle, about 1 m wide, 1.2 m deep, and 2 m high, have stony fl oors and fi reproof brick walls. On the side walls, there are two or three pairs of supporting rails at appropriate distances to allow for sliding the frames on which the meat products hang from spits or rods. On the front of the oven there are three doors: two small ones at the

rations in the rotor. The temperature in the friction zone may be controlled by adjusting the pressure applied to the log and the speed of rotation of the rotor. It is usually 300 – 350 ° C. The smoke is blown into the smoke duct by a fan that may be fi xed on the shaft of the rotor. Because of comparatively limited access of air to the friction zone and low temperature, the smoke contains fewer prod-ucts of thermal degradation and oxidation of lignin than that from the earlier described generators. The assets of the friction - type machine are low consumption of wood and the fact that the smoke production can be started and stopped instantaneously. However, because it makes a lot of noise during operation and needs electrical power for driving the rotor, it is not very often used in the industry.

Other Types of Generators

Thermal degradation of wood can also be accomplished by overheated steam; this takes place in the steam smoke generators covered by numerous patents (Fessman 1971 ). The sawdust is fed by a worm feeder into a reactor formed by a tilted pipe with perforated walls. Overheated steam at temperatures of usually about 200 ° C is blown through the perfora-tions into the sawdust - fi lled reactor. At this considerably lower temperature, lignin does not undergo thermal degradation, and the smoke is rich in carboxylic acids and car-bonyl compounds but relatively poor in phenols and polycyclic aromatic compounds. In some of these generators, additional sections of the reactor serve to oxidize the volatile decomposition products by oxygen - enriched air.

Wood smoke can also be produced by blowing a stream of hot air or air/inert gas mixture countercurrently across a bed of sawdust that is being fed at a controllable rate into a fl uidization chamber that has the form of a truncated cone (Nicol 1962 ). The tem-perature of the fl uidized bed of sawdust or

242 Chapter 12

arranged so that their common back walls can be lifted. In such an arrangement, the loading and pre - drying of the product takes place in the front row of ovens. After lifting the back wall, the meats are pushed into the back row for further heating, smoking, and fi nally unloading. This principle may be applied also in operating other types of smokehouses.

Advanced types of smoking kilns, still with a fi replace in the bottom, may extend over two stories of a building. The drying, heating, and smoking conditions in the lower and higher sections of such ovens differ sig-nifi cantly, which is convenient for processing different assortments of meat products.

Smokehouses with External Smoke Generators

Modern smokehouses (Fig. 12.2 ) make pos-sible full application of the principles of food

bottom and top to handle the fi replace and enable the access of air and to control the airfl ow and humidity, respectively. The third, a large door in the middle, is used for manual loading and unloading of the frames, as well as for observing the process. A duct to the chimney connects the coned ceiling. Its coned shape assures that the water and smoke condensate does not drop on the meats but drains down the walls. The smoke duct is equipped with a shutter to control the fl ow. In order to produce smoked meats of fairly standard quality, the operator may have to rearrange the frames at various distances from the fi replace during smoking to coun-teract the effect of the differences of tem-perature, draft, and humidity on the cross section of the oven. Smoking in such condi-tions is a labor - intensive task if no mechani-zation of the handling of the loaded frames is available. The process may be improved if two smoking ovens, or batteries of ovens, are

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Figure 12.2. A smokehouse with external smoke generator. (1) Smoke generator, (2) sawdust container, (3) electric heater, (4) ashtray, (5) smoke duct, (6) inlet fan, (7) smoke - distributing channel, (8) jet, (9) throttle, (10) smoke inlet collector, (11) outlet fan, (12) afterburner, (13) smoke outlet passage, (14) heater. (Courtesy of Jerzy Balejko.)

Smoking 243

products within tunnel smoke houses, which are designed to operate in a continuous system. In such tunnels, the meats to be smoked are carried through consecutive sec-tions, in which appropriate parameters of temperature, smoke density, humidity, and fl ow rate are maintained.

For electrostatic smoking, the smoke-house is additionally equipped with a high - voltage section, where the smoke deposition takes place within a few minutes (Sikorski 1962 , Tilgner and Sikorski 1962 ). Because the length of treatment is so short, the density of smoke has to be kept very constant in order to assure a uniform degree of smoking of the product. Electrostatic deposition may also be applied in smoking ovens in which smoke preparations are used instead of smoke. The other phases of the process (i.e., pre - drying and cooking) proceed as in con-ventional smoking.

Additional Equipment

To reliably operate a smokehouse, several instruments are necessary for measuring and controlling the temperature, relative humid-ity, and fl ow rate of the smoke and the tem-perature in the meat products. Similar instruments are used for control of other pro-cesses in the food industry. The density of the smoke, a crucial parameter of smoking, can be easily determined by photoelectric mea-surement of the intensity of a light beam transmitted through a layer of smoke.

The spent smoke and other exhaust gases, after leaving the smokehouse, should be cleaned before entering the atmosphere. Depending on the contents of the polluting components, different equipment may be used. Some installations comprise three sec-tions: an electro fi lter, a fi brous fi lter, and activated charcoal. Other systems use after-burners to oxidize the components of the spent smoke at 800 – 1500 ° C. In the presence of catalyzers, the temperature may be reduced to 600 ° C.

engineering regarding heat and mass transfer. They are supplied with smoke of standard quality from an external generator and are heated by steam, gas, or electricity. Sawdust and chips of wood of various species of trees, standardized water content, and mesh size are available commercially. The air and smoke fl ow or circulation is forced mechanically at controlled velocity. The temperature, humid-ity, and density of the air/smoke, as well as the process time, are adjusted according to a computer program to requirements depend-ing on the desired properties of the meat products. The smoke is often fi ltered or con-ditioned under a water spray to control its temperature and humidity, and to separate some tar fractions and soot. In smokehouses working in a half - open system, the smoke is circulated until its density drops below a critical level. At that point, it is discharged into the chimney, and new smoke from the generator is fed into the kiln. In a closed system, the smoke circulates within the kiln during the whole cycle of smoking, and after-ward, a stream of air forces out its residues. The closed system bears the risk of self - igni-tion of the smoke, which may contain CO at a concentration that is too high.

For smoking with smoke preparations or fl avorings, the same smokehouses may be used as in the conventional process. However, additional equipment must be installed for atomization or vaporization of the smoking liquids. Atomization nozzles in the smoke-house create a cloud of smoke droplets in the range of 100 μ m, while the smoke fl avor-ing sprayed onto a heated plate turns into vapors. For processing cooked sausages, hot - water shower or steam injection systems must be fi tted, so that smoking and cooking can be carried out concurrently in the same kiln.

In modern smokehouses, as well as in advanced types of smoking ovens, the mate-rial to be smoked is usually loaded into the smoking chamber on trolleys. Trolleys or conveyors are used also to transport the meat

244 Chapter 12

and less dense smoke. After smoking, the salami is ripened for two to three months, as described in chapter 22 .

Warm smoking is carried out at 23 ° to 45 ° C and relative humidity of 70% – 80% for 4 – 48 hours. The pre - drying and smoke penetration is restricted mainly to the outer layers of the product. It is usually followed by cooking or baking. In smoking frankfurt-ers, the fi rst phase is a tempering period at 32 – 38 ° C, aimed at removing the surface moisture to ensure uniform coloring. Smoking proper, lasting usually about 1 – 1.5 hrs in dense smoke of controlled humidity, brings the internal temperature of the sausages to 60 – 68 ° C and imparts a smoky color and fl avor. This is followed by cooking in a hot - water spray or steam and by chilling. In smoking cooked sausages, too high tempera-ture may lead to excessive fat and weight loss and thus to creased surfaces of the sausages and nonuniformity of color. Smoking at a temperature not exceeding 40 ° C is used also for preparation of salted, spiced, dried pork back fat.

In hot smoking, the fi rst stage (lasting about 30 minutes, without smoke, at 40 – 50 ° C) results in pre - drying of the surface and is followed by several stages of smoking in dense, hot smoke (at temperatures reaching 85 ° C) and further surface drying. In manu-facturing jagdwurst, the links are kept 2 hours for settling at about 30 ° C, surface - dried at 40 – 60 ° C, smoked about 80 minutes at 45 – 80 ° C, dry - heated at 85 ° C during 25 minutes to reach internal temperature 68 – 72 ° C, smoked again at 30 ° C during 12 hours to a dark brown surface color, and dried at 14 – 18 ° C during 5 – 7 days to a water content of 55% – 57% in the product. In man-ufacturing other assortments, the thermal changes in the meat products are caused by dry heating at about 90 ° C, steam cooking in the smokehouse, or cooking in water. The internal temperature of the product should be 68 – 72 ° C.

Many smokehouses are equipped with installations for automatic cleaning that are available as standard units. Alkaline deter-gents may be used for the effi cient removal of smoke deposits and tar.

Typical Procedures for Smoking Meats

Meat may be smoked in the raw state or after previous salting, marinating, cooking, or other treatment, which may also be followed by other processing. However, in industrial practice, meat and meat products are usually smoked after salt curing, as described in chapter 6 . The pre - treatment of the raw mate-rial and the conditions of smoking, mainly the humidity and fat content of the surface layer of the sausage, the temperature, humid-ity, and density of the smoke, and the dura-tion of the process, affect both the characteristic sensory properties and the shelf life of the products.

Many smoking procedures are used in the industry and in artisan meat processing. They lead to very different sensory properties and shelf lives of various products. In these pro-cedures, the impact of drying, heating, and treating with smoke on the quality of the products may be very different.

Cold smoking is used in manufacturing raw, fermented sausages, made from cured meats. The smoke, at 12 – 25 ° C and controlled humidity, is applied for between several hours to about 16 days, depending on the assortment. The loss of water due to drying and the impregnation with smoke compo-nents should be equal on the whole cross section of the product. The surface of the freshly cut sausage should be light brown to dark brown, depending on the duration of the process. In smoking salami, the links are fi rst surface dried one day at 12 ° C in low - density smoke. This is followed by fi ve days of smoking in dense smoke at 15 – 22 ° C, and by the last phase of two days in somewhat colder

Smoking 245

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Seuss , I. 1986 . Einfl uss der R ä ucherung auf n ä hrwert-bestimmende Bestandteile von Fleischerzeugnissen. III. Proteinverdaulichkeit und Aminos ä urengehalt bei Dosenbr ü hwurst und Rohwurst . Fleischwirtschaft 66 ( 4 ): 544 – 550 .

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(in Polish ). Kurko , V. I. 1966 . On the role and importance of indi-

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