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Smoking
17 Smoking Abstract: Smoking is used to preserve protein-rich foods such as meat, fish and cheese by the combined action of heat, which destroys micro-organisms and enzymes, and reduces moisture content, and antimicrobial and antioxidant chemicals in the smoke. The chapter describes the constituents of smoke, liquid smoke and the methods of smoking foods. It concludes with the effects of smoking on the sensory characteristics of smoked foods and their microbial safety, and a discussion of health concerns over potentially carcinogenic chemicals in smoked foods. Key words: smoking, liquid smoke, constituents in smoke, smoking kilns, polycyclic aromatic hydrocarbons, nitrosamines.
Smoking is an ancient process that was used to preserve protein-rich foods for storage at ambient temperatures in times of shortage during winter seasons in temperate climates and during dry seasons in tropical climates. Now, the purpose is to change the flavour and colour of foods, rather than preservation. Smoked foods are preserved by chilling (Chapter 21), and may be packed in modified atmospheres or vacuum packed (Chapter 25, section 25.3) to give the required shelf-life. Smoking is an inexpensive operation that increases the variety of products for consumers, and for processors it adds value to foods. The most commonly smoked foods are fish (e.g. tilapia, mackerel, trout, sable (or black cod), sturgeon and tuna), meats and meat products (e.g. duck, venison, game birds, and paÃteÂs made from these meats, pork, pastrami (pickled, spiced and smoked beef brisket) and beef jerky) and cheeses such as smoked Gouda. Other smoked foods include vegetables such as chipotles (smoked jalapenÄo peppers), seafoods, nuts, lapsang souchong tea, barley malt used in the manufacture of some types of whisky and ingredients used to make German smoked beer (rauchbier). There are four types of smoking operations: 1 Cold smoking, in which the food is flavoured and coloured but not cooked. It is typically used for salmon, salamis, kippers, hams and special cheeses. 2 Warm smoking at 25±40 ëC is used for bacon, sirloin and some types of sausage. 3 Hot smoking of meats and fish at 60±80 ëC, which cooks the food and the heat is sufficient to destroy contaminating micro-organisms. Herring, eel and some sausages are hot-smoked.
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4 Dissolving smoke compounds in water to make smoke concentrate or `liquid smoke' and spraying or coating foods (see also coating, Chapter 24). Each type of smoking is a surface treatment and smoke chemicals penetrate only a few millimetres into the product.
17.1
Theory
In cold smoking, foods are cured at an air temperature <33 ëC for between 6±24 hours and several weeks to produce the required smoked flavour and colour. The texture remains largely unchanged and the products have a milder taste than hot-smoked foods. Microorganisms are not destroyed and for this reason, cold-smoking is preceded by salt curing. Warm smoking has similar effects and foods are also cured. The preservative action of hot-smoking at 60±80 ëC results from a number of factors (see also section 17.4 and hurdle technology (Chapter 1, section 1.3.1)): · dehydration/reduced moisture content to lower the water activity of the product; · antioxidant action of some constituent chemicals in the smoke at the surface of foods (e.g. butyl gallate and butylated hydroxyanisole (BHA)) (Brul et al. 2000); · destruction of micro-organisms and enzymes by heat; · antimicrobial action of some constituent chemicals in the smoke at the surface of foods (e.g. phenolic compounds, organic acids); · antimicrobial action of salt pretreatments where these are used. Fish is the main type of food that is hot-smoked. Details of the process are given by Doe (1998). Goulas and Kontominas (2005) studied the effect of salting and smoking method on the keeping quality of chub mackerel. They found that during the 30 day storage period, salting had a noticeable preservative effect but it was lower than the combined effect of salting and smoking. The heat and humidity cook the products and produce the required smoky taste, golden brown colour, a silky sheen on the skin and uniform weight loss. The products do not require further cooking before consumption. In hot-smoking, the critical factors to control bacteria are (Fletcher et al. 2003): · · · · ·
the core temperatures reached by the product during processing; control over temperature variations in different parts of the kiln; numbers of contaminating bacteria; the D-values of the bacteria of interest (see Chapter 10, section 10.3); and the amount of smoking.
Although smoke and salt have antimicrobial effects (section 17.4), microbial destruction during hot-smoking is based on the heat treatment received. For example, for control of Listeria monocytogenes in smoked salmon, the process should heat the product to a given temperature for a long enough period to ensure that the possibility of any Listeria surviving is less than one in a million (106). If the highest likely contamination by L. monocytogenes is 106 gÿ1 of raw fish, the required reduction is from 106 to 10ÿ6 (12 D-values). For salmon, this requires 12 minutes at 64 ëC or 35.1 seconds at 72 ëC (Table 17.1). However, if contamination by L. monocytogenes is less than 1000 per fish for good quality salmon, then the required reduction is from 103 to 10ÿ6 (a reduction of 109 or a 9D reduction). The D-value for salmon at 64 ëC is 58.44 s and the holding time at 64 ëC to ensure a Listeria-free product is therefore 9 58.44 s 8.77 minutes. At a core temperature of 72 ëC, the time required is 9 2.92 seconds 26.28
Table 17.1 products
D-values for L. monocytogenes and minimum processing times for hot-smoking at different temperatures to ensure 12D reductions in selected
Temperature (ëC)
Cod D-value min
58 60 62 64 66 68 70 72 74 76 78 80 82 84
4.29 1.97
Adapted from Fletcher et al. (2003)
s
54.02 24.75 11.34 5.19 2.38 1.09 0.5 0.23 0.1 0.05 0.02 0.01
Salmon Processing time 52 min 24 min 11 min 5 min 57 s 3 min 16 s 2 min 2 s 28.6 s 13.1 s 6s 2.7 s 1.3 s 0.6 s 0.3 s 0.1 s
D-value min 9.21 4.36 2.06
s
58.44 27.64 13.07 6.18 2.92 1.38 0.65 0.31 0.15 0.07 0.03
Smoked mussels Processing time 1 h 51 min 53 min 25 min 12 min 6 min 32 s 3 min 37 s 2 min 14 s 35.1 s 16.6 s 7.8 s 3.7 s 1.8 s 0.8 s 0.4 s
D-value min 16.22 5.49 1.86
s
37.72 12.76 4.32 1.46 0.49 0.17 0.06 0.02 <0.01 <0.01 <0.01
Processing time 3 h 14 min 1 h 5 min 22 min 7 min 33 s 2 min 33 s 51.8 s 17.5 s 5.9 s 2s 0.7 s 0.2 s 0.1 s <0.1 s <0.1 s
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seconds. Further information is given by Bremer and Osborne (1995) and Ben Embarek and Huss (1993). 17.1.1 Constituents in smoke Mostly, smoke is air and other gases and vapours with a mixture of small hydrocarbon particles of different sizes. Some particles are deposited on the surface of the food, but this is of minor importance for the smoking process. More importantly, the absorption of gases by foods gives the characteristic colour changes and flavour. Some softwoods, especially pine and fir, contain resins that produce harsh-tasting retene and other components when burned, and these woods are therefore not used for smoking, except for lapsang souchong tea leaves that are smoked and dried over pine or cedar fires. For other foods, hardwoods are considered to produce superior flavours and colours in smoked foods. Hardwood shavings or logs (e.g. oak, beech, chestnut, hickory), dampened with wet sawdust, are burned to produce heat and dense smoke. Sometimes aromatic woods, such as apple, juniper or cherry, or aromatic herbs and spices are also used to produce distinctive flavours. The important chemical components of smoke are as follows (Anon 1992): · · · · · · ·
nitrogen oxides; polycyclic aromatic hydrocarbons (PAHs); phenolic compounds; furans; carbonylic compounds; aliphatic carboxylic acids; tar compounds.
For components that have a high boiling point (e.g. PAHs and phenolic compounds) there is a correlation between their concentration in the smoke and that in the smoked food, whereas more volatile components are not usually found in the food. When burned, the cellulose and hemicellulose in hardwoods produce sweet, flowery and fruity aromas. Products of pyrolysis of lignin include spicy, pungent phenolic compounds such as guaiacol, responsible for the smoky taste, phenol and syringol, a contributor to a smoky aroma. It also produces sweeter aromas including vanilla-scented vanillin and clove-like isoeugenol (GuilleÂn et al. 2006, Hui 2001). Wood also contains small quantities of proteins that contribute roasted flavours. Lesimple et al. (1995) identified 62 volatile components in smoked duck fillets, including phenols, alcohols, ethers, aldehydes, ketones, hydrocarbons, acids, esters and terpenes, 34 of which were related to the smoking process. Others (e.g. Anon 1992) report >400 volatile chemical compounds identified in wood smoke. Different species of tree have different amounts of these components and hence their woods impart different flavours to food. The chemical composition and hence the flavours in smoke also depend on the temperature of the fire, the moisture content of the wood, the supply of air to the fire and any water added during burning. High-temperature fires break down flavour molecules into unpleasant tasting compounds. The optimal conditions for producing desirable smoke flavours are lowertemperature, smouldering fires at 300±400 ëC. Woods that contain high amounts of lignin burn hotter and a restricted air supply is needed to keep them smouldering, or their moisture content is increased by soaking the pieces in water. The factors that influence absorption of smoke by food include the density of the smoke and its humidity and temperature. The higher the smoke density, the greater the
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absorption. In warm- and hot-smoking, the food surface is dried and condensation of the smoke particles is less than on products that are smoked at lower temperatures. If the relative humidity (Chapter 16, section 16.1.1) of the smoke is high, vapour condenses on the food surface and absorption of water-soluble components of the smoke increases. However, if the surface remains moist, colour formation is inhibited as this increases at low moisture contents. If the surface is too dry, there is less penetration of the smoke into the food and hence loss of flavour and preservative action. A number of wood smoke compounds act as preservatives. Phenol and other phenolic compounds in wood smoke are both antioxidants and antimicrobials. Other antimicrobial chemicals include formaldehyde, acetic acid and other carboxylic acids (section 17.4). These chemicals are also extracted and used as liquid smoke (section 17.1.2). Smoke also contains compounds that have long-term health consequences including PAHs and dioxin-like polychlorinated biphenyls (PCBs) many of which are known or suspected carcinogens (section 17.3). Nitrogen oxides in smoke may also react with amines or amides in the food to form nitrosamines, or with phenols to produce nitro or C-nitroso phenols. Carbonylic compounds and acids react with proteins and carbohydrates in the food. Nitrogen oxides and PAHs are normally only found in small amounts in smoke, but they are of concern because of the potential public health risk of nitrosamines and PAHs (Anon 1992). `Tasteless smoke' was patented in 1999 as a method of preserving fresh fish. The process involves producing hardwood smoke by burning the chips, and then passing it through filters that remove all particles larger than 1 m and most of the odour and colour components. The remaining gases (nitrogen, carbon monoxide (CO), carbon dioxide and methane, together with trace amounts of phenolic compounds and hydrocarbons) are applied to tuna fish or other red meats. The product is removed from the smoke chamber, washed in ozonated water to remove any residual smoke odour and kill bacteria, and it is then frozen. The treatment retains the appearance, taste, texture and colour of the fresh seafood after freezing and defrosting (Walsh 2005). The low concentrations of CO alter the colour, when it reacts with meat pigments to produce a cherry-red carboxymyoglobin pigment, but only for a limited time, and eventually the colour diminishes as the fish ages. The process is approved by the EU as a smoking process because, as a component of wood smoke (a GRAS (generally regarded as safe) substance), CO is a legal treatment and is declared on the product label. In contrast, the treatment of fish with industrial carbon monoxide produces colour changes that do not fade. It was banned in the EU in 2003 because it could mislead consumers over the freshness of meat by maintaining a bright red colour. Although the cherry-red colour is different from the oxymyoglobin pigment in fresh meat, it may mislead customers that the product is fresher than it actually is, or enhance the appearance of inferior products. CO treatment can also mask colour changes caused by decomposition, mask other visual evidence of spoilage, and mask potential safety problems such as production of histamines (section 17.4). In Singapore the use of CO is considered a malpractice and Japan bans fish that have an initial CO content 500 g kgÿ1. Currently (2008) in the USA, the Food and Drug Administration (FDA) regards the use of tasteless smoke on tuna as a preservative and it thus needs to be labelled accordingly (Anon 2007a), although this position is challenged. 17.1.2 Liquid smoke Smoke flavourings or `liquid smoke' are prepared by condensing smoke derived from burning wood, usually followed by fractionation, purification or concentration. The
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fractionation steps produce products that have the desired olfactory properties and also reduce the concentration of undesirable by-products of the smoke. For example, as a marker for PAHs (section 17.3) the benzo(a)pyrene content of smoke concentrates is reduced to a maximum limit of 1 g kgÿ1 of condensate. Much of the tar formed during pyrolysis can also be removed. A typical commercial smoke condensate contains 70% water, 29% volatile organic compounds and 1% tar (Anon 1992). Smoke condensates may be used for flavouring food but are more commonly used as the basis for smoke flavouring preparations that contain carrier materials such as salt or dextrose. Their main advantages are that they can be used as a step in continuous processing and reduced production times, compared with batch smoking in kilns. The smoke flavour can be mixed directly into the food, applied as a powder (Chapter 24, section 24.2.2) or sprayed as a fine mist. The use of smoke flavourings also avoids product weight losses that take place during smoking, which affects the product yield and quality. Although the flavour from liquid smoke is almost the same as that produced by traditional smoking, the product does not necessarily acquire the same texture or colour. Smoke flavourings are therefore often combined with a smoking process to get the desired colour from the smoke and the taste from the smoke flavouring.
17.2
Processing
Foods are cured with salt before warm- and cold-smoking and sometimes before hotsmoking. Cold-smoked meats may be cured or precooked before smoking. The most common type of cure in industrialised countries is a wet cure using brine with added sugar and spices. Curing times vary from a few hours to two weeks. Dry curing, in which coarse salt is rubbed directly into fish or meat, is less common because of higher labour costs and less uniform salting, but it is still widely practised in tropical artisan processing. These heavily salted, hot-smoked products can be stored without refrigeration for several weeks, but require boiling in freshwater to make them palatable before consumption. After curing, fish are drained, rinsed and refrigerated for 6±12 h, and then smoked at low temperature (25±33 ëC) in a kiln or smokehouse for between half a day to 3 weeks. After removal from the kiln and chilling for 24 h, the food may be sliced or packed as whole pieces. The following process, described by Bannerman (1984) using fresh chilled or thawed frozen fish, is an example of the manufacture of a hot-smoked product. Frozen fish is preferred for smoking because freezing for at least 30 days kills parasites in the fish. Fish are brined using an 80 brine (211 g salt lÿ1) to give them flavour and to inhibit the growth of food poisoning micro-organisms without making the product unpleasantly salty. Fish absorb salt more uniformly in weaker brines, but the immersion time is longer, whereas stronger brines can cause salt to crystallise on the surface of the skin after the fish are dried and smoked, creating unsightly white patches. Immersion times in the brine vary according to the size, thickness and fat content of the fish and the aim is to achieve a salt concentration in the finished product of >3%. Yanar et al. (2006) studied the effect of brine concentration on the shelf-life of hot-smoked tilapia and concluded that 5% brine was optimal for a shelf-life of 35 days. After brining, fish are hung on trolleys in a smoking kiln (section 17.2.1) so that the backs face the flow of smoke. Fillets of fish or meat and small products such as shellfish are arranged on wire mesh trays on the trolleys. The process operates in three stages: first drying at 30 ëC for 30±60 min to toughen the skin, then smoking and partial cooking at
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531
50 ëC for 30±45 min, and finally cooking at 80 ëC for a few minutes up to 30 min for large fish. The total processing time and the times spent at each stage depend on the type of food, its size and fat content, and the type of product required. In the traditional method for making Arbroath smokies (from haddock) the fish are first salted overnight and then tied in pairs and left overnight to dry. The dried fish are hung in a barrel containing a hardwood fire and sealed with a lid to create a very hot and humid smoky fire without flames. Within an hour of smoking, the intense heat and thick smoke produce the strong smoky taste and aroma that characterises this product and the fish are then ready to eat. In other processes, smoking takes up to 8 h at temperatures of 60±85 ëC, with the internal temperature of the fish at 60 ëC for at least 30 min to kill pathogenic bacteria. Cold- and hot-smoked fish are cooled to chill temperatures (3 ëC) in a chill room before packing. They are commonly packaged into modified atmosphere or vacuum packs (Chapter 25, section 25.3). Muratore and Licciardello (2005) studied the effects of different packaging methods on shelf-life. Products are maintained at chill temperatures during storage, distribution and retail display. Cold-smoked white fish have a longer shelf-life than fatty fish, although it varies greatly with the type of fish, the amount of salting, the extent of smoking and drying, and the storage temperature. Civera et al. (1995) conducted chemical and microbiological analyses of smoked Atlantic and Canadian salmon and found that the effective shelf-life was 40±50 days and 80 days respectively if the fish is stored at 2±3 ëC. Smoked products can also be frozen and stored at ÿ30 ëC for at least 6 months, or longer when vacuum packed and frozen. Vacuum packaging excludes oxygen and thus slows the development of rancidity, especially in fatty fish products. However, vacuum packing creates an anaerobic environment inside the pack that is suitable for the growth of C. botulinum (section 17.4). Packaging materials are now available that have adequate oxygen transfer to prevent the development of an anaerobic environment within a package (Chapter 25, section 25.4.3). Containers or packages made of materials with an oxygen permeability of 2000 cm3 mÿ2 per 24 h at 24 ëC or higher are permitted for use with products stored at 4 ëC or lower for up to 14 days (Anon 1991). Styrofoam trays with a single film overwrap are permitted if the total permeability of the final package meets the minimum specifications, but these packs should not be stacked in a way that would reduce the oxygen permeability. Irradiation (Chapter 7) has also been studied as a method to increase the shelf-life of smoked fish. For example, Hammad and El-Mongy (1992) irradiated cold-smoked salmon at 2 and 4 kGy, stored at 2±3 ëC. Unirradiated samples reached the maximum accepted mesophilic plate count after one month of storage, while those irradiated reached this level after three and four months at 2 and 4 kGy respectively. No differences in sensory qualities were found between unirradiated samples and those irradiated at 2 kGy, but there was a loss of red colour in samples irradiated at 4 kGy. 17.2.1 Equipment Smoking equipment should allow the controlled development of flavour and colour in foods, with low levels of carcinogenic or toxic components in the smoke, and low levels of environmental pollution by the smoke. Smoke can either be generated in the kiln or produced in a separate smoke generator. Sawdust and wood should be clean and free from wood preservatives or saw lubricants. Sawdust consumption is 13 kg hÿ1 in a kiln of 375 kg capacity (Bannerman 1984). Separate smoke generators have advantages in that
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Fig. 17.1
Smoking kiln (courtesy of AFOS Ltd at www.intlsmokingsystems.com).
the temperature and humidity in the kiln can be independently controlled so that foods can be dried or cooked before being smoked. They also give better control over the temperature, humidity and density of the smoke. Additionally, the smoke can be filtered, treated with water sprays or by electrostatic precipitation to remove carcinogenic compounds such as benzo(a)pyrene (Ranken 2000) and unwanted particles. However, these treatments also remove some of the components that contribute to the flavour of smoked foods. Smoking kilns are similar in design to cabinet dryers (Chapter 16, section 16.2.1) or batch ovens (Chapter 18, section 18.2.1) and, in hot-smoking, foods may be dried, cooked and smoked in the same equipment. Kiln heaters are designed to quickly reach and maintain an operating temperature of 80 ëC when fully loaded. Trolleys that have rails or mesh trays are wheeled into the smoking chamber (Fig. 17.1) and the food is smoked in an automatically controlled cycle. Computer control includes management of the smoke temperature, humidity and density via touch-screen or remote controls, a fire protection system and alarms, and an automatic cleaning cycle. The capacity of commercial kilns is 250±2000 kg (Anon 2007b).
17.3
Effect on foods
The main purpose of smoking is to alter the sensory properties of foods, particularly the flavour and colour. Chemicals in smoke that contribute to the flavour and aroma of smoked foods are described in section 17.1.1. Smoking produces a shiny yellow colour, which darkens as smoking time is increased. Earlier studies showed that the colour is produced by interaction of amino groups on proteins in the food with carbonyls in the smoke in a similar way to the Maillard reaction (Ruiter 1979, Gilbert and Knowles 1975). Iliadis et al. (2004) studied the effects of pretreatment and hot- and cold-smoking on the chemical, microbiological and sensory quality of mackerel. They found that available lysine was reduced to the same extent (32%) in all hot smoked samples, and that loss of available lysine correlated with colour formation in the cold-smoked products. Nitrogen oxides in smoke can also react with myoglobin to produce a modified colour in smoked
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meat products. Riha and Wendorff (1993) studied the acceptability of the colour of smoked cheese. The surface texture of cold-smoked foods is changed by the formation of a firm pellicle, produced by coagulation of proteins by acidic components of the smoke, but the interior of the food is unchanged. In hot-smoked foods, the texture becomes that of cooked foods owing to heat coagulation of proteins. Salting causes liquid exudates from the flesh of meat and fish, causing losses of watersoluble proteins, vitamins and minerals. Some proteins are also denatured by the salt. Some constituent chemicals in the smoke at the surface of foods (e.g. butyl gallate and BHA) have an antioxidant action (Brul et al. 2000). These antioxidant components of smoke reduce oxidative changes to fats, proteins and vitamins. However, hot-smoking also causes nutrient losses due to heat and interaction of the smoke components with proteins. The heat and flow of gases in the smoke cause dehydration of the food and changes to the nutritional and sensory properties described in Chapter 16 (section 16.5) including denaturation of proteins. The loss of moisture also increases the concentration of protein and fat in the food and an increased concentration of salt and other curing agents. Smoked foods give rise to some health concerns, especially with respect to PAHs and nitrosamines. The potential harmful effects of these smoke components are described by Anon (1992). The PAH component benzo(a)pyrene and 10 other compounds from this group are both mutagenic and carcinogenic (Lawley 1990). High levels of PAHs are found on the surface of products that are smoked for a long time at higher temperatures, but Iliadis et al. (2004) found high levels of benzo(a)pyrene, fluoranthene and perylene both in cold- and hot-smoked fish. Witczak and Ciereszko (2006) studied changes in the content of PCBs in mackerel slices during cold and hot smoking. The hot-smoked mackerel showed a decrease in the PCB content, which may have been due to losses with lipid leakage from the product and co-distillation with water vapour. Cold smoking produced an increase in PCB content in the final product compared to the initial raw material. However, their levels may be reduced by using fire temperatures below 400 ëC for smoke generation, by treating smoke (section 17.1.2) and by reduced smoking times. Some products (e.g. frankfurter sausages) are washed after smoking, which also reduces the concentration of these carcinogens. The average intake of PAH components has been calculated to be 1.2 mg per year, but smoked meat and fish contribute only 10% of this, the remainder coming from environmental pollution and tobacco smoking (Anon 1992). The intake of PAHs from food is therefore regarded as of minor public health importance, but some smoked foods can have unacceptably high levels of PAHs due to the smoking process and these are therefore a health concern. In the EU upper limits for smoked food are 5 g of volatile N-nitroso compounds per kg of food and 1 g of benzo(a)pyrene per kg of food, with the exception for smoked fish of 5 g of benzo(a)pyrene per kg. The maximum limit for benzo(a)pyrene in liquid smoke condensates is 10 g kgÿ1, to produce less than 0.03 g kgÿ1 in the food as consumed and for benzo(a)anthracene, the maximum is 20 g kgÿ1, giving less than 0.06 g kgÿ1 in the food (Anon 1992). Smoked foods may contain N-nitroso compounds, such as N-nitrosodimethylamine. These nitrosamines are among the most carcinogenic substances that have been studied. They are formed by reactions between nitrogen oxides in smoke and amines or amides in the foods, especially in those that have higher concentrations of amines, such as fish and meat. These compounds increase the risk of gastrointestinal cancer in populations where there is a high intake of heavily smoked (and/or salted) foods. Phenolic compounds are
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important for the taste of smoked food, but where nitrite is used (e.g. in cured smoked meats), phenols may react with the nitrite to form nitro- and nitrosophenols, some of which have been shown to be mutagenic. Some of the phenols may also catalyse the formation of nitrosamines in foods during smoking. Tar compounds in smoke are not well characterised and their effects on health are therefore difficult to evaluate.
17.4
Effect on micro-organisms
The combined effects of salt, antimicrobial chemicals in smoke, heat and partial dehydration during hot-smoking are effective against Gram-negative rods, micrococci and staphylococci. Vegetative bacteria are more susceptible to smoke than bacterial spores and moulds. This means that spoilage by moulds is more likely than bacterial spoilage. However, there is a significant risk of pathogenic bacterial contamination, especially of cold-smoked fish products, and standards for hygienic production and handling are described in the legislation of many countries (e.g. Anon 1979). Sikorski and Kaøodziejska (2002) reviewed the incidence of contamination by pathogens on hotsmoked fish. They found low numbers of Listeria monocytogenes, Clostridium botulinum, Staphylococcus aureus and Vibrio parahaemolyticus and concluded that the main causes of contamination were unsanitary procedures and airborne micro-organisms during packing of the product. The internal temperature in the fish, which did not exceed 65 ëC, and the low concentration of salt were insufficient to inactivate all pathogens or inhibit bacterial growth during storage. Product safety required very fresh fish, handled under hygienic conditions, chilling the product to 2 ëC and hygienic handling of the product after smoking. Details of safe handling of chilled foods are given in Chapter 21 (section 21.5). Listeria monocytogenes poses a health risk for immunocompromised individuals and pregnant women (Chapter 1, section 1.2.4). It is not destroyed during cold-smoking and it can become established in the processing environment and re-contaminate products. It is therefore not possible to produce cold-smoked fish that is consistently free of L. monocytogenes (Anon 2001). However, the use of good manufacturing practices (GMP) and good hygienic practices (GHP) (Chapter 1, section 1.5.1) enables production of coldsmoked fish with low levels of L. monocytogenes (<1 cell gÿ1). Growth can also be prevented by freezing, by addition of preservatives (e.g. sodium nitrite) or by use of bioprotective bacterial cultures (Chapter 6, section 6.7). It can also be controlled by limiting the shelf-life (at 4.4 ëC) to ensure that not more than 100 cells gÿ1 are present when the food is consumed. Clostridium botulinum occurs naturally in the aquatic environment and can be present in low numbers on fresh fish. Cases of botulism caused by type E toxin from smoked fish are reported by Fletcher et al. (2003) in the anaerobic conditions found in vacuum packs. Korkeala et al. (1998) report cases of botulism after eating hot-smoked whitefish, processed from frozen fish. The fish contained botulinum toxin and Cl. botulinum was isolated from the fish. They identified safety problems associated with vacuum packed hot-smoked fish and described the product as one of the highest risk industrial foods to cause botulism. They recommended temperature monitoring and the use of time± temperature indicators (Chapter 21, section 21.2.4) to ensure adequately low storage temperatures throughout the processing chain and the use of sodium nitrate and nitrite with a sufficiently high salt concentration to prevent Cl. botulinum growth. In the USA, the recommended smoking conditions required to kill Cl. botulinum type E are that
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products achieve a core temperature of 62.5 ëC for 30 min and for products stored in air, the fish should contain >2.5% salt in the muscle. Vacuum packaged or modified atmosphere packaged products should contain either >3.5% salt or 3.0% salt plus 100± 200 g gÿ1 of sodium nitrite with storage at chill temperatures <4.4 ëC (Anon 1995). Sikorski and Kaøodziejska (2002) reviewed studies of other preservative chemicals, including sodium lactate, lactic and sorbic acids, sodium propionate, nisin and lysozyme. The flesh of some fish species (e.g. tuna, mackerel and mullet) contains high levels of the amino acid histidine, which is converted to the biogenic amine, histamine, by bacteria (e.g. Enterobactericae) growing under suitable conditions on the fish. Histamine causes the symptoms of scombroid poisoning, which are similar to an allergic reaction and include facial swelling, itching of the skin, headache, nausea and vomiting (Fletcher et al. 2003). Histamine is not destroyed by subsequent cooking. Iliadis et al. (2004) found unacceptable levels of histamine (600 mg kgÿ1) in unprocessed samples of fish, which increased to levels that would be expected to cause symptoms of scombrotoxin poisoning in both cold- and hot-smoked products (2220 and 2250 mg kgÿ1 respectively). Freezing, salting or smoking may inhibit or inactivate histamine-producing micro-organisms, but growth may take place after thawing before smoking, and post-smoking. Vacuum packaging does not prevent their growth. Handling and processing the fish under sanitary conditions, rapid cooling and continuous refrigeration until consumption each prevent the growth of these bacteria, and hence prevent biogenic amine formation. Other illnesses that result from consumption of improperly processed smoked foods are caused by Clostridium perfringens, Staphylococcus aureus and Salmonella spp. None of these micro-organisms occurs naturally on raw fish and the sources of infection are food handlers who carry these micro-organisms or shellfish harvested from unsanitary water (see Chapter 1, section 1.2.4). The most important methods to control these bacteria are: · temperature control during hot-smoking to kill any contaminating micro-organisms; · holding the product at a sufficiently low temperature before and after smoking to prevent their growth; · good sanitation and hygienic work practices. Fletcher et al. (2003) describe methods used for the safe production of smoked fish and seafoods using GMP and HACCP (see also Chapter 1, section 1.5.1). Internationally recognised controls require that: · the core temperature of the fish should be brought to 10 ëC or less within 6 h of death and to 4 ëC within 24 h; · chilled fish should not be exposed to temperatures above 4 ëC for more than 4 h cumulatively after the initial chilling. · chilled fish should not be stored for more than 14 days at 0 ëC or more than 7 days at 4 ëC before smoking. · frozen fish (stored for 24 weeks or longer) should not be exposed to temperatures above 4 ëC for more than 12 h, cumulatively after the initial chilling period, and it should not be exposed to temperatures above 4 ëC for more than 6 h of uninterrupted storage (Anon 2001).
References ANON,
(1979), Codex Standards for Smoked Fish, available at www.codexalimentarius.net/ download/standards/123/CXP_025e.pdf.
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(1991), Smoked fish: storage conditions, Canadian Food Inspection Agency, available at www.inspection.gc.ca/english/fssa/labeti/retdet/bulletins/smofume.shtml. ANON, (1992), Health aspects of using smoke flavours as food ingredients, Health protection of consumers, Council of Europe, Publishing and Documentation Service, Strasbourg, available at www.coe.int/t/e/social_cohesion/soc-sp/public_health/flavouring_substances/ SMOKE.pdf. ANON, (1995), Advisory group approves food safety guide for smoked fish, Food Chemical News, 37, 13±14. ANON, (2001), Processing parameters needed to control pathogens in cold smoked fish ± conclusions and research needs, US Food and Drug Administration Center for Food Safety and Applied Nutrition, available at www.cfsan.fda.gov/~comm/ift2list.html. ANON, (2007a), Selected countries prohibiting carbon monoxide (CO) gas in fresh meat and fresh fish packaging, Carbon Monoxide In Fresh Meat, available at www.co-meat.com/ countries.html. ANON, (2007b), company information from AFOS Ltd, available at www.intlsmokingsystems.com/ AFOS.pdf. BANNERMAN, A.M., (1984), Hot smoking of fish, Torry Advisory Notes, 82, HMSO, available at www.fao.org/wairdocs/tan/x5953e/x5953e00.htm BEN EMBAREK, P.K. and HUSS, H.H., (1993), Heat resistance of Listeria monocytogenes in vacuum packaged pasteurized fish fillets, International J. Food Microbiology, 20, 85±95. BREMER, P. and OSBORNE, C., (1995), Thermal-death times of Listeria monocytogenes in green shell mussels (Perna canaliculus) prepared for hot smoking, J. Food Protection, 58, 604±608. BRUL, S., KLIS, F.M., KNORR, D., ABEE, T. and NOTERMANS, S., (2000), Food preservation and the development of microbial resistance, in (P. Zeuthen and L Bùgh-Sorensen, Eds.), Food Preservation Techniques, Woodhead Publishing, Cambridge, pp. 524±543. CIVERA, T., PARISI, E., AMERIO, G.P. and GIACCONE, V., (1995), Shelf-life of vacuum-packed smoked salmon: microbiological and chemical changes during storage, Archiv fuÈr Lebensmittelhygiene, 46 (1), 13±17. DOE, P.E., (1998), Fish Drying and Smoking, Woodhead Publishing, Cambridge. FLETCHER, G.C., BREMER, P.J., SUMMERS, G. and OSBORNE, C., (2003), Guidelines for the safe preparation of hot-smoked seafood in New Zealand, New Zealand Institute for Crop and Food Research Ltd, available at www.crop.cri.nz/home/research/marine/pathogens/hotsmoked.pdf. GILBERT, J. and KNOWLES, M.E., (1975), The chemistry of smoked foods ± a review, J. Food Technology, 10, 245±261. GOULAS, A.E. and KONTOMINAS, M.G., (2005), Effect of salting and smoking-method on the keeping quality of chub mackerel (Scomber japonicus): biochemical and sensory attributes, Food Chemistry, 93 (3), 511±520. Â N, J. and CASAS, C., (2006), Headspace volatile components GUILLEÂN, M.D., ERRECALDE, M.C., SALMERO of smoked swordfish (Xiphias gladius) and cod (Gadus morhua) detected by means of solid phase microextraction and gas chromatography±mass spectrometry, Food Chemistry, 94 (1), 151±156. HAMMAD, A.A.I. and EL-MONGY, T.M., (1992), Shelf-life extension and improvement of the microbiological quality of smoked salmon by irradiation, J. Food Processing and Preservation, 16 (5), 361±370. HUI, Y.H., (2001), Meat Science and Applications, Marcel Dekker, New York. ILIADIS, K.N., ZOTOS, A., TAYLOR, A.K.D. and PETRIDIS, D., (2004), Effect of pre-treatment and smoking process (cold and hot) on chemical, microbiological and sensory quality of mackerel (Scomber scombrus), J. Science Food and Agriculture, 84 (12), 1545±1552. È , E., VOGELSANG, B., BOHL, A., WIHLMAN, H., PAKKALA, P. and HIELM, KORKEALA, H., STENGEL, G., HYYTIA S., (1998), Type E botulism associated with vacuum-packaged hot-smoked whitefish, International J. Food Microbiology, 43 (1±2), 1±5. LAWLEY, P.D., (1990), N-Nitroso compounds, in (C.S. Cooper, and P.L. Grover, Eds.), Handbook of ANON,
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Experimental Pharmacology ± Chemical Carcinogenesis and Mutagenesis, Springer-Verlag, Berlin, pp. 410±469. LESIMPLE, S., TORRES, L., MITJAVILA, S., FERNANDEZ, Y. and DURAND, L., (1995), Volatile compounds in processed duck fillet, J. Food Science, 60 (3), 615±618. MURATORE, G. and LICCIARDELLO, F., (2005), Effect of vacuum and modified atmosphere packaging on the shelf-life of liquid-smoked swordfish (Xiphias gladius) slices, J. Food Science, 70 (5), C359±C363. RANKEN, M.D., (2000), Handbook of Meat Product Technology, Blackwell Science, Oxford, p. 151. RIHA, W.E. and WENDORFF, W.L., (1993), Evaluation of color in smoked cheese by sensory and objective methods, J. Dairy Science, 76, 1491±1497. RUITER, A., (1979), Color of smoked foods, Food Technology, 33, 54±63. SIKORSKI, Z.E. and KAèODZIEJSKA, I., (2002), Microbial risks in mild hot smoking of fish, Critical Reviews in Food Science and Nutrition, 42 (1), 35±51. WALSH, E., (2005), What is Clearsmoke, Food and Beverage International, Spring issue, available at www.fbworld.com/Mag_Spring_2005/advertorial/anova%20-%20clearsmoke/ AnovaClearsmoke.html. WITCZAK, A. and CIERESZKO, W., (2006), Effect of smoking process on changes in the content of selected non-ortho- and mono-ortho-PCB congeners in mackerel slices, J. Agriculture and Food Chemistry, 54 (15), 5664±5671. YANAR, Y., CËELIK, M. and AKAMCA, E., (2006), Effects of brine concentration on shelf-life of hotsmoked tilapia (Oreochromis niloticus) stored at 4 ëC, Food Chemistry, 97 (2), 244±247.
Smoking is an ancient process that was used to preserve protein-rich foods for storage at ambient temperatures in times of shortage during winter seasons in temperate climates and during dry seasons in tropical climates. Now, the purpose is to change the flavour and colour of foods, rather than preservation. Smoked foods are preserved by chilling (Chapter 21), and may be packed in modified atmospheres or vacuum packed (Chapter 25, section 25.3) to give the required shelf-life. Smoking is an inexpensive operation that increases the variety of products for consumers, and for processors it adds value to foods. The most commonly smoked foods are fish (e.g. tilapia, mackerel, trout, sable (or black cod), sturgeon and tuna), meats and meat products (e.g. duck, venison, game birds, and paÃteÂs made from these meats, pork, pastrami (pickled, spiced and smoked beef brisket) and beef jerky) and cheeses such as smoked Gouda. Other smoked foods include vegetables such as chipotles (smoked jalapenÄo peppers), seafoods, nuts, lapsang souchong tea, barley malt used in the manufacture of some types of whisky and ingredients used to make German smoked beer (rauchbier). There are four types of smoking operations: 1 Cold smoking, in which the food is flavoured and coloured but not cooked. It is typically used for salmon, salamis, kippers, hams and special cheeses. 2 Warm smoking at 25±40 ëC is used for bacon, sirloin and some types of sausage. 3 Hot smoking of meats and fish at 60±80 ëC, which cooks the food and the heat is sufficient to destroy contaminating micro-organisms. Herring, eel and some sausages are hot-smoked.
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4 Dissolving smoke compounds in water to make smoke concentrate or `liquid smoke' and spraying or coating foods (see also coating, Chapter 24). Each type of smoking is a surface treatment and smoke chemicals penetrate only a few millimetres into the product.
17.1
Theory
In cold smoking, foods are cured at an air temperature <33 ëC for between 6±24 hours and several weeks to produce the required smoked flavour and colour. The texture remains largely unchanged and the products have a milder taste than hot-smoked foods. Microorganisms are not destroyed and for this reason, cold-smoking is preceded by salt curing. Warm smoking has similar effects and foods are also cured. The preservative action of hot-smoking at 60±80 ëC results from a number of factors (see also section 17.4 and hurdle technology (Chapter 1, section 1.3.1)): · dehydration/reduced moisture content to lower the water activity of the product; · antioxidant action of some constituent chemicals in the smoke at the surface of foods (e.g. butyl gallate and butylated hydroxyanisole (BHA)) (Brul et al. 2000); · destruction of micro-organisms and enzymes by heat; · antimicrobial action of some constituent chemicals in the smoke at the surface of foods (e.g. phenolic compounds, organic acids); · antimicrobial action of salt pretreatments where these are used. Fish is the main type of food that is hot-smoked. Details of the process are given by Doe (1998). Goulas and Kontominas (2005) studied the effect of salting and smoking method on the keeping quality of chub mackerel. They found that during the 30 day storage period, salting had a noticeable preservative effect but it was lower than the combined effect of salting and smoking. The heat and humidity cook the products and produce the required smoky taste, golden brown colour, a silky sheen on the skin and uniform weight loss. The products do not require further cooking before consumption. In hot-smoking, the critical factors to control bacteria are (Fletcher et al. 2003): · · · · ·
the core temperatures reached by the product during processing; control over temperature variations in different parts of the kiln; numbers of contaminating bacteria; the D-values of the bacteria of interest (see Chapter 10, section 10.3); and the amount of smoking.
Although smoke and salt have antimicrobial effects (section 17.4), microbial destruction during hot-smoking is based on the heat treatment received. For example, for control of Listeria monocytogenes in smoked salmon, the process should heat the product to a given temperature for a long enough period to ensure that the possibility of any Listeria surviving is less than one in a million (106). If the highest likely contamination by L. monocytogenes is 106 gÿ1 of raw fish, the required reduction is from 106 to 10ÿ6 (12 D-values). For salmon, this requires 12 minutes at 64 ëC or 35.1 seconds at 72 ëC (Table 17.1). However, if contamination by L. monocytogenes is less than 1000 per fish for good quality salmon, then the required reduction is from 103 to 10ÿ6 (a reduction of 109 or a 9D reduction). The D-value for salmon at 64 ëC is 58.44 s and the holding time at 64 ëC to ensure a Listeria-free product is therefore 9 58.44 s 8.77 minutes. At a core temperature of 72 ëC, the time required is 9 2.92 seconds 26.28
Table 17.1 products
D-values for L. monocytogenes and minimum processing times for hot-smoking at different temperatures to ensure 12D reductions in selected
Temperature (ëC)
Cod D-value min
58 60 62 64 66 68 70 72 74 76 78 80 82 84
4.29 1.97
Adapted from Fletcher et al. (2003)
s
54.02 24.75 11.34 5.19 2.38 1.09 0.5 0.23 0.1 0.05 0.02 0.01
Salmon Processing time 52 min 24 min 11 min 5 min 57 s 3 min 16 s 2 min 2 s 28.6 s 13.1 s 6s 2.7 s 1.3 s 0.6 s 0.3 s 0.1 s
D-value min 9.21 4.36 2.06
s
58.44 27.64 13.07 6.18 2.92 1.38 0.65 0.31 0.15 0.07 0.03
Smoked mussels Processing time 1 h 51 min 53 min 25 min 12 min 6 min 32 s 3 min 37 s 2 min 14 s 35.1 s 16.6 s 7.8 s 3.7 s 1.8 s 0.8 s 0.4 s
D-value min 16.22 5.49 1.86
s
37.72 12.76 4.32 1.46 0.49 0.17 0.06 0.02 <0.01 <0.01 <0.01
Processing time 3 h 14 min 1 h 5 min 22 min 7 min 33 s 2 min 33 s 51.8 s 17.5 s 5.9 s 2s 0.7 s 0.2 s 0.1 s <0.1 s <0.1 s
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seconds. Further information is given by Bremer and Osborne (1995) and Ben Embarek and Huss (1993). 17.1.1 Constituents in smoke Mostly, smoke is air and other gases and vapours with a mixture of small hydrocarbon particles of different sizes. Some particles are deposited on the surface of the food, but this is of minor importance for the smoking process. More importantly, the absorption of gases by foods gives the characteristic colour changes and flavour. Some softwoods, especially pine and fir, contain resins that produce harsh-tasting retene and other components when burned, and these woods are therefore not used for smoking, except for lapsang souchong tea leaves that are smoked and dried over pine or cedar fires. For other foods, hardwoods are considered to produce superior flavours and colours in smoked foods. Hardwood shavings or logs (e.g. oak, beech, chestnut, hickory), dampened with wet sawdust, are burned to produce heat and dense smoke. Sometimes aromatic woods, such as apple, juniper or cherry, or aromatic herbs and spices are also used to produce distinctive flavours. The important chemical components of smoke are as follows (Anon 1992): · · · · · · ·
nitrogen oxides; polycyclic aromatic hydrocarbons (PAHs); phenolic compounds; furans; carbonylic compounds; aliphatic carboxylic acids; tar compounds.
For components that have a high boiling point (e.g. PAHs and phenolic compounds) there is a correlation between their concentration in the smoke and that in the smoked food, whereas more volatile components are not usually found in the food. When burned, the cellulose and hemicellulose in hardwoods produce sweet, flowery and fruity aromas. Products of pyrolysis of lignin include spicy, pungent phenolic compounds such as guaiacol, responsible for the smoky taste, phenol and syringol, a contributor to a smoky aroma. It also produces sweeter aromas including vanilla-scented vanillin and clove-like isoeugenol (GuilleÂn et al. 2006, Hui 2001). Wood also contains small quantities of proteins that contribute roasted flavours. Lesimple et al. (1995) identified 62 volatile components in smoked duck fillets, including phenols, alcohols, ethers, aldehydes, ketones, hydrocarbons, acids, esters and terpenes, 34 of which were related to the smoking process. Others (e.g. Anon 1992) report >400 volatile chemical compounds identified in wood smoke. Different species of tree have different amounts of these components and hence their woods impart different flavours to food. The chemical composition and hence the flavours in smoke also depend on the temperature of the fire, the moisture content of the wood, the supply of air to the fire and any water added during burning. High-temperature fires break down flavour molecules into unpleasant tasting compounds. The optimal conditions for producing desirable smoke flavours are lowertemperature, smouldering fires at 300±400 ëC. Woods that contain high amounts of lignin burn hotter and a restricted air supply is needed to keep them smouldering, or their moisture content is increased by soaking the pieces in water. The factors that influence absorption of smoke by food include the density of the smoke and its humidity and temperature. The higher the smoke density, the greater the
Smoking
529
absorption. In warm- and hot-smoking, the food surface is dried and condensation of the smoke particles is less than on products that are smoked at lower temperatures. If the relative humidity (Chapter 16, section 16.1.1) of the smoke is high, vapour condenses on the food surface and absorption of water-soluble components of the smoke increases. However, if the surface remains moist, colour formation is inhibited as this increases at low moisture contents. If the surface is too dry, there is less penetration of the smoke into the food and hence loss of flavour and preservative action. A number of wood smoke compounds act as preservatives. Phenol and other phenolic compounds in wood smoke are both antioxidants and antimicrobials. Other antimicrobial chemicals include formaldehyde, acetic acid and other carboxylic acids (section 17.4). These chemicals are also extracted and used as liquid smoke (section 17.1.2). Smoke also contains compounds that have long-term health consequences including PAHs and dioxin-like polychlorinated biphenyls (PCBs) many of which are known or suspected carcinogens (section 17.3). Nitrogen oxides in smoke may also react with amines or amides in the food to form nitrosamines, or with phenols to produce nitro or C-nitroso phenols. Carbonylic compounds and acids react with proteins and carbohydrates in the food. Nitrogen oxides and PAHs are normally only found in small amounts in smoke, but they are of concern because of the potential public health risk of nitrosamines and PAHs (Anon 1992). `Tasteless smoke' was patented in 1999 as a method of preserving fresh fish. The process involves producing hardwood smoke by burning the chips, and then passing it through filters that remove all particles larger than 1 m and most of the odour and colour components. The remaining gases (nitrogen, carbon monoxide (CO), carbon dioxide and methane, together with trace amounts of phenolic compounds and hydrocarbons) are applied to tuna fish or other red meats. The product is removed from the smoke chamber, washed in ozonated water to remove any residual smoke odour and kill bacteria, and it is then frozen. The treatment retains the appearance, taste, texture and colour of the fresh seafood after freezing and defrosting (Walsh 2005). The low concentrations of CO alter the colour, when it reacts with meat pigments to produce a cherry-red carboxymyoglobin pigment, but only for a limited time, and eventually the colour diminishes as the fish ages. The process is approved by the EU as a smoking process because, as a component of wood smoke (a GRAS (generally regarded as safe) substance), CO is a legal treatment and is declared on the product label. In contrast, the treatment of fish with industrial carbon monoxide produces colour changes that do not fade. It was banned in the EU in 2003 because it could mislead consumers over the freshness of meat by maintaining a bright red colour. Although the cherry-red colour is different from the oxymyoglobin pigment in fresh meat, it may mislead customers that the product is fresher than it actually is, or enhance the appearance of inferior products. CO treatment can also mask colour changes caused by decomposition, mask other visual evidence of spoilage, and mask potential safety problems such as production of histamines (section 17.4). In Singapore the use of CO is considered a malpractice and Japan bans fish that have an initial CO content 500 g kgÿ1. Currently (2008) in the USA, the Food and Drug Administration (FDA) regards the use of tasteless smoke on tuna as a preservative and it thus needs to be labelled accordingly (Anon 2007a), although this position is challenged. 17.1.2 Liquid smoke Smoke flavourings or `liquid smoke' are prepared by condensing smoke derived from burning wood, usually followed by fractionation, purification or concentration. The
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Food processing technology
fractionation steps produce products that have the desired olfactory properties and also reduce the concentration of undesirable by-products of the smoke. For example, as a marker for PAHs (section 17.3) the benzo(a)pyrene content of smoke concentrates is reduced to a maximum limit of 1 g kgÿ1 of condensate. Much of the tar formed during pyrolysis can also be removed. A typical commercial smoke condensate contains 70% water, 29% volatile organic compounds and 1% tar (Anon 1992). Smoke condensates may be used for flavouring food but are more commonly used as the basis for smoke flavouring preparations that contain carrier materials such as salt or dextrose. Their main advantages are that they can be used as a step in continuous processing and reduced production times, compared with batch smoking in kilns. The smoke flavour can be mixed directly into the food, applied as a powder (Chapter 24, section 24.2.2) or sprayed as a fine mist. The use of smoke flavourings also avoids product weight losses that take place during smoking, which affects the product yield and quality. Although the flavour from liquid smoke is almost the same as that produced by traditional smoking, the product does not necessarily acquire the same texture or colour. Smoke flavourings are therefore often combined with a smoking process to get the desired colour from the smoke and the taste from the smoke flavouring.
17.2
Processing
Foods are cured with salt before warm- and cold-smoking and sometimes before hotsmoking. Cold-smoked meats may be cured or precooked before smoking. The most common type of cure in industrialised countries is a wet cure using brine with added sugar and spices. Curing times vary from a few hours to two weeks. Dry curing, in which coarse salt is rubbed directly into fish or meat, is less common because of higher labour costs and less uniform salting, but it is still widely practised in tropical artisan processing. These heavily salted, hot-smoked products can be stored without refrigeration for several weeks, but require boiling in freshwater to make them palatable before consumption. After curing, fish are drained, rinsed and refrigerated for 6±12 h, and then smoked at low temperature (25±33 ëC) in a kiln or smokehouse for between half a day to 3 weeks. After removal from the kiln and chilling for 24 h, the food may be sliced or packed as whole pieces. The following process, described by Bannerman (1984) using fresh chilled or thawed frozen fish, is an example of the manufacture of a hot-smoked product. Frozen fish is preferred for smoking because freezing for at least 30 days kills parasites in the fish. Fish are brined using an 80 brine (211 g salt lÿ1) to give them flavour and to inhibit the growth of food poisoning micro-organisms without making the product unpleasantly salty. Fish absorb salt more uniformly in weaker brines, but the immersion time is longer, whereas stronger brines can cause salt to crystallise on the surface of the skin after the fish are dried and smoked, creating unsightly white patches. Immersion times in the brine vary according to the size, thickness and fat content of the fish and the aim is to achieve a salt concentration in the finished product of >3%. Yanar et al. (2006) studied the effect of brine concentration on the shelf-life of hot-smoked tilapia and concluded that 5% brine was optimal for a shelf-life of 35 days. After brining, fish are hung on trolleys in a smoking kiln (section 17.2.1) so that the backs face the flow of smoke. Fillets of fish or meat and small products such as shellfish are arranged on wire mesh trays on the trolleys. The process operates in three stages: first drying at 30 ëC for 30±60 min to toughen the skin, then smoking and partial cooking at
Smoking
531
50 ëC for 30±45 min, and finally cooking at 80 ëC for a few minutes up to 30 min for large fish. The total processing time and the times spent at each stage depend on the type of food, its size and fat content, and the type of product required. In the traditional method for making Arbroath smokies (from haddock) the fish are first salted overnight and then tied in pairs and left overnight to dry. The dried fish are hung in a barrel containing a hardwood fire and sealed with a lid to create a very hot and humid smoky fire without flames. Within an hour of smoking, the intense heat and thick smoke produce the strong smoky taste and aroma that characterises this product and the fish are then ready to eat. In other processes, smoking takes up to 8 h at temperatures of 60±85 ëC, with the internal temperature of the fish at 60 ëC for at least 30 min to kill pathogenic bacteria. Cold- and hot-smoked fish are cooled to chill temperatures (3 ëC) in a chill room before packing. They are commonly packaged into modified atmosphere or vacuum packs (Chapter 25, section 25.3). Muratore and Licciardello (2005) studied the effects of different packaging methods on shelf-life. Products are maintained at chill temperatures during storage, distribution and retail display. Cold-smoked white fish have a longer shelf-life than fatty fish, although it varies greatly with the type of fish, the amount of salting, the extent of smoking and drying, and the storage temperature. Civera et al. (1995) conducted chemical and microbiological analyses of smoked Atlantic and Canadian salmon and found that the effective shelf-life was 40±50 days and 80 days respectively if the fish is stored at 2±3 ëC. Smoked products can also be frozen and stored at ÿ30 ëC for at least 6 months, or longer when vacuum packed and frozen. Vacuum packaging excludes oxygen and thus slows the development of rancidity, especially in fatty fish products. However, vacuum packing creates an anaerobic environment inside the pack that is suitable for the growth of C. botulinum (section 17.4). Packaging materials are now available that have adequate oxygen transfer to prevent the development of an anaerobic environment within a package (Chapter 25, section 25.4.3). Containers or packages made of materials with an oxygen permeability of 2000 cm3 mÿ2 per 24 h at 24 ëC or higher are permitted for use with products stored at 4 ëC or lower for up to 14 days (Anon 1991). Styrofoam trays with a single film overwrap are permitted if the total permeability of the final package meets the minimum specifications, but these packs should not be stacked in a way that would reduce the oxygen permeability. Irradiation (Chapter 7) has also been studied as a method to increase the shelf-life of smoked fish. For example, Hammad and El-Mongy (1992) irradiated cold-smoked salmon at 2 and 4 kGy, stored at 2±3 ëC. Unirradiated samples reached the maximum accepted mesophilic plate count after one month of storage, while those irradiated reached this level after three and four months at 2 and 4 kGy respectively. No differences in sensory qualities were found between unirradiated samples and those irradiated at 2 kGy, but there was a loss of red colour in samples irradiated at 4 kGy. 17.2.1 Equipment Smoking equipment should allow the controlled development of flavour and colour in foods, with low levels of carcinogenic or toxic components in the smoke, and low levels of environmental pollution by the smoke. Smoke can either be generated in the kiln or produced in a separate smoke generator. Sawdust and wood should be clean and free from wood preservatives or saw lubricants. Sawdust consumption is 13 kg hÿ1 in a kiln of 375 kg capacity (Bannerman 1984). Separate smoke generators have advantages in that
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Food processing technology
Fig. 17.1
Smoking kiln (courtesy of AFOS Ltd at www.intlsmokingsystems.com).
the temperature and humidity in the kiln can be independently controlled so that foods can be dried or cooked before being smoked. They also give better control over the temperature, humidity and density of the smoke. Additionally, the smoke can be filtered, treated with water sprays or by electrostatic precipitation to remove carcinogenic compounds such as benzo(a)pyrene (Ranken 2000) and unwanted particles. However, these treatments also remove some of the components that contribute to the flavour of smoked foods. Smoking kilns are similar in design to cabinet dryers (Chapter 16, section 16.2.1) or batch ovens (Chapter 18, section 18.2.1) and, in hot-smoking, foods may be dried, cooked and smoked in the same equipment. Kiln heaters are designed to quickly reach and maintain an operating temperature of 80 ëC when fully loaded. Trolleys that have rails or mesh trays are wheeled into the smoking chamber (Fig. 17.1) and the food is smoked in an automatically controlled cycle. Computer control includes management of the smoke temperature, humidity and density via touch-screen or remote controls, a fire protection system and alarms, and an automatic cleaning cycle. The capacity of commercial kilns is 250±2000 kg (Anon 2007b).
17.3
Effect on foods
The main purpose of smoking is to alter the sensory properties of foods, particularly the flavour and colour. Chemicals in smoke that contribute to the flavour and aroma of smoked foods are described in section 17.1.1. Smoking produces a shiny yellow colour, which darkens as smoking time is increased. Earlier studies showed that the colour is produced by interaction of amino groups on proteins in the food with carbonyls in the smoke in a similar way to the Maillard reaction (Ruiter 1979, Gilbert and Knowles 1975). Iliadis et al. (2004) studied the effects of pretreatment and hot- and cold-smoking on the chemical, microbiological and sensory quality of mackerel. They found that available lysine was reduced to the same extent (32%) in all hot smoked samples, and that loss of available lysine correlated with colour formation in the cold-smoked products. Nitrogen oxides in smoke can also react with myoglobin to produce a modified colour in smoked
Smoking
533
meat products. Riha and Wendorff (1993) studied the acceptability of the colour of smoked cheese. The surface texture of cold-smoked foods is changed by the formation of a firm pellicle, produced by coagulation of proteins by acidic components of the smoke, but the interior of the food is unchanged. In hot-smoked foods, the texture becomes that of cooked foods owing to heat coagulation of proteins. Salting causes liquid exudates from the flesh of meat and fish, causing losses of watersoluble proteins, vitamins and minerals. Some proteins are also denatured by the salt. Some constituent chemicals in the smoke at the surface of foods (e.g. butyl gallate and BHA) have an antioxidant action (Brul et al. 2000). These antioxidant components of smoke reduce oxidative changes to fats, proteins and vitamins. However, hot-smoking also causes nutrient losses due to heat and interaction of the smoke components with proteins. The heat and flow of gases in the smoke cause dehydration of the food and changes to the nutritional and sensory properties described in Chapter 16 (section 16.5) including denaturation of proteins. The loss of moisture also increases the concentration of protein and fat in the food and an increased concentration of salt and other curing agents. Smoked foods give rise to some health concerns, especially with respect to PAHs and nitrosamines. The potential harmful effects of these smoke components are described by Anon (1992). The PAH component benzo(a)pyrene and 10 other compounds from this group are both mutagenic and carcinogenic (Lawley 1990). High levels of PAHs are found on the surface of products that are smoked for a long time at higher temperatures, but Iliadis et al. (2004) found high levels of benzo(a)pyrene, fluoranthene and perylene both in cold- and hot-smoked fish. Witczak and Ciereszko (2006) studied changes in the content of PCBs in mackerel slices during cold and hot smoking. The hot-smoked mackerel showed a decrease in the PCB content, which may have been due to losses with lipid leakage from the product and co-distillation with water vapour. Cold smoking produced an increase in PCB content in the final product compared to the initial raw material. However, their levels may be reduced by using fire temperatures below 400 ëC for smoke generation, by treating smoke (section 17.1.2) and by reduced smoking times. Some products (e.g. frankfurter sausages) are washed after smoking, which also reduces the concentration of these carcinogens. The average intake of PAH components has been calculated to be 1.2 mg per year, but smoked meat and fish contribute only 10% of this, the remainder coming from environmental pollution and tobacco smoking (Anon 1992). The intake of PAHs from food is therefore regarded as of minor public health importance, but some smoked foods can have unacceptably high levels of PAHs due to the smoking process and these are therefore a health concern. In the EU upper limits for smoked food are 5 g of volatile N-nitroso compounds per kg of food and 1 g of benzo(a)pyrene per kg of food, with the exception for smoked fish of 5 g of benzo(a)pyrene per kg. The maximum limit for benzo(a)pyrene in liquid smoke condensates is 10 g kgÿ1, to produce less than 0.03 g kgÿ1 in the food as consumed and for benzo(a)anthracene, the maximum is 20 g kgÿ1, giving less than 0.06 g kgÿ1 in the food (Anon 1992). Smoked foods may contain N-nitroso compounds, such as N-nitrosodimethylamine. These nitrosamines are among the most carcinogenic substances that have been studied. They are formed by reactions between nitrogen oxides in smoke and amines or amides in the foods, especially in those that have higher concentrations of amines, such as fish and meat. These compounds increase the risk of gastrointestinal cancer in populations where there is a high intake of heavily smoked (and/or salted) foods. Phenolic compounds are
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Food processing technology
important for the taste of smoked food, but where nitrite is used (e.g. in cured smoked meats), phenols may react with the nitrite to form nitro- and nitrosophenols, some of which have been shown to be mutagenic. Some of the phenols may also catalyse the formation of nitrosamines in foods during smoking. Tar compounds in smoke are not well characterised and their effects on health are therefore difficult to evaluate.
17.4
Effect on micro-organisms
The combined effects of salt, antimicrobial chemicals in smoke, heat and partial dehydration during hot-smoking are effective against Gram-negative rods, micrococci and staphylococci. Vegetative bacteria are more susceptible to smoke than bacterial spores and moulds. This means that spoilage by moulds is more likely than bacterial spoilage. However, there is a significant risk of pathogenic bacterial contamination, especially of cold-smoked fish products, and standards for hygienic production and handling are described in the legislation of many countries (e.g. Anon 1979). Sikorski and Kaøodziejska (2002) reviewed the incidence of contamination by pathogens on hotsmoked fish. They found low numbers of Listeria monocytogenes, Clostridium botulinum, Staphylococcus aureus and Vibrio parahaemolyticus and concluded that the main causes of contamination were unsanitary procedures and airborne micro-organisms during packing of the product. The internal temperature in the fish, which did not exceed 65 ëC, and the low concentration of salt were insufficient to inactivate all pathogens or inhibit bacterial growth during storage. Product safety required very fresh fish, handled under hygienic conditions, chilling the product to 2 ëC and hygienic handling of the product after smoking. Details of safe handling of chilled foods are given in Chapter 21 (section 21.5). Listeria monocytogenes poses a health risk for immunocompromised individuals and pregnant women (Chapter 1, section 1.2.4). It is not destroyed during cold-smoking and it can become established in the processing environment and re-contaminate products. It is therefore not possible to produce cold-smoked fish that is consistently free of L. monocytogenes (Anon 2001). However, the use of good manufacturing practices (GMP) and good hygienic practices (GHP) (Chapter 1, section 1.5.1) enables production of coldsmoked fish with low levels of L. monocytogenes (<1 cell gÿ1). Growth can also be prevented by freezing, by addition of preservatives (e.g. sodium nitrite) or by use of bioprotective bacterial cultures (Chapter 6, section 6.7). It can also be controlled by limiting the shelf-life (at 4.4 ëC) to ensure that not more than 100 cells gÿ1 are present when the food is consumed. Clostridium botulinum occurs naturally in the aquatic environment and can be present in low numbers on fresh fish. Cases of botulism caused by type E toxin from smoked fish are reported by Fletcher et al. (2003) in the anaerobic conditions found in vacuum packs. Korkeala et al. (1998) report cases of botulism after eating hot-smoked whitefish, processed from frozen fish. The fish contained botulinum toxin and Cl. botulinum was isolated from the fish. They identified safety problems associated with vacuum packed hot-smoked fish and described the product as one of the highest risk industrial foods to cause botulism. They recommended temperature monitoring and the use of time± temperature indicators (Chapter 21, section 21.2.4) to ensure adequately low storage temperatures throughout the processing chain and the use of sodium nitrate and nitrite with a sufficiently high salt concentration to prevent Cl. botulinum growth. In the USA, the recommended smoking conditions required to kill Cl. botulinum type E are that
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products achieve a core temperature of 62.5 ëC for 30 min and for products stored in air, the fish should contain >2.5% salt in the muscle. Vacuum packaged or modified atmosphere packaged products should contain either >3.5% salt or 3.0% salt plus 100± 200 g gÿ1 of sodium nitrite with storage at chill temperatures <4.4 ëC (Anon 1995). Sikorski and Kaøodziejska (2002) reviewed studies of other preservative chemicals, including sodium lactate, lactic and sorbic acids, sodium propionate, nisin and lysozyme. The flesh of some fish species (e.g. tuna, mackerel and mullet) contains high levels of the amino acid histidine, which is converted to the biogenic amine, histamine, by bacteria (e.g. Enterobactericae) growing under suitable conditions on the fish. Histamine causes the symptoms of scombroid poisoning, which are similar to an allergic reaction and include facial swelling, itching of the skin, headache, nausea and vomiting (Fletcher et al. 2003). Histamine is not destroyed by subsequent cooking. Iliadis et al. (2004) found unacceptable levels of histamine (600 mg kgÿ1) in unprocessed samples of fish, which increased to levels that would be expected to cause symptoms of scombrotoxin poisoning in both cold- and hot-smoked products (2220 and 2250 mg kgÿ1 respectively). Freezing, salting or smoking may inhibit or inactivate histamine-producing micro-organisms, but growth may take place after thawing before smoking, and post-smoking. Vacuum packaging does not prevent their growth. Handling and processing the fish under sanitary conditions, rapid cooling and continuous refrigeration until consumption each prevent the growth of these bacteria, and hence prevent biogenic amine formation. Other illnesses that result from consumption of improperly processed smoked foods are caused by Clostridium perfringens, Staphylococcus aureus and Salmonella spp. None of these micro-organisms occurs naturally on raw fish and the sources of infection are food handlers who carry these micro-organisms or shellfish harvested from unsanitary water (see Chapter 1, section 1.2.4). The most important methods to control these bacteria are: · temperature control during hot-smoking to kill any contaminating micro-organisms; · holding the product at a sufficiently low temperature before and after smoking to prevent their growth; · good sanitation and hygienic work practices. Fletcher et al. (2003) describe methods used for the safe production of smoked fish and seafoods using GMP and HACCP (see also Chapter 1, section 1.5.1). Internationally recognised controls require that: · the core temperature of the fish should be brought to 10 ëC or less within 6 h of death and to 4 ëC within 24 h; · chilled fish should not be exposed to temperatures above 4 ëC for more than 4 h cumulatively after the initial chilling. · chilled fish should not be stored for more than 14 days at 0 ëC or more than 7 days at 4 ëC before smoking. · frozen fish (stored for 24 weeks or longer) should not be exposed to temperatures above 4 ëC for more than 12 h, cumulatively after the initial chilling period, and it should not be exposed to temperatures above 4 ëC for more than 6 h of uninterrupted storage (Anon 2001).
References ANON,
(1979), Codex Standards for Smoked Fish, available at www.codexalimentarius.net/ download/standards/123/CXP_025e.pdf.
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Experimental Pharmacology ± Chemical Carcinogenesis and Mutagenesis, Springer-Verlag, Berlin, pp. 410±469. LESIMPLE, S., TORRES, L., MITJAVILA, S., FERNANDEZ, Y. and DURAND, L., (1995), Volatile compounds in processed duck fillet, J. Food Science, 60 (3), 615±618. MURATORE, G. and LICCIARDELLO, F., (2005), Effect of vacuum and modified atmosphere packaging on the shelf-life of liquid-smoked swordfish (Xiphias gladius) slices, J. Food Science, 70 (5), C359±C363. RANKEN, M.D., (2000), Handbook of Meat Product Technology, Blackwell Science, Oxford, p. 151. RIHA, W.E. and WENDORFF, W.L., (1993), Evaluation of color in smoked cheese by sensory and objective methods, J. Dairy Science, 76, 1491±1497. RUITER, A., (1979), Color of smoked foods, Food Technology, 33, 54±63. SIKORSKI, Z.E. and KAèODZIEJSKA, I., (2002), Microbial risks in mild hot smoking of fish, Critical Reviews in Food Science and Nutrition, 42 (1), 35±51. WALSH, E., (2005), What is Clearsmoke, Food and Beverage International, Spring issue, available at www.fbworld.com/Mag_Spring_2005/advertorial/anova%20-%20clearsmoke/ AnovaClearsmoke.html. WITCZAK, A. and CIERESZKO, W., (2006), Effect of smoking process on changes in the content of selected non-ortho- and mono-ortho-PCB congeners in mackerel slices, J. Agriculture and Food Chemistry, 54 (15), 5664±5671. YANAR, Y., CËELIK, M. and AKAMCA, E., (2006), Effects of brine concentration on shelf-life of hotsmoked tilapia (Oreochromis niloticus) stored at 4 ëC, Food Chemistry, 97 (2), 244±247.