Drastic reduction of hazardous substances in smoke

After drying and salting, smoking is one of the oldest methods of preserving foods. All three techniques remove some of the tissue fluid from the food and thereby slow down spoilage and decay processes. This mode of action is still used today when we smoke food products. However, modern technology and electronic controls ensure an optimum smoke environment to produce healthier, tasty smoked products.

If in their efforts to protect themselves from flies and other pests Fred Flintstone and his relatives used to hang leftover fishes from the ceiling of their cave – and thus in the smoke that rose from the fire – they might have been surprised to notice that the smoke rendered the fish not only drier and more firm but also more durable… and that on top of that it tasted good, too! Archaeologists believe that people were already preserving certain foods with smoke 7,000 years ago, and perhaps even earlier. Mainly meat and fish but also vegetables and anything else that was on the menu in the outgoing New Stone Age. Without having the slightest clue about what actually happened during the smoking process our ancestors discovered over the course of time more and more possibilities and ways to increase the durability of smoked foods and to influence their texture and consistency, their colouring, smell and flavour in the way they wanted. Basically, the methods used at that time were similar or even the same as those that are today decisive for determining the quality of the final smoked products.

In some supermarkets products are today even smoked directly behind the fish counter.

Of the three early methods of preservation smoking is undoubtedly the most sophisticated and complex technique because it also makes use of individual work stages from salting and drying. For fish is usually wet or dry salted prior to smoking in order to intensify the flavour and to withdraw some of the water from the product in advance. After that, the fish has to be dried on the outside before it can be hung in the smoke. Although food products are smoked more gently today the fish still loses even more moisture during the smoking process. On average, the weight loss from the raw product to the finished smoked product is about 10 to 15%. An exception to this rule is products for which the salt solution is injected directly into the tissue to compensate for the weight loss during the subsequent smoking process. In classic smoked products such as bloaters or red herring which hang in smoke for days on end water loss can even be as much as 40%.

Right up to the start of the 20th century when smokehouses were to be found on nearly every farm, smoking mainly served as a method of preservation: it was an important part of the household and a necessity for longer storage of supplies. Today, the focus is sooner on aspects such as product flavour and appearance. The traditional kilns in which the fish is still smoked above an open flame are a reminder of days gone by. At first sight the fully automatic, computer controlled smoking and curing plants that are used today in both small artisanal and large-scale industrial smokehouses might seem to have little in common with the archaic technique of smoking. But closer inspection will reveal that they do in fact have numerous similarities since essential core elements of smoking are basically the same as those used by the cavemen and have remained the “soul” of this technology.

The most striking difference between smoking then and now is that the individual process stages have been broken up as it were, and basic structural elements have been spatially separated from each other. In the past the smoke was usually generated directly beneath the product, and the fishes were hung over the smouldering flame, which makes the term smokehouse seem fully justified. The intensity of the smouldering process and thus the density of the smoke could be controlled via the air draft, even if only to a limited extent. Apart from the choice of wood it was practically impossible to take any influence on the composition of the smoke. And because the smoke was not distributed evenly throughout the smokehouse it could happen quite frequently that fishes from the same batch were differently smoked: some more, some less strongly. Modern smoking systems can rule out such dissimilarities. The biggest change in contrast to the smoking kilns of former times is probably that smoke generation no longer takes place in the chamber in which the fish are hung but in a separate chamber. This was a revolutionary structural change which made it possible for the first time to treat the smoke in different ways prior to its use, for example to purify or modify it.


The smoke used for smoking contains over 400 different substances

Smoke generation is based on an incomplete combustion process (pyrolysis) of wood, wood shavings, sawdust or other plant parts. This smouldering process takes place at temperatures of between about 260 and 500°C (it can also be initiated by steam that has been superheated to 350°C). This leads to the production of a variety of gaseous, liquid and solid substances of which so far only about 350 are precisely known, however. They include, for example, phenols, organic acids, carbonyls (aldehydes, ketones), alcohols and esters, plus vast amounts of microscopically small solid particles such as soot and metal oxide particles that float in finely dispersed forms, often as aerosols, together with the smoke. Some of these substances have positive and quite desirable effects, others are undesired or are suspected of being harmful to health or even carcinogenic. Phenols, for example, slow down spoilage processes in smoked products for they have an antimicrobial and anti-oxidative effect. Aldehydes and ketones which are often grouped together as carbonyls are largely responsible for the typical smoke aroma. Solid particles such as soot, tar and resin are viewed much more critically today, particularly polycyclic aromatic hydrocarbons (PAHs) which can occur during roasting and grilling as well as during incomplete combustion of organic materials such as wood. Of the 250 PAHs, among which benzo[a]pyrene is probably the best known, 16 are considered to be harmful to health, six of these carcinogenic. To protect European consumers Directive (EG) No. 1881/2006 (Annex 6) lays down maximum levels for benzo[a]pyrene (5.0 µg/kg) und total limits for four other PAHs in smoked fish and fish products.

In “normal” smoke these limits are often exceeded, however. Smokehouses mainly use two methods to reduce the content of PAHs and other undesired substances in the smoke. On the one hand, they try to stop the substances from developing at all through using as gentle smoke generation as possible and, on the other hand, they try to filter them out of the smoke. A lot of smokehouses use both methods simultaneously. The separation of smoke generation and smoke usage as is common today in modern smoking facilities offers optimal conditions to achieve this because the externally generated smoke, which arrives in the smoking chamber via pipes, can be cleaned on its way there. Smoke washing holds back numerous harmful substances and solid particles such as soot, ash or charcoal so that they never actually reach the smoking chamber. Some smoke generation methods, such as friction or steam smoke, don’t even need an open flame today. The smouldering temperature which is regulated exactly to a degree by computer can be selected so that less harmful substances are produced.

The composition of the smoke depends strongly on the method used to produce it. Closest to traditional smoking is smouldering: wood shavings or sawdust are smouldered openly or burnt on a burning plate. During this process temperatures of between 500 and 800 °C are produced as well as relatively large amounts of PAHs which necessitates thorough pre-treatment of the smoulder smoke. In contrast, in the case of friction smoke (i.e. without an open flame) a massive wooden block is pressed against a rotating metal wheel. The resulting friction heat at 350 to 400°C makes the wooden stick burn up slowly during which a mild smoke develops, and this is largely free from tar and benzo[a]pyrene residues particularly in the lower temperature ranges. Similar temperatures also occur with vapour smoke in which superheated steam serves as the smoke generator. Although during this process typical pyrolysis products such as tar and ash hardly occur the smoke contains relatively large quantities of PAHs that have to be eliminated prior to smoking. To save this effort more and more smokehouses are today using industrially produced standardised liquid smoke aromas that practically contain no more PAHs and always deliver the same consistently reproducible smoking results. The liquid smoke (or to put it more correctly, purified smoke condensates) is regenerated for the smoking process and finely atomized in the smoking chamber. Its use thus demands specially equipped smoking plants. Liquid smoke products are on a par with traditional smoked products not only in their colouring and flavour but also in their durability and texture.

Cold smoked salmon gets its typical features from the combination with a particular pre-treatment, here for example, hand salting.


New EU directive could give liquid smoke an added boost

Smoke aromas, also called liquid smoke, are preparations from condensed and purified smoke that can be used for all conventional smoking methods such as cold or hot smoking. For the production of liquid smoke wood shavings are first smouldered largely under the absence of oxygen. The resulting smoke is condensed in water and afterwards broken down into three components. One of them remains unused and the other two (watery smoke condensate, water insoluble tar phase) are thoroughly cleaned and freed to as large an extent as possible from substances that are harmful to health such as PAHs. The primary smoke condensate that is produced in this way and the primary tar phase are the basis for the production of smoke aromas that then demand further complicated technical steps. Like conventional smoke, smoke condensates (or liquid smoke) contain more than 400 chemical compounds, among them phenols that produce the smoke aroma and carbonyls that are responsible for the brown colouring. Because, however, smoke aromas are produced from fractionated and purified smoke, their use is considered to be less hazardous to health than traditional smoking.

Whereas smoke aromas are used frequently in meat processing, they are still the exception in fish processing – in spite of their undeniable advantages. One reason for this could be uncertainties about the proper declaration of products that have been treated with smoke aromas. According to the European aroma directive (EG) Nr. 1334/2008 the addition of smoke aromas has to be specifically declared if they give the food a smoky flavour. This puts a lot of consumers off buying such products because the term “aromas” generally has a negative image. Since, however, this does not do justice to the advantages of smoke condensates as a healthier and environmentally friendlier alternative to traditional smoking, the EU Commission is trying to counter this effect and has presented a draft of a further directive (Document SANCO/-11298/2013 Rev.1). Affording to this document, smoke condensates only have to be declared as smoke aromas in the list of ingredients if they have been added directly to the food, for example to a sauce, a soup or a snack. If, on the other hand, the primary smoke condensates are used for the production of smoke, it can simply be declared as “smoke”. This directive could give liquid smoke an additional boost within the fish processing sector, too.

The range of smoked products available at fish counters today has grown considerably. Smoked salmon products are usually a cornerstone of the range.

This means that smokehouses now have a “real” additional option for smoke generation, particularly since liquid smoke is just as good as smoulder and friction smoke for the three smoking techniques that are generally used in practice. Only two of these three techniques, hot and cold smoking, are of significance in the fish sector, however. The term hot smoking is used for smoking techniques that take place at temperatures between 50 and 120°C, mostly at 65 to 85°C. A typical smoking phase takes about 30 to 90 minutes. Hot smoking delivers “semi-preserved products with a medium shelf-life” that have to be eaten within a few days of their production. Typical hot smoked fish products are “buckling” (large fat herring, hot smoked), smoked eel,”schillerlocken” (strips of belly wall of dogfish, hot smoked), mackerel and black halibut. Cold smoked fish products, which include salmon and herring fillets (kippers, brados), gain their characteristic features after appropriate pre-treatment (salting) through relatively long smoking times ranging from several hours to a day, in the case of a few special products even several weeks. During cold smoking the fishes remain in the smoke at temperatures of 15 to 25°C and the smoke is nearly always generated with high-quality hard wood. In combination with dehydrating pre-treatment with salt, durable products produced in the past using cold smoking remained edible for a long time. However, because today products are usually smoked more mildly and more gently, cold smoked fish products such as smoked salmon mostly only have a short shelf-life.


Climate in the smoking chamber is monitored and regulated electronically

Basically, the higher the smoking temperature, the shorter the food should remain in the smoke. If a product is smoked for a longer time at a low temperature it usually has a much longer shelf-life. As already mentioned, the shelf-life of smoked fish plays only a subordinate role today and the focus is more on flavour and colouring. Behind this change are changed taste preferences of consumers who prefer juicy smoked products that remind them of fresh products. The preservative effect of smoking has lost much of its significance because it is inevitably associated with dehydration and drying of the products. Although some components in the smoke inhibit and slow down microbiological and chemical degradation processes in the muscle flesh of the fish the actual preservative effect is achieved through drying. A measure for the degree of dryness is the reduction of the “aw” (activity of water) value which gives the content of free water in foods. An aw value of 0 is equal to absolute dryness, the value 1 to condensing moisture (pure water has an aw value of 1). Most bacteria find optimal conditions for growth at aw values of 0.98 to 1. Lower values of around 0.80 are sufficient for the development of mould fungi.

Although hot and cold smoking take place under different conditions nearly all modern smoking facilities are suitable for both smoking techniques. The electronic control system that is today standard in most smoking plants ensures the right climate in the smoking chamber for the required process. The smoke produced in the smoke generator is measured precisely and mixed with fresh air, cooled or heated, dehumidified or enriched with humidifying steam before it passes into the chamber. Fans keep the smoke/air mixture in the chamber constantly in motion. In more complex smoking systems this smoke circulation is even passed over the product from different sides which ensures particularly uniform, homogeneous results in every corner of the chamber. Sensors monitor the climate in the smoking chamber constantly. As soon as any parameter exceeds the set limits it is corrected immediately.

The computer controlled smoking and curing plants in large-scale industrial smokehouses have little in common with the archaic technique of smoking.

Today nearly all smoking plants work using permanently installed programmes that can be changed as required or programmed completely freely. This means it is possible to repeat smoking routines with high consistency which in turn leads to consistent colouring and flavour results. Reproducibility is an essential prerequisite if brand quality is to be achieved during smoking. But the free programmability of the smoking units offers users numerous creative options since nearly all the process steps can be altered and stored as desired. Important information and data, for example the selected smoking programme, the current programme phase, or the adherence to actual and target values, are visible in a display at all times. Sometimes it is even possible to make changes when the programme is already running. Some smoking units are fitted with automatic cleaning systems which makes the cleaning of the chamber after the smoking cycles considerably easier.

The question remains to be answered as to what happens to the smoke after it leaves the chamber. If the smoke exhaust does not exceed the limits for critical parameters such as carbon monoxide CO and NOx it can usually be released directly into the atmosphere. However, anyone who uses smoulder smoke for smoking is committed almost everywhere in Europe to thermal afterburning of exhaust gases. In contrast, post-combustion is not generally required for friction smoke. And it’s even easier for users of smoke condensates because here the smoking process takes place within a closed system. And where there are no emissions there are understandably no requirements to keep the air clean.