The ultimate guide to food smoking technology

Abstract: Smoking technology is widely used in the processing and cooking of various types of meat, fish, cheeses, and other foods. It is an indispensable processing technology in the food industry. This article introduces in detail the purpose of food smoking, the impact of smoking on food, food smokers and their working principles, different types of smoking, smoking materials, quality and safety control of smoked products, etc. for your reference. If you have any needs for smoking technology and smokehouse, please contact us.

smoking technology for salmon in a smokehouse

The purpose of using smoking technology in the food industry

The main purposes of smoking are:

  • Give the product a special smoky flavor and enhance the aroma
  • It gives the product a unique smoky color and promotes the color development of saltpetered meat products
  • Dehydration, sterilization, and disinfection to prevent spoilage and make meat products durable in storage
  • The smoke components penetrate into the meat to prevent fat oxidation

Smoking can effectively improve the flavor of food

Smoking improves food flavor mainly from three aspects: First, many organic compounds in the smoke adhere to the products, giving them a unique smoky aroma, such as organic acids (formic acid and acetic acid), aldehydes, alcohols, esters, phenols, etc., especially guaiacol and 4-methylguaiacol among phenols are the most important flavor substances. Second, the heating of smoking promotes the decomposition of proteins in meat products, generating amino acids, low molecular peptides, fatty acids, etc., giving meat products a unique flavor.

Smoking improves the color and texture of food

Smoking can make meat products appear dark red, brown or brown-black, etc., with beautiful colors. The color is generated from three aspects: first, the carbonyl compounds in the smoke components can react with the free amino groups in the meat protein or other nitrogen-containing substances to produce the Maillard reaction, giving the product a unique brown color; second, the heating of the smoke promotes the proliferation of nitrate-reducing bacteria and the thermal denaturation of proteins, releasing cysteine, thereby promoting the formation of stable color by nitric oxide hemoglobin; third, fat exudation during heating plays a role in coloring.

Smoking can effectively kill microorganisms in food and extend the shelf life

The bactericidal and antiseptic effects of smoking are mainly the result of the combined effects of the heat of smoking, the drying effect of smoking, and the chemical components produced by smoking. Among the smoke components, organic acids, aldehydes, and phenols have stronger bactericidal effects. Organic acids can neutralize alkaline substances such as ammonia and amines in meat. Due to their own acidity, they increase the acidity of meat, thereby inhibiting the growth and reproduction of spoilage bacteria. Aldehydes generally have antiseptic properties, especially formaldehyde, which not only has antiseptic properties, but also combines with the free amino groups of proteins or amino acids to weaken the alkalinity and increase the acidity, thereby increasing the antiseptic effect; phenolic substances also have weak antiseptic properties.

The bactericidal effect of fumigation is more obvious on the surface. After fumigation, the number of microorganisms on the surface of the product can be reduced to 1/10. Escherichia coli, Proteus, and Staphylococcus are most sensitive to fumigation and die within 3 hours . Only mold and bacterial spores have a strong tolerance to fumigation. It should be noted that the sterilizing and antiseptic effects of smoking are limited. Uncured raw meat is prone to rapid spoilage if simply smoked. The shelf life of smoked products can be extended more effectively through processes such as pickling before smoking and smoking, dehydration and drying after smoking, retort sterilization, and vacuum packaging

Antioxidant effect

Many components in smoke have antioxidant properties. The most powerful antioxidants are phenols, among which catechol, pyrogallol and their derivatives are particularly effective. Tests have shown that when smoked products are placed at 15°C for 30 days, the peroxide value does not change, while the peroxide value of unsmoked meat products increases by 8 times.

food smoking technology

The impact of smoking technology on food

Sediment of smoke

    The smoke is in gaseous state when it is first produced, but it will quickly separate into gas phase and solid phase. The gas phase contains more volatile substances, most of which have a unique smoky aroma and flavor. Experiments using electrostatic deposition of solid phases show that 95% of the smoky flavor in meat products comes from the gas phase. If the solid phase precipitation is removed, the content of harmful tar and polycyclic hydrocarbons in the smoke will be greatly reduced.

    The deposition of smoke on the product first occurs on the surface of the product, and then gradually penetrates into the interior of the product, giving the product a unique color, aroma, and taste, and enhancing its anti-corrosion and antioxidant capabilities.

    Factors that affect the amount and speed of smoke deposition include smoke density, air velocity and relative humidity in the smoking room, and the state of the food surface. The relationship between smoke density and its deposition speed is very obvious. The greater the density, the greater the amount of smoke absorbed. The air velocity in the smoking room is also conducive to absorption, because the faster the flow, the more smoke will contact the food surface. However, under the condition of high-speed air flow, it is difficult to form a high concentration of smoke, so an air velocity of 7.5 to 15 m/min is generally used. Relative humidity not only affects the deposition speed, but also the nature of deposition. Because relative humidity is conducive to accelerating deposition, but not conducive to color formation. The amount of water on the food surface will also affect the absorption of smoke. Moisture is conducive to absorption, while dry surface will delay absorption.

    Smoking promotes further color development of the product

    Smoking has the effect of promoting further color development of pickled products, which can be proved by the changes in the color ring formed during smoking. During the smoking process, if the sausage product is cut open, obvious color development areas and non-color development areas can be seen from the cross section, and the color development area expands with the extension of smoking time, and the color change evenly expands from the periphery of the sausage to the center. The reason for this phenomenon is that the color development of nitrates must be transformed into nitrites under the action of microorganisms, and this reaction usually occurs during the pickling process. However, when the temperature during pickling is low and the time is short, the above reaction is not sufficient, and no large amount of nitrite is formed, so it can only be realized during the smoking process. In other words, smoking creates suitable conditions for nitrate-reducing bacteria to convert nitrates into nitrites. At the same time, because the thermal conductivity of meat is very poor, when the outer layer temperature of the smoked sausage reaches 60°C, it takes a long time for the center to reach this uniform temperature. When the meat filling is kept at 30-40°C within 1 hour of smoking, it is the most suitable temperature for bacterial reproduction. Therefore, the temperature of smoke is not only a requirement for bacterial growth, but also a necessary condition for color development. During the smoking process, there is a temperature difference between the inner and outer layers of the meat. The center does not develop color and maintains its original state, while the outer surface has a suitable temperature condition, bacteria multiply in large numbers, promoting the conversion of nitrates, so the color development area gradually expands from the periphery to the center.

    Changes in product weight during smoking

    During the smoking process, the biggest change in meat products is weight loss due to the evaporation of water. Weight loss is determined by temperature. The higher the temperature, the faster the weight loss and the greater the weight loss. The airflow speed and direction of airflow also have a great impact on weight loss.

    In the smoking process of meat products, the speed of internal moisture transfer is slower than the speed of surface moisture evaporation, so a layer of dry hard shell is often formed on the surface of the meat. The transfer of internal moisture is affected by many factors, such as the type of raw meat, the composition ratio of meat, the thickness of sausage, etc., all of which have an impact on drying. The higher the fat content, the less loss, while the thinner the sausage, the greater the loss.

    The impact of smoke components on products

    The amount of smoke components that penetrate into meat products is a sign of the degree of smoking, which is often expressed by the phenol number. The phenol number refers to the mass of phenol contained in 100g of meat products, measured in milligrams.

    The composition of smoke at each level in the fumigation chamber is different, and phenol is the most important substance. The bottom layer of the smokehouse contains the most phenols, while aldehydes and ketones are the lightest substances and are most abundant in the upper layers. During cold smoking, smoke will accumulate 9 to 32 mg/100g of phenol and 9 to 48 mg/100g of aldehyde in the enema. Aldehydes are easily soluble in fat. The content of phenol in fat tissue is higher than that of muscle tissue. During smoking, phenol and aldehyde The accumulation of these substances is strongest in the first 24 hours. Later, the concentration difference of these substances in the surrounding medium and the enema will gradually decrease, and the amount of infiltration will also decrease.

    Due to the immersion of smoke components, the storage resistance of smoked products is improved, the appearance of the products is improved, and the products are given a special aroma of smoke. Many studies have also pointed out that the accumulation of aldehydes in smoke has a bactericidal effect, thereby increasing the preservation resistance of the product. Smoked products have significant antioxidant effects because smoke contains antioxidant substances, which are mainly phenols and their derivatives.

    smokehouse for food smoking

    Infusion of smoking ingredients and food hygiene

    It has been found that among the ingredients in smoke, not only tar is carcinogenic, but also 3,4-benzopyrene, 1,2,5,6-dibenzanthracene, 9,10-dimethyl-1,2-benzanthracene, etc. are all carcinogenic substances. These polycyclic hydrocarbons have no important antiseptic effect on smoked products, and cannot produce a unique flavor, so they need to be eliminated.

    At present, there are several methods to eliminate these substances in smoke.

    1. Control combustion temperature

    The generation of phenolic substances and benzopyrene is related to the combustion temperature. The generation of benzopyrene increases with the increase of temperature, showing a linear relationship, while the generation of phenolic substances reaches a maximum at about 440°C. Therefore, in order to achieve a good smoking effect, it is best to heat at 380-400°C.

    2. Use liquid smoke preparations instead of fumigation smoke – liquid fumigation

    Studies have shown that polycyclic hydrocarbon compounds such as benzopyrene are attached to the solid phase in smoke. In addition, analysis of several liquid smoke preparations showed that benzopyrene and dibenzanthracene were not present.

    3. Improve the fumigation equipment

    Separate the smoke generator from the fumigation chamber so that the smoke does not hit the product directly but first circulates through the pipe. Practice has shown that this can effectively reduce the content of carcinogens in the fumigation chamber.

    Composition and effects of smoke

    Smoke is produced by incomplete combustion of wood and is a mixture of water vapor, other gases, liquids (resins) and solid particles. The essence of smoking is the process of the product absorbing the decomposition products of wood, so the decomposition products of wood are the key to the smoking effect.

    The composition of smoke is very complex. More than 200 compounds have been separated from wood smoke. Studies have shown that aldehydes, phenols, alcohols, acids, ketones, carbonyl compounds and hydrocarbons are important compounds in smoked products.

    1. Phenols

    There are more than 20 kinds of phenols in smoke, including guaiacol, methylguaiacol, propylguaiacol , etc. In smoke, phenols have four effects: ① Antioxidant effect. Phenols with high boiling points have stronger antioxidant effects than phenols with low boiling points.

    ② Promote the production of smoky color.

    ③ It is conducive to the formation of smoke flavor. The phenols related to flavor are mainly guaiacol, 4-methylguaiacol, 2,6-dimethoxyphenol, etc. The smell of pure phenolic substances is monotonous, but the flavoring effect is much better when combined with other ingredients (carbonyl compounds, amines, pyridine, etc.).

    ④ Antiseptic effect. Phenols have a strong antibacterial and antiseptic effect, which can damage the cell membrane of bacteria and promote the coagulation of bacterial protein, thereby inhibiting the growth of bacteria .

    Phenol and its derivatives are produced by the cracking of lignin. Lignin decomposes vigorously at temperatures between 280 and 550°C, and is most intense at around 400°C.

    2. Alcohols

    There are many types of alcohols in wood smoke, the most common and simplest of which is methanol (wood alcohol), in addition to ethanol, propylene alcohol, pentanol, etc., but they are often oxidized into corresponding acids. The role of alcohols is mainly as a carrier of volatile substances, and their content is also low. Its bactericidal effect is very weak, and it does not play a major role in flavor and aroma.

    3. Organic acids

    The organic acids contained in smoke are simple organic acids with 1 to 10 carbon atoms. Acids with 1 to 4 carbon atoms exist in the vapor phase, and acids with 5 to 10 carbon atoms adhere to the particles in the smoke. Organic acids have little effect on the flavor of smoked products, but can accumulate on the surface of the products, showing a weak antiseptic effect. Acids have the effect of promoting the coagulation of protein on the surface of smoked meat, which will help to remove the casing when producing sausage products without casings.

    Organic acids come from the decomposition of cellulose and hemicellulose in wood. Cellulose decomposes vigorously at a temperature of 240-400°C, and the most intense decomposition temperature is around 300°C. Hemicellulose decomposes vigorously at a temperature of 180-300°C, and the most intense decomposition temperature is around 250°C.

    4. Carbonyl compounds

    There are a lot of carbonyl compounds in smoke, mainly ketones and aldehydes. Like organic acids, they exist in steam distillation components and also on smoke particles. Although most carbonyl compounds are non-steam distillable, the steam distillation components have a very typical smoky flavor and also contain the color formed by all carbonyl compounds. Therefore, carbonyl compounds can give smoked products a unique smoky flavor and brown color.

    5. Hydrocarbons

    Many PAHs can be separated from smoke, including benzanthracene, benzopyrene, dibenzoanthracene and 4-methylpyrene. Among these compounds, 3,4-benzopyrene is the most harmful component, which is the most polluting, has the highest content and is the most carcinogenic.

    3,4-benzopyrene is a common food contamination, especially in smoked meat products. The highest content is in meat products smoked directly in coal stoves and wood stoves. How to reduce the content of 3,4-benzopyrene in smoke is a problem of great concern to the smoked meat industry.

    6. Gaseous substances The gaseous substances produced in smoke, such as CO2, CO, O2, NO, N2O, acetylene, ethylene, propylene, etc., the effects of these compounds on smoking are not very clear, and most of them are insignificant to smoking. CO2 and CO can be absorbed on the surface of fresh meat to produce carbon monoxide myoglobin, which makes the product bright red; O2 can also form oxymyoglobin or metmyoglobin with myoglobin, but there is no evidence to prove that the smoking process produces these substances. NO in the gas component can form nitrosamines during the smoking process, and alkaline conditions are conducive to the formation of nitrosamines.

    commercial smokehouse smoker oven factory from China

    General processes of food smoking technology

    1. Pre-smoking treatment   

    The main purpose of the pre-smoking process is to ensure that all products to be processed have consistent surface conditions before smoking and cooking. However, it is impossible to keep the product exposed in a dry environment and the time it enters the furnace consistent, which results in inconsistent surface color of the product. The solution is to spray the product for a short time before entering the furnace; another method is to maintain a warm and high temperature environment so that the low-temperature product forms a uniform surface layer. At present, when using the smoke furnace for production, the equipment has some control programs, which is actually drying, but the humidity must be controlled, and the purpose is also to promote the color of the product and make the surface color consistent. Generally, the temperature can be set to 50~60℃ and the humidity to 85%-95%.

    2. Pre-drying   

    The purpose of drying is to ensure the consistency of the dryness of the product surface to prevent water from dripping on the surface, so that the product surface can achieve a uniform smoked color when smoking. On the other hand, it can also promote the color development of the product. If a darker smoked color is required, the drying time should be shortened. However, if the drying time is insufficient, the moisture on the product surface will be too much, resulting in the product being dark brown or even black. On the contrary, if the drying time is prolonged, the product will be yellow or brownish red.

    The temperature and time settings for the drying step depend on the type of product. Generally, the temperature is set at 50-70°C and the relative humidity is within 30%. A wet surface absorbs smoke better than a dry surface. If you want a darker surface color, you must shorten the pre-drying process. If you want a lighter surface color, you must extend the pre-drying process. If the meat product is steamed horizontally in the oven, extending the pre-drying process can minimize the chance of two colors. If the product is over-dried during this process, the color will become too light.

    3. Smoking   

    Common smoking methods in meat processing can be divided into cold smoking, warm smoking and hot smoking according to the processing temperature. Cold smoking, the temperature range is 15-25℃; warm smoking, the temperature range is 30-50℃; hot smoking, the temperature range is 50-80℃; above 80℃ is called baking smoking. The color of hot smoked products is obviously better than cold smoked, but due to the high temperature during hot smoking, muscle protein will undergo thermal denaturation, fat will dissolve, and quality will change.

    The raw materials are salted for a period of time until the degree of Baume of the salt solution reaches 18-20, then indigo removal is carried out, and then seasoning and impregnation are carried out, and then smoked and dried at a temperature range of 15-30℃ for 1 to 3 weeks. This smoking method is called cold smoking. Cold-smoked products produced by cold smoking have better storage properties.

    The warm smoking method is to place the raw materials in a seasoning liquid with added salt for a short period of time for several minutes or hours, and then smoke and dry them in a smoking room at a temperature of 30-50°C for several hours to several days. This smoking method is called warm smoking.

    Cold smoking is often used for non-heated high-end products. Some products need to be cold smoked for half a day or several days. The warm smoking method is conducive to improving the preservation of products and the formation of specific and useful bacteria. Generally, the dry bulb temperature is required to be 50-75℃ and the wet bulb temperature is 0-55℃ (relative humidity 30%-60%) during the smoking process.

    When liquid smoking is used in an oven, the oven is usually closed and the liquid is atomized and sprayed into the oven. The liquid smoking process usually consists of an atomization phase of the smoke, followed by a short “rest” before the oven is restarted and the smoking process continues. Regardless of the atomization time, the rest period should not exceed five to ten minutes. Even if the actual atomization time is the same, atomizing liquid smoking in two steps is usually more effective than just one longer step. For example, two 15-minute liquid smoking processes with a 20-minute drying period will be more effective than a single 30-minute smoking process.

    4. Coloring and color fixing   

    Coloring is the process of setting the smoke color before cooking and finishing at higher humidity. In this step, dry heat conditions are used to promote the formation and stabilization of the desired color. The temperature of the wet sensor should be set to 0°C in this step to open the valve and create dry conditions to promote the formation and stabilization of the smoke color. The coloring step should be long enough to form the same smoke color as set.

    Color fixation is the process before the high humidity heat treatment. The surface of the product needs to have a consistent smoke color. During this step, a high temperature and dry environment is conducive to promoting the stability of the smoke color. Generally, the dry bulb temperature is set at 60-70℃ and the wet bulb is set at 0-50℃ (relative humidity is less than 20%). This process has some changes in the actual operation process. For example, if the humidity is high during smoking, a short drying time is used, which is conducive to the formation and fixation of color. After drying, it is best to stay for 2-3 minutes before exhausting and drying. If liquid smoking is used, the color should be fixed immediately after liquid smoking, which is conducive to the stability of the color of the smoke liquid on the surface of the product.

    5. Steaming            

    The cooking process is an intermediate step between the coloring process at low humidity and the finishing process at high humidity. The wet sensor is set to 60°C in this step to slowly change the properties of the surface protein. The properties of meat proteins change very drastically at 60°C. The typical set point for dry sensors during the cooking step is 70°C to 85°C, and the typical set point for wet sensors is 55°C to 65°C. For some products, this step is not necessary and can be omitted. The cooking process in the smoke oven is generally drying, steaming, baking and other procedures to make the product reach the required core temperature. If the smoked color of the product surface is consistent and stable, the high temperature and high humidity environment of steam cooking will accelerate the progress of the Maillard reaction and the absorption of smoke, making the smoked color of the cooked product darker. The dry bulb temperature during cooking is 72~90℃ and the wet bulb temperature is 68-84℃. There are different treatments for setting the cooking time. One is to set the cooking time based on experience; the other is to set it based on the core temperature of the product, which is generally 68-78℃. The cooking time cannot be too long or too short, otherwise it will affect the taste and flavor of the product. After cooking, spray cooling or re-drying, air cooling and other treatments can be carried out according to the characteristics of the product.

    meat in smokehouse

    Different types of food smoking methods (1)

    According to the processing of the product, it can be divided into cooked smoking and raw smoking

    1. Cooked smoking

    This is a very special smoking method. It refers to a smoking method with a smoking temperature of 90-120℃, or even 140℃. Obviously, smoked products at this temperature are fully matured and do not need to be further matured. In addition, products that are already cooked before smoking are also called cooked smoked, such as the smoking of marinated sauces, roast chicken, etc. This type of product is generally smoked at a high temperature and for a short time.

    2. Raw smoking

    This is a common smoking method. The raw materials for smoking only go through the process of sorting and pickling, and are not subjected to heat processing. For example, ham, bacon, sausage, etc. are all smoked raw. The smoking temperature of raw smoking is 30-60℃. The products made by this smoking process need to be steamed or fried before they can be eaten.

    Different types of food smoking methods (2)

    According to the way the smoke contacts the food, it can be divided into direct smoking and indirect smoking.

    1. Direct smoking

    This is an original smoking method, in which the smoking materials are directly and incompletely burned in the smoking room. The wood is burned in the lower part of the smoking room and the products are hung in the upper part. According to the different temperature ranges maintained during smoking, it can be divided into cold smoking, warm smoking, hot smoking, roasting smoking and other methods.

    Direct smoking has a long history and is widely used. It does not require complex equipment and is easily recognized by manufacturers. Its disadvantages are: ① The smoking conditions are affected by many factors (smoking materials, combustion conditions, etc.), and it is almost impossible to obtain smoke with a certain composition. Therefore, the quality of smoked products is difficult to control, which can easily lead to unstable product quality; ② The smoking time is long, especially the cold smoking method, which can take several hours or even dozens of hours. Even the hot smoking method takes tens of minutes to several hours; ③ The working environment is poor, the labor intensity is high, and the tools and rooms are contaminated; ④ The production efficiency is low, the energy consumption is high, and the utilization rate is low, making it difficult to implement mechanized and continuous production; ⑤ The smoke contains benzopyrene, which is difficult to remove directly during the smoking process, causing meat products to carry carcinogens.

    2. Indirect smoking

    The smoke generator is used to send the smoke of a certain temperature and humidity into the smoke chamber to contact the product for smoking. The smoke generator and the smoke chamber are two independent structures. This method can not only overcome the problems of smoke density and uneven temperature and humidity during direct smoking, but also control the composition of smoke by adjusting the temperature and humidity of the smoke material and the amount of oxygen contact, and reduce the generation of harmful substances. Therefore, it is widely used. According to the smoke generation method and the temperature conditions in the smoke chamber, it can be divided into wet heat method, friction smoke method, combustion method, carbonization method, two-step method and other methods.

    Different types of food smoking methods (3)

    According to the temperature range during the smoking process, it can be divided into cold smoking, warm smoking, hot smoking and roasting smoking.

    1. Cold smoking

    Cold smoking refers to smoking at 15-30℃ for a long time (4-7 days). The raw materials need to be pickled for a long time before smoking. This method is generally only used for smoking ham, bacon, dry sausage, etc., to produce products without heating process. This method is easier in winter, but in summer, due to the high temperature, the temperature is difficult to control, especially when there is little smoke, it is easy to go rancid. However, due to drying and post-ripening, the moisture content in the food is about 40%, which improves the preservation and flavor, but the smoked flavor is not as good as the warm smoking method.

    food smoking technique - cold smoking

    2. Warm smoking method

    Warm smoking refers to the smoking of raw materials at a temperature between 30 and 50 degrees Celsius after they have been properly pickled (sometimes with seasonings). It is used for bacon, bone-in ham and back ham. The smoking time depends on the size of the product. For example, pickled meat is smoked for 5 to 10 hours according to the size of the meat, while ham takes 1 to 3 days. The smoking material is usually dried oak or cherry wood. This method can make the product have a good flavor and less quality loss, but because the temperature conditions are conducive to the reproduction of microorganisms, if the smoking time is too long, it may sometimes cause the product to spoil. The smoked product needs to be boiled before it can be eaten.

    3. Hot smoking

    Hot smoking means that the raw materials are properly pickled (sometimes seasonings can be added) and then smoked. The temperature is between 50 and 80 degrees Celsius, mostly 60 degrees Celsius, and the smoking time is 4 to 10 hours. Within this temperature range, the protein is almost completely coagulated. The surface hardening degree of the product is high, but the interior still contains more water and has good elasticity. This method can form a good smoked color in a short time, and the operation is simple and labor-saving. However, it should be noted that the temperature should not be raised too quickly during the smoking process, otherwise there will be uneven color.

    4. Roasting and smoking method The temperature of the roasting and smoking method is 90-120℃. It is a special smoking method that includes the process of steaming or baking. It is used in the production of baked products, and is often used in the production of ham and bacon. Due to the high smoking temperature, the purpose of cooking is achieved while smoking. The product can be eaten directly without heat processing, and the smoking time is short. However, the product has poor storage properties and more fat is dissolved, which is suitable for products with a high lean meat content.

    Other types of food smoking methods

    Electric smoking

    Electric smoking is a method of fumigation using static electricity. The products are hung up, arranged at intervals of 5 cm, connected to positive and negative electrodes, and 15-20 kV high-voltage direct current or alternating current is applied while sending smoke, so that the product itself (product) is used as an electrode for corona discharge. The smoke particles are charged due to the discharge, and are rapidly adsorbed on the surface of the product and penetrate into the inside. The electric fumigation method shortens the time by 1/20 compared with the usual fumigation method, and can extend the storage period. Since the formaldehyde content inside the product is high, it is not easy to mold. The disadvantage is that the smoke is not evenly attached, the tip of the product is more adsorbed, and the cost is high. It is rarely used at present.

    Liquid smoking

    The method of using liquid smoking preparations instead of smoke is called liquid smoking, also known as smokeless smoking. It is currently widely used and represents the development direction of smoking technology.

    (1) Preparation of liquid smoking

    Liquid smoking is a liquid smoke preparation made by condensing the smoke produced during the wood distillation process and further distilling the condensate to remove harmful substances and resins. The generated smoke is introduced into the water of the absorption tower, and the smoke is continuously produced and repeatedly absorbed by the water until the ideal concentration is reached. After a period of time, the relevant components in the solution react and polymerize with each other, tar precipitates, and the insoluble hydrocarbon substances in the solution are filtered out, and the liquid smoke agent is basically prepared. This liquid smoking mainly contains the vapor phase components in the smoke, including phenols, organic acids, alcohols and carbonyl compounds.

    (2) Application of Liquid smoking

    There are four methods of liquid smoking, namely direct addition, spray soaking, casing coloring and spraying, all of which are carried out before cooking.

    ① Direct addition method. Liquid smoke is a food additive. After being diluted with water, it is directly added to the product by injection, tumbling or other methods, and then blended and stirred evenly. It is mostly used in minced meat products such as red sausage, tripe, round ham, luncheon meat, etc. This method mainly focuses on the formation of product flavor, but cannot promote the formation of product color.

    ② Spraying and soaking method. That is, spray the product surface with liquid smoke or directly put the product into the liquid smoke to soak for a period of time, and then take it out and dry it. This method is conducive to the production of product surface color and flavor. Liquid smoke must be diluted before use. Generally speaking, 20 to 30 parts of liquid smoke are diluted with 60 to 80 parts of water. The dilution multiples of different products are marked in the instructions for use of commercially available liquid smoke.

    The formation of smoke color is related to the dilution concentration of smoke liquid, spraying and soaking time, color fixation and drying process, etc. Adding about 0.5% salt during impregnation can improve the flavor of the product.

    Liquid smoke can be recycled, but should be replenished in time to reach the required concentration according to the frequency and amount of soaking products. When producing casing-free products, about 5% citric acid or vinegar is often added to the diluted liquid smoke to facilitate the formation of the outer skin.

    ③ Casing coloring method. Before packaging the product, use smoke liquid to penetrate and color the casing or packaging film or smoke it. When cooking, because the product is close to the processed casing, the smoke color is automatically absorbed on the surface of the product, and it also has a certain smoke flavor. This method is a new and popular method.

    ④ Spray method. This is a method of fumigating the product by atomizing the smoke liquid and sending it into the fumigation furnace. In order to save smoke liquid, intermittent spraying is often used. Generally, the product is first dried for a short time, and the smoke liquid is atomized and sent into the fumigation furnace to fill the entire space with smoke. After a period of time, spray again. Repeat 2 to 3 times as needed, with an interval of 5 to 10 minutes to ensure a uniform smoke concentration throughout the fumigation process. The fumigation process can also be divided into two times, that is, drying for 15 to 30 minutes between two sprays. The air regulating valve is opened during the drying process, and the dry airflow helps to form the smoke color.

    The color change of spray smoking is mainly related to the concentration of smoke liquid, the time of smoke retention after spraying, the time of intermediate drying, the temperature and humidity in the furnace, etc. Although this method must be carried out in a smoking room, it is easy to keep the equipment clean and there will be no tar or other residues deposited.

    (3) Advantages of liquid smoking

    The advantages of liquid smoking are as follows:

    ① The chance of the product being contaminated by carcinogens is greatly reduced because the particulate phase has been removed during the preparation of the smoke liquid;

    ②No need for smoke generator, saving equipment investment;

    ③The product has good reproducibility and the composition of liquid fumigants is generally stable;

    ④High efficiency, a large number of products with smoked flavor can be produced in a short time;

    ⑤ No air pollution, in compliance with environmental protection requirements;

    ⑥ Liquid smoke is very convenient and safe to use and will not cause fire, so it can be used in areas with dense vegetation. However, the flavor, color and storage performance of meat products made with liquid smoke are inferior to those made directly with liquid smoke.

    different types of food smoking technology

    Smoking equipment overview

    Although there are many smoking methods, warm smoking is the most commonly used. Here we focus on the equipment for warm smoking. There are many types of smoking rooms, including large continuous, intermittent, and small and simple ones for home use. Regardless of the type of smoking room, the following requirements should be met as much as possible:

    ① The temperature and smoke generation should be freely adjustable; ② The smoke should be able to diffuse evenly in the smoking room; ③ Fire prevention and ventilation; ④ The amount of smoking materials used is small; ⑤ The construction cost should be as low as possible; ⑥ Convenient, and humidity should be adjustable if possible.

    There are two types of smoking equipment: simple smoking room (natural air circulation type) and fully automatic smoking furnace.

    1. Simple smoking room (natural air circulation)

    It has general smoking equipment and simple smoking room.

    The general structure of a smoking device is shown in Figure 8-1. The smoke stove in the cold smoking room is made of concrete or plaster. The top of the smoking room is equipped with shutters that can adjust the temperature, smoke, and ventilation. For safety and fire prevention, the side walls of the room should be made of bricks, cement, or stones. The size of the smoking room is more suitable at 1.8m×2.7m. If the smoking room is too large, it is inconvenient to load and unload materials, and the work efficiency is not high.

    The structure of a simple smoking room is shown in Figure 8-2. Its inner side is made of wood, and is surrounded by thin iron sheets. There are exhaust holes on the top that can be opened and closed, and vents on the bottom. The warm smoking room is 1.8m wide, 2.7m deep (to the top shutter), and about 3m high, which is easy to operate. The four walls are painted with concrete, the outside is covered with iron sheets, the top is equipped with shutters, and a chimney with a diameter of 30 to 60cm is set. An exhaust device is installed at the tip of the chimney, and an adjustment plate is installed on the chimney to adjust the exhaust volume.

    2. Forced ventilation fumigation device

    The air in the smokehouse is circulated by a fan, and the heating source of the product is gas or steam. This type of smokehouse has uniform air flow and good humidity control. It can not only correctly control the smoking process, but also control the cooking temperature and the shrinkage of the finished product, which are more important than smoking. Compared with the natural air circulation smokehouse, it has the following advantages: ① The temperature in the smokehouse is uniform, which can prevent uneven smoking; ② The temperature and humidity can be automatically adjusted, which is convenient for large-scale smoke production; ③ Because the hot air has a certain temperature, it not only makes the temperature of the center of the product rise quickly, but also prevents the evaporation of water, thereby reducing losses; ④ Spices, etc. will not be reduced. It is precisely because of these advantages that this type of smokehouse should be more widely used. In actual production, this type of smokehouse is not only used for smoking, but also often used for steaming and cooking food.

    3. Tunnel type continuous smoking furnace

    The tunnel type continuous smoking furnace can smoke 1.5-5 tons of products per hour. The heat treatment, smoking heating, hot water treatment, pre-cooling and rapid cooling of the products are all carried out continuously in the tunnel. The raw materials enter from one side and the products come out from the other side. The advantage of this equipment is that it is extremely efficient. For the convenience of observation and control, closed-circuit televisions are installed in the tunnel, and the whole process can be automatically controlled. However, the initial investment is large and the output also limits its use. It is not suitable for small batch and multi-variety production.

    4. Fully automatic smoking oven

    The fully automatic smoke oven is the most advanced meat smoking equipment. In addition to the main functions of drying, smoking and cooking, it also has the functions of automatic spraying and automatic cleaning, which is suitable for the drying, smoking and cooking processes of all smoked or unsmoked meat products. The outer wall of the room is equipped with a PLC electrical control panel to control the smoke concentration, smoking speed, relative humidity, room temperature, material center temperature and operation time, and is equipped with various display instruments. The appearance of the fully automatic smoke oven is shown in Figure 8-3. The fully automatic smoking furnace can be divided into one-door-one-carriage, one-door-two-carriage, two-door-four-carriage and other models according to the capacity. It can also open the front and back doors. The front door is used for loading raw materials and faces the sausage filling workshop. The back door is used for cooling and packaging and faces the cooling and packaging room. In this way, raw and cooked food are separated, which is conducive to ensuring the hygiene of meat products. There are also two-door-one-carriage, two-door-two-carriage, and four-door-four-carriage models.

    tunnel type fully automatic smokehouse smoker oven

    Different types of smoke generators

    1. Combustion device

    The method of generating smoke by combustion is to pour sawdust on an electric burner to burn it, and then send smoke through a fan. This method separates smoke generation and smoking into two places. The smoke generation temperature is the same as the direct smoking method, and needs to be adjusted by reducing the air volume and controlling the humidity of the sawdust, but sometimes it still cannot be controlled within 400℃. The generated smoke is sent into the smoking room together with the air by the blower, so the temperature in the smoking room is basically determined by the temperature of the smoke and the temperature of the mixed air. This method uses the flow of air to attach the smoke to the product. The shorter the flue from the smoke generator to the smoking room, the more tar components will be attached.

    2. Friction smoke device

    Friction smoke generation is a method of generating smoke using the principle of drilling wood to make fire. A heavy stone is pressed on a hardwood stick, and the hardwood stick is pressed against a high-speed rotating wheel with a sharp friction blade. The intense friction generates heat to thermally decompose the cut wood chips to generate smoke, and the temperature of the smoke is adjusted by the water level in the slag container.

    3. Steam smoke generator

    The wet heat decomposition method is to mix water vapor and air appropriately, heat them to 300-400℃, and then pass the heat through the wood chips to produce thermal decomposition. Because the smoke and water vapor flow at the same time, it becomes moist high-temperature smoke. Generally, the temperature of the smoke sent into the smoking room is about 80℃, so the products must be cooled before smoking in the smoking room. Cooling can make the smoke condense and adhere to the product, so it is also called the condensation method.

    4. Mobile heating smoking device

    The flow heating method uses compressed air to make sawdust fly into the reaction chamber. The sawdust floating in the reaction chamber is thermally decomposed by superheated air at 300-400℃, and the smoke generated enters the fumigation chamber with the airflow. Due to the high airflow speed, the ash-residue sawdust can easily mix into the air, and the two need to be separated by a separator.

    smoke generator of smokehouse

    Different types of fuel in food smoking technology

    Smoked meat products can use a variety of materials to produce smoke, but it is best to choose materials with less resin content, good smoke flavor, and more preservatives, generally hardwood and bamboo, while softwood and pine needles are not suitable because they have a lot of resin content and produce a lot of black smoke when burned, making the surface of meat products black, and the smoke smell is not good. Commonly used smoking materials include poplar, birch, beech, walnut, hickory, cherry, alder, sycamore, fir, etc., and sometimes corn cobs, rice husks and corn stalks are also used .

    Smoked materials are usually in the form of sawdust, but firewood (wood), wood chips, or dry small wood particles, small trees, etc. can also be used. When the main purpose of smoking is drying, larger pieces of firewood are often used directly. Smoked materials, whether wood shavings or wood firewood, should be dry and free of wood preservatives. Wet materials will carry mold, and smoke can easily bring it to meat products. Wood preservatives may produce harmful fumes, affecting the safety of smoked products. The degree of dryness and wetness of smoking materials, generally the moisture content is preferably 20% to 30%. Fresh sawdust has a high moisture content and generally needs to be dried in the sun or air before use.

    wood for smokehouse

    Quality and safety control of smoked products

    During smoking, the smoking conditions have a great impact on the product. Due to the influence of smoking conditions, the quality of the products varies. To produce high-quality products, various factors and production conditions must be fully considered.

    Factors affecting the quality of smoked products

    There are many factors that affect the quality of smoked products, including

    • Food material: The freshness, size, thickness, component, fat content, with skin or not
    • Process before smoking: Salting conditions – salting temperature, time, composition of salting solution; Degree of desalination – temperature, time, flow rate, etc.
    • Smoking conditions: Temperature, time, capacity, fuel type, smokehouse size and shape, etc.
    • After smoking: Heating, cooling, hygiene, etc.

    The main influence of smoking materials has been mentioned before. The following is an overview of other major influencing factors.

    1. Temperature

    During the smoking operation, be careful not to have flames. If flames appear, the indoor temperature will inevitably rise, making it difficult to achieve the purpose of fumigation. At this time, according to the cause, either cut off the air source or spray some water. The main cause of flames is too much air supply and the fumigation material is too dry.

    If the smoking temperature is too low, the smoking effect will not be achieved; if the temperature is too high, fat will be smoked out, causing the meat to shrink. Therefore, close attention should be paid to control it within the specified range as much as possible. The opening and closing of doors and the entry and exit of people should be minimized. Especially for smoked sausage products, which are frequently entered and exited, more attention should be paid.

    The highest quality smoke is produced when the combustion temperature of the smoke material is between 340 and 400°C and the oxidation temperature is between 200 and 250°C. Although the combustion temperature of 400°C is most suitable for the formation of the highest amount of phenol, it is also conducive to the formation of benzopyrene and other cyclic hydrocarbons. If the formation of carcinogens is to be reduced to the minimum, the actual combustion temperature should be controlled at 340 to 350°C (343°C).

    2. Humidity

    The humidity in the smokehouse is important as follows:

    (1) Relative humidity affects the fumigation effect. High humidity is conducive to the deposition of fumigation smoke, but is not conducive to the deepening of color. Dry surfaces require a longer deposition time.

    (2) Generally speaking, the higher the humidity, the greater the degree to which smoke penetrates the casing. When the surface of the casing is not too dry, the smoke will settle on the surface, making the surface appear dark brown or brown, and the desired reddish brown cannot be obtained.

    (3) High humidity not only does not reduce the shrinkage of meat products, but on the contrary, it will aggravate their shrinkage.

    (4) High humidity can easily cause oil to leak out. If sausages leak oil, the usual solution is to reduce the humidity.

    (5) High humidity will cause the casing to soften, and even the collagen casing will melt and the meat filling will fall out. Low humidity smoking will promote the hardening of the casing.

    Therefore, before the product enters the smokehouse chamber, it is necessary to remove the surface moisture, air dry or dry (air drying room). After the material is sent to the smoking room, it is pre-dried before smoking. During the smoking process, the humidity is generally required to be lower at the beginning of smoking, so that the water can evaporate as quickly as possible to harden the surface of the intestine; the humidity should be higher in the later stage of smoking, in order to obtain appropriate softening and tenderness. For common meat products with peelable fiber casings, a relative humidity of 38% to 40% is ideal; for quick peeling (machine) casings, the ideal humidity is 24%; animal casings and collagen casings have a slightly higher humidity, which will have a better effect.

    3. Oxygen supply

    Oxygen supply is air supply, which promotes air circulation. It can affect the degree of heating and smoking of products, especially for those natural convection smokehouses that only rely on the difference in relative density of cold and hot air to produce displacement flow. Whether it is natural convection or forced circulation, the density of meat products in the smokehouse should be as uniform as possible, and the temperature and smoke density at each point should be checked.

    Air circulation has a substantial effect on heat transfer: under static air conditions, the temperature of the product is often very different from the room temperature, and the heat exchange rate is very low; especially when rapid heating is desired, strong air circulation is essential. Many air-conditioned smoke rooms exchange air 10 to 12 times per minute. When heating rapidly, air velocity is more important than air humidity.

    The higher the airflow speed, the faster the product dries and heats. At the same time, the amount of acid and phenol increases, and the oxygen supply exceeds about 8 times the oxygen required for complete oxidation, and the formation amount reaches the highest value. If the airflow speed is strictly controlled, the smoke will be black and contain a large amount of carboxylic acid. Such smoke is not suitable for food. Therefore, the air circulation speed must be controlled to keep heating and drying at a balance point.

    fish in smokehouse

    Control of harmful ingredients

    The smoking process has advantages that cannot be replaced by other processes and is widely used in meat processing. However, products made by traditional smoking processes usually contain carcinogens such as 3,4-benzopyrene and can also promote the formation of nitrosamines. Long-term excessive consumption has potential harm to human health, so the reform of the smoking process is imperative, and efforts should be made to take measures to reduce the production of harmful components in smoke and the pollution of products to ensure the safety of the products.

    1. Control the smoke temperature

    The smoke temperature directly affects the formation of 3,4-benzopyrene. When the smoke temperature is below 400℃, a very small amount of 3,4-benzopyrene is produced. When the smoke temperature is between 400 and 1000℃, a large amount of 3,4-benzopyrene is formed. Therefore, controlling the smoke temperature and making the smoke material burn lightly can effectively reduce the formation of carcinogens. It is generally believed that the ideal smoke temperature is 340-350℃, which can achieve the purpose of fumigation and reduce toxicity.

    2. Use wet smoke method

    Mechanically force the hot water vapor and mixture through the sawdust to produce smoke, which is then introduced into the smoking chamber. This can also achieve the purpose of smoking and improve the safety of smoked products.

    3. Use outdoor fume purification method

    By using outdoor smoke generation, the smoke is filtered, washed with cold air, and electrostatically precipitated before being passed into the smoking room to smoke food. This can greatly reduce the content of 3,4-benzopyrene.

    4. Use isolation protection method

    The use of casings, especially artificial casings such as cellulose casings, has a good barrier effect on harmful substances. Harmful substances such as 3,4-benzopyrene molecules are much larger than the molecules of other substances in the smoke components. The contamination of food is mainly concentrated on the surface of the product, so the filtration method can be used to block harmful components such as 3,4-benzopyrene without preventing the beneficial components of the smoke from penetrating into the product, thereby achieving the purpose of fumigation.

    5. Use liquid smoke

    The above methods can only reduce the content of harmful substances in smoke, but cannot completely eliminate the substances. The use of smoking liquid can avoid the harmful substances produced by smoking in the product.

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