PURPOSE:

        Smoking of meat is a technique in which meat is exposed directly to wood smoke which may be generated by a variety of methods. In smoke produced from wood there are various substances which contribute to the flavour and the appearance of the smoked meat product and which have a certain preserving effect on the product.

TYPES OF SMOKE:

A)   Natural smoke:

         Smoke has been applied for around 80,000 years in the production of meat products and is produced by incomplete combustion(pyrolysis) of wood material such as sawdust or woodchips. Friction or steam condensation are two other methods of generating smoke. In the friction method, a piece of wood of a certain size is pressed against a fast runningrotor with a ripped surface and high frictional forces are the result.Friction smoke is obtained, which exhibits a high level of phenols, carbonyls and other acids whilts showing a low level of tar and polycylic aromatic hydrocarbons (PAHs).

          Pyrolysis takes place through the heat obtained during friction between the piece of wood and the rapidly rotating rotor with the ripped surface.Steam smoke is obtained by the use of overheated low-pressure steam and the temperature within this process is between 300 and 450 ⁰C. A spirally shaped element transports  sawdust into an area where such overheated steam is applied on to sawdust or wood-chips, causing pyrolysis. The generated smoke is cooled to around 85 ⁰C until it reaches the smoking chamber  which causes an increase in moisture(RH) within the smoke and this gives the basis for the name ‘steam smoke’. Steam smoke is basically free of tar as well as PAHs.

        Wood consists of around 50% cellulose, 25% hemicellulose and 25% lignin.From those materials, around 50-70% turn via pyrolysis into smoke and pyrolysis results in burnable coal rather than ash by burning wood slowly at a certain temperature. The term hemicellulose describes polysaccharides which are made from different  pentoses and hexose.Hexoses are monosaccharides containing six carbon atoms whilst pentoses contain five carbon atoms within their molecule. Pyrolysis takes place in four steps.

  1.  Drying of the wood up to around 160 ⁰C.

  2.  Pyrolysis of the hemicellulose between 180 and 250⁰C.

  3.  Pyrolysis of the cellulose between 250 and 300⁰C.

  4.  Pyrolysis of the lignin between 300 and 550⁰C.

      The optimal temperature for combustion is between 350 and 500⁰C and temperatures below or above this range cause a considerably higher amount of unwanted substances within smoke afterwards.The most well-known and dangerous of those unwanted substances is 3,4-benzopyrene, which is carcinogenic and belongs to the group of PAHs. If smoke is generated at temperatures between 350 and 500⁰C, PAH contamination is greatly reduced and concentrations of less than 1 ppb per kilogram of smoked meat product are obtained.

       Smoke is a higly complex mixture of gas-like substances, solid particles (particulate phase) and water, and around 600 components within smoke are known today.The particulete phase accounts for around 80% whilst the gaseous phase for around 20%; the gas fraction is not visible to the human eye and is yet very complex.The composition of smoke depends primarily on the type and moisture content of wood utilized as well as on the method used to generate smoke.The main components of smoke, having the largest impact on meat products, are phenols, organic acids and carbonyls.Most of those substances are in the gaseouse phase and not in the particulete phase.

        Phenols are not, as one might think, alcohols but acids (carbolic acid) and are obtained through the pyrolysis of lignin by a temperature between 300 and 450⁰C.Phenols can release a proton from a hydroxyl group and that makes them a weak acid.The visible particle fraction consists of small liquid colloidal smoke particles.Those particles are very small in size, around 1µm, and are distributed within the gas fraction.Much larger particles such as ash and tar are also part of the visible particle fraction. Two phenolic substances coming from lignin, namely syringol or guaiacol(2-methoxyphenol), are of importance towards the smoke colour and flavour in meat products.Guaiacol results from the lignin present in soft wood and is not favoured as it gives a dark dull smoke colour as well as a rough and unpleseant smoke taste.The desired golden-brown smoke colour, as well as pleseant smoke flavour, originates from syringol, which is obtained from lignin present in hardwood such as oak and hickory.

       Wood chips or sawdust should be stored in a dry area and no animals should have access to this area.It happens quite often that woodchips are contaminated with animal faeces and/or urine. Sawdust or woodchips have to be moistaned first with water; otherwise the wood material becomes too hot very quickly (above 450⁰C)  and little smoke is obtained. The amount of water added to the wood material is around 20-30% of the weight of the wood material. Also, the smoking chambers must not be overloaded during the application of smoke in order to avoid hot spots. Air still has to move freely within the smoking chamber in order to achieve an even drying, smoking and cooking impact.

Table 1 shows the three different methos of smoking.

The main functions of smoke are described in the following sections.

METHOD	TEMPERATURE ( 0C )	RH%
Cold smoke	15 – 25	50 – 85
Warm smoke	25 – 50	50 – 80
Hot smoke	50 – 90	30 – 85

Humidity during smoking :

v  The humidity during smoking in the smokehouse is important as it effects
the smoke deposition on the product.

v  High humidity favors smoke deposition but also tend to limit color development.

v  With high humidity there is higher amount of smoke penetration.

v  When it is too low the smoke is deposited on the surface only and desirable color is not achieved. Its
likely to acquire a dull brown tan.

v  High humidity do not necessarily reduce shrinkage of the products rather it may encourage fat rendering.

v  In case of animal or collagen casings somewhat higher humidity give good results.

v  With collagen casings smoking with low humidity hardens the surface and produce a low quality product.

v  where as too high humidity soften the surface therefore a balanced humidity application has to made.

v  In a gravity house humidity is controlled by opening the dampers.

v  In air- conditioned house the humidity is controlled by adding steam or water vapor and using the dampers.

Development of the smoke color:

          Smoking of meat  and meat products results in a nice and appealing golden- brown color, which is very attractive to the human eye. Carbonyls are the main color – forming agents and carbonyls is absorbed into the slightly  moist surface of the product.Ssubsequently, carbonyls react with amine to from the desired smoke colour.To a small degree, phenols also contribute to smoke colour.It is important to have the correct level of moisture on the surface of the meat and meat products in order to obtain a nice smoking colour. Evenly dried products result in a very appealing smoke colour and a dry surface absorbs significantly less smoke (as well as ash and tar) than a wet surface.If the surface is too wet(underdried), a brownish colour and even streaking (tiger stripes) can be the result of smoke absorbed unevenly on products such as frankfurters.Streaking occurs when there is some free moisture on the surface of the product.When the surface is overdried, a lighter than wanted colour is obtained. Hence, moderate levels of moisture must be present within the smoking chamber during the smoking process as such to obtain an evensmoke colour. Uneven smoke colour is also commonly the result of not having achieved the same conditions regarding surface moisture and temperature of the product to be smoked inside the smoking chamber prior to smoking. In this case it might be necessary to ‘condition’ products such as frankfurters by showering all for a short while in order to obtain the same level of moisture and surface temperature.

         Filling of large smoking chambers with trollies of frankfurters can take a while and the product going into the smoking chamber first can exhibit a dry surface, compared with a wet surface from the last products until the chamber is finally full.Basically, the surface of the product to be smoked should be slightly tacky before smoke is applied. For sausages such as hot dogs which are filled into naturel casings, the casing should feel like human skin after drying prior to the application of smoke.

         If a dark, or black, smoke colour is wanted on the meat product, a high level of humidity is required within the smoking chamber and the drying phase must be short or not part of the smoking process at all. Prolonged periods of smoking commencing on a wet product surface without prior drying produce such a dark and almost black colour because a wet surface demonstrates high smoke absorbtion. S ome of those dark-coloured meat products are called pit smoked. High levels of moisture, around 80-85%, are also maintained during the smoking process to obtain such a dark colour.To fixate the dark colour, some of those wet-smoked products are finished off with hot air (baked) and such treatment enhances the darkness of the product in comparison with being finally steam cooked.Generally, the smoking chamber must not be overloaded with products,in order to secure an even airflow and thus an even distribution of smoke and humidity within the chamber.In order to even out  the smoke colour, short drying steps of 3-5 min are commonly introduced between smoking cycles for products such as hot dogs or frankfurters.

Development of the smoke flavour:

 It is a well-known fact that smoked meat products taste differently from nonsmoked products.Substances such as formaldehydes, lactones and up to 20 different phenols(guaiacol and syringol) are primarily responsible for the smoke flavour.Hardwood such as maple, oak, beech, hickory and mahogany is preferred because these types of wood give a clean and non-tar flavour.

Smoke as preservative:   

 Smoked meats and meat products last longer, a well-known fact for thousand of years.Formaldehyde, phenols and acetic acid are the main agents for extending shelf life of smoked products as these are very effective antimicrobial substances.Phenols are acids which denature proteins and disrupt cell membranes.Disrupted cell membranes eventually kill the cell or make it very hard for the cell to survive or even to grow.

Smoke adding to bite:

 Smoked sausages such as farnkfurters obtain a much better bite, or snap, through smoking. Components of smoke, mainly formaldehyde and other organic acids, combine with the activated protein on the surface of the noncooked sausage and subsequent thermal treatment creates a firm layer around the sausage, which is largely responsible for the snap.Smoke contains around 0.6-1.0% formaldehyde.

Mistakes during smoking:

The most common mistakes during smoking are as follows:

1.    Overloading of the smoking chamber leads to uneven airflow and also can cause uneven smoke color.

2.    A negative impact on the flavour can be seen by using mouldy sawdust or chips. Hence, sawdust or chips must be free of wood impregnation substances and urine from animals.

3.    If smoked is too long, the product can aquire a slightly bitter taste and a wrinkly product is often the result as well.

4.    Overdrying of the product leads to toughening of the casing and natural casing lose their ability to strecth and shrink. As a result, wrinkles are obtained and the product also loses its shine. 

LIQUID SMOKE:

        Liqid smoke is produced by burning selected woods under controlled conditions. The smoke obtained is condensed on water and recycled until the desired concentration is given. Liquid smoke can be applied on to meat product via atomization a dipping or shower system, brine addition to meat products and in the form of smoke- impregnated casings. Atomization is the spraying of the liquid smoke under a predetermined pressure through a nozzle into smoking chamber and creating a cloud of smoke made by from very tiny particles.

        Before liquid smoke is introduced into the smoking chamber, the meat product first has to be dried properly. After atomization, the cloud of smoke is allowed to dwell for around 10 -15 min. Within the chamber before a drying steps of 5-10 min is introduced. The spraying process can be repeated several times until the desired smoke colour is obtained and two or three applications of spraying results in a nice golden-brown colour.

        Within a shower or dipping system, products such as frankfurters are showered with or dipped into a liquid smoke solution for a short time. The dipped or showered products are dried afterwards to fixate the smoke on to the surface before being thermally heat treated afterwards, primarily by the application of steam. Processes such as dipping or showering, colour  fixation and thermal heat treatment of products such as frunkfurters run continuously in large-scale operations.

Liquid smoke shows a several advantages compared with natural smoke:

1)  Liquid smoke is standardized , an even smoke colour on the finished products can be obtained all the time.

2)  There is no emission of smoke into the air and therefore smoking with liquid smoke is environmentally friendly.

3)  Smoke chamber are easy to clean as liquid smoke does not contain tar and other tacky substances.

4)  Most of time, the smoking chamber can be cleaned with water only and no chemical cleaning detergent is required.

5)  Liquid smoke is almost free of PAHs.