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.
TEMPERATURE ( 0C )
15 – 25
50 – 85
25 – 50
50 – 80
50 – 90
30 – 85