The aim of smoking is the preservation of food by exposure to smoke from smouldering wood. In addition to preservation of the food, special much appreciated aromas are added to the product.
Field of application
Smoking is commonly used in the processing of fish, cheese, meat and meat products.
Composition of smoke
Wood smoke consists of two phases: a disperse liquid phase, e.g. droplets, and a vapour phase. The former contains particles of smoke, which are not regarded as important in the smoking process. The vapour phase is the more important in imparting flavour to the meat. The vapour contains up to 200 chemical components, not all of which have been identified. They include a range of organic acids, aldehydes, ketones, alcohols and poly-cyclic hydrocarbons.
Description of techniques, methods and equipment
Techniques for generating smoke:
a) Smoke from burning wood
Traditional smoking equipment consists of a chamber with a smoke generator. In the simplest operation, the products are hung on racks in the chamber with a fire of wood dust or chips ignited on the floor. This fire is suitably dampened to maximise the smoke production and to avoid flame production. The product increases in temperature to about 30 ºC. The smoking period may last for up to 48 hours. In more sophisticated systems, the smoke may be generated outside the chamber and fed into the room using fans, which also produce circulation within the chamber and venting to the atmosphere. The equipment may also include an air conditioning unit (ventilator, cooling, heating, moisturising). The smoke generator can be a small oven where hard wood chips or sawdust are slowly added onto a bed of already smouldering wood or onto an electrically heated grid. Air is blown through the small oven and carries the smoke into the smoking chamber where the product is located. The smoke exiting the chamber is vented to atmosphere or partially recirculated.
b) Friction smoke
In this technique smoke is generated by friction; a high speed-rotating wheel is pressed against a piece of wood through which heat is generated and smoke is formed. This method claims to enable a more precise control of the volume of smoke produced, by altering the pressure between the wheel or disc and the wood.
c) Steam pyrolyses smoke
Smoke is generated by passing superheated steam over chips of wood.
d) Liquid smoke
Liquid smoke is produced by condensation of the smoke, followed by fractional distillation to reduce the content of tarry matter and other contaminants. The resulting solution is diluted with water and sprayed onto the product. In some cases, it is incorporated into a curing brine and injected into the product for flavouring purposes.
The smoking can take place at two temperature levels: ambient (upto 30 °C) and at elevated temperatures (between 50 and 90°C). The heat from the smouldering of the wood is not enough to raise the temperature to 50 – 90 °C, so extra heat is added by means of steam or a heat exchanger. The duration of the smoking depends on the product. Some products require pre-drying or drying or maturing between smoking steps. For this drying/maturing conditioned (temperature, moisture) air (heated by steam-pipes or electrical heaters) is used to control the drying of the product. The time the product is in the chamber varies from hours to days. The duration the product is smoked per step can be between 15 minutes and 4 hours.