Drying in meat processing
Thermal treatment of meat is to disinfect and/or preserve meat. This can be achieved by drying, and/or inducing flavour, and/or tenderising meat, and/or encouraging Maillard reaction. Maillard reaction is a reaction between reducing sugar and amino acids from protein that gives browned foods their distinctive flavour, and typically only occurring at temperature above 150-160°C.
There are three ways of drying meat that do not induce Maillard reaction, i.e. at temperatures much lower than 150-160oC. They are:
(i) cold smoking
(ii) hot smoking
Cold smoking is typically 12-22°C, but less than 30°C. The operating temperature for hot smoking and drying are approximately similar (typically 52-80°C), but the latter usually involved lower final moisture content than hot smoking. The smoke is an additional food preservation method on top of drying (lower moisture content), usually to limit fungal growth of surfaces of meat. Brining is usually necessary prior to any smoking to increase preservative effect.
The type of smoked product quality (flavour intensity and moisture content) will further determine whether dry or wet smoking is required. Wet smoking is more appropriate for stronger smoke flavour due to better diffusion of smoke particles through the meat.
Obtained via 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 advantage is that the smoke flavour can be achieved without smoking and, therefore, the components in the smoke, which are suspected to be damaging to health, are, to a large extent, avoided. As there is no emission of smoke, the need for air treatment is considerably reduced.
Smoke is generated from the friction between wood and a rapidly rotating rough roller which creates pyrolysis. The smoke is mild and contains hardly any carcinogenic compounds. The process can be carried out in a closed system with recirculation, so that use of an afterburner or other smoke cleaning system is unnecessary. 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.
Superheated steam with smoke
The pyrolysis of wood chips/shavings can also be carried out by passing superheated steam over chips of wood, which transfers the smoke and flavour to the products. This reduces the number of compounds in the smoke and also allows the air surplus to be reduced to a minimum. As the surplus steam can be condensed, the exhaust is therefore low. Cleaning is also easier due to a lower level of tar deposits in the smoking chamber.
- Processing time is usually much less than 1 day for hot smoking and drying
- Processing time is typically much longer for cold smoking, can be as long as 6 weeks to 3 months.
Meat processors should confirm with smoker/drier manufacturers that the smokehouse/drier is of optimum energy efficiency design with clear specs. The overall heat transfer coefficient of the smoker/drier should be ~3 W m-2 K-1 Major sources of heat requirement are for evaporation of moisture from the meat and for heating up inlet air to operating temperature. Cold smoking does not have any heat requirement (apart from heat released when hardwood released smoke particles) since the process is usually done in countries with cooler climate. This processing usually utilises the existing climatic conditions for partial dehydration of meat. However, in some processors, heat input may be required. The thermal energy demand is usually higher than hot smoking or drying even though cold smoked product may have higher moisture content. This is due to the fact that processing time is much longer.
Mass and heat transfer
- If mass and heat transfer is convection controlled, the Biot number must be less than 1, preferably less than 0.1;
- If mass and heat transfer is convection and diffusion controlled, the Biot number must be between 1 and 100;
- If mass and heat transfer is diffusion controlled, the Biot number must be larger than 100.
Best practice for uniform product quality
Thickness of meat should be <7cm otherwise not all a parts of the product may be of sufficiently low water activity, which may compromise meat preservation and thus meat safety.
It is often combined with salting and curing processes. The end product contains 3-10% humidity.
- Case studies