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Getting under the skin of a sausage

24 February 2022

Markus Huuhtanen explains how in-line refractometers can monitor and control sausage casing processes to help ensure optimum production line speeds. 

Traditionally, sausage skins were made from the small intestine of meat animals. However, in recent decades artificial casings have become popular in many markets. This has been prompted by a number of factors, including the high costs that result from the number of processes necessary to create the natural casing product. Also, natural casings tend to be variable in length, diameter and thickness, so it is more difficult to streamline sausage production, and the process incurs a heavier labour requirement. In contrast, continuous accurate monitoring of the artificial casing process removes the requirement for manual sampling and testing and enhances both process efficiency and product uniformity.

Alginate casing
Alginate is found in the cell walls of brown algae from seaweeds. An important feature of alginate is its ability to hold many times its own weight in water, making it a naturally gelling substance.

Alginate can be used as a casing for sausages through its gel formation with calcium ions. During the production process, meat mixture is extruded to form the sausage, and a layer of sodium alginate is simultaneously applied (co-extruded) to the outer surface, before a calcium chloride brine solution is used to for dehydration and to induce gel formation. This creates a layer of calcium alginate film on the sausage, which provides the strength and flexibility needed of a sausage casing.

The salt content in the brine solution is very important and needs to be closely monitored because it affects the colour, texture and overall quality of the sausage. 

Monitoring alginate brine 
The brine solution is stored in a brine tank, and spent brine is recycled to the same tank. Consequently, the brine is constantly diluted by the moisture that is removed from the casing gel. It is necessary to monitor the solution so that the correct amounts of salt can be added to the brine solution. This function can be performed by a refractometer which monitors the salt content in real-time. 

There are two possible locations for the refractometers – directly in the brine tank itself, with a second refractometer can be installed in the salt supply tank.

The Vaisala refractometer is able to measure the refractive index (RI) of the liquid, which correlates directly with the salt concentration of the brine solution. In-line RI monitoring with automatic feedback control enables process operators to ensure consistent and reliable operations protecting product quality and reducing downtime.  In contrast with many other liquid concentration methods, the refractometer is extremely accurate and reliable and needs no regular maintenance. Importantly, these refractometers are not affected by particles, bubbles, crystals or colour, so they can be employed in a variety of solutions for measuring liquid concentration. 

The refractometers produce mA and Ethernet output signals that allow automatic operation of the process. Moreover, they can be calibrated to read the concentration of NaOH in g/L, wt-% or any other engineering unit preferred by the factory.

Collagen gel
In addition to alginate gel, it is also possible to use collagen gels in sausage manufacture. Collagen casing is largely derived from beef and pig hides, but it offers most of the speed and efficiency advantages presented by alginate gels.

After co-extrusion with collagen gel, sausages are passed through a brine solution in a similar manner to the alginate gel process. Vaisala refractometers are able to continuously monitor the process as outlined above, to ensure that the correct salt concentration is maintained and product quality is protected.

Emphasising the importance of the brine measurement, a sausage manufacturer in the USA said: “We treat co-extruded sausage casing with dipotassium phosphate to control the moisture, which directly affects the colour and texture of the final product. Too much moisture in the casing makes the sausage too dark and the texture too chewy, while too little means the sausage will be too light and the texture too soft. The Vaisala refractometer helps to keep the moisture at the specified level.”

Cellulose sausage casing
Vaisala refractometers are also used in cellulose sausage casing processes. A cellulose fibre cloth is used to create the sausage casing, but first the cloth is desulfurised by passing it through a Sodium Hydroxide bath. This solution is supplied from a tank, and spent caustic is returned to this tank. Consequently, the concentration needs to be replenished because caustic is lost in the cloth during the impregnation process. A Vaisala refractometer is employed to continuously monitor the NaOH concentration and ensure accurate replenishment.

Markus Huuhtanen is business development manager at Vaisala (Finland).


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