Traditionally, sausage skins were made from the small intestine of meat animals, especially pigs, but also sheep, cattle and goats. These so-called natural casings have been utilized for centuries, but in recent decades artificial casings have become popular in many markets. Artificial casings include collagen (often derived from animal skin), cellulose (from plant materials), plastic, and more recently alginate (from seaweed).
The growth in artificial casings 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 is found in the cell walls of brown algae which is a large group of multicellular algae, including many seaweeds located in colder Northern Hemisphere waters. An important feature of alginate is its ability to hold many times its own weight in water, making it a naturally gelling substance.
As a film-forming natural polymer, 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 required in 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.
The brine solution is stored in a brine tank, and spent brine is recycled to this same tank. Consequently, the brine is constantly diluted by the moisture that is removed from the casing gel. It is therefore necessary to monitor the salt solution so that the correct amounts of salt can be added to the brine solution. This function is performed by the Vaisala refractometer which monitors the salt content in real-time.
There are two possible locations for the refractometers; directly in the brine tank itself, and a second refractometer can be installed in the salt supply tank.
The Vaisala refractometer measures 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; thereby protecting product quality and reducing downtime. In contrast with many other liquid concentration methods, the Vaisala refractometer is said to be accurate and reliable and needs no regular maintenance. In addition, they are not affected by particles, bubbles, crystals or colour, so they can be employed in a wide variety of solutions for measuring liquid concentration. The Vaisala K-PATENTS refractometers are also 3-A Sanitary Standards and EHEDG certified, which is essential for food processing equipment.
The refractometers produce mA and Ethernet output signals that allow automatic operation of the process. Moreover, the refractometers can be calibrated to read the concentration of NaOH in g/L, wt-% or any other engineering unit preferred by the factory.
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.
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 K-Patents refractometer helps to keep the moisture at the specified level, ensuring a standardised end product.”
Vaisala refractometers are also used in cellulose sausage casing processes. In this application a cellulose fibre cloth is used to create the sausage casing, but first the cloth is desulphurised by passing it through a sodium hydroxide (NaOH) bath. Also known as caustic soda, this solution is supplied from a tank, and spent caustic is returned to this tank. Consequently, the NaOH concentration needs to be replenished because caustic is lost in the cloth during the impregnation process. A Vaisala refractometer is therefore employed, in a similar manner to the alginate and collagen applications above, to continuously monitor (in this case) the NaOH concentration and ensure accurate replenishment.