Automatic Moisture Control For Perfect Pet Food
The strict quality control standards imposed on extruded pet foods means that a precise and continuous drying process should be applied, to achieve low moisture, long shelf life products.
Flavoured coatings are also commonly applied after extrusion, so an extra drying step and cooling should be added. Because cereal-based raw materials are still produced outside, where they are susceptible to a variety of environmental conditions, their water content on arrival at the processing stage also varies considerably.
Adding steam, oils or fats and other liquid additives to a processed product that retains a relatively high degree of moisture leaves the finished product susceptible to environmental factors (e.g. oxidation through contact with air, contamination with moulds, bacteria) that can be lessened or eliminated by removing water.
Increased product shelf life, reduced transport weight/volume, improved product characteristics (e.g. flow, colour) and improved grinding are all achieved through effective drying.
Belt Driers for Pet Food
The use of belt driers is a popular choice for the pet food industry, where dry extruded products made from fish, meat and grains are commonly produced. Water that is added during the pre-extrusion mixing process needs to be removed from the final product in order to achieve the desired consistency and increase shelf life.
In contrast to many livestock feeds, pet food may be stored for up to two years before use, and the shelf life needs to reflect this. Furthermore, fat and oil based coatings are often added to pet food post-extrusion as flavourings, so an additional drying and/or cooling stage may need to be added.
There are several basic processes available to achieve effective drying. For products with strict quality control standards such as pet foods, belt driers are recommended. In these systems, the drying process is continuous and, for multiple stage devices, it is common for the extrudates to be turned during drying. Some manufacturers have developed a petfood-specific system, which combines flexibility with efficiency, drying and cooling in a “2 in 1” system.
A model of the system is shown in Figure 1.
A two-stage drying process allows cooling for a coating step, if necessary. Cooling before coating is essential so that the fat that is added to the surface of the hot, dried extrudate should soak in quickly, then during cooling it stiffens.
Save energy- recirculate
One of the belt drying system’s particular features is the improved energy efficiency afforded by recovering heat using an air-recirculation system.
By mixing warm air with fresh air and heating this mixture, less energy is wasted on heating cold, fresh air.
Heat control within the system requires that the temperature is known in at least two sites within the dryer.
A basic example of this concept is illustrated in Figure 2.
Control the drying process
Controlling the drying process is vital for maintaining high uniformity and guaranteeing product quality. Furthermore, measuring and controlling the temperature within the drier, as the drying process continues, removes the “black box” problems and thus offers more transparency to the system. Different input moistures of products entering the drier (depending on the processing method used and the moisture content of the raw materials) translate into different moisture levels of the product at the drier outlet. This is not an ideal situation, as it leads to differing product quality and increased energy consumption.
By measuring the moisture content of the product within the drier and adjusting retention time accordingly, the whole drying process becomes more controllable and product quality can be regulated.
The time-domain-reflectometry (TDR) technology used in driers is based on a dielectric measuring method. The running time of electromagnetic impulses for measuring the dielectric coefficients and the water content are defined. The system consists of a pyrometer with metallic conductors, which must have good contact with the product to be measured and an aluminium die cast housing with integrated TDR measuring converter.
A high frequency TDR impulse generated in the measuring converter (1GHz) runs along the wave conductors and builds an electromagnetic field around these conductors and thus in the material around the probe. A patented measuring method has succeeded in measuring the running time of this impulse with a resolution of 10 picoseconds. Hence continuous, rapid measurement can be achieved. The determined moisture content is transmitted to the computerised operating system via an analogue output. The system has a repeating accuracy of 99.7%. Absolute accuracies of 99.0- 99.7% can be achieved.
Compared to officially calibrated capacitive and resistive measuring devices, these had a relatively high repeating accuracy of partly 99.8%. For absolute accuracy, considerably higher deviations were discovered, which amounted not rarely to 2% for almost all kinds of grain in the lower measuring range and up to 5% in the upper measuring range. Further practical advantages of this system are its large representative measuring capacity (approximately 2 litres) and wide measuring range of between 1 and 90% water content. Temporal and spatial resolution have been maximised, and the system is designed to experience minimal interference from factors such as mineral content (salinity) and product temperature.
Economic Advantages
The advantages of the new technology are varied and far-reaching, even with a low level of investment in technology.
The drier operates more economically, because of the following factors:
• Higher process transparency
• Higher process safety
• Lower losses of the product value caused by over- or under-drying
• Higher throughput rate due to the process being optimised
• Reduced energy consumption
• Minimised personnel costs.
The whole process leads to increased and more homogeneous dried product for the consumer, and the environmental costs are reduced through more efficient use of energy resources.
