Temperature in the extruder is what is considered to be the factor that eliminates the bacteria in petfood for safe handling by pet owners. Data shows that the required temperature is 77oC for bacteria elimination with elevated temperatures, 125oC, killing bacteria spores yeast spores and others. Achieving this temperature continuously is done by use of heat, pressure and frictional development.
Preconditioning starts the process by adding water and steam to the mash. Water is added to achieve elevated capacities and for smooth extruder operation. It also increases the thermal conductivity of the mash when steam is added. The water assists in transfer of the heat. The goal is to have the feed temperature elevated to some point prior to the extruder, and the standard range is usually up in the 80 to 95oC range. Temperature measurement is usually at its discharge taken with care as free steam in the cylinder can cause higher reading than actual mash temperatures.
The extruder barrel finishes the job by elevating the temperature to a confirmed level prior to discharge. Historically testing showed salmonella was eliminated under correct extruder operating ranges. Verifying this and knowing exactly when the product is correct or not is the fun part. When starting an extruder and shutting it down the point at which the temperature is correct is at some point after start up and a point before beginning shutdown. Knowing this historically the food was not kept or bagged until the operator said it was good. Either by not sending downstream or by marking the point product was good when entering the dryer. This was done by observation and experience, not confirmed scientifically.
Control systems, temperature measuring devices and extruder barrel components were developed as this process advanced in order to understand and more precisely control the system. The reason was initially the product bypassing the system until it was judged worthy to advance was costly over time. Money is made on product going out the door not rework or waste. Temperature probes were placed in selected extruder and conditioner locations. These reported back the conditions, were they correct? Testing showed where in the extruder the cold spots were thus the critical areas where temperatures needed to be measured. The probes themselves were redesigned until they reported in a reliable fashion.
Devices were developed to control the passage through the extruder to ensure temperatures were reached. The end of the extruder where the heat and pressure are the highest, a slight increase in restriction can greatly elevate the final temperature as the pressure would increase and thus the frictional development creates temperature elevation. Obviously worked in reverse also, open it up some temperature might go down but if still in correct operation range you might slightly increase the capacity. These same devices also have a great effect on density of the product as all of the factors go hand in hand to make an extruded product. When in harmony and all is designed and working as a team so to speak in the extruder is controllable and adjustable to ensure making a clean safe product at high capacities.