There are many important processes in petfood production and weighing or ensuring the formula is correct is a big one.
The list of nutrients on a finished bag are minimums or maximums and usually guaranteed. The function of the raw materials area is to prepare the formula by weight with an assumed coefficient of variance, a plus or minus figure within a range. Adjustments are made to ensure correct levels of each ingredient are added, usually by computer control. The mixes can be checked with an NIR or other methods to guarantee nutrient levels are met. Micro ingredients also a part of the mix and usually added with an exact system due to costly ingredients added at a low level. Calibration of the scales and system periodically are recommended so as to maintain accuracy. Also, downstream equipment such as conveyors, holding bins, hammermills sifters etc. all play a part is getting the formula to the cooking device. Good designs in these areas will promote movement of the mix forward with low levels of mix being lost in the process.
Dependent on the mixture the system might have more than one weight scale allowing for increased weighing speeds and accuracy. Ingredient usage varies thus bin sizes and discharge devices including screw conveyors are sized based on the ingredient usage per batch. It is not unusual to see two speed screw conveyors used in this area to move volumes of materials at the higher speeds then slow down for the final correct weight.
Bacteria in petfoods is now regulated and the idea is to have a salmonella free kibble delivered to the pet owners. I believe this is easier said than done, consider the overall plant design needs to take this into account. Mainly the raw materials must be assumed to be contaminated and thus handled accordingly. Separate areas for raw materials and processing. No dust allowed into the production area and this is handled by plant design with walls and zone-controlled air pressures and movement patterns. If this is managed and procedures are in place the mix can be processed by high temperature short time extrusion or other methods to achieve the kill temperature for salmonella, 77oC. Dr Fung outlined this as well as other contaminates where 125oC is another critical control point. At 125oC all bacteria spores and other questionable elements are eliminated. Properly placed sensors can control the flow and detect the temperature exiting the conditioning cylinder, 77oC minimum. The same is true for the end or the extruder barrel but at 125oC. Again, the clean product exiting the extruder needs to be protected from reinfection. Conveying system and downstream equipment is designed to achieve this with care and properly outlined good manufacturing practices for sanitation and continuation of air movement control and wall placement.
Soft bites or what is known as semi moist, soft moist or other names of higher moisture kibbles are unique in texture and can contain elevated moistures. Typically, these are made via extrusion cooking and formulated to be cooled to maximize moisture retention. Thus, precautions must be made to ensure the shelf life of these products sold separately or sometimes mixed in with dry kibble. A liquid ingredient steam is usually added to the conditioning cylinder where it mixes with the dry ingredients. This mixture has ingredients which control bacteria and water activity. An acid to lower the pH, a water loving ingredient to hold or bind the water and a mold inhibitor. An example would be phosphoric acid, propylene glycol and potassium sorbate respectively. Lowering the pH protects against bacterial attack, a water loving ingredient by design allows for elevated water levels and the mold inhibitor protects against mold development.
Water activity is also a subject of great importance, method of determining if the water in the final product will allow bacterial growth. I recently attended the FIGAP Feed production School in Guadalajara Mexico and Rob Strathman of Famsun gave an excellent presentation on this subject. Bacteria cannot grow at reduced moisture levels thus the measurement of the Aw to effectively control the growth environment, the kibble. Typically, at an Aw of .70 at 25oC this is the upper limit to control microorganism growth. A target level of .63 to .65 is usual in a quality assurance program. Bacteria can multiply at a rapid rate, doubling every 30 minutes a single cell can reach 1 billion in 18 hours. An overall approach is needed at the factory level to have a winning program. Evaluation of incoming ingredients, both dry and liquid as well as environmental effects such as weather, trucks and personnel backed by a Certified Safety Program greatly assist in control of bacteria growth hot spots etc. All is needed is the organism, organic matter, time and water and you will have bacteria growth. Assume it is present and plan for its elimination and control.
Article Contributed by Joe Kearns, Editor, International Petfood Magazine.