Published November 04, 2011 13:32
One of the critical processing steps in the production of extruded, dry pet food is cooling. The cooling step is required after the drying and liquid-coating processes to remove excess heat. If enough heat is not removed, condensation can occur in the product storage bins or in the product package. This article will discuss the major change in dry pet food that has prompted a change in the method that pet food needs to be cooled. The article will also review horizontal and vertical cooler designs, typical processing conditions, and how much cooling is required.
Changes in Pet Food
In the last 10 to 15 years, product changes have caused the cooling process to be redesigned in most pet food production facilities. In the early stages of pet food production, specifically dog food, fat addition levels applied at the coating process were usually less than 10%. The cooling step was normally done before the coating process, as the typical plant flow was extrusion, drying, cooling, coating, storage and packaging. The pre-cooled product would satisfactorily absorb the fat added in the coating system, and the additional heat added from the fat was usually not sufficient to cause storage problems.
As producers began to make dog food with higher levels of added fat, problems were encountered in the coating process. The cool pet food would not absorb the high levels of fat, resulting in pet food with large amount of free fat on the external surface. It was quickly discovered that if the pet food was coated while it was warm, it would absorb the fat much more effectively. The major problem this created was that most plants were not designed to cool the product after the coating system. Since that time, some plants have been modified to incorporate the cooling process after the coating process. The majority of the new plants being built today are designed to cool the product after the coating process.
Types of Coolers
Most pet food production systems built prior to 1990 commonly incorporated a horizontal cooler (Figure 1) between the drying and the coating process. These coolers were normally an extension of the dryer, which is often referred to as a combination, horizontal dryer-cooler. Although horizontal coolers can be arranged as stand-alone machines, it is more common to see them included as an integral part of the dryer. Incorporating the cooler into the main body of the dryer is a very cost-effective method because the capital investment is reduced compared to a stand-alone unit. In addition, the heated air recovered from the cooling process can be used as make-up air in the dryer, making the overall operation quite efficient.
The general design of a horizontal cooler consists of a perforated conveyor on which the pet food is uniformly distributed up to depths of 4 to 12 inches. The conveyor is sized with sufficient length and width to provide ample retention time to cool the product. Ambient or chilled air is pulled through the product bed with a centrifugal fan to remove the heat.