Fat gets a bad rap, but that’s mainly because it’s so easy to get nowadays. Fat used to be a scarce delicacy. Our caveman brains and the cave-canine instincts of domesticated dogs crave fat because it’s rich in energy and tastes delicious. However, as in our diets, too much fat in an extruder can cause problems.
Pet food formulators make tradeoffs between increasing fat content to improve energy density and palatability while maintaining stable extrusion processing. Whole soybeans bring their own internal fat to the extrusion party along with fiber, which may help manufacturers overcome lipid limitations.
“Pet food does not exceed 200 g/kg fat due to lubrication and related processing and product quality issues; however, increased fat is needed for premium pet foods,” Kansas State University researchers wrote in the journal Animal Feed. “By utilizing whole soybeans in the formula in exchange for liquid fat, the pet food industry might be able to increase the energy density and palatability of kibbles, while keeping cost of processing low.”
Researchers compared internal fat supplied by whole soybeans with external fat added as liquid soybean oil during the extrusion of dry dog food. The internal fat from whole soybeans produced more stable extrusion conditions and more consistent kibble characteristics than liquid soybean oil. This may allow manufacturers to maintain similar product density with higher fat levels.
The study evaluated two fat levels and three fat inclusion methods in a 2 × 3 factorial experimental design. Researchers produced six dry dog food formulations using a single screw extruder. Treatments included diets with no additional fat, internal fat supplied by whole soybeans, and external fat supplied by soybean oil added in the preconditioner. Both low-fat (72 grams per kilogram) and high-fat (80 grams per kilogram) formulations were tested.
Fat greases the machine too much
Dry pet food manufacturers commonly limit fat addition during extrusion because liquid fats act as lubricants inside the extruder. This reduces mechanical energy transfer, limits starch gelatinization and can negatively affect expansion and product consistency. Excessive post-extrusion fat coating also presents challenges because surface oil can migrate into packaging during storage.
The researchers found that formulations containing externally added soybean oil required the highest extruder screw speeds to achieve the target bulk density. Average screw speeds reached 404 revolutions per minute for external fat treatments, compared with 351 revolutions per minute for whole soybean treatments and 309 revolutions per minute for formulations without added fat.
Straight soybean oil also introduced processing instability. External fat treatments experienced extruder surging and produced greater variation in kibble dimensions than either the whole soybean or no-fat treatments. Although operators were able to maintain similar bulk density by adjusting screw speed, the soybean oil treatments exhibited greater sectional expansion and shorter kibble length, indicating different expansion behavior during processing.
In contrast, whole soybean inclusion produced processing conditions that more closely resembled the control formulations without added fat. According to the researchers, internal fat contained within the soybean structure appeared to have less of a lubricating effect than liquid oil introduced directly into the extrusion system.
The researchers noted that including whole soybeans also increased dietary fiber while slightly reducing crude protein compared with soybean meal-based formulations. They said that those compositional differences could also have contributed to some processing effects and should be considered when interpreting the results.
