by Dr. Karen Becker
According to the Animal Nutrition Group at Wageningen University in the Netherlands, studies show drying pet food at 160°C (320°F) to 180°C (356°F) can significantly reduce its nutritional value.In the smaller size kibble used in the study (4 mm or about .16 inch), a drying temperature of 200°C (392°F) lowered concentrations of the following amino acids:
- Proline
- Total lysine
- Reactive lysine (lysine that has remained unchanged during processing)
The same size kibble dried at 120°C (248°F) had a higher ratio of reactive to total lysine than the kibble dried at the higher temperature.
The drying temp of 200°C also decreased concentrations of linolenic (omega-3) and linoleic (omega-6) fatty acids, and increased the concentration of oleic acid (omega-9 monounsaturated). The increase in oleic acid may point to lipid oxidation of the smaller kibbles during the drying process. Lipid oxidation can create off-flavors and aromas, as well as potentially toxic compounds.
Larger size kibble (8 mm or .31 inch) showed only a lowered concentration of reactive lysine, according to study results.
Dr. Becker's Comments:
The news that processed dry pet food is lacking in nutrients will be no surprise to regular readers of my newsletter.
But it’s good to see mainstream sites like PetfoodIndustry.com report on research that provides pet owners with a better understanding of the quality of food they feed to their beloved companion animals.
And it’s especially encouraging to see some of the inherent problems in feeding dry processed pet food brought to light.
The Extrusion Process
Extrusion is a process that has been used by the pet food industry for over 50 years. About 95 percent of dry pet diets are manufactured using the extrusion process.
Batches of dog or cat food ingredients are mixed, sheared and heated under high pressure, forced through a spiral shaped screw and then through the die of the extruder machine. There are two types of extruders, single screw and twin-screw.
Extrudate is the result -- a ribbon-like product that is then knife-cut and dried.
The high temperature used in extrusion (nearly 400°F) and the short time frame to process (under five minutes) create continuous chemical and physical alterations to the ingredient mixture. These changes include:
- Starch gelatinization
- Inactivation of nutritionally active factors
- Protein denaturation
- Vitamin loss
When it comes to the ingredients used in dry dog foods, which are often of low quality to begin with, the first two items on the list, despite what you may think, are actually desirable changes. These changes are necessary to form the physical characteristics of kibble and make it more digestible and assimilable.
The other two items, vitamin loss and certain types of protein denaturation, are undesirable results of the extrusion process.
While the article linked above primarily addresses the effects of the drying process that follows extrusion, let’s take a closer look at the four changes that occur during the extrusion process itself.
Starch Gelatinization
As I often point out in my videos and the newsletter, all dry pet foods contain starch, because starch is essential to the formation of kibble.
When pet food ingredients are exposed to heat and moisture during extrusion, the starch in the mixture gelatinizes – it melts. This helps bind the kibble and also causes expansion of the product after it travels through the die.
A high starch content of 30 to 40 percent of the ingredient mixture decreases the density and therefore the weight of the end product. Dry cat and puppy foods normally contain about 30 percent starch, and 40 percent is typical in dog foods.
Those starch content percentages are average. However, some dry pet food formulas can contain twice that amount. The starch is derived primarily from cereal grains, and as most of you are aware, grains are not biologically appropriate nutrition for dogs or cats.
Interestingly, the extrusion process is thought to lessen the biological inappropriateness of the grain content in dry pet food formulas. According to the Animal Nutrition Group’s report linked above:
One of the challenges when using cereals in canine diets is the presence of anti-nutritional factors that are harmful for dogs. Study on NAF [nutritionally active factors] in canine diets show that extrusion cooking inactivates NAF activity especially those of a proteinaceous structure (Purushotham et al., 2007).
I’ll discuss more about nutritionally active factors shortly.
Another starch-related fact mentioned in the report is that extrusion conditions and the type of starch used can affect glucose and insulin response in dogs after a meal of dry food.
Extruded rice causes a higher glucose and insulin response than other extruded starches like barley, corn, wheat and sorghum. Since rice is considered by many pet owners to be a healthier grain than others, this is good information for those of you who have a pet with diabetes or an overweight dog or cat at risk for the disease.
Inactivation of Nutritionally Active Factors
The ingredients used in dry pet food mixtures, in particular grain legumes, contain undesirable nutritionally active factors (NAFs) that interfere with digestion and absorption of nutrients.
An example of nutritionally active factors is trypsin inhibitors, also called protese inhibitors. These toxins are most often associated with soy products. Trypsin inhibitors are chemicals that reduce the availability of trypsin, an enzyme crucial for digestion.
The good news is the extrusion process used in the manufacture of dry pet food inactivates undesirable NAFs in the ingredient mixture. It also reduces the activity of naturally occurring toxins like allergens, glycoalkaloids and mycotoxins present in grains prior to extrusion. Unfortunately, the extrusion process doesn’t entirely eliminate the activity of these toxic substances. Mycotoxins are still a big risk in dry pet foods even after the manufacturing process is complete.
Since the presence of NAFs limits the amount of grain used in pet food ingredient mixes, if the extrusion process deactivates the NAFs, then the not-so-good news is that even higher percentages of grain could be added to dry food formulas. Grain is much cheaper than biologically appropriate ingredients like meat and vegetables.
Protein Denaturation
The protein sources used in dry pet food formulas are often a combination of animal and vegetable, because less costly vegetable protein doesn’t contain amino acids sufficient for the nutritional needs of carnivorous dogs and cats.
And speaking of amino acids, they don’t fare well during extrusion. In fact, a recent study cited in the Animal Nutrition Group report,
“… observed a large overestimation of the available lysine content such that the amino acid pattern relative to lysine in these diets may not be optimal to promote health. In addition to lysine, other amino acids such as arginine, tryptophan, cysteine and histidine can also be affected by the extrusion process. Of particular importance may be the sulphur amino acids (cysteine and methionine) which are often limiting in diets for cats as these amino acids are susceptible to oxidation.”
But back to the topic of protein denaturation, according to the Animal Nutrition Group report:
Mild denaturation of proteins can make them more susceptible to digestive enzymes and, therefore, improve the digestibility of these proteins (Hendriks and Sritharan, 2002).
Denaturation takes place during the extrusion process, and often prior in the case of animal proteins, which are heated after grinding to a target temperature before being added to the ingredient mixture.
Denaturation modifies the structure of protein. In the case of plant-based proteins like soy and corn, denaturation makes these biologically inappropriate foods easier for pets to digest. However, denaturation is only beneficial to meat-based proteins if the protein sources are substandard, which of course they are in the vast majority of popular dry dog and cat food diets. Rendered meat by-products are a common protein source in dry kibble, and they are indeed difficult for dogs and cats to digest and assimilate.
Unfortunately, denaturation of high quality, lean, whole cuts of meat used in superior quality dry pet foods also occurs. As you might guess, denaturation of biologically appropriate protein has the opposite effect of what is achieved with grain-based and low-grade animal meat. Denaturation makes these once healthy proteins more difficult for your dog or cat to digest and assimilate.
The change in the structure of healthy protein that occurs during exposure to high heat is a possible trigger for food allergies. Research shows the immune system may not recognize the altered protein structure and treats it as a foreign invader. This may explain why pets allergic to a particular meat-based dry food oftentimes have no problem eating that same meat in whole, raw form.
According to the report:
Protein digestibility of extrudates was increased compared to nonextruded samples (Peri et al., 1983; Bhattacharya and Hanna, 1985; Fapojuwo et al., 1987). These studies were, however, conducted on feeds composed from vegetable sources only.
Enzymes like lipoxygenase and peroxidase present in pet food ingredient mixes are inactivated during extrusion cooking. Since these enzymes can cause deterioration of product and shorten shelf life, their destruction is considered by pet food manufacturers to be a beneficial effect of the extrusion process.
Vitamin Loss
According to the Animal Nutrition Group report, the extrusion process primarily destroys vitamin A, vitamin E and the B-group vitamins in dry food ingredient mixtures. No data on vitamins D or K was available for the report.
The percentage of vitamin loss during extrusion varies widely, from a low of 4 percent loss of thiamin to a high of 65 percent loss of vitamin A.
Keep in mind that B-group vitamins are water soluble, meaning your pet’s body can’t store them – they must be provided daily through diet.
If You’re Ready to Make the Switch to Healthier Food for Your Pet …
I hope this information has made you decide to toss out that bag of dry pet food and never buy another for your precious dog or cat.
If you’re still on the fence, I encourage you to watch my Pets, Protein, Dry Food and Disease video.
If you’re ready to make the switch, there is a wealth of information right here at MercolaHealthyPets.com that will give you all the help and encouragement you need to upgrade your pet’s diet.
Whether you need to break your kitty’s addiction to dry food or you’re interested in fixing homemade meals for your pet, you’ll find everything you need right here at HealthyPets.