Chicken: flippo; grain: Buriy; beaker: Josie20; food: Daneel | BigStockPhoto.com
While thermal processing of petfoods provides a number of benefits, extensive processing can increase variability, destroy essential nutrients and create unwholesome by-products.
Thermal processing—also known as cooking—of petfoods provides a number of benefits, including convenience, enhanced flavor and texture, improved consistency, pathogen control and decreased spoilage. However, extensive processing can increase variability, destroy essential nutrients and create unwholesome by-products. From a formulator’s perspective, this creates a dilemma regarding how to assure the diet is sufficiently fortified while avoiding excess after accounting for processing effects.
Given we have been sharing our food with dogs and cats since the dawn of civilization some 15,000 years ago, one would think we would have worked out most of the issues. But in comparison to human foods, we have only bits and pieces of information about how the process affects nutrition and variability of petfoods.
Why is this important? By some estimates, more than 300 million dogs and cats around the globe depend on nutritionally complete commercial diets for their daily sustenance. For most pets, this means a food that has undergone extrusion, canning, baking or some other cooking process. Considering that many of the 40-plus required nutrients are reactive or labile under the thermal, pressure and moisture conditions common to cooking, merely accounting for their content prior to processing does not assure proper fortification.
Overlooking these factors has been an issue in our recent past; for example, look at the root causes for nutritionally mediated diseases such as dilated cardiomyopathy in dogs or thiamine deficiency in cats. To do a better job of consistently providing for our pets, we need to gain a better understanding of these processing influences. This will allow us to turn the “art of fortification” into more of a science.
We have only bits and pieces of information about how the process affects nutrition and variability of petfoods.Petfood is unique. Unlike with human foods, many of the starting ingredients used in commercial petfoods have already been extensively processed (e.g., meat and bone meal, tallow, rice bran, etc.), all of the ingredients in the diet are mixed together and then processed under fairly harsh conditions (e.g., extrusion) and the finished product is expected to be shelf-stable for more than a year under ambient conditions.
Human foods are typically singular ingredients or dietary components, only a portion of which are processed, they are eaten a la carte in multiple meals throughout the day and most are refrigerated, frozen or consumed within weeks of production. Livestock feeds use a similar all-in ration approach like petfoods, but the final diet is seldom heat-processed and is commonly fed the same day it’s produced, so shelf life is of little concern. These differences have profound implications on the nutritional value of the final product.
With petfood ingredients, the most prominent processing challenge is a result of the rendering process. This is important because rendered protein meals are found in nearly all dry petfoods. These protein meals, like meat and bone meal and chicken by-product meal, have undergone extensive cooking to reduce moisture and fat. While the intent of the process is to stabilize and homogenize the resulting material, significant nutritional variability exists (Pearl, 2004). The nutrients most affected by the process are essential amino acids such as lysine and the sulfur amino acids methionine and cysteine.
The loss in lysine due to non-enzymatic Maillard browning reaction is well-known; less well-characterized is the 50 to 60% decline in methionine and cysteine utilization reported for these protein meals (Johnson et al., 1998; Hendriks, 2002; Pearl, 2004). The rendering process also affects the viability of fats due to oxidation. If left unpreserved, the oils remaining in rendered protein meals can begin to oxidize within days, resulting in peroxide formation and, ultimately, destruction of essential fatty acids such as linoleic acid (Kirkland and Fuller, 1971).
According to a study by Murray et al. (2001), excessively cooked starch can increase the content of digestively “resistant starch,” which may behave like fiber in the colon.
We know thermal processing of petfoods generally benefits starch utilization. Processes like extrusion and pressure cooking in the retort gelatinize or “cook” the starch and increase its digestibility (Murray et al., 2001; Kienzle, 1993). This serves to improve dietary energy utilization.
With petfood ingredients, the most prominent processing challenge is a result of the rendering process.However, excessively cooked starch can increase the content of digestively “resistant starch,” which may behave like fiber in the colon (Murray et al., 2001; see Figure 1). This is a factor that deserves to be explored more thoroughly. Extensive cooking can also alter fiber utilization, shifting the soluble-to-insoluble fiber ratio and negatively affecting a targeted total dietary fiber contribution to the diet (Dust et al., 2004).
For protein and amino acids, the mechanisms are similar to what has been learned about rendering but with a few twists. With wet petfoods, extensive retort time during the sterilization step can decrease the digestibility of lysine and methionine (Hendriks et al., 1999; see Figure 2 under the More Images tab at top). Most would guess that a similar fate awaits petfood through extrusion; yet there are indications that extrusion by itself may actually improve availability of amino acids like lysine.
But, food exiting the extruder is relatively wet and must be dried to prevent mold growth. The typical petfood dryer uses a large volume of super-heated air to remove moisture, much like a personal hairdryer. Under these extremes in temperature and retention time, lysine bioavailability can be compromised and linoleic acid can be lost, most probably due to oxidation (Tran, 2008). This hot-air baking may also reduce kibble durability and texture, which affects merchandising and palatability.
All vitamins appear to be affected to some degree at every step of the process from production through shelf-storage (see Figure 3 under More Images). The loss in fat-soluble vitamins (A, D, E, K) is the most significant in extruded products, with rates of more than 50% lost before the kibble goes into the bag (Coelho, 2003). In wet foods, the water-soluble B-vitamin thiamine can be almost completely lost due to its reactivity with heat, moisture, sulfites, elevated pH and the thiaminase enzymes found in fish and organ meats.
This is not a comprehensive list, and many effects of thermal processing have yet to be described. From information we have so far, it seems relatively obvious that processing has some significant effects on petfood and pet nutrition. While some process conditions are nutritionally beneficial to animals, in several areas, formulators must compensate with significant fortification to offset processing losses. The big three in order of importance are vitamins, followed by the often overlooked sulfur amino acids and finally essential fatty acids.
In several areas, formulators must compensate with significant fortification to offset processing losses.Today, we support nutritional adequacy by super-fortification before and nutrient analysis after the fact. Generally, this has proven effective, but occasional toxicities and deficiencies resulting in recalls occur. This would suggest that we still need more comprehensive evaluation of the nutritional effects of thermal processing of petfoods with better models to support fortification needs.