Water Activity and Pet Foods
PET FOOD AND ANIMAL FEEDS need to be nutritious, safe, and stable at a specified shelf life. Just like human food, pet diet components are susceptible to microbial, chemical, physical, and insect spoilage. Water activity (aw) is one of the most important parameters in preservation, safety, and quality. Water activity is a very practical tool in developing and producing nutritious, safe, and stable pet food because it is critical for microbial growth, texture, flavor, chemical reactivity (such as browning or lipid oxidation), or enzyme activity.
In fact, water activity has been an important parameter in commercial pet foods since the late 1950's. Soon after Scott (1957) showed that microorganisms have a limiting water activity level below which they will not grow, scientists were looking for other practical application of aw. One of the first products fully developed using water activity technology was "Gainesburgers" made by General Foods Corp. Gainesburgers were an intermediate moisture dog food that were promoted as "the canned dog food without the can". Quaker Oats Company followed in the mid-1960's by introducing a shelf-stable, intermediate-moisture marbled meat pet food. Water activity technology provided the means to form a soft, elastic, marbled product resembling meat. This highly successful dog food is reported to have generated more profit per square foot of display shelf space than any other product.
There are a number of products on the market with multi-textured characteristics that use water activity. These products combined a hard, drybaked pet food and a soft, moist pet food. The hard, dry component has the advantage of teeth cleaning, but is less palatable than a soft, moist food. The soft, moist component may be highly palatable, but lacks the abrasive teeth cleaning property. When the two components are mixed, they equilibrate to a common equilibrium water activity during storage. This equilibrium aw must allow the dry component to remain hard and crunchy while leaving the soft component moist and tender.
Mold growth is the form of spoilage most frequently observed in dry pet foods. Preventing growth and toxin production from microorganisms is a goal for manufacturing of stable and safe pet foods. Once the optimal aw is found for a product, extreme temperature and humidity during storage, which will change a product's aw, should be avoided to prevent microbial growth. Water activity, not water content, determines the lower limit of available water for microbial growth. The lowest aw at which the vast majority of food spoilage bacteria will grow is about 0.90, with the exception of Staphylococcus aureus which has a limit at 0.86aw under aerobic conditions. The practical limit for growth of yeast is 0.88 and for molds is 0.70aw, although some xerophilic molds and yeast will grow below 0.70aw. The lower limit for all microbial growth is 0.60aw. Toxins, notably mycotoxins in the case of feeds or raw materials, may be formed during mold growth but not below 0.80aw.
Insects are another significant potential source of product or raw material deterioration and in some cases can be controlled using aw. For example, mite activity is dependent on water activity and temperature. Mites may be active at 5°C above 0.65aw, at 25°C above 0.63aw, and at 40°C above 0.60aw. Many species need a water activity of not less than 0.65 for development. Therefore, maintaining a product or raw material water activity below 0.60 should minimize the risk of mite activity.
The pet food and feed industries have long used water activity to create novel products and predict shelf life, safety, and quality. Determination of water activity during manufacture allows tight control of pet foods and feedstuffs found to be at high risk for deterioration. Without the use of water activity, the pet food industry would have a hard time developing innovative products or producing nutritious, high quality, stable food.