Water Activity in Pet Food


Headline: After 30 years of no problems, pet food company has to recall moldy product but claims moisture content didn’t change. If they had only been using water activity instead of moisture content.

Pet food production is one of the fasted growing industries worldwide valued at $91 billion dollars in 2018. Gone are the days of feeding your pet scraps from the table; pet food today is carefully formulated to provide healthy diets, avoid allergies, and provide variety. With the increasing value of pet food comes increased expectations for safety, quality, and consistency. Correspondent with
increased expectations has been more intensive governmental oversight and regulations. When the Food Safety Modernization Act (FSMA) was signed back in 2011, it included recommendations to make regulations on pet food production equivalent with human food production. The last of these updates in regulations went in to place for very small businesses as of September 2018. Meeting
these new FSMA regulations while still making a safe, quality product that is profitable is the current goal and challenge of the pet food industry.


Each microorganism has to maintain and ideal water activity. When a microorganism encounters an environment where the water activity is lower than their internal water activity , they experience osmotic stress and begin to lose water to the environment since water moves from high water activity (energy) to low water activity. This loss of water reduces turgor pressure and retards normal metabolic activity. Notice that an organism’s ability to reproduce and grow does not depend on how much water is in its environment (moisture content), only on the energy of the water (water activity) and whether it can access that water for growth. Consequently, each microorganism has a unique limiting water activity below which they cannot grow. The key to controlling microbial safety and its growth in Pet food is to process it to water activities less than 0.85 aw to stop bacterial growth, 0.70 aw to stop mold growth, and 0.60 aw to stop the growth of all microoganisms. Inbetween these limits you may easily controll the stability of pet food.


Water activity influences reaction rates by reducing activation energy, increasing mobility, and increasing the rate constant. Consequently, reaction rates are better correlated to water activity than moisture content. In general, as water activity increases so do reaction rates, but specific correlations depend on the type of product and the reaction. Most reactions will reach a maximum in the range of 0.70-0.80 aw due to dilution at high water activities, but lipid oxidation is the only reaction that increases at low water activity. The reaction that is most likely to impact the quality of pet food is lipid oxidation or rancidity. This is a complex reaction with multiple possible pathways and requires the presence of lipids (fat), oxygen, and free radicals to occur. Consequently, it is most often controlled through the removal of oxygen by nitrogen flush or the use of oxygen absorbers. Rancidity occurs when lipid oxidation results in the formation of odor compounds that result in a musty smell and taste. For Prevention you need to control a chemical stability in the food. Dry pet food is often sprayed with a fat coating to maintain freshness and improve nutrition, making them particularly susceptible to rancidity. Pets will often reject food that has experienced rancidity, or the owner will discard food that smells rancid.


Changes in water activity can affect the physical stability(structure and texture) of pet food and each product has an ideal water activity range where the texture will be optimal. To maximize shelf life, a product must be manufactured to its ideal water activity range and remain at that water activity during transport and storage. For dry kibbles, water activity is low, and the expected texture is crisp and crunchy but if the water activity increases outside of the ideal range, the kibbles will become soft and undesirable. On the other hand, semi-moist pet treats have higher water activity values and are expected to have a soft and pliable structure. If the water activity decreases outside the ideal range in semi-moist products, they will become hard and undesirable.


Once an ideal water activity specification has been identified, the next challenge will be to consistently produce products at that ideal level. Ideally, production settings such as oven temperature and belt speed could be established and remain the same through each production run, producing product with the same water activity each time. Unfortunately, there are outside factors that necessitate adjustments to production settings. These factors include, but are not limited to, inconsistency in the incoming ingredients and changes in the production environment. An effective solution to avoid problems caused by inconsistent incoming ingredients would be to track the water activity of the ingredients and establish an acceptable range that will produce product meeting specifications with a limited amount of production adjustment. Many pet food manufacturers measure the water activity of their end product, but the idea of using water activity to screen incoming ingredients may be a new, but potentially useful concept.


For pet food, setting an ideal water activity specification is a critical step in formulating for safety, quality, physical and chemical stability. Still today, there are many pet food manufacturers trying to use moisture content to determine the safety of their products and finding moldy product as a result. These failures can be completely avoided by using a water activity specification instead of moisture content. The specification can be set to avoid microbial proliferation, chemical reactions, physical and structural degradation, and moisture migration. The ideal value can be determined based on the most likely mode of failure, such as texture loss for dry products, chemical degradation for semi-moist products, and microbial growth for wet pet food. Once the ideal water activity is determined, a combination of processing and formulation can be used to achieve that ideal water activity. This will ensure a safe, quality product with an optimal shelf life and maximum revenue

" For pet food, setting an ideal water activity specification is a critical step in formulating for safety and quality. The specification can be set to avoid microbial proliferation, chemical reactions, physical and structural degradation, and moisture migration. The ideal value can be determined based on the most likely mode of failure, such as texture loss for dry products, chemical degradation for semi-moist products, and microbial growth for wet pet food. Once the ideal water activity is determined, a combination of processing and formulation can be used to achieve that ideal water activity."

Dr. Brady Carter world-renowned specialist in water activity

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