Page 182 - Small Animal Clinical Nutrition 5th Edition

184 Small Animal Clinical Nutrition

growth is no longer possible (Bush, 1993). Table 8-10 describes
Table 8-10. Relationship between water activity and
VetBooks.ir microbial growth.* the effect of a upon microbial growth.
w
Water activity, not moisture content, becomes more impor-
Water
activity (a ) Phenomenon tant in semi-moist pet foods. Most semi-moist pet foods have
w high moisture contents (>30%), but relatively low a values
0.90 Lower limit for general growth of bacteria w
(e.g., Salmonella, Clostridium and Lactobacillus because added humectants bind free water making it unavail-
species) able for microbial growth. Early preservation techniques such
0.80 Lower limit for most enzymatic activity and as salting or adding sugar lowered the a value of meats and,
growth of most fungi w
0.60 Lower limit for osmophilic/xerophilic yeast therefore, increased the shelf life stability of those foods.
and fungi Common pet food humectants include: 1) high-fructose corn
0.55 DNA becomes disordered, no growth possible syrup, 2) salt, 3) propylene glycol, 4) glycerol, 5) sorbitol and 6)
*Adapted from Bush A. Encyclopaedia of Food Science, Food
Technology and Nutrition, vol. 3. London, UK: Academic Press other polyols. Adding large amounts of humectants results in
Ltd, 1993; 1490. an a value of around 0.70, which will inhibit most microbial
w
growth. In the United States, the Food and Drug
Administration has removed propylene glycol from GRAS
(generally recognized as safe) status for use in cats (Christopher
et al, 1989) because of its potential toxic effects. Manufacturers
Box 8-9. Maillard Reactions.
in the United States have responded by removing propylene
glycol in favor of glycerin or polyols. To add a margin of safety,
The Maillard reactions (nonenzymatic browning reaction) between antimycotic agents (e.g., sorbate, propionate salts or both) are
protein and reducing sugars can deteriorate the nutritional quali- added in small amounts to inhibit growth of resistant organ-
ty of foods during processing or storage by affecting the availabil- isms. In addition, acids (phosphoric, hydrochloric, etc.) may be
ity of some amino acids. The reaction occurs when reducing sug- added to lower a food’s pH to maximize antimycotic effects.
ars, such as glucose, fructose, lactose or maltose, combine with
free amino groups found on amino acids such as lysine. Heat Semi-Moist Packaging
accelerates the reaction. Digestive enzymes cannot cleave the
peptide bonds adjacent to an amino acid that has a sugar Exposure to ambient air markedly increases moisture loss in
attached to it. Lysine typically can be made unavailable through semi-moist foods. About 50% loss occurs over a 24-hour peri-
Maillard reactions with the reducing sugars found in most pet od and much of that in the first four hours.Moisture loss affects
foods. Maillard products may also increase microbial degradation product plasticity, and therefore palatability, creating a hard,
of taurine in the large intestine. Amino acid and sugar Maillard crystalline structure considerably tougher than that of dry prod-
reaction products are generally not absorbed or if they are ucts. Barrier packaging retains moisture that is critical to main-
absorbed they are typically excreted in the urine and are of no taining shelf life.
nutritional value to the animal. On the other hand, controlled Most feline semi-moist products are packaged in a
Maillard reactions can be used during the cooking process to pro- polypropylene inner layer bound to an aluminum foil layer that
duce desirable flavors, colors or aromas. inhibits diffusion of water molecules. Most manufacturers use
In addition to Maillard reactions, heat and alkaline pH may
result in unusual cross-linking of certain amino acids and peptides this type of inner layer regardless of the type of outer packag-
that are not normally found naturally. Examples such as lysinoala- ing (pouch, canister, etc.). Canine products usually contain less
nine (lysine linked with alanine), lanthionine (cysteine linked with moisture (around 25%) but contain large amounts of sugars to
alanine) or ornithoalanine (ornithine linked with alanine) may be control the water activity. Because the a value and moisture
w
formed.These compounds are not used well by animals, thus they are lower in these products, the packaging can be less stringent.
reduce the protein quality of the food. This type of amino acid Nonpermeable polypropylene pouches without the foil mem-
cross-linking may occur during the processing of dried meat brane are often used.
meals, ingredients often used in pet foods. However, Maillard
binding of amino acids only becomes a practical nutritional prob- Principles of Canning
lem when the amino acid is limiting. When lysine is abundant, the The Frenchman Nicholas Appert invented canning as a
protein quality is not significantly degraded.
method of food preservation in 1810 in response to the French
government’s need for preserved foods to support its military
campaigns (Lopez, 1987). Later work by Louis Pasteur showed
using the following formula: the relationship between processing and the reduction of food
a = % Equilibrium Relative Humidity ÷ 100. spoilage. In the 1920s, the toxic effects of mesophilic bacteria
w
such as Clostridium botulinum (anaerobic spore-former) were
The value for a w is a number between 0.0 and 1.0 and is documented and the importance of controlling these species
obtained using a glass jar containing the product and a humid- with heat and pH was first understood.
ity probe. The product is allowed to equilibrate in varying lev- Canning is a time/temperature-dependent process that can
els of humidity, which are then measured and the a calculat- be adjusted to create different textural results with the ultimate
w
ed. Each strain of mold or bacteria has an a value at which goal being to achieve commercial sterility. Lower temperatures
w