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Protein in Powdered Milk Replacers


Dietary considerations


Protein is one of the most important nutrients for overall health and well-being. Protein forms part of all cells in the body – and are essential for many functions, including structure, function, and regulation of the body’s tissues and organs. The percentages and amounts of proteins vary substantially by species. Thus, it is absolutely essential that the caregiver become familiar with the natural history and milk composition analysis of the species.


Protein is necessary to create new cells, and therefore it is important for growth. Proteins provide the body with structural framework, fluid balance, and the proper pH. Proteins coordinate bodily functions and allow metabolic reactions to occur. Protein also strengthens the immune system, transports and stores nutrients, and, if needed, can serve as an energy source. Protein is also important to maintain and repair cells.


Proteins have many functions within cells. They are diverse molecules with different sizes and structures. The functions of the individual proteins are varied and complex. Structural proteins, for example, include bone and cartilage. Enzymes, another type of protein, can cause biochemical reactions in cells. Some proteins are involved with intracellular communications, while others can change their shape and activity in response to metabolic signals or messages from outside cells.


Casein and whey


Casein and whey are the major proteins in milks. Since most milk replacer products are made from cows, let’s first look at the proteins in cow’s milk.  Casein is approximately 80% the total protein from cow’s milk, and whey protein is about 20%. Casein is a complete protein source has all of the essential amino acids necessary for growth and cellular repair. Casein digests slowly to feed the cells over a longer period, almost like a “time-release” protein. Whey digests more quickly and easily. Whey also contains other beneficial nutrients not present in casein.  


"Curds and whey"


Casein and whey are byproducts of cheese production, commonly referred to as "curds and whey." Special enzymes or acids added to heated milk during the cheese-making process cause the casein to change to a semi-solid state and separate from the liquid. This remaining liquid contains the whey protein, which is washed and then dried into a powdered form for use in food and dietary supplements. The remaining casein, in curd form, can also be washed and dried to create protein powder or added to other dairy products. Thus, both casein and whey have undergone several separation and processing steps to become dry powders. Manufacturers of animal milk replacer products combine these dry forms of casein and whey proteins with other ingredients to create a product that is sold to feed target species, mostly in the commercial livestock and companion animal industries. The chart below indicates the various sources of proteins used in the formulation of some of the various milk replacer products available from Fox Valley Animal Nutrition and PetAg®:

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The next table below shows the proximate analysis (typical values) for the various types of dried dairy products used in the milk replacer powders. As shown, the concentration of protein can vary significantly based on the classification of the product. For example, "Milk Protein Concentrate" can range anywhere from 40-85%. Another example, as shown, is Whey Protein Concentrate, that can range from 34-80% protein. It is impossible for the consumer to know the exact amount of protein originating from these two ingredients without knowing the exact formulation used by the manufacturer.

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Powdered milk products

Dried casein and whey powders may be very digestible if rehydrated and reconstituted properly. However, the heat and acids used in the processes to dry casein, soy protein isolate, and some other proteins have been found to decrease protein digestibility. Also, some ingredients added to food are based on less digestible protein fractions, insoluble fibers, and products and are responsible for poor digestibility of proteins. For example, components from soybeans and some grain legumes can reduce digestion of amino acids and proteins by up to 50% in some species. This is mentioned to show how manufacturing processes and other ingredients added to food can affect digestibility - whether added by the manufacturer or by people preparing the formula.


Milk replacer analysis


When casein and whey powders have been prepared in a manner that provides for effective rehydration and reconstitution, the protein in the resulting milk formulas are very digestible (96%+) in young rats. Skim milk powder tested in the same study was 92%+ digestible, while (descriptions of tests and results on the rehydration and reconstitution of 6 milk replacer powders prepared to feed wild mammals in rehabilitation are in progress). 


Some manufacturers may combine the dry ingredients (e.g., proteins, fats, carbohydrates, minerals) and then package directly (such as those available from Fox Valley Animal Nutrition). Other manufacturers, such as PetAg®, may combine both liquid and dry ingredients with water, and then dry that homogeneous wet solution into a powder. The mixing and drying methods used by the consumer may also affect the nutrition, shelf life, storage, mixing method, solubility, and more.


All of the milk replacer products have a Guaranteed Analysis (GA) on the package label that discloses a minimum value for protein (as a percent of total weight) that is contained in the product. Many of the products have incorporated this minimum value in the actual product name (e.g. Fox Valley 20/50 or Zoologic 32/40), with the first of the two numbers indicating the minimum guaranteed percent value for protein. The first chart below shows both the GA value for each product (the black dash), as well as the range of lab test values, with the yellow-center dot indicating average values for the lots tested. Each dot is also accompanied by a range that shows high and low values that were reported from the lab tests.  (The ranges shaded in pink are for Fox Valley products, while the blue shaded ranges indicate PetAg® products.)


The next chart below shows the variance from the GA for protein in percentage terms over the period of years that tests were conducted. As shown, most lots tested at or just above the GA for protein.

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References and further reading (not intended as an exhaustive list)


Davoodi, Seyed, et al. 2016. Health related Aspects of Milk Proteins. Iranian Journal of Pharmaceutical Research. V. 15(3) pp. 573-591.


Grant, C. et al. 2005. Randomized, double-blind comparison of growth in infants receiving goat milk formula versus cow milk infant formula. Journal of Paediatrics and Child Health. V. 


Hageman, Jeske, et al. Comparison of bovine milk fat and vegetable fat for infant formula: implications for infant health. International Dairy Journal. V. 92, May, pp. 37-49.


Hamosh, M. et al. 1999. Protective function of human milk: the milk fat globule. Seminars in Perinatology. V. 23(3), pp. 242-9.


Lonnerdal. 2014. Infant formula and infant nutrition: bioactive proteins of human milk and implications for composition of infant formulas. The American Journal of Clinical Nutrition. V 99(3), pp. 713S-717S.


Martin, Camilla, et al. 2016. Review of Infant Feeding: Key Features of Breast Milk and Infant Formula. Nutrients. 8(5), pps. 


Newmark, Lauren Milligan. 2015.  Milk Protein Comparison Unveils Nutritional Gems for Human Infants. Splash: Milk Science Update. International Milk Genomic Consortium. 


Protein Function. 2004. Nature Publishing Group.


Rudloff, S and B Lonnerdal. 1992. Solubility and digestibility of milk proteins in infant formulas exposed to different heat treatments. Journal of Pediatric Gastroenterology and Nutrition. V. 15(1), pp. 25-33.


Rutherford, S.M. et al. 2006. True Ileal Amino Acid Digestibility of Goat and Cow Milk Infant Formulas. Journal of Dairy Science. V 89(7) pp. 2408-2413.


Sawar, Gilani. 2003. Protein Digestibility and Quality in Products Containing Antinutritional Factors are Adversely Affected by Old Age in Rats. The Journal of Nutrition. V. 133(1), pp. 220-25. 


Sarwar, Gilani, et al. 2005. Effects of Antinutritional Factors on Protein Digestibility and Amino Acid Availability in Foods. Journal of AOC International. V 88 (3), pp. 967-987.


Skibiel, Amy, et al. 2013. The Evolution of Nutrient Composition of Mammalian Milks. Journal of Animal Ecology. V.82, pp. 1254-1264.


Van De Walle, Gavin. 2018. 9 Important Functions of Protein in Your Body. Healthline. 


U.S. National Library of Medicine. What are proteins and what do they do?


World Health Organization. 2009. Infant and Young Child Feeding and Recommended Practices: 


Zhou, Shao et al. 2014. Nutritional adequacy of goat milk infant formulas for term infants in double-blind randomized controlled trial. British Journal of Nutrition. V 11(9), pp. 1641-1651. 

The information included on this website for the Proximate Analysis components (Proteins, fats, carbohydrates, fiber and moisture content) is extremely narrow in its scope and nature. It is limited to certain charts and graphs displaying content values (% of total) of various powdered milk replacers as tested by an independent chemical lab. Extremely brief overview information is provided as to the primary nutritional and medical benefits of each component, as well as, in some cases, a limited discussion of issues that may arise from concentration levels in the body that may be considered deficit or toxic. Entire textbooks on these primary dietary components are written for the medical and veterinary professions, in addition to the internet providing ready access to both scholarly and popular literature. Some of those references are included above.


The data values presented above only represent the test values for the presence and concentration of the component conducted according to standard chemical testing methods in a controlled laboratory setting. Any point test value is accompanied by a measurement uncertainty range as indicated in the charts. The concentration values are in no measure an indication of how much of the component may be provided to an animal in reconstituted formula or its bioavailability (its degree of digestibility, absorption, or ultimate utilization). Additionally, no testing was performed as to the source of the component in the product (such as inorganic salts) or the grade of any added supplements containing the component.


What the data can do is inform the reader as to (1) concentration levels in a product as most recently tested (2) changes over time and between lots, and (3) comparisons of relative concentration levels between products. It is merely data that may serve as a starting point when deciding on a milk replacer product(s) and a recipe, or information to consider if certain medical symptoms appear that could be a result of absence or excess of a specific component in the formula. The reader is encouraged to consult veterinary or nutritional professionals prior to providing additional supplementation of any component.

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