Introduction
The first two Notes in this series make one point repeatedly: the ingredient name on a calf milk replacer tag does not tell the whole story. A CMR can contain a large amount of skim milk powder and still fail to form a firm curd. A whey-based formula can be highly digestible and support excellent growth. And two products both labeled “skim milk powder” may have very different functional properties because of how the milk was heated, concentrated, dried, stored, and incorporated into the final formula.
This may be the most important practical lesson of the skim-versus-whey debate. The question is not simply whether a CMR contains skim milk. The question is whether the dairy ingredients are nutritionally and functionally suitable for a young calf.
Heat is necessary – but too much heat changes protein
Milk must be heated during processing for food safety, evaporation, and drying. Those processes inevitably alter proteins to some degree. Whey proteins are particularly heat sensitive. As temperature and heating time increase, whey proteins unfold and can interact with one another and with casein. Severe heating can also increase Maillard reactions between lactose and amino groups, particularly lysine.
These changes are not automatically harmful. Some protein denaturation is unavoidable, and the relationship between heat treatment and calf performance is not perfectly linear. The concern is excessive processing that reduces digestibility, alters amino acid availability, or changes the way the reconstituted powder behaves in the abomasum.
The classic calf experiments
Lister and Emmons (1976) prepared skim milk powders after heating milk for 30 minutes at 60, 73.9, or 85°C. Young calves fed the most severely heated powder had more diarrhea and slower early growth than calves fed the lower-heat powders. Later in the experiment, the growth difference disappeared and nutrient digestibility measured from days 20 to 26 was not affected.
That age response is important. The youngest calf has the least mature digestive system and may be most sensitive to poor ingredient quality. The same ingredient that causes difficulty during the first 2 or 3 weeks of life may be used more effectively by an older calf.
The investigators also connected heat treatment with rennet coagulation. Their conclusion was not merely that “skim milk is good.” It was that severely heat-treated skim milk could be a poor ingredient for very young calves, and that the ability of a reconstituted skim powder to form a curd could provide useful information about its processing history and functionality.
Rennet coagulation: useful, but not a perfect quality test
Emmons and Lister developed a series of laboratory studies to understand curd formation. In a rennet coagulation test, reconstituted skim milk or CMR is exposed to chymosin under standardized conditions, and investigators measure whether a curd forms, how quickly it forms, how firm it becomes, and sometimes how well it expels whey (syneresis).
For a skim milk powder intended to behave like whole milk in the calf, this is a useful functional test. A powder that cannot coagulate under reasonable laboratory conditions is unlikely to produce the same abomasal behavior as minimally processed milk. But the test must be interpreted carefully.
Emmons, Lister, and Campbell (1976) examined commercial skim powders and milk replacers and found that undenatured whey protein nitrogen did not reliably predict rennet coagulability. Some powders with relatively high amounts of undenatured whey protein still produced weak curds or failed to coagulate. Most of the commercial milk replacers they tested did not coagulate even though some had high apparent whey protein nitrogen values.
In other words, a chemical index of heat treatment and a functional clotting test answer different questions. Neither should be used as a universal stand-alone measure of nutritional value.
WPNI: what it tells us
The whey protein nitrogen index (WPNI) is commonly used to classify skim or nonfat dry milk according to heat treatment. The test measures soluble, undenatured whey protein nitrogen. Traditionally, a higher WPNI indicates less heat damage to whey proteins and a lower WPNI indicates more extensive heat treatment.
The conventional categories are roughly low heat at 6.0 mg or more soluble whey protein nitrogen per gram of powder, medium heat from about 1.51 to 5.99 mg/g, and high heat at 1.5 mg/g or less. Those categories are useful for food-processing applications because heat history affects functionality.
However, WPNI is influenced by the protein concentration of the powder. Sikand et al. (2008) showed that standardizing milk powder with lactose or permeate could change the WPNI enough to change its apparent heat classification even though the underlying protein had not been exposed to additional heat. They proposed expressing soluble whey protein nitrogen per gram of protein rather than per gram of powder to reduce this problem.
Górka and Budziński (2026) used a protein-corrected WPNI approach when characterizing the skim milk and whey ingredients in their recent CMR trial. They appropriately cautioned that WPNI reflects protein solubility and does not directly measure protein structure. Their discussion is a good reminder that WPNI is an indicator, not a complete description of ingredient quality.
Why clotting depends on more than heat
Even excellent skim milk powder can behave differently after it becomes part of a complete milk replacer. Curd formation depends on calcium availability, pH, total solids, protein interactions, fat incorporation, and other formulation details.
The older Emmons and Lister series demonstrated this very clearly. Added proteins, fat, calcium, phosphate, and citrate could change rennet coagulation. Later work showed that the method used to disperse fat into a skim-containing CMR changed curd firmness and whey syneresis. High-pressure homogenization produced softer curds than some lower-pressure fat-dispersion methods. Thus, “50% skim milk” is not a functional specification for the finished product.
The 2025 Vorndran and Steinhoff-Wagner study provides a modern illustration. Their CMR contained 50% skim milk powder, yet associated laboratory testing from the same research program found that the finished CMR failed to form a firm clot under the conditions tested. Whatever the reason – heat history, formulation, minerals, fat processing, or interactions among ingredients – the percentage of skim milk alone did not predict clotting behavior.
What should be tested?
There is no single laboratory number that tells us whether a milk ingredient is “good.” A useful quality-control program combines compositional measurements, functional tests, and ultimately animal data. Which tests matter most depends on how the ingredient will be used.
Table 1. Examples of tests that can help characterize dairy ingredients used in calf milk replacer.
| Test or information | What it can tell us | Important limitation |
| Proximate composition | Protein, fat, ash, moisture and lactose; verifies formulation targets and batch consistency. | Does not describe protein digestibility or heat damage. |
| Amino acid profile | Confirms essential amino acid supply and helps formulate balanced CMR. | Total amino acids do not necessarily equal digestible or reactive amino acids. |
| WPNI / soluble whey protein N | Indicator of heat history and whey-protein denaturation in skim powder. | Affected by powder composition; not a direct clotting or digestibility test. |
| Rennet coagulation time and curd firmness | Tests whether a skim-containing ingredient or finished CMR can coagulate under standardized conditions. | In vitro clotting does not prove that every calf will form a curd or perform better. |
| Syneresis | Measures the ability of a curd to contract and expel whey; describes curd functionality. | Useful only when curd formation is a desired product characteristic. |
| Solubility / dispersibility | Indicates mixing behavior and some types of processing damage. | A soluble protein can still have altered nutritional or functional properties. |
| Heat/Maillard damage indicators | Can identify excessive heating and loss of reactive lysine or formation of Maillard products. | Methods and interpretation vary; not all heat-induced changes reduce calf performance. |
| Calf digestibility / performance trials | Integrates ingredient, formulation and calf physiology. | More expensive and slower than routine laboratory QC; must be well designed. |
Do manufacturers use these tests?
Specialized CMR manufacturers generally perform far more quality control than is visible on a feed tag. Denkavit, for example, publicly describes laboratory screening of potential protein ingredients for properties such as solubility, stability, and anti-nutritional factors, followed by digestibility trials in calves before ingredients are adopted. That is the kind of ingredient-evaluation philosophy that matters.
I have not found a public Denkavit document stating that every incoming batch of skim milk powder is routinely subjected to a rennet coagulation test. They may use such a test internally, but without documentation it should not be presented as fact. More generally, buyers who care about clotting should ask manufacturers directly whether they test the skim ingredient, the finished CMR, or both – and what method they use.
A better purchasing question
The discussion of skim milk often becomes a specification problem: “I want at least 30% skim” or “I want at least 50% skim.” That is attractive because the number is simple and easy to put into a purchasing contract. Unfortunately, it can create a false sense of precision.
A more useful specification would ask about the quality and consistency of the entire protein system. What dairy ingredients are used? How are incoming lots qualified? Is skim milk low, medium, or high heat? Is WPNI measured? If clotting is considered important, has the finished CMR been tested for rennet coagulation and curd firmness? Are amino acid concentrations controlled? What evidence is available for digestibility in young calves? How much lot-to-lot variation is accepted?
Those questions are more difficult than reading a single percentage from a tag, but they are much closer to the biology of the calf.
Ingredient quality may explain some old contradictions
This perspective also helps reconcile the historical literature. Some early nonclotting milk replacers performed poorly, leading to the belief that the absence of a curd caused diarrhea and slow growth. But many of those products also contained severely heated milk proteins, poorly processed plant proteins, or other ingredients that young calves digested inefficiently. Later experiments that suppressed clotting while keeping the underlying skim-milk formula constant generally did not reproduce the poor performance.
The same principle applies in the opposite direction. A formula does not become nutritionally superior simply because it contains a large amount of skim milk. If the skim has been excessively heated, or if the finished product has poor functional properties, the ingredient name provides little protection.
Summary
Skim milk powder can be an excellent protein source for calves, but “skim milk powder” is not a complete quality specification. Heat treatment changes whey proteins, casein interactions, Maillard reactions, and rennet coagulation. WPNI provides useful information about heat history, but it does not directly measure clotting or calf digestibility. Rennet coagulation and curd-firmness tests provide functional information, but clotting itself has not consistently improved growth or nutrient utilization. The best quality-control programs combine composition, processing indicators, functional tests, and animal digestibility or performance data.
The practical message from this three-part series is therefore simple: judge the protein system, not just the ingredient name. Percentage skim milk, presence of whey, and ability to clot are pieces of information. None, by itself, defines the quality of a calf milk replacer.
Industry Source
Denkavit. 2019. What’s actually in the calf milk replacer? Theme: raw materials – vegetable proteins. Link
References
Emmons, D. B., and E. E. Lister. 1976. Quality of protein in milk replacers for young calves. I. Factors affecting in vitro curd formation by rennet (chymosin, rennin) from reconstituted skim milk powder. Can. J. Anim. Sci. 56:317-325. doi:10.4141/cjas76-037. Link
Lister, E. E., and D. B. Emmons. 1976. Quality of protein in milk replacers for young calves. II. Effects of heat treatment of skim milk powder and fat levels on calf growth, feed intake and nitrogen balance. Can. J. Anim. Sci. 56:327-333. doi:10.4141/cjas76-038. Link
Emmons, D. B., E. E. Lister, and D. L. Campbell. 1976. Quality of protein in milk replacers for young calves. III. Rennet coagulation and undenatured whey protein nitrogen content of skim milk powders and commercial milk replacers. Can. J. Anim. Sci. 56:335-338. doi:10.4141/cjas76-039. Link
Emmons, D. B., E. E. Lister, and J. D. Jones. 1976. Quality of protein in milk replacers for young calves. IV. Rennet (chymosin, rennin) coagulation of reconstituted skim milk powder containing added proteins, fat, calcium, phosphate and citrate. Can. J. Anim. Sci. 56:339-345.
Emmons, D. B., and E. E. Lister. 1980. Quality of protein in milk replacers for young calves. V. Effect of method of dispersing fat on curd formation and whey syneresis. J. Dairy Sci. 63:417-425. doi:10.3168/jds.S0022-0302(80)82949-3. Link
Górka, P., and W. Budziński. 2026. Effect of inclusion of skim milk in milk replacer on growth performance of newborn dairy calves. J. Dairy Sci. doi:10.3168/jds.2026-29094. Link
Shillam, K. W. G., J. H. B. Roy, and P. L. Ingram. 1962. The effect of heat treatment on the nutritive value of milk for the young calf: 2. The factor in a milk substitute associated with a high incidence of scouring and mortality. Br. J. Nutr. 16:267-277. doi:10.1079/BJN19620029. Link
Sikand, V., P. S. Tong, and J. Walker. 2008. Impact of protein standardization of milk powder with lactose or permeate on whey protein nitrogen index and heat classification. Dairy Sci. Technol. 88:105-120. doi:10.1051/dst:2007011. Link
Vorndran, A. M., and J. Steinhoff-Wagner. 2025. Effects of high-volume voluntary feed consumption on growth and health of Brown Swiss calves: Continued dam’s milk feeding versus early introduction of milk replacer during the first week of life. J. Dairy Sci. 108:5954-5967. doi:10.3168/jds.2025-26405. Link
Wilms, J. N., M. H. Ghaffari, M. A. Steele, H. Sauerwein, J. Martin-Tereso, and L. N. Leal. 2022. Macronutrient profile in milk replacer or a whole milk powder modulates growth performance, feeding behavior, and blood metabolites in ad libitum-fed calves. J. Dairy Sci. 105:6670-6692. doi:10.3168/jds.2022-21870. Link