What is meant by “Seasonal Variation” of milk?

To understand seasonal variation of cow’s milk, one first needs to understand that all cow’s milk is produced by a cow as part of the mammalian lactation cycle for feeing of infant mammals.

Once a cow begins the pregnancy cycle, the lactation cycle begins. When a calf is birthed (as with all mammal births) the mother cow begins to produce milk. For the first 48 hours, the milk is not what most people would think of as “milk” … it is a somewhat clear liquid that is called colostrum. Colostrum contains mostly whey protein fractions to support the baby’s immune system such as lactoferrin, immunoglobulins, and even growth factors (because most mammals are born with a low functioning immune and hormonal system). The colostrum gives mammal newborns a “jumpstart”. Over the first 48 hours, the mother’s milk changes rapidly in composition. The casein levels increase and whey protein levels decrease. After 48 to 72 hours, cow’s milk becomes much closer to the appearance and definition of what we consumers think of as cow’s milk. What people don’t realize, however, is that cow’s milk continues to change in composition throughout the entire lactation cycle of the mother cow. Because the composition of milk does not remain constant throughout the milking season, manufacturers who use milk proteins as ingredients in their applications notice these changes. It is these changes in the milk composition that can cause numerous formulation headaches for end users. See the graph below for a general overview of the changes in milk production through a season.

Within the first 4 to 8 weeks of a cow’s lactation cycle, the volume of milk builds up rapidly, leading to a peak volume. Then, after approximately 10 weeks, the milk volume per cow starts to decrease. Throughout the remainder of the season, milk volume continues to decrease. At the beginning of the season, during the build up and peak volumes, the milk is higher than normal in whey protein content. Higher whey protein levels can cause discoloration of the MPC
powder, decreases in MPC viscosity, decreases in MPC heat stability, gellation during heat processing or lower solubility after heat processing, and poor quality cheese curd or yogurt gels manufactured from the MPC. In the middle of the milking season, the composition is much as would be expected from textbook descriptions of milk. As the season winds down however, the whey protein content continues to decrease and the casein portion increases. At the same time the mineral content of the milk begins to increase, especially those minerals that are known to “interfere” with the desirable functional properties of milk proteins. As the milk minerals increase, MPC functional properties will again change. Viscosities will modify (either higher or lower depending on your formula), the mouth feel of the MPC will become less smooth (more grainy), the MPC will exhibit decreased solution stability in the presence of added minerals, stability after high heat pasteurization could become more pronounced, and MPC used in cheese or yogurt will again exhibit different curding characteristics. Anyone who has ever made Ready to Drink UHT milk based beverages knows that such changes in milk composition pose tremendous challenges for production crews – higher than expected whey protein contents can lead to heat gellation and/or lower solubility of the protein, while higher than expected mineral levels can result in diminished solubility/suspendability of the protein.