Since milk is intended to be a food for young mammals, it needs to supply protein for growth and some energy source for the young. In most mammals there are two such sources: lactose and fat. Lactose gives the young a comparatively fast boost of energy and the fat gives them a slower more sustained source of energy. The fat in milk is a complex mixture of triglycerides (three fatty acids connected to a glycerol backbone). The fatty acids in milk come from three sources: directly from the diet of the animal, synthesized within the mammary gland and synthesized within the whole body of the animal. The actual proportion of each fatty acid in milk depends on the health of the animal, it’s diet, the season of year, temperature, the stage of lactation the species, subspecies variant, the microbiology of the rumen (in cows) and a plethora of other factors. The average fat in cow’s milk runs about 3.7% (again, depending on many factors). The fatty acids in milk are also variable differing in chain length (mostly between 4 and 18 carbons long) with even numbers of carbon atoms being predominate. They also differ in the number (0 to 3) and placement of double bonds. They even differ in the geometric position of the hydrogen around the double bond (cis and trans positions, cis is more common). A fatty acid with single bonds completely along the fatty acid chain is called saturated (saturated with hydrogen). A fatty acid with one or more double bonds is called unsaturated. Milk fat also contains other lipids, such as mono and diglycerides, phospholipids, cerebrosides, gangliosides, sterols (cholesterol and cholesterol esters) and carotenoids. The following table contains the approximate content for the fatty acids in milk fat:
The melting profile for milk fat is correspondingly complicated. It appears to have at least 5 separate melting peaks and at least two crystallization points. (There may be more, and there are some points where the fat is melting as one form of crystal and recrystallizing into another at the same time.)
From: Melting and Crystallization DSC Profiles of Milk Fat Depending on Selected Factors, 2013, Jolanta Tomaszewska-Gras
It is good to note that at -30⁰C (-22⁰F) the fat has not completely hardened (when cooling at 5⁰C/min) and when heating (same rate) it has not completely melted when the temperature is above 35⁰C (95⁰F). Most fats, such as coconut or hydrogenated soy oil, do not have this wide of a melting / crystallization range. This can have a strong effect on products when switching from, and to, milk fat. Melted fat is much easier to churn out and, when handling products containing milk fat, it is important to make sure most of the fat is in the solid form. Shear, such as that found in a pipe elbow or centrifugal pump,s can cause dairy products with liquid or partially liquid fat globules to agglomerate and form visible deposits of fat in the product and on the surfaces that come into contact with the product. This is also the reason for the need for ageing times in ice cream and for chilling whipping cream before whipping. Both ice cream and whipped cream depend on a partially agglomerated (partially churned) structure for their stability after whipping. Fat that is mostly solid can form microscopic connections between particles of fat and this results, in the dry appearance and more ridged structure of properly whipped cream and ice cream. Further, during fat crystallization depending on cooling rates emulsifier types and fat composition the fat in the globules can crystallize in different ways. Like chocolate, the structure of the fat that results can have an effect on the properties of the product. For example, under certain circumstances the fat can be encouraged to form crystals that emerge from the fat globule surface. This causes the fat globules to take on somewhat irregular shapes (as opposed to spherical). These irregular shapes are thought to encourage agglomeration resulting in the ability of a whipped product to maintain its shape after melting.
So far I have explored some of the physical properties of the fat in milk. It is important to understand it’s packaging in the milk as well. It is important to understand this because fat and water do not mix. Anything that is hydrophobic in the vicinity will accumulate at the water – fat interface. Under normal circumstances the fat in milk exists in small particles called fat globules. In milk there are approximately 108 fat globules for every milligram of fat; for homogenized milk there are about 1011. Fat in cow’s milk has an average diameter of 3 microns and ranges between .02 and 10 microns. The diameters are roughly distributed normally (bell shaped curve) about the average. When the mammary cell assembles the fat to be expressed into the milk, the cells push the fat out through their cell membranes. The cell membrane is composed of phospholipids and proteins and these are deposited, along with some cytoplasm components, on to the fat particle during this process. As soon as the milk has been expressed from the secreting cells a sort of equilibrium is formed with the fat globule membrane and the other milk components. Some proteins (phospholipids etc.) from the milk exchange with those within the (original) fat globule membrane. Although this equilibrium is quickly formed, it is always dynamic with molecules coming and going onto and off the membrane constantly. During homogenization (I will talk about homogenization more completely in another chapter) the shear that the fat globules are subjected to easily strips the (native) fat globule membrane from the fat. And in this process the fat globule size will be altered so that there are many more, smaller globules. Because of the increased number of small fat globules there is a corresponding huge increase in total fat globule surface area. This increase in surface is energetically disfavored because of the hydrophobic nature of fat. The deficit in surface active material generated by homogenization is filled by protein, mostly casein (casein has emulsification properties) and any other surface active components in the product being homogenized (original fat globule membrane components, emulsifiers, other protein, phospholipids, etc.) that are in the vicinity of the fat globule. If these materials are insufficient to cover the fat globules completely an interesting phenomenon happens. Cream with a high butterfat and low milk solids nonfat when homogenized will have such a deficit in membrane material. The fat will then be incompletely covered and will then be able to spontaneously agglomerate in order to expose less of the fat surface to the water. The first result of this will be a larger fat globule average size. Second, there will be a tendency to churn if agitation levels are high enough. If the product is handled correctly, on cooling the cream will solidify into a mass similar to sour cream but at a neutral pH. This effect is related to why ice cream requires more emulsifier at lower fat than it does for higher fat. In ice cream, the whipped structure is produced by partially agglomerating the fat in the freezer. In high fat ice creams (above 12% dairy fat) there is a slight deficit in membrane material this allows the fat to agglomerate during whipping. In low fat ice cream, there is an excess of protein surrounding the fat globules, this acts to prevent agglomeration and the formation of a fat structure in the product. Emulsifiers, especially fat soluble emulsifiers are used to displace some of the protein from the fat globule surface, which in turn helps encourage agglomeration, allowing the desired structure to form. The partial agglomeration of fat can lead to accelerated creaming of the fat into a dense layer of fat. This is because the partially coalesced fat forms larger diameter particles. These rising particles gather even more particles, as they raise and this accelerates the creaming. At the top of the container the agglomerated fat can further agglomerate and form a plug of dense cream at the top of a container. This plug can be strong enough to allow the bottle to be turned upside down without the contents escaping. This phenomenon can also cause problems, if the protein has been affected by processing like high temperature pasteurization, drying, acidification or any other process that will affect the protein, the functionality of the membrane will be affected as well. Often, for instance, products made using dry protein sources can cause problems when the same formulation using wet products does not. The reason for this is that the dry protein requires a much longer period of hydration in order to relax into a more native state where it has sufficient interface functionality (it may never regain it all). This can result in differences between plants where one plant uses dry milk powder and the other uses condensed milk. Similarly, production schedules where the hydration times are varied can cause inconsistencies within the schedules.
It is important for dairy product developers to be cognitive of the importance of fat in their products even at very low levels. A grasp of the physical states of fat and how they can interact with the other components in dairy products is critical to producing quality products for the consumer.