I have covered the basic properties of proteins, fats, carbohydrates and water in dairy products in my dairy chemistry series. Other interesting properties are the following weird properties that you will see occasionally in dairy products.
This phenomenon occurs in raw milk and is due to the immunoglobulins found naturally in milk. Immunoglobulins (also called antibodies) are a set of proteins that are part of the immune system. These proteins can flocculate antigens (bacteria and other foreign bodies) present in the raw milk.
Cryoglobulins are groups of immunoglobulins and perhaps other substances. Cryoglobulins are sensitive to cold by becoming less soluble as the temperature decreases. They are also attracted nonspecifically to particles like milk fat globules. This attraction is not an antibody – antigen reaction because it is nonspecific; but the activity may be related to the more specific interaction. On cooling, cryoglobulins gather at the surface of fat globules covering a very small percentage of the surface. This makes the globules + cryoglobulins “sticky” and causes the fat particles to agglomerate. The cryoglobulins are very large molecules and extend away from the fat globule. This allows for the globules to become attached despite the normal repulsion between fat globules. The agglomerated fat globules rise in response to their increasing diameter. As they rise they gather globules and become bigger accelerating the creaming phenomenon far faster than would be expected simply by normal aggregation and creaming. In this way, a cream layer is quickly formed even in a deep vessel. The cryoglobulins are inactivated by pasteurization, long periods of agitation or by homogenization. Goats, sheep and buffalo do not have these proteins and therefore some people say that their milk is pre-homogenized. In humans, if there is an overabundance of cryoglobulin in the blood, there can be a disease called cryoglobulinemia. This disease is characterized by the cryoglobulin precipitating out when the body temperature is lowered. The precipitated protein can block blood vessels and cause gangrene in the extremities. Of course, as expected, this disease is more prevalent in the wintertime and whenever people are exposed to long periods of cold temperatures.
Emulsifiers and Ice Cream
It is common to assume that the reason we would put an emulsifier into a product, whether it is a monoglyceride or something like egg yolk, would be to emulsify the fat in the product. In general, this is true. For instance, adding egg yolk solids to an oil and water dressing will result in a relatively stable emulsion (mayonnaise). It does this by coating the fat particles with lecithin (an emulsifier found in eggs) and reducing the surface tension of the fat droplets. Adding some agitation allows the fat particles to break up and become smaller. Smaller fat droplets will float to the surface slower (Stokes equation). Eventually the speed of this rise becomes so slow that during the shelf life of the product, the fat does not rise or float visibly. In milk, the emulsifier is casein.
Casein is a highly hydrophobic protein that easily associates itself with the fat surface. In products with a lot of casein and relatively small fat globules, the fat will be stable. With larger fat globules (say in raw milk) the fat will still rise, though it is a matter of particle size. Slightly smaller and the density of the fat – casein particles are heavy enough to reduce the difference in density of the milk serum - fat casein particle to zero and the fat will not float up. It is possible to make the particles so small that the density difference switches sides and the fat + casein particles will sink.
This does not happen because of another interesting effect. As the diameter of the particle decreases the surface area of the particle increases by the square of the reduction in radius. Since the protein must coat the fat to maintain the emulsion, as the size of the fat particles is reduced the amount of protein needed to cover the fat quickly increases. When the supply of protein is exhausted, the fat is vulnerable to agglomeration and the particle size of the fat thus increases. We often add emulsifiers to such systems to coat the fat and allow smaller particles. Since emulsifiers like monoglycerides or lecithin are very small molecules compared to protein, the thickness of the emulsifier layer fat is orders of magnitude smaller than that with protein. Therefore, considerably less emulsifier is needed as compared to a protein such as casein. In ice cream and whipped cream, we would like to achieve a state in the fat called partially agglomerated or churned. It is this partially churned fat that forms the structure surrounding air bubbles that helps give them structure and enables air or overrun to be pumped into ice cream.
The question then becomes, how do we encourage the churning (forming partially agglomerated fat structure)? During the churning step, the product is violently agitated. This causes many impacts between the fat globules. If the protein to fat ratio is high (low fat products) the casein will protect the fat globules during these collisions. For higher fat products with a relatively low proportion, say a very high fat, the protein cannot protect the fat and churning will start to happen. If we would like to whip a lower fat product there must be some mechanism to reduce the amount of protein surrounding the fat. This is where emulsifiers come in. Since emulsifiers can displace protein (because they have a stronger affinity for the fat surface), they can reduce the amount of protein on the fat surface and allow the agglomeration to happen. This explains why more emulsifier is needed to achieve proper overrun as you decrease the fat in the system. An astute reader might ask why doesn’t all the fat churn out then? It can, but in general the fat in both whipping cream and in ice cream is cold when it is whipped. Solid fat slows the agglomeration down just enough to prevent all the fat from churning. In some cases, with sensitive formulations and especially high shear processes or with warm product, this churning can be a problem. Evidence of this can be found in elbows in the lines coming from the freezer, on the dasher and elsewhere along the path of the ice cream.
This phenomenon occurs after several months of shelf life where the viscosity stays relatively constant or drops a bit, but then the viscosity increases unexpectedly to the consistency of pudding, a gel can form. No one really knows what causes this phenomenon and it may have several causes.
Age Gelation is associated with high heat treatment or high storage temperatures, so we see it most often in UHT or sterilized products. One potential cause, is the presence of a small number of active enzymes that are released during pasteurization or homogenization. These enzymes can be proteolytic in nature. At normal storage temperatures, the enzymes work very slowly, but with time they can affect the nature of the casein micelle. With the breakdown of the micelle, the protein can form connections between what remains of the micelle allowing a gel to form. Threads between casein particles have been observed using electron microscopy. (Walstra Wouters and Geurts, 2006, Dairy Science and Technology, CRC Press). People have noticed that age gelation has been associated with seasonality. The protein in milk does vary in overall amount and the relative ratio of the various proteins in both whey and casein proteins. It makes some sense that these changes could affect the tendency for a product to gel during shelf life but it is difficult to see how this alone would be the cause.
Of course, dairy products are highly complex mixtures of chemicals and reactions. During heating a huge number of changes happen (the Milliard browning reaction for example). Milk proteins involved in these reactions can sometimes polymerize resulting in a high viscosity or a gel. A well-known association between kappa-casein and ß -lactoglobulin that happens in dairy products that are heated has been suggested as a cause of age gelation. Large complexes of this protein pair could interconnect casein micelles causing a gel to form. This effect is exploited during the processing of many dairy products, and yogurt is an example. In general products like these do not seem to increase viscosity suddenly after a long time. It is possible, though, that more kappa-casein-ß -lactoglobulin complexes could form over time during the shelf life of the product. During UHT processing buffering salts are often used to prevent instability of the protein in products (resulting in “burn on”). Salts like this can be added to delay or inhibit the formation of gels over time. Polyphosphates are often used for these purposes. These phosphates though, can slowly hydrolyze to mono and di phosphates over time. This results in a slow rise in pH and, therefore, an increase in the solubility of some of the protein particles. Under some circumstances this can lead to age gelation. The addition of orthophosphates or citrate salts seem to accelerate the gelation process. However, the actual cause of a rapid increase in viscosity after several months of stability, age gelation remains a mystery.
The effect of filling temperature on heavy cream and like products.
Perhaps less mysterious than age gelation and certainly more useful to processors is the phenomenon of filling temperatures in high fat dairy products. Considerable control on the texture of high fat products can be made simply by changing the temperature at which the product is filled or cooled. The reason for this is the strange or weird part. It lies in the way fat crystalizes during cooling. If a product is cooled slowly, the fat droplets will behave one way and if it is cooled faster they behave differently. The crystallization of fats like dairy fat is complex. If the fat is cooled slowly without much agitation large crystals can form that extend into the serum beyond the globule. These crystals can more easily form connections between the fat globules (partial coalescence or rebodying). If the same product is cooled quickly with agitation, the fat crystals that form will be much smaller and less likely to agglomerate. The result is that cooled slowly with little shear, the product can become very thick or even gelled. If it is cooled quickly with agitation the very same product can be very thin and easily flowable. Heavy cream can be thicker than cup set sour cream when cooled slowly and nearly as thin as milk when cooled quickly.
Of course, this effect changes with homogenization. If the fat globules are small and separated by protein it is more difficult for connections between fat globules to form and the product will be less viscous and more flowable. If there is less protein on the globule and the fat globule can crystallize slowly large crystals can form making more entanglements and connections. This will result in a thicker product. For processors, manipulating temperatures and homogenization regimes, many textures can be obtained from the same cream. Of course, the advantage of having control over texture can quickly become a problem. For instance, filling warm can result in thicker product that is so thick that it no longer can be poured. Cooling product in several tanks that have different cooling rates can result in an inconsistent product. If you are having trouble with your dairy products, keep these weird properties in mind.