Milk homogenization has been practiced for more than a century. Milk comes from the source as a more or less natural emulsion. The milk protein, casein, forms an emulsifying membrane that stabilizes milk fat in its aqueous matrix. But milk fat, in the form of cream, quickly rises to the top once this nutritious beverage leaves the cow. Homogenization serves to prevent fat globules from separating and rising to the top of whole milk to form a cream plug.
Homogenization is a simple mechanical process that has long been used to reduce the particle sizes of fat droplets, stabilizing the emulsion and preventing fat and aqueous layers from separating.
Traditionally, it has been done in conjunction with, or immediately following, heating — pasteurization — to kill bacteria and other microbes. Heating milk to 72° C for 15 seconds is sufficient to kill potential pathogens, for example. In rare cases, even higher temperatures, applied even more briefly, are used to achieve ultra-pasteurized milk with a dramatically extended shelf life.
Heated milk is forced under pressure through small holes, which breaks up fat globules, resulting in a more stable emulsion of milk fat in its aqueous matrix. Homogenization improves characteristics such as appearance, color and viscosity, and renders milk more suitable for use in products such as cream cheese, ice cream or coffee creamer.
The Next Step: A Microfluidizer Processor
In recent years, a more refined homogenizing technology — Microfluidizer technology — has gained favor. This more sophisticated approach to milk processing and stabilization offers additional benefits. For instance, research shows that Microfluidizer high shear processing of milk helps inactivate some of the microbes that might otherwise contribute to milk spoilage. This could be of special interest to cheesemakers, who may wish to craft cheeses from raw milk.
As noted in a 2014 article published in Foods, “The Microfluidizer® is an apparatus that causes homogenization via shear, turbulence and cavitation.” This same article describes the potential benefits of microfluidization of milk for use in yogurt manufacture. Working with low-fat milk, the authors noted, “… resulted in yogurt with modified microstructure, giving more interconnectivity in the protein networks with embedded fat globules …” This yielded a product with properties, such as texture and water retention, that were similar to yogurt made from “conventionally homogenized” milk.
The Goldilocks Phenomenon
The issue of ideal temperatures and pressures to be used during the microfluidization of milk assumes special importance when you consider the differing effects these factors have on various forms of milk. Milk products may be characterized by their relative milk-fat content. Skim milk, for example, features virtually no milk fat. Low-fat 2% milk, of course, contains just 2% milk fat.
The bottom line is this: The relative fat content of a given type of milk affects the optimal microfluidization pressure that is appropriate for its processing. Skim and 2% milks, for example, experience desirable milk fat particle size reductions when processed at 50 to 100 mPa pressure. Whole milk also follows this pattern, but particle sizes actually begin to increase again as pressures are raised to 150 or 200 mPa.
Thus, the optimum homogenization pressure is 100 mPa. It’s not too low, nor too high, but just right for most purposes. At higher pressures, such as 200 mPa, fat droplets begin once again to form clusters. This negatively affects homogenization.
Thus, there is little to be gained by processing skim or 2% milks with a Microfluidizer processor at pressures above 100 mPa, and the practice may be counterproductive if you are working with whole milk or cream (40% milk fat or greater). Milk to be processed for cheeses may benefit from certain variations in temperature or pressure during microfluidization. In any event, there is evidence that processing with a Microfluidizer processor eliminates some mesophilic aerobic and psychrophilic milk microflora, with rising processing temperatures and pressures.