The fat in cow’s milk is immiscible in the water soluble component and so forms globules that can coalesce together as cream. Traditionally when milk was bought in milk bottles this cream could be seen floating on the top of the milk. Milk fat can be dispersed by shaking the bottle which temporarily mixes the fat with the milk forming an emulsion. Leaving milk to stand causes the dispersed fat emulsion to merge into larger globules which eventually rise back to the top of the bottle to re-form the cream layer. Supermarket milk in cartons does not possess these qualities because the milk is homogenised. In this process the milk is passed through a filter of a small pore size at high pressure and this causes the milk fat to disperse into tiny droplets that remain suspended in the milk as a colloid. This effectively creates an emulsion of very small particle size and unlike untreated milk, the fat in homogenised milk will not reform a layer on top of the milk. This is done for commercial reasons because it is perceived consumers do not like milk with cream.
Little research has been performed to determine the health effects of homogenised milk which is surprising, because history is littered with examples of the way that food processing alters the biochemistry of humans, often detrimentally. In fact, there is relatively little research looking at how the structure and the processing of fats affects their absorption and metabolism. In contrast, there is an extensive body of research assessing the digestibility and glycaemic effects of carbohydrates. Generally, refining carbohydrates by removing the the outer bran and germ layers is known to cause an increase to the rate at which glucose rise absorbed to the bloodstream. This increased glycaemia is now well researched and is known to lead to widespread postprandial metabolic changes. In a similar way, emulsification of fatty acids as is common is food processing can increase the rate of fat absorption and this too may cause a number of changes to normal metabolic regulation. However only a handful of studies have researched this effect.
For example, a study published in the American Journal of Clinical Nutrition in 2013 investigated the effects of fat emulsion on postprandial rises in plasma triglycerides in nine healthy and nine obese subjects1. The researchers fed the subjects emulsified or non-emulsified milk-fat in otherwise identical breakfasts, the fats from which they radiolabelled to allow postprandial measurements over time. The results showed that the emulsified fats caused a more rapid rise and sharper peak in plasma chylomicron levels when compared to the non-emulsified fat. In the normal weight subjects this was accompanied by a greater production of apolipoprotein B-48, a protein particle present on chylomicrons. In the obese subjects the emulsified fats resulted in chylomicrons 3 times larger than those present following the non-emulsified fat meal. Emulsified fat also resulted in a larger spillover of dietary fats to the plasma, with a concomitant increase in fatty acid β-oxidation of around 45 to 52 and 40 to 57 % in healthy and obese individuals, respectively.
These results suggest that emulsified fat is absorbed more quickly than non-emulsified fat, and that this alters the postprandial fat kinetics in humans. The homogenisation process in this research was measured by the researchers and the emulsified fat was found to have a surface area around 70,000 times that of the non-emulsified fat. Because fat must be emulsified in the small intestine prior to absorption through the action of bile acids, it is not surprised that pre-emulsification increases the rate of absorption. However, although this process clearly alters the absorption characteristics and subsequent metabolism of the fat, it is unclear as to the long-term consequences of these changes. Interestingly, the obese subjects reported feeling more hungry following the emulsified fat meal, when compared to the non-emulsified fat meal. This may suggest that one of the physiological changes was related to appetite regulation. If appetite regulation and β-oxidation rates are altered through emulsification then long-term body weight changes might result.
Milk fat is highly complex because it is a mixture of around 400 different fatty acids and has some unique properties. One unusual characteristic of milk fat is the location of the fatty acids within the triglycerides molecules. While the sn-2 position of the triglyceride usually contains saturated medium and long-chain fatty acids (C8 to C18), the sn-3 position contains fatty acids with shorter chains such as butyric (C4) and caproic (C5) acids. These fatty acids are in turn surrounded by membrane containing phospholipids forming the fat globules seen within the milk2. Consumption of milk results in hydrolysis of the sn-3 position preferentially, which selectively releases the shorter chain fatty acids. These are absorbed through the stomach wall and passed to the liver through the portal vein where they are oxidised. The remaining fatty acids are absorbed from the small intestine in chylomicrons. Homogenisation disrupts the natural arrangement of milk fat and this is likely what changes the absorption rate of the fat from emulsified milk.
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