Milk is a mixture of fat, protein, carbohydrate and water. When raw milk sits, the fat separates from the rest of the liquid, rises to the top and forms a layer of cream. The purpose of homogenization is to keep the fat molecules dispersed throughout the milk, so separation does not occur and the milk remains uniform.
Homogenization Process
Homogenization uses high pressure to force the milk through very small tubes, which causes the fat globules to break down into smaller particles. The process results in a greater number of fat particles that are much smaller in diameter and have a reduced tendency to rise to the top. It provides a smoother mouthfeel since the fat is spread throughout the milk uniformly.
Hypothesis of Relationship to Heart Disease
Kurt Oster, M.D., formed a hypothesis in the early 1960s that homogenization was a cause of heart disease. He noted that plasmalogen, an essential component of cell membranes, was often missing from heart and artery tissue. He reasoned that the missing plasmalogen was responsible for the lesions in the artery wall that lead to atherosclerosis. He believed that the enzyme xanthine oxidase, which oxidizes the plasmalogen, was the reason plasmalogen was depleted. While xanthine oxidase can be found in the human body, it is not normally found in heart tissue. He and his colleagues, however, did find the enzyme in autopsied tissue.
Xanthine Oxidase from Homogenized Cow's Milk
Osters and colleagues recognized that cow's milk was the food with the largest amount of xanthine oxidase. The researchers noted that atherosclerosis and heart disease began to rise during the same decade homogenization began to be used universally. He believed the xanthine oxidase was encapsulated in the new smaller fat particles, protecting it from digestion in the stomach. He hypothesized that some of the xanthine oxidase must be absorbed into the blood, eventually making its way to the heart and arteries.
Critiques of the Hypothesis
In 1983, Andrew Clifford and colleagues published an article in the "American Journal of Nutrition" refuting the hypothesis that xanthine oxidase could be absorbed from cow's milk, leading to heart disease. The authors pointed out that there had never been evidence xanthine oxidase was absorbed, no evidence of an association between homogenized milk and heart disease, and no evidence xanthine oxidase is the cause of depleted plasmalogen.
References
- Dairy Science and Technology at University of Guelph: Homogenization
- "Milk Homogenization and Heart Disease"; Mary G. Enig PhD; 2003
- "American Journal of Clinical Nutrition"; Homogenized Bovine Milk Xanthine Oxidase: A Critique of the Hypothesis Relating to Plasmalogen Depletion and Cardiovascular Disease: August 1983
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