If I buy full-fat milk and dilute it 50/50 with water, do I effectively have semi-skimmed milk, or is there something more sophisticated about the skimming process? And if I then dilute it again, do I get skimmed milk… for one quarter of the price?

The process of producing semi-skimmed and skimmed milk is more sophisticated than simply diluting full-fat milk with water. Whole milk naturally contains around 3.5-4% fat. To produce semi-skimmed milk, the fat content is reduced to around 1.5-1.8% through a process called centrifugal separation. This involves spinning the milk at high speeds to physically remove some of the fat globules. Skimmed milk goes through a more thorough separation process to remove even more of the fat, resulting in a milk with less than 0.5% fat. Simply diluting full-fat milk with water would not achieve the same nutrient profile as true semi-skimmed or skimmed milk. The diluted milk would have a lower overall nutrient density compared to the standardized lower-fat milks. So in summary, no - diluting full-fat milk with water does not effectively create semi-skimmed or skimmed milk. The industrial skimming process removes a precise amount of fat to produce the standardized lower-fat varieties, which cannot be replicated by simple dilution at home.
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