It was confirmed by means of electron microscopy and differential centrifugation that casein micelles aggregated when concentrated whey protein-free milk (WPF milk) was heated at 135_??_140°C for 15 sec. The amount of large casein micelles, which sedimented at 3000×g or at 12, 000×g, nearly doubled after heating. The casein micelles sedimented at 3000×g in the heated concentrated WPF milk contained smaller amounts of calcium and inorganic phosphorus than those in the unheated one. The κ-casein content of the large casein micelles was low in both the unheated and the heated concentrated WPF milks. The acid caseins prepared from the large casein micelles were more sensitive to the presence of calcium than the whole casein. The κ-casein content of the soluble casein formed by heating was remarkably high, and the soluble casein did not precipitate in the presence of calcium.
From the above results, the large casein micelles were considered certain to be formed from the small casein micelles which released soluble casein upon heating.

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