= Solution
\b[<Cow's milk> contains <water>, dispersed <fat>, <casein>, soluble <milk proteins>, <lactose> and <mineral> constituents, with smaller amounts of other substances.] These constituents occupy different physical states, which makes their separation possible: the <fat> is present as an <emulsion>, much of the <casein> as <colloidal> <casein micelles>, and the <lactose> and many <salts> in <chemical solution>. <Vitamins>, <enzymes> and small nitrogenous constituents accompany these main fractions. The proportions vary between samples and are not fixed by the name <cow's milk>.<The Chemistry of Milk in the Dairy Processing Handbook>[chapter 2, principal constituents and Tables 2.1–2.2]{ref}
First let fresh unhomogenized <milk> stand so that the less dense <fat globules> rise as <cream>; skimming gives a <fat>-enriched layer and <skimmed milk> beneath. This separates fractions rather than pure <chemical substances>: <cream> still contains <water> and other constituents. Churning <cream> coalesces the <fat globules> into butter; extraction of a separately prepared dried portion with a suitable organic <solvent> provides another way to isolate <fat>. A modern <centrifugal separator> accelerates the same density-based separation, but it is not needed to explain an 1864 bench separation.
Next add dilute <acetic acid> carefully to <skimmed milk>. Lowering its <pH> toward the <isoelectric point> of <casein>, about 4.6, produces a curd that can be collected by <filtration> and washed. Excess <acid>, retained <fat>, and shifts of <calcium phosphate> between curd and liquid limit purity. <Rennet> offers another route to a curd, but its <enzyme>-catalysed action and the <mineral> content of the resulting curd differ from simple <acid precipitation>; the two should not be treated as identical mechanisms.<The Chemistry of Milk in the Dairy Processing Handbook>[chapter 2, Casein, precipitation and acidification]{ref}
The liquid after removal of the curd is <whey>. Heat this fraction under suitable acidity to induce <protein denaturation> and aggregate much of its residual <whey protein>; collect the coagulum by <filtration>. Older descriptions commonly grouped this fraction under “albumin”, but modern <milk chemistry> distinguishes several <proteins>. Heating alone does not make every minor constituent fall into a single pure fraction.<Milk and Whey Fractionation in the Dairy Processing Handbook>[chapter 18, Recovery of denatured whey proteins]{ref}
Concentrate the deproteinized <whey> and cool it to crystallize <lactose>. Filter and wash the crystals, then purify them by <recrystallization>; dissolved <salts> and residual <organic matter> remain substantially in the mother liquor. This provides a separation from most <protein>, not an automatic complete separation from all <minerals>.<Milk and Whey Fractionation in the Dairy Processing Handbook>[chapter 18, Lactose recovery; Crystallisation; Lactose separation]{ref}
Finally use separate measured portions for <water> and <mineral> determinations. Controlled drying gives an operational <total solids> measurement; subtraction from the initial <mass> estimates the <water> lost. Ignition of a dried portion destroys most <organic matter> and leaves <ash> for <mineral analysis>. <Ash> is a transformed residue: it does not preserve all original <salts> unchanged, and some constituents can volatilize. <Calcium>, <phosphorus>, <sodium>, <potassium> and <chloride> are among the important <mineral> constituents. \b[Skimming, curd formation, heat coagulation, lactose crystallization, drying and ashing separate or determine the principal fractions; none alone constitutes a complete pure-component analysis.]
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