The mean content of citric acid in milk is about one-tenth of one per cent.[403]
Citric acid is not found in human milk, and probably exists only in the mammary secretions of herbivores.
Among the mineral acids of milk, phosphoric is the most important, but a part of the phosphorus found as phosphoric acid in the ash of milk may come from pre-existing organic phosphorus (lecithin, nuclein).
The sulfuric acid, which is found in the ash of milk, is derived from the sulfur of the proteid matter during ignition.
Lactic acid is developed from lactose during the souring of milk as the result of bacterial activity.
Gases are also found in solutions of milk, notably carbon dioxid, which gives to freshly drawn milk its brothy appearance.
The ash of milk has the following composition expressed as grams per liter of the original milk:[404]
| Component. | Grams per liter. | Probable form of combination. | Grams per liter. |
|---|---|---|---|
| Chlorin | 0.90 | sodium chlorid | 0.962 |
| potassium chlorid | 0.830 | ||
| Phosphoric acid | 2.42 | KH₂PO₄ | 1.156 |
| K₂HPO₄ | 0.853 | ||
| MgHPO₄ | 0.336 | ||
| CaHPO₄ | 0.671 | ||
| Ca₃(PO₄)₂ | 0.806 | ||
| Potassium | 1.80 | (as shown above) | |
| and as potassium citrate | 0.495 | ||
| Sodium | 0.49 | sodium chlorid | 0.962 |
| Lime | 1.90 | (as shown above) | |
| and as calcium citrate | 2.133 | ||
| Magnesia | 0.20 | MgHPO₄ | 0.336 |
The percentage composition of the ash of milk, according to Fleischmann and Schrott, is expressed as follows:[405]
| Per cent. | ||
| Potassium oxid, | K₂O | 25.42 |
| Sodium oxid, | Na₂O | 10.94 |
| Calcium oxid, | CaO | 21.45 |
| Magnesium oxid, | MgO | 2.54 |
| Iron oxid, | Fe₂O₃ | 0.11 |
| Sulfuric acid, | SO₃ | 4.11 |
| Phosphoric acid, | P₂O₅ | 24.11 |
| Chlorin, | Cl | 14.60 |
| 103.28 | ||
| Less | Cl as O | 3.28 |
| 100.00 | ||