QUALITY OF MILK.

New Zealand Herald, Volume LXVI, Issue 20332, 13 August 1929, Page 14

QUALITY OF MILK.

THE MINERAL CONTENT.

SOLUBLE AND INSOLUBLE ASM

The British Journal of Agriculture says: Although the mineral portion of milk tiiiiounts to less than 1 per cent, of the whole, it is a very important and com pics constituent. In the milk itself there arc present (a) inorganic salts, (b) inorganic bases combined with citric acid, and (c) calcium, phosphorus, and sulphur, combined in the ammo-acid groups of the cuseinogen molecule. When tho solids of milk ore burnt to an ash tno citrates aro decomposed, leaving the bases to sotno extent 111 tho form of carbonates. The calcium of the cascinogen moleculo remains, but part of tho phosphorus in this portion of tho milk solids, with practically all tho sulphur, is lost during ashing. Milk-ash therefore does not wholly represent the mineral constituents of milk. lu the course of this investigation, determinations oi tho total ash. water soltiblo and insoluble ash, total calcium, and total phosphorus were made and the results obtained throw some interesting light ori tho variations in tho composition of tho mineral portion of milk

The ash content of milk appears to havo a seasonal variat ion. Iho ash percentage readies a maximum in the summer (the grass months) and a minimum in tho winter It is significant that tho ash content should reach its holiest point when tho solids-not-fat percentage is low.

Tho soluble portion of milk-ash consists chiefly of sodium chloride, while caJc.um phosphate is the main constituent of tho insoluble ash. It is found that tho percentages of solublo and insoluble ash and tho ratio of tho two ore highly correlated with tho percentages of solids not fat.

Deficiency in Solids. It is apparent that milk low in solids not fat is rich ; n solublo ash and poor in soluble ash This point may have considerable physiological importance. Forchor states that sodium chloride (the principal salt in tho soluble ash) is one of the most variable constituents of milk, and tho amount present in milk is influenced by reason of its function in maintaining osmotic equilibrium in milksecretion. Quoting analytical data supplied by Boggild, I'orcher finds that a high chloride content in milk is associated with a low percentage of lactose, and vice versa. This statement, when considered in conjunction with the solids-not-fat and solublo ash correlation, leads one to infer that deficiency in solids not fat is due in somo measure to low lactose content.

Soluble ash shows considerable variation with months of year. Tho percentage of solublo ash is considerably higher in the summer than in the winter. This is the reverse of the solids-not-fat monthly variation. Assuming Porcher's conclusions regarding the relation of lactose to sodium chloride to bo correct, high soluble ash in milk indicates low lactose content, and therefore low percentages of solids not fat in tho summer months aro duo to a deficiency in lactose. The insoluble-ash percentage falls with the solids not fat, but the rate of fall is not of tho same magnitude as the rate oi rise -in the case of tho soluble ash.

Seasonal Variation. With regard to seasonal variation, phos phoric acid appears to bo influenced but little, with the exception of a slight rise in May and Juno and a fall in July. Lime, on tho other hand, shows a steady fall in the lato spring and summer, reach ing a low percentage in August, following which a sharp riso occurs. It therefore appears that in the summer period, when tho solids not-fat percentage is low,. the lime percentage is also low This is not duo fo protein deficiency, since tho ratio solids not fat is low at that period, and any deficiency in solids not fat in the summer months must therefore bo due in the main to lactose. It is suggested that this low lime content in the summer months is duo to a deficiency of calcium compounds other than the combined calcium in tho caseinogen. but this can only be determined with any degree of certainty by moro detailed analyses than were possible in this investigation. An interesting physiological point is thereby raised. Does tho balance of the mineral constituents of milk alter during the year, and, if so, is this change duo to the great difference in the nature of winter compared with summer feeding? In other words, does'grass influence the composition of tho mineral constituents of milk ?