The use of ultrafiltration and dialysis in isolating the aqueous phase of milk and in determining the partition of milk constituents between the aqueous and disperse phases

Abstract

A description is given of various methods for the ultrafiltration and dialysis of milk and of the composition of the sera obtained. Ultrafiltrate prepared by the procedure recommended is reasonably representative of the aqueous phase of milk, but its content of lactose and citric acid, and consequently also of calcium, is determined to a slight degree by the sieving phenomenon known to occur often in ultrafiltration. The composition of diffusate obtained from milk at 20 °C is not thought to be controlled to any significant extent by a Donnan effect and is regarded as identical with that of the aqueous phase of milk. The lactose content of diffusate suggests that about 2 % of the water in milk is bound to protein, and allowance should be made for this when calculating the concentrations of the soluble constituents in milk from the composition of diffusate. Diffusate prepared from milk at 3°C contains slightly more total calcium, ionized calcium and phosphorus than diffusate prepared at 20 °C. These differences are attributed to a change in the partition of calcium and phosphorus between the disperse and aqueous phases at the lower temperature, an explanation that is supported by the reversibility of the change. The composition of diffusate prepared by the procedure recommended indicates that about 5 % of the sodium and about 6 % of the potassium and citric acid in milk are in the. disperse phase.It is sometimes desirable to know not only the total concentration in milk of a given constituent but also the amounts of it in the disperse and aqueous phases, i.e. how much can be regarded as ' colloidal' and how much as ' soluble'. This information is usually obtained by determining the total amount of the constituent and the amount in solution; the colloidal fraction is then obtained by difference. Such a fractionation depends on the preparation of a suitable serum for estimating the soluble constituents. This serum should have the same composition as the aqueous phase of milk and therefore in its preparation the equilibria in milk should be disturbed as little as possible. Several methods of preparing a serum are available and of these the most suitable would appear to be ultrafiltration or dialysis. Other methods of estimating the soluble constituents of milk are the analysis of rennet whey or the serum obtained by high-speed centrifuging, and the treatment of milk with ionexchange resins.

Characterization of the dromedary milk casein micelle and study of its changes during acidification

“…1). This phenomenon is identical to that observed in cows' milk and it was illustrated by similar sigmoid curves [14,19,26,39]. However, the characteristic points of these curves were different.…”

“…1) continued until close to the pH observed for cows' milk (pH ~ 4.6) [9,14]. However, contrary to what had been reported for cows' milk [9], the compared changes in solubilization of these two cations (Fig.…”

The Variation in the Composition of Milk During Successive Stages of Milk Removal from the Mammary Gland of the Cow

“…Whey was considered to be representative of the aqueous phase of milk secretion. In this connection White (1960) anddeMan (1962) have shown that the composition of whey is similar to that of milk ultrafiltrate and milk dialysate. The concentration of the different constituents in whey and milk of the first two samples from cows 1, 2, 4, and 5 was similar to previously reported values for cows in mid-lactation (White and Davies 1958).…”

“…The results have been expressed in millimoles per litre, thus permitting comparison between different constituents on a concentration basis. This was useful in considering the relationship between sodium, potassium, chloride, and lactose which are mainly in a dissociated form in the milk (White and Davies 1960). It is of less value when considering the anions phosphate and citrate and the cations calcium and magnesium since they form soluble complexes in whey (Jenness and Patton 1959).…”

Évolution des caractéristiques physico-chimiques et des paramètres de coagulation du lait de brebis collecté dans la région de Roquefort

“…Holt et Jenness (1984) utilisant la technique d'ultrafiltration pour la séparation des phases, font état de pourcentages légèrement supérieurs, avec 90% de calcium et 70% de phosphore inorganique sous forme colloïdale. Ces diffé-rences de résultats sont probablement liées à la technique de séparation, comme l'ont montré Davies et White (1960).…”