The manufacture of Domiati cheese by ultrafiltration (UF) processing of recombined milk bas been described in several reports (El-Shibiny et al. 1982; Bogaard, 1986). This method has several potential advantages in areas short in milk production. In Egypt, a new plant has been established recently for the manufacture of Domiati cheese on the basis of this technology.

The behaviour of fresh pasteurized skim milk during UF has been described (Glover, 1971; Setti & Peri, 1976; Patel & Reuter, 1985; Chiang & Cheryan, 1986). Similar studies are lacking for reconstituted skim milk or recombined milk. Skim milk powder is usually subjected to variable heat treatments during processing which are known to affect its properties. These heat treatments have been used as a basis for classification of skim milk powders. Therefore, studies on UF of reconstituted milks previously subjected to different heat treatments would be important in the adaptation of this technology to cheese manufacture.

We describe here results obtained for permeation rates of reconstituted skim milk during UF compared with fresh pasteurized skim milk.

The effects of pressure on the association states of enzyme-treated casein molecules were studied by monitoring the turbidity of the solutions under pressures up to 3000 kg/cm2. β-Casein polymers, partly degraded with immobilized trypsin, dissociated with increasing pressure up to a critical pressure (e.g. 1200 kg/cm2) and then reassociated under the higher pressure up to 3000 kg/cm2, following a parabolic turbidity-pressure curve. In the case of chymosin-treated κ-casein, a similar pressure dependence of turbidity was found under the same pressure conditions. Based on these results, the pressure effects on dissociation and reassociation of casein molecules were discussed in terms of the volume change of water around their hydrophobic moieties.

Small-angle X-ray scattering was performed on whole casein under submicellar (Ca2+ removed) and micellar (Ca2+ re-added) conditions. Submicellar scattering curves showed two Gaussian components which were interpreted in terms of a spherical particle with two concentric regions of different electron density, a relatively compact core of higher electron density and a looser shell. Normalized scattering curves and calculated distance distribution functions were consistent with this picture. Micellar scattering data, which can yield only cross-sectional information related to a window of scattered intensities, could be analysed by a sum of three Gaussians with no residual function. The two Gaussians with the lower radii of gyration were again taken to indicate the two concentric regions of different electron density of inhomogeneous spherical particles; the third Gaussian was shown to reflect the packing number of these particles within a cross-sectional portion of the micelle, which was 3:1 for this system. These results are a strong indication that submicellar inhomogeneous particles containing hydrophobically stabilized inner cores exist within the colloidal micelle.

It is of industrial importance to investigate the interaction of Streptococcus lactis with phages. Although it has been long recognized that in phage–bacterial relationships the phage-carrier state can occur (Hunter, 1947), relatively little study has been done on this subject. The terras ‘phage-carrier state’ and ‘pseudolysogeny’ have been used synonymously to describe bacterial cultures which are persistently infected with a virus (Barksdale & Arden, 1974, Lawrence et al. 1976). The phagecarrier state differs from lysogenesis in that the bacteria are easily separated from the bacteriophage by a simple plating and re-isolation procedure (Graham et al. 1952). Süssmuth & Tayran (1986) showed that after lysis of one single strain, phage and phage-insensitive bacteria coexist. This work investigates the proportion of phage-insensitive bacteria remaining after lysis of other Str. lactis strains, the effect of calcium on this proportion, and the number of generations required to return to a normal sensitive population.

Casein micelles were reassembled from mixtures of F2 and F3 subunits, the major subunits of bovine casein micelles, in varying ratios. The average radius of the reassembled micelles was inversely proportional to the F2 content. Based on this relationship, a model for casein micelle assembly is proposed in which the F3 subunit forms the core of the micelle with F2 subunit occurring on the surface. Micelle size can subsequently be defined by the F2 content.

The major problem encountered in the dairy industry is due to the failure of starter in the manufacture of fermented dairy products such as cheese, Yogurt buttermilk and also the Indian fermented milk products srikhand and dahi, in terms of low acid production and absence of flavour and aroma. Usually, in such cases the presence of antibiotics is detected. Little work has been done on the effect of antibiotic resistance in the starter on flavour production, though many workers have investigated acid production and proteolytic activity (Kondratenko et al. 1978 Ottogalli et al. 1975; Shahani & Harper 1958; Wilkowske & Krienke, 1951). The present investigation has been undertaken to develop antibiotic-resistant strains of lactic streptococci, i.e. Streptococcus lactis biovar. diacetylactis and Lactobacillus delbrueckii subsp. bulgaricus which could be used for the preparation of fermanted milk products.

Hydrodynamic radii from inelastic light-scattering experiments and radii of gyration from Zimm plots give an indication of the change of average casein micelle size when the pH is changed. Combination of the results of both types of measurements gives information on changes in the micelle protein matrix, i.e. changes in the voluminosity.

The voluminosity was also determined by the pellet method and by electron microscopy which also provided comparative data on size parameters.

The properties of adsorbed films of αs1-, β- and κ-casein and sodium caseinate are compared at the planar oil–water interface, at the surface of emulsion droplets, and at the surface of polystyrene latex particles. Attention is directed towards understanding (i) the relationship between the molecular characteristics of the caseins and their surface activities and surface viscosities at the oil–water interface, (ii) the nature of competitive adsorption between the caseins (particularly αs1- and β-), and (iii) the effect of Na and Ca ion concentrations on electrophoretic mobilities and flocculation behaviour.

Glycosylation has been recognized as enhancing some functional properties of caseinates. The present work was aimed at elucidating the structure–function relationship of the functionality of caseinates and, more particularly, the influence of hydration on these properties. Measurements of non-frozen water content derived from sorption isotherms are presented. Using spin-probing Electron Spin Resonance we studied the mobilization pattern of solutes as hydration changed.The protein interactions with bound water were not significantly affected by sugar binding. Changes in the functional properties as, for example, in the water-holding capacity, reflected alterations in the structure, which was looser and more flexible after glycosylation, and in the associative properties (increased solubility) of caseinates. Disaccharides seemed more effective than monosaccharides in inducing these changes.

β-Casein, was enzymically modified by incubation with plasmin to yield γ-caseins and proteose peptones. Whole γ-, γ1-, γ2/γ3-caseins and whole proteose peptone (pp) were isolated from the hydrolysate mixture. The time dependence of surface tension at the air-water interface of solutions of β-casein and its plasmin derived fragments, at concentrations of 10−1 to 10−4% (w/v) protein, pH 7.0, was determined, at 25 °C, using a drop volume apparatus. The ranking of the proteins with respect to rate of reduction of surface tension, during the first rate determining step, at 10-2% (w/v) protein, was γ2/γ3 ≫ pp > whole γ- > γ1- > β-casein. The ranking of the proteins with respect to surface pressures attained after 40 min (π40) was concentration dependent. γ2/γ3-Caseins were found to be very surface active, decreasing surface tension rapidly and giving a high π40. γ1 Casein decreased surface activity somewhat faster than β-casein, but generally reached a lower π40. Whole γ-casein reflected the properties of both γ1 and γ2/γ3-caseins. Proteose peptone was found to decrease surface tension rapidly during the initial rate determining step; it gave a relatively high π40 at a bulk phase concentration of 10−3% (w/v) protein, but, it was the least surface active protein at 10−1 and 10−2% (w/v) protein.