The effects of increased somatic cell count, whether caused by infection or by experimental infusion of bacterial endotoxin, on the distribution in milk of caseins between the micellar and soluble forms were investigated. The relationship of somatic cell count to some cheese-making parameters was also studied. With quite modestly elevated cell counts (2–3 × 106/ml) increases of up to 37% in total casein in the soluble phase were observed, most of which was contributed by β-casein, while κ- and αs1-caseins increased only slightly. With storage at 4°C, the concentrations of all the caseins, Ca and phosphate in the soluble phase increased substantially during the first 48 h, but this was followed by a slight decline on further storage. Rennet clotting time, losses of fat in whey, curd moisture, and losses in curd yield and rigidity were all greater the higher the somatic count. Clear differences were detectable in these parameters between milks of very low cell count (e.g. 5 × 104 cells/ml) and milks with counts more typical of those found in bulk supplies (e.g. about 5 × 105 cells/ml). If these findings can be reproduced in commercial practice even a modest reduction in bulk milk somatic cell counts might be expected to bring definite benefits.

Hydrophobic surface areas of αs1- and κ-casein polymers and αs1-casein: κ-casein complex were estimated by the salting-out technique using various salts according to the theory of Melander & Horvath (1977). Calculated hydrophobic surface areas of αs1, κ-casein polymers and αs1-casein: κ-casein complex were 1976, 3571 and 2989 Å2 respectively. Assuming that κ-casein polymer dissociated into 4 particles in complex formation and that 1 mole of αs1-casein: κ-casein complex was produced from 2 mole of αs1-casein polymer and one of these dissociated κ-casein particles, the hydrophobic surface area of αs1-casein: κ-casein complex was less than those of 2 mole of αs1-casein polymer plus a quarter κ-casein polymer. On the other hand, the net charge of αs1-casein: κ-casein complex was nearly equal to that of 2 mole of αs1-casein polymer plus a quarter of κ-casein polymer. From these results, it was concluded that the complex formation of αs1- and κ-casein polymers was hydrophobic and that electrostatic interaction did not participate in complex formation.

For 17 phages active against Streptococcus cremoris, Str. lactis and Str. lactis subsp. diacetylactis, the killing efficiency of pasteurization (log No/N) at 72°C for 15 s in skim-milk showed large variations from > 6 to 0; the efficienty of killing during spray-drying ranged from 3.7 to 0.2 and phages survived well storage of milk powder at room temperature. Destruction in a heat exchanger was found to be greater than that calculated from biphasic curves obtained by heating phages in sealed ampoules. No relationship was established between lytic classification of phages and their heat resistance.

The reactions between 6 strains of mesophilic lactic streptococci and their respective phages were studied quantitatively. Of 30 nonhomologous reactions, the bacteria were fully sensitive in 4 and restricted the phages in 23. A mathematical model was developed that was used to identify at least 4 and probably 5 modification restriction (M/R) systems of which up to 3 were found in the same strain. The model was based on 24 measured values and correctly predicted the values of 5 others. One of the 3 negative reactions was shown to be due to a restriction beyond the limit of detection, a second was due to lysogeny or lack of adsorption, but was shown to have the predicted value when the homologous phage was modified on the host of the challenging phage. In the last reaction a measurable restriction was predicted, but could not be proven by means of a modified phage. These results suggest M/R to be one of the main defenses of the lactic streptococci against their phages. They explain why host range is not a useful criterion in the classification of phages and suggest a rational approach to the definition of a starter rotation.

The effect of ripening cow'milk cream (40% fat) with 4 strains of lactic acid bacteria on the quality of ghee, an Indian product resembling clarified butterfat, was studied by measuring changes in titratable acidity, pH, volatile acids and diacetyl content due to Streptococcus lactis, Str. lactis subsp. diacetylactis (Str. diacetylactis), Str. thermophilus and Lactobacillus bulgaricus in cream prior to conversion into ghee. L. bulgaricus produced the maximum titratable acidity (1·25% lactic acid) followed by Str. diacetylactis (0·80 % lactic acid) after 72 h. Str. diacetylactis produced the highest level of diacetyl (28 ppm), after 48 h. L. bulgaricus produced no detectable diacetyl. Both of these strains produced the maximum amount of volatile acids (2·2–3·5 ml), after 72 h. The flavour and keeping quality of ghee were improved by Str. diacetylactis and Str. lactis whilst Str. thermophilus and L. bulgaricus had no effect.

The production of a range of single-strain cheese starter concentrates by diffusion culture and some of their subsequent properties are described. Five strains of Streptococcus cremoris and 4 strains of Str. lactis were grown to concentrations of above 1.0 × 1011 cells/ml, these concentrates all being of satisfactory cheese-making potential. Ammonium lactate accumulation in the system appeared to be the limiting factor to growth, although some strains were more tolerant than others. A continuous harvesting experiment demonstrated that the equipment was potentially capable of this mode of concentrate production. A cheese-making trial was carried out with concentrates produced by diffusion culture; a higher proportion of extra and upper selected cheese was obtained than from control cheese produced with conventionally propagated commercial mixed starters.

Casein compositional studies. 1. The composition of casein from Friesian herd milks

BY J. Gerard Barry and William J. Donnelly

Journal of Dairy Research Vol. 47 (1980), 71–82, page 77, Fig. 3.

Top (right): For Month, herd 2 read Month, herd 1.

Bottom (left): For Month, herd 1 read Month, herd 2.