Graeme D. Ford

Chemical composition and coagulating properties of renneted milks from different breeds and species of ruminant

The detailed chemical composition of 25 milks from different breeds of cow, sheep and goat were related to their properties of rennet clotting, coagulum development and syneresis at pH 6·4. Experiments in which concentrations of fat and whey proteins in milk were manipulated, and in which milks were homogenized at different pressures, were also carried out and the effects observed were related to the above processes. The composition of milks varied widely and many relations between concentrations of components could be related to their known modes of secretion from the alveolus or to their structural functions in the colloidal phase of milk. Rennet clotting was related to total Ca concentration and also to the proportions of α s - and β-caseins present. Coagulum strength was strongly related to total casein concentration in milk but was unaffected by total fat content or by casein/fat ratios. Syneresis was inversely related to fat content of milk and little affected by coagulum strength. Homogenization retarded syneresis, probably by altering coagulum structure and modifying entrapment of fat globules within the coagulum.

Effects of variations in somatic cell count on the rennet coagulation properties of milk and on the yield, composition and quality of Cheddar cheese

Cheddar cheeses were prepared from low somatic cell count (SCC) milks and from milks containing 10, 20 and 30% added high SCC milk to give modestly elevated SCC. Increased SCO in the milk were associated with a reduction in the coagulum strength and an increase in moisture content of the cheeses. Cheese texture was affected significantly as judged both by the findings of an assessment panel and by instrumental physical measurement. The panel scores for flavour were also slightly altered. It was concluded that even a small increase in SCC is detrimental to cheese composition and quality, although no significant effects on yield were detected. Relationships between cheese yield, composition and properties, coagulation properties and composition of the milk were also studied. Yield was related to the total solids and fat in the milk but not to the coagulation properties or the flavour or texture of the cheeses. Coagulum strength, cheese composition and cheese texture were strongly interrelated.

Some factors affecting the post clotting development of coagulum strength in renneted milk

Effects of pH, added Ca, rennet concentration, temperature, concentration and dilution of milk, on rennet clotting and coagulum development have been investigated. Rennet clotting time (RCT) was decreased by reduced pH, by increased temperature and by increased concentrations of rennet or added Ca. Only extremes of concentration or dilution of milk increased RCT. Strength of milk coagulum at 1 h after RCT was increased by reduction in pH and by increased concentrations of added Ca. Increased temperatures decreased coagulum strength whereas increased rennet concentrations had little effect. A curvilinear relationship between total casein concentration and coagulum strength was found for Friesian and Jersey milks. Two phases of coagulum development after clotting were observed. Changes in pH particularly affected the first phase whereas changes in temperature particularly affected the second. Effects of rennet, casein and Ca were similar for both phases.

Chemical composition and coagulating properties of renneted milks from cows during early lactation

The detailed composition, pH, and the properties of rennet clotting, coagulum development and syneresis at pH 6·4 were estimated in milks from 4 Friesian cows during the first 9 weeks of lactation. The concentrations of casein, fat, citrate, Ca, Mg, P i and Na decreased significantly during early lactation whilst the pH and concentration of lactose increased. Levels of whey protein and K and the casein:fat ratio were unaltered. Considerable variation occurred in the relative proportions of α-, β- and κ-casein. These changes in composition were associated with decreasing coagulum strength and an increased rate of whey drainage although rennet clotting time (RCT) did not follow a significant trend. RCT was positively related to levels of Na and whey protein but negatively correlated with P i , K and lactose. There were significant positive correlations between coagulum strength and casein, fat, non-protein nitrogen, P i , Ca, Mg and citrate. The correlation with α-casein was stronger than that with β- or κ-casein. Syneresis time was positively related to fat, Ca and coagulum strength, but negatively related to the concentration of lactose.