SUMMARYThe effect of varying the casein composition of artificial micelle milk on rennet coagulation time and syneresis was examined in order to determine whether either of these processes is dependent on the concentration of particular casein components. It was found that the levels of κ-and β-caseins had a significant effect on coagulation, whereas syneresis was only affected by the level of β-casein. Partial dephosphorylation of preformed micelles or the incorporation of dephosphorylated or partly dephosphorylated β-casein into artificial micelle milk was found to have an adverse effect on both coagulation and syneresis. It was concluded that the phosphate groups of casein, particularly those of β-casein, are directly involved in the micelle-micelle interactions which occur during coagulation and syneresis.

The effect of preheating skim milk and artificial micelle milk on curd syneresis was studied. The inhibition of syneresis caused by heat was dependent on the presence of /?-lactoglobulin (/?-lg) and to a lesser extent a-lactalbumin. The degree of inhibition increased with increasing amounts of added /?-lg and preheating temperature. This agrees with the hypothesis that the detrimental effect of preheating on syneresis is due to complex formation between /?-lg and /c-casein. This complex appeared to be mediated via thiol-disulphide exchange and its formation appeared to interfere with the micelle-micelle interactions responsible for syneresis.

SummaryA micro scale syneresis assay, using the natural optical density of the whey released as a trace, is reported. This assay is suitable for use with raw, reconstituted skim-milk (RSM) or artificial milk. Using RSM the reproducibility and accuracy of the method compared favourably with larger scale laboratory assays. When the method was used to investigate the syneresis properties of artificial micelle milk (AMM), the rate and extent of syneresis and response to a major variable such as pH were very similar to those of RSM. This suggests that the assay used in conjunction with AMM is suitable for investigating the mechanism of syneresis at a molecular level.

Milk was added to a continuous fermenter to maintain a selected operating pH in the range 5-4-6-0, a fermentation system controlled in this way being called a pH-stat. Below pH 5-4 casein coagulated in the fermenter and on the pH electrodes. At pH 6-0 stirring inhibited the growth of the test culture, Streptococcus lactis C 10, but the inhibitory effect was reduced and the culture grew satisfactorily when the milk reservoir was sparged with 5 % CO 2 in N 2 or with N 2 alone. The dilution rate was highest near pH 6-0, which was close to the optimum pH for the growth of the organism. The rate of production of lactic acid and of bacterial cells in the fermenter was highest at pH 5-4.

SummaryA pH-stat fermentor is a continuous cultivator in which the feed rate is controlled to maintain a constant pH, i.e., end-product acid concentration. This fermentor has applicatian to the continuous cultivation of lactic acid-producing organisms in milkbased media. The equations describing the operation of this fermentor are developed. It is shown that, where the limiting substrate is the carbon and energy source, the operation of t h e fermentor is essentially equivalent to that of a turbidostat. In contrast to this, where the carbon and energy source is in excess and growth is limited by another substrate, pH-stat fermentation is stable both in regions of substrate excess, where D = P , ) ,~~, comparable with turbidostat operation, and substrate limitation where D < pCLlllaX, comparable with chemostat operation. These conditions are met in milk-based media. An analysis is presented, allowing the prediction of the degree of substrate limitation, cell density, and dilution rate in a pH-stat fermentor from batch-growth kinetics. This was confirmed using experimental data for the growth of S t r r p t o c w r i i s thermophilirs TS2 and Lactobacillus bulgaricus LB I in skim milk. Stable simultaneous growth of two organisms in continuous culture occurs if their growth rates are determined by separate conditions, so that, at steady state, their growth rates are separately made equal to the dilution rate. It is then predicted, and confirmed by experiment, that a mixed culture of S. fhermophiliis TS2 and L . biilguriciis LB I in a pH-stat continuous fermentor in yogurt mix at pH 5.5 would be stable with t h e growth of L. hirlguricirs LBI being substrate unlimited and t h e fermentor operating with D = p,l,ar for L. bulguric~us LBI, and the growth of S. t l w m~p k i l i i s TS2 being substrate limited so that its growth rate is equal to the existing dilution rate. Finally, it is predicted and confirmed by experiment that if the conditions are altered so that the growth of S. thannophilus TS2 is substrate unlimited the stable association is broken down, the fermentor operates with D approaching for S. thermopliilirs TS2, and L . biilgaricus LBI is washed out to the level maintained by wall growth.'