The addition of simple aldehydes brought about large increases in the heat stability of both skim-milk and concentrated skim-milk over a comparatively wide milk–pH range. The coagulation time–pH minima of type A milks were completely removed by aldehyde treatment. Some sugars, which react readily as aldehydes on heating, were also shown to stabilize concentrated milk to prolonged heat treatment at 120 °C.

Micelles of milk heated at 95 °C for 10 min or autoclaved at 121·7 °C for 15 min were observed by electron microscopy to bear filamentous appendages, whereas micelles from raw milk had relatively smooth contours with no appendages. Experiments suggested that the appendages were composed of denatured β-lactoglobulin and showed that their development was sensitive to sulphydryl blocking agents. Firmness and syneresis of yoghurt were associated with the extent to which micelles coalesced during fermentation; the appendages of micelles in heated milk appeared to inhibit coalescence giving rise to a firmer curd with a lower tendency to syneresis.

The quantity of γ- and δ-lactones and of methyl ketones produced when bovine butterfat was heated in the presence of water vapour and the absence of air, and hence the flavour quality of the butterfat, varied widely with the diet of the lactating cows. There was a marked increase in the γ-dodecanolactone potential when the cows were transferred from pasture to a mixture of chopped lucerne hay plus crushed oats but not when they were transferred to lucerne hay only. Feeding crushed sunflower seeds, whose oil was protected against biohydrogenation in the rumen, but not feeding sunflower meal from which the oil had been extracted, raised the γ-dodec-cis-6-enolactone potential, lowered the saturated δ-lactone potential and the methyl ketone potential and simultaneously raised the proportion of linoleic acid in the butterfat at the expense of fatty acids up to and including C16.

An improved method for preparing β-lactose crystals was developed by heating a supersaturated solution of lactose under reflux. The product obtained by this method yielded large well-defined crystals.

Differential thermal analysis was used for the characterization of β-lactose. The thermograms of β-lactose obtained when heating conditions were similar to those used in the conventional measurement of the melting point gave an endothermic peak at 229·5–230 °C. A marked difference was observed between the peak temperature on the thermogram and the melting point (m.p.) of β-lactose (252–252·2 °C) given in the literature. The method for determining the m.p. of β-lactose was reexamined in detail and previously reported values could not be confirmed. By application of the conventional method for determining m.p. of organic substances, 229·5 °C (dec.) is proposed as the m.p. of β-lactose.

The reaction of αs1- and β-casein with ferrous ions and O2 has been studied. It has been found that phosphoseryl residues present in these proteins rapidly catalyse the oxidation of iron from the ferrous to the ferric state with the formation of stable ferric-phosphoprotein complexes and the consumption of the theoretical amount of O2. When, however, the phosphoseryl residues occurred in groups, the formation of ferric complexes was accompanied by uptakes of O2 in quantities significantly in excess of that required for the oxidation of ferrous to ferric iron.

The significance of these findings is discussed in relation to hypotheses which have been advanced concerning the possible biological function of phosphoprotein.

The amount of damage caused to the milk-fat globule by mechanical agitation of raw milk was assessed by several methods and related to the level of induced lipolysis resulting from the agitation. The results obtained by measuring the amount of ‘free’ fat formed, the percentage of fat in the skim-milk phase after separation, and the increase in the proportion of alkaline phosphatase associated with the skim-milk phase, showed little correlation with the level of lipolysis induced by agitation over a range of conditions. However, the level of induced lipolysis showed good correlation with the amount of lipolysis caused by addition of an exogenous lipase (from Candida cylindraceae) to the agitated milk. It is concluded that classical methods of assessing milk-fat globule damage are unsuitable for predicting the amount of fat available for lipase action and the lipolytic potential of raw milk subjected to mechanical agitation.

Streptococcus lactis C10, grown in tryptone–yeast extract–lactose broth containing [14C] bicarbonate, incorporated radioactivity into the protein and nucleic acid fractions of the cell as well as into compounds which were excreted by the organism into the growth medium. Aspartic acid was the first compound to be labelled and was the only amino acid labelled in the cell protein. All 4 bases were labelled in the cell RNA. Aspartic, succinic and lactic acids were the radioactive compounds excreted into the growth medium.

The retention of the protein nutritional value during storage of lactosehydrolysed dried milk at different water activities (aw) was studied and compared with that of ordinary milk. In the lactose-hydrolysed milk biologically available lysine decreased much more rapidly than in the ordinary milk at all the different aw studied. Thus, at conditions normally accepted for ordinary dried milk (aw ~0·2; moisture ~ 4 %) there was an available lysine loss in the hydrolysed milk of about 25 % after 2 months and about 40 % after 6-months storage at room temperature. This occurred without any visible browning. It is concluded that drying to very low aw (≦ 0·11) is necessary to obtain good stability of the protein nutritional value (PNV) in lactose-hydrolysed milk. The drying, however, must be done so that losses in nutritional value are minimal during that process. It should also be noted that fat oxidation might be a problem at such low aw. A chemical method for available lysine assay (guanidination and assay of homoarginine) gave values in good agreement with the biological evaluations with rats.

The effect of reductive methylation on the properties of αs-, β- and κ-caseins was studied using preparations of these proteins in which 20 or 60% of the lysyl residues had been reductively methylated. This modification appeared to affect the physicochemical properties of the protein to an insignificant extent and in particular, electrophoretic mobility, sensitivity to precipitation by Ca2+ and solubility of Ca salts of individual proteins were only slightly altered. The use of caseins which had been subjected to methylation with radioactive formaldehyde is suggested as a potentially valuable method for the study of protein interactions.

Resting cells of Streptococcus lactis C10 were able to synthesize aspartic acid de novo but could not actively transport aspartic acid into the cell. Intracellular aspartate was excreted from the cell in the presence of glucose but did not exchange with any extracellular amino acids. The results indicated that Str. lactis C10 obtains the aspartic acid it requires for growth by bicarbonate fixation instead of by the utilization of extracellular aspartic acid.

All 47 non-nisin producing strains of Streptococcus lactis and 12/13 strains of Str. lactis subsp. diacetylactis examined produced bacteriocins, for which the term lactostrepcins is suggested. Seven strains of Str. cremoris examined produced no bacteriocins active against 3 lactic streptococci strains used as indicators. The strains examined were divided into 3 groups: I, those producing lactostrepcins active against only one streptomycin resistant mutant of Str. lactis 60 indicator strain; II, those producing lactostrepcins active against all 3 indicator strains; III, those not producing lactostrepcins active against the indicator strains employed. The lactostrepcins were sensitive to various proteolytic enzymes and to phospholipase D, but retained full or partial activity after dialysis. Most of the bacteriocins studied were fully active only within the pH range 4·2–5·0 and were reversibly inactivated at pH 7·0 or 8·0. Results suggested occurrence of 4 different lactostrepcins. The lactostrepcins produced by all group I strains were the same, but there were differences among the lactostrepcins produced by group II strains. Lactostrepcins killed some β-haemolytic streptococci and some strains of Lactobacillus helveticus. One of the lactostrepcins was also active against certain Leuconostoc strains, but not against other Leuconostoc strains, nor against L. helveticus or other Gram-positive bacteria.

The effect of heat treatment of milk on low molecular weight, volatile compounds was studied in order to relate changes in the flavour of milks to changes in chemical composition. Milks were heat treated in a UHT plant for 3 or 90 s at 140 °C and stored at ambient temperature for periods up to 112 d. Volatile compounds in raw milk and in heated milks were isolated by a low temperature spray distillation technique and identified using gas chromatography and mass spectrometry. Cabbagey defects in heated milks are correlated with total volatile sulphur and it is concluded that the compounds H2S, COS, CH3SH, CS2 and (CH3)2S could be responsible for this defect.

The recovery of proteins from whey by complex formation with carboxymethyl cellulose (CMC) has been studied. After precipitation of the CMC–whey protein complex at pH 3·2, the complex was separated, washed and redissolved at pH 7. The CMC was separated from the protein by precipitation as its Ca salt. By this method protein concentrations in the supernatant liquor of up to 6 times that of the original whey were achieved. Between 10 and 70% of the protein was precipitated with the Ca CMC, depending on the pH and the excess of Ca present. The effective protein binding capacity of the recovered CMC decreased on re-use because of firm binding of both protein and Ca to the CMC.

The precipitation of proteins from whey by modified carboxymethyl cellulose (CMC) was examined. Carboxymethyl cellulose containing acetyl, methoxyl or carboxymethyl secondary amides with iso-propyl, n-propyl, iso-butyl, n-butyl or cyclohexyl side chains precipitated more protein than did CMC of similar carboxyl content. This effect was greatest in CMC containing secondary amide groups. The increase in protein precipitation gained by introduction of secondary amide groups into CMC was no greater however than the increase obtained by introduction of a similar number of carboxymethyl groups into the polymer. Carboxyethyl cellulose did not form insoluble complexes with whey proteins under the conditions examined.

Thirty-one strains of mesophilic and thermophilic lactic acid bacteria and their respective phages were tested for their minimum, optimum and maximum multiplication temperatures. Culture growth was strongly influenced by temperature during the first few hours of incubation, but less so after 24 h. Most of the phages showed the same pattern of development as their hosts, but one phage lysing a thermophilic lactobacillus and 3 phages lysing mesophilic streptococci proved temperature-sensitive, having a lower maximum temperature than that of their hosts. One phage was unusual in that its minimum development temperature was 7 °C above that of its host. Differences in temperature sensitivity were insufficient to reduce risk of phage infection by temperature control in industrial processes.

Separated milk was treated with rennet at 30 °C for known proportions of the visually-determined clotting time. Samples were fixed with glutaraldehyde, which inhibited aggregation of casein micelles, mixed with agar, and stained sections examined. Bias in the preparation of the electron micrographs was minimized by replication, particularly in the early stages of sample preparation, and, for thin (60–90 nm) sections, by systematic selection of the areas examined.

Whole casein micelles, viewed in stereo micrographs of thick (about 1 μm) sections, had open, diffuse structures and their appearance did not change on treatment with rennet. Micrographs of thick sections taken at 90° to each other showed that casein micelles form a network-type of gel, with the same structure in all dimensions. Counts of micelles larger than about 20 nm diam. in fields of thin sections showed that each of 3 sets of fields was similar and representative for each sample. The number of micelles/μm2 appeared to increase between 60 and 130 % of the clotting time, but this change was not statistically significant.

Strains of coryneform bacteria which produce methanethiol from L-methionine in laboratory culture have been isolated from Cheddar cheese and raw milk Many Gram-negative raw milk isolates including Pseudomonas fluorescens, Ps. putida, Ps. fragi, Ps. aureofaciens, Acinetobacter, Aeromonas and Serratia marcescens also produced methanethiol. Representative strains of coryneforms and pseudomonads produced methanethiol only from free L-methionine; glycyl-L-methionine and D-methionine were not degraded. The aerobic nature of the methanethiolproducing bacteria and the instability and alkaline pH-dependence of their thiol producing enzymes explained their failure to increase methanethiol concentrations in experimental cheese, even when large numbers (5 × 107/g milled curd) of pregrown cells were added. Other commonly occurring cheese bacteria such as starter streptococci, micrococci or lactobacilli either did not produce methanethiol or produced only traces. It is concluded that the methanethiol normally present in Cheddar cheese is unlikely to be the product of bacteria or their enzymes.

Peptide and amino acid transport in Streptococcus lactis was both energy-and temperature-dependent. Separate systems were present for the transport of amino acids, dipeptides and oligopeptides as well as for individual amino acids. Both tri- and tetra-peptides were readily transported while an apparent size restriction at the point of uptake resulted in a relatively low rate of uptake of the pentapeptide. The effects of pH, temperature and salt concentration on the uptake of amino acids and peptides were studied. Exchange reactions played a significant role in the uptake of both peptides and amino acids.

The aggregation of casein micelles in milk by the action of rennet, examined by electron microscopy and viscometry, was correlated with visually-observed coagulation. The spatial relationships between micelles were quantified from measurements made on micrographs. Aggregation did not start until about 60% of the rennet-clotting time, when the enzymic action was almost complete. The micelles tended to form chains which then linked into a loose network. This became more extensive with time as the average size of the aggregates increased. By the clotting time, most micelles were in contact with another and a network was beginning to form. Initially, micelles probably linked by bridges. These then appeared to contract, bringing the particles into contact and eventually causing partial fusion.