The Limulus test was used to investigate the occurrence of Gram-negative bacterial lipopolysaccharide (LPS) in milk and dairy products. This test specifically detects the extremely heat-resistant LPS, and so determines the total accumulated quantity of LPS in a product whether it has been heat-treated or not. A positive correlation between number of psychrotrophic bacteria and amount of LPS in fresh farm milk was found (r = 0·63). Fresh farm milk and freshly produced market milk contained on average < 1 ng LPS/ml. The Limulus test is considered highly suitable for use in the dairy industry, being a method which will characterize quickly and specifically the hygienic level of a product by the extent of the cumulative contamination by Gram-negative bacteria.

Regioselective esterification and acetalation reactions of lactose have produced partly protected derivatives which have significant value in the further modification of lactose and as precursors for the synthesis of higher oligosaccharides.

Yoghurts were made with single strain starter organisms Lactobacillus bulgaricus and Streptococcus thermophilus, L. bulgaricus being cultured in milk + 50 ppm Na formate or in autoclaved milk and Str. thermophilus in milk treated with the proteinase Maxazyme(R) or with added casein hydrolysate. They were assessed chemically and organoleptically and compared with conventional mixed starter yoghurt. The single starter yoghurts all had acceptable acidities and acetaldehyde levels and keeping quality was good. The coagulum of each was firm and when stirred no syneresis occurred; the yoghurts remained smooth and all had an excellent appearance.

An unfortunate error arose in our previous paper ‘Electrofocusing and twodimensional electrophoresis of bovine caseins’ by P. Trieu-Cuot & J.-C. Gripon, Journal of Dairy Research (1981) 48 303–310. A paragraph describing the 2-dimensional electrophoresis technique in ‘Materials and Methods’ (p. 304) should read as follows:

The running gel was a linear gradient gel obtained with 2 starting solutions: (i) 28% acrylamide.0·7% bisacrylamide,4·9 M-urea,0·675 M-Tris-HClpH 8·8,0·1% SDS, 6·0 % sucrose, 0·04% TEMED, 0·012% ammonium persulphate; (ii) 16% acrylamide, 0·4% bisacrylamide, 4·9 M-urea, 0·675 M-Tris-HCl pH 8·8,0·1% SDS, 0·04% TEMED, 0·012% ammonium persulphate.

Isoelectric focusing and 2-dimensional electrophoresis were used to study the development of the pH 4·6-insoluble fraction during Camembert cheese ripening. Modifications of this fraction were due mainly to the action of 5 proteinases: rennet (chymosin + bovine pepsin), plasmin and Penicillium caseicolum aspartyl-and metalloproteinases. Rennet was inactive on β-casein, but acted very early on αs1-casein. However, rennet and P. caseicolum aspartyl-proteinase had a very similar action on the latter substrate, which prevented clear definition of the respective actions of these proteinases on αs1-casein after 7 d of ripening. Plasmin action on β-casein was important from 21 and 35 d of ripening at the surface and in the centre of the cheese respectively, suggesting an important influence of pH changes during maturation. The respective activities of the metallo-and aspartyl-proteinases of P. caseicolum were characterized and followed using β-casein degradation products as markers. The metallo-proteinase activity was detectable immediately after the development of the Penicillium (7 d), while that of the aspartyl-proteinase was observed 3 d later. Thereafter, the amount of β-casein degradation peptides resulting from the metalloproteinase decreased while that resulting from the aspartyl-proteinase increased, suggesting a more important role of the latter enzyme.

Treatment of food proteins with heat and/or alkali may convert some of the constituent amino acids into the unnatural amino acid lysinoalanine (LAL), which has been found to cause kidney damage (nephrocytomegaly) when fed to rats. LAL may be produced by β-elimination of a phosphoseryl, of a glycosylated seryl or of a cystyl residue followed by a nucleophilic addition to a lysyl residue. The formation of LAL in αs0-, αsl- and β-casein is discussed in relation to the location of phosphoseryl residues immediately adjacent to lysyl residues. The influence of pH, temperature and exposure time on the formation of LAL in whole casein is described in detail. The formation of LAL can be prevented by the addition of reducing agents, dephosphorylation or chemical modification of the є-amino group of lysine. The LAL content of foodstuffs can be measured accurately by ion-exchange chromatography and a survey is given of the LAL content of commercial milk and milk products.

The biological effects of free and protein-bound LAL are discussed. It seems that renal changes due to the presence of LAL in the diet are species-specific to the rat although recent findings have shown that no effect is observed in rats fed a mixed diet containing proteins with and without protein-bound LAL. When LAL is removed from the diet, nephrocytomegaly disappears. From these results it may be concluded that the health risks for man, connected with the eating of proteins which have been subjected to treatment with heat and/or alkali, are small.

The decrease in lysine content of milk and milk products due to formation of LAL is small (0–1 or 2%) in comparison with the decrease in available lysine possibly caused by the Maillard reaction (0–24%) and so the decrease in nutritive value of food products due to LAL formation may be neglected.

In this method, the curd, formed in a beaker, was retained after cutting by means of a wire grid placed on top of it. This enabled the whey to be poured off easily at timed intervals for measurement of its volume. The method gave results reproducible within 1·5%. Increasing the amount of rennet added to the milk increased the rate of syneresis of the curd slightly. Adding CaCl2 to the milk, raising the temperature and decreasing the pH, all increased the rate of syneresis. Addition of fat, as cream, to skim-milk decreased the rate of syneresis proportionally. The rate of syneresis was decreased if the milk was concentrated by ultrafiltration and increased if the milk was diluted with permeate. In all instances the early stages of syneresis followed first-order kinetics, the rate depending on the amount of whey remaining within the curd. The later stages of syneresis appeared to depend on hydrophobic forces. Probably, the increasing aggregation of the casein within the curd caused the whey to be squeezed out.

Cheesemaking is initiated by specific hydrolysis of κ-casein, followed by random aggregation of casein particles to form a network. The structure of this curd determines its subsequent behaviour and the composition and texture of the cheese. It is affected by previous homogenization or concentration of the milk. The composition of the milk and the distribution of components in it, which are affected by season, mastitic infection and cold storage, also influence its curd-forming properties, and the composition and yield of cheese. During ripening of cheese, the final texture and the development of flavour occur. Both are influenced by the type and extent of proteolysis, catalysed by coagulant, bacterial and milk enzymes. These and other enzymic and non-enzymic reactions responsible for flavour development depend on the composition and environment within the product. Some of these reactions can be accelerated by incorporating enzymes and chemical reagents into the curd, resulting in the production of an enhanced flavour. A number of compounds responsible for the flavour of cheese and yoghurt have been identified and mechanisms for their formation and retention in the product have been postulated.

Cephalotyre (Ras) cheese slurries developed within 7 d of storage at 30 °C a flavour similar to that of conventional Ras cheese ripened for 2 months. Addition of a proteinase/lipase mixture, trace elements or Na citrate to fresh cheese slurries before storage at 30 °C enhanced flavour development. Additive-treated slurries showed after storage for 5 d at 30 °C a strong flavour similar to that of 3–4 months Ras cheese with increased protein degradation, acidity development, volatile fatty acid formation and higher bacterial counts. The effectiveness of the additives in enhancing flavour development and related chemical changes was in the order: proteinaseòlipase mixture > trace elements > Na citrate.

Ion exchange is a unit operation which has several applications in the processing of skim-milk and whey. For example, in the field of whey processing, ion exchange is used in demineralization, lactose hydrolysis and in the pre-treatment of whey prior to protein recovery. In the case of skim-milk, applications include the manufacture of low Na milk and pre-acidification of skim-milk in casein manufacture. However, the ion exchange treatment of skim-milk to replace Ca ions with Na ions leads to far more fundamental changes with regard to the native protein conformation.

The effect of Ca removal from skim-milk is essentially to disrupt the native casein micelle structure into unassociated molecules of Na caseinate. Skim-milk modified in this way shows increased stability to alcohol and heat and increased functionality of the skim-milk protein in terms of water binding, emulsification and foaming capacity. The viscosity of Ca-reduced skim-milk is also substantially increased.

The improved functionality of Ca-reduced skim-milk makes the latter a potentially useful food ingredient in products where the above mentioned properties are important.