Lactobacillus delbrueckii subsp. bulgaricus cells heat treated at 67 or 68 °C for 15·5–16 s had significantly less acidification capacity and much lower endopeptidase activity than untreated cells, while they still retained high aminopeptidase activities against amino acids commonly found in bitter peptides. A stimulatory effect of heat treatment on the leakage of aminopeptidases from cells was observed at pH 5·4. These properties make them suitable as extra sources of important ripening enzymes in cheesemaking.

A total of 555 milk samples was collected from 53 Holstein cows for 1 year. Samples were analysed for N-acetyl neuraminic acid (NANA) content, κ-casein content and somatic cell count. The extent of glycosylation of κ-casein was estimated through the NANA content of κ-casein in each milk sample. The overall mean value for the NANA content was 63·6 ± 20 μg/mg of κ-casein with a range of 23–166 μg/mg. Least squares analysis showed significant (P < 0·01) effects of parity, test day, stage of lactation, phenotype for κ-casein, somatic cell count, and cow within phenotype. The NANA content of κ-casein decreased with increasing parity number and increased significantly when somatic cell count was above 200000 cells/ml. After reaching a minimum value between 2 and 3 months after calving, the NANA content of κ-casein increased during the later part of the lactation. NANA content was higher in the milk of cows phenotyped AB for κ-casein than for cows phenotyped for κ-casein AA, thus suggesting that B variant is more efficiently glycosylated than A variant.

The speed and duration of cutting the coagulum were varied during the commercial manufacture of Cheddar cheese in nine Damrow cheese vats on each of 3 consecutive days. The effects of the variations were assessed by determining the curd particle size distribution and fat and fines losses into the whey at draining. Short durations of cutting at slow speeds produced small curd particles and high fat losses into the whey. As the duration and/or speed of cutting was increased, average curd particle size increased while fat losses into the whey decreased. A maximum particle size and minimum fat level were reached after the Damrow's knife panels had completed approximately 37 revolutions. Further increases in speed and/or duration of cutting decreased curd particle size. Fat losses did not increase significantly. The hypothesis suggested to explain the results of these trials is that curd particle size is determined not by the cutting programme alone, but by a combination of the speed and duration of cutting and the subsequent speed of stirring prior to cooking. Fat losses in the whey are also influenced by this combination. Based on this hypothesis, a model is proposed which explains how variation in cutting speed and duration, followed by a constant stirring speed, determines curd particle size distribution in a Damrow cheese vat. Implications of the present findings in relation to cheese yield (losses of fat and fines into the whey) are discussed.

Preparations of total human casein and its individual fractions were isolated for production of specific polyclonal antibodies. Immunization procedures used differed in the schedule for antigen administration, antigen concentrations, degree of additional purification, and modification of the size of molecules. Immunoprecipitation techniques failed to provide an unambiguous proof of the presence of antibodies to human milk caseins in antisera even after immunization of animals phylogenetically considerably different from man. Immunoblotting, however, allowed antibodies to β-casein to be identified and compared with available monoclonal antibodies.

A rapid fluorogenic medium was evaluated for the detection of Escherichia coli in dairy products. The medium was capable of detecting Esch. coli after 7·5 h incubation at 41·5 °C. Samples of pasteurized milk (136), raw milk (63), soft cheese (60) and pasteurized cream (39) were examined with media based on 4-methylumbelliferyl-β-D-glucuronide (MUG-7) and Violet red bile agar and there were no significant differences between the numbers of Esch. coli detected on the two media. MUG-7 medium had a specificity of 98·6% and the small number of organisms giving a false positive reaction were identified as Klebsiella pneumoniae. The incidence of false negative results was ∼2%. MUG-7 medium was suitable for pour plate, spread plate and membrane filtration methods. Possible applications of the method are discussed.

The influence of lactic starter inoculation on La Serena cheese characteristics was investigated in duplicate experiments, each consisting of a vat inoculated with 1% Lactococcus lactis subsp. lactis skim milk culture and a control vat without lactic starter. Lc. lactis inoculation achieved a significant pH decrease in the interior and on the surface of cheese, which ranged from 0·09 to 0·25 units throughout ripening. Breakdown of αs1- and β-casein and formation of soluble N fractions were retarded by Lc. lactis inoculation. Levels of αs1-casein, β-casein and pH 4·6-soluble N after 60 d were respectively 30·4, 34·9 and 28·8% in cheese made with starter and 18·4, 23·0 and 34·2% in control cheese. More acidic conditions in the interior of cheese made with starter resulted in a reduction of Enterobacteriaceae, coliform and staphylococci counts, which after 60 d were respectively 2·55, 2·40 and 0·29 log units lower than in control cheese. The firmer texture obtained throughout maturation for cheese made with starter was attributed to retarded proteolysis and lower pH value. Cheese flavour quality and intensity were significantly impaired by Lc. lactis inoculation.

The psychrotrophic bacterial flora of milk was examined after aseptic hand milking. A total of 409 strains were tentatively characterized and classified into 17 groups by a reduced set of tests and cluster analysis. Lipolytic organisms (53·7%) exceeded proteolytic organisms (29·6%), and a reported predominance of oxidase-positive organisms (54·5%) was not found. Gram-negatives were mainly Entero-bacteriaceae, Flavobacterium, Acinetobacter and Pseudomonas. In the Gram-positive groups Micrococcaceae were the most abundant (44·4%) followed by coryneforms (16·3%). Only two of seven commonly used disinfectants acted differently with Gram-positive and Gram-negative organisms (P < 0·01), although in general Gram-positives were more resistant than Gram-negatives.

Lactobacilli, often used as effectors of host functions, could play an important role in maintaining human health by controlling other intestinal microorganisms capable of producing harmful effects. Using an experimental model, we studied the effect of different oral doses of Lactobacillus casei on the secretory IgA response and the protective capacity of the microorganism in preventing intestinal infections. The optimization of the protective dose of Lb. casei by previous feeding and the use of the lactobacillus as an immunological way to control enteric infections were investigated. We found that conventional mice were protected against infection with Salmonella typhimurium and Escherichia coli by previous feeding for 2 consecutive days with a daily Lb. casei dose of 1·2 × 109 cfu/mouse. Previous feeding for 7 d proved less effective, and feeding for 5 d afforded no protection at all. We were also able to demonstrate that the protective effect of Lb. casei against Sal. typhimurium and Esch. coli was connected mainly with the high level of IgA antipathogen antibodies present in intestinal secretions. β-Glucuronidase (EC 3.2.1.31) and β-galactosidase (EC 3.2.1.23) activities, measured both in the intestinal fluid and histological samples, showed a marked increase in intestinal inflammatory response on day 5 of feeding. These results show that Lb. casei plays an important role in the prevention of enteric infections, a low dose being enough for protection against intestinal infections by increasing IgA secretion into the intestinal lumen, thus providing adequate defences for the mucosal surface. A previously administered dose of this magnitude could therefore be used as an oral adjuvant in preventing enteric infections.

The blue cheese nitrogenous fractions soluble in water and in 5% phosphotungstic acid (PTA) were analysed by HPLC after 3–180 d ripening. In the water-soluble fraction, in addition to four or five major peaks corresponding to amino acids, there were many minor peaks, which increased during ripening. The low molecular mass peptides, soluble in 5% PTA, showed ripening-induced increments. A method combining precipitation with 5% PTA, gel permeation and subsequent HPLC was used to isolate some peptides of cheese. Four peptides containing between seven and ten residues were isolated and their amino acid composition and N-terminal residues determined.

A new fermented milk was prepared by using capsular polysaccharide-producing Lactobacillus kefiranofaciens K1 isolated from kefir grains. Fermentation was carried out at 30°C for 18 h, when pH 4·5 was attained. The product had a ropy consistency and was resistant to syneresis. However, the product was given lower scores for acceptability by a consumer panel than a similar product made with Lb. delbrueckii subsp. bulgaricus.

Several strains of Lactococcus lactis subsp. lactis, Lactobacillus casei and Lactobacillus plantarum isolated from traditional goats' cheese have been studied for titratable acidity, proteolysis in milk and enzymic activities. Aminopeptidasc activities were measured with whole cells and cells permeabilized with Triton X-100. Caseinolytic activity was investigated using electrophoresis in polyacrylamide gel with sodium dodecyl sulphate. Lc. lactis subsp. lactis had a level of proteolytic activity in skim milk greater than that of Lb. casei, while this activity in Lb. plantarum was very low. Alanine aminopeptidase activity was almost non-existent for all strains tested, while lysine aminopeptidase activity appeared to be of fundamentally intracellular origin. Leucine aminopeptidase activity was also greater in cells that had been permeabilized than in whole cells for Lb. casei and Lb. plantarum. Lc. lactis subsp. lactis leucine aminopeptidase activity was greater in whole cells. No significant hydrolysis of casein was found with Lb. casei I FPL 725 and Lb. plantarum IFPL 722 permeabilized with Triton X-100 after 24 h incubation with whole bovine casein.

Salted whey (7–8% NaCl) was concentrated by ultrafiltration by a factor of 20. Sweet whey equal to the retentate volume was added and ultrafiltration was continued to a concentration factor of 20. Addition of sweet whey and ultrafiltration was repeated twice more for almost complete removal of salt from whey protein concentrate (WPC). The protein content of WPC was adjusted to 3·5% using sweet whey and the mixture was heated to 65°C for 30 min. This was mixed with buffalo milk at the rate of 0, 10, 20 or 30% and then heated at 80°C for 1, 5 or 20 min before use for yogurt manufacture. The chemical, rheological and organoleptic properties of the yogurt were investigated. WPC could be added to buffalo milk at up to 20% without affecting the quality of the yogurt produced. On the contrary, it improved the texture, mouthfeel and wheying-off of yogurt from buffalo milk. Yogurt with 30% WPC had an unacceptably weak body and texture for a set product. Heating at 80°C for 5 min was sufficient to produce good quality yogurt from buffalo milk containing WPC.

The characteristics of La Serena cheese produced using animal rennet in place of the traditional vegetable rennet were evaluated. Total solids, fat and total N were higher in whey from animal rennet than in whey from vegetable rennet. Cheese produced using animal rennet had a higher moisture content and a lower pH. Proteolysis was more rapid in the cheese produced with vegetable rennet than in the cheese produced with animal rennet, but there was less lipolysis. Enterobacteriaceae and coliform counts were respectively 1·7 and 1·4 log units higher in 60 d vegetable rennet cheese than in 60 d animal rennet cheese. Softening of cheese texture was considerably more pronounced in vegetable rennet cheese, which also showed significantly higher flavour quality and intensity.