To compare the resolving power of starch-urea gel (SUGE) and polyacrylamide-agarose gel electrophoresis (PAAGE) in caprine casein analysis, polyacrylamide gel isoelectric focusing (PAGIF) was used as reference method. The PAAGE or SUGE patterns in the first dimension were allowed to migrate by PAGIF in an orthogonal direction giving rise to two-dimensional (2-D) separations. Using this procedure, some individual bands considered to be homogeneous by SUGE or PAAGE were found to be complex mixtures of casein components. A detailed analysis of the analytical capabilities of SUGE and PAAGE was performed. Procedures based on the densitometric reading of SUGE and PAAGE patterns seemed to give inaccurate results on the quantitative composition of caprine casein. Among the electrophoretic methods assayed, the 2-D procedure gave the best resolution of casein fractions.

When 10% citric, lactic or acetic acid was added to sheep's milk cheese whey during its heating for Myzithra cheese preparation, protein retention in the cheese was increased. Acidification of whey to pH 5·2 with lactic acid before heating followed by readjustment to pH 5·8 with NaOH was considered to be the best treatment for practical use, increasing whey protein retention in the cheese and yield without loss of flavour, aroma or texture.

An extracellular metalloproteinase from Aeromonas hydrophila strain A4, isolated from milk, was purified by a factor of 300 by chromatogrpahy on DEAE-cellulose and Sephadex G-150. The enzyme had a mol. wt of 43000 and contained 2 g atom Ca/mol. It was active over a pH range 4·8–9·5 and had optimum activity on casein at pH 7·0 with Km = 0·17 mM. It was strongly inactivated by metal chelators and the apoenzyme was fully reactivated with Ca2+, Mn2+ or Co2+. Heavy metal ions such as Ag+, Hg2+, Fe2+, Zn2+, Cd2+, Ni2+ and Cu2+ totally or partly inactivated the enzymic activity at 5 mM concentration. The enzyme was not inactivated by diisopropylfluorophosphate, soyabean trypsin inhibitor or sulphydryl group reagents. It was optimally active at 45 °C; above 50 °C activity declined rapidly, but significant activity persisted at 4 °C. It was heat labile in phosphate or Tris-maleate buffer but exogenous Ca2+ afforded protection.

The influence of preheating intensity was examined on deposit formation in an indirect ultra high temperature (UHT) plate heat exchanger and on some quality parameters of the UHT treated milk, such as taste, proteinase inactivation and protein destabilization during storage. Response surface methods were used to define the relationship between the forewarming conditions and the responses. More intense forewarming reduced the fouling rate in the UHT plate section, resulting in longer operation times. While a greater inactivation of the proteinase(s) in the milk was obtained, the taste of the UHT milk was adversely affected, as well as the physical stability during storage owing to higher sedimentation. An optimum temperature/time range for the preheating process could be defined within the temperature/time range of 76–80 °C/40–70 s at which adverse effects were minimal, and positive effects were maximized.

Continuous production of concentrated cells of Streptococcus salivarius subsp. thermophilus was achieved in a continuous stirred tank reactor coupled with an ultrafiltration module. A cellular productivity nine times higher than that obtained by conventional methods was observed. The viability of the cells produced was as satisfactory as that obtained by classical culture, but an increase in latency time and a slight decrease in acidification rate during downstream tests in milk were observed. Specific lactic acid productivity decreased as biomass increased. This process produced concentrated starters and lactic acid which could be purified by a downstream processing such as electrodialysis.

The half-life (∼ 2 h) of commercial yeast lactase (Kluyveromyces marxianus) in milk at 45 °C was 20 times greater than in milk salts and 50 times greater than in phosphate buffer. K was a more effective stabilizer than Na, both in milk salts and buffer. Stability was markedly reduced by the absence of divalent cations. Lactose and caseinate both stabilized the enzyme about 5-fold in milk salts, but together they stabilized it almost 50-fold. At equimolar concentrations, galactose was almost as effective as lactose as a stabilizer, but glucose was much less effective and sucrose had no effect. Stability in milk was independent of enzyme concentration, but varied with concentration of milk solids, reaching a maximum at about 25% solids. In milk, enzyme denaturation was more sensitive to changes in temperature in the range 42–53 °C than in the range 30–40 °C. At a concentration of 10 O-nitrophenyl-β-d-galactopyranoside units/ml it took 19 min at 45 °C to achieve 80% hydrolysis of lactose in milk, compared to 87 min at 30 °C.

A method of extracting cheese juice is described. Juice from a single one-month-old Cheddar cheese was used to investigate the relationship between the composition of the cheese juice and the occurrence of crystalline aggregates in the cheese. Two classes of crystalline aggregates were identified by light microscopy and also by electron microscopy in combination with X-ray microanalysis: the larger class contained both Ca and P whereas the smaller class contained Ca but not P. Concentrations of ions in the cheese and cheese juice were determined and corrected for the excluded volume of co-solutes and other phases present. When this was done, comparison of the concentrations of Na, K, Cl and lactate in the cheese and the cheese juice indicated that they were virtually entirely in solution, whereas appreciable amounts of the total Ca, P and P1 in the cheese were not extracted in the cheese juice. The amounts of the non-extractable salts were consistent with the volume fractions of the crystalline aggregates found by microscopy; calculations of the ion equilibria in the cheese juice showed that it was supersaturated with respect to various Ca phosphate salts and to tricalcium citrate. It is concluded that the physicochemical origin of the crystalline aggregates was the nucleation and growth of crystals from a supersaturated solution, though the sites of nucleation may have been formed by the microbial activity in the cheese. Of the free amino acids found in the juice, none was at a sufficiently high concentration to form a saturated solution, indicating that in this relatively young cheese the crystalline aggregates were not formed from amino acids.

Different selective agar media were compared for the recovery and isolation of five species of Listeria from raw milk and cheese. The selective media examined were Beerens medium, MacBride medium and that described by Dominguez et al. (1984) with 6 mg/1 acriflavine, listeria selective agar medium (LSAM), and LSAM with 12 mg/1 acriflavine (LSAM × 2A); a non-selective yeast glucose Lemco agar was included for comparison. When the difference between listeria and the natural microflora of raw milk and cheese was 102 cfu/ml, listeria could be isolated by direct plating on all media tested. When it was lower than 103–104 cfu/ml, listeria were isolated by direct plating only on LSAM and LSAM × 2A. When the difference was greater than 104 cfu/ml, a previous enrichment was necessary to isolate them. LSAM and LSAM × 2A media performed better than the other media tested for isolating listeria by direct plating and improved their isolation from dairy products. This superior performance was evaluated by the ability of these media to support colony formation of different species of Listeria tested, the easy recognition of these colonies from those formed by other microorganisms and by their capacity to inhibit the natural microflora of these foods.

Forty-five strains of staphylococci and 6 strains of micrococci isolated from pasteurized ewes' milk cheeses (Manchego and Burgos varieties) were identified to species level. Three strains of staphylococci were identified as Staphylococcus aureus. Amongst the 42 coagulase-negative staphylococci, 8 were novobiocin-sensitive and 34 novobiocin-resistant. The novobiocin-sensitive species found were Staph. auricularis (5 strains), Staph. caseolyticus (2 strains) and Staph. epidermidis (1 strain). Novobiocin-resistant strains were identified as Staph. gallinarum (5 strains), Staph. xylosus (4), Staph. saprophyticus (9), Staph. cohnii (6), Staph. sciuri (1) and Staph. lentus (3). The other 6 isolates could not be identified. Only 3 strains of micrococci were identified (Micrococcus kristinae, M. varians and M. sedentarius). The possible origin of the isolates is discussed.

When the antimutagenic activity of milk or cultured milk components on the pepper-induced mutagenicity to streptomycin-dependent strain SD 510 of Salmonella typhimurium TA 98 was investigated, whole casein showed a significant antimutagenic activity. Heating at 121 °C for 15 min completely destroyed the antimutagenic activity of casein, which was temperature dependent. While acid or papain digestion resulted in the loss of antimutagenic activity of casein, trypsin digestion had little effect although the digestion of casein by each proteinase was over 75%. β-Casein, egg albumin, and bovine serum albumin also had a similar antimutagenic activity to that of whole casein.

Undenatured whey protein (UWP) content of skim milk acidified with lactic acid or cultured with lactic cultures was estimated by a dye-binding method. The UWP content decreased with increase in acidity and the denaturation was only partly reversible on neutralization to the original acidity. The decrease in UWP was higher in cultured milk than in the milk acidified to the same extent with lactic acid, indicating the effect of lactic cultures in denaturation of whey proteins during fermentation of milk. Among the lactic cultures the denaturation effect of Lactobacillus delbrueckii subsp. bulgaricus was highest, followed by Streptococcus salivarius subsp. thermophilus, Lactococcus lactis subsp. lactis and Lact. lactis biovar diacetylactis. Denaturation of whey proteins by lactic cultures was found to be partly irreversible.

A simple, rapid and sensitive spectrophotometric assay was developed and evaluated for monitoring proteolysis during cheese ripening, based on the fact that α-amino groups released by hydrolysis of cheese proteins react with trinitrobenzenesulphonic acid to form products that absorb strongly at 420 nm. A linear relationship was shown to exist between A420 and concentration of free α amino groups up to 0·5 HIM (r = 0·999, 38 df, P < 0·001). Repeatability of the method was satisfactory. The coefficient of variance was 0·53% for amino acid solutions and 1·19% for cheese extracts. Average recovery of glycine added to the cheese was 104 ± 2·9%. A comparison of the above method with that of determination of water-soluble N to total N ratio showed that there was good agreement between these two methods of assessment of proteolysis in cheese (r = 0·857, 32 df, P < 0·001). Mainly Feta and Teleme cheese were examined, but a similar correlation was obtained with hard Greek cheeses. Analytical conditions of the procedure are discussed.

Effects of the addition of a proteinase (Neutrase 1–5S) and a peptidase (aminopeptidase DP-102) as agents for accelerating the ripening of Cheddar cheese on the survival of some non-starter bacteria (Staphylococcus aureus, Enterococcus faecalis, Escherichia coli and a Salmonella sp.) were studied throughout a 4-month ripening period. The enzymes were found to have no significant effect on the survival of the Gram-positive bacteria but some significant effects were observed, at some stages of the ripening period, with the Gram-negative bacteria in that lower levels were recovered from cheeses treated with the enzyme system.

The microflora of ten batches of La Serena cheese, made from raw milk of Merino ewes and using vegetable rennet as coagulant, was monitored throughout a 60 d ripening period. After 15 d, lactic acid bacteria predominated in the interior of the cheese, whilst lactic acid bacteria and lactic acid-utilizing species of yeasts and moulds predominated on the cheese surface. Coagulase-positive staphylococci were not detected in the interior or on the surface of any of the cheeses after 45 d and faecal coliforms were not found after 60 d. Higher death rates of staphylococci and coliforms were recorded for cheese made in spring than for the cheese made in winter.