The rates of change in light reflectance and in CIELAB tri-stimulus colour values were compared for direct and indirect ultra heat treated (UHT) and sterilized milk in glass and polyethene bottles stored under different conditions. The rate of change of milk reflectance was higher at shorter wavelengths and the milk colour changed more rapidly at 30 and 37 °C than at room temperature. Sterilized milk in polyethene bottles was bleached when stored in an illuminated cabinet. The colour of skimmed milk changed more rapidly than that of whole milk. The rate of change in reflectance of direct UHT milk above 590 nm was found to be higher than in other milks. A statistical interaction was found between the fat content and the storage temperature for the CIELAB values. The homogenization pressure used in processing UHT milk samples did not affect the rate of change of the milk colour. The main implication of these findings may be that milk colour cannot be used to assess the heat treatment of a milk when the age or storage conditions of the milk are unknown.

The effect of heating raw milk at 75 °C for 30 min and at 95 °C for 15 s in the presence of varying concentrations of three oxidizing agents on the denaturation of total and individual whey proteins was measured. At 75 °C, ±-lactalbumin could be protected against denaturation by both iodate and H202 and ²-Lactoglobulins A and B by H202 and dichromate. However, at concentrations above ∼ 15 MM, the oxidizing agents tended to increase denaturation. At 95 °C, where the extent of whey protein denaturation was much reduced in the absence of oxidizing agents, little or no protective effect was observed. It is suggested that H202 may prove a suitable alternative to iodate in reducing deposits in ultra high temperature plants.

Pseudomonas fluorescens NCDO 2085 produced a single heat-stable extracellular proteinase in Na caseinate medium at 20 °C and pH 7·0. The proteinase was purified to electrophoretic homogeneity using chromatofocusing, gel filtration and ion-exchange chromatography. The purification procedure resulted in a 158-fold increase in the specific activity and a yield of 3·5% of the original activity. The enzyme is a metalloproteinase containing Zn and Ca, with an isoelectric point at 5·40±0·05 and a mol. wt of 40200±2100. It is heat-stable having D-values at 74 and 140 °C of 1·6 and 1·0 min respectively; 40 and 70% of the original activity remained after HTST (74 °C/17 s) and ultra high temperature (140°C/4 s) treatments respectively. The amino acid composition of the proteinase was determined and compared with those from other Pseudomonas spp.

Analysis of retentates of milk or whey, ultrafiltered and diafiltered by a pilot batch process with DDS Lab module equipment or (whey only) ultrafiltered by an industrial continuous process showed that nitrogen and ionic contents could not be described mathematically by the use of any value of the retention coefficient K. Analytical data suggested a new concept called segregation for nitrogen and ions in which each of these components consists of a completely permeable fraction and a totally retained fraction that do not exchange. A segregation coefficient Y is then defined as the ratio of the totally retained fraction to the total concentration of the species in the product fed to the equipment. However, this concept does not apply to lactose, where the classic retention concept (K) is retained. The two models are equivalent when K = Y = 0 or K = Y = 1. A first mathematical expression of this model was elaborated for batch ultrafiltration and/or constant volume diafiltration. Another set of equations was established for industrial conditions. These empirical models predict the retentate and permeate composition at any time during processing as well as after drying. The fit of analytical data with computed values was generally fair, with K being 0·1–0·4 in the pilot plant, and 0·1 in the factory. The nitrogen Y value was ∼ 0·95 for milk, and 0·85 for whey. In whey, the calcium Y value varied greatly from 0·06–0·71 depending on the pH, citrate content and heat treatment; in milk it was fairly constant at 0·5 at pH 6·7–5·8.

The physicochemical properties of eight extracellular proteinases secreted by psychrotrophic bacteria of dairy origin have been studied. Seven of these proteinases were able to withstand ultra heat treatment (UHT) with D values at 140 °C ranging from 2 to 300 s. The six Pseudomonas fluorescens proteinases were glycoproteins of mol. wt 47000–49500. The two Serratia marcescens proteinases, of mol. wt of 51000, did not contain carbohydrate but in other respects were similar to the Pseudomonas proteinases. The proteinases were inhibited by various metal chelators and all contained Ca and Zn in similar proportions. Their amino acid compositions were similar, with alanine as the N-terminal group, cysteine completely absent and very low levels of methionine. Isoelectric points ranged from 5·10 to 8·25. Their physical and chemical properties enabled them to be classified as alkaline metalloendopeptidases. A similarity index (S▵n) was used to predict sequence homology between ten proteinases of known amino acid composition. Comparisons of S▵n of these proteinases showed only minor sequence differences except for those of Ps. fluorescens MC60. Heat resistance could not be related wholly to similarities in protein sequence, but could be related both to the strength of stabilizing Ca2+–protein interactions and to the randomness inherent within the folding of the peptide chain.

Deproteinated whey, prepared from sweet whey powder, was digested anaerobically in a downflow fixed-bed reactor at 35 °C. Different pH control treatments were applied over a period of 210 d. Initially NaOH was added at a concentration of 68 mequiv. l-1 substrate. This was successfully replaced by 80 mequiv. Na2CO3l-1 which resulted in a 15·5% increase in biogas production and a 6·7% increase in CH4 production. A decrease in the quantity of Na2C03l-1 added led to signs of instability and a drop in CH4 production. The stepwise addition of urea restored the stability, and a further increase of urea, to a level of about 20 mequiv. l-1, made it possible to replace Na2C03 completely. When all pH control measures were omitted, complete failure resulted within 10 d. Methane yields, under stable conditions, varied between 0·321 and 0·272 m3 kg-1 chemical oxygen demand (COD) removed which corresponds to 92 and 78% of the theoretical yields. Carbon balances indicated that between 60·8 and 68·5% of the carbon fed was converted to biogas. No toxicity was observed at a carbon: nitrogen (C/N) ratio of 7·7. This was in contrast to previous results where toxicity had occurred at the same C/N ratio but at a higher concentration of Na.

A novel method for measuring tensile strength of heat-setting gels is described and is applied to whey protein gels. Contributions made by β-lactoglobulin and α-lactalbumin individually to the tensile strength of the gel are calculated. The tensile strength can be found from power law equations related to concentration of α-lactalbumin and β-lactoglobulin in whey protein mixtures.

Incubation of bacteria with casein at 30 °C (pH 7·2) resulted in the formation of a number of peptide fragments. The peptides were separable when subjected to electrophoresis. A proteinase-positive strain of Streptococcus lactis preferentially hydrolysed the β-casein moiety of whole casein, and after incubation for 5 min produced a characteristic pattern of four peptides on sodium dodecyl sulphate (SDS) polyacrylamide gels. After 30 min incubation only one peptide remained. This single polypeptide was then further hydrolysed to produce one (60 min) and eventually two (120 min) lower molecular weight peptides. Hydrolysis was generally complete after about 480 min. Two proteinase-negative variants of this strain did not hydrolyse casein. Crude proteinase preparations gave the same characteristic peptide patterns. SDS-polyacrylamide gel electrophoresis was used to screen more proteinase-positive strains of Str. Lactis for casein hydrolysing activity. Of those so far tested all produced the same peptide patterns from β-casein. Proteinase negative variants of all these strains did not hydrolyse casein.

Conditions necessary for the determination of crude extracellular lipase activity from Pseudomonas fluorescens B52 using β-naphthyl caprylate (β-NC), an 8-carbon ester, as the substrate were examined. Maximum enzyme synthesis occurred at 20 °C in pyruvate mineral salts medium containing 1 mM-CaCl2. Bile salts were necessary for enzyme activity; 6 mM-Na taurocholate or Na deoxycholate gave maximum activity but the latter compound was inhibitory at higher concentrations. Activity was optimal in N-Tris[hydroxymethyl]methyl-2-aminoethane sulphonic acid buffer pH 8·0 at 40 °C. A comparison of β-NC with β-naphthyl butyrate (a 4-carbon ester) and β-naphthyl myristate (a 14-carbon ester) showed that β-NC was the best substrate; Km values of 0·0415, 0·141 and 0·200 mM and Vmax values of 67·2, 20·1 and 5·28 μmol ml-1 h-1 were obtained for those substrates respectively. The enzyme was inhibited 50% in its activity against β-NC by 0·009 mM-EDTA, 0–0007% (w/v) mixed alkyltrimethylammonium bromide and 0·00275% (w/v) Triton X-100. The biochemical properties determined using β-NC as substrate are consistent with those reported for the lipases of other strains of Ps. fluorescens using natural substrates.

The effect of milk containing increased somatic cells on the starter organisms Streptococcus thermophilus, Lactobacillus bulgaricus and L. acidophilus was examined. Increased somatic cell count resulted in stimulation of Str. thermophilus owing to increased proteolysis, but inhibition of L. acidophilus as a consequence of increased phagocytic activity of the polymorphonuclear leucocytes

A paper chromatography technique is described for the separation and quantification of minor sugars in infant formula milks. The precision and reliability of the technique are described. Infant milk feeds of various types were analysed and found to contain up to 11·4 mmol/l of the non-absorbed sugar lactulose. Soya derived feeds may contain a significant residue of non-absorbable oligosaccharides.

Wateractivity (aw), pH, and several chemical components were determined in 20 samples of Blue cheese belonging to the varieties Bleu de Bresse, Cabrales, Danablu, Edelpilzkäse, Gorgonzola and Roquefort. The aw in these cheeses could be calculated from chemical data on a water content basis (g/100 g H2O) by two simple equations:

where SN is the soluble nitrogen.

Differences between calculated and measured aw using these relationships were similar to those obtained with a more complex equation described by Rüegg & Blanc (1983).

A method based on the hydrolysis of β-naphthyl caprylate (β-NC) has been developed for quantitating extracellular lipase from Pseudomonas fluorescens. The assay was extremely sensitive to skim milk (SM); as little as 0·02 ml raw SM in a 2·0 ml reaction mixture resulted in an apparent loss of 50% of the lipase activity. Activity improved 3-fold when trypsin (50 μg/ml) was included in the reaction mixture. When super-simplex optimization was used to determine the optimum levels of β-NC, Na taurocholate (NaTC), SM/lipase mixture and trypsin for maximum activity, NaTC was found to be unnecessary for activity. Subsequent addition of 15 mM-NaTC resulted in 80% loss of activity. On the other hand, NaTC was required for native lipase activity in the presence of SM. Native lipase was completely inhibited by heating at 70 °C for 2 min, while B52 lipase retained 75% of its activity under the same conditions. The assay was able to detect lipase produced by Ps. fluorescens B52 in SM at 5 °C when the cell density exceeded 108 colony forming units/ml. The presence of butterfat (3°5%) in the SM assay inhibited B52 lipase by 97%. The β-NC assay gave results comparable to the tributyrin agar diffusion assay using cell-free extracts of ten strains of common dairy psychrotrophs. The results suggest that the β-NC assay may be useful for determining lipase activity in raw SM.

Milk was heated at 97 °C/15 s to denature ∼ 30% of the whey protein and then used to make Cheshire cheese. Measurements of para-κ-casein production indicated that heating milk did not inhibit the enzymic action of rennet, but additional Ca and an initial pH of 6·4 were required for normal coagulation and curd-firming. In the experimental cheeses, about 4·5% more dry matter was recovered compared with controls made with pasteurized milk owing to a 6·7 and 0·7% increase in protein and fat recovery respectively. Experimental cheeses tended to be too moist and the curds did not fuse as well but these problems could be overcome by raising the scald temperature and cheddaring the curds. When the moisture in non-fat solids of control and experimental cheeses was similar (61%), the flavours and textures were not significantly different. The procedure requires little modification to existing commercial plant and should be suitable for varieties of cheese with higher moisture content and more crumbly texture than Cheddar.

Cheddar cheeses were prepared from low somatic cell count (SCC) milks and from milks containing 10, 20 and 30% added high SCC milk to give modestly elevated SCC. Increased SCO in the milk were associated with a reduction in the coagulum strength and an increase in moisture content of the cheeses. Cheese texture was affected significantly as judged both by the findings of an assessment panel and by instrumental physical measurement. The panel scores for flavour were also slightly altered. It was concluded that even a small increase in SCC is detrimental to cheese composition and quality, although no significant effects on yield were detected. Relationships between cheese yield, composition and properties, coagulation properties and composition of the milk were also studied. Yield was related to the total solids and fat in the milk but not to the coagulation properties or the flavour or texture of the cheeses. Coagulum strength, cheese composition and cheese texture were strongly interrelated.