A laboratory scale bioreactor was used for continuous acidification and inoculation of milk with a proteinase-negative, lactose-fermenting strain, Lactococcus lactis subsp. lactis C2S. Calcium alginate-entrapped cells were immobilized on a spiral stainless steel mesh incorporated into a column bioreactor and used to acidify and inoculate reconstituted skim milk. Characteristics of the immobilized cell bioreactor (ICB) were compared with those of a free cell bioreactor (FCB) during challenge with a virulent phage. Steady state biomass and lactate productivities were respectively 25-fold and 12-fold larger with the ICB than with the FCB. The ICB and the FCB were inoculated with the prolate phage c2 at multiplicities of infection of 0·25 and 0·02 respectively. Within 90 min of the infection, the FCB viable cell concentration dropped by five orders of magnitude and never recovered, while the plaque forming units/ml increased dramatically. In the ICB, released cells decreased immediately after infection, but subsequently increased, while the plaque forming units/ml steadily declined, indicating that phage were being washed out of the bioreactor. Productivity of FCB decreased to zero, whereas productivity of the ICB only decreased ∼ 60% and subsequently recovered to its initial steady state value.

Two different investigations were conducted on the chemiluminescent activity of bovine milk polymorphonuclear neutrophil leucocytes (PMN) activated by different stimuli: zymosan, derived from the wall of Saccharomyces cerevisiae (Experiment A), and Streptococcus uberis and Escherichia coli (Experiment B). In Experiment A, a quarter with a phagocytic activity of PMN < 20 mV/1000 PMN following stimulation with zymosan was found to be 23 times more likely to be a clinical mastitis case than a quarter with phagocytic activity above this threshold value. In Experiment B, calculation of the odds ratio showed similar results following stimulation with Str. uberis or Esch. coli. These results provide evidence that immunocompromisation of mammary gland defences could predispose to clinical mastitis. They also support the need to challenge phagocytic cells with appropriate stimuli, and the Esch. coli test seems to be the most sensitive.

The nature and composition of the unsaturated (monoenoic) oxo fatty acids in Cheddar cheese have been re-evaluated. Following isolation by column chromatography and silver ion HPLC, the fatty acids were identified in the form of their picolinyl ester derivatives by gas chromatography–mass spectrometry. For confirmation, they were analysed similarly following deuteration of the double bonds with deuterium and Wilkinson's catalyst. Fifteen fatty acids were identified fully, only five of which corresponded to those reported in an earlier study. Isomers with double bonds in positions 5, 7, 9 and 11–15 were identified, with oxo groups in positions 8, 9, 10, 13, 16 and 17, and with chain-lengths varying from C14 to C18, but with C18 predominating.

The effect of brining time and smoking on the free fatty acid content of Idiazabal cheese during ripening was examined. The main free fatty acids considered underwent at least some increase during the first stage of ripening before day 90 and tended to level off around a constant value towards the end of the ripening period. There were significant differences in free fatty acid levels during ripening among cheeses with different brining times and between smoked and unsmoked cheeses. Brining time and smoking exerted marked effects on lipolytic activity during cheese ripening, depending upon the free fatty acid involved and ripening time. In general, brining and smoking led to increases in free fatty acid levels at the end of the ripening period; the different behaviour of butyric acid may be due to a specific lipolytic activity.

The antimutagenic effects of uninoculated milk and milks cultured with Bifidobacterium or Lactobacillus strains towards the mutagenicity induced by two direct mutagens, 4-nitroquinoline N-oxide and 2-nitrofluorene, and three dietary indirect mutagens, aflatoxin B1, benzo(a)pyrene and quercetin, were investigated using the in vitro Salmonella typhimurium test. Each cultured milk sample and control milk had a significant antimutagenic effect, to an extent varying with the mutagen used. Uninoculated milk had a greater inhibitory effect than cultured milks towards dietary indirect mutagens.

Skim milks were pre-acidified to pH 6·4 and concentrated by ultra-filtration to give retentates with protein levels of 210 g/1. Retentates were blended with skim milk and cream to give standardized milks with protein levels ranging from 30 to 82 g/1. These were used for the manufacture of Cheddar cheese in conventional equipment. Increasing milk protein level resulted in reduced gelation times, increased curd firming rates and a decrease in the set-to-cut time when cutting at equal firmness values (i.e. elastic modulus, G′, ∼ 16 Pa). As the curd firming rates increased with milk protein level, it became increasingly difficult to cut the curd cleanly, without tearing, before the end of the cutting cycle. Reflecting the tearing of curd, and consequent curd particle shattering, fat losses in the running wheys were greater than those predicted on the basis of volume reduction (due to ultrafiltration) for milks with protein levels > 50 g/1. Reduction of setting temperatures, in the range 31–27 °C, and the level of added rennet brought the set-to-cut times and curd firming rates of concentrated milks closer to those of the control milk. While increasing milk protein level in the range 30–70 g/1 had little effect on cheese composition, it resulted in slower proteolysis and maturation.

Several methods used for sample preparation and for the determination of volatile flavour compounds in dairy products were tested. Steam distillation was an advantageous method for isolating volatile substances. The extraction yields for acetaldehyde, ethanol and diacetyl were >90%, and acetoin was partly separated from diacetyl. After steam distillation of the sample, gas-liquid chromatography was found to be a rapid method for the determination of acetaldehyde, ethanol and diacetyl in a wide range of concentrations corresponding to those found in dairy products. However, the lower limits for reproducible measurements were high (250, 1250 and 65 μM respectively). From the same sample, colorimetry was shown to be an accurate method for the detection of low levels of diacetyl and acetoin (12 and 57 μM respectively). Diacetyl interfered in the colorimetrie determination of acetoin. However, the interference was < 10% when the diacetyl: acetoin molar ratio was < 0·033. Dynamic head-space gas chromatography proved to be a sensitive method for acetaldehyde and diacetyl determination when a ‘purge and trap’ injector and a capillary column were used. The ranges suitable for determination lay between 25 and 100 μM for acetaldehyde and between 2·9 and 11·4 μM for diacetyl. Acetoin was analysed by HPLC on a cation-exchange column and detected by refractometry for concentrations ranging from 250 to 4000 μM.

Physicochemical, organoleptic and microbiological characteristics of semi-hard ewes' milk cheeses, frozen immediately after manufacture under two different conditions at −35 and −80°C and stored at −20°C for 4 months, were studied during subsequent ripening. Frozen storage after either freezing condition did not result in marked alterations in overall composition or rheological and sensory properties of the cheeses after 45 d ripening. Water activity decreased over the ripening period and this decrease was more pronounced in the frozen batches. The water-holding capacities of the protein in the frozen cheese batches were higher than for the control batch. The level of proteolysis, estimated from the level of amino groups, was higher in the previously frozen cheese. Freezing also reduced the viability of the microbial flora in the cheese. The differences in these characteristics for the two freezing conditions were not significant.

The effect of giving yogurt supplements to Balb/c mice on the various gut-associated lymphoid cells was studied. Animals were fed for 2, 5, 7 and 10 consecutive days. The different lymphoid cell types were identified and counted by haematoxylin–eosin staining of histological slices. The numbers of cells secreting IgA, IgG and IgM and the numbers of T lymphocytes were determined by direct immunofluorescence. The degree of activation of the intestinal macrophages in the small intestine was assessed by measuring the β-glucuronidase (EC 3.2.1.31) released into the intestinal fluid, and also by a histochemical method. Throughout the feeding period, there were no histological alterations in the gut, but there was marked cell infiltration, mainly of plasma cells and lymphocytes. The number of macrophages on the small intestine increased significantly after feeding for 2 d, while the β-glucuronidase activity was only slightly higher that of the controls. After a 7 d feeding period, the number of IgA secreting cells increased, while the values for cells secreting IgM and IgG and for T lymphocytes remained similar to those of the controls. The effect of giving yogurt on lymphoid cells associated with the large intestine was mainly on the numbers of IgA secreting B cells and T lymphocytes, with a marked increase during the whole feeding period in the latter type of cell. Since giving yogurt mainly enhanced the IgA secreting B cells in both small and large intestines, this increase would strengthen the host's defence mechanisms in the intestinal mucosa. Although the number of macrophages was increased, there was no enhancement in their activity, which might have harmed the host by producing an inflammatory response.

The performance of khoa, a local heat-concentrated milk product, as a milk solids source in ice cream was tested by replacing 250, 500, 750 and 1000 g/kg of the condensed whole milk in control ice cream. Proximate composition was little affected by the inclusion of khoa at any level tested. At levels of inclusion > 250 g/kg the viscosity of both fresh and aged experimental mixes was significantly (P < 0·05) increased compared with controls. Some mixes containing khoa appeared slightly off-white, but there were no overall differences from control. Control and experimental mixes were identical in whipping capacity and overrun. In organoleptic evaluation, the colour score of the ice creams declined significantly (P < 0·05) when 750 or 1000 g/kg of the condensed milk was replaced with khoa. Ice creams containing khoa were equivalent or superior to controls in flavour. In objective tests, the experimental ice creams showed significantly (P < 0·05) faster meltdown compared with control, but no differences were found in subjective assessments. We conclude that khoa can be used successfully as a milk solids source for ice cream without any serious adverse effects on quality, the only problem being the dull colour.