This research paper investigated the hypothesis that the high production of 3-methylbutanal by lactic acid bacteria is achieved not only through a direct metabolic pathway but also via an indirect pathway. 3-methylbutanal is a flavour compound that contributes to the sensory profile of various cheeses, often described as nutty, malty, or chocolate-like depending on its concentration and the cheese matrix. Although it is desired in certain cheese varieties, its acceptability varies among different cheese types. Among the various compounds responsible for this nutty scent, 3-methylbutanal plays a pivotal role. Certain strains of lactic acid bacteria are known to produce 3-methylbutanal from leucine, via both direct and indirect pathways. While it has commonly been assumed that the direct pathway is the primary mechanism, this study reports the discovery of Lactococcus cremoris AJ01, which produces 3-methylbutanal predominantly through an indirect pathway. We first investigated the effects of milk fermentation conditions on 3-methylbutanal and acid production by strain AJ01. The effect of leucine supplementation on 3-methylbutanal production was then examined. Finally, we explored the metabolic pathway responsible for 3-methylbutanal synthesis in this strain, determined the nucleotide sequence of the pyruvate dehydrogenase gene involved in the indirect pathway, and evaluated its enzyme activity to assess its contribution to production. When cultured in milk at 20 °C, strain AJ01 produced titratable acidity at >0.6% and 3-methylbutanal at >6 mg/kg within 18 h. Adding leucine to the milk enhanced 3-methylbutanal production, while excessive leucine supplementation inhibited it. Strain AJ01 exhibited activity of pyruvate dehydrogenase, while no activity of α-keto acid decarboxylase, the direct-pathway enzyme, was detected, suggesting that it produces 3-methylbutanal via the indirect pathway. These findings may contribute to the effective management and enhancement of flavour profiles in fermented dairy products, particularly cheeses.

The senescence-accelerated mouse develops normally until 5–6 months of age and then displays rapid and irreversible advancement of senescence manifesting as clinical signs and gross lesions. To clarify the effect of lactic acid bacteria on the physiological changes with increasing age, heat-killed Lactococcus lactis G50 was administered to 1-month-old senescence-accelerated-prone mouse (SAMP)6 mice for 11 months, a senescence-accelerated mouse strain that develops senile osteoporosis. Mice fed G50 gained more weight than the control mice (not fed G50) during the feeding experiment. Faecal IgA levels in the mice fed G50 at 3 months were higher than those of the control mice but decreased to control levels with increasing age. The numbers of viable cells of Bacteroides sp., Lactobacillus sp., Staphylococcus sp., Enterococcus/Streptococcus sp. and Enterobacteriaceae sp. in faeces were similar for mice fed the G50 and control diets at any age, but strain G50 suppressed the intestinal growth of H2S-producing bacteria. Bone density of the thigh bone did not differ between aged G50 and control mice. Strain G50 would be a beneficial bacterium for the enhancement of intestinal immunity during youth and to suppress the growth of harmful intestinal bacteria. The applicability of strain G50 for the food and animal industries has been proposed in the present study.

A treatment containing a live food-grade organism, Lactococcus lactis DPC3147, was compared with conventional antibiotic therapy for its potential to treat bovine chronic subclinical or clinical mastitis in two separate field trials. Effects on disease symptoms and bacteriology were monitored in response to infusion with the culture in each trial. In the first trial, the live culture treatment was compared with an intramammary antibiotic (n=11 quarters for each treatment). Results from this small trial demonstrated that the live culture had potential to be as effective at eliminating chronic subclinical infections as an antibiotic treatment. By day 12, 7 of the 11 quarters treated with the live culture were pathogen-free compared with 5 of the 11 antibiotic-treated infected quarters. Somatic cell counts (SCC) remained relatively unchanged regardless of treatment: average log SCC pre- and post-treatment in the lactococci-treated group were 6·33±0·41 (day 0) and 6·27±0·43 cells/ml (day 12) and average log SCC pre- and post-treatment in the antibiotic-treated group were 6·34±0·37 and 6·22±0·46 cells/ml on day 0 and on day 12, respectively. In the second trial, the live culture was compared with an intramammary antibiotic for the treatment of naturally occurring clinical mastitis cases (n=25 quarters for each treatment). Following a 14-d experimental period, similar bacteriological responses were observed in 7 out of 25 live culture treated quarters and 9 out of 25 antibiotic-treated quarters. Additionally, 15 of 25 cases treated with the culture and 18 of 25 cases treated with the antibiotic did not exhibit clinical signs of the disease following treatment. The results of these trials suggest that live culture treatment with Lc. lactis DPC3147 may be as efficacious as common antibiotic treatments in some instances.

In the accompanying article, we demonstrated that a live culture of Lactococcus lactis compares favourably with antibiotics for treatment of bovine mastitis in two initial field trials. In an effort to explain the mechanism involved, this study investigated the effect of culture administration on the local immune response. In this respect we initially observed that infusion of the live culture Lactococcus lactis stimulated substantial recruitment of polymorphonucleocytes (PMN) and lymphocytes to the udder. For instance, in one assay, quarters infused with the probiotic experienced a dramatic increase (~20 000-fold) in neutrophils over the first 48-h period from an average value of 83·6 cells/ml pre-treatment to 1·78×106 cells/ml 48 h post-infusion. Levels of the acute phase proteins haptaglobin and milk amyloid A were also elevated significantly in comparison with controls following infusion of the culture. The results of flow cytometric assays also demonstrated that while infusion of a live lactococcal culture led to an enhanced recruitment of PMN to the udder (from 1·85×104 cells/ml pre-infusion to 1·45×106 cells/ml 24 h post-infusion) cell-free supernatant from the same culture was not able to do so, indicating that live Lc. lactis can specifically trigger the mammary immune response to elicit PMN accumulation. These results suggest that the mechanism responsible for this probiotic treatment of mastitis is associated with stimulation of the host intramammary immune system.

Milk fermented with mixed dahi cultures NCDC167, Lactococcus lactis ssp diacetylactis NCDC60 and two probiotic strains; Lactobacillus acidophilus NCDC14 and Lb. casei NCDC19 were evaluated after fermentation (14 h) and during 8 d storage at 7 °C. The β-galactosidase activity was found to increase after fermentation leading to the hydrolysis of lactose and production of glucose, galactose and oligosaccharides; that subsequently decreased during storage. The viable counts of lactococci and lactobacilli decreased during storage yet remained >106 cfu/ml after storage. The results of present study indicate that all the selected cultures have ability to produce oligosaccharides (prebiotics) due to transgalactosidal and lactose hydrolysis activities of β-galactosidase. The cultures developed an active synbiotic formula by maintaining sufficient probiotic viable counts to exert health benefits to the consumers.

High pressure processing was investigated for controlling Cheddar cheese ripening. One-month- or 4-month-old Cheddar cheeses were subjected to pressures ranging from 200 to 800 MPa for 5 min at 25 °C. The number of viable Lactococcus lactis (starter) and Lactobacillus (nonstarter) cells decreased as pressure increased. Subsequent storage of the control and pressure-treated cheeses at 10 °C caused viable cell counts to change in some cases. Free amino acid content was monitored as an indicator of proteolysis. Cheeses treated with pressures [ges ]400 MPa evolved free amino acids at significantly lower rates than the control. No acceleration in free amino acid development was observed at lower pressures. Pressure treatment did not accelerate the rate of textural breakdown compared with the non-pressure treated control. On the contrary, pressure treatment at 800 MPa reduced the time-dependent texture changes. Results indicate that high pressure may be useful in arresting Cheddar cheese ripening.