Antimicrobial activity of pediocin-producing Lactococcus lactis on Listeria monocytogenes,Staphylococcus aureus and Escherichia coli O157:H7 in cheese

The antimicrobial activity of two pediocin-producing transformants obtained from wild strains of Lactococcus lactis on the survival of Listeria monocytogenes, Staphylococcus aureus and Escherichia coli O157:H7 during cheese ripening was investigated. Cheeses were manufactured from milk inoculated with the three pathogens, each at approximately 6 log cfu mL⁻¹. Pediococcus acidilactici 347 (Ped⁺), Lc. lactis ESI 153, Lc. lactis ESI 515 (Nis⁺) and their respective pediocin-producing transformants Lc. lactis CL1 (Ped⁺) and Lc. lactis CL2 (Nis⁺, Ped⁺) were added at 1% as adjuncts to the starter culture. After 30 d, L. monocytogenes, S. aureus and E. coli O157:H7 counts were 5.30, 5.16 and 4.14 log cfu g⁻¹ in control cheese made without adjunct culture. On day 30, pediocin-producing derivatives Lc. lactis CL1 and Lc. lactis CL2 lowered L. monocytogenes counts by 2.97 and 1.64 log units, S. aureus by 0.98 and 0.40 log units, and E. coli O157:H7 by 0.84 and 1.69 log units with respect to control cheese. All cheeses made with nisin-producing LAB exhibited bacteriocin activity throughout ripening. Pediocin activity was only detected throughout the whole ripening period in cheese with Lc. lactis CL1. Because of the antimicrobial activity of pediocin PA-1, its production in situ by strains of LAB growing efficiently in milk would extend the application of this bacteriocin in cheese manufacture.     

Effect of combinations of high-pressure treatment and bacteriocin-producing lactic acid bacteria on the survival of Listeria monocytogenes in raw milk cheese

The combined effect of high-pressure (HP) treatment and bacteriocin-producing lactic acid bacteria (BP-LAB) on the survival of Listeria monocytogenes Scott A in cheeses made from raw milk that was inoculated with the pathogen at 4.80 log cfu mL⁻¹, a commercial starter and one of seven strains of BP-LAB was investigated. On day 3, the counts of L. monocytogenes were 7.03 log cfu g⁻¹ in a control cheese (without BP-LAB, not HP treated), 6.06–6.74 log cfu g⁻¹ in cheeses with BP-LAB, 6.13 log cfu g⁻¹ in a cheese without BP-LAB and treated on day 2 at 300 MPa, 2.01 log cfu g⁻¹ in a cheese without BP-LAB and treated on day 2 at 500 MPa, 3.83–5.43 log cfu g⁻¹ in cheeses with BP-LAB and treated on day 2 at 300 MPa, and 1.81 log cfu g⁻¹ or less in cheeses with BP-LAB and treated on day 2 at 500 MPa. HP treatment was more effective on day 51 than on day 2.     

Fresh-cheesemilk formulation fermented by a combination of freeze-dried citrate-positive cultures and exopolysaccharide-producing lactobacilli with liquid lactococcal starters

Milk formulation (4% fat and 5% protein) prepared to simulate fresh cheese production was inoculated with: (1) 10⁷ cfu mL⁻¹ of fresh liquid starters of Lactococcus lactis ssp. lactis T1 and Lc. lactis ssp. cremoris T2, (2) a freeze-dried exopolysaccharide-producing (EPS) strain of Lactobacillus rhamnosus RW-9595M, and (3) freeze-dried Leuconostoc cremoris LM057 or Lc. lactis ssp. lactis var. diacetylactis MD089 strains. The effect of inoculation rate of the freeze-dried starters (between 10⁶ and 10⁷ cfu mL⁻¹) and incubation temperature (between 23.5 and 36.5 °C) on evolution of pH and the various populations during fermentation was examined. Texture (apparent viscosity, syneresis potential) and chemical composition (diacetyl, acetaldehyde) of the fermented milks were also determined. Milk was incubated until a pH of 4.6 was obtained, which required between 6 and 10 h depending on temperature.In the range of inoculation levels used, there was no significant effect of the presence of lactobacilli, Ln. cremoris or Lc. lactis ssp. lactis var. diacetilactis on the growth of the lactococci. There was a direct correlation between the inoculation rates of the freeze-dried cultures and their final populations in the fermented milks. The growth of the cultures were also affected by temperature, Ln. cremoris growing less as incubation temperature increased, while the opposite was noted with Lb. rhamnosus. The apparent viscosity of the fermented milk was significantly affected by incubation temperature, but there was no correlation between apparent viscosity and the final population in lactobacilli. Of the three variables studied, the highest correlation with diacetyl content was obtained with the inoculation level of the Leuconostoc strain.     

The distribution of Staphylococcus sp. on bovine meat from abattoir deboning rooms

In developing countries such as South Africa, Staphylococcus aureus has been shown consistently to be one of the most important micro-organisms responsible for food poisoning outbreaks. In this study, the staphylococci in selected South African abattoirs were quantified, identified and further characterized in terms of coagulase types. The highest staphylococci counts (1.7×10⁶ cfu g⁻¹) were observed in the meat from the high throughput (Grade A) abattoir during week 3. The counts exceeded the National Guidelines (10² cfu g⁻¹) without exception and at least 50% surpassed the levels sufficient to produce toxins (10⁵ cfu g⁻¹) determined for S. aureus. Species were dominated by S. capitis, S. xylosus, S. auricularis, S. aureus and S. intermedius. In terms of the coagulase types of S. aureus, type V was the most dominant and type VI the least. It became evident that the hygiene practices implemented by the abattoirs investigated in this study were not effective enough in reducing the contamination levels of the staphylococci from carcasses. It is therefore recommended that the sampled abattoirs revise their manufacturing strategies in order to reduce the levels of staphylococcal contamination which have been shown to be transferred through food handlers, surfaces, equipment and the environment.     

Biocontrol of Staphylococcus aureus in curd manufacturing processes using bacteriophages

The ability of specific bacteriophages to inhibit Staphylococcus aureus growth in curd manufacturing processes was determined. Two lytic bacteriophages specific against S. aureus were obtained by DNA random deletion from the milk-isolated temperate phages, ΦH5 and ΦA72. A cocktail of these lytic phages, Φ88 and Φ35, at multiplicity of infection (MOI) of 100, produced a complete elimination of 3×10⁶ cfu mL⁻¹ of the pathogen in ultra-high-temperature (UHT) whole milk at 37 °C. Furthermore, the frequency of emergence of bacteriophage-insensitive mutants was reduced up to 200-fold in the presence of the two lytic phages compared with that detected with the combination of the temperate counterparts. The lytic phage derivatives, added to milk, were able to decrease rapidly the viable counts of S. aureus during curd manufacture. In acid curd, the pathogen was not detected after 4 h of incubation at 25 °C, whereas pathogen clearance was achieved within 1 h of incubation at 30 °C in renneted curd. These results indicate that lytic bacteriophages could be used as biopreservatives in the manufacture of particular dairy products.     

Microbial shelf-life of high-pressure-homogenised milk

The anti-bacterial effect of high pressure homogenisation (HPH) on milk is widely reported but the shelf-life of HPH-treated milk, as reported in this communication has not been studied thus far. Raw whole milk was homogenised at 200 or 250 MPa at 55 or 70 °C and counts of total bacteria (TBC), psychrotrophs, pseudomonads, coliforms, lactobacilli, Bacillus cereus and Staphylococcus aureus were determined throughout subsequent storage for 14 days at 4 °C. Immediately after HPH treatment, counts of all bacteria were below the level of detection but after storage for 14 days at 4 °C, TBC, psychrotroph and pseudomonad counts had reached ∼10⁸ cfu mL⁻¹ in all samples treated with HPH. The limited shelf-life obtained indicates that HPH of milk at these processing parameters it is not a suitable alternative to pasteurisation for extending the shelf-life of milk.     

Production of traditional Greek yoghurt using Lactobacillus strains with probiotic potential as starter adjuncts

Lactobacillus plantarum ACA-DC 146 and L. paracasei subsp. tolerans ACA-DC 4037 were examined for their potential application as adjuncts in the production of traditional Greek set-type yoghurt. Both strains displayed low milk acidification activity, while no inhibition was observed towards or from the yoghurt starters used. Yoghurt produced with L. paracasei subsp. tolerans ACA-DC 4037 exhibited the best sensory properties, with a rich traditional smooth taste, and the strain was selected for further trials. Yoghurt produced with this strain as an adjunct had good physicochemical properties. After 2 weeks of refrigerated storage, microbial loads (>7.0 log cfu g⁻¹) were in accordance with international recommendations and guidelines for probiotic and starter cultures in milk products. Increasing the microbial load further, using concentrated and encapsulated inocula (10–11 log cfu g⁻¹), gave yoghurt with long fermentation times and poor organoleptic properties.     

Production of volatile aroma compounds by kefir starter cultures

Production of carbonyl compounds by single-strain cultures, kefir starter (Lactobacillus delbrueckii subsp. bulgaricus HP1+Lb. helveticus MP12+Lactococcus lactis subsp. lactis C15+Streptococcus thermophilus T15+Saccharomyces cerevisiae A13) and kefir grains during fermentation and storage of kefir was studied. The content of carbonyl compounds produced by kefir starter was greater than that produced by kefir grains. The maximum acetaldehyde concentration (18.3 μg g⁻¹) in kefir with starter culture was mainly due to the metabolic activity of Lb. delbrueckii subsp. bulgaricus HP1 isolated from kefir grains. The highest diacetyl production activity was recorded in the starter culture (1.87 μg g⁻¹) and the single-strain culture St. thermophilus T15 (1.62 μg g⁻¹), followed by Lb. helveticus MP12 (0.85 μg g⁻¹) and Lc. lactis subsp. lactis C15 (0.42 μg g⁻¹). The lactobacilli Lb. delbrueckii subsp. bulgaricus HP1 and Lb. helveticus MP12 produced acetone, which was not found in the cocci cultures. The presence of 2-butanone was related to the production ability of Lb. helveticus MP12. In comparison, Lc. lactis subsp. lactis C15 synthesized ethyl acetate more actively than the other single-strain cultures included in the starter. S. cerevisiae A13 produced ethanol and CO₂ in amounts (3975 μg g⁻¹; 1.80 g L⁻¹) that lent cultured kefir distinctive flavour and aroma characteristic of authentic kefir.     

Survival of Listeria monocytogenes in Galotyri,a traditional Greek soft acid-curd cheese,stored aerobically at 4°C and 12°C

Galotyri is a traditional Greek soft acid-curd cheese, which is made from ewes’ or goats’ milk and is consumed fresh. Because cheese processing may allow Listeria monocytogenes post-process contamination, this study evaluated survival of the pathogen in fresh cheese during storage. Portions (0.5 kg) of two commercial types (<2% salt) of Galotyri, one artisan (pH 4.0±0.1) and the other industrial (pH 3.8±0.1), were inoculated with ca. 3 or 7 log cfu g⁻¹ of a five-strain cocktail of L. monocytogenes and stored aerobically at 4°C and 12°C. After 3 days, average declines of pathogen's populations (PALCAM agar) were 1.3–1.6 and 3.7–4.6 log cfu g⁻¹ in cheese samples for the low and high inocula, respectively. These declines were independent (P>0.05) of the cheese type or the storage temperature. From day 3, however, declines shifted to small or minimal to result in 1.4–1.8 log cfu g⁻¹ of survivors at 28 days of storage of all cheeses at 4°C, indicating a strong “tailing” independent of initial level of contamination. Low (1.2–1.7 log cfu g⁻¹) survival of L. monocytogenes also occurred in cheeses at 12°C for 14 days, which were prone to surface yeast spoilage. When ca. 3 log cfu g⁻¹ of L. monocytogenes were inoculated in laboratory scale prepared Galotyri of pH ≅4.4 and ≅3% salt, the pathogen died off at 14 and 21 days at 12°C and 4°C, respectively, in artisan type cheeses fermented with the natural starter. In contrast, the pathogen survived for 28 days in cheeses fermented with the industrial starter. These results indicate that L. monocytogenes cannot grow but may survive during retail storage of Galotyri despite its low pH of or slightly below 4.0. Although contamination of Galotyri with L. monocytogenes may be expected low (<100 cfu g⁻¹) in practice, that long-term survival of the pathogen in commercial cheeses was shown to be unaffected by the artificial contamination level (3 or 7 logs) and the storage temperature (4°C or 12°C), which should be a concern.     

Effect of temperature on growth and metabolism of probiotic bacteria in milk

The growth and metabolism of six probiotic strains with documented health effects were studied in ultra-high temperature (UHT) treated milk supplemented with 0.5% (w/v) tryptone or 0.75% (w/v) fructose at different temperatures. The probiotic strains were Lactobacillus acidophilus La5, Lb. acidophilus 1748, Lb. johnsonii LA1, Lb. rhamnosus GG, Lb. reuteri SD 2112 and Bifidobacterium animalis BB12. Fermentation was followed for 48 h at 20, 30, 37 and 45 °C and the samples were analysed for pH, log cfu mL⁻¹, volatile compounds, organic acids and carbon dioxide. All six probiotic strains showed very different profiles of metabolites during fermentation, however, the two Lb. acidophilus strains were the most alike. All strains, except Lb. reuteri SD 2112, showed viable cell numbers above 6.5 log cfu mL⁻¹ after 48 h fermentation at 30, 37 and 45 °C. The probiotic strains produced different amounts of metabolic products according to temperature and fermentation time illustrating the importance of controlling these parameters.     

The protective effect of monosodium glutamate on survival of Lactobacillus rhamnosus GG and Lactobacillus rhamnosus E-97800 (E800) strains during spray-drying and storage in trehalose-containing powders

The use of trehalose as a means of preserving Lactobacillus rhamnosus GG (LGG) and L. rhamnosus E-97800 (E800) during spray-drying and the effects of incorporated monosodium glutamate (MSG) in the carrier medium on the survival rates during drying and storage were examined. E800 was more resistant to heat than LGG in 20%, w/w, trehalose; the d-values at 65 °C were 14 s and 5.1 s, respectively. An air outlet temperature of 65–70 °C was taken as optimal for the drying process, as the resultant moisture levels in trehalose containing these bacteria were 4.1% (w/w) and 3.79% (w/w) with corresponding viable counts of 3.65 × 10⁸ cfu mL^?1 and 1.80 × 10⁹ cfu mL^?1, respectively. The presence of MSG increased the final viable counts of LGG and E800 to 3.05 × 10⁹ cfu mL^?1 and 1.30 × 10⁹ cfu mL^?1, respectively. Survival of LGG and E800 remained constant at a minimum level of ～10⁸ cfu mL^?1 during storage at 25 °C in trehalose–MSG medium.     

Effect of thermosonication,pulsed electric field and their combination on inactivation of Listeria innocua in milk

The impact of thermosonication (TS) and pulsed electric field (PEF), individually and combined, on the survival of Listeria innocua 11288 (NCTC) in milk was investigated. TS (400 W, 160 s) without pre-heating reduced L. innocua by 1.2 log₁₀ cfu mL^?1, while shorter treatment times produced negligible inactivation, suggesting TS to be a hurdle rather than an effective standalone treatment. PEF (30 and 40 kV cm^?1, 50 μs) at 10 °C caused a reduction of L. innocua of 1.1 and 3.3 log cycles, respectively. The highest field strength (40 kV cm^?1) combined with TS (80 s) led to 6.8 log₁₀ cfu mL^?1 inactivation. Milk pre-heated to 55 °C (over 60 s) prior to TS followed by PEF (30 and 40 kV cm^?1) showed inactivation between 4.5 and 6.9 log₁₀ cfu mL^?1, the latter being comparable (P > 0.05) with thermal pasteurisation. The data indicate that TS followed by PEF represents a valid alternative for L. innocua inactivation in milk.     

Identification of cultivable lactic acid bacteria isolated from Algerian raw goat's milk and evaluation of their technological properties

One hundred and fifty-eight strains of lactic acid bacteria isolated from Algerian raw goat's milk were identified and technologically characterized. Five genera were found: Lactobacillus (50.63%), Lactococcus (25.94%), Streptococcus (14.56%), Leuconostoc (7.59%) and Pediococcus (1.26%). The predominant species were Lactococcus lactis (32 strains), Streptococcus thermophilus (23 strains), Lactobacillus bulgaricus (19 strains), Lb. helveticus (16 strains) and Lb. plantarum (14 strains).Approximately 39% of the lactic acid bacteria isolated produced more than 0.6% lactic acid (w/v) after 18 h of incubation, and belonged to the Lactococcus and Lactobacillus genera. The highest proteolytic activity was approximately 3 mg tyrosine l⁻¹ for mesophilic strains and nearly 5 mg tyrosine l⁻¹ for thermophilic lactobacilli after 72 h. High aromatic activity (more than 0.8 mg diacetyl l⁻¹ after 16 h) was detected in 14% of the strains.Nine strains were used to make dairy products (a yoghurt-like product and Edam-type cheese) on a pilot scale in the laboratory. The best-liked organoleptic characteristics were noted in a yoghurt produced with a mixed culture made up of S. thermophilus (strain 16TMC+) and Lb. helveticus (strain 20TMC) and in a cheese made with a starter composed of Lc. lactis subsp. lactis (strain 10MCM) and L. lactis subsp. lactis (V.P. +) (strain 19MCM).     

A rapid method for the estimation of the microbiological quality of refrigerated raw milk based on the aminopeptidase activity of Gram-negative bacteria

A rapid method for the estimation of the microbiological quality of refrigerated raw milk was developed based on the aminopeptidase activity of Gram-negative psychrotrophic bacteria, the dominant microbial population in refrigerated foods. Two versions of the test for measuring the aminopeptidase activity are reported (direct and indirect). A good correlation (r=0.93–0.95) between bacterial counts estimated by conventional methods and the aminopeptidase activity determined in milk was obtained. The sensitivity of the indirect assay was 2×10⁴ cfu mL⁻¹. This sensitivity limit complies with the level of detection required to satisfy regulations in many countries. The aminopeptidase test allows the assessment of raw milk quality in approximately 2.5 h, does not require either high-cost equipment or specialised operators, and results can be interpreted both spectrophotometrically and visually.     

Probiotic properties of folate producing Streptococcus thermophilus strains

This study aimed at assessing the probiotic potential of two high folate producing Streptococcus thermophilus strains (RD102 and RD104) isolated from Indian fermented milk products by both in vitro and in vivo tests. These strains were able to survive at pH 2.5 and 2% bile with a good bile salt hydrolase activity, cell surface hydrophobicity and sensitivity to most of the clinically important antibiotics. On evaluation for gastrointestinal transit tolerance these showed a viable count of 5 log cfu mL^?1 and 7 log cfu mL^?1, respectively in simulated gastrointestinal juice of pH 2.0 and 2% bile. During the in vivo feeding trial in mice the strains showed a viable count of about 7 log cfu g^?1 faeces and 6 log cfu g^?1 of large intestine, respectively. These strains were hence observed to possess favorable strain specific probiotic properties and have the potential to be a source of novel probiotics.     

HPLC of proteose peptones for evaluating ageing of packaged pasteurized milk

The proteolysis of casein (CN) occurring in packaged pasteurized milk (PM) during refrigerated storage was studied with relation to hygienic and microbiological characteristics of starting raw milk. Six batches of raw milk having standard plate count (SPC) from 1.5×10⁴ to 2.5×10⁵ cfu mL⁻¹ and somatic cell count (SCC) from 1.6×10⁵ to 4.4×10⁵ units mL⁻¹ were pasteurized (73 °C for 15 s), packaged and stored at 4 °C for 12 days. Capillary zone electrophoresis of CN showed breakdown of β-CN in all PM samples during storage. An HPLC method for monitoring proteose peptones (PP) formation was developed. Level of PP in PM samples increased, with keeping time from 667–789 to 947–1383 mg L⁻¹ and PP formation was significantly (P<0.05) related to SCC of starting raw milk. Electrospray ionization–mass spectrometry showed that PP were mainly represented by PP-5 from either A¹ or A² variants of β-CN. Five commercial samples of PM were analysed for PP formation during 14-day storage at 4 °C. Commercial samples prepared by microfiltration process or bactofugation combined with pasteurization showed the slowest formation of PP. The effect of storage temperature on PP formation was evaluated by keeping a conventional PM sample at either 8 or 12 °C for 12 days. Proteolysis of all major CNs upon action of plasmin and bacterial proteinases was observed under these conditions. PP level thus proves to be a reliable analytical index for evaluating the ageing of packaged PM during refrigerated storage.     

Optical instrument for the rapid detection of microorganisms in dairy products

MicroFoss, an optical instrument capable of detecting metabolic changes due to microbial activity, was tested for the detection of various groups of microorganisms in dairy products. Optical changes in the growth medium are monitored in a semi-fluid zone that separates the liquid medium containing the sample. Raw and pasteurised milks were evaluated for total viable counts (TVC), coliform and Enterobacteriaceae. Yoghurt was evaluated for coliform, Enterobacteriaceae and yeast.The MicroFoss TVC method for both pasteurised and raw milk utilised pre-filled vials containing 8 mL of Nutrient Broth with Bromcresol purple with 2 mL of milk sample. In a comparison of the instrument TVC methodology to the standard plate count methodology, correlation coefficients R=−0.93 and R=−0.92 were obtained for raw and pasteurised milk, respectively. An advantage of using the MicroFoss method was that results are obtained in 2–11 h rather than 24–48 h, with samples with bacterial counts above the acceptable levels detecting in most cases within an 8 h shift.The MicroFoss methodology for the estimation of numbers of coliform and Enterobacteriaceae utilises pre-filled vials containing 5 mL of broth. In a comparison between the MicroFoss coliform methodology and the corresponding plate count methodology utilising Violet Red Bile Agar (VRBA), a correlation coefficient of R=−0.95 was obtained. An identical correlation coefficient of R=−0.95 was obtained for the comparison of the MicroFoss Enterobacteriaceae methodology and the corresponding plate count methodology utilising VRBA+1% glucose. In 60 pasteurised milk samples tested, only 2 samples were positive by the MicroFoss method, one of which had 1 cfu mL⁻¹ by the plate method while the other had <1 cfu mL⁻¹. Coliform or Enterobacteriaceae were isolated from the detecting vials demonstrating the higher sensitivity of MicroFoss.The MicroFoss method for the detection of the presence of yeast in yoghurt required a 1:10 dilution of the product followed by a pH adjustment. Presence of yeast in yoghurt and soft cheeses was always detected without any false positive results. Results were available in less than 40 h.     

Baroprotection of vegetative bacteria by milk constituents: A study of Listeria innocua

The baroprotective effect of milk constituents on Listeria innocua 4202 treated at 350 or 500 MPa for 5 min was examined. High-pressure (HP) treatment of L. innocua 4202 (1×10⁹ cfu mL⁻¹) resulted in complete inactivation in simulated milk ultra-filtrate (SMUF), a ∼5.0 log reduction in phosphate-buffered saline and a ∼2.9 log reduction in milk. The addition of micellar casein to SMUF increased survival of the bacterium by 3 logs, compared with SMUF alone, but the protective effect was negated if the minerals associated with the casein micelles were removed. The colloidal minerals calcium (30 mm), magnesium (5 mm), citrate (10 mm) and phosphate (20 mm), suspended in SMUF increased survival by ∼3.3, ∼1.7, ∼3.3 and ∼3.5 logs, respectively. The buffering capacity of the suspending medium was found to be a key factor in microbial baroresistance. Buffering by phosphate and citrate in milk may protect microorganisms against changes in pH during HP treatment, whereas the divalent cations calcium and magnesium may protect cell membranes against HP.     

Effect of acidification on the activity of probiotics in yoghurt during cold storage

The viability of Lactobacillus acidophilus LAFTI^® L10, Bifidobacterium lactis LAFTI^® B94, and L. paracasei LAFTI^® L26 and their proteolytic activities were assessed in yoghurt at different termination pH of 4.45, 4.50, 4.55, and 4.60 in the presence of L. delbrueckii ssp. bulgaricus Lb1466 and Streptococcus thermophilus St1342 during 28 days of storage at 4 °C. All strains achieved the recommended level of 6.00 log cfu g⁻¹ of the product with L. acidophilus LAFTI^® L10 and L. paracasei LAFTI^® L26 exceeding the number to 8.00 and 7.00 log cfu g⁻¹, respectively. Lactobacilli strains showed a good cellular stability maintaining constant concentration throughout storage period regardless of termination pH. On the other hand, the cell counts of B. lactis LAFTI^® B94 decreased by one log cycle at the end of storage. The presence of probiotic organisms enhanced proteolysis significantly in comparison with the control batch containing L. delbrueckii ssp. bulgaricus Lb1466 and S. thermophilus St1342 only. The proteolytic activity varied due to termination pH, but also appeared to be strain related. The increased proteolysis improved survival of L. delbrueckii ssp. bulgaricus Lb1466 during storage resulting in lowering of pH and production of higher levels of organic acids, which might have caused the low cell counts for B. lactis LAFTI^® B94.     

Microbiological and chemical changes during the manufacture of Kefir made from cows’ milk,using a commercial starter culture

The counts of total aerobic mesophilic bacteria, lactic acid bacteria (in three different culture media, M17 agar, MSE agar, and Rogosa agar) and yeasts, and some biochemical parameters (levels of lactose, glucose, galactose, L(+)- and D(−)-lactic acids, ethanol, titratable acidity and pH) were determined during 196 h of fermentation in five batches of Kefir made from cows’ milk using a commercial starter culture. Lactococcus spp. predominated during the first 48 h of fermentation (∼8 log₁₀ cfu g⁻¹); Lactobacillus spp. became the predominant species after 48 h (∼8.5 log₁₀ cfu g⁻¹). During the first 24 h of fermentation, the lactose content decreased from a mean value of 4.92% (w/w) to 4.02% (w/w); the concentration of L(+)-lactic acid increased from 0.01% to 0.76% (w/w) and the pH decreased to 4.24 over the same period. After 24 h of fermentation, the changes in the levels of lactose and L(+)-lactic acid, and in pH, occurred more slowly. Neither glucose nor galactose were detected during fermentation. The production of ethanol was limited, reaching a mean final value of 0.018% (w/w).