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).     

Modeling the respiration of Pseudomonas fluorescens on solid-state lettuce-juice agar

When modeling gas atmospheres within equilibrium-modified atmosphere packaging it is vital to cover all influences on the atmosphere inside. It is necessary to take the microbial respiration into account when considering an extensive growth of microorganisms on fresh-cut vegetables during shelf life. Therefore the respiration of Pseudomonas fluorescens (due to its frequent occurrence on vegetables) was determined under solid-state conditions. A lettuce-juice agar was chosen to provide similar conditions for the microorganisms. Incubated agar plates were stored in air-tight glass jars at 7 °C for 8 days. The change in gas composition was measured and the data were examined by fitting them to Michaelis–Menten equations. All glass jars were filled with air initially; some of them additionally contained sodium hydroxide to bind the carbon dioxide. In other jars, 26.3% carbon dioxide was added after 4 days of storage whereas the rest of the glass jars were used without any modification. The data were successfully fitted to the Michaelis–Menten equation for substrate limitation (neglecting the influence of CO₂) for the three different experiments. The fit with the Michaelis–Menten equation for competitive inhibition was only successful for the respiration experiments with carbon dioxide added. The order of magnitude of the maximum respiration rate (r_max = 0.289–0.305 mL/(incubated plate (1.7 × 10⁷ cfu) · h)) shows by comparing it with reported respiration rates for fresh-cut vegetables, that the influence of microbial respiration should not be neglected, particularly at the end of the shelf life, where the amount of microorganisms may increase to 10⁷ cfu/g.     

The influence of lactoperoxidase,heat and low pH on survival of acid-adapted and non-adapted Escherichia coli O157:H7 in goat milk

In hot climates where quality of milk is difficult to control, a lactoperoxidase (LP) system can be applied in combination with conventional preservation treatments at sub-lethal levels to inhibit pathogenic microbes. This study investigated the effect of combined heat treatments (55 °C, 60 °C and 72 °C) and milk acidification (pH 5.0) on survival of acid-adapted and non-adapted Escherichia coli O157:H7 strains UP10 and 1062 in activated LP goat milk. Heat treatment at 72 °C eliminated E. coli O157:H7. Acid-adapted strains UP10 and 1062 cells showed resistance to combined LP and heat at 60 °C in fresh milk. The inhibition of acid-adapted and non-adapted E. coli O157:H7 in milk following combined LP-activation, heat (60 °C) and milk acidification (pH 5.0) suggests that these treatments can be applied to reduce E. coli O157:H7 cells in milk when they occur at low numbers (<5 log₁₀ cfu mL^?1) but does not eliminate E. coli O157:H7 to produce a safe product.     

Short communication: Relationship between metabolic status and the milk concentrations of haptoglobin and lactoferrin in dairy cows during early lactation

To investigate the relationship between metabolic status and the acute phase proteins haptoglobin (Hp) and lactoferrin (Lf) in milk, the concentrations of β-hydroxybutyrate (BHBA), nonesterified fatty acids (NEFA) and Hp were determined in blood samples collected weekly from 4 wk prepartum until 12 wk postpartum. Haptoglobin and Lf were determined in weekly milk samples. The cows (n = 49) were retrospectively classified according to NEFA and BHBA concentrations using different time intervals and threshold values for NEFA and BHBA, respectively. For BHBA, 4 threshold concentrations, (0.8, 1.0, 1.2, and 1.6 mM) were evaluated either at the first week before calving, at wk 1 or 2 postpartum, or when considering the means of wk 2 and 3 postpartum. For NEFA, the tested thresholds were 0.5 and 0.6 mM at wk 1 prepartum, wk 1 or 2 postpartum, or the means of wk 1 and 2 postpartum. All variables showed changes during the interval of observation. Comparing the time course of the acute phase proteins in the subgroups classified according to BHBA or NEFA, consistently greater concentrations of Hp in serum and milk and of Lf in milk were observed in those animals with BHBA concentrations above 1.6 mM during the last week before calving (n = 3/47) than in those with BHBA concentrations below this threshold. For NEFA, analogous differences for Hp in both serum and milk (0.52 ± 0.07 and 18.1 ± 4.6 for NEFA >0.6 mM vs. 0.36 ± 0.04 mg/mL and 8.46 ± 1.63 &micro;g/mL for NEFA <0.6 mM, respectively) and for Lf in milk (130 ± 8.5 vs. 89.2 ± 7.1 &micro;g/mL, respectively) were detected when a threshold of 0.6 mM at wk 2 postpartum was used. Our results indicated that cows having BHBA and NEFA serum concentrations above these thresholds at defined times could be identified.     

Influence of lupin-based milk alternative heat treatment and exopolysaccharide-producing lactic acid bacteria on the physical characteristics of lupin-based yogurt alternatives

In the present study we investigated the influence of heat treatment of lupin-based (LB) milk alternatives and different exopolysaccharide (EPS)-producing lactic acid bacteria on the physical characteristics of set-type LB yogurt alternatives. LB milk alternatives, obtained from protein isolate of Lupinus angustifolius cv. Boregine, were either pasteurized at 80 °C for 60 s or ultra-high temperature (UHT) heated at 140 °C for 10 s and was fermented with Lactobacillus plantarum TMW 1.460 and 1.1468, Pediococcus pentosaceus BGT B34 and Lactobacillus brevis BGT L150. Fermentation duration was strongly affected by heat treatment: different strains needed between 25 to 35 h in UHT LB milk alternative to reach a pH of 4.5 compared to 14 to 24 h in pasteurized LB milk alternative. EPS extraction revealed slightly higher amounts of EPS for UHT LB yogurt alternatives (~ 0.5–0.9 g/l; pasteurized: ~ 0.4–0.7 g/l). The more intensive heat treatment (UHT) resulted also in better rheological (apparent viscosity, hysteresis loop area, flow point, elastic, viscous and complex modulus) and textural properties (firmness, consistency, cohesiveness and index of viscosity) of the investigated LB yogurt alternatives. Furthermore, LB yogurt alternatives out of UHT milk alternative revealed a lower tendency to syneresis, measured with siphon and centrifugation method. This work contributes to the fundamental knowledge of the textural properties of LB yogurt alternatives.     

Rapid enumeration of Bifidobacterium lactis in milk powders using impedance

An impedance method was evaluated to enumerate Bifidobacterium lactis, a probiotic, added to milk powder. Impedance changes were measured at 40 °C and recorded using the BacTrac™ 4100 microorganism growth analyser. Five different media were compared for the optimum impedance response. A raffinose-based medium (B. lactis medium) produced the fastest and most reproducible results. Good correlations were obtained between cell numbers from pure cultures of B. lactis (DR 10™) on reinforced clostridial agar plates and the impedance changes in the B. lactis medium. Enumeration of these bacteria in milk powder using the BacTrac™ 4100 impedance system showed no significant difference when compared with agar plate count results. The impedance counts estimated the cell counts of 10⁶ cells g⁻¹ within 15 h and was faster than the 3 days required to obtain a result using the agar plate count method.     

Detection of multiple Mycoplasma species in bulk tank milk samples using real-time PCR and conventional culture and comparison of test sensitivities

The objective of this study was to further validate a SYBR PCR protocol for Mycoplasma spp. by comparing it with standard microbial culture in the detection of Mycoplasma spp. in bulk tank milk samples. Additionally, we identified Mycoplasma spp. present by analysis of PCR-generated amplicons [dissociation (melt) temperature (T_m), length, and DNA sequence]. The research presented herein tests the hypothesis that the SYBR PCR protocol is as sensitive as conventional culture for the detection of Mycoplasma spp. in bulk tank milk samples. Mycoplasmas cause several important disease syndromes in cattle, including mastitis in dairy cows. The standard diagnostic method at the herd level has been microbial isolation of mycoplasmas on 1 of several specialized media and speciation through biochemical or immunological techniques; repeated sampling schemes are recommended. The development of a real-time SYBR PCR protocol offers advantages in decrease of time to detection, cost, and complexity. The T_m of the double-stranded DNA generated from the PCR reaction was used to detect the presence of and tentatively identify the species of mycoplasmas other than Mycoplasma bovis. In the SYBR PCR protocol, the presence of multiple species of mycoplasmas is indicated by an atypical dissociation curve. Gel electrophoresis and sequencing of the amplicons was used to confirm the mycoplasma species present when a non-M. bovis organism was detected (T_m not equal to M. bovis) and used to identify all the mycoplasma species present for the samples with atypical dissociation curves. Mycoplasma bovis was identified in 83% of SYBR PCR mycoplasma-positive bulk tank samples. Another mycoplasma was identified either alone or in addition to M. bovis in 25% of SYBR PCR mycoplasma-positive bulk tank milk samples. Four species of mycoplasma other than M. bovis (Mycoplasma alkalescens, Mycoplasma arginini, Mycoplasma bovigenitalium, and Mycoplasma gateae) were identified in bulk tank milk samples tested with this method. Five farms had 2 mycoplasma species occurring at different times in their bulk tanks. Two mycoplasma species were identified in the same bulk tank sample in 7 instances on 2 farms. The finding of multiple Mycoplasma spp. coexisting on a farm and even in the same bulk tank milk sample indicates that the clinical significance of multiple mycoplasma species in the pathology of intramammary infections should be investigated further. In comparison with conventional culture, the SYBR PCR protocol was slightly (but not statistically significantly) more sensitive in the detection of mycoplasmas in bulk tank milk.     

High pressure treatments on the inactivation of Salmonella Enteritidis and the characteristics of beef carpaccio

The effect of high pressure (HP) on Salmonella enterica subsp. enterica serovar Enteritidis in beef carpaccio stored under temperature abuse conditions (8 °C) during 30 days was investigated. After treatment, reductions of S. Enteritidis were 3.68 and 5.94 log cfu/g in samples pressurized at 450 MPa for 5 and 10 min, respectively, whereas the pathogen was only detected after enrichment of samples treated at 450 MPa for 15 min. During storage, counts of S. Enteritidis decreased 0.26 log cfu/g in non-pressurized carpaccio, 1.33 log cfu/g in carpaccio treated at 450 MPa for 5 min and were only detected after enrichment in carpaccio pressurized at 450 MPa for 10 or 15 min. Color (L*, a* and b*) varied with pressurization and storage, with higher changes in carpaccio treated at longer pressurization times. Shear resistance was slightly lower in treated samples just after pressurization, but increased at the end of the storage period. Maximum force was less affected by treatment.     

Application of statistical process control,sampling, and validation for producing Listeria monocytogenes-free chicken leg quarters processed in steam followed by impingement cooking

Chicken leg quarters (180–230 g) were processed for 4 min in steam at 99°C and then in an air impingement oven for 24 min at an oven temperature of 232°C, an air velocity of 2 m/s, and a humidity of 60%. The cooked chicken leg quarters were sampled to measure for the end-point internal temperatures. Sampling size in each subgroup for the internal temperature measurements was determined based on a normal distribution at a confidence level of 95%. The process mean, range, and standard deviation at 95% confidence level were 73.9°C, 1.8°C, and 0.9°C, respectively, for the internal temperatures of the cooked chicken leg quarters. The process lethality was validated for up to 7  log₁₀ cfu/g reductions of Listeria monocytogenes in the cooked chicken leg quarters and verified by an inoculation study in which the chicken leg quarters were injected with 0.1 ml of the culture per cm² of the product surface area to contain 7–8 log₁₀ cfu/g of L. monocytogenes. This paper provided an approach for process control, sampling, and validation to reduce pathogens in fully cooked poultry products.     

A Naturally Buffered Bulk Retentate Starter from Ultrafiltered Milk

Whole milk was ultrafiltered to approximately 4:1 protein concentration, heated to 85°C for 30 min, and cooled to 22°C. It was inoculated with a commercial frozen concentrated lactic starter to give approximately 10⁷ cfu/ml and incubated at 22°C for 12 h. A commercial phage inhibitory medium and 11% nonfat dry milk were used as controls. After 12 h, retentate had significantly higher colony forming units per milliliter (3.2 × 10⁹) and pH (5.21) than phage inhibitory medium (2.5 × 10⁹ and pH 5.02) and nonfat dry milk (2.4 × 10⁹ and pH 4.58). Retentate starter and phage inhibitory medium starter had equal activity in skim milk (.3% developed acidity in 4 h at 32°C) whereas nonfat dry milk starter had significantly lower activity (.26% developed acidity). After a further 8 h incubation at 22°C, retentate starter had the highest pH (4.95) compared with phage inhibitory medium (4.76) and nonfat dry milk (4.51). At this time retentate starter activity was higher (.3%) than phage inhibitory medium (.27%) and nonfat dry milk (.19%). In highly concentrated retentates (3.5:1 and 5:1), retentate starter lowered pH considerably quicker than nonfat dry milk starter.     

Growth of Lactobacillus bulgaricus in Milk. 1. Cell Elongation and the Role of Formic Acid in Boiled Milk

An abnormal elongation of cells occurred when Lactobacillus bulgaricus B5b was cultivated in milk heated at 100°C for 15 min. Nuclear staining revealed that the elongated cells were multinucleate, and septum staining indicated that the septum had not yet formed. Similar cell elongation was confirmed using other strains of L. bulgaricus.Extent of cell elongation varied with strains and heating conditions of milk; severe heating shortened the cell length. This cell elongation was not observed in autoclaved milk (121°C, 15 min), milk containing added formic acid and heated to 100°C for 15 min, and in mixed culture with Streptococcus thermophilus. The formate added to the milk was incorporated into purines of ribonucleic acid and deoxyribonucleic acid of the cells. Addition of formic acid or adenine stimulated ribonucleic acid synthesis in the cells cultivated in milk heated to 100°C for 15 min. The cell elongation was always accompanied by a remarkable depression of ribonucleic acid synthesis.Lactobacillus bulgaricus cells grown in milk heated to 100°C for 15 min are not able to produce adequate amounts of formic acid required for synthesis of purines de novo.     

The growth and survival of Escherichia coli O157:H7 on minced bison and pieces of bison meat stored at 5 and 10 °C

This study investigated the growth and survival of Escherichia coli O157:H7 on minced and whole pieces of bison meat. Growth curves of native microflora, including Pseudomonas spp. and Enterobacteriaceae were also generated. A marked E. coli O157:H7 strain was inoculated onto minced and whole pieces of bison meat at an initial level of 1.5 log₁₀ cfu g⁻¹. The inoculated meat was stored at either 5 °C for 28 days or 10 °C for 21 days. Survival, but no growth, of E. coli O157:H7 was observed on both forms of bison meat stored at 5 °C, while significant growth of the organism was observed at 10 °C. E. coli O157:H7 counts on whole pieces were generally higher than counts observed on minced bison meat, and reached their highest population by 14 days, with a total increase of 3.36 log₁₀ cfu g⁻¹ on whole pieces and 2.12 log₁₀ cfu g⁻¹on minced bison meat stored at 10 °C. Under the same storage temperature, Pseudomonas spp. and total counts displayed similar growth patterns on both pieces and minced bison meat, while the Enterobacteriaceae showed a slower growth rate. This study showed that the growth of E. coli O157:H7 on bison meat is similar to that observed in studies of beef.     

Milk enhances intestinal absorption of green tea catechins in in vitro digestion/Caco-2 cells model

The effect of milk on the absorption of polyphenols is still controversial so far. In order to determine the impact of milk addition on green tea catechins bioaccessibility and intestinal absorption an in vitro digestion/Caco-2 cell model was applied. Green tea extract (GTE) was solubilized in distilled water at 23 °C and 100 °C, combined with skimmed milk (GTE + 10% milk and GTE + 25% milk) and subjected to simulated gastric and intestinal digestion, followed by transepithelial absorption in Caco-2 cells monolayers. In the mixture with milk, gallated catechins: ECG and EGCG showed binding to milk proteins while EC and EGC seemed to have weaker affinity. Catechins were stable during gastric incubation and very sensitive to intestinal digestion. Bioaccessibility of green tea catechins brewed at 100 °C was higher than brewed at 23 °C. Catechins from digested GTE with 10% and 25% milk exhibited enhanced intestinal permeability in Caco-2 model in comparison to non-digested GTE and digested GTE without milk. Apparent permeability coefficients (P_app) of EGCG and ECG in digested GTE with 25% milk were significantly higher compared to those in GTE with 10% milk, and amounted to 2.41 × 10^− 6 cm/s and 1.39 × 10^− 6 cm/s. The recoveries of all catechins in GTE with milk in Caco-2 cells after 2 h incubation were significantly higher than that without milk. To summarize, these data suggest that milk addition may increase catechin bioavailability by enhancing their transepithelial absorption and uptake from green tea extract.     

Estimation of Residual Pepsin and Chymosin Activity in Curd,

Pasteurized milk (225 g) adjusted to pH 6.2 was set with 3.5 milk clotting units of chymosin (EC 3.4.23.4). The same amount of milk at pH 5.8 was set with 3.5 milk clotting units of porcine pepsin (EC 3.4.23.1). Fifteen minutes after clotting, the curd was broken, and curd and whey were separated by centrifugation at 3500 × g for 20 min. The curd (30 g) was extracted at pH 6.8 in 450 ml water or at pH 6.2 (chymosin) or 5.8 (pepsin) in 450 ml 1 M sodium chloride.Chymosin was completely released from the curd and accounted for by both methods of extraction. Pepsin was completely released and accounted for after extraction in 1 M sodium chloride at pH 5.8 but was partly inactivated during extraction at pH 6.8.Assay of curd extracts and whey by a linear agar diffusion test accounted for 102 ± 6% of the pepsin activity added to milk when the curd was extracted in 1 M sodium chloride. Extraction at pH 6.8 allowed recovery of only 63% of the activity. Chymosin recovery was 100 ± 5% by both methods of curd extraction.     

High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h⁻¹ homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100–300 MPa) and milk inlet temperature T_in (4°C, 14°C or 24°C) on milk temperature T₂ at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The T_in values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High T_in and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d_4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at T_in=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (T_in=4°C, 14°C or 24°C) further decreased d_4.3 diameters to about 0.2 μm and narrowed the size distribution. At all T_in tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than 1 log cycle, provided homogenisation pressure was ⩾200 MPa at T_in=24°C (T₂∼60°C), 250 MPa at T_in=14°C (T₂∼62°C), or 300 MPa at T_in=4°C (T₂∼65°C). At all T_in tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria.     

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.     

Survival of Salmonella Enteritidis PT 30 on inoculated almond kernels in hot water treatments

Almonds are blanched by exposure to hot water or steam-injected water to remove the pellicle (skin) from the kernel. This study evaluated the survival of Salmonella Enteritidis PT 30, Salmonella Senftenberg 775W and Enterococcus faecalis on whole raw almond kernels exposed to hot water. Whole, inoculated (7 to 9 log CFU/g) Nonpareil almonds (40 g) were submerged in 25 L of water maintained at 60, 70, 80 and 88 °C. Almonds were heated for up to 12 min, drained for 2 s, and transferred to 80 mL of cold (4 °C) tryptic soy broth. Almonds in broth were stomached at high speed for 2 min, serially diluted, plated onto tryptic soy and bismuth sulfite agars (Salmonella) or bile esculin agar (Enterococcus) and incubated at 37 °C for 24 and 48 h, respectively. D values of 2.6, 1.2, 0.75 and 0.39 min were calculated for exposure of S. Enteritidis PT 30 to water at 60, 70, 80 and 88 °C, respectively; the calculated z value was 35 C°. D values determined for Salmonella Senftenberg 775W and E. faecalis at 88 °C were 0.37 and 0.36 min, respectively. Neither Salmonella serovar could be recovered by enrichment of 1-g samples after almonds inoculated at 5 log CFU/g were heated at 88 °C for 2 min. These data will be useful to validate almond industry blanching processes.     

Thermosonication versus thermal processing of skim milk and beef slurry: Modeling the inactivation kinetics of psychrotrophic Bacillus cereus spores

Non-thermal processed foods are generally cold stored and distributed. The use of ultrasound for food preservation has attracted the interest of many research groups. In the current study, the thermosonication (TS, simultaneous ultrasound and thermal process) inactivation of psychrotrophic Bacillus cereus spores was investigated (24 kHz, 210 μm, 0.33 W/mL or W/g). First, the effectiveness of a 1.5 min TS process at 70 °C in skim milk, beef slurry, cheese slurry, and rice porridge was investigated. The TS was more effective than sole thermal treatment in reducing B. cereus spores in rice porridge, beef slurry and cheese slurry by 7, 6, and 4 fold, respectively. Then, the first-order D- and z-values for TS and thermal processing in skim milk and beef slurry, and the best model to fit TS inactivation of B. cereus spores in beef slurry were determined. The D_70 °C-values in skim milk were 2.9 min for TS and 8.6 min for the thermal treatment. And in beef slurry, values of 0.4 min for TS and 2.3 min for thermal were estimated. It was found that the Log-logistic model better described the TS spore inactivation in beef slurry. The ultrasound technology required 20–30 °C lower temperatures for the same spore inactivation, which resulted in better food quality and energy saving gains.     

Effect of Chilling and Forewarming Treatments on Heat Stability and Proteolysis in Milk from Individual Cows

Milk samples from 12 Brown Swiss cows were partially defatted by centrifugation at 2500 × g for 30 min at 37°C and then examined. Coagulation time at 140°C and tyrosine value, as an index of proteolysis, was determined for raw, chilled (2°C/48 h), and forewarmed (95°C/10 min) portions from each sample. Coagulation times of chilled samples were longer than those of corresponding raw milks. Coagulation times of forewarmed samples were shorter for 9 cows and longer for 3 cows than those of corresponding raw samples. Addition of β-lactoglobulin, bovine serum albumin, or sodium and potassium salts to raw milk did not affect heat stabilities of the resultant systems before or after chilling or forewarming. Tyrosine values increased both for chilled and forewarmed milks.     

Screening for and characterization of Lactococcus lactis bacteriophages with high thermal resistance

Fifty-six Lactococcus lactis phage isolates collected from different German dairies and obtained from a starter culture manufacturer were tested for their heat resistance. About 40% of these isolates resisted treatment at 80 °C for 5 min when they were heated in milk. The most resistant phage isolate, P1532, was collected from sour cream. Plaque-formation was still detectable even after heating at 97 °C for 5 min. The second heat-resistant one, P680, showed some plaque-forming ability after heating at 95 °C for 5 min. Kinetic parameters for the thermal inactivation of these two resistant phages were determined for temperatures ranging from 70 to 97 °C. The inactivation of phage P1532 in skim milk and in buffer medium were found to follow first-order kinetics and did not exhibit tailing, whereas in the inactivation curves of phage P680 tailing was observed. The D-value of P1532 at pasteurization temperature of 72 °C was calculated as 112 min.