Heat-treated Milk Samples Research Articles

Effect of heat treatment on the activity of inhibitors of plasmin and plasminogen activators in milk

Abstract The effect of heat treatment on inhibitors of both plasminogen activator and plasmin was studied. Fractions of milk with inhibitory activities against plasminogen activator and plasmin were isolated. Different approaches were followed to study the thermal stability of the inhibitors. The inhibitors activities were measured after heat treating fractions isolated from raw milk; and after isolating fractions from pasteurized milk. Also plasmin activity was measured in heat-treated milk samples. Thermal inactivation of plasminogen activator inhibitor (81.1%) was considerably different from that of plasmin inhibitor (35.8%) in milk after heat treatment at 75 ∘ C for 15 s. The plasmin inhibitor in the isolated fractions was suggested to be α -2-antiplasmin, since it reacted immunochemically with polyclonal goat anti-human α -2-antiplasmin and competitively inhibited plasmin. Results showed that plasminogen activator inhibitor is less heat-stable than plasmin inhibitor, indicating that plasminogen activation could overcome any inhibition of plasmin resulting after milk pasteurization.

Effect of chemical decontamination and refrigerated storage on the isolation of Mycobacterium avium subsp. paratuberculosis from heat-treated milk.

To assess the impact of chemical decontamination and refrigerated storage before culture on the recovery of Mycobacterium avium subsp. paratuberculosis from heat-treated milk. Five-millilitre samples of ultra heat-treated (UHT) milk spiked with Myco. paratuberculosis NCTC 8578, B4 or 806R (ca 10(6) CFU ml(-1)) were heated at 63 degrees C for 20 or 30 min by submersion in a water bath. Heat-treated milk (0.5 ml) was cultured immediately into BACTEC 12B medium or refrigerated at 4 degrees C for 48 h before culture. Milk samples that received a 20-min heat treatment were also subjected to decontamination with 0.75% cetylpyridinium chloride (CPC) for 5 h at room temperature before inoculation into BACTEC 12B medium when tested immediately and after 48 h at 4 degrees C. BACTEC vials were monitored for evidence of growth over an 18-week incubation period at 37 degrees C. CPC decontamination resulted in a significant reduction in the number of culture-positive milk samples recovered immediately after heating (P < 0.05) and after refrigerated storage for 48 h (P < 0.01). Refrigerated storage for 48 h before testing did not have any significant effect, beneficial or detrimental, on Myco. paratuberculosis recovery rates. CPC decontamination applied to milk immediately or 48 h after heating will adversely affect the recovery of viable Myco. paratuberculosis, possibly leading to nonrecovery of the organism although viable cells are present in the original milk sample. Published pasteurization studies in which milk samples were decontaminated before culture will have underestimated the survival capability of Myco. paratuberculosis after high-temperature, short-time pasteurization. CPC decontamination should not be applied to pasteurized milk in future studies.

Growth and Survival of Escherichia coli O157:H7 in Different Types of Milk Stored at 4 °C or 20 °C

ABSTRACT: Samples of ultra heat‐treated (UHT) milk (whole, semiskimmed, and skimmed milk, whole milk with cinnamon and lemon flavors, and semiskimmed milk with cocoa) were collected. Growth of E. coli O157:H7 was detected in whole milk at 4 °C (generation time 0.89 d) and in all milks at 20 °C during the first 24 h. Growth of nonpathogenice. coli was detected in milk samples without flavors during the same period of time. The fastest death time of E. coli O157:H7 was detected in whole milk with cinnamon and lemon at 4 °C (−1.35 d) The nonpathogenic strain showed the fastest death time in skimmed milk at 20 °C.

Rheological Monitoring of Structure Development and Rebodying of Set-Style Yoghurt

AbstractThe present work was undertaken in order to evaluate the rheological changes of set-style yoghurt at different incubation times, during production. The study of rheological changes of milk destined to set-style yoghurt production, in different processing stages prior to fermentation was accomplished using steady and oscillatory shear, while the study of the influence of storage time was also carried out using steady shear. Set-style yoghurt samples with 2 and 3 hours of fermentation exhibited a major structural breakdown during upward shear rate sweep experiments as expected, since the protein network was initially intact. Both samples exhibited two points of sudden decay. For T2 the these points occured at shear stresses of 38 Pa and 77 Pa and for T3 at 47 Pa and 80-83 Pa. The first point corresponds to the main break-up of protein network and the second one to the disruption of primary-aggregates, breaking the portion of casein network that was associated with exopolyssacharides produced by lactic acid bacteria. Structural analysis performed by means of oscillatory measurements indicate that the protein network is already partially formed after 2 hours, albeit full development is only achieved after longer periods. Heat-treated milk, evaporated milk and set-style yoghurt samples with 0 and 1 hour incubation times showed an apparent yield stress value of approximately 0.4 Pa and a strong shear-thinning behaviour afterwards. At higher shear stresses (approximately 1.6 Pa), however, the behaviour changed to shear-thickening, this effect being attributed to flow-induced interactions between the milk components. For the latter sample, a further change in flow behaviour occurred at a stress of 8 Pa, which is probably due to the breakdown of the weak protein network that had formed during incubation.

Screening procedure for evaluating heat load in commercial milks

The evaluation of heat damages in commercial milk samples has been studied by an ana lytical procedure which uses two different and complementary methods. The first is the rapid total sul- phydryl (SH) group measurement which evaluates whey protein denaturation. The second involves the measurement of absorbance at 340 nm (A340) of milks modified with the Clarifying Reagent', At .this wavelength sorne coloured compounds resulting from the non-enzymic browning still absorb. There is a satisfying correlation between A340 and lactulose content (r =0.98) measured in commercial milk samples by using capillary electrophoresis or high performance liquid chromatography (HPLC). Our analytical procedure using SH and A340 measurements has been applied to 130 raw and commercial milk samples. Analysis by Student's r-test shows that the values of the two parameters obtained for raw and heat-treated milk samples were significantly different (P < 0.01). This study demonstrates that this simple protocol makes possible rapid evaluation of heat damages in commercial milk sam- pies as il could be done with official but more sophisticated methods. © Inra/Elsevier, Paris. heat damage / commercial milk / sulphydryl group / absorbance A340 / transparent-modified miIk / analytical procedure

Assay of microbial enzymes in opaque samples

The paper describes a system of colorimetric analysis of opaque liquid samples performed with a new instrument known as an opaque sample analyzer (OSA). Alkaline phosphatase, peroxidase, and β-glucosidase were assayed with the OSA in two opaque liquid substrates, cow's milk and microbiologically debittered olive brine. The nondestructive OSA analysis of heat-treated and untreated milk samples permitted quantitative evaluation of the reaction products of alkaline phosphatase and peroxidase with a sensitivity clearly better than that of the qualitative methods currently employed in routine analyses. The olive brine analysis with the OSA allowed the demonstration, for the first time in this matrix, of the activity of β-glucosidase produced by Lactobacillus plantarum used as the inoculum to debitter the table olives.

Differentiation of heat-treated milks by using steam distillation solvent extraction

A recently designed system of steam distillation solvent extraction (SDE) was applied to the preparation of samples for GC analysis of volatile components in heat-treated milk. In order to evaluate the effect of the SDE sample temperature on the original composition of the milk, model systems with different sensitivities to heat were submitted to the SDE process at normal pressure. Finally, the usefulness of the method for the differentiation of heat-treated milk samples was evaluated.

Determination of Whey Protein Denaturation in Heat-Processed Milks: Comparison of Three Methods

Fast protein liquid chromatography was used to determine the extent of whey protein denaturation in various heat-treated milk samples: Sordi-indirect UHT (145°C/3s), Dasi-direct UHT (142°C/3s), HTST (80°C/30 s), and batch (63°C/30min). Results were compared with other published methods (differential scanning calorimetry, whey protein nitrogen index, and Kjeldahl nitrogen on salt fractions). Results of the differential scanning calorimetry method were too erratic to be used to quantify whey protein denaturation. The remaining methods (fast protein liquid chromatography, Kjeldahl nitrogen, and whey protein nitrogen index) gave reproducible results and the extent of denaturation (highest to lowest) was consistently predicted as Sordi > Dasi > HTST > batch. There was no difference between fast protein liquid chromatography and Kjeldahl nitrogen, but there was a significant difference between fast protein liquid chromatography and whey protein nitrogen index and between Kjeldahl nitrogen and whey protein nitrogen index. Fast protein liquid chromatography appears to be an effective method to determine whey protein denaturation in heat-treated milks.

Plasma amino acid response to single test meals in humans. VI. Determination of available lysine.

The plasma amino acid response to single test meals in young adults was used to determine the amount of physiologically available lysine in heat-treated milk samples. The plasma amino acid responses were evaluated as PAA ratios according to a modified calculation procedure based on the method proposed by Longenecker and Hause. The amount of available lysine was determined by two separate series of single-meal tests. In the first, a relationship was established between the plasma amino acid response (PAA ratio) and the dietary level of lysine (given as synthetic amino acid mixtures). This relationship could be described by linear regression (r = 0.90). This regression line was then used to quantitatively evaluate the lysine PAA ratios obtained from single test meals with heat-treated milk samples. The heat-treatment of the milk samples caused a marked reduction of the amount of physiologically available lysine. The quantitative estimates obtained by the present plasma amino acid technique were in good agreement with determinations of available lysine in the same milk samples based on rat bioassay and chemical analysis.

A survey of the incidence of coagulase-positive staphylococci in market milk and cheese in England and Wales

SummaryThe occurrence of coagulase-positive staphylococci in 954 samples of raw or heat-treated market milk and in 910 samples of cheese of different varieties, taken at 40 cheese factories or farms in England and Wales was determined. Only 9% of the cheeses contained more than 500 000 staphylococci/g, and these were all made from raw milk. All but the mildest of the heat treatments used were effective in greatly reducing the numbers of staphylococci present in milk and usually resulted in cheese virtually free from staphylococci. The effect of sublethal heat treatment on multiplication of these organisms in cheese curd is discussed.