Evaluation of pulsed electric field and minimal heat treatments for inactivation of pseudomonads and enhancement of milk shelf-life

Pulsed Electric Field (PEF) treatment of milk provides the opportunity to increase the shelf-life of fresh milk for distribution to distant markets. PEF treatments were evaluated in sterile (UHT) milk to determine the inactivation of added spoilage Pseudomonas isolates and the subsequent gains in microbial shelf-life (time taken to reach 10⁷ CFU mL^− 1). Little inactivation of Pseudomonas was achieved at 15 or 40 °C compared with 50 or 55 °C. The greatest inactivation (> 5 logs) was achieved by processing at 55 °C with 31 kV cm^− 1 (139.4 kJ L^− 1). Heat treatment at the application temperature without PEF treatment caused minimal inactivation of Pseudomonas (only 0.2 logs), demonstrating that the inactivation of the Pseudomonas was due to the PEF treatment rather than the heat applied to the milk. At added Pseudomonas levels of 10³ and 10⁵ CFU mL^− 1, the microbial shelf-life of PEF-treated milk was extended by at least 8 days at 4 °C compared with untreated milk. The total microbial shelf-life of the PEF-treated milk was 13 and 11 days for inoculation levels of 10³ and 10⁵ CFU mL^− 1 respectively. The results indicate that PEF treatment is useful for the reduction of pseudomonads, the major spoilage bacteria of milk.Industrial relevancePseudomonads are the major psychrotrophic spoilage microflora of refrigerated, stored HTST pasteurised milk. Long-life (UHT) products are an important component of milk sales in South-East Asia, but in recent years there has been an increasing demand for less processed milk products with extended shelf-life. The recent practice of shipping fresh bulk milk from Australia to South-East Asian countries has necessitated additional heat treatment prior to export and on arrival, to achieve the required shelf-life. Pulsed electric field treatment of HTST milk, applied alone or in combination with mild heat under optimised conditions, offers the opportunity of shelf-life extension, while limiting the reduction in quality attributes of milk associated with more severe additional heat treatments.     

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.     

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.     

Relationship of Protease Activity to Shelf-Life of Skim and Whole Milk

This study was to investigate causes of a possible difference in shelf-life of pasteurized skim milk and whole milk. Samples of skim and whole milk were obtained the day of processing, in 235 ml containers, from commercial dairies throughout South Carolina. They were stored at 4.5°C for 0,4,8,10,12,14, and 16 days; 7°C for 0,4,6,8,10, and 12 days; and 10°C for 0,1,2,3,4,5, and 6 days. On each sampling day milks were tested for coliform count, psychrotrophic count, flavor score, and relative protease activity. The shelf-life of skim milk was significantly less than that of whole milk when both were stored at 4.5°C and 7°C, but not at 10 C. Bacteria counts were not significantly different; thus, they were of no predictive value as anticipated changes in flavor score. Relationship between flavor score and relative protease activity of skim and whole milk was linear. Also, relative protease activity was significantly higher in skim milk as compared to whole milk stored at 4.5 and 7° C. Therefore, a higher protease activity in skim milk may account partially for its decreased shelf-life.     

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.     

Preservation of salted,vacuum-packaged,refrigerated sea bream (Sparus aurata) fillets by irradiation: microbiological,chemical and sensory attributes

The effect of gamma irradiation (1 and 3 kGy) on the shelf-life of salted, vacuum-packaged sea bream (Sparus aurata) fillets stored under refrigeration was studied by monitoring the microbiological, chemical and organoleptic changes occurring in fish samples. Non-irradiated, salted, vacuum-packaged fish served as control samples. Irradiation affected populations of bacteria, namely, Pseudomonas spp., H₂S-producing bacteria, Brochothrix thermosphacta, Enterobacteriaceae and lactic acid bacteria. The effect was more pronounced at the higher dose (3 kGy) applied. Of the chemical indicators of spoilage, trimethylamine (TMA) values of non-irradiated, salted sea bream increased slowly to 8.87 mg N (100 g)⁻¹ flesh (whereas for irradiated, salted samples significantly lower values were obtained, reaching a final TMA value of 6.17 and 4.52 mg N (100 g)⁻¹ flesh at 1 and 3 kGy, respectively (day 42). Total volatile base nitrogen values increased slowly attaining a value of 60.52 mg N (100 g)⁻¹ for non-irradiated, salted sea bream during refrigerated storage whereas for irradiated fish, lower values of 48.13 and 37.21 mg N (100 g)⁻¹ muscle were recorded at 1 and 3 kGy, respectively (day 42). Thiobarbituric acid values for irradiated, salted sea bream samples were higher than respective non-irradiated (salted) fish, and increased slowly until day 28 of storage reaching final values of 1.01 (non-irradiated, salted), 2.15 (1 kGy) and 3.26 mg malonaldehyde kg⁻¹ flesh (3 kGy), respectively (day 42). Sensory evaluation (taste) showed a reasonably good correlation with bacterial populations. On the basis of sensorial evaluation, a shelf-life of 27–28 days was obtained for vacuum-packaged, salted sea bream irradiated at 1 or 3 kGy, compared to a shelf-life of 14–15 days for the non-irradiated, salted sample.     

Application of FT-NIR and FT-IR spectroscopy to study the shelf-life of Crescenza cheese

Crescenza is a fresh cheese in which “freshness” is associated with low acidity, limited proteolysis, and no bitter taste. In this study, FT-NIR and FT-IR spectroscopy was applied to evaluate the shelf-life period in which “freshness” is maintained. Cheese samples were analysed at different times for 20 days. Spectral data were collected using a FT-NIR spectrometer with an optic fibre (from 12000 to 4000 cm⁻¹), and a FT-IR spectrometer with an ATR cell (from 4000 to 700 cm⁻¹). Principal component analysis was able to detect the decrease of Crescenza “freshness” and to define the critical day during shelf-life. These results are in agreement both with those reported in previous studies and with those of physico-chemical and chemical tests. The principal absorption bands relating to the milk components involved were determined. The advantage of using IR spectroscopy is to rapidly draw a profile of the product related to its total quality.     

Influence of different shrinking temperatures and vacuum conditions on the ability of psychrotrophic Clostridium to cause ‘blown pack’ spoilage in chilled vacuum-packaged beef

This study determined the ability of psychrotrophic Clostridium strains isolated from vacuum-packaged beefs and abattoir environments to cause ‘blown-pack’ spoilage of vacuum-packaged beef stored at 2 and 15 °C. The influence of shrinking temperatures (83, 84 and 87 °C) and vacuum pressure (6 and 9 mbar) on the occurrence of such spoilage as well as the effects of simulated transportation (500 km) on the integrity of packages was determined. At 15 °C and 2 °C, twelve and six strains caused ‘blown-pack’ spoilage, respectively. The combination of vacuum pressure (9 mbar) combined with shrinking temperature (87 °C) retarded the occurrence of spoilage. The simulated transportation under the experimental conditions did not affect the integrity of packages. More studies that assess the factors that may contribute for the occurrence of ‘blown-pack’ spoilage should be performed to avoid the occurrence of such spoilage during its shelf-life.     

Effect of ozone on microbial,chemical and sensory attributes of shucked mussels

The effect of ozonation in aqueous solution (O₃ concentration=1 mg/l, time of ozonation: 60 and 90 min) on the shelf-life of shucked, vacuum-packaged mussels, stored under refrigeration was studied by monitoring the microbiological, chemical and sensory changes occurring in mussel samples, for a period of 12 days. Non-ozonated vacuum-packaged mussels served as the control sample. Ozonation affected populations of bacteria namely, aerobic plate count (APC) (0.7–2.1 log cycle reduction), Pseudomonas spp. (0.5–1.1 log cycle reduction) and H₂S-producing bacteria (1.1–2.5 log cycle reduction), Brochothrix thermosphacta (0.3–1.4 log cycle reduction), lactic acid bacteria (0.3–0.8 log cycle reduction) and Enterobacteriaceae (0.5–1.5 log cycle reduction). The effect of ozonation was more pronounced at the longer time of ozonation. Of the chemical indicators of spoilage monitored, trimethylamine values of all mussel samples remained relatively low throughout the entire storage period, attaining values of 7.5, 6.0 and 6.4 mg N/100 g for the control and ozonated for 60 and 90 min samples, respectively, on day 12 of storage. Total volatile basic nitrogen (TVB-N) values similarly remained relatively low (⩽20 mg N/100 g) until day 6 of storage, and increased to 31.9, 24.2 and 26.9 mg N/100 g mussel meat for the control and ozonated for 60 and 90 min samples, respectively, on day 12 of storage. Initial TBA values were surprisingly high (30–35 mg MA/kg) and decreased to 23.0, 21.7 and 13.3 mg MA/kg mussel meat on day 12 of storage for the control and the ozonated for 60 and 90 min samples, respectively. Sensory evaluation (odor, taste and texture) of cooked mussels showed a good correlation with bacterial populations. On the basis of sensory analyses, a shelf-life of 12 days was obtained for vacuum-packaged mussels ozonated for 90 min as compared to a shelf-life of 9 days for the control sample.     

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.     

Milk Somatic Cells and Lactation in Small Ruminants

The milk somatic cell count (MSCC) is the basis for abnormal milk control programs. The current legal MSCC limit for bulk tank milk for goats and sheep in the United States is 1000 and 750 × 10³/ml, respectively. Milk somatic cell counts for goats are higher than MSCC for cows and sheep. The MSCC for goats free from intramammary infection (IMI) range from 270 to 2,000 × 10³/ml. Cell counts for sheep are similar to cows and range from 10 to 200 × 10³/ml. Neutro-phils comprise the major cell type in milk from uninfected goats and constitute 45 to 74% of the MSCC, compared with 2 to 28% for sheep and cows. The macrophage is the major cell type in milk from cows and sheep. Milk secretion in goats and sheep is largely apocrine in nature and cytoplasmic particles, similar in size to milk somatic cells, are normal constituents of their milk. Concentrations of cytoplasmic particles in sheep milk average 15 × 10³/ml, while goat milk averages 150 × 10³/ml. Therefore, to obtain accurate MSCC for goats, only cell counting procedures specific for DNA should be used. While IMI significantly increases MSCC for goats and sheep, noninfectious factors such as parity, stage of lactation, season and milk yield have been related to increased MSCC. An increase in MSCC for goats has been shown to decrease milk and fat yields. Intramammary infusion of antibiotics at dry-off and postmilking teat dipping in goats decreased the rate of new IMI and MSCC. Thus, mastitis control practices shown to be efficacious in cows are also effective in goats.     

Inactivation of Escherichia coli O157:H7 and Listeria monocytogenes in milk by caprylic acid and monocaprylin

Listeria monocytogenes and Escherichia coli O157:H7 are major foodborne pathogens implicated in various outbreaks involving pasteurized or unpasteurized milk, and various dairy products. The objective of this study was to determine the antibacterial effect of caprylic acid (CA, C_8:0) and its monoglyceride, monocaprylin (MC) on L. monocytogenes and E. coli O157:H7 in whole milk. A five-strain mixture of E. coli O157:H7 or L. monocytogenes was inoculated in autoclaved milk (10⁶ CFU/ml) containing 0, 25, or 50 mM of CA or MC. At 37°C, all the treatments, excepting 25 mm CA, reduced the population of both pathogens by approximately 5.0 log CFU/ml in 6 h. At 24 h of storage at 8°C, MC at both levels and CA at 50 mM decreased L. monocytogenes and E. coli O157:H7, respectively by >5.0 log CFU/ml. At 48 h of 4°C storage, populations of L. monocytogenes and E. coli O157:H7 were decreased to below detection level (enrichment negative) by 50 mm of MC and CA, respectively. Results indicate that MC could potentially be used to inhibit L. monocytogenes and E. coli O157:H7 in milk and dairy products, but sensory studies need to be conducted before recommending their use.     

Effect of gutting on microbiological,chemical, and sensory properties of aquacultured sea bass (Dicentrarchus labrax) stored in ice

The effect of gutting on microbiological, chemical, and sensory properties of aqua-cultured sea bass (Dicentrarchus labrax) stored in ice was studied. Pseudomonads and H₂S-producing bacteria (including Shewanella putrefaciens) were the dominant bacteria at the end of the 16-day storage period in ice for both whole ungutted and gutted sea bass. Brochothrix thermosphacta and Enterobacteriaceae were also found in the spoilage microflora of ungutted and gutted sea bass but their counts were always less than those of Pseudomonads and H₂S-producing bacteria. Bacterial counts of whole ungutted sea bass were always higher than those obtained for gutted sea bass samples. Mesophilic counts for gutted and ungutted fish exceeded 7 log cfu g⁻¹ after 9 and 15 days of ice storage, respectively. Of the chemical indicators of spoilage, TMA values of ungutted sea bass increased very slowly whereas for gutted samples higher values were obtained reaching a final value of 0.73 and 4.39 mg N 100 g⁻¹, respectively (day 16). TVB-N values showed no significant increase for whole ungutted sea bass during storage reaching a value of 27.7 mg N 100 g⁻¹ (day 16) whereas for gutted fish 36.9 mg N 100 g⁻¹ was recorded. TBA values remained low for ungutted sea bass samples until day 16 of storage, whereas for gutted fish were variable. Of the chemical indices used, none proved useful means of monitoring early ungutted and gutted sea bass freshness in ice. Sensory assessment using the EC freshness scale gave a grade E for up to 5 days for the ungutted sea bass, a grade A for a further 2 days and a grade B for an additional 4 days, after which sea bass was graded as C (unfit). Gutted sea bass was given a grade E for up to 3 days, a grade A for the 4–7th days, and a grade B for the 8–10th days of storage, whereas on day 11 it was graded as unfit. Acceptability scores for odor, taste and texture of cooked ungutted and gutted sea bass decreased with time of storage. Results of this study indicate that the shelf-life of whole ungutted and gutted sea bass stored in ice as determined by the overall acceptability sensory scores and microbiological data is 13 and 8 days, respectively.     

Selection of Tests for Monitoring the Bacteriological Quality of Refrigerated Raw Milk Supplies

Raw milk samples were collected from 10 producer bulk tanks. Samples were then subdivided so that milks were subsequently stored at 1.7, 4.4, 7.2, and 10.0°C for 24 and 48 h. After storage, samples were analyzed by seven plating methods: standard plate count, psychrotrophic bacterial count, rapid psychrotrophic count, preliminary incubation count, mesophilic plate count, laboratory pasteurized count, and coliform count by violet red bile agar technique. Impedance protocols on a Bactometer^® Model 123 for total count, psychrotrophic count, mesophilic count, and coliform count were also used to evaluate the bacteriological quality of the milks. Bacterial counts and impedance detection times were analyzed using nonparametric statistics. Impedance protocols for total count and psychrotrophic count were the best indicators of bacteriological quality. Preliminary incubation count was the best of the plating methods. The laboratory pasteurized count performed poorly. Impedance measurements provided information in the shortest time.     

Plasmin activity in direct-steam-injection UHT-processed reconstituted milk: Effects of preheat treatment

Ultra-high-temperature (UHT)-processed reconstituted milk that is subjected to a minimal preheat treatment during the direct-steam-injection heating process may have a shortened shelf-life as a result of plasmin-mediated proteolysis. Some manufacturers apply a preheat treatment before UHT treatment (140 °C for 4 s) with the aim of prolonging the shelf-life. Preheat treatments are, however, often arbitrary in terms of temperature and holding time. The aim of the current work was to determine guidelines for the minimum preheat treatment that will effectively inhibit or prevent plasmin-type enzyme activity in UHT milk. A selected range of preheat treatments was applied to milk preparations reconstituted from several batches of low-heat skim milk powder. Increased plasmin-type proteolysis was observed after intermediate preheat treatments at ⩾80 and <90 °C. Effective inhibition of plasmin-type proteolysis was obtained by preheating at 90 °C for 30 or 60 s.     

Developing an antimicrobial packaging of ready-to-eat pomegranate arils based on vapors of brandy or distillery ethanol

Despite the increasing pomegranate consumption, the ready-to-eat (RTE) arils are highly perishable and this negatively impacts their commercialization. Nowadays, mild pre-packaging decontamination interventions (washing with sanitizing agents or exposure to ultraviolet light) in sequence or not with modified atmospheres packaging technologies are applied. Even though, the latter combination of methods provides them a shelf-life of 10–14 days at cold storage, several negative effects have been also reported (i.e., degradation of anthocyanins). Thus, the aim of the study was to evaluate the effect of alternative, mild antimicrobials such as the vapors of distillery ethanol and brandy on microbial, physical, textural, sensorial, and multispectral imaging attributes of RTE arils during storage at different temperatures in perforated bags. Lactic acid bacteria (LAB) and yeasts/moulds were the dominant spoilage microbiota of RTE arils, regardless of storage temperature and antimicrobial. Vapors produced by both volatile antimicrobials significantly inhibited (p < 0.05) the growth of LAB and yeasts/moulds, at all storage temperatures. For instance, at 4 °C, when population of TVC on controls was 6.9 log CFU g^− 1 (day 23), the respective counts on arils treated with distillery ethanol or brandy followed the order: 4.9 log CFU g^− 1 (1 mL of ethanol) > 3.9 log CFU g^− 1 (1 mL of brandy) > 2.2 log CFU g^− 1 (2 mL of ethanol) > 1.2 log CFU g^− 1 (2 mL of brandy). Moreover, arils exposed to distillery ethanol and brandy vapors showed lower weight loss (%) compared to controls, while the firmness was reduced, regardless of treatment and storage temperature. Color measurements and evaluation of multiple sensory attributes revealed that arils exposed to brandy vapors showed more intense red color and look fresher compared to controls for longer storage time. The latter observation was also validated by multispectral image analysis, since the results suggested that arils packaged with distillery ethanol or brandy maintained their anthocyanin and carotenoids content at higher levels than controls, at 4 °C. Thus, such preservation methods may open new perspectives on mild antimicrobial packaging in order to extend shelf-life of perishable minimally processed fruits, like pomegranate RTE arils.     

Storage quality of strawberry fruit treated by pulsed light: Fungal decay,water loss and mechanical properties

The effect of different pulsed light (PL) doses (2.4–47.8 J/cm²) on water loss, fungal spoilage, mechanical properties and structure of strawberries stored for up to 8 days at 6 °C was studied. Incidence of postharvest molds on strawberry fruits was reduced by over 16–42% with PL application. There were no significant differences in maximal rupture force (F_R), mechanical work (W) and deformability modulus (E_d) values between treated and untreated fruits immediately after treatments. After 8 days storage at 6 °C, untreated strawberries showed a pronounced softening (≈ 48% reduction in F_R), but stored strawberries exposed for 10 s and 40 s to PL presented slight or not significant changes in the mechanical parameters regarding day 0, while F_R and W values of 20 s-PL treated samples were increased by 35% and 88% compared to those at 0 day storage. Micro and ultrastructure changes evaluated by LM and TEM images demonstrated ITW cell wall strengthening and a major integrity of walls of hypodermis cells induced by PL stress, while cell wall disassembly and reduction of cell-to-cell contact were detected in stored untreated fruit. There were no significant differences in weight loss among untreated and PL treated fruits after storage, excepting at the highest PL dose. PL technique would be able to simultaneously provide disinfection and delete softening of the tissues along cold storage. Present results make this non-thermal, residue-free alternative promising for extending shelf-life of traditional and organic strawberry production.Industrial relevanceThe present results demonstrated that pulsed light (PL) treatment is a promising alternative for extending the shelf-life of strawberries. A decrease in fungal incidence and a depletion of softening, important factors which limit the strawberry postharvest storage life, were achieved by the application of PL.     

Proteolysis in Milk Associated with Increasing Somatic Cell Counts

Individual cow samples were collected and preserved with potassium dichromate. Somatic cells counts were determined. Tyrosine value was used as an index of proteolysis. Sixty-six samples ranged in somatic cell count from < 50,000 to > 2,000,000/ml. Initial milk tyrosine values and tyrosine values for milks incubated for 24 h at 37°C showed proteolytic activity increased with increasing somatic cell count. The increase in proteolysis in preserved milk refrigerated for 72 h at 6.7°C was over 1.5 times greater in milks with > 1,000,000 cells/ml than in milks with < 60,000 cells/ml. When preserved milks were laboratory pasteurized, cooled, and stored at 6.7°C for 14 d, some proteolytic activity was detected in milks at all concentrations of somatic cells, and proteolysis increased as somatic cell counts increased. Laboratory-pasteurized samples of milk with various somatic cell counts were also incubated at 30°C for 3 and 6 h to duplicate the proteolysis that could occur during the ripening, coagulation, cutting, and cooking steps of cheese making. Again, the greatest increases in tyrosine were in milks with high somatic cell count. Protease(s) associated with elevated somatic cell counts will damage raw milk quality upon storage, pasteurized fluid milk over shelf-life, and milk during cheese making.     

Analysis of cyclopiazonic acid in milk by capillary electrophoresis

A Micellar Electrokinetic Capillary Chromatography (MEKC) method to detect cyclopiazonic acid (CPA) in milk and compare its quantifying efficiency to the Reverse Phase Liquid Chromatography (RPLC) method was evaluated. Alkaline milk samples were defatted, then acidified before being twice liquid–liquid extracted with chloroform. Bare fused-silica capillary-extended light path with 50 μm i.d. and Nova-pak C18-column, were used for the CPA separation in MEKC and RPLC respectively. The analytical response was linear from 40 ppb to 100 ppm CPA in MEKC (correlation coefficient, r=0.99995). Recoveries of spiked CPA in milk were 78–81% over the range of 20 ppb to 500 ppb in MEKC and 71–80% in the range of 50 ppb to 500 ppb in RPLC. The detectable limit of CPA by MEKC was 0.27×10⁻⁰⁷ pg ml⁻¹. Capillary electrophoresis (MEKC) is a better and rapid method for CPA detection in milk.     

Multicommutation hydride generation atomic fluorescence determination of inorganic tellurium species in milk

A multicommutated flow system has been developed for hydride generation, atomic fluorescence (HG-AFS) determination of tellurate (Te^VI) and tellurite (Te^IV) in milk samples. After a batch leaching of Te by sonication at room temperature for 10 min with aqua regia, sample slurries in acidic medium were merged with sodium borohydride and HCl to obtain data on Te^IV. Another portion of the acidic slurry was mixed with KBr and passed through a reaction coil introduced inside a microwave oven to reduce quantitatively Te^VI to Te^IV which was analyzed by HG-AFS. The detection limit was 0.57 ng g⁻¹ in the original samples. The linear range obtained was till 4 ng ml⁻¹ and the average recovery of different amounts of Te^VI and Te^IV added to real milk samples were 98 ± 4% and 98 ± 2%, respectively, indicating the absence of analyte losses or contaminations and original species modification. Average relative standard deviation of 6.3% was found for Te determination in a series of commercially available milk samples containing from 1.0 to 10.1 ng ml⁻¹ total Te. The proposed method provided a high sampling frequency of 24 h⁻¹ for the determination of both, free Te^IV and total Te, in a same sample with a two times reduced waste generation and a four times reduced reagent consumption as compared with the continuous hydride generation. Additionally, the method developed requires a minimum operator attention and sample manipulation.