A Study of Staphylococcus Aureus Growth in Model Systems and Processed Cheese

A survey of a_w and pH in commercial samples of processed cheese produced in Argentina, indicated the a_w varied between 0.950–0.978 and pH ranged from 5.2–6.1 Staphylococcus aureus growth studies in model systems (laboratory media with added NaCl) with a_w and pH adjusted to resemble that of commercial samples of processed cheese showed that S. aureus grows very well at a temperature of 30°C, but complete growth inhibition occurs at refrigerator temperature. The results obtained with model systems were confirmed by S. aureus growth experiments in inoculated cheese.     

Combined Effect of Water Activity, pH and Sub-optimal Temperature on Growth and Enterotoxin Production of Staphylococcus aureus

Staphylococcus aureus growth and enterotoxin production in relation to a, (0.99–0.87), pH (4.0–7.0) and temperature (8–30°C) was determined in Brain-heart-infusion broth with respectively NaCl and sucrose as humectants. Growth of S. aureus was not observed at a_w 0.85, at pH 4.3 or at 8°C. At 12°C no growth occurred at a, 0.90 or at a, 0.93 in combination with pH < 5.5. At a_w 0.96 no growth occurred at pH ≤ 4.9. Production of staphylococcal enterotoxin type A (SEA) occurred under nearly all conditions allowing growth. Production of SEB appears to be determined by a_w; at a_w 0.96 SEB was produced at all temperatures allowing growth, while at a, 0.93 SEB was hardly produced. Production of SEC and SEF was affected both by a_w and temperaure. Production of these toxins was rarely observed at a, 0.93. No SEC and SEF were produced at 12°C. Extrapolation of these findings to food is discussed.     

‘Combined effect of water activity, pH and additives on growth of Staphylococcus aureus in model salami systems’

The combined effect of pH, water activity (a_w) and certain additives (sodium nitrite and nitrate, polyphosphates, wine and pepper) on growth and survival of two strains of S. aureus in model salami systems at 25°C was investigated. These additives are usually used in Argentinean salami formulation. The combinations of a_w 0·925-pH 5·00, or a_w 0·915 - pH 5·50 or a_w 0·900 - pH 6·00, together with the addition of the additives, resulted in synergistic growth inhibition of both strains. Sodium nitrite and nitrate played an important role in the inhibition, while pepper, polyphosphates and wine had no effect in controlling the growth of these organisms.     

The effects of brine concentration and scalding on survival of some pathogens in urfa cheese: a traditional white-brined turkish cheese

The survival of Staphylococcus aureus (St. aureus), Bacillus cereus, Yersinia enterocolitica, Eschericia coli O157:H7, Shigella flexneri (Sh. flexneri) and Salmonella enteritidis (Sa. enteritidis) in urfa cheese (a traditional white‐brined Turkish cheese) which was stored in brine concentrations varying from 12.5 to 17.5% (wt/v) was tested. Two sets of cheeses were made, namely scalded and unscalded cheeses (scalding was done by heating at 95 °C for 3 min). The variations in the counts of pathogenic colonies were monitored throughout a 90‐day storage period at <10 °C. Results indicated that scalding caused statistically significant reductions in the colony counts of Y. enterocolitica, E. coli O157:H7, Sh. flexneri and Sa. enteritidis during the early periods of storage. In contrast, St. aureus and B. cereus were not generally affected by scalding and brine concentrations, although B. cereus in 17.5% (wt/v) brine was affected. In the unscalded cheeses, 12.5 and 15.0% (wt/v) brine concentrations seemed to be insufficient to eradicate the pathogenic organisms examined.     

Effect of Water Activity on the Growth Kinetics of Staphylococcus aureus in Ground Bread Crumb

The kinetics of Staphylococcus aureus (strains A, B, D) growth in bread crumb were determined as a function of water activity (A_w) from 0.836 to 0.909 at pH 5.2 to 5.5 and at 35°c. Adding glycerol to the dough or equilibrating the bread over saturated salt solutions adjusted the A_w of the bread. Growth kinetics data, plotted as enumerated colony counts versus incubation time, were fitted using the logistic function to determine maximum growth rates. Similar maximum growth rates resulted, irrespective of the method used to adjust A_w. Extrapolation of growth rate‐A_w results predicts the A_w corresponding to a zero growth rate.     

Effect of chemical and environmental factors on Aspergillus ochraceus growth and toxigenesis in green coffee

Post-harvest processing (traditional or ecological wet method, and dry method) and coffee pH did not play a significant role in Aspergillus ochraceus growth and OTA production. However, A_w did play a key role: the optimum for growth and toxigenesis was 0.95; below 0.80, coffee was protected. Temperature affected the rate of toxin production, when A_w was compatible: toxigenesis occurred from 10°C with an optimum at 35°C. The critical stage in the process was drying, where conditions propitious to A. ochraceus (A_w of 0.99–0.80) could be found for 2 days or more. Caffeine and chlorogenic acids had an inhibiting effect on OTA production.     

Isolation and characterization of a novel analyte from Bacillus subtilis SC-8 antagonistic to Bacillus cereus

In this study, an effective substance was isolated from Bacillus subtilis SC-8, which was obtained from traditionally fermented soybean paste, cheonggukjang. The substance was purified by HPLC, and its properties were analyzed. It had an adequate antagonistic effect on Bacilluscereus, and its spectrum of activity was narrow. When tested on several gram-negative and gram-positive foodborne pathogenic bacteria such as Salmonella enterica, Salmonella enteritidis, Staphylococcus aureus, and Listeria monocytogenes, no antagonistic effect was observed. Applying the derivative from B. subtilis SC-8 within the same genus did not inhibit the growth of major soybean-fermenting bacteria such as Bacillus subtilis, Bacillus licheniformis, and Bacillus amyloquefaciens. The range of pH stability of the purified antagonistic substance was wide (from 4.0 to >10.0), and the substance was thermally stable up to 60 °C. In the various enzyme treatments, the antagonistic activity of the purified substance was reduced with proteinase K, protease, and lipase; its activity was partially destroyed with esterase. Spores of B. cereus did not grow at all in the presence of 5 μg/mL of the purified antagonistic substance. The isolated antagonistic substance was thought to be an antibiotic-like lipopeptidal compound and was tentatively named BSAP-254 because it absorbed to UV radiation at 254 nm.     

Antibacterial Effects and Cell Morphological Changes in Sfaphylococcus aureus Subjected to Low Ethanol Concentrations

Ethanol in low concentrations (5–10% weight) affected aerobic growth (at 37°C) of two strains of Stuphylococcus aureus. Growth inhibition in laboratory media was observed with ethanol about 6–7% weight water activities (a_w) of ～0.97. This suggested that the ethanol antibacterial effect was not only from its a_w lowering ability. Electron microscopy observations showed intense morphological modifications in cells in broth with 5% to 6.5% ethanol, and clearly indicated that cell wall changes of S. aureus was partly responsible for the antibacterial action of ethanol.     

Staphylococcus aureus Growth and Toxin Production in Imitation Cheeses

The ability of eleven imitation (or substitute) cheeses to support the growth and toxin production of Staphylococcus aureus at 26°C was evaluated. All established enterotoxin serotypes were tested by inoculating suspensions of the requisite strains into 100-g samples of cheese (about 30 staphylococci/g). Water activity (a_w) of the cheese samples ranged from 0.942–0.973; pH values ranged from 5.33–6.14. Seven cheeses supported extensive growth of S. aureus; one or more serotypes of enterotoxin were produced in six cheeses. Enterotoxin in the cheese was detected in 4 days at 3 × 10⁶ staphylococci/g. However, the ability of some cheeses to support growth and toxin production of S. aureus could not be correlated with pH, a_w, or product formulation.     

INFLUENCE OF WATER ACTIVITY, pH, HEAT TREATMENT AND PRESERVATIVE ON CHEMICAL CHANGES IN CANNED INDIAN COTTAGE CHEESE (PANEER) DURING STORAGE

ABSTRACT

Influence of water activity (A_w), pH and heat treatment on the chemical changes such as Maillard browning, oxidation and lipolysis in canned paneer has been studied by response surface methodology. Browning in canned paneer in terms of hydroxymethylfurfural content increased during storage; however, it was pH dependent. At pH above 5.0, the rate of browning decreased as A_w increased from 0.90 to 0.98. At pH below 5.0, the rate increased with increased A_w from 0.90 to 0.98. Maximum browning rates were observed at pH 5.2–5.6, low A_w values (0.90–0.92) and high heat treatment levels (up to F value 0.8). At all pH levels, increased A_w increased the oxidation rate, more so at low pH levels. Up to an F value of 0.3, heat treatment seemed to have aided the oxidation rate, but higher F values retarded the oxidation rate, more so at low A_w. Lipolysis occurred during storage, but pH had little effect on the rate of lipolysis. High A_w supported high lipolysis rates.

PRACTICAL APPLICATIONS

Raw paneer (Indian cottage cheese), popular with people in Indian subcontinent and with Indian diasporas scattered in several countries, has a shelf life of less than a day at ambient temperatures. Several attempts are being made by workers to improve the shelf life of paneer by canning process so that its market within the country and abroad improves. However, canning at usual canning time–temperatures is leading to intense chemical changes in paneer, which is not being accepted by consumers. Authors have tried to obviate such unacceptable chemical changes by reducing the intensity of canning process by adjusting so called “hurdles” of the product by hurdle technology. The present paper is a part of that attempt describing the chemical changes in paneer subjected to low heat intensity canning process with altered “hurdles.” Because the chemical changes occurring in a food product are a function of various parameters called “hurdles” like pH, water activity, heat treatment, etc., the present work attempted to identify the combination of the “hurdles” at which chemical changes are the slowest, which may find better acceptance with consumers.     

Effects of heat-processing regime, pH, water activity and their interactions on the behaviour of Listeria monocytogenes in ground pork. Modelling the boundary of the growth/no-growth areas as a function of pH, water activity and temperature

The influence of four heat‐processing regimes and a storage phase on the behaviour of Listeria monocytogenes in ground pork was studied. The effects of pH and water activity (a_w) were also tested. During the heat process phase, a_w, the heat‐processing regime and its interactions with pH or a_w, had a significant effect on the behaviour of L. monocytogenes. During the storage phase, all parameters tested and their interactions had significant effects. Nevertheless, the area in which the growth of L. monocytogenes was observed at the end of the experiment was not influenced by the heat‐processing regime tested. On the contrary, pH, a_w and their interactions had significant effects on Listeria behaviour. The boundary of the growth area delimited by environmental conditions where growth was higher than 1.0 Log CFU g^?1 from those where growth was lower than this limit was correctly predicted by Augustin's model.     

Growth of pathogenic bacteria in sliced vacuum-packed bologna-type sausage as influenced by temperature and gas permeability of packaging film

Studies on sliced vacuum-packed Bologna-type sausage showed that some pathogenic bacteria may develop quickly at refrigeration temperatures. Yersinia enterocolitica grew rapidly even at 5°C, reaching 10⁶ g^?1 within 2 weeks. Other bacteria did not grow at 5°C, Staphylococcus aureus and Bacillus cereus grew at 8°C and Salmonella typhimurium and Salmonella enteritidis but not Clostridium perfringens grew at 12°C. Therefore even at temperatures which may be found in commercial refrigerators, growth of pathogenic bacteria is possible to levels which have been shown to represent a hazard to health. Gas permeability of the packgging film had relatively little effect.     

Environmental factors affect efficacy of some essential oils and resveratrol to control growth and ochratoxin A production by Penicillium verrucosum and Aspergillus westerdijkiae on wheat grain

This study determined the efficacy of three essential oils (bay, clove and cinnamon oil) and the antioxidant resveratrol (0–500 μg g⁻¹) on the control of growth and ochratoxin A (OTA) production by Penicillium verrucosum and Aspergillus westerdijkiae (=A. ochraceus) under different water activity (a_w, 0.90, 0.95, 0.995), and temperature (15, 25 °C) conditions on irradiated wheat grain. The most effective treatment (resveratrol) was then tested on natural grain. The ED₅₀ values for growth inhibition by the oils were 200–300 μg g⁻¹ at the a_w and the temperatures tested. For resveratrol, this varied from <50 μg g⁻¹ at 0.90–0.95 a_w to >350 at 0.995a_w at both temperatures. The ED₅₀ values for the control of OTA were slightly lower than for control of growth, with approx. 200 μg g⁻¹ required for the oils and 50–100 μg g⁻¹ of the antioxidant, at 0.90/0.95a_w and both temperatures. In wet grain (0.995a_w), higher concentrations were required. For growth there were statistically significant effects of single-, two- and three-way interactions between treatments except for concentration×temperature and concentration×temperature×essential oil/antioxidant treatment. For OTA control, statistically significant treatments were a_w, temperature×a_w, concentration×temperature, treatment×concentration, and three-way interaction of concentration×a_w×treatment for P. verrucosum and A. westerdijkiae. Subsequent studies were done with the best treatment (resveratrol, 200 μg g⁻¹) on natural wheat grain with either P. verrucosum or A. westerdijkiae at 0.85–0.995a_w and 15/25 °C over 28 days storage. This showed that the populations of the mycotoxigenic species and OTA contamination could be reduced by >60% by this treatment at the end of the storage period.     

Antioxidant and antimicrobial properties of Thai propolis extracted using ethanol aqueous solution

The potential of using propolis collected from Thailand as a natural antioxidant and antimicrobial agent for food applications was investigated. The propolis extract was prepared by using different ethanol aqueous solutions, including 30%, 40%, 50% and 70%. Total phenolic content (TPC), phenolic compound and antioxidant activity of the propolis were determined using Folin–Ciocalteau method, high performance liquid chromatography (HPLC) and 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) radical scavenging activity, respectively. The antimicrobial ability was tested against Staphylococcus aureus (TISTR 118), Salmonella enteritidis (DMST 17368), Escherichia coli (TISTR 780) and Pseudomonas aeruginosa (ATCC 27853) using disc diffusion technique. The major phenolic compounds found in Thai propolis were rutin, quercetin and naringin. The TPC and DPPH radical scavenging activity increased with increasing ethanol concentration in the solvent. Propolis extract showed antimicrobial activity, in terms of inhibitory zone for S. aureus and limited growth underneath paper discs, against all tested bacteria.     

Real-time-PCR-System zum Nachweis von Bacillus cereus (emetischer Typ) in Lebensmitteln

Bacillus cereus is a Gram-positive, facultative anaerobic, spore-performing bacterium. Some B. cereus strains have the ability to produce two different types of toxins: (a) diarrhoeic toxin: the disease is similar to a C. perfringens toxin-infection; it is caused by a heat-labile protein, (b) emetic toxin: the disease is similar to Staphylococcus-aureus intoxikation; it is caused by a heat-stable protein. Statements about the frequency of B. cereusfood-poisonings are difficult because a reporting system for this disease is missing in Germany and there exists no valid methodology to diagnose this disease with conventional microbiological or biochemical methods. Nevertheless B. cereus, beside S. aureus, seems to be the most important bacterium to cause food-associated intoxications.     

Mathematical modeling of Bacillus cereus in Saengsik,a powdered ready-to-eat food and its application in quantitative microbial risk assessment

In this study, we developed a mathematical model to describe the fate of Bacillus cereus in Saengsik, a powdered ready-to-eat food with reduced moisture content, and to estimate the probability of B. cereus infection from Saengsik consumption, using quantitative microbial risk assessment (QMRA). The isothermal kinetic behavior of B. cereus was analyzed as a function of temperature (5–40°C). Bacterial cell counts were fitted to the Weibull model using GinaFit, and the obtained delta values (required time for the first decimal reduction) for each temperature were 128.3–17,124.1 hr and concave (ρ < 1) were observed under all experimental conditions. The obtained delta values were analyzed using the Davey model as a function of temperature, and the performance of the developed model of survival of B. cereus was appropriate based on the validation parameters (B_f, A_f, and RMSE) within the acceptable range. QMRA model was developed using data on the prevalence and concentration of B. cereus along with time and temperature along the retail-to-consumer steps. Using a baseline model with the currently available data, the probability of B. cereus infection was zero. These results provide useful information for the risk assessment and management of microbial risk in foods, especially those with very low-moisture content.     

Inhibition of Salmonella enteritidis and Staphylococcus aureus in nutrient broth by mint essential oil

The effect of different concentrations (0–1.2% v/v) of mint (Mentha piperita) essential oil on the growth/survival of Salmonella enteritidis and Staphylococcus aureus was studied in nutrient broth, using the viable count method and conductance measurements. In particular the addition of mint essential oil reduced the total viable counts of St. aureus about 6–7 logs while this of S. enteritidis only ca. 3 logs. The detection time measurements showed also that the inhibitory effect of mint essential oil was affected by the incubation temperature as well as by the concentration of essential oil added in the growth medium. At low concentration of essential oil (<0.1%), the addition of glucose in the growth medium prevented the formation of staphylococcal enterotoxin B, while the reduction of viable cells was only 2 logs. This glucose effect was not evident in Salmonella enteritidis samples. The percentage of glucose utilization in the growth medium of both pathogens, was reduced drastically with the addition of essential oil and as a consequence the assimilation or formation of different compounds, such as lactate, formate and acetate in the growth medium was also affected.     

Variation of cardinal growth parameters and growth limits according to phylogenetic affiliation in the Bacillus cereus Group. Consequences for risk assessment

The growth rates of strains covering the seven major phylogenetic groups of Bacillus cereus sensu lato (as defined by Guinebretiere et al., 2008) at a range of temperature (7 °C–55 °C), pH (4.6–7.5) and a_w (0.929–0.996, with 0.5%–10% NaCl as humectant) were determined. Growth rates were fitted by non-linear regression to determine the cardinal parameters T_min, T_opt, T_max, pH_min, pH_opt, a_wmin and μ_opt. We showed that cardinal parameters reflected the differences in the temperature adaptation observed between B. cereus phylogenetic groups I to VII. The ability of growing at low pH (up to 4.3) or low a_w (from a_w 0.929 and up to 10% NaCl) varied among strains. The strains of groups III and VII, the most tolerant to heat, were also the most adapted to high NaCl (all strains growing at 8% NaCl) and the ones of groups I and VI the least adapted (no growth at 7% NaCl). All strains of groups II and VII were able to grow at pH 4.6, and only a few strains of group VI. Phenotypic differences between the two psychrotrophic groups II and VI were revealed by contrasted acid and salt tolerance. The cardinal values determined in this work were validated by comparing with cardinal parameters of a panel of strains published elsewhere and with predictions of growth in a range of foods.     

Inactivation of Yersinia enterocolitica by pulsed electric fields

The influence of growth conditions, treatment medium characteristics and PEF process parameters on the lethal effect on Yersinia enterocolitica of pulsed electric fields (PEF) treatments in batch has been investigated. Growth phase, temperature of growth, pH, conductivity of the treatment medium, pulse width and frequency of pulses did not influence the sensitivity of Y. enterocolitica to PEF. However, an A_w decrease from >0.99 to 0.93 of the treatment medium increased the PEF resistance of Y. enterocolitica with 3.5 log₁₀ cycles after a treatment of 22 kV/cm, 800 μs and 880 kJ/kg. Inactivation of Y. enterocolitica increased with the field strength, treatment time and total specific energy up to a maximum of 6 log₁₀ cycles after 28 kV/cm, 2000 μs and 3559 kJ/kg. A nonlinear relationship was found among the survival fraction and the treatment time or the specific energy that was accurately described by a mathematical model based on the Weibull distribution. The inactivation of Y. enterocolitica by PEF was characterized by maximum field strength thresholds. Above these thresholds, specific energy necessary to obtain a given level of inactivation scarcely decreased by increasing the electric field strength, and inactivation of Y. enterocolitica only depended on the specific energy applied.     

Model for Combined Effects of Temperature, pH and Water Activity on Thermal Inactivation of Bacillus cereus Spores

Initially, the effect of water activity (a_w) on heat resistance of Bacillus cereus spores (decimal reduction time) was investigated. A linear relationship was found between log D and 1-a_w. The combined effects of temperature (85-105°C), pH (4.5-6.5) and water activity (0.80-1) were then studied. A four parameter model was fitted to the data. This model appeared to be parsimonious with each parameter having a biological significance. Interactions between factors were observed but they accounted for <2.4% of the total variation and they were not taken into account by the model.