Destruction of Salmonella enteritidis inoculated in liquid whole egg by high hydrostatic pressure: comparative study in selective and non-selective media

The destruction of Salmonella enteritidis inoculated in liquid whole egg at approximately 10⁷−10⁸cfu ml^−lwas studied under combinations of pressure (350 and 450 MPa), temperature (50, 20, 2 and −15°C) and time (5, 10, 15 min and cycles of 5+5 and 5+5+5 min). One non-selective medium (tryptone soy agar) and two selective media (brilliant green agar and salmonella-shigella) were used to evaluate viability of S. enteritidis after pressurization. The inactivation rate increased with pressure and exposure time, being minimal at 350 MPa and −15°C for 5 min (over 1 log₁₀of reduction) and reaching total inactivation (8 log₁₀of reduction) in several treatments at 50°C. Treatments in cycles showed greater effectiveness than continuous treatments of the same total time. The effect of pressure was enhanced by elevated temperatures. The higher counts were obtained in the non-selective medium, indicating the presence of injured cells after pressure treatment. D -values obtained for two temperatures (2 and 20°C) and different times (0–60 min) under controlled pressure (400 MPa) showed that microbial inactivation followed a first-order kinetics with a decimal reduction time evaluated in tryptone soy agar medium of 9·5 min at 2°C and 8·8 min at 20°C.     

Injury,Inhibition and Inactivation ofEscherichia coli O157:H7 by Potassium Sorbate and Sodium Nitrite as Affected by pH and Temperature

The effect of potassium sorbate (0–2 g litre⁻¹) and sodium nitrite (0–1 g litre⁻¹) on the growth of four strains of Escherichia coli O157: H7 in tryptic soya broth at various pH levels (pH 4·0–7·0 for sorbate, pH 5·0–8·0 for nitrite) were determined at 37°C and 4°C. Among the pH levels tested, sorbate and nitrite exhibited the highest antimicrobial activity at pH 4·0 and 5·0, respectively. At pH 5·0 and 37°C, the presence of 500 mg litre⁻¹ sorbate or 200 mg litre⁻¹ nitrite completely inhibited the growth of E coli O157: H7. While at higher pH levels, 2 g litre⁻¹ sorbate or 1 g litre⁻¹, nitrite, the highest concentration tested, did not show significant antimicrobial action against the test organisms. At 4°C and pH 5·0, the inoculated test organisms did not showed any significant growth in preservative-free control media. Different degree of inactivation and injury was observed when E coli O157: H7 strain 933 was stored in TSB (pH 5·0) containing 1 g litre⁻¹ sorbate or nitrite at 37°C. At 4°C, inactivation and injury of E coli O157: H7 cells was not observed in the medium containing sorbate or nitrite throughout the 24 h experimental period.     

High Pressure Inactivation of Polyphenoloxidases

Pressure stabilities of polyphenoloxidases (PPO) from apples, avocados, grapes, pears and plums were determined at pH 6-7. These PPOs differed in pressure stability, but all were rather pressure-stable. Inactivation of PPO from apple, grape, avocado and pear at room temperature (25°C) became noticeable at 600, 700, 800 and 900 MPa respectively, and followed first-order kinetics. Plum PPO was not inactivated at room temperature by pressures up to 900 MPa. For the two most pressure-stable PPOs, we investigated whether pressure stability would be reduced by the simultaneous application of mild heat. In case of plum PPO, activity reduction was detectable at 900 MPa and 50°C. Further temperature increase resulted in increase of the inactivation rate constant (E_a 63 kJ/mol). In case of pear PPO, temperature increase up to 35°C resulted in a 3-fold reduction of the inactivation rate constant. Only at higher temperatures, increase of the inactivation rate constant with increasing temperature was noted (Ea 120 kJ/mol).     

Properties of polyphenol oxidase in mango (Mangifera indica) kernel

Polyphenol oxidase (PPO) activity of filtered extract of ground mango kernel suspension (400 g litre⁻¹) was studied spectrophotometrically at 420 nm using catechol as substrate. The enzyme was most active at pH 6·0 and 25°C. Activity was reduced by 50% at pH values of 5·0 and 7·1, and also at temperatures of 14°C and 30°C. The calculated activation energy and the Michaelis constant (K_m) were 21·4 kcal mol⁻¹ °C⁻¹ and 24·6 mM , respectively. The V_max value was 2·14 units g⁻¹ mango kernel. The time to heat inactivate PPO decreased rapidly to < 10 min with increasing temperature of ⩾ 70°C at 50% activity. © 1998 SCI.     

Modified Atmosphere Packaging of Fresh Beansprouts

Freshly harvested beansprouts displayed a respiration rate of about 1 mmol O₂ kg⁻¹ h⁻¹ at 10°C which was strongly dependent on temperature, a 10-fold increase being observed every 16·5°C (z=16·5°C, ie Q₁₀=4·4). This commodity is also characterised by a high initial microbial load (about 10⁷ cells g⁻¹). During storage at various temperatures from 1 to 20°C, oxygen uptake rates dramatically increased with time and this phenomenon was well correlated with the development of aerobic microorganisms which reached 10⁹ cells g⁻¹ after 2 days at 20°C or 9 days at 1°C. Beansprouts were packaged in films, with permeabilities ranging from 950 to 200000 ml O₂ m⁻² day⁻¹ atm⁻¹, and stored at 8°C. Due to plant and microbial metabolism, oxygen concentrations decreased steadily within all packs until the onset of plant tissue decay. The latter occurred after 5–6 days with the least permeable films but did not occur within when the film permeability was over 100000 ml O₂ m⁻² day⁻¹ atm⁻¹. However, such films favoured brown discolouration, exudation texture and breakdown. The orientated polypropylene film (OPP) induced anoxic condition within 2 days and favoured anaerobic metabolism and necrosis of the sprouts. In all packages there was a rapid development of aerobic microorganisms and lactic acid bacteria that resulted in the accumulation of acetate and lactate and a decrease in pH. Thus, it clearly appeared that tissue decay was enhanced by microbial activity. At 8°C, 0·24 m² of film per kg of sprouts provided the optimal atmosphere composition (ie 5% oxygen and 15% carbon dioxide) when a film permeability of 50000 ml O₂ m⁻² day⁻¹ atm⁻¹ was used. These conditions allowed a shelf-life of 4–5 days.     

Modeling the inactivation of pectin methylesterase in pineapple puree during combined high-pressure and temperature treatments

The inactivation of pectin methylesterase (PME) in pineapple puree was studied within the domain of 0.1–600 MPa/30–70 °C/1 s–40 min. The combined effect of pressure-build up and decompression, as characterized by pulse inactivation (PI value), was modeled by the artificial neural network (ANN) through a tan-sigmoidal function of target pressure, target temperature, compression, and decompression time. Besides, nth order kinetic model was fitted during the isobaric-isothermal hold period. The extent of pulse inactivation of PME ranged from 15% (200 MPa/30 °C) to 67% (600 MPa/70 °C) and it increased at a higher temperature and/or pressure. The inactivation orders (n) during thermal (0.1 MPa/30–70 °C) and high pressure (100–600 MPa/30–70 °C) treatments were 1.15 and 1.3, respectively. The rate constant (k) ranged within 4.0 to 71.2 × 10⁻³ Uⁿ⁻¹·min⁻¹. A nonlinear model considering the pressure dependency of activation energy, and temperature dependency of activation volume was developed which adequately described the inactivation behavior of PME within the domain.Industrial relevancePectin methylesterase (PME) in the pineapple puree results in a product with a modified texture and consistency that is usually not entertained by the consumer. Therefore, pineapple puree has to be processed to inactivate PME to avoid the cloud loss. Now-a-days, high-pressure processing is being used for fruit products to retain the heat sensitive nutrients. In this sense, a model capable of predicting the exact inactivation behavior of PME during the treatment is very much obligatory for process design. This combined model developed in the study will help the food industry to come-up with the exact pressure-temperature-holding time combination achieving a certain degree of PME inactivation.     

Purification and Some Properties of Glutaminase fromActinomucor taiwanensis,Starter of Sufu

Glutaminase of Actinomucor taiwanensis was purified approximately 96-fold with a yield of 18%, by sequential fractionation with ammonium sul-phate, anion exchange with DEAE-Sepharose CL-6B and gel filtration with Sephacryl S-200. The pH and temperature optima of purified glutaminase were 8·0 and 45°C, respectively. Glutaminase was stable at a temperature up to 35°C and at pH values of 6·0–8·0. The molecular weight was 80000 as determined from SDS-PAGE. The enzyme activity was markedly inhibited by HgCl₂. In the presence of 100 g litre⁻¹ NaCl, the enzyme activity was inhibited 50%.     

High carbon dioxide pressure inactivation kinetics of Escherichia coli in broth

The inactivation kinetics of Escherichia coli by high pressure carbon dioxide was investigated. Inactivation rates increased with increasing pressure (25, 50, 75 and 100 atm), temperature, and exposure time. Microbial inactivation followed first order reaction kinetics, with inactivation rates (k) and decimal reduction times (D) that varied from 0·0848 to 0·4717 min⁻¹and from 4·90 to 27·46 min, respectively, at treatment temperatures (20, 30 and 40°C). The inactivation rates of E. coli were described by the apparent activation volume (ΔV^*) and a ‘pressure z value’, and they were greatly dependent on both temperature and pressure.     

High-pressure-induced changes in the characteristics of low-fat and high-fat sausages

The purpose of this study was to analyse the consequences of applying high pressures (100 and 300 MPa for 5 and 20 min) on characteristics such as water and fat binding properties, texture, colour, microstructure and microbiology of low-fat (90 g kg⁻¹, LF) and high-fat (247 g kg⁻¹, HF) meat emulsions. Pressurising of LF and HF samples at 300 MPa caused a decline (P<0·05) in emulsion stability, which was more pronounced at the higher pressure. The influence of high pressures on emulsion texture varies according to fat content. In HF samples, high pressure caused a decrease (P<0·05) in Kramer shear force and Kramer energy, regardless of pressure intensity or time. Pressurisation generally caused increase (P<0·05) in the colour lightness parameter in sausages; the effect on redness and yellowness, however, was dependent on fat content, pressure intensity and pressure time. © 1997 SCI.     

Effect of pH and Ionic Strength on the Heat Stability of Rapeseed 12S Globulin (Cruciferin) by the ANS Fluorescence Method

The heat stability of rapeseed 12S globulin (cruciferin) was examined using 8-anilinonaphthalene-1-sulphonic acid (ANS) as a fluorescence probe. Heating cruciferin (0·06–0·3 mg ml⁻¹ in 10 mM glycyl–glycyl piperizine buffer, pH 7·0, with 0·1–1·0 M NaCl) for 20 min increased its hydrophobicity as monitored by ANS fluorescence measurements. The mid-point temperature for the heat effect (T_m) increased linearly with increasing solvent pH (T_m (°C)=4·16 pH+41 (μ=0.1)) or sodium chloride concentration (T_m (°C)=14·7 [NaCl]+71 (pH=7·0)). The range of T_m values for cruciferin was 45–96°C. At 20°C cruciferin was unstable at pH<3·0 but relatively stable under alkaline conditions (pH 8–10). Though possessing an oligomeric structure, cruciferin appears to heat denature in accordance with the two-stage deactivation model for simple globular proteins.     

Inactivation ofEscherichia coliO157:H7 by combinations of GRAS chemicals and temperature

Reported outbreaks of foodborne illness involvingEscherichia coliO157:H7 have increased in the United States during the last decade, with a variety of food products being implicated as vehicles of infection. Studies were carried out to determine the efficacy of combinations of various GRAS chemicals and moderate temperatures to killE. coliO157:H7. A five-strain mixture ofE. coliO157:H7 of approximately 10⁸cfu ml⁻¹was inoculated into 0·1% peptone solutions containing 1·0 or 1·5% lactic acid plus 0·1% hydrogen peroxide, 0·1% sodium benzoate or 0·005% glycerol monolaurate. The solutions were incubated at 8°C for 0, 15 and 30 min; at 22°C for 0, 10 and 20 min; or at 40°C for 0, 10 and 15 min; populations ofE. coliO157:H7 were determined at each sampling time. At 40°C, the pathogen was inactivated to undetectable levels within 10 min of incubation in the presence of 1·0 or 1·5% lactic acid plus hydrogen peroxide, and within 15 min of incubation in the presence of 1·5% lactic acid plus sodium benzoate or glycerol monolaurate. At 22°C, complete inactivation ofE. coliO157:H7 was observed after 20 min of exposure to 1·5% lactic acid plus 0·1% hydrogen peroxide, whereas a reduction of 5 log₁₀cfu ml⁻¹was observed with a treatment of 1·5% lactic acid plus glycerol monolaurate. None of the treatments resulted in total inactivation of the pathogen at 8°C. The aforementioned treatments could potentially be used to inactivate or reduceE. coliO157:H7 populations on raw produce.     

Effect of pH on Enzyme Inactivation Kinetics in High-Pressure Processed Pineapple (Ananas comosus L.) Puree Using Response Surface Methodology

Effect of pH and high-pressure process treatments viz. pressure, temperature, and dwell time on inactivation of polyphenoloxidase (PPO), peroxidase (POD), bromelain (BRM), and pectinmethylesterase (PME) in pineapple puree was studied. Experiments were conducted according to rotatable central composite design (RCCD) within the range (?α to?+?α) of 100–600 MPa, 20–70 °C, and 0–30 min at three different pH levels (3.0, 3.5, and 4.0) followed by analysis through response surface methodology (RSM). Enzyme inactivation was significantly (p?k in min^?1) revealed that PPO was the most resistive (k ranged between 0.0020 and 0.0379 min^?1) when compared with other three enzymes within the experimental domain. Increased k at lower pH with constant pressure and temperature depicted that pH had negative effect on the inactivation process. The optimized conditions targeting maximum inactivation of PPO, POD and PME with simultaneous retention of BRM in pineapple puree, were 600 MPa/60 °C/9 min, 600 MPa/60 °C/10 min and 600 MPa/60 °C/10 min for the samples of pH 3.0, 3.5, and 4.0, respectively.     

The fate of Escherichia coli O157 : H7 in Myzithra,Anthotyros, and Manouri whey cheeses during storage at 2 and 12°C

Myzithra, Anthotyros and Manouri whey cheeses were inoculated the day after production withEscherichia coli O157 : H7 at concentrations of approx. 1·8×10⁶cfu g⁻¹, and stored at 2 and 12°C for 30 and 20 days, respectively. The pH of the whey cheeses decreased from an initial value of approx. 6·20 to 5·83 or 5·60 (Myzithra) 5·75 or 5·20 (Anthotyros) and 5·80 or 5·30 (Manouri) by the end of the corresponding storage periods at 2 and 12°C, respectively. Escherichia coli O157 : H7 populations in the whey cheeses at the end of the 12°C storage period, had grown with an increase of approx. 1·3 log₁₀cfu g⁻¹. E. coli O157 : H7 populations in whey cheeses at the end of the 2°C storage period did not grow and decreased, with an approx. 2·5 log₁₀cfu g⁻¹reduction. Results showed that E. coli O157 : H7 can grow at 12°C and survive at 2°C storage in Myzithra, Anthotyros and Manouri whey cheeses, and therefore post-manufacturing contamination with this pathogen must be avoided by employing hygienic control programmes such as HACCP.     

Inactivation and reactivation of horseradish peroxidase treated with supercritical carbon dioxide

Effects of supercritical carbon dioxide (SCCO₂) on the activity of horseradish peroxidase (HRP) in pH 5.6 acetate buffer solution were investigated. SCCO₂ treatment could effectively inactivate HRP. Higher pressure, higher temperature, and longer treatment time caused more inactivation. The maximum reduction of HRP activity reached nearly 90% at 30 MPa and 55 °C for 60 min. Analysis of first-order reaction kinetic data (characterized by a rate constant k and by a decimal reduction time D) showed that D value was closely related to the pressure and temperature of SCCO₂ treatment. Higher pressures or higher temperatures resulted in lower D values (higher k), the D value of HRP was minimized to 64.52 min treated by the combination of 30 MPa and 55 °C. The Z _p, representing the range of applied pressure between which the D values change by a factor of 10, was 114.81 MPa. The activity of HRP treated by SCCO₂ was reactivated significantly after initial 7-day storage at 4 °C apart from the samples at 30 MPa for 60 min, indicating the HRP inactivation may be reversible and the reactivation of HRP is dependent on the pressure level and treatment time.     

High Hydrostatic Pressure as a Hurdle for Zygosaccharomyces bailii Inactivation

The combined effects of high hydrostatic pressure (HHP, 172, 345, 517 or 689 MPa), duration of HHP treatment (0, 2, 4, or 10 min), water activity (a_w 0.98 or 0.95), and potassium sorbate (PS) concentration (0 or 1000 ppm) on Zygosaccharomyces bailii inactivation were evaluated at pH 3.5 and 21°C in laboratory model systems. Inactivation of the initial inoculum (? 1.0 times; 10⁵ CFU/mL) occurred when the pressure was 689 MPa regardless of a_w PS or duration of treatment. Lower pressure was required for Z. bailii inactivation in the presence of PS.     

Behavior of mustard seed (<Emphasis Type="Italic">Sinapis alba</Emphasis> L.) myrosinase during temperature/pressure treatments: a case study on enzyme activity and stability

The activity of myrosinase, an enzyme found mainly in Brassicaceae, is influenced by some intrinsic (e.g. pH, ascorbic acid) and extrinsic (e.g. temperature, pressure) factors. In this study, the effect of intrinsic and extrinsic factors on the activity of mustard seed myrosinase (Sinapis alba L.) was determined in a buffer system and in broccoli juice. Ascorbic acid and to a much lesser extent MgCl₂ were found to enhance the myrosinase activity. In buffer solution, the optimal temperature for myrosinase activity at atmospheric pressure was 60 °C. At elevated pressure, the reaction rate increased until 200 MPa and the optimal temperature shifted to 40 °C in a buffer system. In broccoli juice, mustard seed myrosinase behaved somewhat different compared to the buffer system. The highest enzyme activity was found at 60 °C, both at atmospheric and elevated pressures. In broccoli juice, the enzymatic reaction rate also increased up to pressures of 200 MPa. Enzyme inactivation could be described by first order kinetics.     

Sensitivity of spores of Bacillus subtilis and Clostridium sporogenes PA 3679 to combinations of high hydrostatic pressure and other processing parameters

The combined effects of pressure, temperature, pH and the presence of nisin or sucrose laurate on the survival of spores of Bacillus subtilis 168 and Clostridium sporogenes PA 3679 were investigated. Spore populations of PA 3679 were reduced by 2.5-log₁₀ when exposed to 404 megapascals (MPa) at 25°C, pH 4.0 for 30 min, but the same treatment at pH 7.0 resulted in a <0.5-log₁₀ reduction in spore counts. Pressurization of B. subtilis spores at 70°C, pH 6.0 or 7.0 for 15 min at 404 MPa resulted in a 5-log₁₀ reduction as compared to a <0.5-log₁₀ reduction for the same pressurization treatment at 25°C. For the inactivation of spores of B. subtilis and PA 3679, the addition of nisin to the plating medium appeared to be synergistic in some instances when combined with pressurization at elevated temperatures and reduced pH. B. subtilis 168 was resistant to 0.1% sucrose laurate, but when combined at ≤6.0 pH with a 15-min treatment of 404 MPa at 45°C, a dramatic synergistic effect eliminated spore suspensions of 1×10⁶/ml.     

Mathematical modeling of Saccharomyces cerevisiae inactivation under high‐pressure carbon dioxide

High‐pressure carbon dioxide inactivation curves of Saccharomyces cerevisiae at different temperatures were analysed using the modified Gompertz model. Comparable λ and μ values were obtained under pressure treatment as function of temperature. The phase of disappearance (λ) and the inactivation rate (μ) of S. cerevisiae were inversely related. Higher μ values were obtained at 50°C than at 40, 30, and 20°C under 10.0 MPa CO₂ pressure. Increased pressure and temperature had significant effects on the survival of S. cerevisiae. Arrhenius, linear and square‐root models were used to analyse the temperature dependence of the inactivation rate constant. For the Arrhenius model the activation energy (E_μ) was 56.49 kJ/mol at 10.0 MPa, and 55.70, 53.83, and 52.20 kJ/mol at 7.5, 5.0, and 2.5 MPa, respectively. Results of this study enable the prediction of yeast inactivation exposed to different CO₂ pressures and temperatures.     

Sensitivity of multidrug-resistant Salmonella typhimurium DT104 to organic acids and thermal inactivation in liquid egg products

A mixture of four Salmonella typhimurium DT104 strains and a mixture of four S. typhimurium non-DT104 strains were examined for their ability to grow in tryptic soy broth (TSB) acidified with acetic, lactic, citric, or malic acids at pH 5·4, 4·4, and 3·7. Significantly (P<0·05) higher numbers of S. typhimurium DT104 cells were detected at pH 4·4 and 4·0 in TSB acidified with acetic acid and at pH 4·4 and 3·7 in TSB acidified with lactic acid compared to non-DT104 cells. Acid-shocked and non-shocked (control) cells were plated on TSA (pH 7·3) acidified with lactic acid at pH 5·4, 4·4, and 4·0 and on TSA (pH 7·0±0·2) containing 0·5, 2·5, and 5% sodium chloride. Populations of acid-shockedS. typhimurium DT104 and non DT104 cells recovered on acidified or salt-supplemented TSA were significantly (P<0·05) lower than those of non-shocked cells. A significantly lower number of acid-shocked non-DT104 cells recovered on TSA at pH 5·4, compared to acid-shocked DT104 cells, suggests that DT104 cells may be more resistant to acid shock and subsequent exposure to acid pH. D values and z values of acid-shocked or non-shocked cells of DT104 and non-DT104 strains in liquid whole egg (WE), egg yolk (EY), egg white (EW), whole egg+10% salt (WES), and egg yolk+10% salt (EYS) were determined. Differences in thermal sensitivity of the two types of cells were few. Rates of thermal inactivation of S. typhimurium DT104 cells indicate that the USDA pasteurization process would eliminate >8 log₁₀cfu ml⁻¹of EW heated at 57°C and >11 log₁₀cfu ml⁻¹of WE, EY, WES, or EYS heated at 61°C. D values of acid-shocked DT104 and non-DT104 cells heated in liquid egg products were significantly (P<0·05) lower than those of respective non-shocked cells.     

Inactivation of Listeria monocytogenes by high hydrostatic pressure: effects and interactions of treatment variables studied by analysis of variance

High hydrostatic pressure is regarded as possible alternative method for food preservation. One of the primary considerations for industrial applications is the ability of this method to eradicate pathogenic micro-organisms. This study subjected L. monocytogenes suspensions, in a phosphate buffer (pH 7·0) or in a citrate phosphate buffer (pH 5·6), to high hydrostatic pressure treatments relative to the following variables: pressure (200–600 MPa), treatment time (3, 10 and 20 min), temperature (4, 20 and 40°C) and the pH of the suspension medium (5·6 and 7·0). An optimal design of 40 runs was obtained using the Fedorov algorithm, and responses were studied by analysis of variance in terms of cell survival on plate count agar. Efficiency was determined by log₁₀comparisons of the numbers of live cells before and after treatment. A statistically significant relationship was found between the four variables considered (pressure, pH, treatment time and temperature), their interactions (treatment time vs pressure, pH vs treatment time, pH vs pressure, pressure vs temperature, treatment time vs temperature) and the inactivation of L. monocytogenes. R -squared statistical analysis indicated that the linear model used accounted for more than 98·5% of the variability in the inactivation ofL. monocytogenes .