Inactivation Kinetics of Listeria monocytogenes by High-Pressure Processing: Pressure and Temperature Variation

The enhanced quasi-chemical kinetics (EQCK) model is presented as a methodology to evaluate the nonlinear inactivation kinetics of baro-resistant Listeria monocytogenes in a surrogate protein food system by high-pressure processing (HPP) for various combinations of pressure (P= 207 to 414 MPa) and temperature (T= 20 to 50 °C). The EQCK model is based on ordinary differential equations derived from 6 "quasi-chemical reaction" steps. The EQCK model continuously fits the conventional stages of the microbial lifecycle: lag, growth, stationary phase, and death; and tailing. Depending on the conditions, the inactivation kinetics of L. monocytogenes by HPP show a lag, inactivation, and tailing. Accordingly, we developed a customized, 4-step subset version of the EQCK model sufficient to evaluate the HPP inactivation kinetics of L. monocytogenes and obtain values for the model parameters of lag (λ), inactivation rate (μ), rate constants (k), and "processing time" (tp). This latter parameter was developed uniquely to evaluate kinetics data showing tailing. Secondary models are developed by interrelating the fitting parameters with experimental parameters, and Monte Carlo simulations are used to evaluate parameter reproducibility. This 4-step model is also compared with the empirical Weibull and Polylog models. The success of the EQCK model (as its 4-step subset) for the HPP inactivation kinetics of baro-resistant L. monocytogenes showing tailing establishes several advantages of the EQCK modeling approach for investigating nonlinear microbial inactivation kinetics, and it has implications for determining mechanisms of bacterial spore inactivation by HPP. Practical Application: Results of this study will be useful to the many segments of the food processing industry (ready-to-eat meats, fresh produce, seafood, dairy) concerned with ensuring the safety of consumers from the health hazards of Listeria monocytogenes, particularly through the use of emerging food preservation technologies such as high-pressure processing.     

Estimating the heat resistance parameters of bacterial spores from their survival ratios at the end of UHT and other heat treatments

Accurate determination of bacterial cells or the isothermal survival curves of spores at Ultra High Temperatures (UHT) is hindered by the difficulty in withdrawing samples during the short process and the significant role that the come up and cooling times might play. The problem would be avoided if the survival parameters could be derived directly from the final survival ratios of the non-isothermal treatments but with known temperature profiles. Non-linear inactivation can be described by models that have at least three survival parameters. In the simplified version of the Weibullian -log logistic model they are n, representing the curvature of the isothermal semilogarithmic survival curves, T(c), a marker of the temperature where the inactivation accelerates and k, the slope of the rate parameter at temperatures well above T(c). In principle, these three unknown parameters can be calculated by solving, simultaneously, three rate equations constructed for three different temperature profiles that have produced three corresponding final survival ratios, which are determined experimentally. Since the three equations are constructed from the numerical solutions of three differential equations, this might not always be a practical option. However, the solution would be greatly facilitated if the problem could be reduced to the solution of only two simultaneous equations. This can be done by progressively changing the value of n by small increments or decrements and solving for k and T(c). The iterations continue until the model constructed with the calculated k and T(c) values correctly predicts the survival ratio obtained in a third heat treatment with a known temperature profile. Once n, k, and T(c) are established in this way, the resulting model can be used to predict the complete survival curves of the organism or spore under almost any contemplated or actual UHT treatment in the same medium. The potential of the method is demonstrated with simulated inactivation patterns and its predictive ability with experimental survival data of Bacillus sporothermodurans. Theoretically at least, the shown calculation procedure can be applied to other thermal preservation methods and to the prediction of collateral biochemical reactions, like vitamin degradation or the synthesis of compounds that cause discoloration. The concept itself can also be extended to non-Weibullian inactivation or synthesis patterns, provided that they are controlled by only three or fewer kinetic parameters.     

On quantifying nonthermal effects on the lethality of pressure-assisted heat preservation processes

Direct experimental identification and quantification of the pressure contribution to a pressure-assisted sterilization process efficacy is difficult. However, dynamic kinetic models of thermal inactivation can be used to assess the lethality of a purely thermal process having the same temperature profile. Thus, a pressure-assisted process' temperature record can be used to generate a corresponding purely thermal survival curve with parameters determined in conventional heating experiments. Comparison of the actual final survival ratio with that calculated for the purely thermal process would reveal whether the hydrostatic pressure had synergistic or antagonistic effect on bacterial spores survival. The effect would be manifested in the number of log cycles subtracted or added to the survival ratio, and in the length of time at the holding temperature needed to produce the final survival ratio of the combined process. A set of combined treatments would reveal how the temperature and pressure profiles affect the pressure's influence on the process' lethality to either vegetative cells or spores. The need to withdraw samples during the thermal and combined processes would be avoided if the thermal survival parameters could be calculated by the "three endpoints method," which does not require the entire survival curve determination. Currently however, this method is limited to thermal inactivation patterns characterized by up to 3 survival parameters, the Weibull-Log logistic (WeLL) model, for example.     

Kinetic Parameters for the Thermal Inactivation of Peroxidase and Lipoxygenase in Precooked Frozen Brassica Species

Thermal inactivation of peroxidase (POD) and lipoxygenase (LOX), both enzymes present in broccoli and Brussels sprouts, is required before freezing, to obtain high‐quality precooked frozen vegetables. Rate constants of a 1st‐order biphasic model for the heat‐labile and heat‐resistant POD and LOX isoenzymes were determined at different temperatures (75, 80, and 90 °C) and the corresponding activation energies were estimated using nonlinear regressions. In the case of Brussels sprouts, the activation energies for the resistant and labile fractions were 56.3 and 62.5 kJ/mol for POD and 63.7 and 65.8 kJ/mol for LOX, respectively. For Brussels sprouts, different precooking times were tested to analyze the effect of residual enzyme activity on quality parameters and sensory attributes, after a frozen storage of 4 mo at ?20 °C. A significant reactivation of enzyme activity after frozen storage was observed (especially in the case of POD) for short precooking times (<6 min) leading to low‐quality parameters at the interior zone of the vegetable. A precooking time of 6 min at 90 °C allowed an adequate inactivation of LOX and POD obtaining a high‐quality final frozen vegetable. A sensory analysis confirmed the global acceptability of the product. The obtained results are relevant to define the precooking stage conditions in the production of frozen cruciferous vegetables.     

A Gompertz Model Approach to Microbial Inactivation Kinetics by High‐Pressure Processing (HPP): Model Selection and Experimental Validation

A recently proposed Gompertz model (GMPZ) approach describing microbial inactivation kinetics by high‐pressure processing (HPP) incorporated the initial microbial load (N₀) and lower microbial quantification limit (N_lim), and simplified the dynamic effects of come‐up time (CUT). The inactivation of Listeria innocua in milk by HPP treatments at 300, 400, 500, and 600 MPa and pressure holding times (t_hold) ≤10 min was determined experimentally to validate this model approach. Models based on exponential, logistic‐exponential, and inverse functions were evaluated to describe the effect of pressure on the lag time (λ) and maximum inactivation rate (μ_max), whereas the asymptote difference (A) was fixed as A = log₁₀(N₀/N_lim). Model performance was statistically evaluated and further validated with additional data obtained at 450 and 550 MPa. All GMPZ models adequately fitted L. innocua data according to the coefficient of determination (R² ≥ 0.95) but those including a logistic‐exponential function for μ_max(P) were superior (R² ≥ 0.97). These GMPZ versions predicted that approximately 597 MPa is the theoretical pressure level (P_λ) at which microbial inactivation begins during CUT, mathematically defined as λ (P = P_λ) = t_CUT, and matching the value observed on the microbial survival curve at 600 MPa. As pressure increased, predictions tended to slightly underestimate the HPP lethality in the tail section of the survival curve. This may be overseen in practice since the observed microbial counts were below the predicted log₁₀ N values. Overall, the modeling approach is promising, justifying further validation work for other microorganisms and food systems.     

Model of the Inactivation of Bacterial Spores by Moist Heat and High Pressure

ABSTRACT: Formulae for the prediction of inactivation and accumulated lethality of bacterial spores under moist heat and high pressures were derived on the basis of classic thermodynamic and kinetics principles. The capability of the model to describe the inactivation of bacterial spores was verified using 2 independent data sets corresponding to Clostridium botulinum processed at 60°C to 75°C and Bacillus stearothermophilus processed at 92°C to 110°C. Both sets included pressures between 5 × 10⁸ Pa and 7 × 10⁸ Pa. The equation fit explained more than 86% of the variation of the rate constant data. The developed equations establish a strong foundation on which to compare high-pressure processing treatments of different systems. This is especially useful because most systems have different transient temperature-pressure conditions.     

甘油与氯化钠模型热反应中生成3-氯-1,2-丙二醇的研究

以甘油、氯化钠、水为原料建立热反应模型,结合气相色谱-质谱法（gas chromatography-mass spectrometry,GC-MS）分析,对其生成的3-氯-1,2-丙二醇（3-MCPD）进行了研究。通过正交实验,考察了反应时间、温度和各原料用量对3-MCPD生成量的影响,同时研究了在热反应模型中分别加入不同种类单糖和氨基酸后3-MCPD生成量的变化规律,并对其反应机理进行了探讨。结果表明,反应时间2h、反应温度140℃、甘油用量0.05g（占原料总量0.31%）、NaCl用量1.0g（6.23%）、水用量15g（93.46%）,3-MCPD生成量最少;葡萄糖、果糖、木糖、甘露糖、半乳糖、缬氨酸、精氨酸、赖氨酸、丝氨酸、亮氨酸、苯丙氨酸、苏氨酸、组氨酸使3-MCPD的生成量减少;谷氨酸、蛋氨酸、丙氨酸、天冬氨酸、甘氨酸使3-MCPD的生成量增加;半胱氨酸、酪氨酸、脯氨酸、色氨酸对3-MCPD的生成无明显作用。      

脉冲强光对黄曲霉菌孢子的杀菌效果、微观结构及动力学的影响

脉冲强光是一种新兴的非热杀菌技术。为研究和预测脉冲强光对黄曲霉菌孢子的杀菌作用,在强度2.86,1.60,1.08,0.93 W/cm2脉冲强光下对固体培养基中黄曲霉菌孢子处理1~60 s,并对处理前、后孢子的微观形态进行扫描电镜分析。运用Log-linear模型、Weibull模型和Weibull+tail模型对杀菌致死曲线进行动力学分析。研究表明,随着照射强度增加和时间的延长,脉冲强光对黄曲霉菌孢子的杀菌效果增强。初始菌落数为5.15 lg(CFU/mL)的孢子在2.86 W/cm2下处理19 s可被全部杀死。电镜照片显示脉冲强光处理后孢子细胞结构崩溃。Weibull模型和Weibull+tail模型较Log-linear模型能更好地拟合脉冲强光处理黄曲霉菌孢子的杀菌曲线(R2>0.95)。其中Weibull+tail模型能更加精确地拟合杀菌动力学过程(R2=0.9727)。     

Copper in foods,beverages and waters from South East Spain: influencing factors and daily dietary intake by the Andalusian population

The copper content of 225 food, 49 beverage and twelve potable water samples were determined using atomic absorption spectrometry (AAS). Analyses of NIST and BCR reference materials demonstrated the accuracy of this technique. The highest copper levels were found in dried fruit and legumes, followed by organ meats, molluscs and crustaceans, cephalopods, cereals and sausages, respectively. In cereals, legumes and fruit, copper levels increased significantly with increasing levels of protein and decreasing carbohydrate content (p?<?0.001). In meat and meat by-products, copper concentrations found in organ meats were significantly higher (p?<?0.01). In fresh fish products, copper levels in shellfish were significantly higher than those measured in fish (p?<?0.001). In vegetables, the copper concentrations found in mushrooms were significantly higher (p?<?0.005). Mean copper concentrations analysed in cheese were statistically higher than those determined in other dairy products (p?<?0.01). In beverages, copper levels determined in rum and juices were significantly higher (p?<?0.001). Beverages for which a vegetable component was directly used in their manufacturing process (juices, wines and beers) had statistically higher copper levels when compared with fresh drinks. The daily dietary intake (DDI) of copper in the Andalusian diet was 1979 μg day^?1 per person. Cereals, meat, meat by-products and vegetables are the food categories that are the main source of copper in the daily diet. Taking into account the dietary reference intakes and upper levels (900 and 10, 000 μg Cu day^?1 for healthy adults, respectively), the mean copper DDI found indicate that for most of healthy adult individuals from the area, no adverse effects occur in relation to copper nutrition (deficiency or toxicity). Potable waters supplied 53 μg day^?1, which constitutes on average 0.025% of the maximum tolerable daily intake of this element set by the Joint FAO/WHO Expert Committee.     

Occurrence of the vancomycin-resistant genes vanA, vanB, vanCl, vanC2 and vanC3 in Enterococcus strains isolated from poultry and pork

It is suspected that the use of avoparcin as a feeding antibiotic for the fat stock contributes to development of cross-resistance against vancomycin and teicoplanin. After isolating enterococci strains from poultry and pork meat by cultivation on citrate azide Tween carbonate agar (CATC) and screening the vancomycin resistance on Columbia colistin nalidixic acid agar (CNA, supplemented with 5% sheepblood and 5 mg vancomycin/l) the polymerase chain reaction (PCR) was used for the detection of the vancomycin resistance genes vanA (‘high level’), vanB (‘moderate high level’), vanC1, vanC2 and vanC3 (‘low level’). Out of 1643 E.-isolates from 115 poultry and 50 pork samples, 420 isolates could be identified as vancomycin resistant, 202 isolates of which carry the vanA, one isolate both the vanA and the vanC1, 38 isolates the vanC1, 14 isolates the vanC2, nine isolates both the vanC1 and the vanC3 gene and 156 isolates carry no gene. The vanB gene was not found in these isolates. Comparing vanA-positive food isolates with those from different human sources by means of the pulsed field gel electrophoresis (PFGE) it could clearly be demonstrated that they do not show homological fingerprints according to the source of origin. It is therefore unlikely that there is a close genetic relationship between isolates from animal foodstuff and humans.     

Inactivation kinetics of inoculated Escherichia coli O157:H7, Listeria monocytogenes and Salmonella Poona on whole cantaloupe by chlorine dioxide gas

The objectives of this study were to examine inactivation kinetics of inoculated Escherichia coli O157:H7, Listeria monocytogenes and Salmonella Poona inoculated onto whole cantaloupe and treated with ClO(2) gas at different concentrations (0.5, 1.0, 1.5, 3.0 and 5.0 mg l(-1)) for different times (0, 2.0, 4.0, 6.0, 8.0 and 10.0 min). The effect of ClO(2) gas on the quality and shelf life of whole cantaloupe was also evaluated during storage at 22 degrees C for 12 days. A 100 microl inoculation of each targeted organism was spotted onto the surface (5 cm(2)) of cantaloupe rind (approximately 8-9 log CFU 5 cm(-2)) separately, air dried (60 min), and then treated with ClO(2) gas at 22 degrees C and 90-95% relative humidity for 10 min. Surviving bacterial populations on cantaloupe surfaces were determined using a membrane transferring method with a non-selective medium followed by a selective medium. The inactivation kinetics of E. coli O157:H7, L. monocytogenes and S. Poona were determined using nonlinear kinetics (Weibull model). A 3 log CFU reduction of E. coli O157:H7, L. monocytogenes and S. Poona were achieved with 5.0 mg l(-1) ClO(2) gas for 5.5, 4.2 and 1.5 min, respectively. A 5l og CFU reduction of S. Poona was achieved with 5.0 and 3.0 mg l(-1) ClO(2) gas for 6 and 8 min, respectively. A 4.6 and 4.3 log reduction was achieved after treatment with 5.0 mg l(-1) ClO(2) gas at 10 min for E. coli O157:H7 and L. monocytogenes, respectively. Treatment with 5.0 mg l(-1) ClO(2) gas significantly (p<0.05) reduced the initial microflora (mesophilic bacteria, psychrotrophic bacteria, and yeasts and molds) on cantaloupe by more than 2 log CFU cm(-2) and kept them significantly (p<0.05) lower than the untreated control during storage at 22 degrees C for 12 days. Treatment with ClO(2) gas did not significantly (p>0.05) affect the color of whole cantaloupe and extended the shelf life to 9 days compared to 3 days for the untreated control, when stored at ambient temperature (22 degrees C).