Inactivation of barotolerant Listeria monocytogenes in sausage by combination of high-pressure processing and food-grade additives

Food-grade additives were used to enhance the efficacy of high-pressure processing (HPP) against barotolerant Listeria monocytogenes. Three strains of L. monocytogenes (Scott A, OSY-8578, and OSY-328) were compared for their sensitivity to HPP, nisin, tert-butylhydroquinone (TBHQ), and their combination. Inactivation of these strains was evaluated in 0.2 M sodium phosphate buffer (pH 7.0) and commercially sterile sausage. A cell suspension of L. monocytogenes in buffer (10(9) CFU/ml) was treated with TBHQ at 100 ppm, nisin at 100 IU/ml, HPP at 400 MPa for 5 min, and combinations of these treatments. Populations of strains Scott A, OSY-8578, and OSY-328 decreased 3.9, 2.7, and 1.3 log with HPP alone and 6.4, 5.2, and 1.9 log with the HPP-TBHQ combination, respectively. Commercially sterile sausage was inoculated with the three L. monocytogenes strains (10(6) to 10(7) CFU/g) and treated with selected combinations of TBHQ (100 to 300 ppm), nisin (100 and 200 ppm), and HPP (600 MPa, 28 degrees C, 5 min). Samples were enriched to detect the viability of the pathogen after the treatments. Most of the samples treated with nisin, TBHQ, or their combination were positive for L. monocytogenes. HPP alone resulted in a modest decrease in the number of positive samples. L. monocytogenes was not detected in any of the inoculated commercial sausage samples after treatment with HPP-TBHQ or HPP-TBHQ-nisin combinations. These results suggest that addition of TBHQ or TBHQ plus nisin to sausage followed by in-package pressurization is a promising method for producing Listeria-free ready-to-eat products.     

Pressure death and tailing behavior of Listeria monocytogenes strains having different barotolerances

The objectives of this study were to investigate the variability among Listeria monocytogenes strains in response to high-pressure processing, identify the most resistant strain as a potential target of pressure processing, and compare the inactivation kinetics of pressure-resistant and pressure-sensitive strains under a wide range (350 to 800 MPa) of pressure treatments. The pressure resistance of Listeria innocua and nine strains of L. monocytogenes was compared at 400 or 500 MPa and 30 degrees C. Significant variability among strains was observed. The decrease in log CFU/ml during the pressure treatment was from 1.4 to 4.3 at 400 MPa and from 3.9 to >8 at 500 MPa. L. monocytogenes OSY-8578 exhibited the greatest pressure resistance, Scott A showed the greatest pressure sensitivity, and L. innocua had intermediate resistance. On the basis of these findings, L. monocytogenes OSY-8578 is a potential target strain for high-pressure processing efficacy studies. The death kinetics of L. monocytogenes Scott A and OSY-8578 were investigated at 350 and 800 MPa. Survivors at 350 MPa were enumerated by direct plating, and survivors at 800 MPa were enumerated by the most-probable-number technique. Both pressure-resistant and pressure-sensitive strains exhibited non-first-order death behavior, and excessive pressure treatment did not eliminate the tailing phenomenon.     

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.     

High-pressure processing of Gorgonzola cheese: influence on Listeria monocytogenes inactivation and on sensory characteristics

The presence of Listeria monocytogenes on the rind of Gorgonzola cheese is difficult to avoid. This contamination can easily occur as a consequence of handling during ripening. The aims of this study were to determine the efficiency of high-pressure processing (HPP) for inactivation of L. monocytogenes on cheese rind and to evaluate the influence of HPP treatments on sensory characteristics. Gorgonzola cheese rinds, after removal, were inoculated (about 7.0 log CFU/g) with L. monocytogenes strains previously isolated from other Gorgonzola cheeses. The inoculated cheese rinds were processed with an HPP apparatus under conditions of pressure and time ranging from 400 to 700 MPa for 1 to 15 min. Pressures higher than 600 MPa for 10 min or 700 MPa for 5 min reduced L. monocytogenes more than 99%. A reduction higher than 99.999% was achieved pressurizing cheese rinds at 700 MPa for 15 min. Lower pressure or time treatments were less effective and varied in effectiveness with the cheese sample. Changes in sensory properties possibly induced by the HPP were evaluated on four different Gorgonzola cheeses. A panel of 18 members judged the treated and untreated cheeses in a triangle test. Only one of the four pressurized cheeses was evaluated as different from the untreated sample. HPP was effective in the reduction of L. monocytogenes on Gorgonzola cheese rinds without significantly changing its sensory properties. High-pressure technology is a useful tool to improve the safety of this type of cheese.     

Temperature-assisted pressure inactivation of Listeria monocytogenes in turkey breast meat

Ready-to-eat turkey breast meat samples were surface-inoculated with a five-strain cocktail of Listeria monocytogenes cultures to a final concentration of approximately 10(7) CFU/g. The inoculated meat samples were vacuum-packaged and pressure treated at 300 MPa for 2 min, 400 MPa for 1 min, and 500 MPa for 1 min at initial sample temperatures of 1, 10, 20, 30, 40, 50, and 55 degrees C. L. monocytogenes was most resistant to pressure at temperatures between 10 and 30 degrees C. As temperature decreased below 10 degrees C or increased over 30 degrees C, its pressure sensitivity increased. This enhanced inactivation effect was more pronounced when meat samples were treated at higher temperature than at lower temperature. For example, a 1-min treatment of 500 MPa at 40 degrees C reduced the counts by 3.8 log(10), while at 1 and 20 degrees C the same treatment reduced counts by 1.4 and 0.9 log(10), respectively (P<0.05). The survival curves of L. monocytogenes were obtained at 300 MPa and 55 degrees C, 400 MPa and 50 degrees C, and 500 MPa and 40 degrees C. With increasing treatment time, the three survival curves showed a rapid initial drop in bacteria counts with a diminishing inactivation rate or tailing effect. The survival data were fitted with a linear and a nonlinear, Weibull, models. The Weibull model consistently produced better fit to the survival data than the linear model.     

Enhanced inactivation of foodborne pathogenic and spoilage bacteria by FD&C Red no. 3 and other xanthene derivatives during ultrahigh pressure processing

Variability among microorganisms in barotolerance has been demonstrated at genus, species, and strain levels. Identification of conditions and additives that enhance the efficacy of ultrahigh pressure (UHP) against important foodborne microorganisms is crucial for maximizing product safety and stability. Preliminary work indicated that FD&C Red No. 3 (Red 3), a xanthene derivative, was bactericidal and acted synergistically with UHP against Lactobacillus spp. The objective of this study was to determine the antimicrobial efficacy of Red 3 and other xanthene derivatives, alone and combined with UHP, against spoilage and pathogenic bacteria in citrate-phosphate buffer (pH 7.0). Xanthene derivatives tested were fluorescein, Eosin Y, Erythrosin B, Phloxine B, Red 3, and Rose Bengal. Halogenated xanthene derivatives (10 ppm) were effective at reducing Listeria monocytogenes survivors but ineffective against Escherichia coli O157:H7. When combined with UHP (400 MPa, 3 min), the presence of derivatives enhanced inactivation. Because Red 3 was the only xanthene derivative to produce synergistic inactivation of both pathogens, further studies using this colorant were warranted. Efficacy of Red 3 against gram-positive bacteria (Lactobacillus plantarum and L. monocytogenes) was concentration dependent (1 to 10 ppm). E. coi O157: H7 strains were resistant to Red 3 concentrations up to 300 ppm. When Red 3 was combined with UHP, the lethality against gram-positive and gram-negative bacteria was dose dependent, with synergy being significant for most strains at > or = 3 ppm. Additional gram-positive and gram-negative bacteria showed lethalities similar to those observed for L. plantarum or L. monocytogenes, and E. coli O157:H7, respectively. Red 3 is a potentially useful additive to enhance the safety and stability of UHP-treated food products.     

The effects of growth temperature and growth phase on the inactivation of Listeria monocytogenes in whole milk subject to high pressure processing

The aim of this study was to explore the effect of a wide range of growth temperatures, growth phases and plating media on the inactivation of Listeria monocytogenes by high pressure processing (HPP). In part one, L. monocytogenes was grown to mid-stationary phase at 4, 15, 25, 35 or 43 degrees C, inoculated into whole UHT milk at approximately 10(7) CFU/ml and high pressure processed at 400 MPa at room temperature (20-25 degrees C). Afterward, the HPP milk was plated on Tryptic Soy Yeast Extract Agar (TSYEA) and Modified Oxford Agar (MOX) to determine the degree of injury. For part two, cells were grown to mid-exponential, late-exponential or mid-stationary phase at 15 or 43 degrees C and processed in the same way. Time to reach a 5-log reduction was determined and data were analysed by ANOVA. The results from part one showed that both growth temperature and plating medium had a significant effect (P < 0.001) on the inactivation of stationary phase L. monocytogenes by HPP. Tukey's pairwise comparisons revealed that the effects of all temperatures, except 35 and 43 degrees C, were significantly different (P < 0.05). Cells grown at 15 degrees C were most sensitive to HPP, followed by cells grown at 4, 25 or 35 degrees C, with cells grown at 43 degrees C appearing to be the most resistant. Inactivation of cells grown at 4, 15 or 25 degrees C followed first order kinetics, whereas cells grown at 35 or 43 degrees C displayed non-linear inactivation kinetics due to tailing. In part two, both growth phase and plating medium had significant effects on the inactivation (P < or = 0.001) of L. monocytogenes by HPP. Cells grown at 15 degrees C to mid-stationary phase were the most pressure-resistant when tested on both media, and were significantly more resistant (P < 0.05) than cells grown at the same temperature to the other two phases of growth. There was no significant difference between mid- and late-exponential phase cells grown at 15 degrees C. When cells were grown at 43 degrees C, mid-exponential phase cells were significantly more sensitive (P < 0.05) than either late-exponential or mid-stationary phase cells, with no difference between late-exponential or mid-stationary phase cells. It was postulated that membrane composition, stationary phase proteins and/or stress proteins may affect pressure resistance.     

Use of mild-heat treatment following high-pressure processing to prevent recovery of pressure-injured Listeria monocytogenes in milk

This study examined the inactivation of Listeria monocytogenes in milk by high-pressure processing (HPP) and bacterial recovery during storage after HPP. We developed a technique to inhibit the bacterial recovery during storage after HPP (550 MPa for 5 min) using a mild-heat treatment (30-50 degrees C). Various mild-heat treatments were conducted following HPP to investigate the condition on which the bacterial recovery was prevented. Immediately after HPP of 550 MPa at 25 degrees C for 5 min, no L. monocytogenes cells were detected in milk regardless of the inoculum levels (3, 5, and 7 log(10)CFU/ml). However, the number of L. monocytogenes cells increased by >8 log(10)CFU/ml regardless of the inoculum levels after 28 days of storage at 4 degrees C. Significant recovery was observed during storage at 25 degrees C; the bacterial number increased by >8 log(10)CFU/ml after 3 days of storage in the case of an initial inoculum level of 7 and 5 log(10)CFU/ml. Even in the case of an initial inoculum level of 3 log(10)CFU/ml, the bacterial number reached the level of 8 log(10)CFU/ml after 7 days of storage. No bacterial recovery was observed with storage at 37 degrees C for 28 days. Milk samples were treated by various mild-heat treatments (30-50 degrees C for 5-240 min) following HPP of 550 MPa at 25 degrees C for 5 min, and then stored at 25 degrees C for 70 days. The mild-heat treatment (e.g., 37 degrees C for 240 min or 50 degrees C for 10 min) inhibited the recovery of L. monocytogenes in milk after HPP. No recovery of L. monocytogenes in milk was observed during 70-day storage at 25 degrees C in samples that received mild-heat treatments such as mentioned above following HPP (550 MPa for 5 min). Moreover, the mild-heat treatment conditions (temperature and holding time) required to inhibit the recovery of L. monocytogenes in milk was modelled using a logistic regression procedure. The predicted interface of recovery/no recovery can be used to calculate the mild-heat treatment condition to control bacterial recovery during storage at 25 degrees C after HPP (550 MPa for 5 min). The results in this study would contribute to enhance the safety of high-pressure-processed milk.     

Inactivation kinetics of three Listeria monocytogenes strains under high hydrostatic pressure

High hydrostatic pressure (HHP) inactivation of three Listeria monocytogenes strains (EGDe, LO28, and Scott A) subjected to 350 MPa at 20 degrees C in ACES buffer resulted in survival curves with significant tailing for all three strains. A biphasic linear model could be fitted to the inactivation data, indicating the presence of an HHP-sensitive and an HHP-resistant fraction, which both showed inactivation according to first-order kinetics. Inactivation parameters of these subpopulations of the three strains were quantified in detail. EGDe showed the highest D-values for the sensitive and resistant fraction, whereas LO28 and Scott A showed lower HHP resistance for both fractions. Survivors isolated from the tail of LO28 and EGDe were analyzed, and it was revealed that the higher resistance of LO28 was a stable feature for 24% (24 of 102) of the resistant fraction. These HHP-resistant variants were 10 to 600,000 times more resistant than wild type when exposed to 350 MPa at 20 degrees C for 20 min. Contrary to these results, no stable HHP-resistant isolates were found for EGDe (0 of 102). The possible effect of HHP survival capacity of stress-resistant genotypic and phenotypic variants of L. monocytogenes on the safety of HHP-processed foods is discussed.     

High-pressure processing of Turkish white cheese for microbial inactivation

High-pressure processing (HPP) of Turkish white cheese and reduction of Listeria monocytogenes, total Enterobacteriaceae, total aerobic mesophilic bacteria, total molds and yeasts, total Lactococcus spp., and total Lactobacillus spp. were investigated. Cheese samples were produced from raw milk and pasteurized milk and were inoculated with L. monocytogenes after brining. Both inoculated (ca. 10(7) to 10(8) CFU/g) and noninoculated samples were subjected to HPP in a high-pressure food processor at 50 to 600 MPa for 5 and 10 min at 25 degrees C. Reductions in L. monocytogenes, total aerobic mesophilic bacteria, Lactococcus spp., and Lactobacillus spp. in both pasteurized- and raw-milk cheese samples and reductions in total molds and yeasts and total Enterobacteriaceae counts in raw-milk cheese samples increased with increased pressure (P < or = 0.05). The maximum reduction of the L. monocytogenes count, ca. 4.9 log CFU/g, was obtained at 600 MPa. Because of the highly inhibitory effect of pasteurization, the total molds and yeasts and total Enterobacteriaceae counts for the cheese samples produced from pasteurized milk were below the detection limit both before and after HPP. There was no significant difference in inactivation of L. monocytogenes, total aerobic mesophilic bacteria, Lactococcus spp., and Lactobacillus spp. under the same treatment conditions for the raw milk and pasteurized milk cheeses and for 5- and 10-min treatment times (P > 0.05). No significant change was detected in pH or water activity of the samples before and after HPP. Our findings suggest that HPP can be used effectively to reduce the microbial load in Turkish white cheese.     

Inactivation of Shigella boydii 18 IDPH and Listeria monocytogenes Scott A with power ultrasound at different acoustic energy densities and temperatures

ABSTRACT:  The effect of acoustic energy density (AED) on inactivation of Shigella boydii 18 IDPH and Listeria monocytogenes Scott A in a cell suspension was studied at sublethal temperatures and at AEDs of 0.49, 0.85, and 1.43 W/mL. The effect of temperature on ultrasonic inactivation of L. monocytogenes Scott A at 35, 50, and 65 °C was examined at an AED of 1.43 W/mL. Increasing AED increased the rate of inactivation for both S. boydii and L. monocytogenes . The destruction of S. boydii and L. monocytogenes followed 1st order kinetics in a 20-min treatment, except for S. boydii inactivation at 1.43 W/mL where a tailing effect was observed after 15 min. At sublethal temperatures, the D-values of S. boydii were 8.8, 4.3, and 2.5 min for AEDs of 0.49, 0.85, and 1.43 W/mL, whereas those for L. monocytogenes at the 3 AED levels were 31.5, 13.5, and 7.3 min, respectively. Ultrasonic treatment of L. monocytogenes at 35 and 50 °C enhanced inactivation. However, at 65 °C, application of ultrasound did not result in additional inactivation compared to thermal treatment alone at the same temperature. With the experimental conditions and the ultrasound system used in this study, an upper temperature limit for thermosonication was evident above which no added killing due to ultrasound was observed.     

A novel approach to predicting microbial inactivation kinetics during high pressure processing

The inactivation kinetics of Escherichia coli (ATCC 25922) during high pressure processing (HPP) was examined from 200 to 400 MPa in 50 MPa increments at 15 degrees C. Although the time course of HPP-induced E. coli inactivation in 0.1% peptone water successfully fitted the Weibull function, this procedure involved curve fitting, and not prediction. The objective of this study was to develop a novel HPP-induced microbial inactivation model to simulate the inactivation kinetics under various pressure conditions. The maximum inactivation rate during HPP was calculated from the inactivation curves at different pressure conditions on a semi-log plot. The relationship between the square root of the absolute value of the inactivation rate (k(max)) and treatment pressure was linear (R(2)=0.99). The linear relationship between k(max) and treatment pressure also successfully described independent data from other studies in the literature. Overall, the newly developed differential equation model, into which was substituted the square root function of the inactivation rate, was capable of simulating the inactivation kinetics during HPP at constant pressure. Additionally, the model could successfully describe the inactivation kinetics during HPP using other researchers' data. The accuracy of prediction of the new model was comparable to that derived from Weibull or modified Gompertz fitting to the observed data. Furthermore, the new model could successfully simulate the inactivation kinetics during dynamic pressure conditions, which included come-up time, changes in holding pressure during treatment, and pressure-release time. Moreover, the effect of pulsed pressure treatment was also simulated successfully using this model. Therefore, the modeling procedure presented in this study will contribute to the advancement of predictive modeling for HPP-induced microbial inactivation.     

Water activity of bacterial suspension media unable to account for the baroprotective effect of solute concentration on the inactivation of Listeria monocytogenes by high hydrostatic pressure

Inactivation of Listeria monocytogenes (10(8) CFU/ml) by high hydrostatic pressure (HHP) from 400 to 600 MPa at 25 degrees C for 10 min was investigated with various concentrations of sodium chloride, sucrose, and sodium phosphate buffer solutions. Sodium chloride significantly inhibited HHP-induced inactivation of L. monocytogenes at concentrations higher than 2.6 M. A low concentration of sodium chloride within 1.7 M had no effect on HHP-induced inactivation. Almost complete inactivation at relatively low sodium chloride concentration solution was observed with treatments above 500 MPa. Sucrose also significantly inhibited HHP-induced inactivation of L. monocytogenes when greater than 1.2 M sucrose solutions were used. HHP-treatment at 400 MPa reduced the number of L. monocytogenes in 1.2 M, 1.5 M, and 1.8 M sucrose solutions by 4.8, 2.0, and 0.7 log cycles, respectively. Higher pressure did not yield significant reductions. Sodium phosphate buffer significantly inhibited HHP-induced inactivation of L. monocytogenes. In particular, 1 M phosphate buffer completely inhibited HHP-induced inactivation even at 600 MPa. HHP-treatment at 400 MPa reduced the number of L. monocytogenes in 0.1 M, 0.25 M, and 0.5 M phosphate buffer solutions by 5.6, 4.1, and 3.2 log cycles, respectively. The effect of HHP-induced inactivation of L. monocytogenes in the three kinds of solution was evaluated by adjusting water activity (a(w)). However, the baroprotective effect differed depending on the kind of solute even at the same a(w). This result showed no consistent correlation between a(w) and solute concentration in terms of the baroprotective effect. As an alternative approach, saturation of suspension solution was used for evaluating the effect of HHP-induced inactivation of L. monocytogenes. As the saturation of suspension media increased, the effect of HHP-induced inactivation of L. monocytogenes decreased regardless of the kinds of solute. The saturation of solution would be an alternative parameter of inhibition in terms of HHP-induced inactivation of bacteria.     

Modelling the bacterial survival/death interface induced by high pressure processing

The survival/death interface model was developed for prediction of inactivation of Listeria monocytogenes by high pressure processing (HPP). The model was derived from data sets comprising 360 combinations of environmental factors such as pressure (200, 300 400, and 500 MPa), pressure-holding time (1, 3, 5, 10, 20, 30 min), pH (3, 4, 5, 6, 7), and inoculum level (3, 5, 7 log(10) CFU/ml). The determination of survival/death of L. monocytogenes after HPP was confirmed by the presence/absence of colony forming ability on non-selective agar plates after 30 days of incubation at 20 degrees C in broth to take into account recovery of HPP-induced injured cells. The developed linear logistic model with time logarithmically transformed gave a degree of agreement between probabilities predicted by the fitted model and all observations as 99.3% concordant. The model provided a good fit to the data as shown by performance statistics. The developed interface model in the present study provided requisite process conditions for the target effect of HPP on L. monocytogenes. In addition to using the simple linear logistic model, a polynomial logistic model was also fitted to the data where pressure-holding time was not logarithmically transformed. That model did not produce a better fit to the data and resulted in some potentially misleading predictions. Optimization of HPP could be accomplished using the model developed in this study. Furthermore, choice in processing factors allows for processing flexibility in HPP and specifies the process criteria that are incorporated into the HACCP plan.     

pH and solute concentration of suspension media affect the outcome of high hydrostatic pressure treatment of Listeria monocytogenes

The effect of pH and solute concentration of suspension media on high hydrostatic pressure (HHP) induced inactivation of Listeria monocytogenes (approximate 10(8) CFU/ml) was investigated by the using treatment between 300 MPa and 600 MPa at 25 degrees C for 10 min. The suspension media used in this study represented different concentrations (0.1% to 10%) of buffered peptone water (BPW) with an adjusted pH of 4 to 7. An increase in the concentration of BPW resulted in a decreased HHP-induced inactivation of L. monocytogenes that was dependent on the pH of the medium. HHP-treatment at 300 MPa showed no bactericidal effect at neutral pH regardless of the BPW concentration. When the pH of BPW (0.1% to 5%) was reduced to 4, L. monocytogenes was completely inactivated (more than an 8 log cycle reduction) with a HHP-treatment of at least 300 MPa. HHP-treatment above 400 MPa completely inactivated L. monocytogenes in a relatively dilute BPW (0.1% and 1%) with an adjusted pH below 6. While only a 2 log cycle reduction was observed in 10% BPW at the pH ranging from 5 to 7 after treatment with 600 MPa, L. monocytogenes in 10% BPW at pH 4 was completely inactivated. Even though a significant bactericidal effect of HHP-treatment was not observed when applied with a low pressure such as 300 MPa or suspended in higher BPW at neutral pH, a reduction of the pH greatly affected the HHP-induced inactivation of L. monocytogenes. These results indicated that information concerning the pH of food or media would greatly assist an optimization of HHP-treatment for the inactivation of bacteria.     

Screening for Listeria monocytogenes surrogate strains applicable to food processing by ultrahigh pressure and pulsed electric field

Ultrahigh pressure (UHP) and pulsed electric field (PEF) are emerging processing technologies developed to enhance the safety while maintaining the fresh-like quality of food. For each food and process combination, a pathogen of concern (i.e., target pathogen) must be determined, and a low-risk microorganism that serves as the pathogen surrogate for process validation must be identified. The objective of this study was to identify a surrogate for Listeria monocytogenes for UHP and PEF process validation. Potential surrogates tested include four Lactobacillus spp., a Pediococcus sp., and a Listeria innocua strain. These were compared with nine L. monocytogenes strains, with regard to sensitivity to UHP and PEF processing. For UHP treatment, the strains were suspended in citrate-phosphate buffer (pH 7.0 or 4.5), sweet whey, or acidified whey and pressure processed at 500 MPa for 1 min. For PEF treatment, the strains were suspended in NaCl solution, acid whey, or sweet whey and processed at 25 kV/cm. The lethality of UHP or PEF treatment varied considerably, depending on medium types and pH and the treated strain. Treating the tested microorganisms with UHP inactivated 0.3 to 6.9 log CFU/ml for L. monocytogenes strains and 0.0 to 4.7 log CFU/ml for the potential surrogates. When PEF was employed, populations of tested microorganisms decreased < 1.0 to 5.3 log CFU/ml. L. monocytogenes V7 and OSY-8578 were among the most resistant strains to UHP and PEF treatments, and thus are candidate target strains. Lactobacillus plantarum ATCC 8014 demonstrated similar or greater resistance compared with the target organisms; therefore, the bacterium is proposed as a surrogate of L. monocytogenes for both processes under the conditions specified in the food matrices tested in this study.     

超高压处理对黑莓酒香气成分的影响

运用超高压技术处理黑莓酒以期改善其香气品质,并采用固相微萃取与气相色谱-质谱联用对不同超高压条件处理的黑莓酒挥发性香气成分进行检测,探究超高压处理对黑莓酒香气成分的影响及作用机理.结果表明:经超高压处理的黑莓酒香气种类与未处理样品相同,但含量发生较大变化,200MPa处理20min的黑莓酒醇类、酯类和醛类无显著变化(p＞0.05),酸类减少了7.44％;600MPa处理20min的黑莓酒醇类、酯类分别增加了4.63％、13.38％,醛类、酸类无显著变化(p＞0.05);400MPa处理20min的黑莓酒醇类、酯类和醛类分别增加了4.89％、43.04％和1.12％,酸类减少了11.28％,整体香气趋向柔和、协调、饱满,达到改善黑莓酒香气品质的目的.     

Effect of high-pressure processing on flavonoids,hydroxycinnamic acids,dihydrochalcones and antioxidant activity of apple ‘Golden Delicious’ from different geographical origin

The influence of high-pressure processing (HPP) (400, 500 and 600 MPa at 35 °C for 5 min) on different classes of phenolic compounds and antioxidant activity (AA) of ‘Golden Delicious’ apple from two different growing regions, northeastern of Spain (lowland climate) (S-apples) and north of Italy (mid-mountain climate) (I-apples) was investigated. Total hydroxycinnamic acids, total dihydrochalcones and total flavan-3-ols content were higher in S-apple (untreated and HPP-treated) than in I-apples and total flavonols content was higher in I-apples. HPP affected phenolic compounds and AA depending on the apple geographical origin. 400 MPa/35 °C/5 min increased total flavonols (30%) and maintained total phenolic compounds determined by HPLC (TP-HPLC) in S-apples. The higher increase of TP-HPLC (54%) was achieved when I-apple was treated at 600 MPa. Untreated and HPP-treated I-apples displayed higher AA than S-apples. HPP (400 and 600 MPa) increased AA in I-apple. Positive correlations were found between TP-HPLC and AA (TP-FC, DPPH^·, ABTS^·+ and FRAP) in both Italian and Spanish apples.Industrial relevanceThe apples of cultivar ‘Golden Delicious’ are one of the most consumed fruits in the UE. High-pressure processing (HPP) of these fruits acquires great importance to obtain ingredients and apple functional foods highly demanded by consumers. For this, it is necessary to know the process variables and plant material that favor greater extraction of phenolic compounds and antioxidant activity characteristics. This paper provides useful results to help fruit processor to select the appropriate HPP conditions and the geographical origin of ‘Golden Delicious’ apple to obtain apple-based products with high content on different classes of phenolic compounds with beneficial health effects.     

Heat shock induces barotolerance in Listeria monocytogenes

The aim of this study was to investigate the effect of heat shock on the resistance of Listeria monocytogenes to high pressure processing (HPP). L. monocytogenes ATCC 19115 was grown to stationary phase at 15 degrees C and inoculated into whole ultrahigh-temperature milk at approximately 10(7) CFU/ml. Milk samples (5 ml) were placed into plastic transfer pipettes, which were heat sealed and then heated in a water bath at 48 degrees C for 10 min. Immediately after heat shock, the milk was cooled in water (20 degrees C) for 25 min and then placed on ice. The samples were high pressure processed at ambient temperature (approximately 23 degrees C) at 400 MPa for various times up to 150 s. Following HPP, the samples were spread plated on tryptic soy agar supplemented with yeast extract. Heat shock significantly increased the D400 MPa-value of L. monocytogenes from 35 s in non-heat-shocked cells to 127 s in heat-shocked cells (P < 0.05). Addition of chloramphenicol before heat shock eliminated the protective effect of heat shock (P < 0.05). Heat shock for 5, 10, 15, or 30 min at 48 degrees C resulted in maximal barotolerance (P < 0.05); increasing the time to 60 min significantly decreased survival compared with that at 5, 10, 15, or 30 min (P < 0.05). These results indicate that prior heat shock significantly increases the barotolerance of L. monocytogenes and that de novo protein synthesis during heat shock is required for this enhanced barotolerance.