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Consumers’ demands for ready-to-eat, fresh-like products are on the rise during the last years. This type of products have minimal processing conditions that can enable the survival and replication of pathogenic microorganisms. Among them, Listeria monocytogenes is of special concern, due to its relatively high mortality rate and its ability to replicate under refrigeration conditions. Previous research works have shown that nanoemulsified essential oils in combination with thermal treatments are effective for inactivating L. monocytogenes. However, previous research works were limited to isothermal conditions, whereas actual processing conditions in industry are dynamic. Under dynamic conditions, microorganism can respond unexpectedly to the thermal stress (e.g. adaptation, acclimation or increased sensitivity). In this work, we assess the combination of nanoemulsified D-limonene with thermal treatments under isothermal and dynamic conditions. The nanoemulsion was prepared following an innovative methodology using soya lecithin, a natural compound as well as the essential oil. Under isothermal heating conditions, the addition of the antimicrobial enables a reduction of the treatment time by a factor of 25. For time-varying treatments, dynamic effects were relevant. Treatments with a high heating rate (20 °C/min) are more effective than those with a slow heating rate (1 °C/min). This investigation demonstrates that the addition of nanoemulsified D-limonene can greatly reduce the intensity of the thermal treatments currently applied in the food industry. Hence, it can improve the product quality without impacting its safety.

The effects of high hydrostatic pressure (HHP) compared to thermal pasteurization (TP) were studied in healthy citrus-maqui beverages. The impact of the processing technologies on the microbiological and phytochemical profile was assessed by applying two HHP treatments at 450 and 600 MPa for 180 s and TP at 85 °C for 15 s. The shelf life under refrigeration (4 °C) and room temperature (20 °C) was monitored for 90 days. All treatments ensured microbiological stability at both storage temperatures. Aside from that, the physicochemical parameters were not significantly different after processing or throughout the storage period. Regarding color parameters, an increase in the reddish coloration was observed during storage for those beverages treated by HHP. In general, phenolic compounds were little affected by the processing technique, even when treatment under HHP was more stable than by TP during storage. On the other hand, vitamin C showed great degradation after processing under any condition. It can be concluded that HHP is an effective alternative to thermal treatments, achieving effective microbial inactivation and extending the shelf life of the juices by contributing to a better preservation of color and bioactive compounds.

Microbial cells respond to sub-lethal stresses with several physiological changes to increase their chance of survival. These changes are of high relevance when combined treatments (hurdle technology) are applied during food production, as the cells surviving the first hurdle may have greater resistance to subsequent treatments than untreated cells. In this study, we analyzed if Salmonella develops increased resistance to thermal treatments after the application of an acid shock. We compared the heat resistance of acid-shocked (pH 4.5 achieved with citric acid) Salmonella cells with that of cells maintained at pH 7 (control cells). Thermal treatments were performed between 57.5 and 65°C. We observed a differential response between the two strains studied. Acid-shocked cells of Salmonella Senftenberg exhibited reduced heat resistance, e.g., for a treatment at 60.0°C and pH 7.0 the time required to reduce the population by 3 log cycles was lowered from 10.75 to 1.98min with respect to control cells. Salmonella Enteritidis showed a different response, with acid-shocked cells having similar resistance than untreated cells (the time required to reduce 3 log cycles at 60.0°C and pH 7.0 was 0.30min for control and 0.31min for acid-shock cells). Based on results by differential plating (with or without adding the maximum non-inhibitory concentration of NaCl to the recovery medium), we hypothesize that the differential response between strains can be associated to sub-lethal damage to the cell membrane of S. Senftenberg caused by the acid shock. These results provide evidence that different strains of the same species can respond differently to an acid shock and highlight the relevance of cross-resistances for microbial risk assessment.

Cooking vegetables in microwave bags is becoming a popular domestic cooking method, being relevant to know how this cooking method affects health-promoting phytochemicals of staples such as broccoli. The aim of this work was to study the effect of microwave bag cooking versus conventional microwaving on bioactive compound content (glucosinolates and hydroxycinnamic acid derivatives) and other quality parameters (such as antioxidant capacity, mineral content and microbial load) of broccoli florets. The influence of cooking time on bioactive compounds content was also evaluated. The study was carried out in two independent experiments; using intact broccoli and broccoli preprocessed in industry. Microwave bag cooked broccoli for 5 min (following label recommendation) showed higher glucosinolate content retention compared to conventional microwaving. Results suggest that volatilization could be an important phenomenon in reduction of glucosinolates during microwave cooking of broccoli florets. Glucosinolate profile did not change after cooking, regardless of cooking method applied. Furthermore, microwave bag cooked broccoli presented higher antioxidant capacity (by DPPH assay) than conventional microwaved broccoli. Hydroxycinnamic acid derivatives content was reduced in microwave cooking, regardless of method applied. Altogether, the use of microwave bags for microwaving is a novel method that retains main bioactive components of broccoli. This option is a fast, easy and considerably clean cooking option to fulfill modern consumer needs., This work was supported by Comision ´ Sectorial de Investigacion ´ Científica, Universidad de la República. Authors are indebted to Agencia Nacional de Innovacion ´ e Investigacion ´ for granting Erika Paulsen a PhD scholarship and an internship at Phytochemistry and Healthy Foods Laboratory of the Department of Food Science and Technology at CEBAS-CSIC (scholarship reference: POS_EMHE_2018_1_1007740)

In the present study, it was evaluated how two different thermal treatments (Mild and Severe) may affect the anthocyanin content, antioxidant capacity (ABTS(+), DPPH, and FRAP), quality (CIELAB colour parameters), and microbiological safety of a new isotonic drink made of lemon and maqui berry over a commercial storage simulation using a shelf life of 56days at two preservation temperature (7°C and 37°C). Both heat treatments did not affect drastically the anthocyanins content and their percentage of retention. The antioxidant capacity, probably because of the short time, was also not affected. The CIELAB colour parameters were affected by the heat, although the isotonic drinks remained with attractive red colour during shelf life. From a microbiological point of view, the Mild heat treatment with storage at 7°C is the ideal for the preservation of microbial growth, being useful for keeping the quality and safety of beverages in commercial life.

The application of oily antimicrobials in form of nanoemulsion has been proved to solve the problem of their immiscibility in aqueous media, still preventing microbial growth and even improving the antimicrobial effect observed when applied directly. At present, only a few documented studies have evaluated the combined effect of nanoemulsions with other factors of stress for the microorganism. The present research shows very promising results on the combination of nanoemulsified d -limonene with thermal treatments on the inactivation of Listeria monocytogenes . The thermal resistance of L . monocytogenes was reduced two to five times when 0.5 mM d -limonene was added directly to the heating medium. However, when the same concentration of d -limonene was present in the heating medium in form of nanoemulsion, the heat resistance was reduced by one hundred times at all heating temperatures tested. The addition of nanoemulsified antimicrobials would allow to reduce greatly the intensity of the thermal treatments currently applied in the food processing industry.

The present work evaluated the antibacterial effect of nanoemulsions from natural compounds d-limonene and nisin against Listeria monocytogenes in tryptic soy broth growth medium, chicken broth, and vegetable cream. Experiments were performed by means of optical density growth curves and studies of viability in culture media and foods. Stability of nanoemulsions was evaluated by a Mastersizer 2000 equipment. Results showed greater effectiveness when applying d-limonene in form of nanoemulsion than when applying it directly, and when using both compounds together, both directly and in the form of nanoemulsion. Concentration of L. monocytogenes decreased at least in three log cycles in all the culture media and foods used within the first 90 min after the addition of the natural antimicrobials combined in form of nanoemulsion. Moreover, the growth of L. monocytogenes was inhibited with the combination of antimicrobials in the four weeks that the experiment lasted. Nanoemulsion technology would solve present problems of solubility and stability of oily antimicrobials in the food industry.

The present work studies the effect of acid shock on cell viability of Bacillus cereus and Bacillus weihenstephanensis. Experiments were performed by means of flow cytometry (FCM) combined with fluorescent labelling. Propidium iodide, for membrane evaluation, and carboxyfluorescein diacetate, for esterase activity were used for differential staining of both strains. pH values of 3.4, 3.8 and 4.2 were selected on the basis of previous screening tests. FCM analysis showed to clearly discriminate between different populations, viable and damage cells, leading to successful assessment of acid shock effect on B. cereus and B. weihenstephanensis vegetative cell viability. Cellular viability decreased at lower pH values and longer exposure times, being B. weihenstephanensis more acid resistant than B. cereus. Reliability of FCM analysis for detection of B. cereus and B. weihenstephanensis vegetative cells was compared with classical viable plate count techniques (VPC). Although FCM and VPC data were not directly correlated and classical analytical methods show higher reliability, FCM analysis provides high-speed information at real time on damage at single cell level, whereas VPC only gives an indication of cells able to grow at a certain time. These data could help to establish more accurately the potential risk associated to foods or processing conditions where pH is involved, taking damage into consideration.

Yeasts are tolerant to acid pH values, are able to grow in anaerobic media and have minimum nutrition requirements. These capabilities enable them to survive and even grow in foods prepared from acid fruits or vegetables. Among yeasts, Candida is one of the genus most frequently isolated from fruit juices. Bacteriocins and essential oils from spices and aromatic herbs are an alternative to preservatives and other technological treatments and have the advantage that their natural origins do not lead to consumer rejection. However, before the food industry uses them on a large scale, it is necessary to know their effects on microorganisms. The objective of this research was to study the effect of different concentrations of nisin, thymol, carvacrol and cymene on the growth of Candida lusitaniae in pH 5 broth at 25 ℃, and their potential uses as food preservatives. The addition of nisin at the concentrations tested (up to 3 µmol L−1) did not affect the yeast growth. Thymol, carvacrol and cymene completely inhibited the yeast growth at concentrations over 1 mM for at least 21 days at 25 ℃. Below this concentration, inhibitions on yeast growth were observed at increasing concentrations. The effect of thymol was also proved in tomato juice. This study indicates the potential use of essential oils for preservation of minimally processed foods.

The use of non-thermal methods for food preservation is due to consumer demands for microbiological safe products, without changes in the sensory and nutritional qualities of the product. High hydrostatic pressure (HHP) has emerged as an alternative to traditional thermal processing methods for foods. Listeria monocytogenes CECT 5672 was treated under HHPs (350, 400 and 450 MPa) for 3, 16 and 23 min. The effects of pH (5, 6 and 7) and sodium chloride concentration (0, 0.5 and 1.0) of the recovery medium were studied on L. monocytogenes . The kinetic parameters were estimated by the method described by Metris, George, and Baranyi (2006) . From results obtained, histograms of the lag phase were generated and distributions were fitted. The duration of the lag phase of HHP damaged cells increased with the application of additional stresses. Histograms showed a shift to longer lag phases and an increase in variability with high stress levels in the recovery medium. Using a primary model together with Monte Carlo simulation, predictions of time to growth (100 cfu/g) of L. monocytogenes were established and they were compared with deterministic predictions. It was evidenced that deterministic predictions do not give a good indication of the probability of a certain level of growth.

Heat resistance of microorganisms can be affected by different influencing factors. Although, the effect of heating rates has been scarcely explored by the scientific community, recent researches have unraveled its important effect on the thermal resistance of different species of vegetative bacteria. Typically heating rates described in the literature ranged from 1 to 20°C/min but the impact of much higher heating rates is unclear. The aim of this research was to explore the effect of different heating rates, such as those currently achieved in the heat exchangers used in the food industry, on the heat resistance of Escherichia coli. A pilot plant tubular heat exchanger and a thermoresistometer Mastia were used for this purpose. Results showed that fast heating rates had a deep impact on the thermal resistance of E. coli. Heating rates between 20 and 50°C/min were achieved in the heat exchanger, which were much slower than those around 20°C/s achieved in the thermoresistometer. In all cases, these high heating rates led to higher inactivation than expected: in the heat exchanger, for all the experiments performed, when the observed inactivation had reached about seven log cycles, the predictions estimated about 1 log cycle of inactivation; in the thermoresistometer these differences between observed and predicted values were even more than 10 times higher, from 4.07 log cycles observed to 0.34 predicted at a flow rate of 70 mL/min and a maximum heating rate of 14.7°C/s. A quantification of the impact of the heating rates on the level of inactivation achieved was established. These results point out the important effect that the heating rate has on the thermal resistance of E. coli, with high heating rates resulting in an additional sensitization to heat and therefore an effective food safety strategy in terms of food processing. This research was financially supported by the Ministry of Economy and Competitiveness of the Spanish Government and European Regional Development Fund (ERDF) through project AGL2013-48993-C2-1-R

Stochastic models can be useful to predict the risk of foodborne illness. The presence of Bacillus cereus in liquid egg can pose a serious hazard to the food industry, since a mild heat treatment cannot guarantee its complete inactivation. However, most of the information available in the scientific literature is deterministic, including growth of B. cereus. In this paper, a stochastic approach to evaluate growth of B. cereus cells influenced by different stresses (presence of nisin and lysozyme separately or in combination) was performed, using an individual-based approach of growth through OD measurements. Lag phase duration was derived from the growth curves obtained. From results obtained, histograms of the lag phase were generated and distributions were fitted. Normal and Weibull distributions were ranked as the bestfit distributions in experiments performed at 25 °C. At 16 °C, lag values (obtained in presence of combinations of both antimicrobials) were also fitted by a Gamma distribution. Predictions were compared with growth curves obtained in liquid egg exposed to mild heat, nisin and/or lysozyme to assess their validity.

The microbial safety and stability of minimally processed foods are based on the application of combined preservative factors. Since microorganisms are able to develop adaptive networks to survive under conditions of stress, food safety may be affected, and therefore understanding of stress adaptive mechanisms plays a key role in designing safe food processing conditions. In the present study, the viability and the sublethal injury ofListeria monocytogenesexposed to moderate heat (55°C) and/or essential oil compounds (carvacrol and thymol, 0.3 mM) treatments were studied. Synergistic effects were obtained when combining mild heat (55°C) with one or both essential oil compounds, leading to inactivation kinetics values three to four times lower than when using heat alone. All the treatments applied caused some injury in the population. The injury levels ranged from around 20% of the surviving population under the mildest conditions to more than 99.99% under the most stringent conditions. Protein extracts of cells exposed to these treatments were analysed by two-dimensional gel electrophoresis. The results obtained revealed that stressed cells exhibited differential protein expression to control cells. The proteins upregulated under these stressing conditions were implicated, among other functions, in stress response, metabolism, and protein refolding.

The antibacterial action of carvacrol and cymene on two Listeria monocytogenes strains (STCC4031 and NCTN4032) was studied. Carvacrol or cymene showed inhibitory effect on the growth of L. monocytogenes during lag and exponential growth phases and was more evident with increasing concentrations in brain heart infusion broth at 30 degrees C. Carvacrol or cymene also decreased the survival of mid-exponential-growth-phase L. monocytogenes STCC4031 cells in potassium-N-2-hydroxy-ethylpiperazine-N-ethanesulfo nic acid, at 30 degrees C. The combination of carvacrol and cymene resulted in an increased antibacterial effect on the growth and a synergistic effect on the viability of L. monocytogenes compared with the natural compounds applied separately. The analysis of survival curves by the Weibull frequency distribution function allowed an accurate prediction of the level of inactivation achieved. Interestingly, an important bactericidal effect (4.7-log reduction) of low concentrations of both antimicrobials combined (0.75 mM) was observed on L. monocytogenes in carrot juice. This study indicates the potential use of carvacrol and cymene applied simultaneously for preservation of minimally processed foods.

The bactericidal action of thymol and cymene on two Bacillus cereus strains (INRA-AVTZ415 and INRA-AVZ421) was studied. Increasing concentrations of thymol (0.2–1.0 mmol l−1) or cymene (0.2–2.0 mmol l−1) showed higher bactericidal effect on exponential growth phase B. cereus cells suspended in HEPES buffer (pH 7), at 30°C. The two strains tested presented different sensitivity to these natural antimicrobials. When thymol and cymene were combined, it resulted in a greater bactericidal effect on B. cereus cells than when these compounds in the essential oil fraction of aromatic plants were applied separately. There was a synergistic effect of both natural antimicrobials on the viability of exponential growth phase B. cereus cells in pH 7 HEPES buffer, at 30°C. Survivor curves did not follow a first-order kinetics, which makes it difficult to establish processing conditions. Experimental data were modeled using a frequency distribution function (Weibull). This model guaranteed a good description of the experimental results and allowed predictions of time to a specific decrease in bacterial populations. This study indicates the potential use of thymol and cymene applied separately or simultaneously for preservation of minimally processed foods, as well as the validity of the Weibull distribution to describe the resulting nonlinear data and establish effective treatment conditions with these compounds.

The antibacterial action of thymol and cymene on two Bacillus cereus strains (INRA-AVTZ415 and INRA-AVZ421) was studied. Thymol and cymene alone showed higher antibacterial effect on the growth of B. cereus in BHI at 30 °C, both in lag and in exponential growth phases, with increasing added concentrations. However, the two strains tested showed different sensitivity to these natural compounds. When thymol and cymene were applied simultaneously, it resulted in a greater antibacterial effect than when the compounds were applied separately. The synergistic behaviour of both compounds was also shown for lag phase B. cereus INRA-AVZ421 cells in food substrates (carrot juice, chicken soup and pumpkin cream) at 30 °C, though a clear food-related effect was observed. This study indicates the potential use of thymol and cymene applied simultaneously for preservation of minimally processed foods.

The effect of irradiation with accelerated electrons on Bacillus cereus and Bacillus subtilis spore counts and spore heat resistance was investigated. Spore counts reduced approximately two log cycles for B. cereus and up to five log cycles for B. subtilis after a radiation dose of 7.6 kGy, with D10 values ranging from 1.5 to 3.8 kGy. Radiation had an important heat-sensitising effect on bacterial spores. After a 3.3 kGy treatment DT values were reduced more than three times for both B. cereus and B. subtilis spores. This study indicates the potential use of accelerated electrons, alone or followed by a heat treatment, for food preservation.

The influence of mild heat pretreatment on the bactericidal action of nisin, carvacrol and thymol on stationary and exponential phase cells of two Bacillus cereus strains was studied. Carvacrol or thymol concentrations of 0.3 mmol/L had no bactericidal effect on unheated, and a minor effect on heated B. cereus cells either in stationary or exponential growth phases. Nisin (0.15 mg/mL) alone decreased the viable counts in all cases (unheated and heated cells in stationary and exponential phase) although the two strains tested showed different sensitivity to this natural antimicrobial between them. When carvacrol or thymol were combined with nisin, it resulted in a greater loss of viability of heated cells than when nisin was applied alone. There was a synergistic effect of nisin and both essential oils with a mild heat pretreatment on the viability of B. cereus cells. This study pointed out the potential use of nisin alone and in combination with carvacrol or thymol applied after a mild heat pretreatment for preservation of minimally processed foods.

A risk analysis was applied to experimental heat resistance data. This analysis is an approach for processing experimental thermobacteriological data in order to study the variability of D and z values of target microorganisms depending on the deviations range of environmental factors, to determine the critical factors and to specify their critical tolerance. This analysis is based on sets of sensitivity functions applied to a specific case of experimental data related to the thermoresistance of Clostridium sporogenes and Bacillus stearothermophilus spores. The effect of the following factors was analyzed: the type of target microorganism; nature of the heating substrate; pH, temperature; type of acid employed and NaCl concentration. The type of target microorganism to be inactivated, the nature of the substrate (reference or real food) and the heating temperature were identified as critical factors, determining about 90% of the alteration of the microbiological risk. The effect of the type of acid used for the acidification of products and the concentration of NaCl can be assumed to be negligible factors for the purposes of engineering calculations. The critical non-uniformity in temperature during thermobacteriological studies was set as 0.5% and the critical tolerances of pH value and NaCl concentration were 5%. These results are related to a specific case study, for that reason their direct generalization is not correct.

Thermobacteriological studies using Bacillus stearotherrnophilus spores were carried out by heating the spores under isothermal and non-isothermal conditions followed by an isothermal period. Ex perimental data obtained after isothermal heating were analyzed using a two-step linear regression procedure and a one-step nonlinear regression method. Results obtained using both analytical tech niques were close, but the 90% interval of confidence for predictions was lower when the one-step nonlinear regression was used. These results indicated the convenience of using the one-step nonlin ear regression method to obtain thermal kinetic parameters for bacterial spores. Also, the parameters obtained by a non-isothermal program followed by an isothermal heating period were similar to those obtained after isothermal heating. Nevertheless, the joint confidence regions at 90% did not overlap, indicating that there are significant differences. The z values obtained under isothermal (7.0 °C) and non-isothermal (8.5 °C) conditions are close and characteristic for this microorganisms.

The effect of different concentrations of sodium chloride (NaCl) in the recovery medium on the apparent thermal resistance of Bacillus stearothermophilus spores (strain ATCC 12980) was studied. Spores were heated at different temperatures (115–125°C) and subcultured in medium with different NaCl concentrations [1, 2, 3 and 3.5% (w/v)]. The results indicate that increasing NaCl levels decreases the apparent heat resistance of the spores, over a temperature range of 115–125°C. No significant difference was found between 3 and 3.5% (w/v) NaCl although both reduced B. stearothermophilus heat resistance considerably. A second-order polynomial equation provided a good description of the spores’ D value, as influenced by the temperature and NaCl concentration. Predictions from the model compared well with published data, and therefore the model is suitable for predicting the effect of NaCl concentration on thermal inactivation of B. stearothermophilus.

A time temperature integrator (TTI) was developed by immobilizing Bacillus stearothermophilus spores in a cylindrical particle consisting of an alginate-starch-mushroom puree. The particle showed homogeneous spore distribution, and when heated over a temperature range of 121 – 130° C negligible spore leakage was observed after the thermal process. The experimental data on spore survivor levels obtained for each temperature-time combination were compared with theoretical predictions using a mathematical model. The results showed a good correlation between the experimental and theoretical data. All these results provide evidence that this artificial particle could be a very reliable TTI for monitoring the thermal impact on micro-organisms during validating sterilization processes in continuous aseptic systems.

Two different effects of calcium were studied, respectively, in plasma membrane vesicles and in protoplasts isolated from roots of control pepper plants (Capsicum annuum L cv. California) or of plants treated with 50 mM NaCl, 10 mM CaCl(2) or 10 mM CaCl(2) + 50 mM NaCl. Under saline conditions, osmotic water permeability (P ( f )) values decreased in protoplasts and plasma membrane vesicles, and the same reduction was observed in the PIP1 aquaporin abundance, indicating inhibitory effects of NaCl on aquaporin functionality and protein abundance. The cytosolic Ca(2+) concentration, [Ca(2+)](cyt), was reduced by salinity, as observed by confocal microscope analysis. Two different actions of Ca(2+) were observed. On the one hand, increase in free cytosolic calcium concentrations associated with stress perception may lead to aquaporin closure. On the other hand, when critical requirements of Ca(2+) were reduced (by salinity), and extra-calcium would lead to an upregulation of aquaporins, indicating that a positive role of calcium at whole plant level combined with an inhibitory mechanism at aquaporin level may work in the regulation of pepper root water transport under salt stress. However, a link between these observations and other cell signalling in relation to water channel gating remains to be established.

6 pages, 3 tables, 4 figures., Dietary ellagic acid and related polyphenols are metabolized in humans to dibenzopyran-6-one derivatives, and the microbial origin of these metabolites has been suggested. However, this has not been demonstrated so far. Fecal samples donated by six volunteers were incubated under anaerobic conditions, and aliquots were used to evaluate the fecal metabolism of ellagic acid, the ellagitannin punicalagin, and an ellagitannin rich extract from walnuts. The isoflavone daidzein was also incubated with the same fecal samples to follow the production of the microbial metabolites previously reported (dihydrogenistein, O-demethylangolensin, and equol) as a positive control of the system and to evaluate similarities between isoflavone and ellagic acid fecal flora metabolism. After fermentation the metabolite “urolithin A” (3,8-dihydroxy-6H-dibenzo[b,d]pyran-6-one) was produced from ellagic acid, punicalagin, and the ellagitannin extract in all the fecal cultures from different volunteers, but with very different production rates and concentrations. This large variability in the concentration of metabolite and kinetics of metabolite production is consistent with the large variability found in the excretion of these metabolites in urine in vivo after human consumption of ellagitannins, and with differences in the composition of the fecal microflora. No correlation between isoflavone and ellagic acid metabolism by fecal microflora was observed. The present study confirms the microbial origin of the recently reported in vivo generated hydroxy-6H-dibenzo[b,d]pyran-6-one derivatives in humans and is a further step in the study of the bioavailability and metabolism of ellagic acid and ellagitannins., This investigation has been supported by Spanish CICYT, projects AGL2003-02195 and AGL-2004-03989, and by the “Fundación Séneca”, project PB/18/FS/02. B.C. and P.P. have fellowships from the Spanish “Consejo Superior de Investigaciones Científicas” (CSIC) and ESF (I3P Programme).

The influence of pH and temperature on the bactericidal action of nisin and carvacrol on vegetative cells of different Bacillus cereus strains was studied. The five strains tested showed significant differences in sensitivity towards nisin, at pH 7.0 and 30 degrees C. Carvacrol concentrations of 0.3 mmol l(-1) had no effect on viability of B. cereus cells. When the same carvacrol concentration was combined with nisin, however, it resulted in a greater loss of viability of cells than when nisin was applied alone. The concentration of carvacrol played an important role on the bactericidal effect of nisin and, therefore, on the synergistic action of both compounds combined. At lower pH values (6.30 and 5.75), nisin was more active against B. cereus cells than at pH 7.0 at 30 degrees C, with a different sensitivity of the strains tested. The combined effect of nisin and carvacrol was found to be significantly different at pH 7.0 and 5.75. When the temperature was 8 degrees C, nisin was significantly less active against B. cereus IFR-NL 94-25 than at 30 degrees C, both at pH 7.0 and 6.30. At 8 degrees C, there was a significant increased effect of nisin at lower pH values. Also at this low temperature, a synergistic effect between nisin and carvacrol on B. cereus cells was observed at the pHs tested. This study indicates the potential of nisin and carvacrol at lower pHs to be used for preservation of minimally processed foods.

The combined effect of pH and NaCl on the apparent thermal resistance of Bacillus stearothermophilus ATCC 12980 spores was studied. Spores were heated at different temperatures (115-125 degrees C) in mushroom substrate, acidified using glucono-delta-lactone to different pH levels (from 5.75 to 6.7), which contained concentrations of NaCl that ranged from 0.5 to 3% (w/v). The recovery medium was acidified to the same pH level and contained the same NaCl concentration as the heating menstruum. A factorial experimental design allowed a predictive model to be developed, which described the combined effect of heating temperature, pH and NaCl on the thermal resistance of B. stearothermophilus spores. Predictions from the model provided a valid description of the data used to generate the model, and agreed with observations from the literature and from an independent experiment performed using asparagus and bean substrates.