Effect of single and combined UV-C and ultra-high pressure homogenisation treatments on inactivation of Alicyclobacillus acidoterrestris spores in apple juice

Alicyclobacillus acidoterrestris is a spore-forming bacterium that can survive thermal pasteurization and acidic conditions. It produces changes in the odour and flavour of fruit juices leading to economical loses. A. acidoterrestris CECT 7094 spores were inoculated in clarified and cloudy apple juices (Golden delicious var.) in the range of 5–6 log₁₀ spores/mL and submitted to different short-wave ultraviolet light (UV-C) doses (7.2–28.7 J/mL) and ultra-high pressure homogenisation (UHPH) treatments (100–300 MPa), including their combination. A. acidoterrestris could be inactivated in clarified apple juice at a level of 4.8 log₁₀ CFU/mL by a 300 MPa-UHPH treatment when the inlet temperature was 80 °C. UV-C treatments showed to be more efficient achieving a lethality of 5.5 log₁₀ CFU/mL with a dose of 21.5 J/mL at 20 °C. In cloudy apple juice (2357 NTU) UV-C treatments were less efficient with a maximum lethality of 4.07 CFU/mL after a dose of 28.7 J/mL. A previous application of UHPH contributed with UV-C to obtain higher reductions of A. acidoterrestris spores at the doses of 14.3 and 21.5 J/mL compared with UV-C single treatments. On the other hand, this previous treatment also changed the properties of particles in the matrix which apparently reduced the effectiveness of UV-C at 28.7 J/mL.     

The impact of high power ultrasound for controlling spoilage by Alicyclobacillus acidoterrestris: A population and a single spore assessment

Consumer demand for healthier, microbiologically safe and stable, higher quality and minimally processed foods has increased in the last decades, promoting the application of non-thermal process technologies. Ultrasound processing is gaining attention because of its potential for improving quality and safety while retaining product flavor, texture, color and nutrient composition. Recently, Alicyclobacillus acidoterrestris has been recognized by manufacturers and processors in the fruit industry as a new type of thermoacidophilic, endospore-forming spoilage bacterium for commercialized fruit juices that can withstand the high temperatures applied in pasteurization process and spoil them producing taint compounds. Based on the above, the present study was undertaken to investigate the separate and combined effect of ultrasound treatment operating at 26 kHz, 90 μm, 200 W for 5, 10, 15 min and heat stress (at 80 °C for 10 min) on the recovery of A. acidoterrestris spores in K broth adjusted to pH = 4.5 with 25% (w/v) citric acid at temperatures between 35 and 45 °C at population and individual spore level. At the population level, statistically significant differences in average lag time of A. acidoterrestris spores from different treatments at 35 and 45 °C have been found. After the applied ultrasound (at 100% Amplitude for 10 min) and heat shock (at 80 °C for 10 min), the average lag time of A. acidoterrestris spores at 35 °C (7.24 ± 0.34 h) and 45 °C (6.38 ± 0.18 h) has been shown to be longer compared to the control at 35 °C (5.68 ± 0.36 h) and at 45 °C (2.87 ± 0.19 h), while for this combination, the maximum specific growth rate was not significantly different from the control samples. Additionally, the findings of this study have shown that λ among individual spores was greatly variable when they were treated by ultrasound (at 100% Amplitude for 10 min) and/or thermal treatment (at 80 °C for 10 min). The application of ultrasound and thermal treatment resulted in a more pronounced effect on median lag phase duration (25.74 h) than the heat shock (11.63 h) and affected both the position and the spread of the λ distributions of A. acidoterrestris spores. This work would help to better assess and optimize the proposed combined treatments, since ultrasound and thermal treatments could work in a synergistic way on delaying the lag time of the tested bacterial spores.     

High pressure thermal processing for the inactivation of Clostridium perfringens spores in beef slurry

The spores of Clostridium perfringens can survive and grow in cooked/pasteurized meat, especially during the cooling of large portions. In this study, 600 MPa high pressure thermal processing (HPTP) at 75 °C for the inactivation of C. perfringens spores was compared with 75 °C thermal processing alone. The HPTP enhanced the inactivation of C. perfringens spores in beef slurry, resulting in 2.2 log reductions for HPTP vs. no reductions for thermal processing after 20 min. Then, the HPTP resistance of two C. perfringens spore strains in beef slurry at 600 MPa was compared and modeled, and the effect of temperature investigated. The NZRM 898 and NZRM 2621 exhibited similar resistance, and Weibull modeled well the log spore survivor curves. The spore inactivation increased when HPTP temperature was raised from 38 to 75 °C. The results confirm the advantage of high pressure technology to increase the thermal inactivation of C. perfringens spores in beef slurry.Industrial relevanceC. perfringens spores may cause food/meat poisoning as a result of improperly handled and prepared foods in industrial kitchens. Thermal processes at 100 °C or higher are generally carried out to ensure the elimination of these pathogenic spores. High pressure processing (HPP) is a food pasteurization technique which would help to maintain the sensorial and nutritional properties of food. Preservation of foods with HPP in conjunction with mild heat (HPTP) would enhance the spore inactivation compared to thermal processing alone at the same temperature, due to a known germination–inactivation mechanism. This technology, together with the application of Good Manufacturing Practices, including rapid cooling, is a good alternative to the traditional methods for producing safe processed meat and poultry products with enhanced sensory and nutritional quality.     

Growth inhibition and inactivation of Alicyclobacillus acidoterrestris endospores in apple juice by hyperbaric storage at ambient temperature

Control of endospores of Alicyclobacillus acidoterrestris in pasteurized apple juice using hyperbaric storage at 18 to 23 °C was compared to storage at atmospheric pressure and 18 to 23 °C, as well as refrigeration at ~4 °C for up to 30 days. The juice samples were inoculated with approximately 1 × 10⁵ CFU/mL spores. The juice spoiled quickly at atmospheric pressure and ambient temperature, while under refrigeration spore levels remained unchanged for 30 days. Hyperbaric storage of inoculated apple juice at 25, 50 and 100 MPa at 18 to 23 °C resulted in spore inactivation at more rapid rates as pressure magnitudes increased, reaching levels below the detection limit of 10 CFU/mL at 50 and 100 MPa. In highly acid foods such as apple juice, hyperbaric storage at pressures ≤100 MPa and ambient temperature was effective in inactivating spores of A. acidoterrestris for periods up to 30 days.These results indicate hyperbaric storage at ambient temperature as a clearly more efficient preservation procedure to control the development of A. acidoterrestris endospores, compared to ambient temperature and refrigerated storage, in highly acidic foods as apple juice.     

Thermal inactivation of <Emphasis Type="Italic">Alicyclobacillus acidoterrestris</Emphasis> spores under conditions simulating industrial heating processes of tangerine vesicles and its use in time temperature integrators

The aim of the present study was to characterize the thermal inactivation of Alicyclobacillus acidoterrestris spores under isothermal and non-isothermal conditions simulating industrial heating processes applied to tangerine vesicles. A microbiological time temperature integrator (TTI) suitable for estimating the severity of thermal processes applied to acid foods was also developed. The heat resistance of A. acidoterrestris was characterized by D _105 °C = 0.63 min and z = 10.8 °C in tangerine juice, showing linear survival curves, without shoulders and tails. Under non-isothermal conditions, the use of isothermal data allowed for an accurate prediction of the inactivation. The spores were included in alginate TTIs and they were used to estimate the severity of thermal treatments applied both in a thermoresistometer Mastia and in a food pilot plant scale system, which allows fast heating of the product to 93 °C and then a short holding time (2 min). In the thermoresistometer, tangerine juice was used as heating medium. In the food pilot plant scale system, thermal treatments were applied in batch to unpackaged tangerine vesicles. In both equipments, the TTIs accurately predicted the lethality of the thermal treatments applied. The percent coefficients of variation for survivor counting in TTIs showed that distribution of heat is homogeneous both in the thermoresistometer and in the reactor, being lower than 10% in all cases. The logistic and normal distributions were found to be the best for fitting the different survivor datasets.     

Application of electro-activated potassium acetate and potassium citrate solutions combined with moderate heat treatment on the inactivation of Clostridium sporogenes PA 3679 spores

Combined effects of moderate temperatures and the electro-activated aqueous solutions of potassium acetate and potassium citrate on the inactivation of C. sporogenes PA 3679 spores (D_121°C = 1.18 min) were studied. Four types of solutions (potassium acetate with/without KCl and potassium citrate with/without KCl) were activated at 400 mA for 60 min. The oxidation reduction potential (ORP) and pH values ranged from + 417.50 to + 1043.33 mV and 3.18 to 3.47, respectively. The combination of these solutions with a moderate heat treatment (95 °C, 105 °C, and 115 °C) for different time (5, 10, 20, and 30 min) was sufficient to reach a 100% of spore destruction (inactivation) in a medium with an initial contamination level comprised between 7.0 and 7.8 log CFU/mL. The sporicidal effect of solutions was also present even if activated solutions were applied alone against spores without being combined with heat treatment. Spore morphology was determined under transmission electron microscopy and showed that there were important damages, such as rupture of spores and release of spore components in all of the treated spores. Thus, the sporicidal effect detected was the result of inactivation mechanisms of electro-activated solutions on spores. In almost all of observed micrographs, there were coreless spores, deformed spores, or debris of spores. The current investigation can be used for achieving further studies in order to better understand the mechanisms of inactivation of C. sporogenes spores by electro-activated solutions.Industrial relevanceThis research article aims to study the combined effect of electro-activated potassium acetate and citrate solutions and moderate heat treatment on the viability of Clostridium sporogenes in model solutions as a non-pathogenic surrogate of Clostridium botulinum. The objective was to use hurdle technology to produce nutritious, minimally processed foods while ensuring food safety. Moreover, this approach allowed for a reduced level of sodium in canned foods since the solutions were sodium-free.     

Thermal inactivation of Bacillus anthracis Sterne in irradiated ground beef heated in a water bath or cooked on commercial grills

The thermal stability of heat-shocked and non-heat-shocked spores of the virulence-attenuated Sterne strain of Bacillus anthracis was evaluated at select temperatures in irradiated, raw ground beef (25% fat) heated in a water bath or cooked using two different commercial grills. For the former, 3-g portions of inoculated ground beef were packaged in bags that were completely immersed in a temperature-controlled circulating water bath held at 65 °C (149 °F), 70 °C (158 °F), 75°(167 °F), and 80 °C (176 °F) for a predetermined length of time. For the latter, formed ground beef patties (95-g each) were inoculated with spore stock A or B of the Sterne strain and then cooked on a commercial open-flame gas grill or on a commercial clamshell electric grill to achieve target internal temperatures of either 71.1 °C (160 °F), 82.2 °C (180 °F), or 93.3 °C (200 °F). Cooking ground beef patties on commercial grills, resulted in reductions of ca. 0.8 to 3.5 log₁₀ CFU/g for spore stocks A and B of B. anthracis Sterne after heating to 71.1 °C (160 °F), 82.2 °C (180 °F), or 93.3 °C (200 °F) on either the open-flame gas grill which required ca. 9.6 min to reach the target internal temperatures or on the clamshell electric grill which required ca. 4.0 min to reach the target internal temperatures. In comparison, our data using a water bath system and heating at 65° to 80 °C predict nearly 4 log reductions in spore levels for short times, ~½ min, depending possibly on the temperature. Thus, our data suggest that models based on heating ground beef in a water bath is not a good predictor of reductions of levels of spores of B. anthracis Sterne strain that would be obtained when cooking ground beef patties on commercial grills under conditions that may be typically used by consumers and/or retail establishments. Nevertheless, our data validated that cooking ground beef patties on a commercial grill at a temperature considered to be “well-done” and a temperature (71.1 °C;160 °F) recommended by the USDA/FSIS, is effective at killing spores of B. anthracis Sterne.Industrial relevanceHeating ground beef in a water bath or cooking ground beef patties on commercial grills under conditions simulating those that are used by consumers and/or that occur in retail food service establishments is effective at killing spores of B. anthracis Sterne.     

Optimization and evaluation of heat-shock condition for spore enumeration being used in thermal-process verification: Differential responses of spores and vegetative cells of Clostridium sporogenes to heat shock

To evaluate a heat-shock condition for the enumeration of Clostridium sporogenes spores, a surrogate for C. botulinum spores, we examined the heat tolerance of C. sporogenes spores and vegetative cells exposed to a heat shock at 90°C. From the D values of the spores determined in the temperature range of 113–121°C, z value (±SD) and D_90°C value were estimated to be 10.16±0.90°C and 1,071.52 min, respectively, and the inactivation rates were predicted to be only approximately 2% at 90°C for up to 10 min. Meanwhile, the viable count of spores was significantly higher when activated under a heat-shock condition of 90°C for over 9 min than those activated for shorter time periods. The heat tolerance of vegetative cells was extremely low, showing a D_90°C value (±SD) of 0.21±0.01 min. Finally, 3 different heat-shock conditions were compared: 70°C for 30 min, 80°C for 20 min, and 90°C for 10 min, and the experimental comparative data showed no significant differences in viable spore counts. Consequently, these results support that the heat-shock treatment at 90°C for 10 min is suitable to activate spores and to inactivate vegetative cells of C. sporogenes.     

Bacterial spore inactivation at 45-65 °C using high pressure processing: Study of Alicyclobacillus acidoterrestris in orange juice

High pressure processing (HPP) is a new non-thermal technology commercially used to pasteurize fruit juices and extend shelf-life, while preserving delicate aromas/flavours and bioactive constituents. Given the spoilage incidents and economic losses due to Alicyclobacillus acidoterrestris in the fruit juice industry, the use of high pressure (200 MPa – 600 MPa) in combination with mild temperature (45 °C–65 °C) for 1–15 min, to inactivate these spores in orange juice were investigated. As expected, the higher the temperature, pressure and time, the larger was the A. acidoterrestris inactivation. The survival curves were described by the first order Bigelow model. For 200 MPa, D_45 °C = 43.9 min, D_55 °C = 28.8 min, D_65 °C = 5.0 min and z-value = 21.3 °C. At 600 MPa, D_45 °C = 12.9 min, D_55 °C = 7.0 min, D_65 °C = 3.4 min and z-value = 34.4 °C. Spores were inactivated at 45 °C and 600 MPa, and at 65 °C only 200 MPa was needed to achieve reduction in spore numbers.     

Effect of vacuum cooling followed by ozone repressurization on Clostridium perfringens germination and outgrowth in cooked pork meat under temperature-abuse conditions

The effect of combining vacuum cooling with an ozone-based inhibition process (InhVac) on Clostridium perfringens spore germination and outgrowth in cooked pork meat after exponential chilling (from 54.4 to 7.2 °C in 12, 15, 18, or 21 h) and isothermal storage (20, 25, 30, 36, or 45 °C) was evaluated. Ice cooling (IC) and vacuum cooling (VC) were used to compare the effects with InhVac. The samples were inoculated with a three-strain mixture of C. perfringens spores to obtain concentration of ca. 3 log₁₀ CFU/g. C. perfringens growth in samples treated by InhVac were 0.1, 0.37 and 0.9 log₁₀ CFU/g after 15, 18 and 21 h of cooling from 54.4 to 7.2 °C respectively, significantly lower (P<0.05) than those in samples subjected to IC (1.01, 2.10 and 2.8 log₁₀ CFU/g) and VC (0.56, 1.01 and 2.13 log₁₀ CFU/g). Compared to VC and IC, InhVac treatment increased the lag phase (λ), decreased the growth rates (μ_max), and extended the sample shelf-life (the time until a 1 log₁₀ CFU/g increase in C. perfringes from the initial concentration value) at all storage temperatures. InhVac-treated samples not only had a longer shelf-life than those treated by VC, but also exhibited almost two times longer shelf-life compared to those subjected to IC regardless of storage temperatures. Additionally, statistical indexes showed that a primary modified Gompertz model and a secondary Square Root model could fit the data well.Industrial relevanceIn this study, an innovative inhibition approach (InhVac) was found to show a better antimicrobial effect on C. perfringens germination and outgrowth in cooked pork meat compared to ice cooling and vacuum cooling under temperature-abuse conditions. A primary modified Gompertz model and a secondary Square Root model could be used to predict the C. perfringens growth in samples subjected to InhVac treatment.     

Clostridium sporogenes-ATCC 7955 Spore Destruction Kinetics in Milk Under High Pressure and Elevated Temperature Treatment Conditions

Clostridium sporogenes (ATCC 7955) spores inoculated in milk (2% fat) were subjected to high-pressure (HP) treatments (700–900 MPa) and at elevated temperatures (80–100 °C) for selected times up to 32 min. Samples were sealed in 1-mL plastic vials and placed in a specially constructed insulated chamber to prevent temperature drop during the treatment. Both pressure pulse (with no hold time) and pressure hold techniques were employed for treatment. Pressure pulse resulted in a small, but consistent, destruction (up to 0.5 log kill) of spores. During the pressure hold treatment, the destruction followed a first-order model (R ² > 0.90). The kinetic data were compensated for the small variations in temperature during the treatment. As expected, higher pressures and higher temperatures resulted in a faster rate of spore destruction. Temperature-corrected D values ranged from 13.6 to 2.4 min at 700 MPa and 7.0 to 1.3 min at 900 MPa, respectively, with process temperatures set at 90 and 100 °C. In comparison, thermal treatments gave D values ranging from 156 min at 90 °C to 12.1 min at 100 °C. The temperature sensitivity Z _P values (16.5 to 20.3 °C) under high pressure (700–900 MPa) were higher than under conventional thermal processing (9.0 °C), indicating the spore’s thermal resistance to increase at HP processing conditions. The pressure sensitivity Z _T values varied between 450 and 680 MPa under the elevated temperature (80–100 °C) processing conditions. Overall, C. sporogenes 7955 spores were relatively more sensitive to temperature than pressure.     

Thermosonication versus thermal processing of skim milk and beef slurry: Modeling the inactivation kinetics of psychrotrophic Bacillus cereus spores

Non-thermal processed foods are generally cold stored and distributed. The use of ultrasound for food preservation has attracted the interest of many research groups. In the current study, the thermosonication (TS, simultaneous ultrasound and thermal process) inactivation of psychrotrophic Bacillus cereus spores was investigated (24 kHz, 210 μm, 0.33 W/mL or W/g). First, the effectiveness of a 1.5 min TS process at 70 °C in skim milk, beef slurry, cheese slurry, and rice porridge was investigated. The TS was more effective than sole thermal treatment in reducing B. cereus spores in rice porridge, beef slurry and cheese slurry by 7, 6, and 4 fold, respectively. Then, the first-order D- and z-values for TS and thermal processing in skim milk and beef slurry, and the best model to fit TS inactivation of B. cereus spores in beef slurry were determined. The D_70 °C-values in skim milk were 2.9 min for TS and 8.6 min for the thermal treatment. And in beef slurry, values of 0.4 min for TS and 2.3 min for thermal were estimated. It was found that the Log-logistic model better described the TS spore inactivation in beef slurry. The ultrasound technology required 20–30 °C lower temperatures for the same spore inactivation, which resulted in better food quality and energy saving gains.     

Influence of sporulation awon heat resistance and germination ofPenicillium roquefortispores

Spores ofPenicillium roquefortiwere produced using potato dextrose agar which was solute agjusted using either NaCl, sucrose, or glycerol to water activity (a_w) levels of: 0·99 (control), 0·97, 0.·90 and 0·88. All spore crops, regardless of the sporulation a_won which they were produced, exhibited near total germination within 24 h at 22°C in sterile water. However, when spores were initially heated either in water (42°C, 2 h or 55°C, 10 min) or in air (60°C, 20 min) and then germinated, levels were observed to decrease particularly for spores produced on 0·88 a_wadjusted media. Germination levels were also observed to decrease with time when harvested spores were initially suspended in sterile distilled water (0 to 24 h, 2°C) and then heated. Control spores (0·99 a_w) showed no effects. Heat studies of spores suspended in water were used to calculate thermal decimal reduction times (D). Control spores and spores produced at 0·88 a_wwith glycerol, exhibitedD_55°Cvalues of 10·3and 2·3 min, respectively. Specific effects attributable to the nature of the solute appeared to influenceDvalues; however, no clear trend was observed. The results of this study indicated that spores that were produced at reduced a_wbecame heat-sensitized perhaps owing to injury and/or damage incurred during sporogenesis.     

Accurate quantification of thermophilic spores in dairy powders

Internationally, there are no official guidelines for the quantification of thermophilic spores in dairy products, which leads to variations in applied methodology. In this study, we assess the heat sensitivity of thermophilic spores, vegetative cells grown under laboratory conditions and spores in German dairy powders to determine appropriate heating conditions for accurate quantification of total thermophilic spores. The heat inactivation effect (80–95 °C) is limited for spores of Anoxybacillus flavithermus and Geobacillus stearothermophilus grown under laboratory conditions. However, for spores originating from whey, whey powder and skimmed milk powder (mostly identified as A. flavithermus), a different trend was observed; spore counts continuously reduced when heating time and temperature increased (80–98 °C, 10–30 min). The results indicate that data obtained using laboratory cultures cannot be extrapolated to commercial powders, and in this case, applying temperatures above 80 °C leads to an underestimation of spore counts in dairy powders.     

Mechanism of Bacillus subtilis spore inactivation induced by moderate electric fields

Bacterial endospores are the key safety targets for inactivation within low-acid foods. Herein, we investigated the inactivation of Bacillus subtilis CGMCC 1.1087 spores (10⁷ CFU/mL) in sterile distilled water using moderate electric fields (MEF, 300 V/cm) under various temperatures (<30, 55, 65 and 75 °C). MEF treatment at below 30 °C resulted in 0.6-log reduction of spores, while treatments for 60 min without electric fields showed no inactivation. Inactivation induced by MEF in the same treatment time increased to 1.8-, 2.0- and 2.5-log as temperature increased to 55, 65 and 75 °C. Spores treated with MEF at <30, 55, 65 and 75 °C or mild heat (55, 65 and 75 °C) scarcely lost heat resistance, suggesting that spores did not germinate during MEF or mild heat treatment. The viability of MEF-treated spores did not increase by addition of lysozyme (3 μg/mL) in recovery plates, preincubation for 1 h in a 1:1 mixture of 60 mM Ca²⁺ and DPA, or lysozyme treatment in hypertonic medium. Confocal laser scanning microscopy photomicrographs showed that exposure to MEF induced a marked increase in the permeability of inner membrane and cortex. These findings suggested that damage of the cortex and inner membrane, rather than spore nutrient germinant receptors or cortex lytic enzymes, are possible reasons contributing to inactivation of B. subtilis spores by MEF. This study indicates that MEF at mild temperatures (55 to 75 °C) have the potential for spore inactivation.Industrial relevanceLiterature in the past few years has shown that moderate electric fields (MEF), typically associated with ohmic heating, have nonthermal effects on bacterial spores, leading to accelerated inactivation. The current work extends the range of temperatures to those well below thermally lethal conditions, and shows that some spore inactivation occurs under MEF, even when temperatures are sublethal. Little or no germination is observed, and spore inner membranes are increasingly compromised over time. The elucidation of such nonthermal effects would be significant to the food industry as it seeks increasingly nonthermal methods for inactivation of spores.     

Germination and inactivation of Bacillus coagulans and Alicyclobacillus acidoterrestris spores by high hydrostatic pressure treatment in buffer and tomato sauce

Acidothermophilic bacteria like Alicyclobacillus acidoterrestris and Bacillus coagulans can cause spoilage of heat-processed acidic foods because they form spores with very high heat resistance and can grow at low pH. The objective of this work was to study the germination and inactivation of A. acidoterrestris and B. coagulans spores by high hydrostatic pressure (HP) treatment at temperatures up to 60 °C and both at low and neutral pH. In a first experiment, spores suspended in buffers at pH 4.0, 5.0 and 7.0 were processed for 10 min at different pressures (100-800 MPa) at 40 °C. None of these treatments caused any significant inactivation, except perhaps at 800 MPa in pH 4.0 buffer where close to 1 log inactivation of B. coagulans was observed. Spore germination up to about 2 log was observed for both bacteria but occurred mainly in a low pressure window (100-300 MPa) for A. acidoterrestris and only in a high pressure window (600-800 MPa) for B. coagulans. In addition, low pH suppressed germination in A. acidoterrestris, but stimulated it in B. coagulans. In a second series of experiments, spores were treated in tomato sauce of pH 4.2 and 5.0 at 100 - 800 MPa at 25, 40 and 60 °C for 10 min. At 40 °C, results for B. coagulans were similar as in buffer. For A. acidoterrestris, germination levels in tomato sauce were generally higher than in buffer, and showed little difference at low and high pressure. Remarkably, the pH dependence of A. acidoterrestris spore germination was reversed in tomato sauce, with more germination at the lowest pH. Furthermore, HP treatments in the pH 4.2 sauce caused between 1 and 1.5 log inactivation of A. acidoterrestris. Germination of spores in the high pressure window was strongly temperature dependent, whereas germination of A. acidoterrestris in the low pressure window showed little temperature dependence. When HP treatment was conducted at 60 °C, most of the germinated spores were also inactivated. For the pH 4.2 tomato sauce, this resulted in up to 3.5 and 2.0 log inactivation for A. acidoterrestris and B. coagulans respectively. We conclude that HP treatment can induce germination and inactivation of spores from thermoacidophilic bacteria in acidic foods, and may thus be useful to reduce spoilage of such foods caused by these bacteria.     

Mechanisms of Bacillus subtilis spore inactivation by single- and multi-pulse high hydrostatic pressure (MP-HHP)

Herein we investigate the effect of multi-pulse high hydrostatic pressure (MP-HHP) on the inactivation of Bacillus subtilis spores. B. subtilis spores were subjected to MP-HHP under pressures at 200–500 MPa at temperatures of 40 and 60 °C with 3 pulses (holding time of 3 min) with a total processing time of 10 min and compared it with a single pressurization (S-HHP).Mechanism of spore inactivation by S- or MP-HHP was explored by assessing germination by heat shock treatment, spore susceptibility to lysozyme and hydrogen peroxide (H₂O₂), release of dipicolinic acid (DPA), and the permeability of inner membrane and cortex. Our results presented the highest spore inactivation (5.8 log reduction), when MP-HHP was applied under the highest temperature and pressure. The increased inactivation appears to be largely due to mechanical disruption of spore coat and inner and outer membranes, as evidenced by DPA release, increased susceptibility to lysozyme and H₂O₂ (indicative of breakage of disulfide bonds in the spore coat), and membrane permeability as assessed by spore staining and fluorescence microscopy. No differences were seen in germination between MP-HHP and S-HHP. There was no evidence of any loss of cortex lytic enzymes or degradation of small acid-soluble proteins (SASPs) during both MP-HHP and S-HHP treatments.     

Effects of plasma-activated water on microbial growth and storage quality of fresh-cut apple

Fresh-cut ‘Fuji’ apples were immersed for 5 min in plasma-activated water (PAW) generated, by plasma generated with sinusoidal voltages at 7.0 kHz with amplitudes of 6 kV, 8 kV, and 10 kV, designated PAW-6, PAW-8, and PAW-10, respectively. The control group was soaked in distilled water for 5 min instead of PAW. The results indicated that the growth of bacteria, molds, and yeasts was inhibited by PAW treatments during storage at 4 ± 1 °C, especially the microbial inactivation with PAW-8, which was the most efficient. PAW-8 reduced the microbial counts by 1.05 log₁₀CFU g⁻¹, 0.64 log₁₀CFU g⁻¹, 1.04 log₁₀CFU g⁻¹ and 0.86 log₁₀CFU g⁻¹ for aerobic bacteria (aerobic plate counts), molds, yeasts and coliforms on day 12, respectively. In addition, the bacterial counts of fresh-cut apples treated with PAW were <5 log₁₀CFU g⁻¹, which did not exceed to the existing China Shanghai local standard (DB 31/2012–2013) during 12 days of storage. PAW treatments reduced superficial browning of fresh-cut apples without affecting their firmness and titratable acidity. In addition, no significant change was observed in antioxidant content and radical scavenging activity between the PAW-treated and control groups. It is suggested that PAW is a promising method for preservation of fresh-cut fruits and vegetables, which is usually beneficial to the quality maintenance of fresh-cut fruits and vegetables during storage.     

Predictive model for growth of Clostridium perfringens during cooling of cooked ground chicken

Traditional methodologies for development of microbial growth models under dynamic temperature conditions do not take into account the organism's history. Such models have been shown to be inadequate in predicting growth of the organisms under dynamic conditions commonly encountered in the food industry. The objective of the current research was to develop a predictive model for Clostridium perfringens spore germination and outgrowth in cooked chicken products during cooling by incorporating a function to describe the prior history of the microbial cell in the secondary model. Incorporating an assumption that growth kinetics depends in an explicit way on the cells' history could provide accurate estimates of growth or inactivation.Cooked, ground uncured chicken was inoculated with C. perfringens spores, and from this chicken, samples were formed and vacuum packaged. For the isothermal experiments, all samples were incubated in a constant temperature water baths stabilized at selected temperatures between 10 and 51 °C and sampled periodically. The samples were cooled from 54.4 to 27 °C and subsequently from 27 to 4 °C at different time periods (cooling rates) for dynamic cooling experiments. The standard model provided predictions that varied from the observed mean log₁₀ growth values by magnitudes up to about 0.65 log₁₀. However, for a selected memory model, estimates of log₁₀ relative growth provided predictions within 0.3 log₁₀ of the mean observed log₁₀ growth values. These findings point to an improvement of predictions obtained by memory models over those obtained by the standard model. More study though is needed to validate the selected model.Industrial relevanceMention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.     

Spores in dairy – new insights in detection,enumeration and risk assessment

The performance of the ISO method for the ‘Enumeration of the Specially Thermoresistant Spores of Thermophilic Bacteria in Dried Milk’ (ISO/TS27265; 2009) was compared with a more practical method. Both were tested for predictability of spoilage of UHT treated reconstituted milk. The data show that heating for 30 min at 100 °C has the same predictive value as heating for 30 min at 106 °C, provided that specifications are increased 1 log₁₀ and the use of TSA as a cultivation medium is recommended over PCMA. Predictability of spoilage using classical plating methods is furthermore discussed in relation to variation in spore heat resistance of spores commonly present in the dairy industry.