Pressure-assisted Thermal Processing Research Articles

Role of Dipicolinic Acid in Heat Resistance of Spores of Clostridium botulinum and Clostridium sporogenes PA3679 by Thermal and Pressure-assisted Thermal Processing

Dipicolinic acid (DPA) is a major constituent of spores and reportedly provides protection against inactivation by various thermal processes; however, the relationship between DPA and resistance towards pressure-assisted thermal processing is not well understood. Thermal and pressure-assisted thermal inactivation studies of Clostridium botulinum nonproteolytic strains QC-B and 610-F, proteolytic strain Giorgio-A, and thermal surrogate Clostridium sporogenes PA3679 spores suspended in ACES buffer (0.05 M, pH 7.0) were performed to determine if a relationship exists between DPA release and log reduction of spores. Thermal inactivation at 80, 83, and 87 °C for nonproteolytic strains and 101, 105, and 108 °C for the proteolytic strain and thermal surrogate were conducted. Pressure-assisted thermal inactivation for nonproteolytic strains at 83 °C/600 MPa and for the proteolytic strain and thermal surrogate at 105 °C/600 MPa were performed. Surviving spores were enumerated by 5-tube MPN method for log reductions and analyzed for released DPA by liquid chromatography-tandem mass spectrometry. The correlation between MPN log reductions, released DPA, and D-values were calculated. A positive correlation between released DPA and log reduction of spores was observed for QC-B and 610-F at 80 and 83 °C (r = 0.6073 − 0.7755; P < 0.01). At 87 °C, a positive correlation was detected for 610-F (r = 0.4242, P < 0.05) and no correlation was observed for QC-B (r = 0.1641; P > 0.05). A strong, positive correlation (r = 0.8359 − 0.9284; P < 0.05) between released DPA and log reduction of spores was observed for Giorgio-A at 101, 105, and 108 °C, and a strong, positive correlation (r = 0.8402; P < 0.05) was observed for PA3679 at 101 °C. A positive correlation (r = 0.5646 − 0.6724; P < 0.01) was observed for QC-B, 610-F, and Giorgio-A after pressure-assisted thermal treatment. No correlation (r = 02494; P > 0.05) was found for PA3679 after pressure-assisted thermal treatment. These results suggest a correlation exists between DPA release and heat resistance; however, the level of correlation varied between strains and temperatures. The findings from this research may aid in the development of spore inactivation strategies targeting the thermal resistance profiles of various strains of C. botulinum spores.

Inactivation of Clostridium sporogenes PA 3679 spores by synergistic pressure-assisted thermal processing and antimicrobial compound combinations

Inactivation of Clostridium sporogenes PA 3679 spores by synergistic pressure-assisted thermal processing and antimicrobial compound combinations

Enhancing vertical thermal conductivity of carbon fiber reinforced polymer composites using cauliflower-shaped copper particles

Carbon fiber reinforced polymer (CFRP) composites are lightweight and mechanically robust, and therefore have received considerable attention with a wide range of applications from the aerospace to automotive industries. However, their low vertical thermal conductivity seriously limits the potential applicability of high-performance CFRPs in metal replacement. Herein, we propose a simple and effective, scalable strategy to fabricate thermally conductive and mechanically robust CFRP composites on the basis of formation of vertical heat transfer pathways using pressure-assisted thermal migration of cauliflower-shaped dendritic copper particles. A new type of CFRP composite was achieved by facile layer-by-layer coating of prepregs with dendritic coppers and subsequent pressure-assisted thermal processing under elevated temperature. The cauliflower-shaped dendritic copper particles efficiently infiltrated into the CFRP matrix and thereby provided an effective heat transfer pathway, resulting in a CFRP composite with an excellent vertical thermal conductivity of 2.321 W m−1 K−1, which is 155% higher than that of pristine CFRP. Moreover, the proposed CFRP composite afforded a high thermal diffusivity of 1.125 × 10−6 m2 s−1 and a low specific heat capacity of 1.267 J g−1 K−1, as well as an excellent flexural strength of about 2.4 GPa.

High-Pressure-Based Strategies for the Inactivation of Bacillus subtilis Endospores in Honey

Honey is a value-added product rich in several types of phenolic compounds, enzymes, and sugars recently explored in biomedical and food applications. Nevertheless, even though it has a low water activity (aW ≈ 0.65) that hinders the development of pathogenic and spoilage microorganisms, it is still prone to contamination by pathogenic microorganisms (vegetative and spores) and may constitute harm to special groups, particularly by immunosuppressed people and pregnant women. Thus, an efficient processing methodology needs to be followed to ensure microbial safety while avoiding 5-hydroxymethylfurfural (HMF) formation and browning reactions, with a consequent loss of biological value. In this paper, both thermal (pressure-assisted thermal processing, PATP) and nonthermal high-pressure processing (HPP), and another pressure-based methodology (hyperbaric storage, HS) were used to ascertain their potential to inactivate Bacillus subtilis endospores in honey and to study the influence of aW on the inactivation on this endospore. The results showed that PATP at 600 MPa/15 min/75 °C of diluted honey (52.9 °Brix) with increased aW (0.85 compared to ≈0.55, the usual honey aW) allowed for inactivating of at least 4.0 log units of B. subtilis spores (to below detection limits), while HS and HPP caused neither the germination nor inactivated spores (i.e., there was neither a loss of endospore resistance after heat shock nor endospore inactivation as a consequence of the storage methodology). PATP of undiluted honey even at harsh processing conditions (600 MPa/15 min/85 °C) did not impact the spore load. The results for diluted honey open the possibility of its decontamination by spores’ inactivation for medical and pharmaceutical applications.

High-pressuretreatment of water-filled co-extruded polylactide films: Effect on microstructure, barrier, thermal, and rheological properties.

The impact of high-pressure treatments (450 and 600MPa) on the morphological, thermal, structural, and barrier properties of commercial coextruded polylactide (PLA) packaging films has been explored to evaluate their applicability in food processing. Pouches filled with water as a food simulant were subjected to high-pressure treatment for 15min at ambient temperature. Results indicated no significant changes in the visual appearance, color, integrity, or water barrier properties of the post-process pouches. However, high-pressure treatment affected mechanical property results. Thermal analysis of the film showed endothermic double melting peaks (165.12 and 170.55°C), which did not change with the pressurization; however, the exothermic crystallization peak (118.08°C) varied significantly. Both SEM and AFM micrographs demonstrated that the surface morphology and roughness parameters (arithmetic mean [Sa ] and root mean square height [Sq ]) of the films were significantly affected by the HP treatment, which is further complemented by the FTIR spectra and XRD diffractogram. Melt rheology (175-205°C) of the pressure-treated films showed a significant drop (20-30%) in mechanical rigidity (G') when compared to the untreated sample. Changes in the microstructure/crystallinity in the PLA films were indicated by van Gurp and Palmen plot. PRACTICAL APPLICATION: The results reported here can help to improve the design of the coextruded packaging materials so that it can be successfully implemented to high-pressure processing and high pressure-assisted thermal processing of food.

Enhancing the Inactivation of Bacterial Spores during Pressure-Assisted Thermal Processing

High pressure treatments have been the best pasteurization alternative to thermal processing due its capacity to reduce microbial safety risks and increase shelf life by inactivating microorganisms and key food spoilage–causing enzymes while retaining food freshness. In spite of these advantages, an important drawback limiting a wider application of this technology is its inability to inactivate bacterial spores which are resistant to several stress conditions, including high pressure. An approach to pressure-mediated spore inactivation is to promote spore germination which reduces their resistance to inactivation treatments. However, the germination response, and thus the spore inactivation rate achieved by these treatments, is strongly dependent on the food matrix, process conditions, spore physiology factors, and also on their interactions. Statistical experimental designs, such as the use of the central composite design as an optimization tool to identify effective PATP treatments as opposed to one-factor-at-a-time experimental designs, can reveal the importance of the effect of individual and combined factors on the inactivation response. A general review of these factors and examples of agents that could lower the severity of pressure treatments required to inactivate spores is here presented including the modelling of germination as affected by these factors.

Retention of Ascorbic Acid, Retinol, β-Carotene, and α-Tocopherol in Milk Subjected to Pressure-Assisted Thermal Processing (PATP)

Thermal treatments can cause undesirable sensory quality and nutritional changes in food, whereas high-quality retention has been widely reported for high-pressure processing (HPP). The aim of this study was to assess the retention of whole-fat milk vitamins after pressure-assisted thermal processing (PATP). The retention of ascorbic acid (AA), retinol, β-carotene, and α-tocopherol was determined by HPLC after 0.5 to 10 min treatments at 46 to 75 °C and 95 to 705 MPa. AA content was particularly sensitive to PATP treatments, particularly at 705 MPa and 75 °C for 5 min, reaching a loss of 55.1 ± 0.9%. Liposoluble vitamin losses were much lower than those observed for AA. For all treatments, retinol loss remained below 10.2 ± 1.7% and it was independent of the processing times tested (0.5, 5, and 10 min) for treatments at 400 MPa and 75 °C with losses of 6.3 ± 1.3, 6.1 ± 1.5, and 8.1 ± 3.2%, respectively. For PATP treatments at 75 °C, 5 min, and 95–705 MPa, α-tocopherol losses were minor and ranging from 5.0 ± 2.1 to 6.4 ± 0.6%. These values were similar to those observed after conventional thermal pasteurization. Low β-carotene losses were also observed at 75 °C for all PATP treatments with losses of −0.6 ± 2.9 to 4.4 ± 0.5% for 95–705 MPa/5 min and 2.4 ± 0.3 to 4.9 ± 2.1% for 400 MPa/0.5–10 min treatments. This study showed that in whole-fat milk subjected to PATP treatments, AA is significantly more sensitive than retinol, β-carotene, and α-tocopherol.

Study on the changes induced by the Pressure-Assisted Thermal Processing (PATP) in polymer films used as packaging by the meat industry

The purpose of this study was to assess the changes induced by pressure-assisted thermal processing (PATP) treatment in two multilayer polymer films used as packaging material by the meat industry. The changes induced by the treatment at 600 MPa, 70 ºC for 10 minutes on the structural, mechanical and thermal properties of two multilayer polymer films (Pev-70 coextruded high barrier multilayer film with the structure PA / EVOH / PA / PE and Pob-60 combination of biaxially oriented polyamide with a co-extruded barrier film of the polyethylene / EVOH / m-polyethylene structure) were studied. Numerous changes induced by the PATP treatment on the structural, mechanical and thermal properties of the two multi-layered polymer films used as packaging materials in the meat industry have been identified. The choice of a packaging material according to the PATP treatment requires a better understanding of all the changes induced by this treatment. So, this study has preliminarily identified a part of those changes that occurs and could irreversible affect the quality of foods.

Inactivation of peroxidase and polyphenoloxidase in coconut water using pressure-assisted thermal processing

Abstract The effect of pressure assisted thermal processing (PATP) was evaluated on the inactivation kinetics of polyphenol oxidase (PPO) and peroxidase (POD) and selected quality attributes of coconut water. Coconut water from green young coconuts was treated at 200, 400 and 600 MPa, 40–90 °C, and 60–1800 s of holding time. The activities of PPO and POD were determined using spectrophotometric methods. No enzymatic activity was detected for both enzymes within 300 s at 90 °C/400–600 MPa. The combination of 400 MPa/90 °C/300 s yielded POD and PPO inactivation, and could be used in the industrial development of PATP treated-coconut water. The POD showed to be more pressure-temperature resistant than the PPO in coconut water. The pressure-temperature inactivation kinetics of PPO and POD in coconut water were well described by the Weibull model. The activation energy for the inactivation of POD and PPO were 107–192 and 41–191 kJ mol−1, respectively, while the activation volume varied from −13.2 to 10.2 and −37 to 9.2 cm3 mol−1, respectively. Total phenolic content extractability significantly increased after PATP treatments at all conditions evaluated compared to the control. Low ∆E values of PATP treated coconut water were obtained, indicating imperceptible change of color. Industrial relevance Pressure Assisted Thermal Processing (PATP) is an emerging technology that requires further research. The results of this study highlighted for the first time the potential of PATP on polyphenoloxidase and peroxidase inactivation of coconut water, maintaining color characteristics of coconut water. The pink color after PATP treatment was not observed. In addition, the use of kinetic models helped to determine the optimal conditions for enzyme inactivation. The outcomes of this study can be used for further industrial development of PATP treated-coconut water.

Study on the Influence of Pressure-Assisted Thermal Processing on PET/PE via the Change of Melting Enthalpy

The effect of Pressure-assisted thermal processing (PATP) on mechanical and barrier properties of polyethylene terephthalate (PET)/polyethylene (PE) for sausage packaging was studied. The samples were subjected to 150, 300 and 450 MPa for 10 minutes at temperatures of 25, 40 and 50C, separately. The delamination and bubbling occurred. The mechanical properties of PET/PE increased after the processing except that under 150 MPa, 50C and 450 MPa at 40C. Both tensile strength and barrier properties presented significant decrease and fluctuations under 450 MPa at 40 and 50C. The results indicated that high pressure and thermal processing might have opposite effects on PET/PE. Practical Applications Pressure-assisted thermal processing (PATP) is a combined process to inactive bacterial spores especially for low-acid foods. In 2009, the US FDA approved a petition for the commercial use of PATP for low-acid foods, which opened up opportunity for processing a variety of heat sensitive products. Study on the impact of PATP on packaging material, such as seal integrity, barrier properties to oxygen and water vapor, can ensure food quality during the shelf life.

Microbiological Design and Validation of Thermal and High Pressure Processing of Acidified Carrots and Assessment of Product Quality

Modification of pH and combined use of novel processing methods may be a good strategy to improve the quality of canned vegetables. In this study, selected thermal (TP) and high pressure-assisted thermal (HP-T) processing methods were validated for citric acid-infused carrots (pH ≤ 4.5) using Bacillus licheniformis spores. Previously established thermal inactivation kinetics data were used to setup the target process times (to achieve 7-log kill of B. licheniformis). The microbial spores were inoculated at the center of simulated carrot alginate beads and subjected to different processing methods. Delivered process lethalities, evaluated by the microbial count/re-count method and measured time–temperature data, were equal to or higher than the targeted values. No survivors were found after the treatments, demonstrating the adequacy of the processes. Texture, color and β-carotene retention in processed carrots, evaluated and compared with those processed under conventional canning, showed higher texture retention (P < 0.05) in the modified processing methods. Residual hardness values of carrots were 86% with HP-T, 70% with ohmic heating and 8% with conventionally canned product. The same trend was observed with chewiness value. However, processing methods showed no differences (P > 0.05) with respect to color change. In terms of β-carotene, carrots subjected to a relatively more severe heat treatment (water immersion mode in static retort) showed better β-carotene extractability than samples from HP-T. Practical Applications Conventional thermal processing of “low acid” foods (pH > 4.6) experiences significant quality loss due to the long thermal processing times required to inactivate spores of the key pathogen Clostridium botulinum. Alternative thermal processing techniques have evolved to shorten the processing times by enhancing heating rate to the product through process modifications or using thin profile packages. Even though quality retention can be enhanced through these modifications, thermal damage to quality is still indispensable since the sensitively microbial destruction to heat remain unaltered. Product acidification moves the “low acid” foods to “acid” category, thereby shifting the process regulation from the high temperature sterilization to the lower temperature pasteurization conditions. Savings in energy and reduction in process time immediately become obvious. Novel acidification methods are necessary for making the process efficient. Bacillus licheniformis is important and a suitable microorganism for validating thermal processing of acidified foods. The combined use of acid infusion methods and alternative thermal processing technologies could play a significant role to improve quality of acidified foods as compared to current practices.

Microbiological efficacy of pressure assisted thermal processing and natural extracts against Bacillus amyloliquefaciens spores suspended in deionized water and beef broth

Effect of aqueous extracts of pomegranate peel (PPE) and tamarind pulp (TPE) on lethality of pressure assisted thermal processing (PATP) against Bacillus amyloliquefaciens Fad 82 spores was investigated. B. amyloliquefaciens spores (≈108CFUml−1) were suspended in deionized water (DIW), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer PPE, and TPE (1% w/v in DIW, pH-6) and beef broth with or without PPE and TPE (0.45% w/v). Samples were subjected to pressure assisted thermal processing (PATP, 600MPa, 105°C), and thermal processing (TP; 0.1MPa, 105°C) for 0 (come-up time), 0.5, 1, 3, and 5min. Beef broth samples were also pressure treated (HPP; 600MPa, 35°C) for 3 and 5min. Dormant spore survivors were enumerated by spread-plate technique. Heat shock (80°C for 15min) was used to enumerate the heat sensitive spores. A 5-min PATP reduced spore survivors counts to below 1-logCFUml−1 (≈6.5–7.0log reduction) in PPE and TPE as compared to 2.5-log and 3.1-log in DIW and HEPES buffer, respectively. 3min PATP treatment reduced the spore survival to 3.58 and 2.1logCFUml−1 in beef broth and beef broth+natural extract, respectively. TP and HPP processing could inactivate the spores only by 0.5–1.2logCFUml−1. PPE and TPE are found to enhance the efficacy of PATP.

Lethality enhancement of pressure-assisted thermal processing against Bacillus amyloliquefaciens spores in low-acid media using antimicrobial compounds

Pressure-assisted thermal processing (PATP; 500–700 MPa, 90–121 °C) offers new opportunities to sterilize low-acid foods while preserving quality attributes to an extent greater than is possible with traditional thermal processing (TP). This study was conducted to evaluate the possibility of enhancing PATP lethality against the spores of Bacillus amyloliquefaciens, by sensitizing the spores with selected antimicrobial compounds (including emphasis on the use of natural antimicrobials) prior to treatment. A spore crop of B. amyloliquefaciens TMW 2.479 Fad 82, that had previously shown high resistance to combined pressure-heat treatment, was prepared on Nutrient Agar medium supplemented with 10 mg L−1 MnSO4·H2O and incubated at 32 °C for 3 d. Spores were inoculated (at ∼107–108 CFU mL−1 inoculum level) in HEPES buffer (pH ≤ 7.0) or selected low-acid foods (pH 5.2–5.6) with or without added antimicrobial compounds. The samples were then treated at 600 MPa and 105 °C (PATP) or 0.1 MPa and 105 °C (TP) for various holding times. Among different compounds tested, low-molecular-weight chitosan, and combination of chitosan with some surfactants were most effective (P < 0.05) in enhancing the PATP and TP lethality. This study suggests that certain antimicrobials can be added to the low-acid media prior to PATP or TP treatment to enhance the efficacy of the process. The treatment allows sterilization of low-acid foods at lower process temperatures thus ensuring better preservation of quality attributes.

Combined Effect of Pressure-Assisted Thermal Processing and Antioxidants on the Retention of Conjugated Linoleic Acid in Milk.

The effect of pressure-assisted thermal processing (PATP) in combination with seven synthetic antioxidants was evaluated on the retention of conjugated linoleic acid (CLA) in enriched milk. Milk rich in CLA was first saturated with oxygen, followed by the addition of either catechin, cysteine, ascorbic acid, tannic acid, gallic acid, caffeic acid or p-coumaric acid (500 mg kg−1 untreated milk). Samples were treated at 600 MPa and 120 °C up to 15 min of holding time. During PATP, CLA not only oxidized at a slower rate, but also less oxygen was consumed compared to the control (0.1 MPa and 120 °C). In addition, phenolic antioxidants were able to quench dissolved oxygen in samples treated with PATP. For those samples added with gallic acid and catechin, 85% and 75% of the CLA was retained after 15 min of holding time at 600 MPa and 120 °C, respectively. The retention of CLA was enhanced by the application of PATP in combination with gallic acid.

Kinetics of lactulose formation in milk treated with pressure-assisted thermal processing

Abstract The combined effect of temperature and pressure on the kinetics of lactulose formation was investigated in the range of 100–120 °C and 100–600 MPa up to 15 min. The kinetics data were fitted using the Weibull model and the parameters were evaluated through joint confidence regions. The scale parameter ( α ) was affected by temperature and pressure, following Arrhenius-type and Eyring-type equations, respectively. The calculated kinetic parameters, E a = 182 ± 8 kJ mol − 1 and ∆ V # = − 7.5 ± 0.1 cm 3 mol − 1 , revealed that the combination of temperature and pressure accelerated the formation of lactulose, reaching a maximum concentration of 650 mg L − 1 after 15 min of holding time at 120 °C and 600 MPa. Interestingly, lactulose contents obtained were lower after PATP treatments compared with values reported in the literature for equivalent UHT treatments. The formation of lactulose was also represented in pressure–temperature diagrams. This information can be used to determine the impact of pressure-assisted thermal processing in milk-based products. Industrial relevance The dairy industry uses the lactulose concentration as a heat damage indicator to distinguish between sterilized and pasteurized milk. Pressure-assisted thermal processing (PATP) is a relative new technology with potential applications in the dairy industry. This study provides new kinetic data on the formation of lactulose at PATP conditions. The information obtained in this study can be used to assess the impact of PATP on other shelf-stable dairy products, including functional beverages that contain milk.

Effect of Sporulation Temperature on the Resistance of Clostridium botulinum Type A Spores to Thermal and High Pressure Processing

The purpose of this study was to determine the effect of sporulation temperature on the resistance of Clostridium botulinum type A spores of strains 62A and GiorgioA to thermal and high pressure processing (HPP). Spore crops produced in Trypticase-peptone-glucose-yeast extract broth at four incubation temperatures (20, 27, 37, and 41°C) were harvested, and heat resistance studies were conducted at 105°C (strain 62A) and 100°C (strain GiorgioA). Resistance to HPP was evaluated by subjecting the spores to a high pressure (700 MPa) and temperature combination (105°C, strain 62A; 100°C strain GiorgioA) in a laboratory-scale pressure test system. The decimal reduction time (D-value) was calculated using the log-linear model. Although the time to sporulation for GiorgioA was shorter and resulted in higher spore concentrations than for 62A at 20, 27, and 37°C, GiorgioA did not produce a sufficient spore crop at 41 °C to be evaluated. The heat resistance of 62A spores was greatest when produced at 27°C and decreased for spore crops produced above or below 27°C (D105°C-values: 20°C, 1.9 min; 27°C, 4.03 min; 37°C, 3.66 min; and 41°C, 3.5 min; P < 0.05). Unlike 62A, the heat resistance behavior of GiorgioA spores increased with rising sporulation temperature, and spores formed at the organism's optimum growth temperature of 37°C were the most resistant (D100°C-values: 20°C, 3.4 min; 27°C, 5.08 min; and 37°C, 5.65 min; P < 0.05). Overall, all spore crops were less resistant to pressure-assisted thermal processing than thermal treatment alone. Sporulation temperature has an effect on the resistance of C. botulinum spores to heat and HPP, and is characteristic to a particular strain. Knowledge of the effect of sporulation temperature on the resistance of C. botulinum spores is vital for the production of spores utilized in thermal and high pressure inactivation studies.

Inactivation kinetics and injury recovery of Bacillus amyloliquefaciens spores in low-acid foods during pressure-assisted thermal processing

Pressure-assisted thermal processing (PATP) inactivation kinetics of Bacillus amyloliquefaciens spores in deionized water (DIW) were evaluated for egg patty mince (EPM) and green pea puree (GPP). Recovery of PATP-injured spores during storage was determined. The number of B. amyloliquefaciens spores in DIW was reduced more than 6 log when treated at 121°C and 700 MPa, including a come-up time reduction (3.32 log MPN/g) and a pressure holding time reduction (3.55 log MPN/g). Treatments at 700 MPa in combination with 105°C for 16 min, 115°C for 5 min, or 121°C for 3 min, decreased B. amyloliquefaciens spore populations in EPM to levels undetectable using an enrichment procedure. No significant (p>0.05) recovery of PATP-injured spores was observed in EPM and GPP during storage for 8 weeks, compared with controls. These results provide useful information for enhancing microbial lethality of PATP-resistant bacterial spores in low-acid foods.

Inactivation of Geobacillus stearothermophilus spores in low-acid foods by pressure-assisted thermal processing.

The effect of pressure-assisted thermal processing (PATP) on the inactivation of Geobacillus stearothermophilus spores was determined in deionized water, cooked ground beef, egg patty mince, whole milk and mashed potatoes at 105 °C under 500 and 700 MPa. The numbers of G. stearothermophilus spores in deionized water and milk were reduced by more than 6 log CFU mL(-1) at 700 MPa and 105 °C, whereas those in cooked beef were reduced by 4.27 log CFU g(-1). The inactivation patterns of G. stearothermophilus spores in all food matrices followed nonlinear behavior, showing that Weibull model fitted well to the inactivation curves of G. stearothermophilus spores in low-acid foods. The complex food matrices caused a protective effect on the inactivation of G. stearothermophilus spores during PATP. The results provide useful information in inactivation kinetics of bacterial spores for validating PATP-processed low-acid foods.

High-pressure Processing: Kinetic Models for Microbial and Enzyme Inactivation

High-pressure processing (HPP) has become the most widely accepted nonthermal food preservation technology. The pressure range for commercial processes is typically around 100–600 MPa, whereas moderate temperature (up to 65 °C) may be used to increase microbial and enzymatic inactivation levels. However, these industrial processing conditions are insufficient to achieve sterilization since much higher pressure levels (>1,000 MPa) would be required to inactivate bacterial endospores and enzymes of importance in food preservation. The next generation of commercial pressure processing units will operate at about 90–120 °C and 600–800 MPa for treatments defined as pressure-assisted thermal processing or pressure-assisted thermal sterilization if the commercial food sterilization level required is achieved. Most published HPP kinetic studies have focused only on pressure effects on the microbial load and enzyme activity in foods and model systems. Published work on primary and secondary models to predict simultaneously the effect of pressure and temperature on microbial and enzymatic inactivation kinetics is still incomplete. Moreover, few references provide a detailed and complete analysis of the theoretical, empirical, and semiempirical kinetic models proposed to predict the level of microbial and enzyme inactivation achieved. This review organizes these published kinetic models according to the approach used and then presents an in-depth and critical revision to define the modeling research needed to provide commercial users with the computational tools needed to develop and optimize pasteurization and sterilization pressure treatments.

Quality of shelf-stable low-acid vegetables processed using pressure–ohmic–thermal sterilization

Pressure ohmic thermal sterilization (POTS) involves ohmic heating of foods by applying an electric field under elevated pressure. Experiments were conducted using a custom made POTS apparatus using carrot as the model food. The samples were processed at a target temperature of 105 °C at 600 MPa and treated for 0, 1, 3, and 5 min by regulating the electric field (30 V/cm). POTS treated carrot sample quality attributes were compared against those processed using ohmic heating (0.12 MPa, 30 V/cm, 105 °C) and pressure-assisted thermal processing (PATP; 600 MPa, 0 V/cm, 105 °C). Within the experimental conditions of the study, the thermal come-up time of the POTS, Ohmic and PATP were 1.45 min, 3.58 min and 6.84 min respectively. PATP come-up time also included the pre-heating time (≈6 min). Results indicate the POTS samples had the least textural damage due to minimal thermal exposure. POTS samples had higher crunchiness index (CI) of 0.76 compared to PATP (CI = 0.57) and ohmic heated (CI = 0.62) carrots. All the technologies investigated had minimal impact on product color. This study demonstrates the potential to produce high quality shelf stable low-acid vegetable products using POTS.