Combined pressure-thermal inactivation kinetics of Bacillus amyloliquefaciens spores in egg patty mince

Synergistic Effects of Nisin, Lysozyme, Lactic Acid, and CitricidalTM for Enhancing Pressure-Based Inactivation of Bacillus amyloliquefaciens, Geobacillus stearothermophilus, and Bacillus atrophaeus Endospores

The inactivation of bacterial endospores continues to be the main curtailment for further adoption of high-pressure processing in intrastate, interstate, and global food commerce. The current study investigated the effects of elevated hydrostatic pressure for the inactivation of endospore suspension of three indicator spore-forming bacteria of concern to the food industry. Additionally, the effects of four bacteriocin/bactericidal compounds were studied for augmenting the decontamination efficacy of the treatment. Elevated hydrostatic pressure at 650 MPa and at 50 °C was applied for 0 min (untreated control) and for 3, 7, and 11 min with and without 50K IU of nisin, 224 mg/L lysozyme, 1% lactic acid, and 1% Citricidal^TM. The results were statistically analyzed using Tukey- and Dunnett’s-adjusted ANOVA. Under the condition of our experiments, we observed that a well-designed pressure treatment synergized with mild heat and bacteriocin/bactericidal compounds could reduce up to >4 logs CFU/mL (i.e., >99.99%) of bacterial endospores. Additions of nisin and lysozyme were able, to a great extent, to augment (p < 0.05) the decontamination efficacy of pressure-based treatments against Bacillus amyloliquefaciens and Bacillus atrophaeus, while exhibiting no added benefit (p ≥ 0.05) for reducing endospores of Geobacillus stearothermophilus. The addition of lactic acid, however, was efficacious for augmenting the pressure-based reduction of bacterial endospores of the three microorganisms.

Bacterial diversity in six species of fresh edible seaweeds submitted to high pressure processing and long-term refrigerated storage

Seaweeds are highly perishable foods due to their richness in nutrients. High pressure processing (HPP) has been applied for extending the shelf life of fresh seaweeds but there is no information on the effect of HPP on the bacterial diversity of seaweeds. The culturable bacteria of six species of fresh edible seaweeds (green seaweeds Codium fragile and Ulva lactuca, brown seaweeds Himanthalia elongata, Laminaria ochroleuca and Undaria pinnatifida, and red seaweed Chondrus crispus) were investigated and compared to those of HPP-treated (400 and 600 MPa for 5 min) seaweeds, at the start and end of their refrigerated storage period. A total of 523 and 506 bacterial isolates were respectively retrieved from untreated and HPP-treated seaweeds. Isolates from untreated seaweeds belonged to 18 orders, 35 families, 71 genera and 135 species whereas isolates from HPP-treated seaweeds belonged to 13 orders, 23 families, 43 genera and 103 species. HPP treatment significantly reduced the number of isolates belonging to 6 families and greatly increased the number of Bacillaceae isolates. At the end of storage, decreases in bacterial diversity at the genus and species level were observed for untreated as well as for HPP-treated seaweeds.

Inactivation of non-proteolytic Clostridium botulinum type E in low-acid foods and phosphate buffer by heat and pressure

The effect of high pressure thermal (HPT) treatments on the inactivation of spores of non-proteolytic type E Clostridium botulinum TMW 2.990 was investigated at high pressures (300 to 600 MPa) and elevated temperatures (80 to 100 °C) in four low-acid foods (steamed sole, green peas with ham, vegetable soup, braised veal) and imidazole phosphate buffer (IPB). In addition, corresponding conventional thermal treatments at ambient pressure were performed to expose possible synergisms of pressure and temperature on spore inactivation. In general, spore count reduction was more efficient by combining pressure and temperatures < 100 °C and the overall process duration could be shortened due to accelerated heating rates (adiabatic effect). Processing at 90 °C and 600 MPa resulted in inactivation below the detection limit after 5 min in all foods except steamed sole. Traditional thermal processing of spores at 90 °C for 10 min, on the other hand, did not result in an estimated 6-log reduction. Additional HPT treatments in steamed sole and IPB did not reveal pronounced food matrix dependent protective effects. Here, varying pressure levels did not appear to be the driving force for spore count reduction in steamed sole at any temperature. By applying a Weibull distribution on destruction kinetics of isobaric/isothermal holding times, 6D-values were calculated. Compression and decompression phase (1 s pressure holding time) had a considerable impact on spore count reduction (max. -2.9 log units) in both, foods and buffer. Hence, compression and decompression phases should directly be included into the total lethal effect of HPT treatments to avoid prolonged holding times and overprocessing.

Application of High Pressure with Homogenization, Temperature, Carbon Dioxide, and Cold Plasma for the Inactivation of Bacterial Spores: A Review

Formation of highly resistant spores is a concern for the safety of low-acid foods as they are a perfect vehicle for food spoilage and/or human infection. For spore inactivation, the strategy usually applied in the food industry is the intensification of traditional preservation methods to sterilization levels, which is often accompanied by decreases of nutritional and sensory properties. In order to overcome these unwanted side effects in food products, novel and emerging sterilization technologies are being developed, such as pressure-assisted thermal sterilization, high-pressure carbon dioxide, high-pressure homogenization, and cold plasma. In this review, the application of these emergent technologies is discussed, in order to understand the effects on bacterial spores and their inactivation and thus ensure food safety of low-acid foods. In general, the application of these novel technologies for inactivating spores is showing promising results. However, it is important to note that each technique has specific features that can be more suitable for a particular type of product. Thus, the most appropriate sterilization method for each product (and target microorganisms) should be assessed and carefully selected.

Selection of indigenous indicator micro-organisms for validating desiccation-adapted Salmonella reduction in physically heat-treated poultry litter

AIMS

The thermal resistance of desiccation-adapted Salmonella Senftenberg 775/W was compared with those of indigenous enterococci and total aerobic bacteria in poultry litter.

METHODS AND RESULTS

Aged broiler litter and composted turkey litter with 20, 30, 40 and 50% moisture contents were inoculated with desiccation-adapted Salm. Senftenberg 775/W, and then heat-treated at 75 and 85°C. Compared to total aerobic bacteria, there were better correlations between mean log reductions of desiccation-adapted Salm. Senftenberg 775/W and indigenous enterococci in broiler litter samples with 20, 30, 40 and 50% moisture contents at 75°C (R²  > 0·91), and 20, 30 and 40% moisture contents at 85°C (R²  > 0·87). The mean log reductions of Salm. Senftenberg 775/W were better correlated with those of indigenous enterococci in turkey litter samples with 20, 30, 40 and 50% moisture contents at 75°C (R²  > 0·88), and 20 and 30% moisture contents at 85°C (R²  = 0·83) than those of total aerobic bacteria, which had a better correlation in turkey litter sample with 40% (R²  = 0·98) moisture content at 85°C.

CONCLUSION

Indigenous enterococci may be used to validate the thermal processing of poultry litter, as it predicts the survival behaviour of Salmonella under some treatment conditions.

SIGNIFICANCE AND THE IMPACT OF THE STUDY

This study provides some scientific data for poultry litter processors when validating the effectiveness of thermal processing.

High-Pressure Processing of Broccoli Sprouts: Influence on Bioactivation of Glucosinolates to Isothiocyanates

Effects of high-pressure processing (HPP, 100-600 MPa for 3 min at 30 °C) on the glucosinolate content, conversion to isothiocyanates, and color changes during storage in fresh broccoli sprouts were investigated. A mild heat treatment (60 °C) and boiling (100 °C) were used as positive and negative controls, respectively. Glucosinolates were quantified using liquid chromatography-mass spectrometry, and isothiocyanates were quantified using high-performance liquid chromatography-photodiode array detection. A formation of isothiocyanates was observed in all high-pressure-treated sprouts. The highest degree of conversion (85%) was observed after the 600 MPa treatment. Increased isothiocyanate formation at 400-600 MPa suggests an inactivation of the epithiospecifier protein. During storage, color changed from green to brownish, reflected by increasing a* values and decreasing L* values. This effect was less pronounced for sprouts treated at 100 and 600 MPa, indicating an influence on the responsible enzymes. In summary, HPP had no negative effects on the glucosinolate-myrosinase system in broccoli sprouts.

Selection of Surrogate Bacteria for Use in Food Safety Challenge Studies: A Review

Nonpathogenic surrogate bacteria are prevalently used in a variety of food challenge studies in place of foodborne pathogens such as Listeria monocytogenes, Salmonella, Escherichia coli O157:H7, and Clostridium botulinum because of safety and sanitary concerns. Surrogate bacteria should have growth characteristics and/or inactivation kinetics similar to those of target pathogens under given conditions in challenge studies. It is of great importance to carefully select and validate potential surrogate bacteria when verifying microbial inactivation processes. A validated surrogate responds similar to the targeted pathogen when tested for inactivation kinetics, growth parameters, or survivability under given conditions in agreement with appropriate statistical analyses. However, a considerable number of food studies involving putative surrogate bacteria lack convincing validation sources or adequate validation processes. Most of the validation information for surrogates in these studies is anecdotal and has been collected from previous publications but may not be sufficient for given conditions in the study at hand. This review is limited to an overview of select studies and discussion of the general criteria and approaches for selecting potential surrogate bacteria under given conditions. The review also includes a list of documented bacterial pathogen surrogates and their corresponding food products and treatments to provide guidance for future studies.

Thermal Inactivation Kinetics of Sporolactobacillus nakayamae Spores, a Spoilage Bacterium Isolated from a Model Mashed Potato-Scallion Mixture

Sporolactobacillus species have been occasionally isolated from spoiled foods and environmental sources. Thus, food processors should be aware of their potential presence and characteristics. In this study, the heat resistance and influence of the growth and recovery media on apparent heat resistance of Sporolactobacillus nakayamae spores were studied and described mathematically. For each medium, survivor curves and thermal death curves were generated for different treatment times (0 to 25 min) at different temperatures (70, 75, and 80°C) and Weibull and first-order models were compared. Thermal inactivation data for S. nakayamae spores varied widely depending on the media formulations used, with glucose yeast peptone consistently yielding the highest D-values for the three temperatures tested. For this same medium, the D-values ranged from 25.24 ± 1.57 to 3.45 ± 0.27 min for the first-order model and from 24.18 ± 0.62 to 3.50 ± 0.24 min for the Weibull model at 70 and 80°C, respectively. The z-values determined for S. nakayamae spores were 11.91 ± 0.29°C for the Weibull model and 11.58 ± 0.43°C for the first-order model. The calculated activation energy was 200.5 ± 7.3 kJ/mol for the first-order model and 192.8 ± 22.1 kJ/mol for the Weibull model. The Weibull model consistently produced the best fit for all the survival curves. This study provides novel and precise information on thermal inactivation kinetics of S. nakayamae spores that will enable reliable thermal process calculations for eliminating this spoilage bacterium.

Pressure-Based Strategy for the Inactivation of Spores

Since the first application of high hydrostatic pressure (HHP) for food preservation more than 100 years ago, a wealth of knowledge has been gained on molecular mechanisms underlying the HHP-mediated destruction of microorganisms. However, one observation made back then is still valid, i.e. that HHP alone is not sufficient for the complete inactivation of bacterial endospores. To achieve "commercial sterility" of low-acid foods, i.e. inactivation of spores capable of growing in a specific product under typical storage conditions, a combination of HHP with other hurdles is required (most effectively with heat (HPT)). Although HPT processes are not yet industrially applied, continuous technical progress and increasing consumer demand for minimally processed, additive-free food with long shelf life, makes HPT sterilization a promising alternative to thermal processing.In recent years, considerable progress has been made in understanding the response of spores of the model organism B. subtilis to HPT treatments and detailed insights into some basic mechanisms in Clostridium species shed new light on differences in the HPT-mediated inactivation of Bacillus and Clostridium spores. In this chapter, current knowledge on sporulation and germination processes, which presents the basis for understanding development and loss of the extreme resistance properties of spores, is summarized highlighting commonalities and differences between Bacillus and Clostridium species. In this context, the effect of HPT treatments on spores, inactivation mechanism and kinetics, the role of population heterogeneity, and influence factors on the results of inactivation studies are discussed.

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

BACKGROUND

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.

RESULTS

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.

CONCLUSION

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.

Combined high pressure and thermal processing on inactivation of type E and nonproteolytic type B and F spores of Clostridium botulinum

The aim of this study was to determine the resistance of multiple strains of the three nonproteolytic types of Clostridium botulinum (seven strains of type E, eight of type B, and two of type F) spores exposed to combined high pressure and thermal processing. The resistance of spores suspended in N-(2-acetamido)-2-aminoethanesulfonic acid (ACES) buffer (0.05 M, pH 7) was determined at a process temperature of 80°C with high pressures of 600, 650, and 700 MPa using a laboratory-scale pressure test system. Spores of C. botulinum serotype E strains demonstrated less resistance than nonproteolytic spores of type B or F strains when processed at 80°C and 600 MPa for up to 15 min. All C. botulinum type E strains were reduced by . 6.0 log units within 5 min under these conditions. Among the nonproteolytic type B strains, KAP 9-B was the most resistant, resulting in reductions of 2.7, 5.3, and 5.5 log, coinciding with D-values of 7.7, 3.4, and 1.8 min at 80°C and 600, 650, and 700 MPa, respectively. Of the two nonproteolytic type F strains, 610F was the most resistant, showing 2.6-, 4.5-, and 5.3-log reductions with D-values of 8.9, 4.3, and 1.8 min at 80°C and 600, 650, and 700 MPa, respectively. Pulsed-field gel electrophoresis was performed to examine the genetic relatedness of strains tested and to determine if strains with similar banding patterns also exhibited similar D-values. No correlation between the genetic fingerprint of a particular strain and its resistance to high pressure processing was observed.

Effect of high hydrostatic pressure processing on microbiological shelf-life and quality of fruits pretreated with ascorbic acid or SnCl2

In the current study, the processing conditions required for the inactivation of Paenibacillus polymyxa and relevant spoilage microorganisms by high hydrostatic pressure (HHP) treatment on apricot, peach, and pear pieces in sucrose (22°Brix) solution were assessed. Accordingly, the shelf-life was determined by evaluating both the microbiological quality and the sensory characteristics (taste, odor, color, and texture) during refrigerated storage after HHP treatment. The microbiological shelf-life of apricots, peaches, and pears was prolonged in the HHP-treated products in comparison with the untreated ones. In all HHP-treated packages for apricots, peaches, and pears, all populations were below the detection limit of the method (1 log CFU/g) and no growth of microorganisms was observed until the end of storage. Overall, no differences of the L*, a*, or b* value among the untreated and the HHP-treated fruit products were observed up to the time at which the unpressurized product was characterized as spoiled. HHP treatment had no remarkable effect on the firmness of the apricots, peaches, and pears. With regard to the sensory assessment, the panelists marked better scores to HHP-treated products compared to their respective controls, according to taste and total evaluation during storage of fruit products.

Advanced retorting, microwave assisted thermal sterilization (MATS), and pressure assisted thermal sterilization (PATS) to process meat products

Conventional thermal processes have been very reliable in offering safe sterilized meat products, but some of those products are of questionable overall quality. Flavor, aroma, and texture, among other attributes, are significantly affected during such processes. To improve those quality attributes, alternative approaches to sterilizing meat and meat products have been explored in the last few years. Most of the new strategies for sterilizing meat products rely on using thermal approaches, but in a more efficient way than in conventional methods. Some of these emerging technologies have proven to be reliable and have been formally approved by regulatory agencies such as the FDA. Additional work needs to be done in order for these technologies to be fully adopted by the food industry and to optimize their use. Some of these emerging technologies for sterilizing meat include pressure assisted thermal sterilization (PATS), microwaves, and advanced retorting. This review deals with fundamental and applied aspects of these new and very promising approaches to sterilization of meat products.

Screening foods for processing-resistant bacterial spores and characterization of a pressure- and heat-resistant Bacillus licheniformis isolate

This study was carried out to isolate pressure- and heat-resistant indicator spores from selected food matrices (black pepper, red pepper, garlic, and potato peel). Food samples were processed under various thermal (90 to 105°C) and pressure (700 MPa) combination conditions, and surviving microorganisms were isolated. An isolate from red pepper powder, Bacillus licheniformis, was highly resistant to pressure-thermal treatments. Spores of the isolate in deionized water were subjected to the combination treatments of pressure (0.1 to 700 MPa) and heat (90 to 121°C). Compared with the thermal treatment, the combined pressure-thermal treatments considerably reduced the numbers of B. licheniformis spores to less than 1.0 log CFU/g at 700 MPa plus 105°C and at 300 to 700 MPa plus 121°C. The inactivation kinetic parameters of the isolated B. licheniformis spores were estimated using linear and nonlinear models. Within the range of the experimental conditions tested, the pressure sensitivity (zP) of the spores decreased with increasing temperature (up to 121°C), and the temperature sensitivity (zT) was maximum at atmospheric pressure (0.1 MPa). These results will be useful for developing a combined pressure-thermal inactivation kinetics database for various bacterial spores.

Combined high pressure and thermal processing on inactivation of type A and proteolytic type B spores of Clostridium botulinum

The aim of this study was to determine the resistance of multiple strains of Clostridium botulinum type A and proteolytic type B spores exposed to combined high pressure and thermal processing and compare their resistance with Clostridium sporogenes PA3679 and Bacillus amyloliquefaciens TMW-2.479-Fad-82 spores. The resistance of spores suspended in N-(2acetamido)-2-aminoethanesulfonic acid (ACES) buffer (0.05 M, pH 7.0) was determined at a process temperature of 105°C, with high pressures of 600, 700, and 750 MPa by using a laboratory-scale pressure test system. No surviving spores of the proteolytic B strains were detected after processing at 105°C and 700 MPa for 6 min. A . 7-log reduction of B. amyloliquefaciens spores was observed when processed for 4 min at 105°C and 700 MPa. D-values at 105°C and 700 MPa for type A strains ranged from 0.57 to 2.28 min. C. sporogenes PA3679 had a D-value of 1.48 min at 105°C and 700 MPa. Spores of the six type A strains with high D-values along with C. sporogenes PA3679 and B. amyloliquefaciens were further evaluated for their pressure resistance at pressures 600 and 750 MPa at 105°C. As the process pressure increased from 600 to 750 MPa at 105°C, D-values of some C. botulinum strains and C. sporogenes PA3679 spores decreased (i.e., 69-A, 1.91 to 1.33 min and PA3679, 2.35 to 1.29 min). Some C. botulinum type A strains were more resistant than C. sporogenes PA3679 and B. amyloliquefaciens to combined high pressure and heat, based on D-values determined at 105°C. Pulsed-field gel electrophoresis (PFGE) was also performed to establish whether strains with a similar restriction banding pattern also exhibited similar D-values. However, no correlation between the genomic background of a strain and its resistance to high pressure processing was observed, based on PFGE analysis. Spores of proteolytic type B strains of C. botulinum were less resistant to combined high pressure and heat (700 MPa and 105°C) treatment when compared with spores of type A strains.

Kinetics of Bacillus cereus spore inactivation in cooked rice by combined pressure-heat treatment

The efficacy of pressure-heat treatment was evaluated for the inactivation of Bacillus cereus spores in cooked rice. The spores of B. cereus ATCC 9818 were inoculated (1.1 × 10(8) CFU/g) in a parboiled rice product (pH 6.0, water activity of 0.95) and inactivated to an undetectable level (<10 CFU/g) by treatment of 600 MPa and process temperatures of 60 to 85 °C or 0.1 MPa and 85 °C. Kinetic inactivation parameters were estimated with linear and nonlinear models. The potential recovery of injured bacteria was also evaluated during storage of the treated product for 4 weeks at 4 and 25 °C. Depending on the process temperature, a 600-MPa treatment inactivated spores by 2.2 to 3.4 log during the 30-s pressure come-up time, and to below the detection limit after 4- to 8-min pressure-holding times. In contrast, a 180-min treatment time was required to inactivate the spores to an undetectable level at 0.1 MPa and 85 °C. The decimal reduction time of spores inactivated by combined pressure-heat treatment ranged from 1.08 to 2.36 min, while it was 34.6 min at 85 °C under atmospheric conditions. The nonlinear Weibull model scale factor increased, and was inversely related to the decimal reduction time, and the shape factor decreased with increasing pressure or temperature. The recovery of injured spores was influenced by the extent of pressure-holding time and process temperature. This study suggests that combined pressure-heat treatment could be used as a viable alternative to inactivate B. cereus spores in cooked rice and extend the shelf life of the product.

Effect of packaging systems and pressure fluids on inactivation of Clostridium botulinum spores by combined high pressure and thermal processing

Several studies have been published on the inactivation of bacterial spores by using high pressure processing in combination with heat. None of the studies investigated the effect of the packaging system or the pressurizing fluid on spore inactivation. The objective of this study was to select and validate an appropriate packaging system and pressure transfer fluid for inactivation of Clostridium botulinum spores by using high pressure processing in combination with thermal processing. Inactivation of spores packaged in three packaging systems (plastic pouches, cryovials, and transfer pipettes) was measured in two pressure test systems (laboratory-scale and pilot-scale) at 700 MPa and >105°C. Total destruction (>6.6-log reduction) of the spores packaged in the graduated tube part of transfer pipettes was obtained after processing for up to 10 min at 118°C and 700 MPa in both pressure test systems, compared with the spores packaged either in plastic pouches or cryovials. Reduction of spores packaged in plastic pouches was lowest (<4.8 log) for both pressure test systems when processed at the same conditions (i.e., 700 MPa and 118°C). Within the pilot-scale pressure system, increasing the process temperature from 118 to 121°C at 700 MPa for 10 min resulted in only a small increase in spore reduction (<5.1 log) for spores packaged in plastic pouches, whereas there were no recoverable spores for either of the other two packaging systems. Use of plastic pouches for packaging spores in inactivation kinetic studies could lead to erroneous conclusions about the effect of high pressure in combination with heat. BioGlycol is the pressure-heat transfer fluid of choice, as compared with Duratherm oil, to maximize the temperature response rate during pressurization within the laboratory-scale pressure test system.