Inactivation of Bacillus cereus spores using a combined treatment of UV-TiO2 photocatalysis and high hydrostatic pressure

Bacillus cereus spores are resistant to high hydrostatic pressure (HHP) processing treatment. A combination of UV-TiO₂ photocatalysis (UVTP for 10 min) and two cycles of 600 MPa HHP treatment for 10 min for the first cycle and 1 min for the second cycle (UVTP-2HHP) at ambient temperature was applied to inactivate B. cereus spores inoculated on a solidified agar matrix (SAM) used as a model matrix. Two cycles of HHP treatment were used as a strategy for induction of spore germination, followed by inactivation. UVTP and 2 cycles of HHP resulted in a 5.0-log CFU/cm² spore reduction (initial spore count was 6.6 log CFU/cm²), including an approximate 0.8-log CFU/cm² reduction due to a synergistic effect. The inactivation mechanism of UVTP pretreatment was related to lipid peroxidation of the spore membrane based on the level of malondialdehyde (MDA) making spores susceptible to the HHP treatment. Flow cytometry and transmission electron microscopic (TEM) analyses showed severe physiological alteration and structural damage to spores after the combined treatment. UVTP and 2 cycles of HHP showed potential for effective inactivation of B. cereus to ensure food safety from B. cereus spores on food products.Practical applicationsInactivation of bacterial spores remains a technical challenge for HHP and other interventions because spores are highly resistant to high pressure. Pretreatment with UVTP followed by two cycles of HHP resulted in reduction in B. cereus spores due to a synergistic effect. This hurdle technology of UVTP and HHP can help food industry in ensuring food safety against the Bacillus spores.     

Effect of high hydrostatic pressure on the viability of Streptococcus thermophilus bacteriophages isolated from cheese

The effect of high hydrostatic pressure (HHP) on the inactivation of Streptococcus thermophilus bacteriophages ϕAbc2, ALQ13.2 and DT1 was investigated. Reductions in viability were proportional to pressure (400–600 MPa) and holding time (0–30 min), though the inactivation extent was phage-dependent. HHP treatment at 600 MPa for less than 5 min was sufficient to completely inactivate phages ϕAbc2 and DT1. However, ALQ13.2 only suffered a 3.7-log reduction which showed markedly higher resistance than ϕAbc2 and DT1. Weibull model was applied to describe the inactivation behavior of phages. Regression coefficients (R²), root mean square error (RMSE) and residual plots strongly suggested that Weibull model had a good fit to the data. Additionally, the Weibull models were simplified with a binary model since b values had a linear relationship at 400–600 MPa. The simplified Weibull model provided reasonable predictions of survival data at different pressures for ϕAbc2 and DT1.Industrial relevancePhage infection remains today the most serious and common problem in the manufacture of fermented cheese products. In particular, some S. thermophilus bacteriophages exhibited high thermal resistance, high burst size values and remarkably short latent periods. The use of alternative preservation techniques such as high hydrostatic pressure processing is an efficient method to improve cheese quality and achieve bacteriophages safety. The results showed HHP treatment was capable for complete inactivation of S. thermophilus bacteriophages. The lethal kinetics of S. thermophilus bacteriophages using the Weibull model could help the application of HHP in cheese-making industry.     

Influence of high hydrostatic pressure processing on physicochemical characteristics of a fermented pomegranate (Punica granatum L.) beverage

The effect of high hydrostatic pressure at 500 MPa/10 min (HHP1), 550 MPa/10 min (HHP2) and 600 MPa/5 min (HHP3) on the microbiological, physicochemical, antioxidant and sensory characteristics of a fermented pomegranate (FP) beverage, stored for 42 days (4 ± 1 °C), was evaluated. The FP beverage was also pasteurized at 63 °C/30 min (VAT) and 72 °C/15 s (HTST). The high hydrostatic pressure (HHP) and VAT pasteurized beverages did not show microbial growth (<10 CFU/mL) throughout 42 days of storage. The physicochemical characteristics were not affected (p > 0.05) by HHP or pasteurization. Color of the samples showed significant differences (p ≤ 0.05) in all HHP processed and pasteurized beverages. Antioxidant activity, total phenolic compounds, flavonoids and anthocyanins increased slightly after HHP processing. Antioxidants decreased throughout the storage in all treatments. Both HHP processed and pasteurized beverages were well accepted by average consumers when evaluated using a 9-points hedonic scale.Industrial relevanceThe high hydrostatic pressure (HHP) improves the microbiological, antioxidant and sensorial stability of fermented pomegranate beverages during storage. The HHP is more common for processing fruit juices than for fermented beverages; therefore, it can be expanded to the fermented beverages industry, which could modify the today usual thermal processing methods and, or the addition of preservatives, that are not natural, for delivering high quality and healthier pomegranate fermented beverages to consumers.     

Model for Listeria monocytogenes inactivation on dry-cured ham by high hydrostatic pressure processing

The aim of the work was to develop and validate a model of the inactivation of Listeria monocytogenes on dry-cured ham by high hydrostatic pressure (HHP) processing, as a function of the technological parameters: intensity, length and fluid temperature. Dry-cured ham inoculated with L. monocytogenes was treated at different HHP conditions (at 347-852 MPa; for 2.3 to 15.75 min; at 7.6 to 24.4 °C) following a central composite design. Bacterial inactivation was assessed in terms of logarithmic reductions of L. monocytogenes counts on selective media. According to the best fitting and most significant polynomial equation, pressure and time were the most important factors determining the inactivation extent. The significance of the quadratic term of pressure and time indicated that little effect was observed below 450 MPa, whereas holding time longer than 10 min did not result in a meaningful reduction of L. monocytogenes counts. Temperature did not show significant influence at the range assayed. The model was validated with results obtained from further experiments and bibliographical data within the range of the experimental domain. The accuracy factor and bias factor were within the proposed acceptable values indicating the suitability of the model for predictive purposes, such as prediction of the process criteria to meet the Food Safety Objectives. The results of this work may help food processors to select optimum processing conditions of HHP.     

High hydrostatic pressure processing on microstructure of probiotic low-fat yogurt

The effect of milk processing on the microstructure of probiotic low-fat yogurt was studied. Skim milk fortified with skim milk powder was subjected to three treatments prior to innoculation: thermal treatment at 85 °C for 30 min, high hydrostatic pressure at 676 MPa for 5 min, and combined treatments of high hydrostatic pressure (HHP) and heat. The processed milk was then fermented by using two different starter cultures containing Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus acidophilus, and Bifidobacterium longum. The microstructure of heat-treated milk yogurt had fewer interconnected chains of irregularly shaped casein micelles, forming a network that enclosed the void spaces. On the other hand, microstructure of HHP yogurt had more interconnected clusters of densely aggregated protein of reduced particle size, with an appearance more spherical in shape, exhibiting a smoother more regular surface and presenting more uniform size distribution. The combined HHP and heat milk treatments led to compact yogurt gels with increasingly larger casein micelle clusters interspaced by void spaces, and exhibited a high degree of cross-linking. The rounded micelles tended to fuse and form small irregular aggregates in association with clumps of dense amorphous material, which resulted in improved gel texture and viscosity.     

Effect of Compression and Decompression Rates of High Hydrostatic Pressure on Inactivation of Staphylococcus aureus in Different Matrices

Staphylococcus aureus ATCC6538 was inoculated in skimmed milk, orange juice, and Tris buffer samples. Inoculated samples were subjected to high hydrostatic pressure (HHP) treatments at 700 MPa for 5 min at 4 °C starting temperature with fast, medium, and slow rates of compression and decompression. The objective of this study was to determine the effects of changing rates of compression and decompression on inactivation of S. aureus during HHP processing. Immediate effect of different HHP treatments was not significantly different. However, during subsequent storage in refrigeration, highest microbial inactivation was the result of treatments with fast compression and slow decompression rates in all matrices.     

INACTIVATION KINETICS OF LIPOXYGENASE IN PRESSURIZED RAW SOYMILK AND SOYMILK FROM HIGH-PRESSURE TREATED SOYBEANS

High hydrostatic pressure (HHP) treatment (275, 345, 483, 550 and 620 MPa), combined with thermal treatment (19, 50, 65, 70 and 80C), was applied for selected times to raw soymilk and to previously soaked soybeans before preparation of soymilk. Lipoxygenase (LOX) activity in HHP-treated raw soymilk was described using a first-order kinetics model. At low or moderate pressures, alone or in combination with thermal treatment, LOX activity exhibited high stability. Soymilks treated at 550 MPa exhibited high LOX inactivation rate constant values. Temperature dependence on the inactivation rate constants of soymilk treated at 550 MPa indicated that high activation energy was required to inactivate the enzyme in soymilk from HHP-treated soybeans compared with that for HHP-treated raw soymilk. Lipoxygenase activity was not detected in soymilk from HHP (620 MPa and 80C) treated soybeans.

PRACTICAL APPLICATIONS

Soybean processed products have to have the minimum amount of antinutritional factors. Soymilk has to be pasteurized to deliver a soy product with low microbial load and enzymes to avoid deterioration during storage. The traditional process to obtain this type of soy product is using thermal treatment; however, this treatment can change nutritional and sensory characteristics. An alternative approach to process soymilk is using high hydrostatic pressure treatments, alone or in combination with thermal processing, to deliver milk with fresh like sensory characteristics free from pathogens and deteriorative enzymes. Lipoxygenase could deteriorate soymilk fat to produce chemicals and off-flavor.     

A combination of TiO2–UV photocatalysis and high hydrostatic pressure to inactivate Bacillus cereus in freshly squeezed Angelica keiskei juice

High hydrostatic pressure (HHP) is an effective nonthermal food processing method for microbial inactivation with minimal change to sensory and nutritional values. However, the resistance of endospores to high pressure hinders application of HHP to freshly squeezed vegetable juice, especially from leafy vegetables that are susceptible to contamination of soil bacteria, such as endospore-forming Bacillus cereus. A combination of TiO₂–UV photocatalysis and HHP was used in the processing of freshly squeezed Angelica keiskei juice, and inactivation of naturally occurring microbes, especially B. cereus, was investigated. Yeasts and molds, coliform bacteria, and Pseudomonas were inactivated by HHP to levels below the detection limit, but B. cereus survived HHP and grew during 8 days of subsequent refrigerated storage (4 °C). However, no colonies of yeasts and molds, coliform bacteria, Pseudomonas, or B. cereus were detected in A. keiskei juice after processing with a combination of TiO₂–UV photocatalysis and HHP. Although B. cereus in juice processed with the combination treatment recovered and grew to 2.02 log CFU/mL on day 6 of storage, this level was less than the population of B. cereus in unprocessed Angelica keiskei juice immediately after squeezing.     

Evaluation of the technological properties of rice starch modified by high hydrostatic pressure (HHP)

This aim of the study was to evaluate the technological properties of rice starch modified by high hydrostatic pressure (HHP). Black rice starch (BRS) was dispersed in 20% water and then HHP was applied at pressures of 200, 400 and 600 MPa for 30 min, where morphological, structural, functional and thermal parameters were evaluated. High pressure (BRS600) provided greater morphological damage, such as surface cavities and loss of crystallinity. The treatment HHP > 400 MPa the type of diffraction pattern was changed from type A to type V. The FT-IR spectra showed differences in intensity, especially for control, which revealed better defined peaks of greater intensity. The modified starch showed a greater affinity for water and oil absorption than the native starch as well as for milk absorption, exhibiting a higher binding capacity for the whole milk. HHP treatment is a fast and efficient non-thermal method to improve the technological properties of BRS.     

A combined treatment of UV-assisted TiO2 photocatalysis and high hydrostatic pressure to inactivate internalized murine norovirus

Human norovirus (HuNoV) is a major cause of foodborne illness associated with shellfish consumption. A solidified agar matrix (SAM) was experimentally prepared using agar solution for inactivation of murine norovirus (MNV-1) as a surrogate for HuNoV in a simulation model approach. MNV-1 was injected inside the SAM for virus internalization, and the effects of single and combined UV-assisted TiO₂ photocatalysis (UVTP) and high hydrostatic pressure (HHP) treatments were determined. The internalized MNV-1 were reduced by 2.9-log₁₀ and 3.5-log₁₀, respectively, after single treatments of UVTP (4.5 mW/cm², 10 min) and HHP (500 MPa, 5 min, ambient temperature). However, the internalized MNV-1 was reduced by 5.5-log₁₀ (below the detection limit) when UVTP was followed by HHP, indicating a synergistic inactivation effect. Analysis of viral morphology, proteins, and genomic RNA allowed elucidation of mechanisms involved in the synergistic antiviral activity of combined treatments, which appeared to disrupt the MNV-1 structure and damage both the capsid protein and genomic RNA.Industrial relevanceHHP treatment of raw oysters has proved commercially successful, but there is a less evidence available regarding the potential of HHP for inactivation of localized viruses present inside foods. A sequential combination of UV-assisted TiO₂ photocatalysis (UVTP) and high hydrostatic pressure (HHP) achieved significantly higher inactivation of localized virus compared to individual treatments due to a synergistic mechanism. An experimentally prepared model food system was found useful to simulate foods with morphological variations and unpredictable viral internalization patterns. This UVTP-HHP combined treatment for inactivation of localized MNV-1 can be useful for disinfection of raw oysters and other similar foods.     

Microbial and Sensory Effects of Combined High Hydrostatic Pressure and Dense Phase Carbon Dioxide Process on Feijoa Puree

High hydrostatic pressure (HHP) is used for microbial inactivation in foods. Addition of carbon dioxide (CO₂) to HHP can improve microbial and enzyme inactivation. This study investigated microbial effects of combined HHP and CO₂ on Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae, and evaluated sensory attributes of treated feijoa fruit puree (pH 3.2). Microorganisms in their growth media and feijoa puree were treated with HHP alone (HHP), or saturated with CO₂ at 1 atm (HHPcarb), or 0.4%w/w of CO₂ was injected into the package (HHPcarb+CO₂). Microbial samples were processed at 200 to 400 MPa, 25 °C, 2 to 6 min. Feijoa samples were processed at 600 MPa, 20 °C, 5 min, then served with and without added sucrose (10%w/w). Treated samples were analyzed for microbial viability and sensory evaluation. Addition of CO₂ enhanced microbial inactivation of HHP from 1.7‐log to 4.3‐log reduction in E. coli at 400 MPa, 4 min, and reduction of >6.5 logs in B. subtilis (vegetative cells) starting at 200 MPa, 2 min. For yeast, HHPcarb+CO₂ increased the inactivation of HHP from 4.7‐log to 6.2‐log reduction at 250 MPa, 4 min. The synergistic effect of CO₂ with HHP increased with increasing time and pressure. HHPcarb+CO₂ treatment did not alter the appearance and color, while affecting the texture and flavor of unsweetened feijoa samples. There were no differences in sensory attributes and preferences between HHPcarb+CO₂ and fresh sweetened products. Addition of CO₂ in HHP treatment can reduce process pressure and time, and better preserve product quality.     

Modification of dietary fibers from purple-fleshed potatoes (Heimeiren) with high hydrostatic pressure and high pressure homogenization processing: A comparative study

In this study, we evaluated the effects of high hydrostatic pressure (HHP) and high pressure homogenization (HPH) treatments on the physicochemical, functional, and structural properties of dietary fibers (DFs) obtained from purple-fleshed potatoes. DFs subjected to HHP and HPH exhibited increased content of soluble dietary fiber. HHP and HPH treatments did not improve water holding capacity, but increased oil holding and swelling capacities, and emulsion activity and stability. DFs treated with HPH showed the increased antioxidant activities (DPPH 0.89, ABTS 2.65, FRAP 3.39 mg Trolox/g DF), content of total phenol, and α-glucosidase inhibition (98.3%). HHP and HPH treatments changed monosaccharide compositions and structural characteristics of DFs. Therefore, DFs from purple-fleshed potatoes could be used as a fiber-rich ingredient in functional foods, and HPH was more useful in the modification of dietary fiber than HHP at the same treatment conditions.Industrial relevance: This article deals with the modification of dietary fibers from purple-fleshed potatoes (Heimeiren) with HHP and HPH treatments. Results suggest that HPH treated dietary fiber showed a higher ratio of soluble fraction, increased physicochemical and functional properties than HHP at 200 MPa. There outcomes could help the food industry identify the optimal high pressure processing type to improve physicochemical and functional properties of dietary fiber.     

Effect of high pressure processing on gastrointestinal fate of carotenoids in mango juice: Insights obtained from macroscopic to microscopic scales

High hydrostatic pressure (HHP) and high-pressure homogenization (HPH) were applied to mango juice to explore their effects on gastric retention rate (G-CRR), bioaccessibility (BAC) of total and individual carotenoids, and the corresponding mechanisms from macroscopic to microscopic scales. Compared to the control, both HHP and HPH at 50 MPa had no significant effect on BAC and G-CRR, whereas HPH at 100 MPa significantly increased BAC by 44.33% and G-CRR by 11.84%. Further HHP treatments (particularly at 400 MPa) on the 100 MPa-HPH-pretreated samples significantly increased BAC by 71.37% and G-CRR by 24.24%. Violaxanthins/esters were less stable than carotenes in the stomach, resulting in lower bioaccessibility of violaxanthins/esters. G-CRR and BAC were negatively correlated with the viscosity and particle size of juice, whereas they were positively correlated with the solubility/dispersibility of carotenoids. In addition, pectin-carotenoid interactions may also be an important factor affecting the digestive fate of carotenoids in juice.Industrial relevanceHigh pressure processing (High hydrostatic pressure, HHP, and high pressure homogenization, HPH) is a non-thermal technique and its effect on the bioaccessibility of carotenoids in fruits and vegetables have attracted attention from researchers. Our research found that HPH and HHP combined treatment could decrease the particle size of mango juice, and increase the viscosity and turbidity as well as the bioaccessibility of carotenoids therein. This technology can be used to preserve the physical stability of mango juice and improve the nutritional value.     

Effect of nonthermal technologies on the native size distribution of fat globules in bovine cheese-making milk

Milk-fat globule membranes are susceptible to damage by mechanical and thermal processes. This damage is translated into alterations of milk fat structure and functionality of cheese-making milk. The objective of this work was to evaluate the effect of pulsed electrical fields (PEF), high hydrostatic pressure (HHP), and conventional thermal treatments on fat globule size distribution and ζ-potential. Milk was processed by HHP at 400 and 500 MPa for 0–20 min, and with PEF at 36 kV/cm and 42 kV/cm up to 64 pulses. The ζ-potential of HHP and PEF treated milk were − 15.47 mV and − 14.63 mV respectively. HHP treatments induced fat globules flocculation, increasing their mass moment mean diameter. Although PEF processing did not modify the true mean diameter of MFG, it induced small globules to clump together, causing an apparent increment in the population of larger milk-fat globules.

Industrial relevance

The market for traditional raw dairy products has increased in recent times in several regions of the world due to their unique flavor and texture attributes. However, the potential negative implications of consuming raw products limit the growth of this market segment. Manufacture of raw-like cheese from thermally pasteurized milk is not feasible, among other things, because of milk fat globule membrane damage caused by elevated temperatures. Nonthermal food preservation technologies offer the potential to produce milk technically suitable for the industrial manufacture of microbiologically safe raw-like dairy products.     

Changes induced by high hydrostatic pressure in acidified and non-acidified milk during Oaxaca cheese production

Oaxaca cheese, produced using the pasta filata method, is a very popular Mexican dairy products. In this work, the effect of high hydrostatic pressure (HHP) and the acidification process before and after HHP treatment of raw cow milk was studied at different pressure levels (150, 300 and 500 MPa) and holding times (10 and 30 min). Clotting time, proximal composition, microstructure, secondary protein structure and electrophoretic profile were evaluated. HHP did not influence clotting time in samples acidified before HHP, but it appears to have a positive effect at lower pressure treatments on non-acidified milk. Moisture, protein and fat were similar in cheeses treated at most HHP conditions regardless of the acidification. HHP did not influence the microstructure of cheese and the secondary structure of proteins. The use of HHP during the manufacture of Oaxaca cheese allowed preserving quality parameters evaluated without advantages in processing time and the product's proximal composition.     

Inactivation Kinetics of Peach Pulp Pectin Methylesterase as a Function of High Hydrostatic Pressure and Temperature Process Conditions

A kinetic study of the inactivation of endogenous pectin methylesterase (PME) in Greek commercial peach pulp under high hydrostatic pressure (HHP; 100–800 MPa) combined with moderate temperature (30–70 °C) was conducted. Thermal inactivation of the enzyme at ambient pressure conditions was also studied. PME inactivation was modeled by first order kinetics at all conditions tested. High pressure and temperature acted synergistically on PME inactivation, except at the high temperature of 70 °C at the middle pressure range (100–600 MPa), where an antagonistic effect of pressure and temperature was observed. At this specific middle pressure range, an increase of pressure processing led to increased inactivation rate constants of peach PME. A multiparameter model was developed to express the PME inactivation rate constant as a function of temperature and pressure process conditions, taking into account the dependence of both activation energy and activation volume on pressure and temperature, respectively. A good correlation between experimental and predicted values of inactivation rate constants was established. This modeling approach enables the quantitative estimation of the HHP–temperature conditions needed to achieve a targeted PME inactivation in the peach pulp.     

Effects of combination treatments of nisin and high-intensity ultrasound with high pressure on the microbial inactivation in liquid whole egg

The consecutive combinations of nisin with high hydrostatic pressure (nisin-HHP) and ultrasound with high hydrostatic pressure (US-HHP) were explored to achieve enhanced microbial inactivation in liquid whole egg processing. The HHP processing conditions were fixed to either 250 MPa for 886 s or 300 MPa for 200 s at the treatment temperature of 5 °C, which have been determined as the optimum HHP processing conditions considering egg protein coagulation and microbial inactivation kinetics. Between the two types of combinations, the nisin-HHP combination showed more promising results. The addition of nisin prior to pressure treatments significantly increased the lethal effects of HHP against Listeria seeligeri up to 5 log cycles. Because the individual effects of each nisin and HHP on the Listeria were almost negligible, the Listeria reductions are considered to be due to the synergistic action of nisin and HHP. However, the combination of nisin-HHP on E. coli showed exactly the same degree of inactivation by HHP alone, which supports protection mechanism of gram negative bacteria against the action of nisin. The US-HHP combination caused no reductions of Listeria and only a slightly increased inactivation of E. coli under the experimental conditions.     

Inactivation of Listeria monocytogenes in raw and hot smoked trout fillets by high hydrostatic pressure processing combined with liquid smoke and freezing

High hydrostatic pressure (HHP; 200 MPa for 15 min), liquid smoke (0.50%, v/v) and freezing (−80 °C, overnight) was used to eliminate Listeria monocytogenes in BHI broth, raw and smoked trout. The bactericidal effect of liquid smoke solutions (L9 and G6), HHP and their combinations was evaluated against L. monocytogenes LO28, EGD-e and 10403S and further continued with the most resistant strain (10403S) to the combined treatment. For first time, a synergistic effect of liquid smoke and HHP was observed and was further enhanced by freezing prior to HHP. The effect of HHP and liquid smoke, prior to freezing was highest in BHI compared to raw and smoked trout. A major synergistic effect of HHP, liquid smoke and freezing was observed, reaching a 5.48 or 1.93 log CFU/g reduction when smoked or raw trout was used respectively. Furthermore, high injury levels occurred, among treatments reaching up to 55.98%.Industrial relevanceThis paper illustrates for first time, the possibility of using a very low pressure in combination with liquid smoke and freezing to eliminate L. monocytogenes. It was demonstrated that treatment of trout samples with liquid smoke followed by freezing prior to pressurization at 200 MPa for 15 min reduced the number of L. monocytogenes by more than 5-log CFU/g. Such a remarkable bacterial inactivation at a very low pressure (compared to common industrial practices) is a significant achievement that could allow production of safer and novel products by HHP at an affordable price, as the cost of equipment manufacture as well as the maintenance and running costs could be reduced substantially at lower operation pressures.     

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.     

Effect of high isostatic pressure on the peptidase activity and viability of Pseudomonas fragi isolated from a dairy processing plant

To produce safe and high quality processed milk, high pressure (HP) technology was tested to inactivate undesired microorganisms and their caseinolytic activity. A Pseudomonas fragi strain, isolated from the inner surface of a cheese-making machine from a dairy plant, was shown to harbour the aprX gene and cause casein proteolysis. Single-cycle HP processing of P. fragi-spiked milk at 450 MPa and 25 °C for 20 min decreased bacteria viability to lower levels and reduced peptidase activity by 14%. However when HP processing was performed at 50 °C, a synergistic effect on peptidase was observed, reaching 40% inactivation. Multiple HP treatment cycles at 450 MPa and 25 °C were less effective and reduced peptidase activity by only 23%. HP treatment could aid in the challenge to reduce AprX peptidase activity produced by microbial contaminants, but partial inactivation of peptidase was not effective in preventing UHT milk coagulation during storage at room temperature.