Baroprotective Effect Research Articles

Depiction of the In Vitro and Genomic Basis of Resistance to Hop and High Hydrostatic Pressure of Lactiplantibacillus plantarum Isolated from Spoiled Beer.

Among the beer-spoiling microorganisms, the dominant ones belong to the genera Lactobacillus, Leuconostoc, Oenococcus, and Pediococcus. It is assumed that resistance to hop bitters correlates with resistance to other factors and can significantly impact the brewing industry. Beer preservation with high hydrostatic pressure eliminates the spoiling microorganisms while preserving all desired properties of the beer. Here, we present comprehensive in vitro and genomic analysis of the beer-spoiling Lactiplantibacillus plantarum KKP 3573 capacity to resist hop and high hydrostatic pressure. Lp. plantarum KKP 3573 is a strain isolated from spoiled beer. Our finding suggests that the growth rate of the strain depends on the medium variant, where a small concentration of beer (5 IBU) stimulates the growth, suggesting that the limited concentration has a positive effect on cell growth. At the same time, increased concentrations of 20 IBU, 30 IBU, and pure beer 43.6 IBU decreased the growth rate of the KKP 3573 strain. We observed that higher extract content in the pressurized beer increased microbial survivability. The wort and Vienna Lager beer can stimulate the baroprotective effect. The taxonomy of the novel strain was confirmed after whole genome sequencing (WGS) and comparative genomic analysis. More specifically, it contains a chromosome of 3.3 Mb with a GC content of 44.4%, indicative of the Lp. plantarum species. Accordingly, it possesses high genomic similarity (>98%) with other species members. Annotation algorithms revealed that the strain carries several genes involved in resistance to stress, including extreme temperature, hop bitters and high pressure, and adaptation to the brewing environment. Lastly, the strain does not code for toxins and virulence proteins and cannot produce biogenic amines.

Impact of high hydrostatic pressure on the stability of lytic bacteriophages cocktail Salmonelex™ towards potential application on Salmonella inactivation

This work consisted in the first comprehensive study in which the potential to exploit the Salmonella lytic bacteriophages cocktail, Salmonelex™, in association with high hydrostatic pressure (HHP) towards potential application in egg matrices decontamination was evaluated. The impact of HHP (200–600 MPa) on the bacteriophages' viability pointed out a stability in the range of 200–400 MPa. From 400 MPa onwards, the inactivation was potentiated by an increase in the pressure magnitude, being matrix dependent. Salmonelex™ possessed a prominent baroresistance, requiring 600 MPa to completely lose its infectivity. Egg yolk presented the highest baroprotective effect, followed by whole egg and egg white. Transmission electron microscopy unveiled that 500 and 600 MPa elicited a detrimental impact on the bacteriophages’ structural integrity. It was noteworthy the barotolerance (200–300 MPa) of Salmonelex™, previously exposed to different pH conditions (5.0–9.0), which proved not to undermine its infectivity. Regarding the influence of ovalbumin, lysozyme, l-α-phosphatidylcholine, palmitic and oleic acids on the mild HHP-induced inactivation of Salmonelex™, a baroprotective effect was observed, particularly conferred by those compounds comprising egg yolk. The promising results highlighted the feasibility of utilizing Salmonelex™ as an adjuvant to mild HHP processing of egg matrices.

Effects of High Hydrostatic Pressure and Beef Patty Formulations on the Inactivation of Native Strains of Shiga Toxin-Producing Escherichia coli O157:H7

This study aimed to evaluate the effects of beef patty formulations and pressure levels on the inactivation of spoilage microbiota and native Shiga toxin-producing Escherichia coli (STEC) O157:H7. Beef patties were made with different salt and fat content (A: 1% NaCl – 10% fat, B: 2% NaCl – 10% fat, C: 1% NaCl – 20% fat and D: 2% NaCl – 20% fat) and treated at 400 and 600 MPa. STEC reductions after 400 and 600 MPa were 2 and 4 log CFU/g, respectively. STEC inactivation was similar in patties with 10 and 20% of fat content. However, patties with 1% of NaCl showed higher STEC reductions than those made with 2% of NaCl. The baroprotective effect of salt was probably related with the accumulation of compatible solutes, a well-known bacterial response to osmostress. After HPP treatment, all counts of spoilage microbiota were under the detection limit regardless of the formulation.

High hydrostatic pressure treatment of Arthrospira (Spirulina) platensis extracts and the baroprotective effect of sugars on phycobiliproteins

The effect of High Hydrostatic Pressure (HHP) (600 MPa; 300 s) upon phycobiliproteins from Spirulina platensis extract, in aqueous systems, has been investigated along with the potential baroprotective ability of several sugars (sucrose, trehalose and glucose) at different concentrations (20%, 40% w/w). Phycobiliproteins stability and technological functionality were assessed through spectroscopical and colour analysis, and radical scavenging ability.HHP treatment resulted in a decrease of the phycobiliproteins concentration with allophycocyanin being the most sensitive towards HHP stress (p ≤ 0.05). Addition of saccharides had a positive effect on phycocyanin stability with a higher residual concentration after HHP treatment (p ≤ 0.05) irrespective of the type and concentration of sugars added in the system, whilst baroprotective effects on allophycocyanin and phycoerythrin were observed only at the highest (40%) concentrations tested (p ≤ 0.05). After the HHP treatment, colouring ability of Spirulina platensis extract resulted in being directly correlated to phycocyanin concentration variation (R2 = 0.75). Addition of sugars preserved the antioxidant activity of the extract subject to high pressure regardless of the sugar type. Industrial relevanceSpirulina platensis has become a popular ingredient among manufacturers and consumers, especially for its technological functionality and nutritional properties. Nevertheless, processing and storage may result detrimental for its performance in foods. High hydrostatic pressure is a successful, non-thermal technology that is finding increasing applications in the food industry though processing costs of food products stabilized by HHP result, nowadays, being 7 to 15-fold higher than thermal pasteurization. However, consumers today are willing to pay higher prices for higher quality products with better sensory profiles and nutritional properties. Moreover, compared to thermal pasteurization, HHP is a rather new technology whose cost-effectiveness and environmental impact is expected to improve over time, with a consequent reduction of the gap among performance of these two technologies. Hence, this study focused on the effects of HHP treatments on aqueous Spirulina platensis extracts, alone or added with small saccharides to evaluate their potential baroprotective abilities, providing results of interest for application in food product design and development.

Interaction of fat and aqueous phase parameters during high-hydrostatic pressure inactivation of Lactobacillus plantarum in oil-in-water emulsions

High hydrostatic pressure (HHP) processing of food can inactivate spoilage microorganisms, while largely preserving quality characteristics like color, flavor, texture, and vitamin content. However, the composition of the food matrix can strongly affect HHP inactivation efficiency. As systematic data on the interplay of different food matrix components are rare, we used defined oil-in-water (O/W) emulsions as a model system to investigate the impact of fat on the effect of the aqueous phase parameters sodium chloride (NaCl), sucrose, pH, and proteins on HHP inactivation of Lactobacillus (L.) plantarum strains. To identify a potential interconnection with cell surface characteristics, strains with hydrophobic and hydrophilic surface characteristics were considered. HHP inactivation varied widely among the investigated strains. In general, high NaCl and sucrose concentrations exerted a baroprotective effect, whereas low-pH environments enhanced inactivation, and the impact of proteins followed no clear trend. The presence of fat did not generally influence any of the aqueous phase parameters in its impact on HHP inactivation, but caused sporadic matrix and strain specific; however, cell surface hydrophobicity-independent alterations in inactivation efficiency. Still, fat was able to significantly change, and sometimes even invert, the effect of other food matrix parameters.

The protective effect of food matrices on Listeria lytic bacteriophage P100 application towards high pressure processing

The application of lytic phages as biocontrol agents is emerging as a promising strategy towards elimination or reduction of foodborne pathogens in a variety of food products. This technology is particularly advantageous for minimally processed and ready-to-eat (RTE) foods. In this study, the potential use of Listex™ P100 combined with high hydrostatic pressure (HPP), to enhance the control of Listeria monocytogenes in food was evaluated. For that, the effect of three pressures (200, 300 or 400 MPa; 5 min, 10 °C) on phage P100 stability was tested when inoculated in six different matrices: phosphate buffered saline (PBS, pH 7.4); apple juice (pH 3.41); orange/carrot nectar (pH 3.54); UHT whole milk (pH 6.73); and, two traditional Portuguese fermented products, “Serra da Estrela” cheese (pH 5.66) and “Alheira”, a meat sausage (pH 6.07). The results showed that treatment at 400 MPa reduced phage titres to below the detection level in all matrices, whereas at milder pressures the survival of the phage was matrix dependent. “Alheira”, “Serra da Estrela” cheese and UHT whole milk were shown to be baroprotective matrices that support phage P100 application in HHP up to 300 MPa; however, an accentuated phage inactivation was observed in apple and orange/carrot nectar, which may be related to the acidic pH values of these matrices. The initial phage load did not affect the inactivation rate during HHP processing (300 MPa, 5 min, 10 °C) in PBS, cheese, sausage or milk matrices, and the phage titres were stable in these matrices during storage at 4 °C for 28 days for milk and 60 days for “Alheira” and “Serra da Estrela” cheese. In addition, a baroprotective effect on phage stability was observed when PBS was supplemented with reducing sugars, dextrin, casein, and tween 80. In conclusion, at mild HHP treatment, phage P100 remained active in specific matrices and seems to present potential to be added in non-thermal inactivation of L. monocytogenes.

Mathematical approach for the Listeria monocytogenes inactivation during high hydrostatic pressure processing of a simulated meat medium

Abstract Based on a central composite experimental design, the effects of five factors: pH (4.6–6.6), sodium chloride (NaCl, 0–10%), sodium nitrite (NaNO2, 0–152 ppm), pressure (P, 300–600 MPa) and pressure-holding time (t, 0–10 min) on the inactivation of Listeria monocytogenes by high hydrostatic pressure processing on a simulated meat medium (SMM) were evaluated and quantified. Results showed that L. monocytogenes reductions during pressure treatments ranged from 0 to >6 log cfu/g. A polynomial inactivation model was developed being P, t and NaCl the only significant factors influencing HP-lethality (p ≤ 0.05). Inactivation on SMM increased with the increase in P and t, and a synergistic effect between both factors on process effectiveness was remarked. By increasing NaCl concentration (i.e. lowering aw values), a baroprotective effect on L. monocytogenes cells was evidenced. Besides the well-known effects of technological parameters, this study highlights the relevant influence of meat products formulations on the inactivation of L. monocytogenes induced by HHP processing.

An insight on the relationship between food compressibility and microbial inactivation during high pressure processing.

This paper investigates the effect of high pressure liquid food compressibility on S. cerevisae inactivation. Honey with various adjusted sugar with different values of compressibility was selected as a model food. S. cerevisiae cells in different honey concentrations (0-80°Brix), 600MPa (at ambient temperature) showed an increasing resistance to inactivation with increasing °Brix. D-values of S. cerevisiae at 200, 400 and 600MPa, for 20min/80°Brix were 136.99±7.97, 29.24±6.44 and 23.47±0.86min, respectively. These D-values resulted the Z p -value of 526±39MPa. A significant correlation (p<0.05) of cell reduction, °Brix and compressibility was found. Cell reduction in high pressure-treated samples varied linearly with °Brix suggesting that the baroprotective effect of the food was not solely due to sugar content, but also due to its compressibility. This research could have significant implications on the success of HPP (high pressure processing) preservation of foods containing high sugar content.

Effect of Pressure-Induced Changes in the Ionization Equilibria of Buffers on Inactivation of Escherichia coli and Staphylococcus aureus by High Hydrostatic Pressure

Survival rates of Escherichia coli and Staphylococcus aureus after high-pressure treatment in buffers that had large or small reaction volumes (ΔV°), and which therefore underwent large or small changes in pH under pressure, were compared. At a low buffer concentration of 0.005 M, survival was, as expected, better in MOPS (morpholinepropanesulfonic acid), HEPES, and Tris, whose ΔV° values are approximately 5.0 to 7.0 cm(3) mol(-1), than in phosphate or dimethyl glutarate (DMG), whose ΔV° values are about -25 cm(3) mol(-1). However, at a concentration of 0.1 M, survival was unexpectedly better in phosphate and DMG than in MOPS, HEPES, or Tris. This was because the baroprotective effect of phosphate and DMG increased much more rapidly with increasing concentration than it did with MOPS, HEPES, or Tris. Further comparisons of survival in solutions of salts expected to cause large electrostriction effects (Na2SO4 and CaCl2) and those causing lower electrostriction (NaCl and KCl) were made. The salts with divalent ions were protective at much lower concentrations than salts with monovalent ions. Buffers and salts both protected against transient membrane disruption in E. coli, but the molar concentrations necessary for membrane protection were much lower for phosphate and Na2SO4 than for HEPES and NaCl. Possible protective mechanisms discussed include effects of electrolytes on water compressibility and kosmotropic and specific ion effects. The results of this systematic study will be of considerable practical significance in studies of pressure inactivation of microbes under defined conditions but also raise important fundamental questions regarding the mechanisms of baroprotection by ionic solutes.

Factors influencing the inactivation of Alicyclobacillus acidoterrestris spores exposed to high hydrostatic pressure in apple juice

Alicyclobacillus acidoterrestris, a thermoacidophilic and spore-forming bacterium, survives the typical pasteurization process and can cause the spoilage of juices, producing compounds associated with disinfectant-like odour (guaiacol, 2,6 – dibromophenol, 2,6 – dichlorophenol). Therefore, the use of other more effective techniques such as high hydrostatic pressure (HHP) is considered for preserving juices. The aim of this study was to search for factors affecting the resistance of A. acidoterrestris spores to HHP. The baroprotective effect of increased solute concentration in apple juice on A. acidoterrestris spores during high pressure processing was observed. During the 45 min pressurization (200 MPa, 50°C) of the spores in concentrated apple juice (71.1°Bx), no significant changes were observed in their number. However, in the juices with a soluble solids content of 35.7, 23.6 and 11.2°Bx, the reduction in spores was 1.3–2.4 log, 2.6–3.3 log and 2.8–4.0 log, respectively. No clear effect of age of spores on the survival under high pressure conditions was found. Spores surviving pressurization and subjected to subsequent HHP treatment showed increased resistance to pressure, by even as much as 2.0 log.

The effect of high hydrostatic pressure on the survival of Saccharomyces cerevisiae in model suspensions and beetroot juice

The inactivation of Saccharomyces cerevisiae NCFB 3191 using high hydrostatic pressure of 300 MPa at 20°C with a holding time of 0, 1, 5 and 10 min was investigated with model suspensions in phosphate-buffered saline and in beetroot juice. The reduction in S. cerevisiae NCFB 3191 in model suspensions was about 5 log after 10 min of pressurization, irrespective of the initial level of cell concentration in the samples (5.4–8.7 log cfu/mL). The baroprotective effect of beetroot juice on yeast cells during pressurization was observed; the reduction was lower and was only 3.5 log (the inoculum was 5.4 log cfu/mL). No sublethal injury among the surviving cells of the studied yeast strain was found.

Evaluation of high hydrostatic pressure effect on human adenovirus using molecular methods and cell culture

Human adenoviruses (HAdV) are shed in human faeces and can consequently contaminate environmental waters and possibly be transferred to foods by irrigation. Therefore, efficient inactivation technologies for water and foods are needed. High hydrostatic pressure (HHP) processing is a non-thermal, energy-efficient and rapid emergent inactivation technology, which has been widely studied to eliminate pathogenic microorganisms in foods. We have applied HHP to HAdV-2 in water and cell culture medium (CCM) and measured the effect on virus infectivity and genome and capsid integrity, by using infectivity assay, real-time PCR (qPCR) and qPCR with prior enzymatic treatment (ET-qPCR) with Proteinase K and DNase I. While lower pressures did not provide satisfactory inactivation levels, 400 and 600MPa treatments were estimated to reduce virus infectivity by approximately 6 log10 units when effectively applied for 93s and 4s, respectively (i.e., excluding come up times of the pressure unit). However, virus genome remained intact even when higher pressures were applied. While acidic pH protected HAdV-2 from inactivation with HHP, no baroprotective effect was observed when 1% sucrose was added to the CCM. On the other hand, 10mM CaCl2 added to the CCM was estimated to protect HAdV-2 from HHP with longer treatment times (>10min). When virus was treated in bottled mineral water, significantly higher infectivity reduction was observed compared to the same treatment in CCM. In conclusion, HHP was shown to effectively reduce HAdV-2 infectivity up to 6.5 log10 units within 4s and can thus contribute to public health protection for food- and water-borne virus transmission. However, its precise effect is matrix dependent and therefore matrix-specific evaluations need to be considered for assuring reliable inactivation in practice.

Resistance of Mexican Fruit Fly to Quarantine Treatments of High-Pressure Processing Combined with Cold

Mexican fruit fly Anastrepha ludens Loew (Diptera: Tephritidae) is one of the most important insects infesting fruits. Although high pressure has been proposed as an alternative quarantine process for this pest, conditions applied to destroy eggs and larvae can also damage the fruits. The objective of this study was to assess the biological viability of A. ludens eggs treated by high-pressure processing at 0°C, establishing whether nondestroyed eggs and larvae preserved their ability to develop and reproduce. One-, 2-, 3-, and 4-day-old eggs were pressurized at 50, 70, or 90 MPa for 0, 3, 6, or 9 min. The hatching ability of pressurized eggs and their capacity to develop larvae, pupae, and reproductive adults were evaluated. The ability of pressurized larvae to pupate was also registered. The results showed that most of the eggs that resisted the treatments were able to produce adults with capability to reproduce a new generation of A. ludens flies. Larvae were less resistant than eggs. All larvae were killed at 90 MPa for 9 min. The pressure induced the expression of heat shock proteins (HSP) in second- and third-instar larvae. The HSP showed a baroprotective effect in A. ludens larvae. These results are relevant to the industry because they show that eggs of insects infesting fruits treated by high-pressure processing were able to pupate after treatments. This indicates that efforts must be addressed to destroy all eggs because most of the surviving organisms could reach an adult stage and reproduce.

Inactivation of Human and Murine Norovirus by High-Pressure Processing

The effect of high hydrostatic pressure (HPP) was evaluated for inactivation of murine norovirus (MNV), a propagable norovirus (NoV), and human NoV genogroup II.4. Inactivation of MNV was assessed by viral culturing (50% tissue culture infectious dose [TCID(50)]) and real-time reverse-transcription-polymerase chain reaction (RT-qPCR), whereas NoV survival was determined only by RT-qPCR. A treatment of 450 MPa for 15 min at 45°C was sufficient to inactivate 6.5 log(10) of infectious MNV in culture medium as determined by TCID(50). Further, the inactivation of MNV was enhanced when pressure was applied at an initial temperature of 25°C. On the other hand, a baroprotective effect was observed when MNV suspensions were supplemented with 10 mM of CaCl(2). A 400 MPa treatment at 45°C inactivated >5 log(10) of infectious MNV, whereas the addition of CaCl(2) increased the pressure resistance of MNV, with <0.5 log(10) reduction observed. MNV decay as determined by TCID(50) was generally greater than that determined by RT-qPCR; for instance, MNV genomes were detected even after 15 min treatment at 450 MPa, with <0.5 log(10) reduction. Experiments with NoV suspensions showed that all tested HPP treatments reduced the numbers of NoV by <0.5 log(10) units as determined by RT-qPCR. Additionally, RNA of human NoV was more resistant to certain HPP treatments than the RNA of MNV.

High hydrostatic pressure for disinfection of bone grafts and biomaterials: an experimental study.

Background:Autoclaving, heat, irradiation or chemical detergents are used to disinfect autografts, allografts and biomaterials for tissue reconstruction. These methods are often associated with deterioration of mechanical, physical, and biological properties of the bone grafts and synthetic implants. High hydrostatic pressure has been proposed as a novel method preserving biomechanical and biological properties of bone, tendon and cartilage. This is the first study to assess the inactivation of clinically relevant bacteria on biomaterials and human bone by high hydrostatic pressure.Methods:Bacterial suspensions of Staphylococcus aureus, Pseudomonas aeruginosa and Enterococcus faecium, implants covered with infected blood, human bone infected in vitro, and biopsies of patients with chronic osteomyelitis were subjected to different protocols of high hydrostatic pressure up to 600 MPa. Bacterial survival after high hydrostatic pressure treatment was determined and compared with bacterial growth in untreated controls.Results:S. aureus and P. aeruginosa in suspension were completely inactivated by high hydrostatic pressure (> 5log levels), whereas E. faecium showed barotolerance up to 600 MPa. Blood and adherence to metal implants did not significantly alter inactivation of bacteria, and complete disinfection was achieved with barotolerant bacteria (S. aureus and P. aeruginosa). However, osteoarthritic bone demonstrated a non-homogeneous baroprotective effect, with single bone samples resistant to treatment resulting in unaltered bacterial growth, and complete disinfection of artificially infected bone specimens was achieved in 66% for S. aureus, 60% for P. aeruginosa and 0% for E. faecium. Human bone samples of patients with chronic osteomyelitis could be completely disinfected in 2 of 37 cases.Conclusion:High hydrostatic pressure offers new perspectives for disinfection of sensitive biomaterials and bone grafts, and contamination by blood did not significantly affect bacterial inactivation rates. However, a significant baroprotective effect was demonstrated in bone. Effectiveness is currently limited to colonization and / or infection with barosensitive micro-organisms.

Influence of pH, salt, and temperature on pressure inactivation of hepatitis A virus

The effects of pH (3–7), NaCl (0–6%), and temperature on pressure inactivation of hepatitis A virus (HAV) were determined. The HAV samples were treated at 400 MPa for 1 min at 5, 20, and 50 °C. Decreasing solution pH enhanced pressure inactivation of HAV. This enhanced inactivation effect was most evident at 20 °C. A baroprotective effect was observed for NaCl concentrations from 1 to 6%. For example, a treatment of 400 MegaPascals (MPa) for 1 min at 50 °C reduced the HAV titers by 4.0, 4.1, 1.3 and 0.4 log plaque forming units (PFU)/ml for NaCl concentrations of 0, 1, 3, and 6%, respectively. Overall, increasing the treatment temperature enhanced pressure inactivation of HAV in the solutions. The pressure resistance of HAV in oysters was also examined. Temperature in the range of 5 to 50 °C did not significantly affect the pressure inactivation of HAV within oyster homogenates. It is concluded that HPP treatment of oysters at temperatures above room temperature would not provide any additional benefit for inactivation of HAV. However, the observation that HAV inactivation is enhanced in acidic matrices is information that may be useful for designing product formulations and processing parameters for high pressure processing of products such as low pH fruit juices and salsa.

Effects of fat, casein and lactose on high-pressure destruction of Escherichia coli K12 (ATCC-29055) in milk

High-pressure (HP) destruction of Escherichia coli K12 suspended in commercial pasteurized milk (0, 3.25 and 5% fat), peptone water (0.1%), and phosphate buffer (0.2 mol/l Na 2HPO 4 and 0.2 mol/l NaH 2PO 4) supplemented with different amounts of casein or lactose (1–4%) was investigated. HP treatment resulted in the highest level of E. coli destruction in the buffer (possibly because it contained no nutrients), followed by those in peptone water and least in milk. Therefore, milk and peptone provided some baro-protection to the destruction of E. coli during the HP treatment. Fat content in milk between 0, 3.25 and 5% had no significant ( P > 0.05) influence on the HP destruction of E. coli. The supplement of either casein or lactose to milk did not influence the level of HP destruction of E. coli. However, the baro-protection became significant during HP treatment when casein or lactose was added the buffer. The buffer solution supplemented with 1–4% casein showed a baro-protective effect equivalent to that in milk. Lactose supplement to buffer also caused baro-protection, though the effect was relatively less pronounced. However, no significant difference in the baro-protection effect was observed among the buffer samples supplemented with different levels (1, 2 and 4%) of lactose or casein. Therefore the major contributors for baro-protection of E. coli in milk during HP treatment appear to be casein and lactose, rather than the fat content.

Effect of Sodium Chloride on Glassy and Crystalline Melting Transitions of Wheat Starch Treated with High Hydrostatic Pressure: Prediction of Solute‐induced Barostability from Nonmonotonic Solute‐induced Thermostability

AbstractWheat starch was high hydrostatic pressure (HHP)‐treated in aqueous solutions with various sodium chloride (NaCl) concentrations (0 to near‐saturation), in order to explore the effects of salt on glassy and crystalline transitions of starch during the treatment, using differential scanning calorimetry (DSC). For wheat starch at atmospheric pressure, glass and crystalline melting transitions of amylopectin (reported as gelatinization peak temperature) increased up to 2 M NaCl, and then decreased with further increase in NaCl concentration, but the gelatinization peak temperature was higher in all NaCl solutions than in water alone. In contrast, the melting transition for the amylose‐lipid complex (reported as peak temperature) increased continuously with increasing NaCl concentration. When 50% (w/w) starch slurries were HHP‐treated in water and various NaCl concentrations (0.1, 2 and 5 M) for 15 min at 25°C, the presence of salt significantly protected glass and crystalline transitions of starch during the HHP treatment. Although the baroprotective effect was maximal near the lyotropic 2 M NaCl concentration [1‐5], all NaCl concentrations were more baroprotective than was water alone for HHP‐treated wheat starch. As reported previously [6] for corn starches in a lyotropic concentration of NaCl (ionic solvent) or a non‐equilibrium concentration of sucrose (glass‐forming solvent) [7], solute‐induced thermostabilization of the wheat starch gelatinization transition predicted solute‐induced barostabilization.

Baroprotective effect of increased solute concentrations on yeast and moulds during high pressure processing

The baroprotective effect of increasing solute concentrations on yeast cells and fungal conidia subjected to high pressure processing (HPP) was studied. Suspensions of yeast cells (Saccharomyces cerevisiae, Pichia anomala and Hanseniaspora uvarum) or fungal spores (Penicillium expansum, Fusarium oxysporum and Rhizopus stolonifer) in citrate phosphate buffer formulated with sucrose at 40, 50 and 60 °Brix, or with glycerol and NaCl at equivalent water activity (aw) values (0.925, 0.903 and 0.866 aw) were subjected to 600 MPa pressure for varying times, and then were enumerated by spread plate technique to assess survival. There was an increasing resistance to inactivation by high pressure with an increase in solute concentration. The two moulds with easily wetted spores, R. stolonifer and F. oxysporum, showed strongest resistance to HPP at 0.866 aw. Differing responses to the three solutes were observed among the fungal species tested, indicating that the chemical nature of the solute may also be important in protecting yeasts and moulds during and after pressure treatment. Sucrose had a stronger baroprotective effect for S. cerevisiae than the other solutes, at two of the three investigated aws. For P. expansum at 0.903 aw, NaCl gave the best protective effect. Scanning electron microscopy of HPP treated cells showed the protective effects of increased sucrose concentration. The results reported here have practical implications for the food industry in the application of HPP for production of fruit preparations or syrups, and should be taken in account in establishing efficient process design. As high concentrations of sugar, salt and glycerol provide protection for yeasts and moulds during high pressure processing, foods containing high levels of solutes may need longer processing times or higher pressures to achieve inactivation of these fungi.

Water activity of bacterial suspension media unable to account for the baroprotective effect of solute concentration on the inactivation of Listeria monocytogenes by high hydrostatic pressure

Inactivation of Listeria monocytogenes (10 8 CFU/ml) by high hydrostatic pressure (HHP) from 400 to 600 MPa at 25 °C for 10 min was investigated with various concentrations of sodium chloride, sucrose, and sodium phosphate buffer solutions. Sodium chloride significantly inhibited HHP-induced inactivation of L. monocytogenes at concentrations higher than 2.6 M. A low concentration of sodium chloride within 1.7 M had no effect on HHP-induced inactivation. Almost complete inactivation at relatively low sodium chloride concentration solution was observed with treatments above 500 MPa. Sucrose also significantly inhibited HHP-induced inactivation of L. monocytogenes when greater than 1.2 M sucrose solutions were used. HHP-treatment at 400 MPa reduced the number of L. monocytogenes in 1.2 M, 1.5 M, and 1.8 M sucrose solutions by 4.8, 2.0, and 0.7 log cycles, respectively. Higher pressure did not yield significant reductions. Sodium phosphate buffer significantly inhibited HHP-induced inactivation of L. monocytogenes. In particular, 1 M phosphate buffer completely inhibited HHP-induced inactivation even at 600 MPa. HHP-treatment at 400 MPa reduced the number of L. monocytogenes in 0.1 M, 0.25 M, and 0.5 M phosphate buffer solutions by 5.6, 4.1, and 3.2 log cycles, respectively. The effect of HHP-induced inactivation of L. monocytogenes in the three kinds of solution was evaluated by adjusting water activity ( a w). However, the baroprotective effect differed depending on the kind of solute even at the same a w. This result showed no consistent correlation between a w and solute concentration in terms of the baroprotective effect. As an alternative approach, saturation of suspension solution was used for evaluating the effect of HHP-induced inactivation of L. monocytogenes. As the saturation of suspension media increased, the effect of HHP-induced inactivation of L. monocytogenes decreased regardless of the kinds of solute. The saturation of solution would be an alternative parameter of inhibition in terms of HHP-induced inactivation of bacteria.