Effect of simultaneous application of heat and pressure on the survival of bacterial spores

High Hydrostatic Pressure as a Method to Reduce Microbial Contamination of Porcine Blood Plasma

The suitability of high hydrostatic pressure as an alternative method to produce porcine blood plasma with a sufficient microbial stability without affecting its functional properties was evaluated. The effects of high pressure on plasma microorganisms were highly dependent on processing temperature. Treatments of 15 min at 450 MPa carried out at 5 'C led to reductions of about 90% in microbial counts and to 20-50% decreases in the growth ability of the survivors. At 25 'C and 40 'C, the efficiency was increased to reduction values of 99.82 and 99.97%, respectively. The lowering of the growth capacity was about 50% at 25 'C and up to 80% at 40 'C. The most efficient treatment (450 MPa, for 15 min at 40 'C) did not lead to appreciable negative effects on the functional properties of heat-induced gels from porcine blood plasma.

Monitoring biochemical changes in bacterial spore during thermal and pressure-assisted thermal processing using FT-IR spectroscopy

Pressure-assisted thermal processing (PATP) is being widely investigated for processing low acid foods. However, its microbial safety has not been well established and the mechanism of inactivation of pathogens and spores is not well understood. Fourier transform infrared (FT-IR) spectroscopy was used to study some of the biochemical changes in bacterial spores occurring during PATP and thermal processing (TP). Spore suspensions (approximately 10(9) CFU/mL of water) of Clostridium tyrobutyricum, Bacillus sphaericus, and three strains of Bacillus amyloliquefaciens were treated by PATP (121 degrees C and 700 MPa) for 0, 10, 20, and 30 s and TP (121 degrees C) for 0, 10, 20, and 30 s. Treated and untreated spore suspensions were analyzed using FT-IR in the mid-infrared region (4000-800 cm(-1)). Multivariate classification models based on soft independent modeling of class analogy (SIMCA) were developed using second derivative-transformed spectra. The spores could be differentiated up to the strain level due to differences in their biochemical composition, especially dipicolinic acid (DPA) and secondary structure of proteins. During PATP changes in alpha-helix and beta-sheets of secondary protein were evident in the spectral regions 1655 and 1626 cm(-1), respectively. Infrared absorption bands from DPA (1281, 1378, 1440, and 1568 cm(-1)) decreased significantly during the initial stages of PATP, indicating release of DPA. During TP changes were evident in the bands associated with secondary proteins. DPA bands showed little or no change during TP. A correlation was found between the spore's Ca-DPA content and its resistance to PATP. FT-IR spectroscopy could classify different strains of bacterial spores and determine some of the changes occurring during spore inactivation by PATP and TP. Furthermore, this technique shows great promise for rapid screening PATP-resistant bacterial spores.

High-pressure-mediated survival of Clostridium botulinum and Bacillus amyloliquefaciens endospores at high temperature

Endospores of proteolytic type B Clostridium botulinum TMW 2.357 and Bacillus amyloliquefaciens TMW 2.479 are currently described as the most high-pressure-resistant bacterial spores relevant to food intoxication and spoilage in combined pressure-temperature applications. The effects of combined pressure (0.1 to 1,400 MPa) and temperature (70 to 120 degrees C) treatments were determined for these spores. A process employing isothermal holding times was established to distinguish pressure from temperature effects. An increase in pressure (600 to 1,400 MPa) and an increase in temperature (90 to 110 degrees C) accelerated the inactivation of C. botulinum spores. However, incubation at 100 degrees C, 110 degrees C, or 120 degrees C with ambient pressure resulted in faster spore reduction than treatment with 600 or 800 MPa at the same temperature. This pressure-mediated spore protection was also observed at 120 degrees C and 800, 1,000, or 1,200 MPa with the more heat-tolerant B. amyloliquefaciens TMW 2.479 spores. Inactivation curves for both strains showed a pronounced pressure-dependent tailing, which indicates that a small fraction of the spore populations survives conditions of up to 120 degrees C and 1.4 GPa in isothermal treatments. Because of this tailing and the fact that pressure-temperature combinations stabilizing bacterial endospores vary from strain to strain, food safety must be ensured in case-by-case studies demonstrating inactivation or nongrowth of C. botulinum with realistic contamination rates in the respective pressurized food and equipment.

Inactivation of Bacillus spores by the combination of moderate heat and low hydrostatic pressure in ketchup and potage

The combination effect of moderate heat and low hydrostatic pressure (MHP) on the reduction of Bacillus subtilis, Bacillus coagulans and Geobacillus stearothermophilus spores in food materials (potage and ketchup) was investigated. These bacterial spores were suspended in potage (pH 7), acidified potage (pH 4), neutralized ketchup (pH 7) and ketchup (pH 4). The suspensions were treated with and without pressure (100 MPa) and temperatures of 65-85 degrees C for 3 to 12 h. The bacterial spores were inactivated by 4-8 log cycles during MHP treatment in potage, acidified potage and ketchup, whereas the spores were highly resistant to long time heat treatment in potage and neutralized ketchup. The degrees of spore destruction were mostly dependent on pH and medium composition during MHP treatment. The inactivation effect in MHP treatment was higher at the pH 7 than at pH 4 both in ketchup and potage. The bacterial spores showed higher inactivation in potage than ketchup during MHP treatment.

Pressure inactivation of Bacillus endospores

The inactivation of bacterial endospores by hydrostatic pressure requires the combined application of heat and pressure. We have determined the resistance of spores of 14 food isolates and 5 laboratory strains of Bacillus subtilis, B. amyloliquefaciens, and B. licheniformis to treatments with pressure and temperature (200 to 800 MPa and 60 to 80 degrees C) in mashed carrots. A large variation in the pressure resistance of spores was observed, and their reduction by treatments with 800 MPa and 70 degrees C for 4 min ranged from more than 6 log units to no reduction. The sporulation conditions further influenced their pressure resistance. The loss of dipicolinic acid (DPA) from spores that varied in their pressure resistance was determined, and spore sublethal injury was assessed by determination of the detection times for individual spores. Treatment of spores with pressure and temperature resulted in DPA-free, phase-bright spores. These spores were sensitive to moderate heat and exhibited strongly increased detection times as judged by the time required for single spores to grow to visible turbidity of the growth medium. The role of DPA in heat and pressure resistance was further substantiated by the use of the DPA-deficient mutant strain B. subtilis CIP 76.26. Taken together, these results indicate that inactivation of spores by combined pressure and temperature processing is achieved by a two-stage mechanism that does not involve germination. At a pressure between 600 and 800 MPa and a temperature greater than 60 degrees C, DPA is released predominantly by a physicochemical rather than a physiological process, and the DPA-free spores are inactivated by moderate heat independent of the pressure level. Relevant target organisms for pressure and temperature treatment of foods are proposed, namely, strains of B. amyloliquefaciens, which form highly pressure-resistant spores.

Comparison of pressure and heat resistance of Clostridium botulinum and other endospores in mashed carrots

Inactivation of bacterial endospores in food requires a combination of pressure and moderate heat. Endospore resistance of seven Clostridium botulinum strains was compared with those of Bacillus spp. (B. cereus, B. subtilis, B. licheniformis, B. smithii, B. amyloliquefaciens) and Thermoanaerobacterium thermosaccharolyticum with respect to pressure (600 to 800 MPa) and temperature (80 to 116 degrees C) treatments in mashed carrots. A large variation was observed in the pressure resistance of C. botulinum spores. Their reduction after treatments with 600 MPa at 80 degrees C for 1 s ranged from more than 5.5 log units to no reduction. Spores of the proteolytic C. botulinum TMW 2.357 exhibited a greater resistance to pressure than spores from all other bacteria examined, with the exception of B. amyloliquefaciens. Heat resistance of spores did not correlate with pressure resistance, either within strains of C. botulinum or when C. botulinum spores were compared with spores of T. thermosaccharolyticum. A quantitative release of dipicolinic acid was observed from C. botulinum spores on combined pressure and temperature treatments only after inactivation of more than 99.999% of the spores. Thus, dipicolinic acid is released by a physicochemical rather than a physiological process. The resistance of spores to combined pressure and temperature treatments correlated with their ability to retain dipicolinic acid. B. amyloliquefaciens, a mesophilic organism that forms highly pressure-resistant spores is proposed as a nonpathogenic target organism for high-pressure process development.

Effects of minerals on resistance of Bacillus subtilis spores to heat and hydrostatic pressure

Among Bacillus subtilis IFO13722 spores sporulated at 30, 37, and 44 degrees C, those sporulated at 30 degrees C had the highest resistance to treatments with high hydrostatic pressure (100 to 300 MPa, 55 degrees C, 30 min). Pressure resistance increased after demineralization of the spores and decreased after remineralization of the spores with Ca(2+) or Mg(2+), whereas the resistance did not change when spores were remineralized with Mn(2+) or K(+), suggesting that former two divalent ions were involved in the activation of cortex-lytic enzymes during germination.

Mechanism of the inactivation of bacterial spores by reciprocal pressurization treatment

AIMS

The mechanism of the inactivation of Bacillus subtilis spores by reciprocal pressurization (RP) was unclear. Therefore, the mechanism was investigated.

METHODS AND RESULTS

To investigate the effects of RP and continuous pressurization (CP) treatments on the inactivation and injury of B. subtilis spores, spores were treated at 25, 35, 45 and 55 degrees C under 200, 300 and 400 MPa. RP treatment was effective in injuring and inactivating spores. Scanning electron microscopy and transmission electron microscopy observation showed that spores treated by RP treatment were more morphologically and structurally changed than the ones treated by CP treatment. There were significant differences between the release of dipicolinic acid (pyridine-2,6-dicarboxylic acid) by RP and CP treatments. From this result, it was concluded that the core fraction was released into the spore suspension.

CONCLUSIONS

The mechanism of RP treatment is believed to work as follows: hydrostatic pressure treatment initiated germination of bacterial spores, and the repeated rapid decompression caused disruption, injury and inactivation of the germinated spores.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study indicated that the physical injury of bacterial spores was effective to inactivate the bacterial spores through the disruption of spores and leakage of their contents.

Inhibitory effects of high pressure and heat on Alicyclobacillus acidoterrestris spores in apple juice

The effectiveness of combined high pressure and heat treatment for reducing spore levels of Alicyclobacillus acidoterrestris, a thermoacidophilic spore-forming bacterium, in commercial pasteurized apple juice was investigated. Spores suspended in apple juice were successfully destroyed by combining high pressure with a mild or high temperature (45, 71, or 90 degrees C).

Influence of temperature and pressure on the lethality of ultrasound

A specially designed resistometer was constructed, and the lethal effect on Yersinia enterocolitica of ultrasonic waves (UW) at different static pressures (manosonication [MS]) and of combined heat-UW under pressure treatments (manothermosonication [MTS]) was investigated. During MS treatments at 30 degrees C and 200 kPa, the increase in the amplitude of UW of 20 kHz from 21 to 150 mum exponentially decreased decimal reduction time values (D(MS)) from 4 to 0.37 min. When pressure was increased from 0 to 600 kPa at a constant amplitude (150 mum) and temperature (30 degrees C), D(MS) values decreased from 1.52 to 0.20 min. The magnitude of this decrease in D(MS) declined progressively as pressure was increased. The influence of pressure on D(MS) values was greater with increased amplitude of UW. Pressure alone of as much as 600 kPa did not influence the heat resistance of Y. enterocolitica (D(60) = 0.094; z = 5.65). At temperatures of as much as 58 degrees C, the lethality of UW under pressure was greater than that of heat treatment alone at the same temperature. At higher temperatures, this difference disappeared. Heat and UW under pressure seemed to act independently. The lethality of MTS treatments appeared to result from the added effects of UW under pressure and the lethal effect of heat. The individual contributions of heat and of UW under pressure to the total lethal effect of MTS depended on temperature. The inactivating effect of UW was not due to titanium particles eroded from the sonication horn. The addition to the MS media of cysteamine did not increase the resistance of Y. enterocolitica to MS treatment. MS treatment caused cell disruption.

The effect of ethylenediaminetetraacetic acid on heat resistance and recovery of Clostridium sporogenes PA 3679 spores treated in HTST conditions

The effect of ethylenediaminetetraacetic acid (EDTA) on the heat resistance of Clostridium sporogenes PA 3679 spores was studied. EDTA was added to heating substrates and recovery media in order to establish which stage of the heat treatment registered the greatest EDTA activity. The heating substrates assayed were phosphate buffer (pH 7.0) and white asparagus purée, at natural pH (5.8) and acidified with citric acid and glucono-delta-lactone (GDL) to pH 5.5, 5.0 and 4.5. Recovery of survivors was carried out in MPA3679A medium in various conditions of acidification with citric and GDL (250 and 500 ppm), at pH 7.5 6.5 and 6.0. The results show greater activity of EDTA on spores when it was applied in recovery of heat injured spores, than during heating. The strongest influence of EDTA during heating was found in phosphate buffer (pH 7.0), with the effect being most evident at 121 and 126 degrees C, and in asparagus purée, at 121 degrees C and pH 5.8 rather than acidified. In recovery, the inhibiting activity of EDTA was more evident in spores subjected to more severe heat treatment, either by increasing the exposure time or by raising the temperature to 130 or 135 degrees C. The pH level of the recovery medium also affected the antimicrobial activity of EDTA, which had a greater inhibiting effect at pH 7.5 than at lower pH levels (6.5, 6.0).