Effects of pulsed electric fields on inactivation kinetics of Listeria innocua

The effects of pulsed electric field (PEF) treatment and processing factors on the inactivation kinetics of Listeria innocua NCTC 11289 were investigated by using a pilot plant PEF unit with a flow rate of 200 liters/h. The electric field strength, pulse length, number of pulses, and inlet temperature were the most significant process factors influencing the inactivation kinetics. Product factors (pH and conductivity) also influenced the inactivation kinetics. In phosphate buffer at pH 4.0 and 0.5 S/m at 40 degrees C, a 3. 0-V/microm PEF treatment at an inlet temperature of 40 degrees C resulted in > or = 6.3 log inactivation of strain NCTC 11289 at 49.5 degrees C. A synergistic effect between temperature and PEF inactivation was also observed. The inactivation obtained with PEF was compared to the inactivation obtained with heat. We found that heat inactivation was less effective than PEF inactivation under similar time and temperature conditions. L. innocua cells which were incubated for a prolonged time in the stationary phase were more resistant to the PEF treatment, indicating that the physiological state of the microorganism plays a role in inactivation by PEF. Sublethal injury of cells was observed after PEF treatment, and the injury was more severe when the level of treatment was increased. Overall, our results indicate that it may be possible to use PEF in future applications in order to produce safe products.     

Pulsed Electric Field Alters Molecular Chaperone Expression and Sensitizes Listeria monocytogenes to Heat

Pulsed electric field (PEF)-resistant and PEF-sensitive Listeria monocytogenes strains were sublethally treated with electric pulses at 15 kV/cm for 29 μs and held at 25°C for 5 to 30 min prior to protein extraction. The levels of the molecular chaperones GroEL, GroES, and DnaJ were determined by immunoblotting. After 10 to 20 min after sublethal PEF treatment, a transient decrease in molecular chaperone expression was observed in the PEF-sensitive strain (Scott A). The levels of GroEL and DnaJ increased back to the basal expression level within 30 min. A substantial decrease in GroES expression persisted for at least 30 min after PEF treatment. Chaperone expression was suppressed after PEF treatment to a smaller extent in the PEF-resistant (OSY-8578) than in the PEF-sensitive strain, and no clear expression pattern was identified in OSY-8578. Inactivation of Scott A and OSY-8578 in phosphate buffer was compared when lethal PEF (27.5 kV/cm, 144 μs) and heat (55°C, 10 min) were applied in sequence. When PEF and heat treatments were applied separately, the populations of L. monocytogenes Scott A and OSY-8578 decreased 0.5 to 0.6 log CFU/ml. Cells treated first with PEF and incubated at 25°C for 10 min showed substantial sensitivity to subsequent heat treatment; the decrease in counts for Scott A and OSY-8578 was 6.1 and 2.8 log CFU/ml, respectively. The sequence and time lapse between the two treatments were crucial for achieving high inactivation rates. It is concluded that PEF sensitized L. monocytogenes to heat and that maximum heat sensitization occurred when chaperone expression was at a minimum level.     

Inactivation of Mycobacterium paratuberculosis by Pulsed Electric Fields

The influence of treatment temperature and pulsed electric fields (PEF) on the viability of Mycobacterium paratuberculosis cells suspended in 0.1% (wt/vol) peptone water and in sterilized cow's milk was assessed by direct viable counts and by transmission electron microscopy (TEM). PEF treatment at 50°C (2,500 pulses at 30 kV/cm) reduced the level of viable M. paratuberculosis cells by approximately 5.3 and 5.9 log₁₀ CFU/ml in 0.1% peptone water and in cow's milk, respectively, while PEF treatment of M. paratuberculosis at lower temperatures resulted in less lethality. Heating alone at 50°C for 25 min or at 72°C for 25 s (extended high-temperature, short-time pasteurization) resulted in reductions of M. paratuberculosis of approximately 0.01 and 2.4 log₁₀ CFU/ml, respectively. TEM studies revealed that exposure to PEF treatment resulted in substantial damage at the cellular level to M. paratuberculosis.     

Inactivation of Escherichia coli and Listeria innocua in Milk by Combined Treatment with High Hydrostatic Pressure and the Lactoperoxidase System

We have studied inactivation of four strains each of Escherichia coli and Listeria innocua in milk by the combined use of high hydrostatic pressure and the lactoperoxidase-thiocyanate-hydrogen peroxide system as a potential mild food preservation method. The lactoperoxidase system alone exerted a bacteriostatic effect on both species for at least 24 h at room temperature, but none of the strains was inactivated. Upon high-pressure treatment in the presence of the lactoperoxidase system, different results were obtained for E. coli and L. innocua. For none of the E. coli strains did the lactoperoxidase system increase the inactivation compared to a treatment with high pressure alone. However, a strong synergistic interaction of both treatments was observed for L. innocua. Inactivation exceeding 7 decades was achieved for all strains with a mild treatment (400 MPa, 15 min, 20°C), which in the absence of the lactoperoxidase system caused only 2 to 5 decades of inactivation depending on the strain. Milk as a substrate was found to have a considerable effect protecting E. coli and L. innocua against pressure inactivation and reducing the effectiveness of the lactoperoxidase system under pressure on L. innocua. Time course experiments showed that L. innocua counts continued to decrease in the first hours after pressure treatment in the presence of the lactoperoxidase system. E. coli counts remained constant for at least 24 h, except after treatment at the highest pressure level (600 MPa, 15 min, 20°C), in which case, in the presence of the lactoperoxidase system, a transient decrease was observed, indicating sublethal injury rather than true inactivation.     

Inactivation of a Norovirus by High-Pressure Processing

Murine norovirus (strain MNV-1), a propagable norovirus, was evaluated for susceptibility to high-pressure processing. Experiments with virus stocks in Dulbecco's modified Eagle medium demonstrated that at room temperature (20°C) the virus was inactivated over a pressure range of 350 to 450 MPa, with a 5-min, 450-MPa treatment being sufficient to inactivate 6.85 log₁₀ PFU of MNV-1. The inactivation of MNV-1 was enhanced when pressure was applied at an initial temperature of 5°C; a 5-min pressure treatment of 350 MPa at 30°C inactivated 1.15 log₁₀ PFU of virus, while the same treatment at 5°C resulted in a reduction of 5.56 log₁₀ PFU. Evaluation of virus inactivation as a function of treatment times ranging from 0 to 150 s and 0 to 900 s at 5°C and 20°C, respectively, indicated that a decreasing rate of inactivation with time was consistent with Weibull or log-logistic inactivation kinetics. The inactivation of MNV-1 directly within oyster tissues was demonstrated; a 5-min, 400-MPa treatment at 5°C was sufficient to inactivate 4.05 log₁₀ PFU. This work is the first demonstration that norovirus can be inactivated by high pressure and suggests good prospects for inactivation of nonpropagable human norovirus strains in foods.     

Selection and Identification of a Listeria monocytogenes Target Strain for Pulsed Electric Field Process Optimization

Nine Listeria monocytogenes strains were treated individually with a continuous pulsed electric field (PEF) apparatus, and their sensitivities to the treatment were compared at 25 kV/cm. When cell suspensions of these strains in 0.1% NaCl (pH 7.0) were treated at 23°C for 144 μs, inactivation ranged from 0.7 to 3.7 log₁₀ CFU/ml. Inactivation by 72-μs PEF treatments at 37°C ranged from 0.3 to 2.5 log₁₀ CFU/ml. L. monocytogenes OSY-8578 was substantially more resistant than other strains when cells were PEF treated in 0.1% NaCl, whereas Scott A was one of the most sensitive strains. The superiority of OSY-8578's resistance to that of Scott A was confirmed in 50% diluted acid whey (pH 4.2). Changes in sensitivity to PEF during phases of growth were minimal in OSY-8578 and substantial in Scott A. Use of L. monocytogenes OSY-8578, therefore, is recommended in studies to optimize PEF processes that target L. monocytogenes. The nine L. monocytogenes strains were genotyped with pulsed-field gel electrophoresis (PFGE) and arbitrarily primed PCR (AP-PCR) techniques. These strains were better differentiated with PFGE than with AP-PCR. The target strain (OSY-8578) was characterized by both molecular typing techniques, but resistance to PEF, in general, was not associated with a particular genotype group.     

Effect of Electropermeabilization by Ohmic Heating for Inactivation of Escherichia coli O157:H7, Salmonella enterica Serovar Typhimurium,and Listeria monocytogenes in Buffered Peptone Water and Apple Juice

The effect of electric field-induced ohmic heating for inactivation of Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in buffered peptone water (BPW) (pH 7.2) and apple juice (pH 3.5; 11.8 °Brix) was investigated in this study. BPW and apple juice were treated at different temperatures (55°C, 58°C, and 60°C) and for different times (0, 10, 20, 25, and 30 s) by ohmic heating compared with conventional heating. The electric field strength was fixed at 30 V/cm and 60 V/cm for BPW and apple juice, respectively. Bacterial reduction resulting from ohmic heating was significantly different (P < 0.05) from that resulting from conventional heating at 58°C and 60°C in BPW and at 55°C, 58°C, and 60°C in apple juice for intervals of 0, 10, 20, 25, and 30 s. These results show that electric field-induced ohmic heating led to additional bacterial inactivation at sublethal temperatures. Transmission electron microscopy (TEM) observations and the propidium iodide (PI) uptake test were conducted after treatment at 60°C for 0, 10, 20, 25 and 30 s in BPW to observe the effects on cell permeability due to electroporation-caused cell damage. PI values when ohmic and conventional heating were compared were significantly different (P < 0.05), and these differences increased with increasing levels of inactivation of three food-borne pathogens. These results demonstrate that ohmic heating can more effectively reduce bacterial populations at reduced temperatures and shorter time intervals, especially in acidic fruit juices such as apple juice. Therefore, loss of quality can be minimized in a pasteurization process incorporating ohmic heating.     

Biphasic Thermal Inactivation Kinetics in Salmonella enteritidis PT4

The thermal inactivation kinetics of Salmonella enteritidis PT4 between 49 and 60°C were investigated. Using procedures designed to eliminate methodological artifacts, we found that the death kinetics deviated from the accepted model of first-order inactivation. When we used high-density stationary-phase populations and sensitive enumeration, the survivor curves at 60°C were reproducibly biphasic. The decimal reduction time at 60°C (D_60°C) of the tail subpopulation was more than four times that of the majority population. This difference decreased with decreasing temperature; i.e., the survivor curves became more linear, but the proportion of tail cells remained a constant proportion of the initial population, about 1 in 10⁴ to 10⁵. Z plots (log D versus temperature) for the two populations showed that the D values coincided at 51°C, indicating that the survivor curves should be linear at this temperature, and this was confirmed experimentally. Investigations into the nature of the tails ruled out genotypic differences between the populations and protection due to leakage from early heat casualties. Heating of cells at 59°C in the presence of 5 or 100 μg of chloramphenicol per ml resulted in reductions in the levels of tailing. These reductions were greatest at the higher chloramphenicol concentration. Our results indicate that de novo protein synthesis of heat shock proteins is responsible for the observed tailing. Chemostat-cultured cells heated at 60°C also produced biphasic survivor curves in all but one instance. Cells with higher growth rates were more heat sensitive, but tailing was comparable with batch cultures. Starved cells (no dilution input) displayed linear inactivation kinetics, suggesting that during starvation a rapid heat shock response cannot be initiated.     

Synergistic Inactivation of Spores of Proteolytic Clostridium botulinum Strains by High Pressure and Heat Is Strain and Product Dependent

The combined high pressure and heat resistances of spores of five proteolytic Clostridium botulinum strains and of the nonpathogenic surrogate strain Clostridium sporogenes PA3679 were compared with their heat-only resistances on the basis of equivalent accumulated thermal lethality, expressed as equivalent minutes at a reference temperature of 105°C (F_105°C). Comparisons were made with three model (i.e., diluted) products, namely, 30% (wt/wt) Bolognese sauce, 50% (wt/wt) cream sauce, and rice water agar. Pressure was determined to act synergistically with heat during high-pressure thermal (HPT) processing for C. botulinum FRRB 2802 (NCTC 7273) and C. botulinum FRRB 2804 (NCTC 3805 and 62A) in the Bolognese and cream sauces and for C. botulinum FRRB 2807 (213B) in the Bolognese sauce only. No synergy was observed for C. botulinum FRRB 2803 (NCTC 2916) or FRRB 2806 (62A) or C. sporogenes FRRB 2790 (NCTC 8594 and PA3679) in any of the model products. No significant protective effect of pressure against spore inactivation was determined for any Clostridium strain in any product. Because synergy was not consistently observed among strains of C. botulinum or among products, the prediction of inactivation of C. botulinum spores by HPT sterilization (HPTS) for the present must assume a complete lack of synergy. Therefore, any HPTS process for low-acid shelf-stable foods must be at least thermally equivalent to an F₀ process of 2.8 min, in line with current good manufacturing practices. The results of this study suggest that the use of C. sporogenes PA3679 as a surrogate organism may risk overestimating inactivation of C. botulinum by HPT processing.     

Persistence of Viruses in Desert Soils Amended with Anaerobically Digested Sewage Sludge

Pima County, Ariz., is currently investigating the potential benefits of land application of sewage sludge. To assess risks associated with the presence of pathogenic enteric viruses present in the sludge, laboratory studies were conducted to measure the inactivation rate (k = log₁₀ reduction per day) of poliovirus type 1 and bacteriophages MS2 and PRD-1 in two sludge-amended desert agricultural soils (Brazito Sandy Loam and Pima Clay Loam). Under constant moisture (approximately -0.05 × 10⁵ Pa for both soils) and temperatures of 15, 27, and 40°C, the main factors controlling the inactivation of these viruses were soil temperature and texture. As the temperature increased from 15 to 40°C, the inactivation rate increased significantly for poliovirus and MS2, whereas, for PRD-1, a significant increase in the inactivation rate was observed only at 40°C. Clay loam soils afforded more protection to all three viruses than sandy soils. At 15°C, the inactivation rate for MS2 ranged from 0.366 to 0.394 log₁₀ reduction per day in clay loam and sandy loam soils, respectively. At 27°C, this rate increased to 0.629 log₁₀ reduction per day in clay loam soil and to 0.652 in sandy loam soil. A similar trend was observed for poliovirus at 15°C (k = 0.064 log₁₀ reduction per day, clay loam; k = 0.095 log₁₀ reduction per day, sandy loam) and 27°C (k = 0.133 log₁₀ reduction per day, clay loam; k = 0.154 log₁₀ reduction per day, sandy loam). Neither MS2 nor poliovirus was recovered after 24 h at 40°C. No reduction of PRD-1 was observed after 28 days at 15°C and after 16 days at 27°C. At 40°C, the inactivation rates were 0.208 log₁₀ reduction per day in amended clay loam soil and 0.282 log₁₀ reduction per day in sandy loam soil. Evaporation to less than 5% soil moisture completely inactivated all three viruses within 7 days at 15°C, within 3 days at 27°C, and within 2 days at 40°C regardless of soil type. This suggests that a combination of high soil temperature and rapid loss of soil moisture will significantly reduce risks caused by viruses in sludge.     

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°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°C) accelerated the inactivation of C. botulinum spores. However, incubation at 100°C, 110°C, or 120°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°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°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.     

Ultraviolet Inactivation and Photoproducts of Transforming DNA Irradiated at Low Temperatures

Solutions of Haemophilus influenzae transforming DNA were irradiated at temperatures ranging from 25°C to - 196°C. Temperature dependence of the formation of thymine-containing dimers was closely correlated with inactivation of transforming activity; in general, both dimerization and inactivation decreased with decreasing temperature. The fraction of nonphotoreactivable damage increased with increasing dose at low temperatures. The nonphotoreactivable spore-type photoproduct was formed at low temperatures with a maximum at - 100°C, a temperature at which the nonphotoreactivable biological inactivation was also a maximum. Intrastrand cross-linking, like dimer formation, decreased with decreasing irradiation temperature.     

Combined Effects of High Hydrostatic Pressure and Temperature for Inactivation of Bacillus anthracis Spores

Spores of Bacillus anthracis are known to be extremely resistant to heat treatment, irradiation, desiccation, and disinfectants. To determine inactivation kinetics of spores by high pressure, B. anthracis spores of a Sterne strain-derived mutant deficient in the production of the toxin components (strain RP42) were exposed to pressures ranging from 280 to 500 MPa for 10 min to 6 h, combined with temperatures ranging from 20 to 75°C. The combination of heat and pressure resulted in complete destruction of B. anthracis spores, with a D value (exposure time for 90% inactivation of the spore population) of approximately 4 min after pressurization at 500 MPa and 75°C, compared to 160 min at 500 MPa and 20°C and 348 min at atmospheric pressure (0.1 MPa) and 75°C. The use of high pressure for spore inactivation represents a considerable improvement over other available methods of spore inactivation and could be of interest for antigenic spore preparation.     

Thermal Tolerance of Zymomonas mobilis: Temperature-Induced Changes in Membrane Composition

The membrane composition of Zymomonas mobilis changed dramatically in response to growth temperature. With increasing temperature, the proportion of vaccenic acid declined with an increase in myristic acid, the proportion of phosphatidylcholine and cardiolipin increased with decreases in phosphatidylethanolamine and phosphatidylglycerol, and the phospholipid/protein ratio of the membrane declined. These changes in membrane composition were correlated with changes in thermal tolerance and with changes in membrane fluidity. Cells grown at 20°C were more sensitive to inactivation at 45°C than were cells grown at 30°C, as expected. However, cells grown at 41°C (near the maximal growth temperature for Z. mobilis) were hypersensitive to thermal inactivation, suggesting that cells may be damaged during growth at this temperature. When cells were held at 45°C, soluble proteins from cells grown at 41°C were rapidly lost into the surrounding buffer in contrast to cells grown at lower temperatures. The synthesis of phospholipid-deficient membranes during growth at 41°C was proposed as being responsible for this increased thermal sensitivity.     

Developmental history affects the susceptibility of spinach leaves to in vivo low temperature photoinhibition

Room temperature chlorophyll a fluorescence was used to determine the effects of developmental history, developmental stage, and leaf age on susceptibility of spinach to in vivo low temperature (5°C) induced photoinhibition. Spinach (Spinacia oleracea cv Savoy) leaves expanded at cold hardening temperatures (5°C day/night), an irradiance of 250 micromoles per square meter per second of photosynthetic proton flux density, and a photoperiod of 16 hours were less sensitive than leaves expanded at nonhardening temperatures (16 or 25°C day/night) and the same irradiance and photoperiod. This differential sensitivity to low-temperature photoinhibition was observed at high (1200) but not lower (500 or 800 micromoles per square meter per second) irradiance treatment. In spite of a differential sensitivity to photoinhibition, both cold-hardened and nonhardened spinach exhibited similar recovery kinetics at either 20 or 5°C. Shifting plants grown at 16°C (day/night) to 5°C (day/night) for 12 days after full leaf expansion did not alter the sensitivity to photoinhibition at 5°C. Conversely, shifting plants grown at 5°C (day/night) to 16°C (day/night) for 12 days produced a sensitivity to photoinhibition at 5°C similar to control plants grown at 16°C. Thus, any resistance to low-temperature photoinhibition acquired during growth at 5°C was lost in 12 days at 16°C. We conclude that leaf developmental history, developmental stage, and leaf age contribute significantly to the in vivo photoinhibitory response of spinach. Thus, these characteristics must be defined clearly in studies of plant susceptibility to photoinhibition.     

Assessment of Heat Resistance of Bacterial Spores from Food Product Isolates by Fluorescence Monitoring of Dipicolinic Acid Release

This study is aimed at the development and application of a convenient and rapid optical assay to monitor the wet-heat resistance of bacterial endospores occurring in food samples. We tested the feasibility of measuring the release of the abundant spore component dipicolinic acid (DPA) as a probe for heat inactivation. Spores were isolated from the laboratory type strain Bacillus subtilis 168 and from two food product isolates, Bacillus subtilis A163 and Bacillus sporothermodurans IC4. Spores from the lab strain appeared much less heat resistant than those from the two food product isolates. The decimal reduction times (D values) for spores from strains 168, A163, and IC4 recovered on Trypticase soy agar were 1.4, 0.7, and 0.3 min at 105°C, 120°C, and 131°C, respectively. The estimated Z values were 6.3°C, 6.1°C, and 9.7°C, respectively. The extent of DPA release from the three spore crops was monitored as a function of incubation time and temperature. DPA concentrations were determined by measuring the emission at 545 nm of the fluorescent terbium-DPA complex in a microtiter plate fluorometer. We defined spore heat resistance as the critical DPA release temperature (T_c), the temperature at which half the DPA content has been released within a fixed incubation time. We found T_c values for spores from Bacillus strains 168, A163, and IC4 of 108°C, 121°C, and 131°C, respectively. On the basis of these observations, we developed a quantitative model that describes the time and temperature dependence of the experimentally determined extent of DPA release and spore inactivation. The model predicts a DPA release rate profile for each inactivated spore. In addition, it uncovers remarkable differences in the values for the temperature dependence parameters for the rate of spore inactivation, DPA release duration, and DPA release delay.     

Inactivation of Geobacillus stearothermophilus Spores by High-Pressure Carbon Dioxide Treatment

High-pressure CO₂ treatment has been studied as a promising method for inactivating bacterial spores. In the present study, we compared this method with other sterilization techniques, including heat and pressure treatment. Spores of Bacillus coagulans, Bacillus subtilis, Bacillus cereus, Bacillus licheniformis, and Geobacillus stearothermophilus were subjected to CO₂ treatment at 30 MPa and 35°C, to high-hydrostatic-pressure treatment at 200 MPa and 65°C, or to heat treatment at 0.1 MPa and 85°C. All of the bacterial spores except the G. stearothermophilus spores were easily inactivated by the heat treatment. The highly heat- and pressure-resistant spores of G. stearothermophilus were not the most resistant to CO₂ treatment. We also investigated the influence of temperature on CO₂ inactivation of G. stearothermophilus. Treatment with CO₂ and 30 MPa of pressure at 95°C for 120 min resulted in 5-log-order spore inactivation, whereas heat treatment at 95°C for 120 min and high-hydrostatic-pressure treatment at 30 MPa and 95°C for 120 min had little effect. The activation energy required for CO₂ treatment of G. stearothermophilus spores was lower than the activation energy for heat or pressure treatment. Although heat was not necessary for inactivationby CO₂ treatment of G. stearothermophilus spores, CO₂ treatment at 95°C was more effective than treatment at 95°C alone.     

Catabolite Inactivation in the Methylotrophic Yeast Pichia pastoris

Inactivation of the alcohol oxidase enzyme system of Pichia pastoris, during the whole-cell bioconversion of ethanol to acetaldehyde, was due to catabolite inactivation. Electron microscopy showed that methanol-grown cells contained peroxisomes but were devoid of these microbodies after the bioconversion. Acetaldehyde in the presence of O₂ was the effector of catabolite inactivation. The process was initiated by the appearance of free acetaldehyde, and was characterized by an increase in the level of cyclic AMP, that coincided with a rapid 55% drop in alcohol oxidase activity. Further enzyme inactivation, believed to be due to proteolytic degradation, then proceeded at a constant but slower rate and was complete 21 h after acetaldehyde appearance. The rate of catabolite inactivation was dependent on acetaldehyde concentration up to 0.14 mM. It was temperature dependent and occurred within 24 h at 37°C and by 6 days at 15°C but not at 3°C. Alcohol oxidase activity was psychrotolerant, with only a 17% decrease in initial specific activity over a temperature drop from 37 to 3°C. In contrast, protease activity was inhibited at temperatures below 15°C. When the bioconversion was run at 3°C, catabolite inactivation was prevented. In the presence of 3 M Tris hydrochloride buffer, 123 g of acetaldehyde per liter was produced at 3°C, compared with 58 g/liter at 30°C. By using 0.5 M Tris in a cyclic-batch procedure, 140.6 g of acetaldehyde was produced.     

Temperature characteristics and adaptive potential of wheat ribosomes

The translational efficiency of wheat ribosomes was studied as a function of an in vivo temperature pretreatment of wheat seedlings (Triticum aestivum L.). Ribosomes were isolated from heat-pretreated (36°C) and reference (4°C, 20°C) wheat seedlings. The efficiency of the ribosomes in translating polyuridylic acid was assayed. Ribosomes from heat-pretreated seedlings exhibit a threefold enhanced incorporation rate of phenylalanine as compared to ribosomes from wheat seedlings adapted to 20 or 4°C. This difference develops within 24 hours after onset of the heat treatment of seedlings following a 3 hour lag phase. The temperature induced changes can be traced back to the cytoplasmic ribosomes, since cycloheximide inhibits translation almost completely. Thermal inactivation of ribosomes occurs at 45°C, irrespective of the temperature pretreatment of the wheat seedlings. Specific differences in the yield of ribosomes, in the polyribosomal profiles, and in the apparent Arrhenius' activation energy of protein synthesis were observed depending on the age and the temperature pretreatments. The results presented here are considered an important molecular correlation to phenotypical temperature adaptation of in vivo protein synthesis in wheat (M Weidner, C Mathée, FK Schmitz 1982 Plant Physiol 69: 1281-1288).