Thermal Death Kinetics Research Articles

Thermal death kinetics in dry air of Diplodia seriata fungus inoculated in vitro in pruned vine shoots

The upsurge in fungal diseases that cause grapevine wood rot in young and adult plants remains a major concern for the wine sector. Since pruned vine shoots act as a reservoir of these fungi, there is a need for sanitization methods that can prevent their spread, avoiding plant reinfection. In this work, the effectiveness of dry heat treatments for the disinfection of pruning waste has been investigated. The thermal death of two inocula of Diplodia seriata fungus, associated to black dead arm disease, was studied in vitro at four different temperatures (40, 50, 60 and 65 °C) for different time periods (20, 40, 60, 90, 120 and 150 min) in a dry air incubator. Obtained data confirmed that inhibition increased as a function of temperature and exposure time. First-order and Weibull models were compared to describe D. seriata fungus survival curve kinetics, with high R 2 values (0.923 and 0.930, respectively) and high significances (p < 0.001). Both models predict that the time required to reach 99 % mortality at 65 °C would be 143 min, slightly higher than the experimentally determined minimum time of 120 min. The thermal death kinetics data reported herein has direct applications in process design and validation studies of other grapevine-pruning-waste sanitization methodologies.

Simulation of heating uniformity in a heating block system modified for controlled atmosphere treatments

Reliable and repeatable insect thermal mortality data rely on the performance of a heating device. Computer simulation has been widely used to optimize structures, design parameters and process conditions. To improve the temperature uniformity of a heating block system (HBS), a computer model was developed using finite element software COMSOL. Good agreement was obtained between the simulated and experimental block surface temperatures at three positions of the HBS and three heating rates. The validated computer model was further used to predict the effects of heating rates, the position of test insects and the addition of gases on the block and air temperature distributions. Simulation results showed that increasing heating rate reduced heating uniformity. The position of test insects in the treatment chamber largely affected their heating rate, with a position closer to the surface of the heat block providing a better temperature match between test insects and the HBS. When gas was added, block temperatures within the treatment chamber, particularly near the gas inlet, were influenced by gas speeds, temperatures and the gas channel design. The heating uniformity in the treatment chamber of the HBS was improved by heating the gas before adding it to the HBS, by routing the gas channel through the heating block to preheat the gas, and by using a relatively slow gas speed. The simulation results demonstrated that the validated computer model could be a reliable tool to evaluate the heating performance of the HBS for studying insect thermal death kinetics and optimize treatment conditions for the HBS when modified to include controlled atmospheres.

Thermal Death Kinetics of Conogethes Punctiferalis (Lepidoptera: Pyralidae) as Influenced by Heating Rate and Life Stage.

Thermal death kinetics of Conogethes punctiferalis (Guenée) (Lepidoptera: Pyralidae) at different life stages, heating rate, and temperature is essential for developing postharvest treatments to control pests in chestnuts. Using a heating block system (HBS), the most heat-tolerant life stage of C. punctiferalis and the effects of heating rate (0.1, 0.5, 1, 5, and 10°C/min) on insect mortality were determined. The thermal death kinetic data of fifth-instar C. punctiferalis were obtained at temperatures between 44 and 50°C at a heating rate of 5°C/min. The results showed that the relative heat tolerance of C. punctiferalis was found to be fifth instars>pupae> third instars> eggs. To avoid the enhanced thermal tolerance of C. punctiferalis at low heating rates (0.1 or 0.5°C/min), a high heating rate of 5°C/min was selected to simulate the fast radio frequency heating in chestnuts and further determine the thermal death kinetic data. Thermal death curves of C. punctiferalis followed a 0th-order kinetic reaction model. The minimum exposure time to achieve 100% mortality was 55, 12, 6, and 3 min at 44, 46, 48, and 50°C, respectively. The activation energy for controlling C. punctiferalis was 482.15 kJ/mol with the z value of 4.09°C obtained from the thermal death-time curve. The information provided by thermal death kinetics for C. punctiferalis is useful in developing effective postharvest thermal treatment protocols for disinfesting chestnuts.

Empirical model based on Weibull distribution describing the destruction kinetics of natural microbiota in pineapple (Ananas comosus L.) puree during high-pressure processing

High pressure inactivation of natural microbiota viz. aerobic mesophiles (AM), psychrotrophs (PC), yeasts and molds (YM), total coliforms (TC) and lactic acid bacteria (LAB) in pineapple puree was studied within the experimental domain of 0.1–600MPa and 30–50°C with a treatment time up to 20min. A complete destruction of yeasts and molds was obtained at 500MPa/50°C/15min; whereas no counts were detected for TC and LAB at 300MPa/30°C/15min. A maximum of two log cycle reductions was obtained for YM during pulse pressurization at the severe process intensity of 600MPa/50°C/20min. The Weibull model clearly described the non-linearity of the survival curves during the isobaric period. The tailing effect, as confirmed by the shape parameter (β) of the survival curve, was obtained in case of YM (β<1); whereas a shouldering effect (β>1) was observed for the other microbial groups. Analogous to thermal death kinetics, the activation energy (Ea, kJ·mol−1) and the activation volume (Va, mL·mol−1) values were computed further to describe the temperature and pressure dependencies of the scale parameter (δ, min), respectively. A higher δ value was obtained for each microbe at a lower temperature and it decreased with an increase in pressure. A secondary kinetic model was developed describing the inactivation rate (k, min−1) as a function of pressure (P, MPa) and temperature (T, K) including the dependencies of Ea and Va on P and T, respectively.

Tolerance of Sitophilus zeamais (Coleoptera: Curculionidae) to heated controlled atmosphere treatments

Combination heat and controlled atmosphere (CA) postharvest phytosanitary treatments are environmentally friendly alternatives to chemical fumigants. A controlled atmosphere/heating block system (CA-HBS) was used to rapidly assess tolerances of adult maize weevil, Sitophilus zeamais, both under regular air (RA) and CA (1% O2 and 15% CO2) conditions. In the RA treatment, thermal death kinetics for S. zeamais adults were determined at temperatures between 46 °C and 52 °C at a heating rate of 5 °C/min. The results showed that thermal death curves of S. zeamais adults followed a 0th-order kinetic reaction model. The required holding times for achieving 100% mortality were 165, 40, 14, and 4 min at 46, 48, 50 and 52 °C, respectively. The activation energy for killing S. zeamais adults was 526.7 kJ/mol. The effects of CA at various temperature-time combinations and heating rates on insect mortality were evaluated. The mortality of S. zeamais adults was higher under CA treatment than in the heat treatment alone. The slowest heating rate (0.1 °C/min) achieved the highest insect mortality in CA treatments but lowest mortality in RA treatments. The information obtained from the CA-HBS can be used to develop combination heat and CA treatments against S. zeamais.

Thermal death kinetics of fifth-instar Corcyras cephalonica (Lepidoptera: Galleriidae).

The infestation of rice moth, Corcyras cephalonica (Lepidoptera: Galleriidae), causes severe losses in postharvest walnuts. Heat has been studied as a phytosanitary treatment to replace chemical fumigation for controlling this pest. Information on kinetics for thermal mortality of C. cephalonica is needed for developing effective postharvest phytosanitary thermal treatments of walnuts. Thermal death kinetics of fifth-instar C. cephalonica were investigated at temperatures between 44°C and 50°C at a heating rate of 5°C min−1 using a heating block system. The results showed that thermal-death curves for C. cephalonica larvae followed a 0 order of kinetic reaction. The time to reach 100% mortality decreased with increasing temperature from 150 min at 44°C to 2.5 min at 50°C. The activation energy for controlling C. cephalonica was 466–592 kJ/mol, and the z value obtained from the thermal death time curve was 3.3°C. This kinetic model prediction could be useful in designing the thermal treatment protocol for controlling C. cephalonica in walnuts.

Thermal death kinetics of adult Sitophilus oryzae and effects of heating rate on thermotolerance

Information on thermal death kinetics of targeted stored insects under different heating conditions is essential for developing postharvest disinfestation treatment protocols. Using a heating block system, the thermal death kinetics of adult rice weevil, Sitophilus oryzae (L.), were determined at temperatures from 44 to 50 °C at 2 °C intervals and a heating rate of 5 °C/min. The effects of heating rates (0.1, 0.5, 1, 5 and 10 °C/min) on mortality were also examined. The results showed that thermal death curves of S. oryzae followed a 0-order kinetic reaction model. The required holding times for achieving 100% mortality were 130, 50, 12, and 4 min at 44, 46, 48, and 50 °C, respectively. The activation energy for killing S. oryzae was 505 kJ/mol and the z value obtained from the thermal-death-time curve was 3.9 °C. Insect mortality after a 20 min exposure to 46 °C at low heating rates (0.1 or 0.5 °C/min) was significantly lower than that at high heating rates (1–10 °C/min). The information provided by thermal death kinetics for S. oryzae is useful in developing effective postharvest thermal treatment protocols.

Lethal Temperature against Japanese Pine Sawyer, Monochamus alternatus, in Infested Wood using Microwave Energy

Thermal death kinetics of the Japanese pine sawyer, Monochamus alternatus, was investigated to develop a heat treatment method that controls the pest in wood packing materials used in the export of goods. To determine the feasibility of microwave irradiation as an alternative treatment, laboratory experiments were performed on blocks (200x200x250 mm) of Douglas fir artificially infested with live larvae and pupae irradiating the blocks with 2.45 GHz of microwave energy. One hundred percent mortality of larvae and pupae of Japanese pine sawyer was observed by hot water treatment at 64°C and 68°C, respectively. One hundred percent mortality of treated Japanese pine sawyer was achieved above 67°C for larvae and 70°C for pupae based on the measured wood temperature, regardless of the irradiation time.

Occurrence of molds on laminated paperboard for aseptic packaging, selection of the most hydrogen peroxide- and heat-resistant isolates and determination of their thermal death kinetics in sterile distilled water

This study aimed at enumerating molds (heat-labile and heat-resistant) on the surface of paperboard material to be filled with tomato pulps through an aseptic system and at determining the most heat- and hydrogen peroxide-resistant strains. A total of 118 samples of laminated paperboard before filling were collected, being 68 before and 50 after the hydrogen peroxide bath. Seven molds, including heat-resistant strains (Penicillium variotii and Talaromyces flavus) with counts ranging between 0.71 and 1.02 CFU/cm(2) were isolated. P. variotii was more resistant to hydrogen peroxide than T. flavus and was inactivated after heating at 85 °C/15 min. When exposed to 35 % hydrogen peroxide at 25 °C, T. flavus (F5E2) and N. fischeri (control) were less resistant than P. variotti (F1A1). P. citrinum (F7E2) was shown to be as resistant as P. variotti. The D values (the time to cause one logarithmic cycle reduction in a microbial population at a determined temperature) for spores of P. variotii (F1A1) and N. fischeri (control) with 4 months of age at 85 and 90 °C were 3.9 and 4.5 min, respectively. Although the contamination of packages was low, the presence of heat- and chemical-resistant molds may be of concern for package sterility and product stability during shelf-life. To our knowledge, this is the first report that focuses on the isolation of molds, including heat-resistant ones, contaminating paperboard packaging material and on estimating their resistance to the chemical and physical processes used for packaging sterilization.

Evaluation and Quantification of Heat Treatments Applied to Food Preserves

The quantitative study of heat treatments for sterilisation uses the Bigelow model to calculate the sterilising value (F). Calculation of F requires the previous determination of parameters D (decimal reduction time at experimental temperature) and Z (thermal-death time parameter), obtained from the thermal-death kinetics. Herein we compare two different methods, namely the Bigelow model and a predictive-type statistical method, to calculate the sterilisation effect against Bacillus coagulans spores when heat was applied to runner bean preserves (variety: Helda). Samples were subjected to various autoclave treatments at working temperatures (T ai) of 105, 107, 110, and 115°C for periods from 3 to 35 min. The microorganism used was B. coagulans. Sterilisation achieved by these autoclave treatments was determined by using the equation based on the Bigelow model (n probe = F z Ti/D Ti) where n is the fractional concentration of colony-forming units (or some quality factor), F z Ti is F at temperature T i, and D Ti is D at temperature T i. The Bigelow model can be used to obtain Z (thermal-death time parameter), which is needed to calculate the traditional sterilisation factor F, but not to determine the reduction factor n for the heat treatments, particularly when microbial indicators with low decimal reduction times (D) are studied. The thermokinetic parameters for B. coagulans in runner bean solution resulted to be Z = 10.64°C and D 121 = 0.0264 min (Af = 1.04). Treatment at 115°C for 20 min resulted in the most efficient sterilisation effect for B. coagulans.

Thermal death kinetics of B. stearothermophilus in coriander leaf puree

Coriander leaves are widely used in culinary throughout the world. Thermal death kinetics of Bacillus stearothermophilus in coriander leaf puree was investigated at varying levels of pH (4.5–8.5) and processing temperatures (105–145 °C). The thermal death kinetics data was statistically treated using non-linear regression. The first order kinetics model provided a good description of the heat inactivation of B. stearothermophilus in coriander puree for the entire range of temperatures tested and was therefore found to be appropriate for predictive purposes. The effect of temperature on death rate was modelled using Arrhenius equation (coefficient of determination R2 > 0.90). This study utilised the principles of predictive microbiology in real food and thus will be useful for process design and validation studies.

Insertional mutagenesis of Listeria monocytogenes 568 reveals genes that contribute to enhanced thermotolerance

The objectives of this study were to identify molecular mechanisms of thermotolerance using transposon mutants of Listeria monocytogenes 568, serotype 1/2a, and to compare their thermal death kinetics at 52, 56 and 60 °C. Sixteen Tn 917 transposon mutants with enhanced heat resistance were acquired from a library of 4300 mutants following a multi-step screening process. Genetic regions with Tn 917 insertions encompassed a broad range of functionalities including; transport, metabolism, replication and repair, general stress, and structural properties. Modeling of the heat inactivation data using the Geeraerd et al. and Whiting (Fermi) models showed that the mutants' enhanced thermal resistance was manifested mostly through a significant ( p ≤ 0.05) extension of the lag period on the thermal death curve. This new knowledge impacts our understanding of molecular mechanisms affecting the kinetics of thermally induced cell death and enables the development of safer thermal processes.

Lethal Effects of Heat and Use of Localized Heat Treatment for Control of Bed Bug Infestations

Bed bugs, Cimex lectularius L., hide in cracks and crevices in furniture and are difficult to control. The bed bug thermal death kinetics were examined to develop a heat treatment method to eliminate bed bug infestations in room contents. High temperatures caused temporary immobilization (knockdown) of bed bugs even with exposures that did not have lethal effects. Exposure of bed bug adults to 39 degrees C for 240 min caused no mortality; however, as temperatures increased from 41 to 49 degrees C, exposure times that caused 100% mortality decreased. The temperature difference to provide a 10-fold change in the mortality was estimated at 4 degrees C, and the estimated activation energy (EA) was between 484 and 488.3 kJ/mol. This demonstrates that bed bugs are not more resistant or susceptible to changes in temperature than other tested insects and that the temperatures needed to kill bed bugs are relatively low. In room treatment tests, heat treatment times varied from 2 to 7 h with complete mortality of exposed bed bugs within the treatment envelope created by surrounding the treated furniture with polystyrene sheathing boards. Containment and circulation of heat around the treated material were crucial factors in an efficient heat treatment for bed bug control. The room floor material greatly affected containment of the heat. The tested method for limited heat treatment of furniture and other room contents required equipment costing less than US$400 and provided opportunity for residual pesticide application around the room with minimal disruption in use of treated room.

Thermal Death Kinetics of Mediterranean, Malaysian, Melon, and Oriental Fruit Fly (Diptera: Tephritidae) Eggs and Third Instars

The late-aged egg and third-instar life stages of laboratory-reared Malaysian fruit fly, Bactrocera latifrons (Hendel); Mediterranean fruit fly, Ceratitis capitata (Wiedemann); melon fly, B. cucurbitae Coquillett; and oriental fruit fly, B. dorsalis (Hendel), (Diptera: Tephritidae); and the third instars of wild Mediterranean fruit fly were exposed to thermal treatments. A heating block system was used to determine the thermal death kinetics of the four fruit fly species. Treatments consisted of heating the fruit fly life stages to 44, 46, 48, and 50 degrees C and holding for different times ranging from 0 to 120 min depending on the thermal mortality response and time required to obtain 100% mortality for each species and life stage. The 0.5-order kinetic model had the best fit to the survival ratio for all the treatment temperatures and was used to predict lethal times. The thermal death time (TDT) curves showed a tolerance order of Mediterranean fruit fly eggs < or = third instars at 44, 46, and 50 degrees C, third instars < or = eggs at 48 degrees C, and wild third instars < the laboratory-reared third instars. Comparison between Mediterranean fruit fly third instar thermotolerance from Hawaii and Israel showed that Israel Mediterranean fruit fly was more thermotolerant. A comparison of minimum treatment times at a given temperature required to obtain 100% mortality of laboratory-reared Malaysian, Mediterranean (Hawaii and Israel strains), melon, Mexican, and oriental fruit fly eggs or third instars and wild Mediterranean fruit fly (Hawaii strain) eggs or third instars showed that oriental fruit fly was the most thermotolerant among the third instars, and the difference in heat tolerance between third instars and eggs was negligible at 50 degrees C.

Reaction Orders for Thermal Mortality of Third Instars of Mexican Fruit Fly (Diptera: Tephritidae)

Mexican fruit fly, Anastrepha ludens (Loew), is a quarantine pest of several fruit, including citrus, avocados, and mangoes, from extreme southern Texas to Costa Rica. To provide information for modeling heat phytosanitary treatments, third instars were heated with an aluminum heating block between 44 and 50 degrees C for time intervals up to those causing 100% mortality. At 44 and 50 degrees C, 100% mortality was achieved at 100 and 2 min, respectively. Each 2 degrees C increase in temperature resulted in a three-fourths reduction in the amount of time required to achieve 100% mortality. Mortality was modeled using thermal death kinetics, and the most suitable reaction order was the 0.5th. The thermal death activation energy was 560.7 kJ/mol, which is very similar to the value found for Mediterranean fruit fly, Ceratitis capitata (Wiedemann), in a previous study, indicating similar modes of action for heat mortality. However, the Mexican fruit fly had a lower threshold for heat-induced mortality, resulting in less time at all temperatures studied to achieve 100% mortality compared with the Mediterranean fruit fly. This type of information being gathered for fruit flies could lead to the development of generic phytosanitary heat treatments, which are available for other major phytosanitary treatments, such as cold storage, methyl bromide fumigation, and ionizing irradiation.

Quality of oranges as influenced by potential radio frequency heat treatments against Mediterranean fruit flies

There has been an increased interest in developing alternative quarantine treatment methods for control of fruit flies under growing international pressure to replace the remaining use of methyl bromide fumigation because of concerns over its role in ozone depletion. The present work explored the possibility of using radio frequency (RF) heating as a means to increase the internal fruit heating rate in water to control pests. Based on the thermal death kinetics of the Mediterranean fruit fly (Medfly), thermal treatments were designed that could provide quarantine security against fruit flies. The main objective of this research was to study the influence of those RF heat treatments on the quality of treated fruit. Treated ‘Navel’ and ‘Valencia’ oranges were evaluated for post-harvest quality after 10 days of 4 °C storage. The quality parameters included: weight loss, loss in firmness, color change, total soluble solids, acidity, and change in volatiles. The volatile analysis was done by the SPME-GC/MS technique. The results indicated a significant change in volatile flavor profiles upon RF heat treatments even when there was no significant difference in the other quality parameters. The reduction in process time due to RF heating helped in retention of many volatile compounds in comparison with conventional hot water heating. The treatment that raises fruit temperature from 19 to 48 °C by RF heating in saline water and held then for 15 min in 48 °C hot water would meet the quarantine security without impairing the quality of the treated oranges. However, sensory evaluation for market acceptability of treated oranges should be carried out for complete treatment protocol development.

Design and development of apparatus to provide repeatable surface temperature–time treatments on inoculated food samples

The majority of data relating heat treatments to thermal death kinetics has previously been obtained by carrying out in vitro tests on small samples of microorganisms in growth medium or food slurries. However, strong evidence exists that suggests microbial death on heat-treated food surfaces cannot be predicted accurately from such systems. An apparatus was therefore designed to enable microbial death on food surfaces to be quantified. The apparatus uses hot air to provide a ‘dry’ heat treatment to raise the surface of a food sample to a given temperature, up to 100 °C, hold and then cool it. The surface temperature was measured using an infra-red (IR) thermometer and the air heater was controlled to give a specified surface temperature history. Steam was also used to provide a ‘wet’, but less controlled heating cycle.

Thermal Death Kinetics of Red Flour Beetle (Coleoptera: Tenebrionidae)

While developing radio frequency heat treatments for dried fruits and nuts, we used a heating block system developed by Washington State University to identify the most heat-tolerant life stage of red flour beetle, Tribolium castaneum (Herbst), and to determine its thermal death kinetics. Using a heating rate of 15 degrees C/min to approximate the rapid heating of radio frequency treatments, the relative heat tolerance of red flour beetle stages was found to be older larvae > pupae and adults > eggs and younger larvae. Lethal exposure times for temperatures of 48, 50, and 52 degrees C for the most heat-tolerant larval stage were estimated using a 0.5th order kinetic model. Exposures needed for 95% mortality at 48 degrees C were too long to be practical (67 min), but increasing treatment temperatures to 50 and 52 degrees C resulted in more useful exposure times of 8 and 1.3 min, respectively. Red flour beetle was more sensitive to changes in treatment temperature than previously studied moth species, resulting in red flour beetle being the most heat-tolerant species at 48 degrees C, but navel orangeworm, Amyelois transitella (Walker), being most heat tolerant at 50 and 52 degrees C. Consequently, efficacious treatments for navel orangeworm at 50-52 degrees C also would control red flour beetle.

Thermal Death Kinetics of Red Flour Beetle (Coleoptera: Tenebrionidae)

While developing radio frequency heat treatments for dried fruits and nuts, we used a heating block system developed by Washington State University to identify the most heat-tolerant life stage of red flour beetle, Tribolium castaneum (Herbst), and to determine its thermal death kinetics. Using a heating rate of 15°C/min to approximate the rapid heating of radio frequency treatments, the relative heat tolerance of red flour beetle stages was found to be older larvae > pupae and adults > eggs and younger larvae. Lethal exposure times for temperatures of 48, 50, and 52°C for the most heat-tolerant larval stage were estimated using a 0.5th order kinetic model. Exposures needed for 95% mortality at 48°C were too long to be practical (67 min), but increasing treatment temperatures to 50 and 52°C resulted in more useful exposure times of 8 and 1.3 min, respectively. Red flour beetle was more sensitive to changes in treatment temperature than previously studied moth species, resulting in red flour beetle being the most heat-tolerant species at 48°C, but navel orangeworm, Amyelois transitella (Walker), being most heat tolerant at 50 and 52°C. Consequently, efficacious treatments for navel orangeworm at 50–52°C also would control red flour beetle.

Heating condition effects on thermal resistance of fifth-instar Amyelois transitella (Walker) (Lepidoptera: Pyralidae)

Successful development of a thermal treatment protocol depends on reliable information on fundamental thermal death kinetics of targeted insects under different heating conditions. The effects of heating rates (1, 10, and 15 °C min −1), pre-treatment conditioning (30 °C+6 h), and the difference between long-term laboratory cultures and recently isolated cultures on thermal mortality of fifth-instar navel orangeworm, Amyelois transitella (Walker), were studied using a heating block system. There was no significant difference in insect mortality resulting from heating rates of 10 and 15 °C min −1. Temperature control at 1 °C min −1 was more uniform than for the other heating rates, resulting in reduced variability for insect mortality. The mean mortality at the heating rate of 1 °C min −1 was significantly lower than for the two faster heating rates only at 48 °C+30 min. The pre-treatment conditioning of fifth-instar Amyelois transitella enhanced their thermotolerance only at certain temperature–time combinations. Fifth-instars from long-term laboratory and recently isolated cultures were equally susceptible to elevated temperatures. Therefore, thermal death kinetic information obtained from the long-term laboratory cultures can be used to develop thermal protocols against field pests.