Short communication: Temperature sensibility of Prototheca blaschkeae strains isolated from bovine mastitic milk

Dairy cow mastitis associated with microalgae of the genus Prototheca has been reported worldwide. This alga is extremely resistant to most antimicrobials commonly used in mastitis therapy. In milk processing, different thermal treatments are generally efficient at inactivating and eliminating microorganisms. Until recently, no reports on Prototheca blaschkeae susceptibility to heat treatment have been described. Thus, considering the potential zoonotic risk that Prototheca may represent, the objective of this study was to test the susceptibility of P. blaschkeae field isolates retrieved from bovine mastitis to different temperature/time ratios that are generally used in the milk processing industry: 62°C/15 min and 30 min; 70°C/20 s, 15 min, and 30 min; 75°C/20 s; 90°C/1 s; and 100°C/1 s. The results showed a growth reduction of all isolates after the heat treatments, but only at 100°C was a total growth inhibition observed.     

Inactivation of Bacillus stearothermophilus spores in egg patties by pressure-assisted thermal processing

The use of pressure-assisted thermal processing (PATP) to inactivate bacterial spores in shelf-stable low-acid foods, without diminishing product quality, has received widespread industry interest. Egg patties were inoculated with Bacillus stearothermophilus spores (10⁶ spores/g) and the product was packaged in sterile pouches by heat sealing. Test samples were preheated and then PATP-treated at 105 °C at various pressures and pressure-holding times. Thermal inactivation of spores was studied at 121 °C using custom-fabricated aluminum tubes; this treatment served as a control. Application of PATP at 700 MPa and 105 °C inactivated B. stearothermophilus spores, suspended in egg matrix rapidly, (4 log reductions in 5 min) when compared to thermal treatment at 121 °C (1.5 log reduction in 15 min). Spore inactivation by PATP progressed rapidly (3 log reductions at 700 MPa and 105 °C) during pressure-hold for up to 100 s, but greater holding times (up to 5 min) had comparatively limited effect. When PATP was applied to spores in water suspension or egg patties, D values were not significantly different. While thermal inactivation of spores followed first-order kinetics, PATP inactivation exhibited nonlinear inactivation kinetics. Among the nonlinear models tested, the Weibull model best described PATP inactivation of B. stearothermophilus spores in the egg product.     

Inactivation of Cronobacter sakazakii by manothermosonication in buffer and milk

The inactivation of Cronobacter sakazakii by heat and ultrasound treatments under pressure at different temperatures [manosonication (MS) and manothermosonication (MTS)] was studied in citrate-phosphate pH 7.0 buffer and rehydrated powdered milk. The inactivation rate was an exponential function of the treatment time for MS/MTS treatments (35−68 °C; 200 kPa of pressure; 117 μm of amplitude of ultrasonic waves) in both media, and for thermal treatments alone when buffer was used as heating media. Survival curves of C. sakazakii during heating in milk had a concave downward profile. Up to 50 °C, the lethality of ultrasound under pressure treatments was independent of the treatment temperature in both media. At temperatures greater than 64 °C in buffer and 68 °C in milk, the inactivating effect of MTS was equivalent to that of the thermal treatments alone at the same temperature. Between 50 and 64 ºC for buffer and 50 and 68 °C for milk, the lethality of MTS was the result of a synergistic effect, where the total lethal effect was higher than the lethal effect of heat added to that of ultrasound under pressure at room temperature. The maximum synergism was found at 60 °C in buffer and at 56 °C in milk. A heat treatment of 12 min (60 °C) or 4 min of an ultrasound under pressure at room temperature treatment (35 °C; 200 kPa; 117 μm) would be necessary to guarantee the death of 99.99% of C. sakazakii cells suspended in milk. The same level of C. sakazakii inactivation can be achieved with 1.8 min of a MTS treatment (60 °C; 200 kPa; 117 μm). Damaged cells were detected after heat treatments and after ultrasound under pressure treatments at lethal but not at non-lethal temperatures.     

Heat resistance of Listeria species to liquid whole egg ultrapasteurization treatment

The lethality of ultrapasteurization treatments (70 °C/1.5 min.) applied at constant temperature (isothermal condition) and at a constantly raising temperature of 2 °C/min (non-isothermal condition) in liquid whole egg (LWE) against two strains of Listeria monocytogenes (STCC 5672 and 4032) and one of Listeria innocua has been investigated. Isothermal survival curves up to 71 °C were obtained, which followed first-order inactivation kinetics. The obtained D_t values indicated that L. innocua was significantly (p < 0.05) more heat resistant than L. monocytogenes strains. Non-significant (p > 0.05) differences were observed among z values (12.4 ± 0.4 °C, 13.1 ± 0.4 °C and 12.2 ± 0.7 °C for L. innocua and L. monocytogenes 5672 and 4032, respectively). Based on obtained D_t and z values, isothermal ultrapasteurization treatment (70 °C/1.5 min.) would provide 3.5-, 5.0-, and 6.5-Log₁₀ cycles of L. innocua and L. monocytogenes 5672 and 4032, respectively. Non-isothermal heating lag phase increased the thermotolerance of Listeria species in LWE. The simulated industrial pasteurization treatment for LWE (heating-up phase from 25 to 70 °C followed by 1.5 min. at 70 °C) would attain 5-Log₁₀ reductions of L. monocytogenes 5672 and 4032, and 3.7-Log₁₀ reductions of L. innocua. Therefore, the safety level of industrial ultrapasteurization concerning L. monocytogenes could be lower than that estimated with data obtained under isothermal conditions.     

Heat resistance of Neosartorya fischeri in various juices

Heat resistance of Neosartorya fischeri was studied in three different juices (apple, pineapple and papaya). The optimum heat activation temperature and time for the ascospores of the N. fischeri (growth for 30 days at 30 °C) was 85 °C for 10 min. Of the three juices tested, apple juice exhibited maximum 1/k values at 80, 85 and 90 °C (208.3, 30.1 and 2.0 min, respectively). The 1/k values for papaya juice (129.9, 19.0 and 1.9 min) and pineapple juice (73.5, 13.2 and 1.5 min) decreased with acidity and °Brix/acidity (ratio) level. The Z^* values for apple, papaya and pineapple juices were 5, 5.5 and 5.9 °C, respectively. The sterilization F values (4-log reduction) for apple, pineapple and papaya juices were 56.3, 38.0 and 7.2 s, respectively. Considering the thermal treatments commercially applied to pineapple (96 °C/30 s) and apple juices (95 °C/30 s), it is concluded that such treatments will not guarantee that less than 1 ascospore in each set of 10³ packs survive. Only the treatment applied to papaya juice (100 °C/30 s) will be sufficient because the F value is less than 30 s.     

A mathematical model of inactivation kinetics for a four-strain composite of Salmonella Enteritidis and Oranienburg in commercial liquid egg yolk

The goal of this study was to develop a general model of inactivation of salmonellae in commercial liquid egg yolk for temperatures ranging from 58 °C to 66 °C by studying the inactivation kinetics of Salmonella in liquid egg yolk. Heat-resistant salmonellae (three serovars of Enteritidis [two of phage type 8 and one PT 13] and one Oranienburg) were grown to stationary phase in Tryptic Soy Broth and concentrated 10-fold by centrifugation. Each inoculum was added to liquid egg yolk and mixed thoroughly, resulting in a final population of ca. 7 log CFU/ml egg yolk. Inoculated yolk was injected into sterile glass capillary tubes, flame-sealed and heated in a water bath at 58, 60, 62, 64, and 66 °C. Capillary tubes were ethanol sanitized, rinsed, and contents were extracted. Yolk was diluted, surface plated onto Tryptic Soy Agar + 0.1% sodium pyruvate and 50 μg/ml nalidixic acid and incubated at 37 °C for 24 h before colonies were enumerated. Decimal reduction values were calculated from survivor curves with a minimum inactivation of 6 log CFU/ml at each temperature. Survival curves (except for 66 °C) featured initial lag periods before first order linear inactivation. Estimated asymptotic D-values were 1.83 min at 58 °C, 0.69 min at 60 °C, 0.26 min at 62 °C, 0.096 min at 64 °C and 0.036 min at 66 °C. The estimate of the asymptotic z-value was ca. 4.7 °C with standard error of 0.07 °C. A linear relationship between the log₁₀ of the lag times and temperature was observed. A general kinetic model of inactivation was developed. The results of the study provide information that can be used by processors to aid in producing safe pasteurized egg yolk products and for satisfying pasteurization performance standards and developing industry guidance.     

Strategy to inactivate Clostridium perfringens spores in meat products

The current study aimed to develop an inactivation strategy for Clostridium perfringens spores in meat through a combination of spore activation at low pressure (100–200 MPa, 7 min) and elevated temperature (80 °C, 10 min); spore germination at high temperatures (55, 60 or 65 °C); and inactivation of germinated spores with elevated temperatures (80 and 90 °C, 10 and 20 min) and high pressure (586 MPa, at 23 and 73 °C, 10 min). Low pressures (100–200 MPa) were insufficient to efficiently activate C. perfringens spores for germination. However, C. perfringens spores were efficiently activated with elevated temperature (80 °C, 10 min), and germinated at temperatures lethal for vegetative cells (≥55 °C) when incubated for 60 min with a mixture of l-asparagine and KCl (AK) in phosphate buffer (pH 7) and in poultry meat. Inactivation of spores (∼4 decimal reduction) in meat by elevated temperatures (80–90 °C for 20 min) required a long germination period (55 °C for 60 min). However, similar inactivation level was reached with shorter germination period (55 °C for 15 min) when spore contaminated-meat was treated with pressure-assisted thermal processing (568 MPa, 73 °C, 10 min). Therefore, the most efficient strategy to inactivate C. perfringens spores in poultry meat containing 50 mM AK consisted: (i) a primary heat treatment (80 °C, 10 min) to pasteurize and denature the meat proteins and to activate C. perfringens spores for germination; (ii) cooling of the product to 55 °C in about 20 min and further incubation at 55 °C for about 15 min for spore germination; and (iii) inactivation of germinated spores by pressure-assisted thermal processing (586 MPa at 73 °C for 10 min). Collectively, this study demonstrates the feasibility of an alternative and novel strategy to inactivate C. perfringens spores in meat products formulated with germinants specific for C. perfringens.     

Thermal inactivation of Salmonella spp. during conching

Although chocolate is a microbiologically stable product it has been described as a vehicle for Salmonella spp. Because of the low water activity (a_w) and the high fat content of chocolate Salmonella spp. shows an increased heat resistance, even during the thermal process of chocolate making. The aim of this study was to evaluate the thermal inactivation of Salmonella spp. during conching in various masses of chocolate and cocoa butter at different temperatures (50-90 °C). The effect of thermal treatment on Salmonella spp. was determined with the MPN (Most-Probable-Number) method. Results of thermal treatment showed approximate D-values for cocoa butter from D_50°C = 245 min to D_60°C = 306 min, for cocoa liquor from D_50°C = 999 min to D_90°C = 26 min and for dark chocolate of D_50°C = 1574 min. z-values were found to be z = 20 °C in cocoa liquor and z = 14 °C in dark chocolate. This study demonstrates that the conching process alone does not ensure the inactivation of Salmonella spp. in different chocolate masses and that an additional decontamination step at the beginning of the process as well as an HACCP concept is necessary during chocolate production to guarantee the absence of Salmonella spp. in chocolates and related products.     

Effect of heat treatment of bovine colostrum on bacterial counts, viscosity, and immunoglobulin G concentration

A study was conducted to identify the optimal temperature and time at which heat treatment of bovine colostrum would least change viscosity and IgG concentrations yet reduce bacterial count. First-milking colostrum with >50 g of immunoglobulins/L (measured by colostrometer) was collected from 30 Holstein cows. Aliquots of colostrum were heated for 0, 30, 60, or 90 min at 57, 60, or 63°C in a water bath. Samples were examined for viscosity, IgG₁, and IgG₂ concentrations, standard plate count, coagulase-negative staphylococci, environmental streptococci, coliform, gram-negative noncoliform, Streptococcus agalactiae, and Staphylococcus aureus counts. All heat treatments reduced counts of all bacteria groups measured compared with untreated colostrum samples. Heat treatment at ≥60°C denatured IgG₁ compared with untreated colostrum; however, colostral IgG₂ levels were not reduced when temperature was held at 60°C for <60 min. Viscosity was not affected when temperature was held at 60°C for <60 min. In this study, heat treatment of bovine colostrum at 60°C for 30 or 60 min reduced bacterial count, slightly reduced IgG concentration, and did not affect viscosity.     

A predictive model for heat inactivation of Listeria monocytogenes biofilm on buna-N rubber

The purpose of this study was to develop a predictive model for the heat inactivation of Listeria monocytogenes in monoculture (strains Scott A and 3990) and with competing bacteria (Pseudomonas sp. and Pantoea agglomerans) formed on buna-N rubber with and without the presence of food-derived soil. Biofilms were produced on rubber disks in dilute Tryptic Soy broth (dTSB) with incubation for 48 h at 25 °C. Duplicate biofilm samples were heat treated for 1, 3, 5, and 15 min at 70, 72, 75, 77 and 80 °C and tested for survivors using enrichment media. The experiment was repeated six times. A predictive model was developed and plots were generated showing the percent probability of L. monocytogenes inactivation in biofilms after heat treatment. For example, to achieve a 95% probability level of complete inactivation required heat treatment of 76 °C for 6 min. The predicted model was validated using a five-strain cocktail of L. monocytogenes. The validated prediction model indicates that with proper maintenance of the time/temperature controls L. monocytogenes in biofilms on rubber surfaces will be inactivated. This model can be used as a tool in the selection of hot water sanitation processes for rubber surfaces.     

Bacteriocinogenic Lactobacillus plantarum ST16Pa isolated from papaya (Carica papaya) — From isolation to application: Characterization of a bacteriocin

Strain ST16PA, isolated from papaya was identified as Lactobacillus plantarum based on biochemical tests, PCR with species-specific primers and 16S rDNA sequencing. L. plantarum ST16PA produces a 6.5 kDa bacteriocin, active against different species from genera Enterobacter, Enterococcus, Lactobacillus, Pseudomonas, Streptococcus and Staphylococcus and different serotypes of Listeria spp. The peptide is inactivated by proteolytic enzymes, but not when treated with ??-amylase, catalase, lipase, Triton X-100, SDS, Tween 20, Tween 80, urea, NaCl and EDTA. However, presence of 1% Triton X-114 deactivates the bacteriocin. No change in activity was recorded after 2 h at pH values between 2.0 and 12.0, and after treatment at 100 °C for 120 min or 121 °C for 20 min. The mode of activity against Lactobacillus sakei ATCC 15521, Enterococcus faecalis ATCC 19443 and Listeria innocua 2030C was bactericidal, resulting in cell lysis and enzyme-leakage. No significant differences in cell growth and bacteriocin production were observed when strain ST16Pa was cultured in MRS broth at 26 °C and 30 °C for 24 h (25 600 AU/ml). However, even though strain ST16PA grows well in MRS broth at 15 °C and 37 °C, a reduction of bacteriocin production was observed (400 AU/ml and 1600 AU/ml, respectively). In addition, effect of MRS medium components, different initial pH and additions of glycerol or vitamins to the media on bacteriocin ST16Pa production was studied.Peptide ST16PA adsorbs (400 AU/ml) to producer cells. However, bacteriocin ST16Pa was adsorbed at 50% to cells of L. innocua 2030C and at 75% to L. sakei ATCC 15521 and E. faecalis ATCC 19433 when experiments were conducted at 30 °C and pH 6.5. Adsorption of bacteriocin ST16Pa to target cells at different temperatures, pH and in presence of potassium sorbate, sodium nitrate, sodium chloride, ascorbic acid, Tween 80 and Tween 20 were also studied. To the best of our knowledge, this is the first report on detection of L. plantarum in papaya.     

Bactericidal activity of lauric arginate in milk and Queso Fresco cheese against Listeria monocytogenes cold growth

Lauric arginate (LAE) at concentrations of 200 ppm and 800 ppm was evaluated for its effectiveness in reducing cold growth of Listeria monocytogenes in whole milk, skim milk, and Queso Fresco cheese (QFC) at 4°C for 15 to 28 d. Use of 200 ppm of LAE reduced 4 log cfu/mL of L. monocytogenes to a nondetectable level within 30 min at 4°C in tryptic soy broth. In contrast, when 4 log cfu/mL of L. monocytogenes was inoculated in whole milk or skim milk, the reduction of L. monocytogenes was approximately 1 log cfu/mL after 24 h with 200 ppm of LAE. When 800 ppm of LAE was added to whole or skim milk, the initial 4 log cfu/mL of L. monocytogenes was nondetectable following 24 h, and no growth of L. monocytogenes was observed for 15 d at 4°C. With surface treatment of 200 or 800 ppm of LAE on vacuum-packaged QFC, the reductions of L. monocytogenes within 24 h at 4°C were 1.2 and 3.0 log cfu/g, respectively. In addition, the overall growth of L. monocytogenes in QFC was decreased by 0.3 to 2.6 and by 2.3 to 5.0 log cfu/g with 200 and 800 ppm of LAE, respectively, compared with untreated controls over 28 d at 4°C. Sensory tests revealed that consumers could not determine a difference between QFC samples that were treated with 0 and 200 ppm of LAE, the FDA-approved level of LAE use in foods. In addition, no differences existed between treatments with respect to flavor, texture, and overall acceptability of the QFC. Lauric arginate shows promise for potential use in QFC because it exerts initial bactericidal activity against L. monocytogenes at 4°C without affecting sensory quality.     

Decomposition kinetics of umami component during meat cooking

We developed a kinetic model for the decomposition reaction of inosine monophosphate (IMP), which is a umami component, and obtained kinetic parameters based on the amount of IMP in an isothermal experiment. The amount of remaining IMP decreased with heating time, and its reduction rate was the highest at 40 °C. We assumed that the activity of IMP decomposition enzyme is temperature-dependent above 40 °C, and constant below 40 °C. The predicted results using this kinetic model are in good agreement with the experimental ones. Unsteady-state three-dimensional heat transfer analysis of meat during sous-vide cooking was conducted, and the distribution of remaining IMP was predicted. By the end of sous-vide cooking, the ratio of the amount of IMP in the interior of the meat decreased, whereas at the surface region, it was almost the same as the initial value, because the surface temperature reached the inactivation temperature immediately.     

Impact of environmental stress desiccation, acidity, alkalinity, heat or cold on antibiotic susceptibility of Cronobacter sakazakii

Cronobacter sakazakii is an emerging foodborne pathogen that has been implicated in severe forms of meningitis, septicemia or necrotizing colitis in pre-term neonates. Although illness outbreaks (primarily associated with powdered infant formula, PIF) caused by this pathogen are rare, the case-fatality rate may reach 50%. Successful treatment of C. sakazakii infection is reliant upon clinical use of antibiotics (AB) such as ampicillin. Recent reports showed increased resistance of C. sakazakii to broad-spectrum antibiotics. The objective of this study was to evaluate the effect of extreme pH (3.5 for 30 min or 11.25 for 5 min), cold (4 °C for 24 h), heat (55 °C for 5 min), and desiccation (cells were dried at 40 °C for 2 h and held at 21 °C for 4 d) stresses on susceptibility of five isolated strains of C. sakazakii to streptomycin, gentamicin, kanamycin, neomycin, tetracycline, doxycycline, tilmicosin, florfenicol, ampicillin, amoxicillin, vancomycin, ciprofloxacin and enrofloxacin. All unstressed strains of C. sakazakii were sensitive to streptomycin, gentamycin, kanamycin, ciprofloxacin, enrofloxacin, ampicillin and amoxicillin, but were moderately resistant or resistant to the rest. Exposing cells to alkaline or acidic stress did not change their sensitivity toward streptomycin, gentamycin, kanamycin or ciprofloxacin, but their resistance toward the other AB was increased. Cells stressed by desiccation showed increased sensitivity toward streptomycin, gentamicin, kanamycin, ciprofloxacin, enrofloxacin, ampicillin and doxycycline, but showed resistance toward the others. Cold-stressed cells were more sensitive to streptomycin, gentamicin, kanamycin, and ciprofloxacin compared with heat-stressed cells, but both heat and cold-stressed cells showed increased resistance toward all the other AB. Results obtained will help in understanding the effect of environmental stresses during processing on C. sakazakii susceptibility to AB.     

Analysis and formation of trans fatty acids in hydrogenated soybean oil during heating

Hydrogenated oil has been widely used for production of shortenings or margarine, however, the presence of trans fatty acids may be detrimental to human health. The objectives of this study were to develop an improved method for analysis of trans fatty acids and evaluate their formation in both unhydrogenated and hydrogenated soybean oil during heating at 160, 180 and 200 °C for varied length of time. Results showed that among the four columns tested, an Agilent HP-88 column (100 × 0.25 mm I.D., 0.2-μm film thickness) could resolve eight trans fatty acids and nine cis fatty acids simultaneously within 31 min with injector temperature 240 °C, detector temperature 250 °C, and column temperature 170 °C in the beginning, maintained for 24 min, increased to 220 °C at 7.5 °C/min, 230 °C at 10 °C/min, and maintained for 5 min. The contents of both cis and trans fatty acids showed a decreased trend for the increase of heating time or temperature. No trans fatty acid formation was observed even after extensive heating of unhydrogenated and hydrogenated soybean oil for 24 h. This phenomenon demonstrated that trans fatty acids can only be formed under severe conditions.     

Inactivation of Saccharomyces cerevisiae in pineapple, grape and cranberry juices under pulsed and continuous thermo-sonication treatments

Pineapple, grape and cranberry juice were thermo-sonicated (24 kHz, 400 W, 120 μm) at 40 °C, 50 °C and 60 °C during 10 min at continuous and pulsed mode. Inactivation of Saccharomyces cerevisiae was tested from 0 to 10 min; color and pH were measured. Survivor’s curves were fitted with Weibull distribution, four parameter model and modified Gompertz equation. The acoustic energy (AE) was also calculated. S. cerevisiae was inactivated in the treatments at 60 °C, with the continuous mode being more effective. Grape juice showed total inactivation (7-log) after 10 min. Results showed that pH and color changed significantly (p < 0.05); ultrasound may promote chemical reactions and extract some components. The modified Gompertz equation showed the best fit. Energy analysis showed that pineapple juice (4287.02 mW/ml) required a higher amount of energy; grape juice showed the lowest value (3112.13 mW/ml). Ultrasound represents a viable option for juice pasteurization.     

Effects of combining microbial transglutaminase and high pressure processing treatments on the mechanical properties of heat-induced gels prepared from arrowtooth flounder (Atheresthes stomias)

The objective of this study was to evaluate the effect of setting conditions (25 °C for 2 h or 40 °C for 30 min) and combining of microbial transglutaminase (MTGase) and high pressure processing (HPP) on the mechanical properties of heat induced gels obtained from paste from arrowtooth flounder (Atheresthes stomias). Treatments included fish paste control without added MTGase, fish paste incubated with MTGase but not pressurized (MTGase + cooking), fish paste incubated with MTGase and pressurized at 600 MPa for 5 min (MTGase + HPP + cooking) and fish paste pressurized at 600 MPa for 5 min and incubated with MTGase (HPP + MTGase + cooking). The controls and the treated samples were then subjected to one of two thermal treatments: 90 °C for 15 min or 60 °C for 30 min before cooking at 90 °C for 15 min. Samples of fish paste heated at 60 °C before cooking could not be used to prepare gels for texture profile analysis (TPA). TPA showed that pressurization improved the mechanical properties of gels made from paste treated with MTGase and set at 25 °C. The opposite was observed for samples set at 40 °C. Setting at 40 °C appeared to induce proteolytic degradation of myofibrillar proteins.     

Insecticidal efficacy of diatomaceous earth against Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and Tribolium confusum du Val (Coleoptera: Tenebrionidae) on stored wheat: influence of dose rate, temperature and exposure interval

Laboratory experiments were conducted in order to assess the insecticidal effect of a diatomaceous earth formulation (Silicosec^®, Biofa GmbH, Germany) against Sitophilus oryzae and Tribolium confusum on stored wheat. Adults of the two species were exposed on wheat treated with diatomaceous earth at four dose rates: 0.25, 0.5, 1 and 1.5 g/kg of wheat, respectively. For each dose rate, the treated wheat was placed at 22°C, 25°C, 27°C, 30°C and 32°C. Dead adults were counted after 24 and 48 h, 7 and 14 d of exposure. After the 14-d interval, the live adults were removed and placed for 7 d in untreated wheat (in the case of S. oryzae) or untreated flour (in the case of T. confusum), and the production of F₁ was recorded. For both species, dose rate, temperature and exposure interval significantly affected mortality (P<0.001). Mortality was higher at longer exposure intervals. The efficacy of SilicoSec against S. oryzae increased with temperature, but for T. confusum mortality was lower at 32°C, compared to 30°C, for 24 and 48 h exposure intervals. Tribolium confusum proved less susceptible to SilicoSec than S. oryzae. In general, the rates of 1 and 1.5 g/kg of wheat provided a satisfactory level of protection against the two species examined. For S. oryzae, F₁ emerged only at 22°C, in wheat treated with 0.25 or 0.5 g/kg. However, for T. confusum, F₁ were recorded at 22°C for 0.5 g/kg and at 22°C, 25°C, 27°C and 30°C for 0.25 g/kg.     

Initial and delayed mortality of late-instar larvae, pupae, and adults of Tribolium castaneum and Tribolium confusum (Coleoptera: Tenebrionidae) exposed at variable temperatures and time intervals

Late-instar larvae, pupae, and adults of Tribolium castaneum (Herbst), the red flour beetle, or Tribolium confusum (DuVal), the confused flour beetle, were exposed for variable durations at 36-54 °C. Beetles were placed in laboratory ovens set at a baseline of 27 °C, the temperature was increased by 0.1 °C per minute until the target temperature was achieved, and beetles were then held for specified exposure durations. There was no mortality after initial exposure or after a 1-week holding period of any life stage of T. castaneum or T. confusum exposed for 32 h to 36, 39, or 42 °C. At 45 °C, there was no initial mortality of either species exposed for different time intervals except for those exposed for 28 h. However, there was a significant increase in mortality after the 1-week holding period of those beetles exposed initially for at least 16 h to 45 °C. There was a sharp increase in mortality after the initial exposures of 4 h at 48 °C; mortality of T. confusum larvae was 90.0±5.7% but was only 10.0±10.0% for larvae of T. castaneum, and no pupae of either species were dead. All life stages of both species were killed after the initial exposure of 12 h, and 1-week mortality of beetles exposed for 4 and 8 h was generally greater than initial mortality. At 51 and 54 °C, 2- and 1-h exposures, respectively, killed all life stages of each species. Mortality in conditions of gradual temperature increase was less than previous studies with sudden temperature increases.     

Validation of radio-frequency dielectric heating system for destruction of Cronobacter sakazakii and Salmonella species in nonfat dry milk

Cronobacter sakazakii and Salmonella species have been associated with human illnesses from consumption of contaminated nonfat dry milk (NDM), a key ingredient in powdered infant formula and many other foods. Cronobacter sakazakii and Salmonella spp. can survive the spray-drying process if milk is contaminated after pasteurization, and the dried product can be contaminated from environmental sources. Compared with conventional heating, radio-frequency dielectric heating (RFDH) is a faster and more uniform process for heating low-moisture foods. The objective of this study was to design an RFDH process to achieve target destruction (log reductions) of C. sakazakii and Salmonella spp. The thermal destruction (decimal reduction time; D-value) of C. sakazakii and Salmonella spp. in NDM (high-heat, HH; and low-heat, LH) was determined at 75, 80, 85, or 90°C using a thermal-death-time (TDT) disk method, and the z-values (the temperature increase required to obtain a decimal reduction of the D-value) were calculated. Time and temperature requirements to achieve specific destruction of the pathogens were calculated from the thermal destruction parameters, and the efficacy of the RFDH process was validated by heating NDM using RFDH to achieve the target temperatures and holding the product in a convection oven for the required period. Linear regression was used to determine the D-values and z-values. The D-values of C. sakazakii in HH- and LH-NDM were 24.86 and 23.0 min at 75°C, 13.75 and 7.52 min at 80°C, 8.0 and 6.03 min at 85°C, and 5.57 and 5.37 min at 90°C, respectively. The D-values of Salmonella spp. in HH- and LH-NDM were 23.02 and 24.94 min at 75°C, 10.45 and 12.54 min at 80°C, 8.63 and 8.68 min at 85°C, and 5.82 and 4.55 min at 90°C, respectively. The predicted and observed destruction of C. sakazakii and Salmonella spp. were in agreement, indicating that the behavior of the organisms was similar regardless of the heating system (conventional vs. RFDH). Radio-frequency dielectric heating can be used as a faster and more uniform heating method for NDM to achieve target temperatures for a postprocess lethality treatment of NDM before packaging.