Sanghyup Jeong

Quantifying the Performance of Pediococcus sp. (NRRL B-2354: Enterococcus faecium) as a Nonpathogenic Surrogate for Salmonella Enteritidis PT30 during Moist-Air Convection Heating of Almonds

Pediococcus sp. NRRL B-2354 was investigated as a potential nonpathogenic surrogate for Salmonella enterica serovar Enteritidis phage type 30 (SE PT30) on the surface of almonds subjected to moist-air heating. Both microorganisms were subjected to various time, temperature, and humidity regimens on almonds processed in a computer-controlled, laboratory-scale, moist-air convection oven. Overall, the mean log reductions for Pediococcus sp. were 0.6 log and 1.4 log lower than those for SE PT30 (P < 0.05) at predicted reductions of 3 and 5 log, respectively. Also, the D(ref)-values for Pediococcus sp., calculated using a modified inactivation model (accounting for moisture) for SE PT30 on the surface of almonds subjected to moist-air heating (30 to 90% moisture by volume) were ~30% larger than those for SE PT30. Based on these findings, Pediococcus sp. NRRL B-2354 can be used as a conservative surrogate for SE PT30 during moist-air heating, and this organism is also likely to be an acceptable surrogate for steam heating.

Thermal inactivation kinetics for Salmonella Enteritidis PT30 on almonds subjected to moist-air convection heating.

A traditional thermal inactivation kinetic model (D- and z-value) was modified to account for the effect of process humidity on thermal inactivation of Salmonella Enteritidis PT30 on the surface of almonds subjected to moist-air heating. Raw almonds were surface inoculated to approximately 10(8) CFU/g and subjected to moist-air heating in a computer-controlled laboratory-scale convection oven. Time-temperature data were collected for 125 conditions (five dry bulb temperatures, 121 to 232 degrees C; five process humidity levels, 5 to 90% moisture by volume; and five process durations). Moisture status at the surface of the almond, rather than the humidity of the bulk air, was a primary factor controlling the rate of inactivation; therefore, the D-value could not be a simple function of process temperature. Instead, the traditional D- and z-value model was modified to account for the dynamic water status at the surface of the product under humid heating conditions. The modified model needs only the dew point temperature of the processing air and dynamic surface temperature history of the almonds during moist-air heating. The modified model was more robust and accurate than the traditional model. The accuracy of the modified model was improved by 32 to 44% (in terms of the root mean squared error [RMSE] for the model fit) when compared with the traditional model in all moist-air heating conditions. Also, the prediction error of the modified model (RMSE = 1.33 log reductions) against an independent validation data set was approximately one-half that of the traditional model (RMSE = 2.56 log reduction) in the humidity range of 5 to 90% moisture by volume.

Growth of Escherichia coli O157:H7 and Listeria monocytogenes in Packaged Fresh-Cut Romaine Mix at Fluctuating Temperatures during Commercial Transport, Retail Storage, and Display

Temperature abuse during commercial transport and retail sale of leafy greens negatively impacts both microbial safety and product quality. Consequently, the effect of fluctuating temperatures on Escherichia coli O157:H7 and Listeria monocytogenes growth in commercially-bagged salad greens was assessed during transport, retail storage, and display. Over a 16-month period, a series of time-temperature profiles for bagged salads were obtained from five transportation routes covering four geographic regions (432 profiles), as well as during retail storage (4,867 profiles) and display (3,799 profiles). Five different time-temperature profiles collected during 2 to 3 days of transport, 1 and 3 days of retail storage, and 3 days of retail display were then duplicated in a programmable incubator to assess E. coli O157:H7 and L. monocytogenes growth in commercial bags of romaine lettuce mix. Microbial growth predictions using the Koseki-Isobe and McKellar-Delaquis models were validated by comparing the root mean square error (RMSE), bias, and the acceptable prediction zone between the laboratory growth data and model predictions. Monte Carlo simulations were performed to calculate the probability distribution of microbial growth from 8,122,127,472 scenarios during transport, cold room storage, and retail display. Using inoculated bags of retail salad, E. coli O157:H7 and L. monocytogenes populations increased a maximum of 3.1 and 3.0 log CFU/g at retail storage. Both models yielded acceptable RMSEs and biases within the acceptable prediction zone for E. coli O157:H7. Based on the simulation, both pathogens generally increased <2 log CFU/g during transport, storage, and display. However, retail storage duration can significantly impact pathogen growth. This large-scale U.S. study-the first using commercial time/temperature profiles to assess the microbial risk of leafy greens-should be useful in filling some of the data gaps in current risk assessments for leafy greens.

The effect of X-ray irradiation on Salmonella inactivation and sensory quality of almonds and walnuts as a function of water activity.

The overall goal of this study was to develop a set of process design principles for low-energy X-ray irradiation of tree nuts. Almonds and walnuts were inoculated with Salmonella Enteritidis PT30 and Salmonella Tennessee, and conditioned to four different water activities (0.23, 0.45, 0.64, and 0.84 a(w)). Thereafter, the inoculated/conditioned samples were irradiated to achieve up to a 5-log reduction in Salmonella using a pilot scale low-energy X-ray food irradiator. Greater efficacy (D(10)-value: the dose required to eliminate 90% of the microbial population) for inactivating SE PT30 and S. Tennessee was seen on the surface of almonds (0.226-0.431 kGy) than on walnuts (0.474-0.930 kGy) at all water activities. Also, the efficacy did not change monotonically with water activity. Overall, no significant difference (P>0.05) in sensory characteristics was seen between non-irradiated almonds and those irradiated to achieve a 5 log reduction in Salmonella. However, irradiating walnuts to the dose corresponding to a 5 log reduction caused a perceivable change in flavor. Post-irradiation storage tests revealed that surviving bacterial counts did not change over 120 days, regardless of nut type, Salmonella serovar, and a(w). Therefore, low-energy X-ray irradiation technology appears to be a promising non-thermal pasteurization strategy for certain types of nuts.

Inactivation of Escherichia coli O157:H7 on Lettuce, Using Low-Energy X-Ray Irradiation

Low-energy X-ray irradiation was assessed as a means of eliminating Escherichia coli O157:H7 on lettuce. Round-cut iceberg lettuce samples (2.54-cm diameter) were dip or spot inoculated with a three-strain cocktail of E. coli O157:H7, stored for 24 h at 4 degrees C, and then irradiated at four dose levels up to 0.25 kGy using a prototype low-energy (70 kV) X-ray irradiator. E. coli O157:H7 survivors were quantified by plating on sorbitol MacConkey agar containing cefixime and tellurite. Dip inoculation yielded a D(10)-value of 0.040 +/- 0.001 kGy, which is 3.4 times lower than a previously reported value of 0.136 kGy using gamma radiation. The D(10)-value for E. coli O157:H7 on spot-inoculated samples was 0.078 +/- 0.008 kGy, which is about twice that of dip-inoculated samples. When 10 stacked leaves were irradiated from both sides, a dose of 0.2 kGy was achieved at the center of the stack with a surface dose of 1 kGy, corresponding to a approximately 5-log reduction of E. coli O157:H7 at the center of the stack. Based on these findings, low-energy X-ray irradiation appears to be a promising microbial inactivation strategy for leafy greens and potentially for other types of fresh produce.

X-Ray irradiation as a microbial intervention strategy for food

First recognized in 1895, X-ray irradiation soon became a breakthrough diagnostic tool for the dental and medical professions. However, the food industry remained slow to adopt X-ray irradiation as a means for controlling insects and microbial contaminants in food, instead using gamma and electron beam (E-beam) irradiation. However, the reinvention of X-ray machines with increased efficiency, combined with recent developments in legislation and engineering, is now allowing X-ray to actively compete with gamma irradiation and E-beam as a microbial reduction strategy for foods. This review summarizes the historical developments of X-rays and discusses the key technological advances over the past two decades that now have led to the development of several different X-ray irradiators capable of enhancing the safety and shelf life of many heat-sensitive products, including lettuce, spinach, tomatoes, and raw almonds, all of which have been linked to high profile outbreaks of foodborne illness.

Comparing Thermal Process Validation Methods for Salmonella Inactivation on Almond Kernels

Ongoing regulatory changes are increasing the need for reliable process validation methods for pathogen reduction processes involving low-moisture products; however, the reliability of various validation methods has not been evaluated. Therefore, the objective was to quantify accuracy and repeatability of four validation methods (two biologically based and two based on time-temperature models) for thermal pasteurization of almonds. Almond kernels were inoculated with Salmonella Enteritidis phage type 30 or Enterococcus faecium (NRRL B-2354) at ~108 CFU/g, equilibrated to 0.24, 0.45, 0.58, or 0.78 water activity (aw), and then heated in a pilot-scale, moist-air impingement oven (dry bulb 121, 149, or 177°C; dew point <33.0, 69.4, 81.6, or 90.6°C; vair = 2.7 m/s) to a target lethality of ~4 log. Almond surface temperatures were measured in two ways, and those temperatures were used to calculate Salmonella inactivation using a traditional (D, z) model and a modified model accounting for process humidity. Among the process validation methods, both methods based on time-temperature models had better repeatability, with replication errors approximately half those of the surrogate ( E. faecium ). Additionally, the modified model yielded the lowest root mean squared error in predicting Salmonella inactivation (1.1 to 1.5 log CFU/g); in contrast, E. faecium yielded a root mean squared error of 1.2 to 1.6 log CFU/g, and the traditional model yielded an unacceptably high error (3.4 to 4.4 log CFU/g). Importantly, the surrogate and modified model both yielded lethality predictions that were statistically equivalent (α = 0.05) to actual Salmonella lethality. The results demonstrate the importance of methodology, aw, and process humidity when validating thermal pasteurization processes for low-moisture foods, which should help processors select and interpret validation methods to ensure product safety.

Kinetics of surface color change during moist-air impingement processing of meat patties

Non-enzymatic browning in food processing is a major contributor to surface color, aroma, and flavor. The reaction is very complex and is affected by many factors, such as temperature, pH, and water activity. Color development on the surface of meat products cooked in commercial-scale, moist-air impingement processes is highly influenced by the dynamic surface water state caused by condensation and then evaporation. Therefore, the goal of this project was to modify conventional kinetic modeling for color change to be a function of product surface temperature and the water status at the surface of the product. To measure surface color, surface temperature, and crust moisture content, beef patties (three replicates per treatment) were processed at three temperatures (121, 177, and 232°C), three humidity levels (10, 70, and 90%Mv), and five cooking durations in a pilot-scale moist-air impingement oven. During the cooking process, center and surface temperature of the sample were recorded. After cooking, the surface color was measured using a colorimeter, and the surface moisture content was determined by the air-oven method. Across all conditions, surface lightness (L*) decreased from ~50 to 25, and surface (crust) moisture content decreased to ~48%wb. A conventional model was compared with a modified model, in which a dew point temperature and product surface temperature were used to quantifying the dynamic water state on the product surface. The modified model was ~20% more accurate than the conventional model.