Joy Waite-Cusic

Screening for Listeria monocytogenes surrogate strains applicable to food processing by ultrahigh pressure and pulsed electric field.

Ultrahigh pressure (UHP) and pulsed electric field (PEF) are emerging processing technologies developed to enhance the safety while maintaining the fresh-like quality of food. For each food and process combination, a pathogen of concern (i.e., target pathogen) must be determined, and a low-risk microorganism that serves as the pathogen surrogate for process validation must be identified. The objective of this study was to identify a surrogate for Listeria monocytogenes for UHP and PEF process validation. Potential surrogates tested include four Lactobacillus spp., a Pediococcus sp., and a Listeria innocua strain. These were compared with nine L. monocytogenes strains, with regard to sensitivity to UHP and PEF processing. For UHP treatment, the strains were suspended in citrate-phosphate buffer (pH 7.0 or 4.5), sweet whey, or acidified whey and pressure processed at 500 MPa for 1 min. For PEF treatment, the strains were suspended in NaCl solution, acid whey, or sweet whey and processed at 25 kV/cm. The lethality of UHP or PEF treatment varied considerably, depending on medium types and pH and the treated strain. Treating the tested microorganisms with UHP inactivated 0.3 to 6.9 log CFU/ml for L. monocytogenes strains and 0.0 to 4.7 log CFU/ml for the potential surrogates. When PEF was employed, populations of tested microorganisms decreased < 1.0 to 5.3 log CFU/ml. L. monocytogenes V7 and OSY-8578 were among the most resistant strains to UHP and PEF treatments, and thus are candidate target strains. Lactobacillus plantarum ATCC 8014 demonstrated similar or greater resistance compared with the target organisms; therefore, the bacterium is proposed as a surrogate of L. monocytogenes for both processes under the conditions specified in the food matrices tested in this study.

Effect of commercial hauling practices and tanker cleaning treatments on raw milk microbiological quality

Consolidation of the US milk industry has led to use of tankers for up to 24 h in between thorough cleanings. As the heavy use of tankers has not been previously studied, the effect of this form of hauling on raw milk quality is unknown. This study focused on the effect of frequent tanker use during hauling on raw milk quality at a commercial facility. Standard tanker use (cleaned-in-place once per 24 h) served as our control and incremental cleaning treatments (water rinse after each load, water rinse after each load with a sanitizer treatment after 12 h, and 12 h of sanitizer treatment) were added to the study to understand if any effect could be mitigated by more frequent cleaning. Producer samples were collected from the farm before loading milk into the tanker as well as sampling the same milk directly out of the tanker truck before unloading at the manufacturer. The study was repeated at 2 different dairy manufacturing facilities, once during the summer and once during the winter. Milk quality was quantified through industry-relevant microbiological tests: individual bacteria count, thermophilic spore count, and preliminary incubation count. Within the study we defined a negative effect on milk quality as a statistically significant difference between the tanker and producer samples in any of the 3 microbial tests conducted between treatments. Results from the study showed no clear effect due to hauling in individual bacteria count, thermophilic spore count, or preliminary incubation counts. There was also no difference in milk quality between the 2 plants, suggesting that neither season nor location affected our results in the standard use variable. As we did not see a negative effect on milk quality in the standard use variable, the addition of cleaning treatments did not appear to provide any clear benefit. Tanker surface swabs and ATP swabs were also used to monitor tanker sanitation and the efficacy of cleaning treatments. Both surface and ATP swabs revealed differences between cleaning efficacy at the 2 facilities. Although the differences in efficacy did not influence tanker milk quality within our study, variability in sanitation may provide a source of contamination that could negatively affect raw milk quality in other areas. Based on this study, current hauling practices appear to be effective in mitigating any measurable effect on raw milk quality; however, further investigation is needed before making industry-wide recommendations.

Intracellular Free Iron and Its Potential Role in Ultrahigh-Pressure-Induced Inactivation of Escherichia coli

ABSTRACT Intracellular free iron of Escherichia coli was determined by whole-cell electron paramagnetic resonance spectrometry. Ultrahigh pressure (UHP) increased both intracellular free iron and cell lethality in a pressure-dose-dependent manner. The iron chelator 2,2′-dipyridyl protected cells against UHP treatments. A mutation that produced iron overload conditions sensitized E. coli to UHP treatment.

Short communication: Microbial quality of raw milk following commercial long-distance hauling

Hauling is a critical part of the commercial milk supply chain, yet very few studies have aimed to understand its effect on raw milk quality. This study focused on the effect of extended-duration tanker use during hauling on raw milk quality at a commercial facility. Standard tanker use [cleaned-in-place (CIP) once per 24h] served as a control and an incremental between-load water rinse with sanitizer treatment (RS) was evaluated to mitigate any effect from extended duration hauling. During this study, 1 commercial truck with 2 trailers was monitored for 10d. The truck collected milk at a large dairy farm, transported the milk to a manufacturing facility, and then returned to the same farm for a second load. Each round-trip journey took between 10 and 12h, allowing for 2 loads per 24-h use period. Following the second delivery, the truck was cleaned by CIP treatment starting a new treatment day. Producer samples were collected from the raw milk bulk tank on the farm before loading milk into the tanker. The same milk was sampled directly out of the tanker truck before unloading at the manufacturer. Effect on individual bacteria count, thermophilic spore count, and preliminary incubation count was quantified through common industry tests. Surface sponge swabs were also used to monitor tanker sanitation and the efficacy of cleaning treatments. Results did not identify a negative effect on raw milk quality due to extended duration hauling. Whereas the addition of RS did not provide any measurable quality benefits for the microbial milk quality, swab results demonstrated that the RS treatment was able to reduce surface bacteria in the tanker, although not to the same level as the full CIP treatment. Based on this study, current CIP practices for long distance milk hauling appear to be effective in mitigating any measurable effect on raw milk quality.

Evidence of terroir in milk sourcing and its influence on Cheddar cheese

The concept of local food is rapidly gaining importance within the United States. The foundation of local food is terroir, which links a food to its production environment. The purpose of this study was to investigate evidence of terroir in milk sourcing and its influence on Cheddar cheese flavor. Specifically, the study was designed to assess if consumers could differentiate between Cheddar cheeses made with milk from different dairy farms. Milk from 5 locations, including single dairy farms and commingled sites, was collected from around the state of Oregon. Using raw and pasteurized counterparts of the milk, Cheddar cheese was made and aged. At 5 and 9mo into aging, Cheddar cheese consumers were asked to group the samples based on perceived similarity/dissimilarity of cheese flavor. Grouping data were subjected to multidimensional scaling and subsequent cluster analysis. Results at 5mo into aging revealed that cheeses made by milk originating from different farms (80km apart) within the same region were perceived as different, whereas cheeses made with milk from neighboring farms (5km apart) were grouped together, irrespective of heat treatment (i.e., raw vs. pasteurized). Cheeses made with commingled milk from different regions grouped together. At 9mo of aging, in contrast, a clear separation of perceived flavor was present between the pasteurized and raw cheese samples, whereas the effect of milk sourcing was less pronounced. These data suggest that the geographical location of the milk source has an effect on the flavor of Cheddar cheese, but that the practices of milk commingling and heat treatment likely reduce the effect of geographical location, particularly as cheese ages.

Adjusting Processing Parameters in an Entry-Level Commercial Dehydrator To Achieve a 5-Log Reduction of Salmonella during the Cooking Step

The U.S. Department of Agriculture, Food Safety and Inspection Service (FSIS) guidance encourages jerky processors to use a single lethality step that will achieve a 5-log reduction of Salmonella. Many processors rely on internal temperature-time combinations recommended in this guidance. However, the efficacy of convective heating is highly dependent on relative humidity (RH). RH recommendations are vague because of variability in processing conditions and limited available data. This study was conducted to establish processing conditions for an entry-level commercial dehydrator (Harvest Saver R5A) to achieve a 5-log reduction of Salmonella. Unseasoned, unmarinated top round beef strips (65 mm thick) were inoculated with Salmonella (>7 log CFU/g) and processed with the chamber temperature set to 82.2°C for a total cook time of 60 min with the intake closed (closed oven). Modifications (product load and fan speed) were made in subsequent trials to improve lethality. After incubation at 37°C for 24 to 48 h, surviving Salmonella populations were enumerated on tryptic soy agar. In trial A, the maximum fan speed (2.5 m/s) with 30 kg of product resulted in 45 to 48.5% RH at 60 min, and a 5-log reduction of Salmonella was achieved in only 35.5% of the meat samples (54 of 152 samples). Increasing the product load by 40% (42 kg; trial B) increased RH in the chamber (57 to 85%) and resulted in improved lethality; a 5-log reduction was achieved in 95.0% of samples (131 of 138 samples). Because samples with reduced lethality were located on the windward side of the chamber, the fan speed was reduced (0.9 m/s; trial C1) to increase the RH, resulting in a 5-log reduction in 100% of the samples (138 of 138 samples). A replicate trial (trial C2) was conducted, and a 5-log reduction again was achieved in 100% of the samples. All trials exceeded recommendations by the FSIS; however, adequate Salmonella reduction was achieved only when the RH was >65% throughout. Product load and fan speed are practical parameters for processors to manipulate to increase the RH in closed systems and thus improve Salmonella lethality.

Field Evidence Supporting Conventional Onion Curing Practices as a Strategy To Mitigate Escherichia coli Contamination from Irrigation Water

The Produce Safety Rule of the U.S. Food Safety Modernization Act includes restrictions on the use of agricultural water of poor microbiological quality. Mitigation options for poor water quality include the application of an irrigation-to-harvest interval of <4 days; however, dry bulb onion production includes an extended irrigation-to-harvest interval (<30 days). This study evaluated conventional curing practices for mitigating Escherichia coli contamination in a field setting. Well water inoculated with rifampin-resistant E. coli (1, 2, or 3 log CFU/mL) was applied to onion fields (randomized block design; n = 5) via drip tape on the final day of irrigation. Onions remained undisturbed for 7 days and were then lifted to the surface to cure for an additional 21 days before harvest. Water, onions, and soil were tested for presence of rifampin-resistant E. coli. One day after irrigation, 13.3% of onions (20 of 150) receiving the poorest quality water (3 log CFU/mL) tested positive for E. coli; this prevalence was reduced to 4% (6 of 150 onions) after 7 days. Regardless of inoculum level, E. coli was not detected on any onions beyond 15 days postirrigation. These results support conventional dry bulb onion curing practices as an effective strategy to mitigate microbiological concerns associated with poor quality irrigation water.

Effect of leaving milk trucks empty and idle for 6 h between raw milk loads

The US Pasteurized Milk Ordinance (PMO) allows milk tanker trucks to be used repeatedly for 24 h before mandatory clean-in-place cleaning, but no specifications are given for the length of time a tanker can be empty between loads. We defined a worst-case hauling scenario as a hauling vessel left empty and dirty (idle) for extended periods between loads, especially in warm weather. Initial studies were conducted using 5-gallon milk cans (pilot-scale) as a proof-of-concept and to demonstrate that extended idle time intervals could contribute to compromised raw milk quality. Based on pilot-scale results, a commercial hauling study was conducted through partnership with a Pacific Northwest dairy co-op to verify that extended idle times of 6 h between loads have minimal influence on the microbiological populations and enzyme activity in subsequent loads of milk. Milk cans were used to haul raw milk (load 1), emptied, incubated at 30°C for 3, 6, 10, and 20 h, and refilled with commercially pasteurized whole milk (load 2) to measure cross-contamination. For the commercial-scale study, a single tanker was filled with milk from a farm known to have poorer quality milk (farm A, load 1), emptied, and refilled immediately (0 h) or after a delay (6 h) with milk from a farm known to have superior quality milk (farm B, load 2). In both experiments, milk samples were obtained from each farms bulk tank and from the milk can or tanker before unloading. Each sample was microbiologically assessed for standard plate count (SPC), lactic acid bacteria (LAB), and coliform counts. Selected isolates were assessed for lipolytic and proteolytic activity using spirit blue agar and skim milk agar, respectively. The pilot-scale experiment effectively demonstrated that extended periods of idle (>3 h) of soiled hauling vessels can significantly affect the microbiological quality of raw milk in subsequent loads; however, extended idle times of 6 h or less would not measurably compromise milk quality in subsequent loads in commercial tankers. Current tanker sanitation practices appear to be sufficient for maintaining raw milk SPC, LAB, and coliform levels, which are important measures of milk quality.

Impact of Feeding Status on the Efficacy of Depuration for Decontaminating Vibrio parahaemolyticus in Pacific Oysters (Crassostrea gigas)

ABSTRACT Vibrio parahaemolyticus causes acute human gastroenteritis and is often linked to consumption of raw oysters. Previous investigations indicated that refrigerated seawater depuration at 12.5°C could significantly reduce V. parahaemolyticus contamination in Pacific oysters; however, further optimization is necessary to achieve the regulatory target of >3.52 log most probable number (MPN)/g reduction. The current study investigated influences of algal feeding on efficacy of depuration to reduce V. parahaemolyticus in raw oysters. A V. parahaemolyticus cocktail (10290, 10292, 10293, BE 98–2029, 027-1c1) was mixed in artificial seawater (70 L) to inoculate oysters (n = 35) at 4–5 log MPN/g. Inoculated oysters were subjected to depuration with feed (algae = 0.036 ml/gram of oyster) and without feed at 12.5°C. Oysters (n = 5) were analyzed for V. parahaemolyticus using a three-tube MPN method after 0, 1, 3, 5, and 6 days of depuration. Depuration (6 days) achieved average V. parahaemolyticus reductions of 2.75 log MPN/g and 3.03 log MPN/g in the fed and unfed systems, respectively; however, feeding status did not significantly impact the efficacy of depuration to reduce V. parahaemolyticus in Pacific oysters. Further optimization of depuration is necessary to achieve the regulatory target for V. parahaemolyticus decontamination in raw oysters.