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GROWTH KINETICS OF LISTERIA MONOCYTOGENES DURING REHYDRATION OF DEHYDRATED PLANT FOODS AND SUBSEQUENT STORAGE
Title
GROWTH KINETICS OF LISTERIA MONOCYTOGENES DURING REHYDRATION OF DEHYDRATED PLANT FOODS AND SUBSEQUENT STORAGE
Creator
Wu, Zihui
Date
2020
Description
Dehydrated plant foods do not support the growth of pathogenic bacteria. However, once rehydrated, the high-water activity and neutral pH of...
Show moreDehydrated plant foods do not support the growth of pathogenic bacteria. However, once rehydrated, the high-water activity and neutral pH of these foods may support the growth of pathogens, such as L. monocytogenes, during storage. The goal of this study was to examine the growth kinetics of L. monocytogenes during 5, 10, and 25°C storage on potatoes, carrots, and onions after rehydration with 5 or 25°C water. Fresh plant foods were dehydrated at 140°F (60°C) for 24 h. A 4-strain rifampicin-resistant L. monocytogenes cocktail was inoculated onto dehydrated plant foods at 4 log CFU/g and dried for 24 h. Plant foods were rehydrated in 4-volumes of 5 or 25°C water for 24 h. At various timepoints during rehydration, 30 g of sample was removed and drained for 10 min. Samples were homogenized 1:10 with BLEB and the homogenate was plated onto BHIRif for enumeration. After rehydration, samples were drained and portioned into deli-style containers for storage at 5, 10, and 25°C and L. monocytogenes was enumerated at 1, 3, 5, and 7 d. Triplicate samples were assessed at each timepoint and three independent trials were conducted. Growth rates were determined using DMFit and data were statistically analyzed using Student t-test (α=0.05). Overall, the growth rates of L. monocytogenes during storage of potatoes and carrots were higher when rehydrated with 5°C water compared to 25°C water. The highest growth rate on potatoes was 3.51±0.43 log CUF/g per d with 5°C water rehydration and 25°C storage, resulting in a 1 log CFU/g increase in 0.29 d (7.0 h). When rehydrated with 25°C water and 25°C storage, the growth rate was significantly lower at 1.03±0.01 log CFU/g per d. The highest growth rate of L. monocytogenes on carrots was 0.68±0.07 log CUF/g per day when rehydrated with 5°C water and 10°C storage, resulting in a 1 log CFU/g increase in 1.47 d (35.3 h). For onion, L. monocytogenes was below the level of enumeration during storage at 5°C for both water rehydration temperatures and also for 10°C storage with 5°C water rehydration. The highest growth rate was 0.46±0.11 log CFU/g per d, resulting in a 1 log CFU/g increase in 2.17 d. The results of this study can aid in determining appropriate time and temperature control for safety for dehydrated potatoes, carrots and onions during rehydration and subsequent storage.
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Efficacy of Organic Acid Treatments for the Reduction of Listeria Monocytogenes on Hard Boiled Eggs
Title
Efficacy of Organic Acid Treatments for the Reduction of Listeria Monocytogenes on Hard Boiled Eggs
Creator
Khouja, Bashayer
Date
2022
Description
Ready-to-eat hard-boiled eggs (HBEs) are a popular and convenient choice for consumers and food servicers. Recentrecalls of hard-boiled eggs...
Show moreReady-to-eat hard-boiled eggs (HBEs) are a popular and convenient choice for consumers and food servicers. Recentrecalls of hard-boiled eggs have highlighted the susceptibility of contamination with Listeria monocytogenes. HBEs are generally treated with antibacterials to ensure the safety and quality of the product. While citric acid is often used, research has determined it is not effective in some situations; therefore, the assessment of additional organic acids is necessary. This study examined the efficacy of acetic, lactic, and malic acid on the reduction of L. monocytogenes on HBEs after a 24- hour treatment trials and 28 days storage trials. Fresh eggs were cooked in boiling water, peeled, and stored at 4°C for 24h before use. For treatment trials, HBEs were dip- inoculated with a 4-strain cocktail of rifampicin resistant L. monocytogenes, resulting in 8 log CFU/egg. Following air drying, hard-boiled eggs were treated at 5 or 25°C with 2% acetic, lactic, or malic acid. L monocytogenes populations were enumerated in intervals up to 24h by homogenization of HBEs with BLEB and cultivation on BHIrif. For pre- treatment storage trials, HBEs were first dip- inoculated with a rifampicin- resistant 4- strain L. monocytogenes cocktail for 20 min, resulting in 1 log CFU/egg, air dried for 10 min, followed by treatment with 2% acetic, lactic, or malic acid for 24 h at either 5 or 25°C. For post- treatment inoculation trials, HBEs were first soaked in 2% acetic, lactic, or malic acid for 24 h at either 5 or 25°C, air dried for 10 min, spot-inoculated at 1 log CFU/egg, and then dried for 20 min. All HBEs were individually stored in bags at 5°C for up to 28 days. The presence of L. monocytogenes was determined at intervals during storage by enrichment with BLEB on BHIrif and Brilliance Listeria Agar. Triplicate eggs were assessed for each timepoint, and three independent trials were conducted. Data were analyzed by Student’s T-test, ANOVA, and Fisher’s exact test, p≤0.05. The initial inoculation level of L. monocytogenes on HBEs was 8.27±0.37 log CFU/egg. After 24 h treatment, all L. monocytogenes populations were significantly reduced on HBEs. At 5°C, populations were reduced by 3.15±0.70, 3.46±0.02, and 4.78±0.23 log CFU/egg. Compared to 5°C, a significantly higher population reduction occurred with acetic and lactic acid when treatment occurred at 25°C. The inactivation of L. monocytogenes on HBEs for the storage trials was associated with the order of the contamination: pre-or post-the acid treatment. Prior storage, L. monocytogenes was detected on 100% of the HBEs. Malic acid pre-treatment was significantly effective in eliminating L. monocytogenes on HBEs at 5 and 25°C, while acetic acid was effective only at 5°C. All acids did not eliminate L. monocytogenes in the case of post-treatment contamination at any tested temperature. The results of this study aid in understanding the efficacy of organic acid treatments against L. monocytogenes on HBEs. Results are useful in the development of preventive controls and guidelines to ensure the safety of HBEs.
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