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Title:Characterization of US Queso Fresco and endolysins for Queso Fresco implementation
Author(s):Holle, Maxwell James
Director of Research:Miller, Michael J
Doctoral Committee Chair(s):Stasiewicz, Matthew J
Doctoral Committee Member(s):Garrow, Timothy A; Engeseth, Nicki J
Department / Program:Food Science & Human Nutrition
Discipline:Food Science & Human Nutrition
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):L. monocytogenes
food safety
Abstract:Queso Fresco (QF) is a white, soft, and crumbly fresh Hispanic-style cheese. QF is also the most widespread Hispanic-style cheese in the United States. Possessing a near neutral pH, low salt content, and high moisture content, QF’s intrinsic properties support the survival and growth of a number of bacteria including one of the deadliest foodborne pathogen Listeria monocytogenes. Dairy products are highly associated with L. monocytogenes outbreaks and in particular QF has been frequently implicated in multiple multi-state outbreaks of listeriosis. The bulk of my dissertation research has revolved around characterizing US QF and evaluating potential antilisterials for implementation in QF. Standards of identity allow for consistent production of certain products and are helpful for categorizing food outbreaks as well as analyzing food safety parameters. The wide variety of Hispanic style cheeses vary amongst each other based on their ingredients and/or production methods. However, in the US, there are no standards of identity for any of the Hispanic style cheeses. As a result, the food safety data (i.e. recalls and outbreaks) may possess an additional variable of error. Furthermore, QF has been reported to possess relatively wide ranges of moisture content, pH, and salt content which could be the result of the lack of standard of identity. In Chapter 2, I describe a sampling study of all of the available QF in the Urbana-Champaign area over a 7-month time period, where the pH, salt content, moisture content, and microbial communities were evaluated. We hypothesized that there would be notable differences amongst QF brands not only because of the lack of standard of identity but also because their labels showed variation in ingredients such as added cultures, enzymes, or the addition of antimicrobials. Our analysis found very little variation between the brands for the evaluated chemical characteristics, and all of the QFs were able to support the growth of L. monocytogenes. However, the microbial contents differed and highlighted that each brand of QF possessed a unique QF microbe fingerprint. The differences in microbial community may account for sensory differences and help manufacturers evaluate their production processes for quality control over time. Common food safety measures for protecting a product against pathogenic contamination are not easily implemented in QF. Organic acids are ineffective at QF’s pH, evaluated essential oils have been shown either ineffective or shown to negatively impact the sensory characteristics of QF, and HPP is incapable of addressing post processing contamination. Endolysins are a class of cell wall hydrolases derived from bacteriophages, which have been shown to be promising candidates for controlling pathogens in food systems. Endolysins derived from gram positive bacteriophages are incredibly diverse. Further fundamental enzymatic characterization is necessary for comprehending their application opportunities. In Chapter 3 presents a study where the only two listerial endolysins with crystal structures (PlyP40 and PlyPSA) are characterized and compared to a previously evaluated endolysin (PlyP100). The lytic spectrum was determined and showed that both endolysins were effective against a broad range of Listeria, however PlyPSA showed very little activity against L. monocytogenes serovars 1/2a, 1/2b, 1/2c, 3b, and 3c. As a result, PlyPSA might be a poor candidate for implementation as a food safety control mechanism because L. monocytogenes 1/2a and 1/2b are the two most common serovars associated with outbreaks of listeriosis. The lytic activities were also compared across varying QF relevant pH ranges, NaCl concentrations, and temperatures. Both the salt content and the pH impacted the lytic activity of the two endolysins in different capacities. Since QF is refrigerated product and L. monocytogenes is a psychrotrophic organism, it was important to evaluate the endolysins for their activity at 4°C. Although both endolysins still functioned at this temperature, they showed approximately an 80% decrease in function, whereas PlyP100 only lost approximately 40% of its optimal lytic activity. However, L. monocytogenes’ growth rate at 4°C is also much slower. Finally, the endolysins were implemented in a miniaturized laboratory scale Queso Fresco to evaluate their efficacy against L. monocytogenes in a food matrix. Although both hindered L. monocytogenes growth over 28 d shelf life of QF, L. monocytogenes was still able to grow over 2 log CFU/g in 14 d whereas PlyP100 held the growth to less than 2 log CFU/g over the entire 28 d at the evaluated concentration. This characterization alongside future characterization will continue to open the opportunities for endolysin optimization and implementation in new food matrices. In recent days the number of online courses offered by universities in the US has dramatically increased. These courses frequently have limited direct interactions with students and can struggle to support student engagement. Chapter 4 details the development and implementation of an at home food fermentation project for an online course within the Food Science and Human Nutrition Department at the University of Illinois Urbana-Champaign. The project requires students to create their own fermented foods at home. The students are then assessed on their ability to explain and apply the fermentation process which is evaluated through a written submission with accompanying pictures. The students explain the scientific reasoning behind each step, such as describing where the cultures originate from, what microorganisms are part of the culture, and what metabolic products the microorganisms will produce.
Issue Date:2020-07-13
Rights Information:Copyright 2020 Maxwell Holle
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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