Linda J Harris

Linda J Harris
University of California, Davis | UCD · Department of Food Science and Technology

Ph.D.

About

156
Publications
29,665
Reads
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6,902
Citations
Additional affiliations
January 1996 - present
University of California, Davis
Position
  • Specialist in Cooperative Extension
January 1991 - December 1995
University of Guelph
Position
  • Professor (Assistant)
Education
January 1987 - March 1991
North Carolina State University
Field of study
  • Microbiology
September 1984 - August 1986
University of Alberta
Field of study
  • Food Microbiology
September 1979 - January 1983
University of Alberta
Field of study
  • Food Science

Publications

Publications (156)
Article
The impact of water application method on bacterial survival at or after the final irrigation was evaluated in bulb onions during commercially relevant field drying (curing). A three-strain rifampin-resistant cocktail of Escherichia coli was introduced to onions via a single overhead spray application in two separate trials (5.22 [trial 1] or 2.40...
Article
Salmonella enterica subsp. enterica is one of the leading causes of human foodborne infections and several outbreaks are now associated with the consumption of fresh fruit and vegetables. This study aims at evaluating whether Salmonella virulence can be linked to an enhanced ability to survive successive digestive environments. Thirteen S. enterica...
Article
BACKGROUND Both reusable and single-use gloves can be employed during hand harvesting of lettuce and leafy greens. The impact of glove type on survival and transfer of Escherichia coli was evaluated using agar or lettuce in a laboratory setting and during simulated lettuce harvesting in the field. RESULTS Textured and smooth reusable latex and smo...
Article
In-home or food service antimicrobial treatment options for fresh produce are limited. Hot water treatments for whole (unpeeled) produce have been proposed, but data to support this practice for onions are not available. Separate cocktails of rifampin-resistant Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella were cultured on agar a...
Article
Full-text available
Food manufacturers are required to obtain scientific and technical evidence that a control measure or combination of control measures is capable of reducing a significant hazard to an acceptable level that does not pose a public health risk under normal conditions of distribution and storage. A validation study provides evidence that a control meas...
Article
This publication serves as a reference for anyone concerned about the safety of fresh and frozen berry products. Providing information for those who grow, harvest, process, transport, and serve berries to consumers is important for improving science-based food safety programs for the entire supply chain. Table 1 lists the reported outbreaks of food...
Article
This document provides an overview of juice-related foodborne outbreaks between 1922-2019. This information is useful to juice processors to aid in the identification of pertinent microorganisms, and also reviews locations, venues of juice preparations, and severity of juice-associated outbreaks.
Article
Full-text available
The practice of soaking almonds prior to consumption is popular both commercially and at home. The food safety implications of soaking almonds was investigated through analysis of blogs and videos (n = 85 recipes) to identify both the reasons for soaking almonds and the common practices employed. Among the recipes analyzed, the most common reasons...
Article
Full-text available
Over a 2-year period, drag swabs of orchard soil surface, and air, soil, and almond leaf samples were collected in an almond orchard adjacent to (35 m from first row of trees) and downwind from a poultry operation, and in two almond orchards (controls) that were surrounded by other orchards. Samples were evaluated for aerobic plate count, generic E...
Article
Process control validations require knowledge of the resistance of the pathogen(s) of concern to the target treatment and, in some cases, the relative resistance of surrogate organisms. Selected strains of Escherichia coli O157:H7 (five strains), Listeria monocytogenes (five strains), and Salmonella enterica (five strains) as well as Salmonella Ent...
Article
Full-text available
The impact of plant development, environmental conditions at the time of inoculation, and inoculum concentration on survival of attenuated BSL1 Escherichia coli O157:H7 strain ATCC 700728 on field-grown romaine lettuce was evaluated over 3 years. E. coli 700728 was inoculated onto 4- and 6-week-old romaine lettuce plants in the Salinas Valley, CA,...
Article
During harvest, pistachios are hulled, separated in water into floater and sinker streams (in large part on the basis of nut density), and then dried before storage. Higher prevalence and levels of Salmonella were previously observed in floater pistachios, but contributing factors are unclear. To examine the behavior of pathogens on hulled pistachi...
Article
Full-text available
A Scientific Integrity Consortium developed a set of recommended principles and best practices that can be used broadly across scientific disciplines as a mechanism for consensus on scientific integrity standards and to better equip scientists to operate in a rapidly changing research environment. The two principles that represent the umbrella unde...
Article
Salmonella has been isolated from dried pistachios in both postharvest and retail surveys. The source of Salmonella in pistachios is unknown, but introduction is possible at points during production, harvest, and postharvest activities. To examine the behavior of Salmonella on pistachios during simulated postharvest conditions, early-, mid-, and la...
Article
Full-text available
Recalls and cases of listeriosis have been associated with the presence of Listeria monocytogenes in fresh-cut refrigerated or frozen onions or associated processing environments. The survival or growth of L. monocytogenes on the outer surface of whole onions and in diced onion was evaluated during simulated retail or consumer storage. Whole and di...
Article
The risk of salmonellosis from consumption of pistachios produced and consumed in the U.S. was assessed through quantitative microbial risk assessment. Data on Salmonella prevalence and concentration on pistachios, nut crop volume, storage times and temperatures during processing and handling, and reductions during storage or from roasting were der...
Book
Full-text available
Many pathogens, such as Salmonella, due to their enhanced thermal resistance in dry environments, can survive the drying process and may persist for prolonged periods in low-moisture foods, especially when stored in refrigerated environments. Bacterial contamination of low-moisture foods, such as peanut butter, present a vexing challenge to food sa...
Chapter
This chapter provides an introduction to modeling Salmonella in low water activity foods focusing on the statistical issues related with model development and the development of a quantitative microbial risk assessment (QMRA). Salmonella on peanuts are used in an example case study. Predictive models in food microbiology are used to estimate microb...
Chapter
Processors of low-moisture foods (LMF) must ensure that Salmonella and other bacterial pathogens are significantly minimized or eliminated to ensure food safety. The control of pathogens in LMF typically involves the implementation of processing control measures, often heating processes. This chapter describes validation of control measures for pat...
Chapter
This chapter covers the persistence of Salmonella and other bacterial pathogens in low-moisture foods (LMF). The moisture conditions in which pathogenic and spoilage microorganisms cannot grow is of great importance for food preservation. Some bacterial pathogens (vegetative cells and spore formers), toxin-producing molds, and viruses, are able to...
Chapter
This chapter describes heat resistance of Salmonella and other bacterial pathogens in low-moisture foods (LMF). It is well established that the thermal tolerance of microorganisms is enhanced as water activity decreases, thus presenting a unique challenge LMF. Thermal inactivation kinetic values are determined experimentally in the laboratory by co...
Article
Full-text available
The Salmonella Syst-OMICS consortium is sequencing 4,500 Salmonella genomes and building an analysis pipeline for the study of Salmonella genome evolution, antibiotic resistance and virulence genes. Metadata, including phenotypic as well as genomic data, for isolates of the collection are provided through the Salmonella Foodborne Syst-OMICS databas...
Article
This study quantifies the cross-contamination rates between fresh-cut produce and hands using a nalidixic acid-resistant nonpathogenic Enterobacter aerogenes and cocktails of rifampin-resistant Salmonella or Escherichia coli O157:H7 strains. Volunteers performed the E. aerogenes experiments (n = 20), and one of the authors performed the Salmonella...
Article
After harvest, pistachios are hulled with mechanical abrasion and then separated in a float tank containing water; the nuts that float (∼15%; floaters) and those that sink (∼85%; sinkers) are dried and stored separately. To determine the prevalence of Salmonella in pistachios, a total of 3,966 samples (1,032 floaters and 2,934 sinkers) were collect...
Article
Association of foodborne pathogens with tree nuts has led to interest in microbial mitigation strategies that can be applied during postharvest handling and processing. The efficacy of commercial peracetic acid (PAA)-based sanitizer spray applications was evaluated for efficacy in reducing aerobic plate counts (APC) and Escherichia coli/coliform co...
Article
After harvest, inshell walnuts are dried using low-temperature forced air and are then stored in bins or silos for up to 1 year. To better understand the survival of bacteria on inshell walnuts, aerobic plate counts (APCs) and Escherichia coli?coliform counts (ECCs) were evaluated during commercial storage (10 to 12°C and 63 to 65% relative humidit...
Article
Inshell walnuts collected from California walnut handlers over four harvests were evaluated for the presence of Escherichia coli O157:H7 and Salmonella. E. coli O157:H7 was not detected in any of 2,903 375-g samples evaluated in 2011, 2012, and 2013 (<0.034% prevalence; 95% confidence interval [CI], 0 to 0.13%). Salmonella was not isolated from any...
Article
Changes in aerobic plate count (APC) and Escherichia coli/coliform count (ECC) of inshell walnuts and walnut kernels were evaluated during commercial harvest and postharvest handling. APC and ECC for inshell walnuts collected from the tree were 6 and 4 log CFU/nut, respectively; counts increased by 1 log during harvest and hulling and decreased by...
Article
Failure to secure safe and affordable food to the growing global population leads far too often to disastrous consequences. Among specialists and other individuals, food scientists have a key responsibility to improve and use science-based tools to address risk and advise food handlers and manufacturers with best-practice recommendations. With coll...
Article
Full-text available
Cocktails of lawn-collected cells were used to determine the survival of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes on the surface of raw peanut and pecan kernels. Kernels were inoculated with mixtures of four to five strains at 3 or 6 log CFU/g, dried at room temperature, and then stored at –24 ± 1, 4 ± 2, and 22 ± 1°C for 28...
Article
Lettuce and leafy greens have been implicated in multiple foodborne disease outbreaks. This study quantifies cross contamination between lettuce pieces in a small-scale home environment. A five-strain cocktail of relevant Escherichia coli O157:H7 strains was used. Bacterial transfer between single inoculated lettuce leaf pieces to 10 non-inoculated...
Article
Hand washing is recognized as a crucial step in preventing foodborne disease transmission by mitigating crosscontamination among hands, surfaces, and foods. This research was undertaken to establish the importance of several keys factors (soap, soil, time, and drying method) in reducing microorganisms during hand washing. A nonpathogenic nalidixic...
Article
Whole and diced yellow onions (Allium cepa) were inoculated with five-strain cocktails of rifampin-resistant Escherichia coli O157:H7 or Salmonella and stored under conditions to simulate food service or consumer handling. The inoculum was grown in broth (for both whole and diced onion experiments) or on agar plates (for whole onion experiments). M...
Chapter
Worldwide nut production has expanded rapidly in recent years with a corresponding increase in consumption. Large outbreaks of salmonellosis have been associated with nuts and their products during this same time period, which has resulted in a major shift in the approach used to process these products. A brief overview of the history and use of nu...
Article
Postharvest contamination of in-shell walnuts may occur when the fruit is dropped to or harvested from the orchard floor or as the outer hull is removed with mechanical abrasion and water. To evaluate the effect of maturity on the potential for microbial contamination, 'Howard' walnut fruits were collected weekly from the tree canopy, from 6 to 7 w...
Article
The ability to predict the behavior of Escherichia coli O157:H7 on contaminated field lettuce is essential for the development of accurate quantitative microbial risk assessments. The survival pattern of the species was assessed from several data sets derived from field-based experiments, which were analyzed by regression analysis fitting one monop...
Article
Full-text available
Enterococcus faecium NRRL B-2354 is a surrogate microorganism used in place of pathogens for validation of thermal processing technologies and systems. We evaluated the safety of strain NRRL B-2354 based on its genomic and functional characteristics. The genome of E. faecium NRRL B-2354 was sequenced and found to comprise a 2,635,572-bp chromosome...
Article
This 9-page fact sheet serves as a reference for anyone concerned about the safety of fresh and frozen berry products. Providing information for those who grow, harvest, process, transport, and serve berries to consumers is important for improving science-based food safety programs for the entire supply chain. Table 1 lists the reported outbreaks o...
Article
This chapter discusses microbiological safety of almonds and pistachios. An overview of the basic production or growing practices is provided along with general post-harvest handling and processing schemes that compare and contrast the two nuts. Sources and routes of contamination of these nuts with foodborne pathogens and mycotoxigenic molds is co...
Article
Cross-contamination between foods and surfaces in food processing environments and home kitchens may play a significant role in foodborne disease transmission. This study quantifies the cross-contamination rates between a variety of fresh-cut produce and common kitchen surfaces (ceramic, stainless steel, glass, and plastic) using scenarios that dif...
Conference Paper
Introduction: Cool temperatures and high relative humidity (RH) are known to improve the survival of enteric foodborne pathogens on produce. Purpose: To determine the impact of temperature and RH on the survival of attenuated Escherichia coli O157:H7 and generic E. coli on field‑grown cilantro. Methods: In two Salinas, California field trials (summ...
Article
Full-text available
Leafy green produce has been associated with numerous outbreaks of foodborne illness caused by strains of Escherichia coli O157:H7. While the amounts of culturable E. coli O157:H7 rapidly decline after introduction onto lettuce in the field, it remains to be determined whether the reduction in cell numbers is due to losses in cell viability, cell i...
Data
Effect of increasing PMA concentration on detection of live and dead cells by real-time PCR. Live or isopropanol-inactivated dead cells of E. coli O157:H7 ATCC 700728 (5×106 CFU) were incubated for 30 min in 0, 50, or 100 µM PMA prior to real-time PCR with the lpfA primer set. Each bar represents the mean ± stdev for three independent replicates. (...
Data
Standard curves for detection of E. coli O157:H7 with hlyA primers (A) and lpfA primers (B). DNA was extracted by boiling as described in the Materials and Methods from a stationary phase culture of E. coli O157:H7 diluted to a cell density of 106 CFU/ml. Real-time PCR was performed for each dilution in triplicate. The limit of detection for E. col...
Data
Ct values for real-time PCR measurements performed using different annealing temperatures and gene targets of E. coli O157:H7 ATCC700728 and E. coli K12. (DOCX)
Data
Primers used in this study. (DOCX)
Article
Full-text available
The survival of Salmonella enterica Enteritidis PT 30 or five-strain cocktails of S. enterica, Escherichia coli O157:H7, and Listeria monocytogenes was evaluated on inshell walnuts during storage. Inshell walnuts were separately inoculated with an aqueous preparation of the pathogens at levels of 10 to 4log CFU/nut, dried for 24h, and then stored a...
Article
Full-text available
The developmental and temporal succession patterns and disturbance responses of phyllosphere bacterial communities are largely unknown. These factors might influence the capacity of human pathogens to persist in association with those communities on agriculturally-relevant plants. In this study, the phyllosphere microbiota was identified for Romain...
Data
Correlation between Proteobacteria and Firmicutes abundances in the Romaine lettuce phyllosphere. The phyla are inversely correlated with an R2 of 0.93. A total of 223 plants were examined during E09, L09, E10, and L10. (TIF)
Data
Rank abundance of the top 20 taxonomic groups at the genus level. The ranked average percentage of the most abundant taxa across all samples (n=223) is shown. The samples are ranked in order of abundance from left to right on the x-axis. When sequences were unable to be classified to a genus, the next most specific designation is shown (i.e. Family...
Data
PCoA of UniFrac community distances between E. coli O157:H7 inoculated and control plants. Differences between inoculated and control plants are highlighted at each timepoint in all four trials. Unweighted UniFrac community distance data was used to make the E09 14 dpi drip-irrigated PCoA, the remaining analyses used the weighted UniFrac data. All...
Data
Comparison of microbiota on control plants examined in 2010. PCoA plots of weighted UniFrac community distances of control plants in the L10 trial. Plant samples are colored according to days post inoculation (dpi) (A) and control group type (unsprayed controls (control) vs. peptone sprayed controls (control + peptone) (B). (TIF)
Data
Microbial diversity at the genus level in the Romaine lettuce phyllosphere. The top 20 most abundant taxonomic groups at the genus level are listed. The relative abundance for each of the 20 groups is shown for each plant, listed from 7 dpi (left) to 21 dpi (right) for the four field trials. When sequences were unable to be classified to a genus, t...
Data
Classification of unknown 16S rRNA sequences from the Enterobacteriaceae family. The similarity of 16S rRNA sequences from OTU’s classified as belonging to unknown genera of Enterobacteriaceae (numbers) and representative Enterobacteriaceae genera (Genus_species) is investigated here. Those OTU’s that grouped together and not closely related to kno...
Conference Paper
Hulls are removed from in-shell walnuts with physical abrasion and water. Introduction of an effective antimicrobial step post hulling would supplement existing walnut food safety programs.The study objective was to evaluate a peracetic acid (PAA) spray for reducing the microbiota on hulled in-shell walnuts and conveyor belts in a commercial walnut...
Conference Paper
Introduction: After harvest, hulling and dehydration, walnuts are stored in-the-shell for up to 1 year prior to cracking. Purpose: Our objective was to determine the survival of Salmonella, Escherichia coli O157:H7 and Listeria monocytogenes on inoculated inshell walnuts under conditions that mimic commercial storage. Methods: Inshell walnuts were...
Article
Full-text available
Many outbreaks of foodborne illness associated with the consumption of fresh-cut melons have been reported. The objective of our research was to develop a mathematical model that predicts the growth rate of Salmonella on fresh-cut cantaloupe over a range of storage temperatures and to validate that model by using Salmonella and Escherichia coli O15...
Article
Full-text available
Application of manure or soil amendments of animal origin (untreated soil amendments; UTSAs) to agricultural land has been a long-standing practice to maintain or improve soil quality through addition of organic matter, nitrogen, and phosphorus. Much smaller quantities of these types of UTSAs are applied to land used for food crops than to land use...
Article
A dry-inoculation method for almonds and walnuts was developed to eliminate the need for the postinoculation drying required for wet-inoculation methods. The survival of Salmonella enterica Enteritidis PT 30 on wet- and dry-inoculated almond and walnut kernels stored under ambient conditions (average: 23 °C; 41 or 47% RH) was then compared over 14...
Data
Peanuts and peanut-containing products have been linked to at least seven salmonellosis outbreaks worldwide in the past two decades. In response, the Technical Committee on Food Microbiology of the North American Branch of the International Life Sciences Institute collaborated with the American Peanut Council to convene a workshop to develop a fram...
Article
Full-text available
Peanuts and peanut-containing products have been linked to at least seven salmonellosis outbreaks worldwide in the past two decades. In response, the Technical Committee on Food Microbiology of the North American Branch of the International Life Sciences Institute collaborated with the American Peanut Council to convene a workshop to develop a fram...
Article
Full-text available
Agricultural water may contact fresh produce during irrigation and/or when crop protection sprays (e.g., cooling to prevent sunburn, frost protection, and agrochemical mixtures) are applied. This document provides a framework for designing research studies that would add to our understanding of preharvest microbial food safety hazards and control m...
Article
Full-text available
Enterococcus faecalis ATCC 29212, a vancomycin-sensitive strain, has been extensively used as a representative control strain for clinical and laboratory experiments. Here we report the draft genome and annotation of this strain, containing 3,027,060 bp, with a G+C content of 37.2% in 126 contigs (≥500 bp).
Article
The survival of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes was determined on almonds and pistachios held at typical storage temperatures. Almond kernels and inshell pistachios were inoculated with four- to six-strain cocktails of nalidixic acid-resistant Salmonella, E. coli O157:H7, or L. monocytogenes at 6 log CFU/g and then...