Progress 09/01/10 to 08/31/14
Outputs
Target Audience:Fresh produce growers in Texas and Arizona who are interested in new research findings on produce safety so they can apply them to their operations in order to stay compliant with the Food Safety Modernization Act provisions. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?Three graduate students and one undergraduate studentin the U.S.,have benefitted professionally, having presented the results of their work at several local and national conferences, and at one international conference held in Mexico. Moreover, one of the graduate students was the winner of the Nutritional Sciences Graduate Student Association student poster competition, held in College Station, Texas. How have the results been disseminated to communities of interest?Workshops targeted for producers have been held in Texas and Arizona. In Texas, three workshops of 4 to 6 hours duration were held, reaching 80 participants in total. A37% increase in knowledge gained by the participants was achieved by the workshops in Texas. This is significant, given that many of the participants had an average or above average knowledge of food safety on the farm because they were either food safety managers or had third party Good Agricultural Practice certification. In Arizona, there were 10 workshops held, with a total of 245 participants. These consisted of producers as well as third-party auditors of the produce industry in Arizona. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Goal #1:Parallel field studies were conducted in Texas and Mexico to determine the microbial content of spinach, lettuce, parsley (leafy greens), andcantaloupe, tomatoes, and peppers (fruit). Samples were collected in two seasons (Winter/Spring, and Summer/Fall). Samples were analyzed for several categories of bacteria:coliforms, lactic acid bacteria, mesophilic bacteria, and psychrotrophic bacteria. Spinach was the most contaminated leafy green, followed by lettuce, then parsley. Cantaloupes were the most contaminated fruit, with pepper and tomatoes the least.All commodities tested had similar surface polarity. Hydrophobicity of the surface,as measured by contact angle, differed, being highest for spinach compared to parsley and lettuce, indicating a high degree of correlation between hydrophobicity and the presence of microbial contamination. Regarding roughness, spinach, lettuce, and cantaloupes hadmeasurably deeper and narrower indentations, compared with parsley, tomatoes, and peppers.These indentations not only served to shield bacteria from changes in environmental temperature and moisture, but also served to trap water doplets as to minimize evaporation, creating a more humid microenvironment where bacteria were able to thrive. Similar results were observed in produce grown in Sonora and Baja California regions of Mexico. The main exception wasMexican parsley, found to be more contaminated than spinach, in direct contrast to the findings in Texas. This can be attributed to the fact that spinach varieties grown in Mexico are smoother in terms of surface properties, creating a less suitable surface for bacterial adhesion. Sugar residues on the surface were also measured as a way to determine whther the presence of microorganisms is related to availability of nutrients. All commodities had similar levels of sugar on the surface, with the exception of spinach, which had the lowest. Given that spinach from Texas was the most contaminated leafy green, this suggests that the presence of sugar is not a significant factor by itself on the ability of organisms to be present on the surface of produce, but rather it is the degree of hydrophobicity and roughness of the surface that play a major role. Goal #2: In general, the highest contamination levels for most categories of bacteria on Texas-grown produce were in the Summer and Fall, with the lowest in Winter and Spring. This included categories of bacteria within which most pathogenic organisms belong (mesophiles), suggesting that pathogens may be present at higher numbers in produce during Summer and Fall than in Winter and Spring. The exception were those bacteria that typically grow better at low temperatures (psychrotrophs), which were foundto be in higher numbers in the Winter and Spring.This is significant because some pathogens, such as Listeria monocytogenes, which have been found to contaminate produce, belong to this category. Overall, produce grown in Mexico was less contaminated than produce grown in Texas. Mexican produce was irrigated with well water, compared with river water used in Texas. Well water is typically less contaminated than river water, which may explain this observed difference. Psychrotrophic bacteria were found in very low nubmers in produce grown in Mexico, even in the Winter and Spring. This may be due to the fact that this region has a much higher temperature and lower humidity than the Rio Grande Valley of Texas during these seasons. Goal #3:Of 9,324 isolates tested, 914, or roughly 10%, were found to be antagonistic against one or both of the target pathogens (E. coli O157:H7 and Salmonella Saintpaul).In produce grown in Texas, most of the antagonistic bacteriawere found on lettuce and on tomatoes. The least number of antagonistic bacteriawere found on spinach and on peppers. In produce grown in Mexico, the highest number of antagonistic bacteria were foundon canatloupe, spinach, and pepper in the Fall and Winter, and on lettuce and tomatoes in the Spring and summer. Antagonists from lettuce were: Bacillus licheniformis and pumilus, Gemella bergei, Leuconostoc mesenteroides, Listeria grayi, Pantoea spp, Pediococcus pentosaceus, Staphyloccus sciuri, Streptococcus sanguini, mutans, and alactolyticus, and Vagococcus fluvialis. Antagonitic isolates from spinach included: Cupriavidus pauculus, Enterococcus cecorum, and Kocuria kristinae. In parsley, pathogen antagonists included Gemella morbillorum, Staphylococcus intermedius and gallinarum, Pseudomonas paucimobilis, Serratia plymuticha, and Providencia rettgeri. Antagonistic isolates recovered from multiple commodities included: Enterococcus casseliflavus, and Pseudomonas pseudoalcaligenes (spinach and parsley), Enterococcus gallinarum, Myroides spp, and Staphylococcus lentus (parsley and lettuce), Aerococcus viridans and Alcaligenes faecalis (all commodities). Thus, the populations of epiphytic bacteria on leafy greens were affected by comodity type. Most of these antagonistic bacteria synthesize significant amounts of lactic acid, which lowers the pH of the surface of produce, displayinga bactericidal effect on pathogenicorganisms.Most are alsopsycrhotrophs which are typically present more frequently in the Winter andSpring than in the Summer and Fall. Therefore, it is possible that produce grown in Winter and Spring may be least likely to have pathogens present because of the higher presence of antagonistic bacteria. Conversely, produce grown in Summer and Fall may be most likely to have pathogens present because of the lower presence of antagonistic bacteria. Goal #4: Organisms targeted were the pathogens E. coli O157:H7, E. coli O26, E. coli 0121, E. albertii, and Listeria monocytogenes. The most effective antimicrobial treatment applied against pathogenic bacteria consisted of carvacrol encapsulated in Sodium Dodecyl Sulfate (SDS) particles. Also effective was treatment with eugenol or carvacrol when encapsulated in CytoGuard particles. Specifically, E. coli O121 was highly sensitive to Tween 20 micelles bearing eugenol, with an MIC (Minimum Inhibitory Concentration) of 0.4%. E. coli O26 was sensitive to a lower concenrtration of eugenol (0.25%). E. coli O157:H7 and E. albertii were inhibited at 033% eugenol. Similar assaysusing micelles formed from SDS demonstrated an increase in the apparent MIC, in some cases requiring more than double the oil content, indicative of strong antagonistic interactions between oil and surfactant species. Further exploration of MICs against E. coli O157:H7 and Salmonella Saintpaul confirmed preliminaryfindings such that these results appear to be applicableagainst Gram-negative enteric pathogens in general. MICs of micelles against E. coli O157:H7 ranged from 0.11 to 2.0% oil delivery, in a surfactant selection and concentration, as well as in an oil (eugenol, carvacrol)-dependent manner. Similar trends were observed against Salmonella Saintpaul, with MICs ranging from 0.11 to 2.% oil deliery. MBCs (Minimum Bactericidal Concentrations, or the amount required for a 3.0 log cycle reduction in the population)of oils against both pathogens ranged from 0.23 to 4.5% oil, approximately twice that of the MIC. The lowest MIC and MBC against both pathogens was observed with CytoGuard LA 20 + eugenol or carvacrol, while the highest MIC and MBC was observed with Tween 20 + carvacrol. This suggests that eugenol and carvacrol in CytoGuard LA 20 micelles was the most effective system against these two important foodborne pathogens.Given that spinach and cantalopues grown in Texas, and parsley and cantaloupes grown in Mexico, in the Summer and Fall had the highest level of microbial contaminants, they would benefit the most from antimicrobial treatments with Cytoguardcontaining carvacrol or eugenol. Goal #5: Materials were developed in both English and Spanish, and used as PowerPoint presentations and handouts at workshops targeted for producers, held in Texas and Arizona.
Publications
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2013
Citation:
Differential quantification of microorganisms on skin or rind and stem scar of tomatoes and cantaloupes harvested over two seasons in south Texas (Abstract #P3-136), International Association for Food Protection 102nd Annual Meeting. Charlotte, NC, July 28-31, 2013.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2015
Citation:
Epithytic bacteria from fruit and leafy green surfaces with antagonistic effect against enteric bacterial pathogens. Research Symposium, Nutritional Sciences Graduate Association, Texas A&M University, College Station, TX, 2015.
- Type:
Conference Papers and Presentations
Status:
Accepted
Year Published:
2015
Citation:
Bacterial populations present on various leafy greens and their antagonistic activity against enteric pathogens. International Association for Food Protection, 104th Annual Meeting, July 25-28, 2015, Portland, Oregon.
- Type:
Conference Papers and Presentations
Status:
Accepted
Year Published:
2015
Citation:
Quantification of native microbiota on surface of leafy green produce commodities grown in South Texas. International Association for Food Protection, 104th Annual Meeting, July 25-28, 2015, Portland, Oregon.
- Type:
Conference Papers and Presentations
Status:
Accepted
Year Published:
2013
Citation:
Presencia de bacterias psicrotrofas en cultivo de cilantro y su actividad antagonista in vitro contra Salmonella Saintpaul y E. coli O157:H7. Congreso Internacional de Inocuidad de Alimentos, October 31-November 1, 2013, Guadalajara, Mexico.
- Type:
Journal Articles
Status:
Submitted
Year Published:
2015
Citation:
Ruengvisesh, S., A. Loquercio, E. Castell-Perez, and T.M. Taylor. Inhibition of bacterial pathogens in medium and on spinach leaf surfaces using plant-derived antimicrobials loaded in surfactant micelles. J. Food Sci. Submitted.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2013
Citation:
Role of surface-related factors on contamination and survival of pathogens in fresh produce grown in Texas and Mexico. Congreso Internacional de Inocuidad de Alimentos, Mexico, July, 2013.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2014
Citation:
Inhibition of Escherichia coli O157:H7 and Salmonella Saintpaul using plant-derived antimicrobial essential oils in surfactant micelles. International Association for Food Protection 103rd Annual Meeting. Indianapolis, IN, August 3-6, 2014.
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Progress 09/01/12 to 08/31/13
Outputs
Target Audience: The target audience of this project is two-fold: the scientific community of food microbiologists conducting research, teaching, andextension activitiesin this area of study, and fruit and vegetable producers who can benefit from the information provided as a result of this project in terms of enabling them to know the levels and types of bacterial contamination they can expect in their products in the field, the relative ability of certain crops to have antimicrobial properties, and the possible application of essential oils post-harvest to kill pathogenic organisms that may be on the surface of these crops. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided? The results reported here will form part of the development of training modules for fruit and vegetable producers. It is anticipated that the training will consist of an overview of microbial contamination of produce on the farm, the effect of season on the types and levels of microorganisms found on the products, the differences due to the product surface and season, and the relative ability of microorganisms on these products to antagonize, or prevent, the presence and growth of pathogenic bacteria. Finally, such workshops can include demonstrations on the effect of treatment with essential oils after harvest as a method of decontamination of these products. How have the results been disseminated to communities of interest? Results have been disseminated via participation and presentations in various conferences within the food microbiology scientific community over the last two years. We are preparing to develop materials for the general public that can be used in workshop modules as described above. What do you plan to do during the next reporting period to accomplish the goals? We plan to finish the work on the antagonistic effect of normal flora isolates, to include a study to better describe the possible causes of these effects, especially with respect to psychrotrophic bacteria and the differential effect they appear to have on E. coli O157:H7 compared with Salmonella. Secondly, we plan to further develop the delivery system of essential oils, and to test its effectiveness on produce. Lastly, we will develop training materials and will conduct training sessions/workshops for producers to enhance the understanding of the underlying reasons for contamination, and what they can do to miminize it in the field, and post-harvest, to include application of antimicrobial spraying with essential oils encapsulated in surfactants.
Impacts
What was accomplished under these goals?
Three major activities were conducted since the last reporting period. First, we completed work related to determining the microbial content of various fruit and vegetable commodities grown in Texas. Commodities tested included spinach, lettuce, and parsley (vegetables), and cantaloupe, tomatoes, and peppers (fruit). Samples were collected in two farms. For fruit, collection took place in the Winter/Spring season, as well as in the Fall season. For vegetables, only Winter/Spring collection was carried out. 25 samples were obtained from each commodity per farm and per season, for a total of 300 fruit samples (150 in Winter/Spring and 150 in Fall) and 150 vegetable samples (Winter/Spring). These samples were analyzed for the presence and levels of coliforms, lactic acid bacteria, mesophilic bacteria, and psychrotrophic bacteria. With regard to the vegetables, the highest counts in all four categories of bacteria were found in spinach, and the lowest in parsley. With regard to fruit, the highest counts in all four categories of microorganisms were found in cantaloupes, with peppers and tomatoes having the lowest, and relatively similar counts. Spinach and cantaloupe have very rough outer surfaces, which can allow for bacteria to adhere more easily, and resist removal when compared with smoother surfaces, as found on parsely, tomatoes, and peppers. In fact, bacterial counts were found to be (from highest to lowest): spinach>cantaloupes>lettuce>parsley>tomatoes and peppers.There were no significant differences in these results with regard to farm 1 vs. farm 2. However, significant differences were found according to season, and these also differed according to the fruit tested. For example, for cantaloupes, coliforms andlactic acidorganisms were found to be highest in the Fall, while mesophiles and psychrotrophs were found to be highest in the Winter/Spring for cantaloupes. However, for peppers and tomatoes, coliforms, lactic acid bacteria, and mesophileswere highest in the Fall, whereas only psychrotrophs were highest in the Winter/Spring.The contact angle, a measure of hydrophobicity of surfaces, wasrecorded for these commodities, as a way to further explain whether the prevalence of microorganisms is related to the conditions on the surface that may allow bacteria to better adhere to it. In addition, sugar residues on the surface were also measured, as away to determine whether the presence of microorganisms is related to availability of nutrients. We found that cantaloupe and spinachhadthe highest level of hydrophobicity, with tomato and peppers in the medium range of measurements, while lettuce and parsley, as well as the inside of cantaloupeshad the lowest. Given that spinach and cantaloupes had the highest overall counts, these results suggest that indeed, hydrophobicity can enhance the ability of microorganisms to attach to these surfaces. All commodities had similar levels of sugar on the surface, with the exception of spinach, which had the lowest. This suggests that the presence of sugar is not a significant factor on the ability to organisms to be present on the surface of these products. Secondly, we conducted a studyto determinethe presence of bacteria in the normal flora of these commodities, and whether these are antagonistic to the pathogens E. coli O157:H7 and Salmonella Saintpaul. Randomly selected isolates were obtained out of a total of 12,000 isolates collected in the Winter/Spring and 6,000 collected in the Fall from all commodities. Most antagonistic isolates were found in lettuce, then on tomatoes, parsley, with the least number of antagonists being found in spinach and peppers. Most antagonistic bacteria were lactic acid bacteria.Similar numbers of lactic acid bacteria isolates wereantagonistic towards E. coli O157:H7as well asSalmonella,whereas morepsychrotrophic bacteria were found to be antagonistic to E. coli O157:H7 than to Salmonella. The fact that most antagonists were lactic acid bacteria is likely due to their ability to lower the pH of their surroundings, which would have a detrimental effect on pathogenic organisms. The antagonistic effect of psychrotrophs towards E. coli O157:H7 being superior than towards Salmonella maybe due toa specific ability of psychrotrophic organisms, such as Pseudomonads and Klebsiella species, which may be present on these commodities, to either outcompete or directly antagonize one pathogen more than the other. However, further study will be carried out in order to ascertain the exactnature ofthe antagonistic effectobserved in these studies. Thirdly, we beganthe study to determine thebestnanoparticle delivery of essential oils, and the inhibitory, as well as bactericidal effects of these oils on E. coli O157:H7 and Salmonella Saintpaul in vitro. Four surfactants (surfynol 485W, Tween 20, Sodium Dodecyl Sulfate, and Cytoguard LA) were tested for their effectiveness at forming micelles that could carry a load of essential oil for eventual delivery onto surfaces of fruits and vegetables post-harvest as a decontamination process. The essential oils tested were eugenol and carvacrol. SDS micelles were found to bear the highest load of essential oils, with surfynol 485W micelles being moderately successful, and Tween 20 and Cytoguard LA not able to encapsulate the oils at all. The lowest minimum inhibitory concentration (MIC), which was determined by measuring the optical density ofthe solution,was found with 0.5% SDS micelles carrying 0.1125% carvacrol. The lowest minimum bactericidal concentration (MBC), which was determined by plating onto bacteriological media, was with 0.0625% Cytoguard LA and 0.0016 % eugenol or carvacrol. The next step in these studies is to determine the ease of shearing of the micelles in vitro, in order to ascertain ease of delivery, followed by delivery of this treatment to pathogens on the surface of fruits and vegetables.
Publications
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2013
Citation:
Differential quantification of microorganisms on skin and rind and stem scar of tomatoes and cantaloupes harvested over two seasons in south Texas. Abstract #P3-136 International Association for Food Protection 102nd Annual Meeting, Charlotte, NC, July 28-31, 2013.
- Type:
Conference Papers and Presentations
Status:
Submitted
Year Published:
2014
Citation:
Inhibition of Escherichia coli O157:H7 and Salmonella Saintpaul using plant-derived antimicrobial essential oils in surfactant micelles. International Association for Food Protection 103rd Annual Meeting. Indianapolis, IN, August 3-6, 2014.
- Type:
Conference Papers and Presentations
Status:
Awaiting Publication
Year Published:
2013
Citation:
Role of surface-related factors on contamination and survival of pathogens in fresh produce grown in Texas and Mexico. Congreso Internacional de Inocuidad de Alimentos, Mexico. July, 2013
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Progress 09/01/11 to 08/31/12
Outputs
OUTPUTS: Samples of cantaloupe, peppers, and tomatoes were collected during the summer growing season from three farms in the Rio Grande Valley of Texas. These commodities were irrigated by a drip system. Samples of spinach and lettuce were collected during the spring growing season from three farms. These commodities were irrigated by the furrow method. Samples were evaluated for a series of microbial populations both on the surface (rind or leaf) vs. the stem scar. Populations identified included total microorganisms, pseudomonads, yeasts, streptococci, coliforms, Escherichia coli, lactobacilli, and lactic acid bacteria. Psudomonads were used as an indicator for the formation of biofilms by slow-growing organisms. Yeasts, streptococci, coliforms, and E. coli were used as indicators of water quality and of the possible presence of fecal contamination. Lactobacilli and lactic acid bacteria were used as indicators of the presence of spoilage organisms. Readings of ambient field temperature and relative humidity were also recorded. In addition, nanoparticles were developed containing either eugenol or carvacrol and evaluated for maximum antimicrobial delivery for future evaluation as an antimicrobial treatment rinse for the commodities being studied. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
No significant differences were observed among the three farms sampled. With regard to temperature and relative humidity, the ambient temperature was highest in the summer, while the relative humidity was highest in the spring. The readings during the summer ranged from 84 to 104F and 42 to 62% RH, while readings during the spring ranged from 60 to 75F and 51 to 90% RH. In general, every category of microorganism tested on the surface of these commodities was higher in the summer than in the spring. This is to be expected, given that, in general, these organisms are mesophilic and thus require at least a temperature of 68F to grow. However, when testing was performed in the stem scar, warmer temperatures during the summer only resulted in higher counts for coliforms, E. coli, lactobacilli, and lactic acid bacteria. These are organisms that typically originate from the intestinal tract of animals, where the environment is at least 95F, but also in environments where available water is moderate. In contrast, total counts, pseudomonads, and yeast counts in the stem scar were higher in the spring. These organisms thrive in humid environments. However, they also require temperatures above 68F. It may be that being located within the stem scar offered insulation from low temperatures while providing the high humidity for these organisms to grow better than in the summer months, when the temperatures were warmer but the humidity was lower. Within commodity types, cantaloupes had the highest overall microbial counts (6.1 log), regardless of temperature or relative humidity, followed by leafy greens (4.6 log), and then lastly by tomatoes and peppers (3.4 log). This trend may be due to roughness of the surface, with the rougher the surface the higher the microbial counts. In addition to total microbial counts, the highest counts were observed in lactic acid bacteria, lactobacilli, coliforms, and yeasts. The latter two types of organisms are very good competitors, able to metabolize complex carbohydrates and other nutrients on the surface of fruits and vegetables. The former do not grow as well at first, but once they are able to produce lactic acid and lower the pH of the microenvironment, they are better able to compete for nutrients. By contrast, the pseudomonads, streptococci, and E. coli are not as tolerant of pH below 6.5, which would explain the lower counts observed in these organisms. In conclusion, microbial load was highest on the stem scar than on the surface, regardless of type of product or of season. With regard to the surface, produce with the roughest surface properties had the highest microbial counts, regardless of season. Also, counts on the surface were highest in the summer than in the spring. With regard to the specific populations of microorganisms, season resulted in significant differences only in the stem scar, with higher numbers observed in mesophilic organisms in the summer, and with organisms typically found in humid environments during the spring. Thus, surface roughness, location on the surface itself, and season play a role in the type and level of organism contaminating fruits and vegetables.
Publications
- No publications reported this period
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Progress 09/01/10 to 08/31/11
Outputs
OUTPUTS: Samples of spinach, lettuce, and parsley were collected from two farms located in the Rio Grande valley of Texas. Factors such as method of irrigation, fertilizers used, ambient temperature, and relative humidity during time of harvesting were recorded. Samples were evaluated for the presence of coliform bacteria, mesophilic bacteria, psychrotrophic bacteria, and lactic acid bacteria. Ten isolates from each type of bacteria have been obtained and stored for further study on the antagonistic effect these may have toward the pathogens Escherichia coli O157:H7 and Salmonella Saintpaul, to be performed in the next phase of the project. In addition, samples have been analyzed for total aerobic bacteria, Pseudomonas, total Enterococci, and Escherichia coli, as well as yeasts and mold counts in preparation for conducting activities pertaining to determining the effect of antimicrobial treatments on produce, to be performed in the next phase of the project. PARTICIPANTS: The PI Murano led the organization of the project and has been conducting teleconference meetings every three weeks with all other investigators in order to keep abreast of activities, as well as to coordinate activities among the various investigators, such as sample collection. Murano has also communicated with the produce industry with regards to the project at the annual meeting of the Texas Produce Association in 2011. Co-PI Anciso has been directly supervising the collection of smples at the farms, as well as recording information regarding ambient temperature, relative humidity, method of irrigation, and other pertinent information at time of collection. He has also been in charge of securing permission from farms to allow for sample collection. Sample collection is timed according to harvest seasons, and as a result, he has developed a schedule for the collection of samples for the various products involved in this project, as follows: spinach, lettuce, and parsley from February to March, and cantaloupe, tomato, and peppers from May to June. Co-PI Castillo has been conducting microbial analysis of all samples. Co-PI Taylor has been conducting preliminary studies on the specific types of organisms present on the spinach and lettuce, particularly Pseudomonas and E. coli as marker organisms for spoilage and pathogenic bacteria, respectively. Co-PI Cisneros is beginning preliminary studies with regard to the effect of surface properties on adherence of microorganisms to spinach and lettuce, in preparation for the next phase of the project in which several variables will be analyzed in terms of their effect on bacterial attachment to these surfaces. TARGET AUDIENCES: Producer groups and individual farmers in the Rio Grande valley of Texas have been contacted with regard to securing their collaboration for sample collection, and to determine the type of information they find most useful that this project may provide in terms of production practices that will minimize contamination with bacterial pathogens, and that will enhance the overall microbial quality of these products. Preliminary discussions on a tentative schedule for conducting training workshops have been conducted and are in progress. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
In general, bacterial counts for mesophiles, coliforms, psychrotrophs, and lactic acid bacteria were significantly higher in samples collected when the ambient temperature in the field was at least 70F, compared with samples collected below 70F. Specifically, the log counts for spinach samples collected when the temperature was 75F ranged from 0.8 to 2.2 log higher than samples collected when the temperature was 65F. A similar trend was observed in lettuce samples, although the difference between log counts in samples collected when the temperature was 75F compared with those collected when the temeperature was 60F was only 0.1 to 1.2 log higher. The relative humidity (RH) during spinach harvest was relatively high and fairly constant compared to that when lettuce samples were obtained, with the RH for spinach at time of sample collection ranging from 81 to 90%, compared with that for lettuce, which ranged from 51 to 81%. Given that in both spinach and lettuce samples the counts were higher when the temperature was higher, temperature is more important than RH in providing suitable conditions for bacterial growth. However, the fact that the difference in counts for samples collected at the higher temperature was greater for spinach than for lettuce suggests that there are other factors at play. We speculate that because of the many-layered arrangement of lettuce leaves, there may be an insulating effect that allows for bacteria to grow at similar rates, achieving similar counts, in spite of differences in ambient temperature. In contrast, spinach grows in a single-leaf arrangement, causin bacteria to be exposed to the elements more directly than in lettuce, thereby being subject to the effect of variations in ambient temperature to a greater extent. Overall, the bacterial counts were highest in spinach, with mesophiles and psychrotrophs reaching 7.3 log, coliforms at 6.5 log, and lactic acid bacteria at 6.3 log when harvested at 75F. The bacterial counts were lowest in lettuce harvested at the lower temperature (60F), with mesophiles at 6.2 log, but coliforms, psychrotrophs, and lactic acid bacteria registering as low as 4.0 log. Of all the various types of organisms, the mesophilic bacteria grew to the highest counts, regardless of product. This is to be expected, given the fact that mesophilic organisms grow well at the temperatures we observed, and to their ability to utilize complex carbohydrates and other nutrients found on the surface of produce. By contrast, lactic acid bacteria have more exacting nutritional requirements due to their lack of some biosynthetic capabilities, such as for metabolyzing proteins into amino acids, as well as their inability to synthesize certain vitamins. For this reason, they rely on the action of other microorganisms to digest food components into simpler compounds that can then be utilized by the lactic acid bacteria. Psychrotrophic counts were the lowest of any microbial type in lettuce samples only. Given that these organisms are aerobes, it is possible that due to the layered arrangement of lettuce leaves, conditions were less aerobic than in spinach, reducing the ability of psychrotrophs to grow.
Publications
- No publications reported this period
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