Microbial ecology of mushroom casing soils and preharvest strategies to enhance safety and quality of fresh mushrooms

Open Access
Chikthimmah, Naveen
Graduate Program:
Food Science
Doctor of Philosophy
Document Type:
Date of Defense:
January 13, 2006
Committee Members:
  • Robert Bruce Beelman, Committee Chair
  • Luke F Laborde, Committee Chair
  • Hassan Gourama, Committee Member
  • David Meigs Beyer, Committee Member
  • food microbiology
  • food safety
  • mushrooms
  • Agaricus
  • Listeria monocytogenes
  • peat
  • casing layer
Agaricus bisporus mushrooms of good quality immediately after harvest normally develop brown blotches at retail, even while kept at refrigeration temperatures. The brown blotch discoloration, caused due to high bacterial populations, diminishes the quality and value of the product. Food safety is another significant issue of concern to the mushrooms industry. In August 2003, the Georgia Dept. of Agriculture recalled a brand of ¡°ready-to-eat¡± mushrooms grown in Pennsylvania for testing positive for Listeria monocytogenes. Prior to that incident, in 2001, a health alert was issued by the Food Safety Authority of Ireland (FSAI) after Salmonella Kedougou was detected on mushrooms.1,2 These recent incidents and several anecdotal reports within the mushroom industry of the presence of Listeria sp. in mushroom casing soils have been an impetus to develop preharvest Hazard Analysis Critical Control Points (HACCP) programs at the mushroom farm level. The first goal of this research was to evaluate crop irrigation with modified acidic electrolyzed oxidizing water, < 1% hydrogen peroxide (H2O2), 0.3% calcium chloride (CaCl2), and casing soil pasteurization as preharvest cultural practices to reduce bacterial populations on fresh mushrooms and enhance quality. The cultural practices were also evaluated for crop yield. Crops were grown using commercial mushroom growing practices except for the treatments to the irrigation water or casing layer. Mushrooms were aseptically sampled from the production beds for enumerating microbial populations by standard microbiological plating procedures. Whiteness (L-value) and color (delta E) following harvest and postharvest storage were measured using a Minolta Chromameter. Mushrooms were separated by treatment and weighed to determine yield. Preliminary studies indicated that 0.5% H2O2 in irrigation water reduced symptoms of brown blotch in mushrooms during postharvest storage. Further studies evaluated 0.75% H2O2 and/or 0.3% CaCl2 in irrigation water to reduce bacterial populations. Mushrooms irrigated with water (control) had ca. 7.3 log CFU of aerobic bacterial populations per gram of fresh mushroom tissue. Compared to the control, irrigation with 0.75% H2O2 and 0.3% CaCl2 reduced the bacterial populations on fresh mushrooms by 87% to 6.4 log CFU/g. The irrigation treatment significantly enhanced mushroom whiteness following harvest as well as 6-days of postharvest storage at 12¨¬C, and had no significant effect on crop yields. Unpasteurized casing soil prior to irrigation contained 5.9 log CFU/gm of aerobic bacterial populations. Pasteurization of the casing soil resulted in a 2.9 log reduction in aerobic bacterial population numbers (reducing the total aerobic bacterial population from 5.9 to 3 log CFU/gm of casing soil (wet wt.). However, the aerobic bacterial population increased by 3.9 log (from 3 log to 6.9 log CFU/gm) following irrigation and holding for 1 week at 17¨¬C. The aerobic bacterial population of unpasteurized casing soil increased by 1.4 log CFU/gm following irrigation and holding for 1 week (from 5.9 log to 7.3 log). There was no significant difference in bacterial numbers in mushrooms grown using either unpasteurized or pasteurized casing soil. However, mushrooms grown on pasteurized casing soil had better postharvest quality. Crop yield decreased by 11.0% when mushrooms were grown using pasteurized casing soil. While casing soil pasteurization demonstrated potential to enhance mushroom quality, the technology was not optimized for commercial recommendations due to food safety factors discussed below. The second goal of this research was to determine the survival and/or growth of Listeria monocytogenes and Salmonella sp. in whole and sliced mushrooms during postharvest storage at 12¨¬C. The slicing operation was conducted following inoculation of the foodborne pathogens over the mushroom cap surface. Inoculated and packaged mushroom were stored at 12¡ÆC and 75-85% relative humidity. Mushroom packages were removed over the shelf-life period to enumerate populations of L. monocytogenes and Salmonella sp. Whole mushrooms did not significantly support the growth of L. monocytogenes or Salmonella sp. However, slicing mushrooms significantly promoted the growth of the pathogens. Populations of L. monocytogenes in unwashed sliced mushrooms rapidly increased from 4 log CFU/gm to 6.8 log CFU/gm during 5 days of postharvest storage at 12¨¬C. Populations of Salmonella sp. in unwashed sliced mushrooms increased from 4.47 log CFU/gm to 6.7 log CFU/gm during 5 days of postharvest storage at 12¨¬C. The third goal of this research was to understand the microbial ecology of the casing soil, determine the survival of L. monocytogenes and Salmonella sp. in casing soils, and study the influence of indigenous culturable casing soil microflora on the survival of L. monocytogenes and Salmonella sp. Because mushrooms are in close proximity to the casing soil and, in fact, often adhere to the mushroom surface at retail display, there are new demands from buyers for growers to implement food safety control measures for casing soils. A recommendation to implement a control measure for casing soils has been reserved since the microbial ecology of the casing soil and its influence on the survival of foodborne pathogens is currently not known. Freshly prepared casing soil samples were used to isolate pure cultures of indigenous microflora. Batches of casing soil were either untreated or autoclaved at 121¨¬C for 90 min to destroy populations of indigenous casing microflora. The casing soils were inoculated with L. monocytogenes and/or Salmonella sp., maintained under simulated mushroom-growing conditions (80% moisture, 22¡ÆC), and periodically sampled for enumerating populations of the foodborne pathogens. To determine the influence of soil biotic factors on food safety, pure cultures of indigenous microflora were re-introduced into sterile casing soil, allowed to establish, and challenged with the foodborne pathogens. Indigenous culturable casing soil microflora comprised predominantly of the Pseudomonas and Pantoea bacterial genera, the Streptomyces genera from the actinomycetes, the Penicillium fungal genera, and a high population of native yeast. Inoculated population levels of L. monocytogenes and Salmonella sp. remained largely unchanged in autoclaved casing soil over a period of even upto 8 weeks. However, the foodborne pathogens were rapidly destroyed in untreated casing soil. A 4 log reduction in populations of L. monocytogenes occurred within 10 days of introduction into the casing soil. The presence of Penicillium sp. and Streptomyces sp. in sterile casing soil inhibited the growth of L. monocytogenes and Salmonella sp. Penicillium chrysogenum, indigenous to casing soil, produced the ¥â-lactam class of antibiotic molecule in a casing soil broth system. The studies demonstrate that unpasteurized casing soils do not support the survival or growth of L. monocytogenes or Salmonella sp. The extended survival of the foodborne pathogens in casing soil with no background microflora emphasizes the importance of a viable and robust casing soil microflora.