GROWTH OF MICROORGANISMS IN FOOD
 
by O. Peter Snyder, Jr., Ph.D.
Hospitality Institute of Technology and Management
 

Introduction
The FDA Food Codes (1993, 1995, 1997, 1999) specify that raw seed sprouts and melon are potentially hazardous food. However, there is no mention that other salad vegetables and high-pH fruits may also be potentially hazardous. The following research studies indicate that salad vegetables and high-pH fruits, along with other foods, support the growth of Listeria monocytogenes and Escherichia coli O157:H7 and other pathogens at refrigerator temperatures. Salad vegetables and neutral-acidity fruits are, in fact, hazardous foods in the context of NACMCF HACCP (1998). Since L. monocytogenes is a natural contaminant of raw salad vegetables, it is essential that acceptable levels of contamination be established for fresh salad vegetables. Since vegetables can easily become contaminated with fecal pathogens in handling, this is another reason for time-temperature control.  Otherwise, all retail operations are open to extreme liability costs if the pathogens are found in the food being sold.

1. Listeria monocytogenes on lettuce.  Beuchat and Brackett studied the effects of shredding, chlorine treatment, and modified atmosphere packaging on survival and growth of L. monocytogenes, mesophilic aerobes, psychrotrophs, and yeast and molds on lettuce stored at 5C and 10C. With the exception of shredded lettuce that had not been chlorine treated, no significant changes in population of L. monocytogenes were detected during the first 8 days of incubation at 5C; significant increases occurred within 3 days when lettuce was stored at 10C. After 10 days, populations reached 108 to 109 CFU/g. Chlorine treatment, modified atmosphere (3% O2, 97% N2), and shredding did not influence growth of L. monocytogenes. It was concluded that L. monocytogenes is capable of growing on lettuce subjected to commonly used packaging and distribution procedures used in the food industry.
     Generation times for microorganisms on shredded lettuce (chlorine treated and modified atmosphere storage) calculated from graphed data are as follows:

L. monocytogenes
G [10C (50F)] = 10.8 hr.

mesophilic aerobic microorganisms
G [5C (41F)] = 12.6 hr.
G [10C (50F)] = 4.5 hr.

psychrotrophic microorganisms
G [5C (41F)] = 17.3 hr.
G [10C (50F)] = 5.4 hr.

     These data indicate that L. monocytogenes is capable of multiplication every 10.8 hours in lettuce at 50F, a common display temperature for fresh vegetables in food markets and in foodservice preparation and service systems.

2. Listeria monocytogenes on deli meat.  Glass and Doyle (1989) reported the growth of L. monocytogenes in processed meat and poultry products at 4.4C. The generation times as calculated are:

Ham I: G [4.4C (40F)] = 21.7 hr.
Ham II: G [4.4C (40F)] = 22.7 hr.
Sliced chicken: [4.4C (40F)] = 17.14 hr.
Sliced turkey I: [4.4C (40F)] = 37.8 hr.
Sliced turkey II: G [4.4C (40F)] = 15.6 hr.
Wieners: G [4.4C (40F)] = 20.6 hr.
Bratwurst: G [4.4C (40F)] = 32.8 hr.
* Roman numerals I and II designate different producers.
     These data indicate that 41F (5C) is not a "safe" storage/display temperature for deli meats if they are contaminated with L. monocytogenes, which could be very likely in today's delicatessens if sanitation SOPs (standard operating procedures) are not strictly followed--which, of course, they are not, because HACCP is not required.

3. Listeria monocytogenes on lettuce.  Steinbruegge et al. (1988) reported the ability of L. monocytogenes to survive and grow on head lettuce obtained from a retail outlet over a period of 10 months. Lettuce was torn into bite-sized pieces, inoculated with L. monocytogenes, placed into plastic bags, and held under a variety of storage conditions. Samples were stored at 5C and 12C, were subjected to aerobic plate count analysis, and levels of L. monocytogenes were determined after 7 and 14 days. Aerobic plate counts increased by 1 or 2 log cycles, and L. monocytogenes increased by 1 log cycle, except for occasional trials where the organism did not grow or survive. L. monocytogenes serotype 1 was isolated from some uninoculated samples, indicating that the organism was naturally present on some of the lettuce heads purchased from retail outlets.
     The generation times for natural flora and L monocytogenes on lettuce are calculated as follows:

APC: G [5C (41F)] = 29.5 hr.
APC: G [12C (53.6F)] = 25.1 hr.

L. monocytogenes: G [5C (41F)] = 89.5 hr.
L. monocytogenes: G [12C (53.6F)] = 33.3 hr.

 4. Listeria monocytogenes on corned beef and ham.  Grau and Vanderlinde (1992) reported the growth of L. monocytogenes on some vacuum-packaged processed meat products.
     The generation times as calculated are as follows: Vacuum packaged corned beef
L. monocytogenes:
G [0C (32F)] = 109.9 hr.
G [2.5C (53.6F)] = 49.4 hr.
G [5C (41F)] = 24.5 hr.
G [10C (50F)] = 32.7 hr.
G [15C (59F)] = 4.5 hr.

Vacuum packaged ham
L. monocytogenes:
G [0C (32F)] = 181.3 hr.
G [5C (41F)] = 33.1 hr.
G [10C (50F)] = 13.4 hr.
G [15C (59F)] = 6.1 hr.

5. Listeria monocytogenes on fresh vegetables.  Berrang et al. (1989) reported the growth of L. monocytogenes on fresh vegetables stored under controlled atmosphere (lower O2 and higher CO2 concentration than found in air). CAS (controlled atmosphere storage) lengthened the time that all vegetables were considered acceptable for consumption by subjective inspection. Populations of L. monocytogenes increased during storage. CAS did not influence the rate of growth of L. monocytogenes.
    The generation times in air are as follows: L. monocytogenes:
G [15C (59F)] = 9.0 hr. -- asparagus (air)
G [15C (59F)] = 11.5 hr. -- broccoli (air)
G [15C (59F)] = 9.6 hr. -- cauliflower (air)
6. Salmonellae and shigellae on papaya, jicama, and watermelon.  Escartin et al. (1989) reported the ability of 5 strains of enteropathogenic bacteria (Shigella sonnei, S. flexneri, S. dysenteriae, Salmonella derby, and S. typhi) to survive and grow on sliced jicama, papaya, and watermelon.
     The generation times are as follows: 7. E. coli on lettuce and other vegetables.  Abdul-Raouf et al. (1983) reported the survival and growth of Escherichia coli O157:H7 on salad vegetables.
     The generation times in air and modified atmosphere are as follows: E. coli O157:H7:
G [12C (53.6F)] = 46.4 hr. -- shredded lettuce (air)
G [21C (69.8F)] = 14.5 hr. -- shredded lettuce (air)
G [12C (53.6F)] = 38.7 hr. -- shredded lettuce (modified atmosphere)
G [21C (69.8F)] = 15.1 hr. -- shredded lettuce (modified atmosphere)

E. coli O157:H7:
G [12C (53.6F)] = 21.0 hr. -- cucumbers (air)
G [21C (69.8F)] = 9.8 hr. -- cucumbers (air)
G [12C (53.6F)] = 63.2 hr. -- cucumbers (modified atmosphere)
G [21C (69.8F)] = 10.9 hr. -- cucumbers (modified atmosphere)

E. coli O157:H7:
G [12C (53.6F)] = 12.1 hr. -- carrots (air)
G [21C (69.8F)] = 10.6 hr. -- carrots (air)
G [12C (53.6F)] = 15.5 hr. -- carrots (modified atmosphere)
G [21C (69.8F)] = 19.0 hr. -- carrots (modified atmosphere)

Abdul-Raouf et al. (1983) also reported the growth of populations of psychrotrophic and mesophilic microorganisms on salad vegetables.
     The generation times in air and modified atmosphere are as follows: G [5C (41F)] = 15.1 hr. -- carrots (modified atmosphere)
G [12C (53.6F)] = 13.0 hr. -- carrots (modified atmosphere)
G [21C (69.8F)] = 6.5 hr. -- carrots (modified atmosphere)
8. Salmonella on cantaloupe, honeydew, and watermelon.   Golden et al. (1993) reported the ability of Salmonella spp. to grow on the interior tissues of cantaloupe, watermelon, and honeydew melons.  The fruit was inoculated with a mixed culture containing equal portions of 5 species of Salmonella (S. anatum, S. chester, S. havana, S. poona, and S. senftenberg).  This study indicates that Salmonella grows well on the interior surface of cut melons at room temperature, such as those found at roadside stands.
     The generation times are as follows: 9.  Salmonella on tomatoes.  Asplund and Nurmi (1991) reported the growth of Salmonella enteritidis, S. infantis, and S. typhimurium in tomatoes at room temperatures of 22C and 30C.
     The generation times are as follows: 10. Salmonella on tomatoes.  Zhuang et al. (1993) reported the ability of Salmonella montevideo to grow on the exterior and interior tissues of tomatoes.  This study indicates that Salmonella grows well on the exterior and interior surface of tomatoes at room temperature.
     The generation times are as follows: Summary
There is no question that pathogenic microorganisms can grow on some common vegetables and deli meats at refrigeration temperatures typical of retail food operations. The only control that is available is time, along with specification of critical limits for pathogen populations. It is naive to specify zero L. monocytogenes in retail ready-to-eat food, salad vegetables, and deli foods.
 
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  2. Asplund, L. and Nurmi, E.  1991.  The growth of salmonellae in tomatoes.  Internat. J. Food Microbiol.  13:177-182.
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  15. Zhuang, R.Y., Beuchat, L.R., and Angulo, F.J.  1995.  Fate of Salmonella montevideo on and in raw tomatoes as affected by temperature and treatment with chlorine. Applied Environ. Microbiol.  61(6):2127-2131.
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