SECTION 4: Contamination Levels and Microbiological Control

  • Pathogen Contamination from Human Sources
  • Foodborne Illness Hazards:  Threshold and Quality Levels
  • Assumed Contamination Levels for Raw Food
  • Assumed Microbiological Criteria for Food Handlers and Food Contact Surfaces
  • Food Pathogen Control Data Summary



    Pathogen Contamination from Human Sources
            In addition to the contamination of raw food supplies that occurs during growing, shipping, and processing, there is the problem of food contamination caused by people who are carriers of pathogens.  (See Table 4-1.)  While most food codes require that when an employee is sick, he or she should stay home, people actually shed pathogenic bacteria a few hours to many days before they have major symptoms of illness.  Food workers can become permanent carriers of pathogens and yet exhibit no signs of illness (e.g., Salmonella carriers).  Therefore, the only safe assumption is that all employees who work with food every day carry pathogens (on their skin and in their urine and feces) that must be kept out of the food.  The following table is a list of some pathogens that commonly originate from human sources.

    Table 4-1. Pathogen Contamination Common from Human Sources*

    Shigella spp., hepatitis A, Norwalk virus, E. coli, Salmonella spp., Giardia lamblia Feces
    Norwalk virus Vomit
    Staphylococcus aureus  Skin, nose, boils, skin infections
    Streptococcus Group A Throat and skin

    * 1 in 50 (2%) of the employees who come to work each day are highly infective.
    (This calculation is based on 16,000,000 people who are ill each year for 2.5 days, of which 5,000,000 work in food operations each day.)

    Foodborne Illness Hazards:  Threshold and Quality Levels
            The next step in the systematic approach to hazard control is to recognize that certain pathogens can be selected as the basis for process control criteria.  Table 4-2 shows the levels of microorganisms that caused illness, based on volunteer feeding tests of healthy people.  Hazardous levels of chemicals and hard foreign objects in food are also listed.
            Spores of the pathogenic bacteria B. cereus, C. botulinum, and C. perfringens must germinate after the food is cooked, and the vegetative cells must multiply to a level of at least 104 to 106 in order to make the food a hazard to consumers.  Low levels of these spore-forming organisms (102 to 103) in food are not a hazard, with the exception of C. botulinum found in honey.  Honey must not be fed to infants at an age of less than one year (103).  Infants have few competitive gut microflora, and the C. botulinum in the honey is able to germinate and then multiply in the infant's intestinal tract and produce toxin at a level sufficient to cause illness and even death.  This hazard can be controlled by educating family members not to feed honey to babies.
            Campylobacter jejuni is a principal cross-contamination problem.  This pathogenic microorganism may be present in poultry at a "natural" level after slaughter that will cause illness without having to multiply.  Raw chicken and other poultry can be contaminated at high levels (i.e., 106 to 107 organisms / chicken) (84).  The threshold for illness in healthy people is approximately 500 organisms in 180 ml of milk, which means approximately 2 organisms per ml.  Comparing this infective level with those for Salmonella, E. coli, Vibrio, etc., it is obvious that C. jejuni, in poultry that is grossly contaminated, will be a major cause of foodborne illness. (If Table 4-2 does not print properly, click here for separate image.)

            Other human fecal pathogens of concern include Shigella spp., pathogenic E. coli, Salmonella spp., hepatitis A virus, and Norwalk virus.  For example, human fecal material from infected individuals can contain millions of shigellae per gram.  Since toilet paper is unreliable in preventing fingertips from coming in contact with fecal material, fingertip washing becomes critical.
            Note that safe food for immune-compromised people in the U.S. has been basically defined by the government as no detectable Salmonella spp. or L. monocytogenes in one or two 25-gram samples from a lot.  A level of less than 100 CFU L. monocytogenes per gram in ready-to-eat food, as set by Canada (46, 47), is probably realistic.  Immune-compromised people must take the responsibility themselves for staying healthy.  For example, they must understand that typical foodservice salads are high-risk food items for containing L. monocytogenes and should be avoided.  These individuals should insist on well-cooked food.
            There are very few sources of data to indicate the threshold levels for hepatitis A virus and Norwalk virus.  If Shigella spp. is controlled, hepatitis A and Norwalk virus will probably also be controlled.  Assuming toilet paper is 99.9% effective, 106 of the 109 pathogens in the feces of an ill employee will get on the employee's fingertips and underneath his or her fingernails.  The fingertip washing process must reduce these organisms to below 10 in order to prevent an employee from transferring this pathogen to food that will make customers ill.  Since antimicrobial chemicals used on hands only reduce pathogens about 100 to 1, the key safety strategy involves using a good hand soap or detergent; physical agitation of the fingertips with a fingernail brush; a lot of warm, flowing water; followed by a second hand wash without the brush.  A greater than 105 reduction can be achieved using this double hand wash procedure.
            Threshold levels for some chemical additives are also listed in Table 4-2.  There are hundreds of additives that are accepted for food use.  The addition of these compounds to food must comply with level of use defined by the Code of Federal Regulations (29, 30, 31).  It is essential that all chemical additives must be measured correctly before being added to food.  For instance, in foodservice, there are no guidelines for the amount monosodium glutamate (MSG) in food.  Hence, MSG is commonly overused in some restaurants.  The 0.5% limit of use for monosodium glutamate suggested in Table 4-2 is based on 1/3 of the levels recommended by a noted supplier of MSG (4).

    Assumed Contamination Levels for Raw Food
          A series of beginning contamination levels for raw food coming into typical foodservice systems is shown in Table 4-3.  It is based on threshold levels that make healthy people ill and normal contamination as listed in the literature.
            These figures pertain to food in the U.S.  Contamination levels in other countries may be higher or lower.  When designing a safe food process, these are the levels of contamination that must be controlled and eliminated by methods of processing, preservation, and storage.

            Often there is the question for retail food operators, "What is a microbiological standard for good food quality?"  Neither the USDA nor the FDA provides an answer to this question.  Table 4-4 lists suggested microbial specifications that can be used when communicating with suppliers as to the microbial quality of raw food (171).

    Table 4-4.  Microbial Criteria for Food Quality

    Number of Spoilage Microorganisms Aerobic Plate Count at 70°F (21.1°C) Rating
    < 10,000 CFU / gram  Good
    10,000 to 5,000,000 CFU / gram Average
    5,000,000 to 50,000,000 CFU / gram  Poor
    > 50,000,000 CFU / gram  Spoiled

            The purpose of foodservice is to provide food that is safe and pleasurable to consume.  Therefore, in addition to providing safe food by controlling and eliminating pathogens in food, the growth of spoilage microorganisms must be limited and controlled as much as possible in order to provide food products of a specified quality.

    Assumed Microbiological Criteria for Food Handlers and Food Contact Surfaces
            In addition to microbial contamination expected in food, it is also necessary to define contamination levels for food handlers, facilities, and equipment.  Based on the estimate that as many as 16 million people may get foodborne illness each year (15), for a period of 2 to 5 days.  It can be estimated through calculations that 1 in 50 people working in the 500,000 food establishments in the U.S. is shedding billions of pathogens in his or her feces.   If  0.001 gram of fecal material leaks through or gets around toilet paper, fingertips and underneath fingernails will become contaminated with fecal pathogens.  Hands and fingertips can also become contaminated when changing diapers, cleaning up vomit, or cleaning up after animals at home.  The double hand wash method using a fingernail brush must be used to reduce fecal microorganisms to less than 10 on the fingertips and under the fingernails.
            The facilities and equipment will also be contaminated.  If the food contact surfaces are washed and rinsed every 4 hours, and if the facilities are cleaned and sanitized adequately at the end of production, the pathogenic build-up on the equipment and facilities can be kept to a safe level.  Note that L. monocytogenes is an environmental pathogen that arrives on food or on the people who enter the facility.  It is essential that the facility be as well maintained as possible in critical locations, so that cracks in the floors, walls, or ceilings (where L. monocytogenes can accumulate) are minimal.  When cleaning is done regularly, L. monocytogenes can be reduced to an undetectable level immediately after cleaning.  It is also essential that food processing areas be kept at a humidity of less than 65% to minimize mold multiplication in dry food and in the environment of the facility.  In a food production area where pasteurized, cooked, cooled food is being assembled and packaged for refrigerated meals, there must be no pathogens on food contact surfaces.  This is defined as no vegetative pathogens (e.g., Salmonella spp., Shigella spp., and C. jejuni) in a 50-sq.-cm. swabbed area of a surface.
            The Standards for Number of Spoilage Microorganisms on Food Contact Surfaces are shown in Table 4-5.  If these standards are maintained, industry experience has shown that foods with long shelf lives can be produced.

    Table 4-5.  Standards for Number of Spoilage Microorganisms on Food Contact Surfaces

    Number of Spoilage Microorganisms
    <1 CFU / 50 sq. cm. or <1/ml. of rinse solution Excellent
    2 to 10 CFU / 50 sq. cm. Good
    11 to 100 CFU / 50 sq. cm. Clean-up time
    101 to >1,000 CFU / 50 sq. cm. Out of control, shut down and find the problem

    Food Pathogen Control Data Summary
            The next step is to develop the microbiological basis for the time and temperature and pH process standards.  Table 4-6 provides the database for process standards development.
            Since Y. enterocolitica and L. monocytogenes both begin to multiply at 29.3ºF (-1.5ºC) (76), food must be kept below a temperature of 30ºF (-1.1ºC) if the multiplication of these pathogens is to be totally stopped.  This means only having frozen food, which is impractical.  Another solution is to use time with temperature from 29.3 to 127.5ºF (-1.5 to 52.2ºC) so that the pathogens do not multiply to an unsafe level.  Salmonella spp. will multiply at a pH as low as 4.1.  Therefore, it is essential that if food such as mayonnaise is made with raw eggs (notorious for being contaminated with Salmonella spp.), the pH of the product must be below 4.1.  Smittle (167) determined that there is no Salmonella spp. growth at pH 4.1 (when acidified with acetic acid), and in fact, Salmonella spp. in mayonnaise is destroyed when held at 70ºF (21.1ºC) for 72 hours.
            The data for C. jejuni point out that it grows very poorly over a limited range and is quite easily destroyed.  Therefore, the major problem with C. jejuni is cross-contamination, as mentioned earlier.  It can be assumed that 10,000 Campylobacter spp. per 50 square cm (177) will be deposited on the food contact surface by raw food such as chicken, and it must be reduced to less than 100 per 50 square cm to be tolerable.
            Spores of C. botulinum type E are inactivated at 180ºF (82.2ºC).  However, many foods do not reach this temperature when cooked or pasteurized.  It must be assumed, then, that C. botulinum type E and other non-proteolytic C. botulinum spores will survive the cooking process.  (If Table 4-6 does not print properly, click here for separate image.)

            Many foods, 0.25 inch below the surface, have an oxidation reduction potential at which C. botulinum will grow.  For control, if the food is a probable carrier of non-proteolytic C. botulinum, the food must be stored below 38ºF (3.3ºC) in order to prevent outgrowth of the spores.  Staphylococcus aureus begins to multiply at 43ºF (6.1ºC) but does not begin to produce a toxin until it reaches a temperature of 50ºF (10.0ºC).  Since there may be recontamination of food with S. aureus when people make salads with their hands, if salad ingredients are pre-chilled to less than 50ºF (10.0ºC) and are kept below this temperature when mixed, there will be no chance of S. aureus toxin production.  Food containing 1,000 S. aureus organisms per gram is not hazardous.  A population of 106S. aureus per gram of food is necessary to produce a sufficient amount of toxin to cause illness.   However, if the toxin is produced, it is virtually impossible to destroy the toxin when the food is cooked or reheated.  Therefore, reheating food to 165ºF (73.9ºC) should never be used as a critical control procedure.  After food is cooked, the best method of control is to prevent the production of toxin by controlling cross-contamination and by keeping the temperature of food below 50ºF (10.0ºC).
            Some types of B. cereus begin to multiply below 40ºF (4.4ºC) (192).  It is a very common contaminant of many cereal products, spices, and even fresh sprouts.  This spore-forming pathogen will survive cooking.  To be safe for long-term storage (greater than 10 days), cooked food must be cooled and maintained at less than 38ºF (3.3ºC) in order to prevent the growth of both type E C. botulinum and B. cereus.
            Proteolytic strains of C. botulinum (types A and B) do not begin to multiply and produce a toxin until they reach a temperature of 50ºF (10.0ºC).  Therefore, if produce such as fruits and vegetables, which today are frequently vacuum packaged and contaminated with low levels of C. botulinum, are kept at a temperature of less than 50ºF (10.0ºC), types A and B C. botulinum will not cause illness.
            Finally, C. perfringens is considered to be another control organism.  Actually, the highest known growth temperature for a foodborne pathogenic bacterium is that of C. perfringens at 126.1°F (52.3°C) and is referred to as the Phoenix phenomenon (164).  Therefore, the upper temperature limit for pathogenic microorganism control is 126.1°F (52.3°C) [rounded to 130ºF (54.4ºC)].  Because of its rapid growth, as rapidly as once every 7.2 minutes at 105.8ºF (41.0ºC) (196), this pathogen dictates the heating and cooling rates for food.  Food must be heated from 40 to 130ºF (4.4 to 54.4ºC) in less than 6 hours in order to assure no multiplication.  Food must be cooled from 130 to 45ºF (54.4 to 7.2ºC) within 15 hours in order to prevent the outgrowth of C. perfringens spores during cooling (98).

    to Section 3
    to Section 5 (part A)
    to Table of Contents
    to References