The establishment and use of microbiological criteria serve as standards and guidelines or specifications for ensuring food safety. Microbiological standards and criteria are set as specifications to determine the usefulness of a food or food ingredient for a particular purpose.
As a standard, there is zero tolerance set for Salmonella in all ready-to-eat (RTE) foods. The criteria are most effectively applied as part of quality assurance programs in which Hazard Analysis and Critical Control Points (HACCP) and other prerequisite programs are in place.
On this premise, microbiological criteria for foods continue to evolve as new information becomes available. The establishment of microbiological criteria for foods in international trade serves as guidelines for national standards and policies, which will culminate in food safety for all.
Types and Variations of Microbiological Criteria
Microbiological criteria vary considerably for different countries. There are no general criteria suitable for all foods; however, the standard is to ensure that the limit of indicator organisms is not exceeded. Microbiological criteria fall into two main categories: mandatory and advisory.
A mandatory criterion is a microbiological standard that normally should contain limits only for pathogens of public health significance, but limits for nonpathogens may be set. An advisory criterion is either a microbiological end-product specification intended to increase assurance that hygienic significance has been met or a microbiological guideline that is applied in a food establishment at a point during or after processing to monitor hygiene (it, too, may include nonpathogens).
Before recommending a criterion, the product must be in international trade, there must be good epidemiological evidence that it has been implicated in foodborne disease, and there must be associated with it good evidence that a criterion will reduce the potential hazard(s) in principle.
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Components of a Microbiological Criterion
The Codex definition of a microbiological criterion consists of five components:
- A statement of the organisms of concern and/or their toxins.
- The analytical methods for their detection and quantification.
- A sampling plan, including when and where samples are to be taken.
- Microbiological limits considered appropriate to the food.
- The number of sample units that should conform to these limits.
These five components are embodied in a sampling plan.
Understanding Sampling Plans
A sampling plan is a statement of the criteria of acceptance applied to a lot based on appropriate examinations of a required number of sample units by specified methods. It consists of a sampling procedure and decision criteria and may be a two-class or a three-class plan. A two-class plan consists of the following specifications: n, c, m; while a three-class plan requires n, c, m, and M.
- n = the number of sample units (packages, beef patties, and so forth) from a lot that must be examined to satisfy a given sampling plan.
- c = the maximum acceptable number, or the maximum allowable number of sample units that may exceed the microbiological criterion m. When this number is exceeded, the lot is rejected.
- m = the maximum number or level of relevant bacteria per gram; values above this level are either marginally acceptable or unacceptable. It is used to separate acceptable from unacceptable foods in a two-class plan, or, in a three-class plan, to separate good quality from marginally acceptable quality foods. The level of the organism in question that is acceptable and attainable in the food product is m. In presence/absence situations for two-class plans, it is common to assign m = 0. For three-class plans, m is usually some nonzero value.
- M = a quantity that is used to separate marginally acceptable quality from unacceptable quality foods. It is used only in three-class plans. Values at or above M in any sample are unacceptable relative to health hazard, sanitary indicators, or spoilage potential.
Microbiological Criteria for Listeria monocytogenes in RTE Foods
| COUNTRY | n | c | m (per g) | M (per g) |
|---|---|---|---|---|
| United States | 5 | 0 | 0 (negative/25 g) | – |
| United Kingdom | 5 | – | 100 | – |
| Canada (Category 1, 2) | 5 | 0 | 0 (0/25 g) | – |
| Canada (Category 2) | 5 | 0 | 100 | – |
| Germany | – | – | 10,000 | – |
Source: International Commission on Microbiological Specifications for Foods, Microorganisms in Foods: Microbiological Testing in Food Safety Management.
International Standards and Risk Management
International microbiological standards have been developed. These standards recommend that a series of steps must be taken to manage microbiological hazards for foods intended for international trade.
These steps include conducting a risk assessment and an assessment of risk management options, establishing a food safety objective (FSO), and confirming that the FSO is achievable by application of Good Microbiological Practices and Hazard Analysis and Critical Control Points (HACCP) for all foods meant for public consumption.
A food safety objective (FSO) is a statement showing the frequency or maximum concentration of a microbiological hazard contained in a food that is considered acceptable for consumer protection.
For example, in cheese, the staphylococcal enterotoxin levels must not exceed 1 µg/100 g. The concentration of aflatoxin in peanuts should not exceed 15 µg/kg. It is the responsibility of the food industry to apply Good Manufacturing Practices (GMPs) and establish appropriate control measures, including critical control points, in all their processes and HACCP plans to ensure quality and safety.
Hazard Analysis and Critical Control Points (HACCP) System
Hazard is a biological, chemical, or physical agent that is reasonably likely to cause illness or injury in the absence of its control. Hazard Analysis is the process of collecting and evaluating information on hazards associated with the food under consideration to decide which are significant and must be addressed in the HACCP plan.
Hazard Analysis and Critical Control Points (HACCP) is a system that provides the framework for monitoring the total food system, from harvesting to consumption, to reduce the risk of foodborne illness. The system is designed to identify and control potential problems before they occur. The application of HACCP is based on technical and scientific principles that assure safe food.
HACCP consists of seven steps used to monitor food as it flows through the establishment, whether it is a food processing plant or foodservice operation. The seven steps of the HACCP system address the analysis and control of biological, chemical, and physical hazards.
HACCP Terminology
- 1. Critical Control Point (CCP): A procedure/practice (control) in food handling/preparation that will reduce, eliminate, or prevent hazards. It is a “kill” step that kills microorganisms or a control step that prevents or slows their growth.
- 2. Hazard: Unacceptable contamination, microbial growth, persistence of toxins, or survival of microorganisms that are of concern to food safety.
- 3. Monitoring: Checking to determine if the criteria established by the critical control point(s) (CCP) have been achieved.
- 4. Risk: Probability that a condition(s) will lead to a hazard.
- 5. Severity: Seriousness of the consequences of the results of a hazard.
Seven Steps of HACCP
- Conduct a Hazard Analysis
The purpose of a hazard analysis is to develop a list of hazards that are likely to cause injury or illness if they are not controlled. Points to be considered in this analysis can include: skill level of employees; transport of food; serving elderly, sick, very young children, immune-compromised; volume cooling; thawing of potentially hazardous foods; high degree of food handling and contact; adequacy of preparation and holding equipment available; storage, and method of preparation. The next step is to determine if the factors may influence the likely occurrence and severity of the hazard being controlled. Finally, the hazards associated with each step in the flow of food should be listed along with the measures necessary to control the hazard. - Determine Critical Control Points (CCPs)
A critical control point is any step in which hazards can be prevented, eliminated, or reduced to acceptable levels. CCPs are usually practices/procedures that, when not done correctly, are the leading causes of foodborne illness outbreaks. Examples of critical control points include cooking, cooling, reheating, and holding. - Establish Critical Limits
A critical limit ensures that a biological, chemical, or physical hazard is controlled by a CCP. Each CCP should have at least one critical limit. Critical limits must be something that can be monitored by measurement or observation. They must be scientifically and/or regulatory-based. Examples include temperature, time, pH, water activity, or available chlorine. - Establish Monitoring Procedures
The monitoring system should be easy to use and meet the needs of the food establishment, as well as the regulatory authority. It is important that the job of monitoring be assigned to a specific individual and they be trained on the monitoring technique. - Establish Corrective Actions
Corrective actions may range, for example, from “continue cooking until the established temperature is reached” to “throw out the product,” depending on the severity of the situation. HACCP plans should be established in advance to include the following: who is responsible for implementing the corrective action and what corrective action was taken. - Establish Verification Procedures
Verification can be accomplished by expert advice and scientific studies and observations of the flow of food, measurements, and evaluations. Another means of verification is an onsite review of the established critical limits. Each CCP will have one independent authority. This verification step provides an opportunity to make modifications to the plan if necessary. - Establish Record-Keeping and Documentation Procedures
Record-keeping and documentation procedures should be simple to complete and include information that illustrates that the established standards are being met. Employees need to be trained on the record-keeping procedures and why it is a critical part of their job. Examples of records include time/temperature logs, checklists, forms, flowcharts, employee training records, etc.
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Food Safety Objectives (FSOs)
An FSO is a statement of the frequency or maximum concentration of a microbiological hazard in a food considered acceptable for consumer protection. Some examples of specific FSOs include:
- Staphylococcal enterotoxin in cheese must not exceed 1 µg/100 g.
- Aflatoxin in peanuts should not exceed 15 µg/kg.
- Listeria monocytogenes in ready-to-eat foods should not exceed 100/g at the time of consumption.
- Salmonellae on raw poultry meat should be 10^6 to 10^9 cfu/g.
Application of Microbiological Criteria to Food Products
The application of criteria to products in the absence of an HACCP program is much less likely to be successful than when the two are combined. Thus, microbiological criteria are best applied as part of a comprehensive program.
Prior to the development of the HACCP and sampling plan concepts, microbiological criteria (generally referred to as standards at the time) were applied to a variety of products. Below are foods and food ingredients that are covered under microbiological standards in the United States.
A. Standards for Starch and Sugar (National Canners Association)
1. Total thermophilic spore count: Of the five samples from a lot of sugar or starch, none shall contain more than 150 spores per 10 g, and the average for all samples shall not exceed 125 spores per 10 g.
2. Flat-sour spores: Of the five samples, none shall contain more than 75 spores/10 g, and the average for all samples shall not exceed 50 spores per 10 g.
3. Thermophilic anaerobe spores: Not more than three (60%) of the five samples shall contain these spores, and in any one sample, not more than four (65%) of the six tubes shall be positive.
4. Sulfide spoilage spores: Not more than two (40%) of the five samples shall contain these spores, and in any one sample, there shall be no more than five colonies per 10 g.
B. Standard for “Bottlers” Granulated Sugar, Effective July 1, 1953 (American Bottlers of Carbonated Beverages)
- Mesophilic bacteria: Not more than 200 per 10 g.
- Yeasts: Not more than 10 per 10 g.
- Molds: Not more than 10 per 10 g.
C. Standard for “Bottlers” Liquid Sugar, Effective in 1959 (American Bottlers of Carbonated Beverages)
All figures are based on dry-sugar equivalent (D.S.E.)
1. Mesophilic bacteria: (a) Last 20 samples average 100 organisms or less per 10 g of D.S.E.; (b) 95% of last 20 counts show 200 or less per 10 g; (c) 1 of 20 samples may run over 200; other counts as in (a) or (b).
2. Yeasts: (a) Last 20 samples average 10 organisms or less per 10 g of D.S.E.; (b) 95% of last 20 counts show 18 or less per 10 g; (c) 1 of 20 samples may run over 18; other counts as in (a) and (b).
3. Molds: Standards like those for yeasts.
D. Standards for Dairy Products (1965 Recommendations of the U.S. Public Health Service)
1. Grade A raw milk for pasteurization: Not to exceed 100,000 bacteria per milliliter prior to commingling with other producer milk; and not exceeding 300,000 per milliliter as commingled milk prior to pasteurization.
2. Grade A pasteurized milk and milk products (except cultured products): Not over 20,000 bacteria per milliliter, and not over 10 coliforms per milliliter.
3. Grade A pasteurized cultured products: Not over 10 coliforms per milliliter.
Note: Enforcement procedures for (a), (b), and (c) require a three-out-of-five compliance by samples. Whenever two of four successive samples do not meet the standard, a fifth sample is tested; and if this exceeds any standard, the permit from the health authority may be suspended. It may be reinstated after compliance by four successive samples has been demonstrated.
E. Certified Milk (American Association of Medical Milk Commissions, Inc.)
1. Certified milk (raw): Bacterial plate count not exceeding 10,000 colonies per milliliter; coliform colony count not exceeding 10 per milliliter.
2. Certified milk (pasteurized): Bacterial plate count not exceeding 10,000 colonies per milliliter before pasteurization and 500 per milliliter in route samples. Milk not exceeding 10 coliforms per milliliter before pasteurization and 1 coliform per milliliter in route samples.
F. Milk for Manufacturing and Processing (USDA, 1955)
Class 1: Direct microscopic clump count (DMC) not over 200,000 per milliliter.
Class 2: DMC not over 3 million per milliliter.
Milk for Grade A dry milk products: Must comply with requirements for Grade A raw milk for pasteurization above.
Dry Milk
- Grade A dry milk products: At no time a standard plate count over 30,000 per gram, or coliform count over 90 per gram.
- Standards of Agricultural Marketing Service (USDA):
- Instant nonfat: U.S. Extra Grade, a standard plate count not over 35,000 per gram, and coliform count not over 90 per gram.
- Nonfat (roller or spray): U.S. Extra Grade, a standard plate count not over 50,000 per gram; U.S. Standard Grade, not over 100,000 per gram.
- Nonfat (roller or spray): Direct microscopic clump count not over 200 million per gram; and must meet the requirements of U.S. Standard Grade. U.S. Extra Grade, such as used for school lunches, has an upper limit of 75 million per gram.
Dried Milk (International Dairy Federation Proposed Microbiological Specifications, 1982)
- Mesophilic count: n = 5, c = 2, m = 5 × 10^4, M = 2 × 10^5.
- Coliforms: n = 5, c = 1, m = 10, M = 100.
- Salmonella: n = 15, c = 0, m = 0.
Frozen Desserts
States and cities that have bacterial standards usually specify a maximal count of 50,000 to 100,000 per milliliter or gram. The U.S. Public Health Ordinance and Code sets the limit at 50,000 and recommends bacteriological standards for cream and milk used as ingredients.
Standard for Tomato Juice and Tomato Products—Mold-Count Tolerances (Food and Drug Administration)
The percentage of positive fields tolerated is 2% for tomato juice and 40% for other comminuted tomato products, such as catsup, purée, paste, and so forth. A microscopic field is considered positive when an aggregate length of not more than three mold filaments present exceeds one-sixth of the diameter of the field (Howard mold count method). This method has also been applied to raw and frozen fruits of various kinds, especially berries.
Frequently Asked Questions (FAQs)
- What are microbiological criteria in food safety?
Microbiological criteria are standards or guidelines that specify the acceptable levels of microorganisms or their toxins in food to ensure safety. They include details on organisms of concern, analytical methods, sampling plans, microbiological limits, and the number of sample units that must conform to these limits. - How do mandatory and advisory microbiological criteria differ?
Mandatory criteria are legally enforced standards that typically set limits for pathogens of public health significance, though nonpathogens may also be included. Advisory criteria are guidelines or end-product specifications used to ensure hygiene during or after processing and may include nonpathogens. - What is a sampling plan in microbiological testing?
A sampling plan outlines the criteria for accepting or rejecting a food lot based on the examination of a specified number of sample units using defined methods. It includes parameters like n (number of samples), c (maximum allowable defective samples), m (acceptable microbial level), and M (unacceptable microbial level in three-class plans). - What is the role of HACCP in ensuring food safety?
Hazard Analysis and Critical Control Points (HACCP) is a systematic approach to monitor the food production process from harvesting to consumption, identifying and controlling potential biological, chemical, or physical hazards to prevent foodborne illnesses. - What is a Food Safety Objective (FSO)?
An FSO is a statement defining the maximum frequency or concentration of a microbiological hazard in a food that is considered acceptable for consumer protection, such as limiting staphylococcal enterotoxin in cheese to 1 µg/100 g or aflatoxin in peanuts to 15 µg/kg. - Why is there zero tolerance for Salmonella in ready-to-eat foods?
Salmonella is a significant pathogen that can cause severe foodborne illness. Zero tolerance in ready-to-eat foods ensures consumer safety by preventing any detectable presence of this organism, as even low levels can pose a health risk. - How are microbiological standards applied to dairy products in the U.S.?
Standards for dairy products, such as Grade A raw milk, specify bacterial limits (e.g., not exceeding 100,000 bacteria per milliliter before commingling) and coliform counts (e.g., not over 10 per milliliter for pasteurized products). Compliance is enforced through a three-out-of-five sample testing protocol. - What is the significance of the Howard mold count method for tomato products?
The Howard mold count method assesses mold contamination in tomato products by determining the percentage of microscopic fields with mold filaments exceeding one-sixth of the field diameter. It sets tolerances, such as 2% for tomato juice and 40% for other tomato products like catsup or purée.
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