Chilled and frozen foods

Microbiological risks and designing safe processes

М. Г. Браун, Unilever Research, г. Шарнбрук


Many different ingredients and raw materials are used to produce chilled foods. When harvesting or slaughtering livestock, various microorganisms can be present in these materials - both inside and on their surface. Some products are carriers of microorganisms that ultimately cause spoilage (for example, bacilli or lactic acid bacteria), while others get into them when harvesting or processing. Many foodborne bacteria are usually found in pets and agricultural products (e.g. Salmonella, E. coli 0157 and Campylobacter) and therefore can infect meat and poultry, milk and vegetable products. The number of microorganisms and their types present are different in different ingredients, and often the safety of the product at the place of consumption will depend on the conditions of production, the use of the product by the consumer, the presence of pathogens in the raw materials and, ultimately, in the finished product. In order to guarantee the production of safe products with a reliable shelf life, the manufacturer must determine (for example, using microbiological studies) which bacteria are most likely to cause food poisoning and spoilage for certain raw materials and products. Therefore, it is important to design food processing technology in accordance with principles that guarantee a reduction in the risk of food poisoning. This is especially important for semi-finished and finished chilled products, the safety of which is based on the control of a number of characteristics of the production process [48, 56]. Appropriate controls should be implemented in production operations, and often these controls operate at several stages of the product movement chain. For example, in [37] it was suggested that the general sanitary and hygienic condition of hamburgers with different venom can only be improved by using hygienically clean beef in production (that is, having the lowest possible level of infection with pathogens) and by improving storage organization and cooking hamburgers at retail outlets. For many chilled product sectors, there are good technology manuals (eg, [50]). These guidelines outline the areas of responsibility for the manufacture of safe products, and adhering to the principles set forth in these manuals will ensure that the safety and usefulness of the products are maintained under the specified conditions for their use.

Product and technology development is always a compromise between safety and quality requirements on the one hand, and cost and production constraints on the other. The main means of ensuring product safety and eliminating bacteria that cause spoilage is heat. Possible heat treatment can sometimes be limited by changes in product properties. Typically, minimal heat treatment at work or at home is designed to kill certain bacteria that cause infections or product spoilage. The product developer’s skill is to balance the conflicting demands of quality and safety. At the same time, quality and safety are determined by several stages of production: for example, cold storage is used to slow down or prevent the growth of plant cells and spores that have undergone heating in the workplace. Therefore, the safety of chilled products that do not have internal preservative properties depends almost exclusively on maintaining appropriate cooling temperatures throughout the product supply chain, including, for example, thawing frozen ingredients and loading refrigerated trucks. When using certain methods of preservation (for example, lowering the pH, increasing acidity, or applying vacuum (sealed packaging)), cooling helps to increase the efficiency of the preservation system and requires additional control during processing.

Risk assessment methods (formal or more often informal) can help the manufacturer achieve a predictable and acceptable balance between the sale of raw or non-disinfected components, heat treatment and the likelihood of survival of pathogenic microorganisms. For successful development of the technological process, it is necessary to consider not only the contaminants that will probably come from the raw materials, but also the shelf life of the product, the expected storage conditions in the wholesale / retail trade and at the consumer [17]. In this sense, the consumer is a component of the security chain, and therefore, when developing products whose safety and shelf life depends on the consumer’s handling (for example, during heat treatment or cold storage), the manufacturer always takes into account some additional risk due to possible violations of the consumer’s regimes processing and use of the product. It was noted in [15] that chilled products contain little (or no at all) antimicrobial additives to prevent the growth of pathogenic microorganisms, and if they are improperly cooled, such products can grow. This work also highlights related issues - such as excessive trust in the shelf life as a measure of quality and the need to take into account when developing products the needs of people with hypersensitivity (for example, with impaired immune systems). If during product development it is assumed that the consumer will perform some operation that eliminates pathogenic microorganisms in the product (for example, salmonella), it is important that the manufacturer provides him with clear, accurate and verified instructions for heating or cooking - so that following these instructions leads to high product quality. For product safety, proper control of heat treatment and hygiene in production, at home or at a catering facility is important. Prevention of repeated and mutual contamination of products after heating plays an even more important role when products are sold ready-to-eat.

It is important that products whose safety depends on refrigerated storage are preserved during production, marketing and storage at a given temperature (from -1 to + 8 ° C) or at a lower temperature. Storing foods at higher temperatures can contribute to the growth of the dangerous microorganisms that may be present in them. Inadequate processing, combined with violations of the temperature or time regime during storage, will certainly lead to the growth of microorganisms, causing spoilage and premature loss of quality. To control the conditions and degree of violation of the temperature regime in the distribution chain, the use of temporary temperature integrators (777) [58] is proposed. Based on the prediction of the dynamics of the development of microorganisms, one can evaluate the effect of storage temperatures on the safe shelf life of meat and poultry products. The risks associated with any particular product can be investigated using practical tests (such as “provocative” testing) or mathematical modeling.

Applying prognostic models to the destruction of microorganisms by heating (changing the time and temperature to calculate the lethality of a process based on Dnz values) or to the growth rate of microorganisms can improve the distribution chain management. Microbiological predictive computer databases applicable to chilled products include the Food MicroModel in the UK and the Pathogen Modeling Program in the United States. In [75] FMM was used to predict the growth of pathogens with changing pH and salt content in the product, in particular, the effect of lowering the pH of the paste. [98] took the next step in this direction and described modeling to predict the effect of processing on the growth of microorganisms in the production, storage and marketing of food products. The process models described in this work were based on mass and energy balances along with simple dynamics of the growth and death of microorganisms. The resulting models were evaluated on technological lines for the production of meat products and hamburgers. Such models can predict the effect of each individual stage of the process on the level of microorganisms in the product.

In [99-101], the influence of temperature, time and changes in processing temperature on the growth of Lactobacillus plant arum was simulated. Such prognostic models can be used to determine the conditions necessary to control the growth of microorganisms, or to calculate the lag phase during production and marketing. where there may be temperature fluctuations allowing their growth. [49] described similar behavioral patterns of bacterial populations during processing, taking into account time and temperature, but taking into account inactivation at temperatures higher than the maximum temperature rise.

[1] proposed the development of expert systems based on prognostic models to assess the microbiological safety of chilled products. Such systems can be used to interpret microbiological data, as well as to predict the microbiological safety of products, taking into account data on their processing, formulation and use. However, if approached realistically, such models cannot be better than the initial data, and currently the available data are inaccurate and variable. [12] also discusses the practical application of microbial growth models to determine the shelf life of chilled products, notes the usefulness of models that accelerate product development, and the importance of testing models on real products. Simulation technology can provide benefits in time and cost, but is still in its infancy [77]. Its applicability is limited, because not only the types of microorganisms present in raw materials and products, but also their activity and interactions that change growth or survival, as well as the production of metabolites that are perceived by consumers as product spoilage, are changing.

Various manufacturers in the development of food products provide, not surprisingly, very different degrees of violation of the time and temperature that their products must withstand, that is, manufacturers differ significantly in what risks they are willing to take relative to consumers. This can lead to significant differences in the applied technological processes, ingredients and packaging, as well as in the shelf life specified for seemingly similar products.


Below we give some definitions - firstly, to avoid misunderstanding, and secondly, to give general comments and recommendations on the development of processes that properly control microbiological risks. These definitions are divided into groups:

  • raw materials;
  • chilled foods;
  • safety and quality control;
  • technological process.


Neobezzarazhennye materials

This group includes any food components of the finished product that have not been disinfected enough to be virtually free of harmful bacteria, reducing the microbiological safety or shelf life of the finished product. Such starting materials should be handled at the factory so that the number of microorganisms does not increase and they cannot infect any other components that have already been disinfected. For example, the location of production areas to prevent mutual contamination of product components should be based on the principle of its unidirectional movement; personnel working with raw materials should not work with the finished product (unless proper hygiene measures and separation of zones are followed) and should not be allowed into high-purity zones (see below). If it is assumed that these materials may contain pathogens, the degree of risk should be assessed. To prevent cross contamination or the production of products that may be inadvertently made hazardous to consumers, the transportation, handling and use of these materials should be adequately controlled (see below for more details).

decontaminated materials

These materials are processed to reduce the number of microorganisms - usually by heating. If such materials are intended for direct introduction into ready-to-eat foods, then the heat treatment used in their preparation should be sufficient to ensure product safety (i.e., the predicted absence of pathogenic microorganisms) depending on whether the product has a short or long shelf life (see the section entitled “Developing Safe Technology” below). Appropriate precautions should also be taken to prevent re-contamination of materials after processing and when working with them in production, in connection with which the primary packaging should be removed from disinfected materials only in areas of high purity.

chilled food

This large group covers all products for which refrigerated storage (originally defined as storage at temperatures from -1 to + 8 ° C [5]) is used as part of their preservation system. Thus, this group may include products consisting entirely of raw or not cooked ingredients. Some of these foods, in order to make them edible, may require processing before consumption (for example, raw fish and meat products). It is generally accepted that pathogenic microorganisms can inevitably be found in such products from time to time.

Chilled cooked or ready-to-eat products

These chilled products may contain raw or unprepared ingredients (risk classes 1 and 2, see below and table 11.1) - such as salad or cheese. However, the preparation of these products by the manufacturer is such that the product is either clearly ready for use, or requires only re-heating, and not complete preparation before use. The manufacturer must do everything possible to ensure that such products do not contain dangerous pathogenic microorganisms or their dangerous levels at the end of the shelf life, and the ingredients should be delivered with this in mind. The layout of the technological lines used in the production of such products is shown in Fig. 11.1 and 11.2.Typical production of chilled products made only from raw ingredients (class 1)

Fig. 11.1. Typical production of chilled products made only from raw ingredients (class 1)Typical production of chilled products made of processed and raw components (class 2)

Fig. 11.2. Typical production of chilled products made of processed and raw components (class 2)

Cooked foods, ready to eat

Such products (risk classes 3 and 4, see below and table. 11.1) consist entirely of prepared ingredients and therefore infectious pathogens can be destroyed in them during processing. This is achieved by the appropriate organization of the cooking operation, and the processing procedures after it, including cooling, should be designed so as to prevent re-infection of the product or its components (including primary packaging materials). Often the appearance of such products clearly tells the consumer that before use they do not require heating at all or require little heating. Heating requirements should be clarified with clear instructions. A typical layout of technological lines is shown in Fig. 11.3 and 11.4.A typical scheme of production pre-cooked

Fig. 11.3. Typical production scheme for pre-cooked and chilled food from cooked ingredients (class 3)Typical production of chilled foods cooked in the package in front of their sales (Class 4)

Fig. 11.4. Typical production of chilled foods cooked in the package in front of their sales (Class 4)

11.1 Table. Classes risk for chilled products


risk *








The minimum required heat treatment



production **

software 43 zpch
1 2 3 4 5 6 7 8
1 1 Week



Tall At the consumer (minimum 70 ° С, 2 min) + capacitor positive (+) lead
1 2 3 4 5 6 7 8

1 – 2

of the week



Low Pasteurization at the manufacturer (minimum 70 ° С, 2 min) + + • +



Infectious pathogens and spore-forming bacteria Also Pasteurization at the manufacturer (minimum 90 ° С, 10 min) + +





Also Pasteurization at the manufacturer (minimum 90 ° С, 10 min) + +


* 1 class: crude products stable under cooling (for example, meat, fish, etc...); class 2: products made of a mixture of cooked and raw foods with a low


3 class: products subjected to varying heat treatment, assembled or packaged primarily in areas of high purity; class 4: foods cooked in the package.

** PZ - production area; 43 - clean zone; ЗПЧ - high purity zone.

Chilled Pasteurized long-term storage products (REPFEDS)

For a wide range of products pasteurized in packaging, a more informative name was proposed in [69,72]: chilled, pasteurized, shelf-stable products (REPFED, REfrigerated Pasteurized Foods of Extended Durability). These include products that are prepared in packaging, and other products to which a particular combination of preservation and pasteurization is applied, which ensures long shelf life in a refrigerated state.

These products are processed to destroy the spoilage bacteria and pathogens that can grow at low temperatures. In this way, very long storage periods (about 42 days) can be achieved. Thus, processing, transportation and packaging should, in particular, ensure the removal of infectious and spore-forming pathogens capable of growth under cooling conditions. Realistic safety limits and risks associated with these products and due to the most dangerous non-proteolytic Clostridium botulinum are not yet known [76]. Factors determining the effectiveness of complex combined canning systems based on poor heating and cold storage are still not entirely clear.

Safety and Quality Control

Proper organization of production

The correct organization of production is understood as the permissible limits and basic principles, procedures and tools necessary to develop an environment suitable for the manufacture of food products of acceptable quality. The proper sanitary and hygienic organization of production provides for basic sanitary and hygienic requirements that production must meet and which are a prerequisite for the implementation of other methods, in particular, HACCP.

The rules for the proper organization of production and the hygiene requirements included in them serve as important boundary conditions for hygienic food production. National governments (see [6, 7]), the Codex Alimentarius Committee on Food Hygiene, the United Nations Food and Agriculture Organization (FAO), the World Health Organization (WHO) and the food industry, often operating together with the food inspection, control authorities and other organizations, developed the relevant requirements [54]. Usually they relate to the following issues:

  •  design and construction of premises for food production, taking into account sanitary and hygienic requirements;
  •  design and proper use of the equipment, taking into account health and hygiene requirements;
  •  procedures for cleaning and disinfection (including pest control);
  •  general hygiene and safety measures in food production, including:
  •  account the microbiological quality of raw materials;
  •  hygiene performance of each activity;
  •  personnel hygiene and training in hygienic and safe food production methods.


Hazard Analysis at Critical Control Points (HACCP) is a food safety management system that uses a method for identifying risks and monitoring critical points in food processing. It is a system or methodology that can be used to ensure safety at any type and scale of food production, and is an important element in the overall quality and food safety management system.

The widespread adoption of the HACCP methodology facilitated the shift of emphasis from control and testing of finished products to preventive control of hazardous factors at all stages of their production, especially at critical control points (SSR). As a quality management method, this method is ideally suited for the production of chilled products, in which many process parameters affect safety and shelf life, with a limited shelf life, and any delay in obtaining microbiological test results reduces the remaining shelf life.

HACCP includes:

  • identification of real (microbiological) risks - such as the presence of pathogenic microorganisms and the conditions leading to their appearance, growth or survival (HACCP is also used to control harmful physico-chemical factors);
  • determination of specific requirements for the control of harmful factors and determination of the stages of the technological process in which control is carried out;
  • procedures and equipment for measuring and documenting control effectiveness;
  • Documentation of the limits and the necessary measures when they are exceeded.

For sites that are not considered critical control points, the use of the right production methods provides assurance of the use of appropriate controls and standards. Risk identification and analysis according to the HACCP method provides information for interpreting requirements and training personnel, calibrating equipment, etc. for specific products or processes. The Committee on Microbiology and Food Safety of the National Association of Food Processing Enterprises [71] reviewed HACCP systems for chilled products manufactured at enterprises and sent chilled to retail. In the study, a chicken salad was used as a model for determining critical control points and developing practical recommendations for planning HACCP, developing a production scheme, identifying risks, establishing critical limits, monitoring requirements and procedures for checking the effectiveness of the HACCP system. There are also recommendations from the US Department of Agriculture (USD A) and general HACCP schemes for chilled, packaged, and cooked and then packaged products [84].

risk Analysis

Ensuring microbiological safety and usefulness of food products requires the identification of real risks and means of their control, that is, risk assessment. The ability of the food manufacturer to assess the impact of changes in the process, the actual effect of the product and the market on the level and type of risk, are important to ensure consistent maintenance of food safety. The impact of changes on risk and harmful factors should be determined; These changes may include the development of new products and processes, the use of other sources of raw materials, or the re-targeting of the product to new consumer groups (for example, children). Food manufacturers have always evaluated these risks using empirical or experimental methods. When a causal relationship was established between foodborne diseases and the presence or activity of microorganisms causing food poisoning, the control of certain microbiological risks gradually became a means of ensuring food safety. Such practical methods have thus far been formed into a formal system with clearly defined procedures, called the “microbiological risk assessment”. The description of this system is given in the preliminary report [6] or in the diagram (Fig. 1 in the Codex Alimentarius, 1996).

The ultimate goal of risk analysis is to reduce them using the following measures:

  • determination of real microbiological risks and their description in accordance with the degree of danger;
  • determination of the impact on the level of risk of the degree of infection of raw materials, the type of processing and use of the product;
  • clear and consistent information to bring to the level of risk people are prepared on the basis of research results.

The combination of risk assessment and communication about it (distribution of risk information and decisions taken to deal with it) used to implement effective risk management (actions aimed at eliminating or minimizing risk) forms a risk analysis [3].

Risk Assessment Phase

The beginning of a risk assessment is a clear statement of the problem. The reason for the risk assessment may be changes in the technological process or in the ingredients, the emergence of a new pathogen or a change in the attitude of society, and this may lead to a revision of the control system, layout of production, sources of raw materials or revision of instructions for preparing the product.

The first stage of the assessment is to identify risks, that is, identify them - for example, concern about the presence of salmonella in the product, since the consumption of products containing infectious cells can cause salmonella. The probability of harm depends on many parameters specific to a certain harmful factor (virulence, scope and concentration) and its presence in the raw material.

An impact assessment describes the likely impact of a harmful factor on consumers and is based on the size of the portion consumed, the effects of previous processing at the factory, etc., on the amount of infectious agent (e.g., Salmonellae) present during consumption. For a cooked product, this effect will depend on the number of Salmonella bacteria entering the heat treatment, the characteristics of the product when heated, and the heat treatment at the factory or at home. The combination of these factors determines the number of pathogenic microorganisms that persist at the time of consumption. If the heat sensitivity of Salmonella and the heat treatment of the product are known, the likely number of microorganisms that will undergo the treatment can be estimated. For many chilled products (such as hamburgers or quickly fried poultry products), microbiological safety is not necessarily guaranteed by the process, but can be achieved with the participation of the consumer [74]. This makes the consumer part of the process of ensuring the safety of the finished product, and the assessment of its actions is a necessary part of the overall risk assessment.

Quantifying the risks of infection after consuming a product is called “determining hazard parameters." It relates the sensitivity of consumers to infection (usually based on expert judgment or knowledge of the dependence of the reaction on the dose consumed among individual population groups) and the concentration of the agent in the consumed portion of the product. The result of these three stages is a “definition of risk parameters” that describes the risks of infection (salmonella) for a particular consumer associated with the consumption of a particular product manufactured and delivered under certain conditions.

To facilitate the transfer of information about risk decisions and the rationale for these decisions, information should be available to administrations, consumers and staff. Where information on risk decisions or conclusions is disseminated effectively, risk management methods can be easily implemented, agreed standards applied and dangerous changes prevented. The introduction of effective means of exchange of information and coordination on a reliable scientific, but at the same time practical basis is still an unresolved problem. Risk assessment is considered in [53], and regarding specific problems - listeriosis, the role of indicators and communication with HACCP - in [67, 78 and 27], respectively.

The principle of prevention

Typically, actions taken to protect the health of consumers have a solid scientific basis, but sometimes decisions have to be made if there is incomplete information (for example, if the prevalence or effectiveness of a new pathogen is unknown). Any decision made on the basis of the “prevention principle” should control health risks without resorting to unnecessarily strict control measures and be consistent with the seriousness of the food safety problem. An example is the measures taken to control the presence of E. coli 0157: H7 in vegetables (pasteurization) or in a lettuce crop (during disinfection and washing), as well as the proper methods of organizing agricultural work [27] for cases when the prevalence of the pathogen is unknown, and the disease caused is serious. The measures taken should be clear and justified, and any assumptions and ambiguities should be clearly described. The main thing is that the achieved risk reduction should be acceptable to all parties involved. Decisions made in this way should be considered temporary until additional information is obtained that will provide a more reliable risk assessment and take appropriate control measures.

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