Chilled and frozen foods

Processes. The heat treatment (in particular, cooking)

Heat treatment refers to a process in which enough heat is supplied or supplied so that all parts of the food product achieve the desired organoleptic quality, and a significant reduction in the amount of any infectious pathogens that may be present in the product is expected.

It is generally assumed that when kept at 70 ° C for two minutes, the number of infectious pathogens should decrease by 106 times (see below, “Developing a safe technology”). At some stages of heat treatment, much more heat can be supplied (for example, when long boiling periods are used). If the heat treatment stage is used as a pasteurisation stage, subsequent re-contamination must be prevented. It is important that the requirements for heat treatment make a distinction between the conditions inside the product and the conditions of the technological process necessary to ensure it. For example, for a given heat treatment process conditions will vary depending on the diffusion of heat in the product, its size and heat supply to the product surface or through it.


Pasteurization is a process operation using heat that serves to predict and reproducibly reduce the number of certain types of microorganisms in food products or their ingredients. Requirements for the safety and shelf life of the product should determine the minimum degree of any stage of pasteurization, and the technologies used do not necessarily heat the ingredients to such an extent as to achieve the required organoleptic quality or destroy all microorganisms present. The effectiveness of a particular type of heat treatment can be determined by various factors, including the canning system used in a particular product. For example, the heat resistance of bacteria and their spores usually increase with low water activity, but decreases with low pH. To make recommendations for the development of the process and the selection of control points based on published data on the heat resistance and growth of microorganisms, [34] provides an overview of the types of pasteurization and their microbiological objects with an example of a pre-cooked and cooled product. Minimal pasteurization should be directed at food pathogens, but in practice most types of pasteurization are tougher and are aimed at more heat-resistant bacteria that cause spoilage of the product (see [34] and [18]).

pasteurizing effect

Pasteurization units indicate the efficiency of pasteurization heat treatment. They are used to determine the equivalent heat treatment corresponding to a given heating time at a given initial temperature.


The z-number is an empirical value that has the dimension of temperature (in ° C or T), which is used to calculate the increase or decrease in temperature necessary to change the rate of inactivation of a particular microorganism 10 times. It is assumed that the dynamics of the death of microorganisms at a constant temperature is exponential (that is, log-linear). Although the number r is the basis for calculating the equivalence of the sterilization process, it should be used very carefully for pasteurization processes, since the death dynamics of many types of microorganisms is not described by a logarithmic-linear relationship, especially when it comes to vegetative forms of microorganisms, and the heating rate is low. For example, adaptation to heat may occur, which may even increase the resistance of the microorganism to heat [68]. Speakers and flat areas on the “survival curves” are very common [39], so at low temperatures (used in pasteurization) the validity of the concept of 2 numbers is very limited in practice. If there are other factors (such as the relationship of preservatives to heat treatment, or if the technology is designed to to cause significant (more than ten thousand times) logarithmic decreases), the flat parts of the survival curves may become especially important. To create guaranteed safe processes, real or “provocative” tests must be undertaken.


The consumer usually performs reheating, which the manufacturer provides only to ensure optimum culinary quality of the product.

Depending on the product, this process may or may not provide the heating necessary to achieve product safety; This is especially true if the products are intended to be reheated in a microwave. The reheating process must therefore be recommended to the consumer if, during pre-treatment, the removal of hazardous contaminants was carried out efficiently and the product was kept clean during subsequent processing, packaging, marketing and storage operations. The physicochemical characteristics of the finished products, including salt content, tray type, geometry, and component layout, affect the uniformity and rate of heating in a microwave oven. In [79], the cooled finished products of the four components were heated in a home microwave oven; It was found that heating uniformity depends mainly on the placement and geometry of the components, as well as the type of tray. If microwave heating is used to prepare food, its effectiveness must be confirmed; for this, a method based on the use of alginate beads containing microorganisms with known heat resistance [46] can be used.


Cooling reduces the temperature of the product after it is cooked in a factory. Its purpose is to minimize the time the product is in the temperature range that allows the growth of dangerous microorganisms, that is, between 55 and 10 ° C. Cooling rates are specified in regulatory enactments - for example, in accordance with the EU Directive on meat and meat products 77 / 99, finished meat should be cooled to a temperature below 10 ° C during 2 h after cooking. The work of [29] indicates the importance of cooling and compliance with the requirements for cooked and chilled products adopted in the UK [5]. This guide recommends cooling eight-centimeter trays to a temperature below 10 ° C in 2,5 hours and 1- and 4-centimeters below 3 ° C in 1,5 hours. Considering the use of a simple one-step operation and the need to avoid surface freezing, within these time limits only trays centimeter depth are cooled.

Cooling of liquids or suspensions can be performed in the stream using heat exchangers. If cooling solids and suspensions is performed in tanks, the size of the tank or the amount of the product should not impede rapid cooling. Do not use the product depth greater than 10-15 cm, since with greater thickness the cooling rate of the entire mass of the product will limit the supply of heat to the surface, and not the removal of heat from it, and therefore the possibility of microbial growth may occur.

When a warm or hot substance is loaded into air-cooled containers, the material from which they are made has a significant effect on the cooling rate, with thick-walled plastic containers being cooled much more slowly than metal ones.

The cooling rate is determined by the design of the cooler, especially its distribution system, speed and air temperature, as well as how it is located in the container with the product. In order to maximize the cooling rate, the filling or packaging systems must allow the flow of cold air over the surface of the container. Special attention should be paid to hygiene and the prevention of condensation in coolers, since if air flows over open tanks move condensate in the form of an aerosol, this is the main potential source of re-infection of Listeria. The importance of cooling after cooking, refrigerated storage and distribution are considered in [9] as critical control points in the production of raw and finished chilled products. In [30], American rules and regulations are given with recommendations for preparing safe chilled foods; they cover refrigeration, refrigerated storage, pre-sale storage, product handling methods and temperature control.

Cold storage

Refrigeration should be stored in such a way as to maintain the existing or desired product temperature. Containers with products or ingredients must have a predetermined temperature when entering the storage, since the operation of the air system (temperature and air velocity) and the storage system of the product usually do not allow for a significant reduction in temperature.

Production zone

The production area (P3) is part of an enterprise where all types of ingredients are processed. Semi-finished products obtained in this zone are subjected to heat treatment before being sold as finished products and pass through a clean zone or a zone of high purity.

clean area

Clean Zone (CH3) is a technological zone intended for processing raw materials with a low degree of risk of contamination and products containing mixtures of ingredients, both cooked and non-disinfected (class 1). This zone should be designed and constructed in such a way that it can be easily cleaned and disinfected, achieving a high level of hygiene and, most importantly, preventing the spread of bacteria infecting products (for example, Listeria). If such a zone is used to build final products with non-disinfected ingredients (eg, cheese), it should not be used to process or prepare any “ingredients that pathogenic microorganisms may contain, and therefore may increase the risks of getting products with infectious pathogens. Zones,

so appropriate hygiene requirements, must be used to perform operations with the already processed foods, pasteurized in the package (4 class).

Zones of high purity

The Over Frequency Zone (HRA) is a well-defined, physically separated part of an enterprise, designed and working specifically to prevent reinfection of prepared ingredients and products after the heat treatment process is completed, during cooling, packaging (assembly) and primary packaging. This is a necessary part of the enterprise, shown in Fig. 11.3, used for cooking products of classes 2 and 3. Typically, there are specific hygiene requirements regarding the location, standards for construction and equipment, training and hygiene for operators, engineers and management personnel, as well as certain sets of technological operations (especially regarding the reception and dispensing of food components and packaging materials). All these norms are aimed at limiting the likelihood of infection. The HRA should strive to eliminate the use of recycled packaging and recycled materials, and if this is not possible, strict time division measures should be implemented.

air Treatment

Air is an important carrier of pollution, and therefore special attention should be paid to the direction of air flow in the production area and between zones. In the production areas, the air must move from clean materials to dirty in such a way as to minimize the likelihood of pollution transfer from raw products to disinfected ones. Air quality must comply with the requirements for the hygienic category of the zone (see the sections “Production zone”, “Clean zone” and “Zone of high purity”) [19].

Washing and cleaning

Washing and cleaning should remove food residues from process equipment, from production areas and warehouses. Effective cleaning should completely remove food debris from work surfaces, machines or areas so that microorganisms cannot grow and the product is not contaminated. Effective cleaning is only possible if the equipment is designed to meet hygienic requirements. In practice, complete removal of residues is rarely achieved using the methods used to clean an open installation (for example, a cutting machine and dispenser). At enterprises producing chilled products, residues from cleaning can serve as growth medium for microflora, and experience has shown that many modern methods and chemical means for washing and cleaning when used in cooled areas can actually be used to remove Listeria. During cleaning at high pressure and low consumption, when it is uncontrolled, aerosols are formed that can contaminate the products and equipment with food residues and microorganisms. To minimize the risk of contamination, food and packaging materials must be removed from the areas to be cleaned during cleaning. The HACCP system can make a valuable contribution to this area, identifying those stages of the process in which hygienic factors are critical to ensuring the quality and safety of the product, as well as checking on the basis of the technological process scheme the availability of good access for cleaning with regard to the layout of the enterprise.


Disinfection procedures should destroy any microorganisms remaining on the surfaces to be cleaned and be used at those technological stages where possible repeated contamination of the product is one of the safety factors. In practice, these procedures must destroy or suppress microorganisms remaining in food residues, which are invariably present after cleaning, which in itself allows a satisfactory level of hygiene to be achieved in clean areas, but in order to gain additional confidence that there are no viable bacteria, high-purity zones require additional disinfection. Heating and some chemicals are used as disinfectants. The effectiveness of disinfection will be reduced if food residues impede the access of disinfectants to microorganisms, and therefore thorough cleaning is always required to ensure effective disinfection. Providing access to disinfectants along with the development of cleaning schedules is the main goal of designing equipment with hygiene requirements. For visual or microbiological verification of the effectiveness of cleaning is the systematic monitoring of certain areas of equipment or industrial premises. The efficacy of disinfection can also be tested with tampons or chemicals.

Microbiological risks

For refrigerated products, microbiological risks can be roughly divided depending on whether harmful microorganisms can infect a consumer or multiply in products and produce toxins that can cause diseases soon after eating foods. Microorganisms representing the greatest danger are listed in Table. 11.2. Products and processes must be designed to withstand all the real microbiological risks. To determine whether a particular risk (factor) is real for a given product, the real, and * not specified storage temperatures and shelf life during storage should be assessed.

Infectious pathogens in very small quantities may already be dangerous, while toxigenic microorganisms are dangerous only if they are present or reproduce in significant quantities. When developing a product or technology, it is very risky to assume that certain microbiological risks will be absent (for example, on the grounds that they are not found in separate components). Technological processes must be specifically designed to prevent all real risks.

11.2 Table. The most dangerous microorganisms that cause food poisoning, their thermal resistance and temperature rise

Minimum Heat resistance
temperature Low Medium High
growth plant cells Споры

Listeria monocytogenes (inf.)

Yersinia enterocolitica (inf.) Vibrio parahaemolyticus (inf.)

Clostridium botulinum type E, non-proteolytic B & F (current)

Bacillus cereus (current).

Average Aeromonas hydrophilia (inf.) Species of Salmonella (inf.) Bacillus subtillis (current.) Bacillus licheniformis (current.) Clostridiumperfringens (inf).

Escherichia coli 0157 (inf.)

Staphylococcus aureus (current) Camplylobacterjejuni & coli (inf.)

Clostridium botulinum type A & proteolytic B (current)

Note ', inf. - infectious; current. - toxicogenic.

Infectious pathogens (see chapter 8) include, in particular, Salmonella, E. coli 0157: 7 / 7 and Listeria monocytogenes. They may be present in raw materials (meat, vegetables, and cheese made from unpasteurized milk). If they are not destroyed during processing, they can maintain viability for long periods of time in cooled products (for example, in a crisp salad of E. coli 0157: 7 / 7 retains the viability of 22 days at 8 ° С). All infectious pathogens are heat sensitive and are destroyed under the conditions that are created during pasteurization (for example, 70 ° C for 2 min or 72 ° C for 16,2 s). The growth of Salmonella and E. coli 0157: 7 / 7 in food or in the environment of an enterprise can be stopped by cooling (that is, at a temperature below about 10 ° C). E. coli 0157: 7 / 7 has a low infectious dose and causes serious diseases (especially in children and the elderly), as it attaches to the walls of the intestinal tract and causes acute hemorrhagic diarrhea (hemocolitis) or hemolytic uremic syndrome (kidney disease).

Several outbreaks of disease were associated with chilled foods, and they usually arose from beef (for example, products from insufficiently cooked ground beef), although they could have been caused by unpasteurized fruit drinks and mayonnaise. In the latter case, it is believed that the cause is improper handling of draft mayonnaise or cross-infection with meat sauces or meat products. It was found that E. coli is more resistant to the action of acidic environments than other known strains, and therefore it can remain viable in fermented dry sausage and yogurt. Salmonella enteritidis poses a potential hazard in products made from poultry and eggs, while Salmonella typhimurium DT 104 is resistant to many drugs and is found in many products, with outbreaks of diseases caused by it in the UK associated with poultry, meat, meat products and unpasteurized milk.

Campilobacter can cause intestinal infections, leading to fever, diarrhea and sometimes vomiting. Their sources can be water, milk or meat.

S. jejuni is regularly found in the retail on raw poultry, with outbreaks associated with undercooked birds and cross-contamination of ready-to-eat materials through the hands of kitchen personnel or through production areas. This microorganism does not grow at temperatures below 30 ° C, and therefore the conditions affecting its viability are important, since a sufficient number of cells must be maintained for the formation of an infectious dose. The survival rate of microorganisms in chilled products is better than at ambient temperature and freezing. Vibrio cholerae can remain viable on chilled, raw or boiled vegetables and grains, if they are obtained from tropical or warm areas where the infection is endemic. Of particular danger are seafood and other types of products collected in estuarine or coastal waters, waters exposed to effluent, or in fields irrigated with polluted wastewater. Contamination can also occur if the product itself is cooled, washed or refreshed with polluted water. During the preparation of products of such origin (raw, pre-cooked or processed shellfish, crustaceans, fish and vegetables), all measures should be taken to minimize the likelihood of cross-contamination and pasteurized before sale.

Psychrotrophic pathogens, such as Listeria monocytogenes [95], can grow at cooling temperatures and settle on poorly designed or maintained equipment and in the environment of enterprises. They are found in small quantities in environmental samples in the production of raw food [31] and therefore are likely to be contaminated due to production conditions.

It was shown that under optimal (with the exception of temperature) conditions, some L. monocytogenes strains are capable of slow growth at temperatures down to -0,1 ° C; Yersinia enterocolitica - at -0,9 ° C and Aeromonashydrophila - at -0,1 ° C [96]. For example, I. monocytogenes can grow well in components such as stored ready-to-eat vegetables and many products that lack strong chemical preservation systems (for example, stored ready-made lunches and pies stored chilled).

The most important toxigenic pathogens are non-proteolytic strains of Clostridium botulinum growing in the cold. Their growth in pasteurized products is particularly dangerous if, during processing, the competing microflora is destroyed and their growth may precede deterioration. Proteolytic strains are less dangerous because they are able to grow only at higher temperatures, and unlike non-proteolytic strains they usually cause spoilage, which makes the product inedible. At refrigerated storage temperatures, the growth rate of non-proteolytic species is small and therefore requires control only in products in which the expected shelf life in refrigerated form exceeds 10-14 days. [41] suggested that non-proteolytic strains of Clostridium botulinum can grow at cold storage temperatures, creating a potential hazard for refrigerated products subjected to minimal heat treatment. In [41], prognostic models are compared with published data and their suitability for fish, meat and poultry products is shown. These models describe the relationship between pH values ​​(5,0-7,3), salt concentrations (0,1-5,0%) and temperatures (4-30 ° C) and are based on the growth of the non-proteolytic strain of C. botulinum in a laboratory culture medium. Fortunately, these strains, capable of growing at cold storage temperatures, are relatively heat-sensitive, and therefore they can be dealt with using fully realizable heat treatment or pasteurization processes (90 ° С x 10 min) - processes that are recognized as suitable for chilled products with a long life. storage.

Although heat treatment can eliminate microorganisms that grow at low temperatures, storage temperature remains the most important means of restraining the growth of clostridia. In the UK, the Advisory Committee on Microbiological Food Safety considered the potential risks of refrigerated products manufactured using vacuum packaging and related processes (such as heat treatment in packaging), with particular attention paid to the risks associated with botulism. The committee outlined methods for preventing and / or limiting the risks of botulism, including appropriate heating, taking into account the temperature sensitivity of the spores and limiting the shelf life. The use of HACCP to solve this problem is described in detail in [90].

At temperatures above 12 — 15 ° C, mesophilic species (forming more heat-resistant spores) are capable of growing, and the technologies used in the production of chilled products certainly do not inactivate their spores. Bacillus cereus is sometimes referred to as representing a potential hazard in refrigerated foods, although data on its ability to form harmful toxins in such foods (perhaps with the exception of dairy products) are rather uncertain. In the case of refrigerated products, toxins forming golden streptococcus (Staphylococcus aureus) are also of concern, although it is dangerous only when the product does not contain competing microflora and its storage temperature has been significantly impaired. Nevertheless, it is important that the zones of high purity and the operations used in them be organized in such a way as to prevent the infection of the heat-treated S. aureus products.

risk Classes

Consumers can consume chilled, ready-to-eat products without sufficient heat, destroying infectious pathogens in them, and therefore the risk to consumers depends on the number and type of microorganisms in the products after production and their growth during marketing and storage. Therefore, the processing and hygienic principles applied in the production, marketing and trade of finished and chilled stable products must be designed to control the risks associated with the content of infectious or toxicogenic microorganisms. Control of spoilage microorganisms in the development of technological processes must be secondary, although it may often require more stringent heat treatment or more stringent hygiene or preservation measures than safety control. Sometimes the control of microflora is impossible without damage to the organoleptic properties of the food product, and therefore a compromise must be made based on an acceptable balance between controlled loss of quality and spoilage. To improve the organoleptic properties in the development of the technological process and the product should not violate the standards of microbiological safety. If the required processing conditions cannot ensure food safety in the actual practice of their use by the consumer, then the product should not be marketed (see [46, 90]).

Cooled products can be divided into clear risk classes (Table 11.1). Some finished chilled foods (class 1) are made entirely from raw ingredients and clearly require consumers to cook them. Others that contain a mixture of raw and already prepared components that have been processed or packaged to ensure a satisfactory shelf life (class 2) may not so explicitly require heat treatment and may contain infectious pathogens that are capable (for example, L. monocytogenes) or not (for example, Salmonella) to growth during refrigerated storage. By minimizing the levels and incidences of the presence of pathogenic microorganisms in the materials supplied (for example, by careful selection of suppliers), the manufacturer can control the safety of products of only the second category (2 class). To reduce the risk of storage and processing procedures should not introduce additional hazardous microorganisms or allow an increase in the number already present. The time and storage temperature of such products should be set so that when products are stored for the entire planned period, ensure that only safe amounts of infectious pathogens are present. Currently there is no generally accepted estimate of the infectious dose of Listeria, and manufacturers or industry associations should independently determine acceptable risks. In such products, Salmonella should be absent, because its infectious dose is very low. Since Listeria can grow at low temperatures (for example, during storage, sale and at home), only its complete absence from production ensures the safety of ready-to-eat products regardless of the circumstances. If this microorganism is present after production, the manufacturer assumes the risk associated with the sensitivity of consumers of its products to any L. monocytogenes that may be in the product.

Other cooled products may contain only heat-treated or otherwise disinfected components (3 class), or the heat treatment may be performed by the manufacturer in the primary packaging (4 class). When produced under well controlled conditions, such products will be free from infectious pathogens (such as Listeria and Salmonella) and spoilage microorganisms, and therefore they will have significantly longer shelf life (more than 42 days) than products containing raw materials. components (since they will not be susceptible to microbial spoilage). Such a significant increase in shelf life without showing signs of deterioration has important consequences, as this raises the question of what potential microbiological changes should be considered to limit the safety of the product during storage (see below), and therefore what controls (especially pasteurization conditions) are suitable for production.

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