Chilled and frozen foods

The development of sound technology, equipment and technology

The production of chilled products is a complex process. From the point of view of microbiology, technological processes should be designed so that it is possible to control the presence, growth and activity of certain types of microorganisms. Some operations that make up a typical technological process allow you to destroy bacteria or reduce their number, while others allow re-infection or the growth of microorganisms. Product development, its planned shelf life (see table 11.2), as well as, apparently, sanitary and hygienic conditions at the enterprise, are determined by the bacteria, the appearance of which must be monitored at each stage of the process. At the very beginning of the supply chain, agricultural and livestock products can be sources of bacteria that cause food poisoning (for example, Salmonella, Campylobacter and E. coli 0157). Therefore, it is important to take this into account when working with raw materials, and when developing technology, provide reliable means of destroying microorganisms or preventing food contamination. The scope of precautions necessary to ensure effective prevention of certain risks is proportional to the length and complexity of the supply chain.

For finished chilled products with a short shelf life (less than 10 - 14 days), the main risk is the presence of infectious pathogens, and the technology should be designed to predictably reduce their number. If these types of bacteria undergo heat treatment or re-infection occurs after a technological operation designed to destroy them, they can be a danger to the health of consumers. There are two main ways of re-infection: through personnel working with products, or cross-contamination from other products. When designing technological routes and operations, it is necessary to proceed from the assumption that raw foods always contain a small amount of bacteria that cause food poisoning, and therefore it is necessary to effectively separate the flows of raw materials. Risks increase even more if the product is ready for use, and the shelf life and temperature of its storage make possible the growth of microorganisms.

Extending the shelf life of the product in a refrigerated form carries an additional danger arising from the growth of toxicogenic bacteria, and therefore, special technological processes must be developed for their destruction. The reason is that under normal cooling conditions (i.e. below 10 ° C) in about two weeks the number of microorganisms of the Clostridium botulinum strain growing in cold conditions can grow to levels at which the formation of toxin is possible. The spores of this microorganism can remain viable in foods pasteurized using mild regimes designed to kill infectious pathogens. In the production of products with a long shelf life, it is necessary to use more stringent heat treatment, which should cause a predicted decrease in the number of heat-sensitive spores. If a product preservation system can prevent the germination of these spores, it is necessary to develop a heating process to deal only with microorganisms that cause spoilage. Many chilled products have effective internal preservation systems and are therefore safe despite very poor heat treatment.

Currently, manufacturers and regulatory authorities have not reached a consensus on the seriousness of the danger of botulism due to the use of chilled non-canned foods. There is sufficient evidence that in model systems based on prepared dinners contaminated with spores, toxin [72] occurs and forms under the conditions used in real conditions.

If the product was not heated in its primary packaging, to prevent re-infection by spores and infectious pathogens, unpacked components intended for long-term storage products and which have undergone heat treatment must be cooled, processed and collected in an area of ​​high purity. Even if the risks of reinfestation are controlled, there remains the risk of the viability of heat-resistant spores of bacteria that can grow in individual components of the product at refrigerated storage temperatures. These are mainly varieties of sticks (Bacillus), which can ultimately cause mold damage.


Many of the most important quality properties of chilled products and their safety are determined by the technical characteristics of the technological installation and equipment. The degree of influence of heat treatment on the lethality of microorganisms determines its influence on the probability of survival of microorganisms and the level of their presence in the product, and therefore, proper equipment design and reliable heating are very important. The periods of delay and the growth rate of any harmful microorganisms depend on a number of technological parameters (including the cooling rate and the accuracy with which the storage temperature is maintained). Critical factors also include the uniformity with which preservatives (preservative salt and acidifiers) are metered and mixed, as well as the effectiveness of packaging machines in the manufacture of gas-tight packages (e.g. containing C0 inhibitor gas mixture)2 and N2).

The main cause of infection by microorganisms is food products that remain in the installation or in the production area for some time after cleaning, and if these residues are not regularly removed, they pose a risk to the finished products, which is much less associated with infection from the air or from the staff. Many rules and regulations indicate that "food processing equipment must be designed so that it can be cleaned or disinfected." However, behind these instructions are the real issues of design, operation and maintenance of technological equipment for the production of food products, which often boil down to finding an acceptable balance between costs, production efficiency and hygienic design (see chapter 15).

heating processes. The use of heat for disinfection products

heating methods

In the production of chilled products, heating is the most widespread means of inactivating microorganisms and the cause of favorable texture and color changes in products. Different equipment is used for heating, depending on the type of product, some examples of which are given below.

  • Direct heating of mixtures or particles suspended in sauces steam or hot water in open vessels.
  • Individual pieces of meat, fish or vegetables can be cooked in trays, molds or in packaging with air heating. If the products are hermetically sealed, they can be heated in water baths. Open vessels or furnaces can be heated indirectly with a jacket, direct injection, circulation of steam, air or a mixture thereof. Substances in direct contact with the product must be of appropriate quality.
  • Liquids or pumped ingredients can be heated indirectly using jacketed equipment or using heat exchangers. Conventional processing equipment can be used at product temperatures up to 100 ° C; if higher temperatures are used or planned, equipment such as an autoclave should be selected and used.
  • Solids (such as pieces of meat or vegetables) can be heated by contact heating or deep-frying to obtain temperatures higher than 100 ° C on the surface and lower temperatures in the center. These temperatures depend on the initial temperature and the characteristics of heat transfer and thermal conductivity of the product.

Packaged products or ingredients can be heated in autoclaves or other pressure vessels using coolants with temperatures above 100 ° C; often use water baths to heat them. It is important that in order to ensure good heat transfer, air is removed from the packages processed in this way or that the heating processes are developed taking into account the insulating properties of the space above the packaged product and its expansion when heated.

heating control

The heating process must be designed to provide the above defined minimum heat treatment. Since this is very important, critical control parameters and tolerances should be defined in the requirements available to process operators. The training of these operators should enable them to reliably heat, control and record the process. This is most effectively accomplished by controlling time and temperature conditions in a vessel, chamber, or sometimes in the product itself. For each batch of the product, it is necessary to confirm the completion of a given treatment, and therefore, the achievement of a given lethality of microorganisms.

To ensure reliable destruction of microorganisms by means of heat treatment, food and equipment manufacturers should take into account some important points, namely:

 the equipment, its use and maintenance must ensure an accurate and reliable supply of heat to the surface of the product in such a way that the specified heat transfer rate is achieved, and therefore the autoclave must always be loaded in packs arranged in the same way, and the contents of the vessel with the jacket to ensure uniform exposure of the coolant to each the product or packaging must be mixed;

 the rate of penetration of heat into the product must also be known and controlled so that the necessary general temperature-time effects are reliably achieved at the points of weakest and slowest heating; the achievement of such a regime is determined by the formulation of the product (for example, particle size, viscosity and other physical characteristics), package size, shape and thermal conductivity of the packaging material;

 the design of equipment, control systems and the supply of steam, cooling water or air should ensure a constant supply of the same amount of heat.

The suitability of the equipment is not the only factor determining how successfully or reliably the specified product temperatures are achieved. Process control can affect the characteristics of the material being processed. For example, the temperature of the component at the beginning of processing, regardless of whether it is frozen, thawed, or warm, determines the heating rate, and such parameters should be reflected in the technological maps. To obtain safe products, it is necessary that, regardless of the heating method used, it can provide a certain amount of heat to all parts of the product or packaging.

equipment Jobs

Upon delivery, many types of industrial heating equipment (for example, stoves) are not supplied with information about the distribution of heat in them. In addition, heating of the product depends on the type of heat input to the surface of the product, the unit of the product or package, and the penetration of heat into the package. Researching these characteristics is an essential part of developing a process or product. For example, the uniformity of heating in a vessel with a jacket depends on the degree of mixing, which can be performed by an additional device or operator. In a furnace or autoclave, the distribution of heat may depend on the density of the package or the placement of the product, forming or blocking the channels between the units of production, which violates the uniform circulation of the coolant. The temperature measurement in the center of the product and (at a more complex level) the control of the heat flow in the product depends on the type of equipment [32,33], and therefore the user must create a heat distribution scheme for his specific conditions so that the technological processes can be calculated taking into account units product located in the coldest part of the equipment. [86] proposed an optimization process procedure based on the use of a two- or three-component temperature-time sensor for evaluating thermal processes. The proposed procedure for assessing the influence of specific temperature-time characteristics can take into account various z values ​​associated with the destruction of microorganisms and loss of quality.

The more variable and heterogeneous the heat supply to achieve the required minimum, the higher the intensity of the thermal process should be. An essential part of the development of thermal processes is the study of the ranges of heat treatment achieved on this equipment in the predicted range of conditions. The mortality criteria of microorganisms for the development of technological parameters are given below, taking into account the types and number of microorganisms that must be destroyed. It is important that the number of these microorganisms is controlled at the entrance; if at the input it is higher than the technology provided by the developer, subsequently microorganisms that have retained their viability will be detected (possibly by the consumer).


Although heat is effective for killing microorganisms, the effectiveness of the cooling processes [90] and the hygiene status of the cooling equipment [51] must also be carefully determined. Even in equipment designed specifically to achieve high cooling rates, the risk of re-infection of the product remains either from microorganisms endemic to the cooler, or introduced into it, and then distributed by forced air circulation, often associated with rapid cooling. The cooling rate is also important, since it determines the degree of rest of any spores surviving in the product, which affects their readiness to grow during storage of the food product. This degree of rest is especially important for determining shelf life when using heat in combination with the use of preservatives such as salt and nitrites.

Microbiology of heat treatment

The heat treatment of packaged products is usually carried out with hot water, steam, in an autoclave or (less commonly) using microwave or electric heating. The degree of heating is selected so as to meet the intended sales conditions and the shelf life of the product, as well as specific (“target”) microorganisms, taking into account the type of raw materials and product (see below the minimum processing modes offered for products with short or long shelf life). Target microorganisms include pathogenic non-spore forming microorganisms (Salmonella), enteropathogenic (E. coli, Campylobacter, Listeria monocytogenes and Yersinia enterocolitica), as well as spore-forming non-proteolytic strains (Clostridium botulinum type E and some types B and F). It is possible that some strains of Bacillus cereus that can grow slowly at temperatures up to 4 ° C [91] may be dangerous, but epidemiological data on this issue are insufficient. Since raw materials are supplied from all over the world, other pathogens are also possible, and in order to ensure proper heat treatment, it should be analyzed in the HACCP plan.

While heat treatment at 70 ° C for 2 min in the coldest part of the package will reduce the number of L. monocytogenes (the most heat-resistant of the above-mentioned vegetative microorganisms) at least in 106 times, such treatment will not have any effect on the spores of psychrotrophic strains C /, botulinum. Therefore, in the UK, heat treatment with weak heating at 70 ° C for 2 minutes is recommended only for products with a short shelf life or for food products that will be accurately stored at 3 ° C [38]. In the Netherlands, heat treatment at 72 ° C for 2 min is also recommended to ensure the inactivation of L. monocytogenes in more than 108 time [70].

Although prolonged soft heating (as in the heat treatment of packaged products) may sometimes be desirable for organoleptic reasons, it is important to remember that slow heating can cause so-called thermal shock, in which the resistance of vegetative microorganisms to subsequent heating increases [62]. Therefore, for reasons of microbiological safety, the duration of warming up or maintaining in a warm state during processing should be short, because otherwise it is possible to increase the heat resistance of microorganisms.

Although psychotrophic strains of Ci botulinum cannot grow at 3 ° C or lower, the possibility of their slow growth in products with a long shelf life at somewhat higher temperatures requires a more stringent heat treatment. This process should be designed so that substantially (more than in 106 times) reduce the likelihood of survival of the spores, but there is still debate about the minimum required heating. For example, in [72] it was concluded that there is currently insufficient data on the thermal stability of spores of non-proteolytic C /, botulinum to provide their necessary mortality during traditional processing in packaging (see below). It has been found that surviving spores can grow and form a toxin at 8 ° C for a period of about three weeks. Pre-incubation at 3 ° C shortened the subsequent time until the formation of the toxin at 8 ° C. It was concluded that if storage cannot be guaranteed (as is often the case in retail and at home) below 3,3 ° C, then the shelf life should be limited. At the same time, it should be noted that these products have been present on the market for many years and so far there have not been fixed problems associated with their microbiological safety.

The above remarks apply to pasteurized products packaged in vacuum, in which the preserving factors that determine shelf life and safety are only heating, vacuum packaging and maintaining the temperature in the distribution chain. Such products usually have high water activity close to neutral pH and do not contain preservatives. Many other pasteurized vacuum-packed products have additional internal preservative factors for the development, which further increase shelf life and safety [18]. Examples include, for example, foods containing salt and nitrites (ham and other canned meat products), acidified pasteurized meat sausages (sausages) and a wide range of traditional foods with reduced water activity, some of which are stable when chilled, and some ( the so-called SSP (shelf-stable products), products stable at room temperature) are stable even at ambient temperature [59]. The processing, safety and stability requirements of these products differ from those of traditional pasteurized chilled products.

 Pasteurization of products with short shelf life (1 and 2 classes)

Products with a short shelf life are designed to be suitable for 10-14 days. The heat treatment of such products during production should reduce the number of infectious pathogens (Salmonella and Listeria) by at least 106 times, and handling food after heating and packaging should prevent re-contamination. At a neutral pH in antiseptic-free products with high water activity, a combination of temperatures and processing times equivalent to 70 ° C for 2 minutes is more than enough for the indicated reduction in the number of pathogens. Experience has shown that a longer exposure in this temperature range is required to effectively combat some non-spore forming bacteria that cause spoilage (for example, lactic acid). This combination of time and temperature can be used by the consumer to destroy these infectious pathogens in products.

 Pasteurization of products with a long shelf life (3 and 4 classes)

Products with a long shelf life in a refrigerated state are stored long enough so that any psychotrophic spores that survive in them can germinate. In order to ensure the safety of such products and prevent their spoilage during the indicated storage period, it is necessary to use thermal processes to destroy any spores capable of growth. This means that technologies designed to ensure food safety should reduce the number of cold-growing strains of Clostridium botulinum by at least 106 times. It is generally believed that to ensure safety, it is sufficient to use heating at 90 ° C for 10 min or equivalent technology, but this mode is not sufficient to destroy to the same extent the spores of all psychotrophic Bacillus species. In non-canned foods, some species are able to grow and reach levels of spoilage for approximately three weeks at temperatures of 7-10 ° C, which are known to be used in the chilled food distribution system in many countries [14]. These cold-growing spores often have D values ​​at 90 ° C to 11 min [66].

Microwave processing

Cooking or warming it up with microwave heating, especially at home, has become quite common in recent years. At the same time, in response to consumer demand, the variety and sales of products intended for subsequent microwave heating and stored at ambient temperature, in a chilled or frozen form, increased. In addition, it is highly likely that the use of microwaves for the preparation and pasteurization of food will continue to grow. The key issues are the development and preparation of products with predicted microwave absorption. It is known that heating is determined by the dielectric properties, location and thickness of the product in the furnace chamber [92]. A problem in practice is sufficiently accurate dosing on an industrial scale to ensure uniform and predictable heating of the product, taking into account absorption in the microwave range.

There is no fundamental difference in the effect on heating microorganisms using microwaves or other forms of energy, although there is no reliable data on the additional bactericidal non-thermal effects of industrial or domestic microwave equipment. However, the microbiological safety of products heated in domestic microwave ovens is of some concern; Thus, the inconstancy of heating and its effect on the survival of microorganisms are considered in [45, 80, 87]. Of particular concern are cases where products are not fully cooked, may be contaminated after cooking, or may contain raw ingredients. Concern was mainly expressed after the presence of Listeria monocytogenes on the retail market was demonstrated in a wide range of products, including some suitable for microwave heating. For example, a study in the UK showed that this microorganism can be detected in 25 gram samples in 18% of tested chilled products manufactured industrially and commercially available [36]. The resulting concern led the UK Department of Agriculture to conduct a thorough study of this problem involving oven manufacturers, various food industries, retailers, and consumers, and then circulated recommendations regarding the proper use of microwave ovens to achieve effective pasteurization when heating products.

It has been suggested that the unanticipated survival of microorganisms in foods heated by microwave radiation may be due to an increase in the heat resistance of microorganisms (eg, Listeria monocytogenes; see [57]). It is now generally accepted that the survival of microorganisms occurs solely as a result of inhomogeneous heating, leading to unheated places in certain parts of the product [23,61]. This is a consequence of heating using the energy of microwaves and the fact that its absorption (and therefore the heating rate) depends on the composition and quantity of ingredients, the geometry of the product and its packaging to a much greater extent than in the case of using traditional heating means. Measurement of the rates of inactivation of Listeria monocytogenes by heating in various food substrates showed that a ten-fold decrease in the number of microorganisms is achieved by heating at 70 ° C for 0,14-0,27 min (D70 = 0,14-27 min) [35]. Therefore, recommendations that products in which Listeria can develop should receive a minimum of heating with microwaves at 70 ° C for 2 min (as indicated in the UK Department of Health guidelines for finished and chilled products), aim to reduce the number of these microorganisms more than 106 times [8]. If microwave processing is used in production for the preparation of chilled products with a short shelf life, it must reliably ensure the supply of the minimum allowable amount of heat to all parts of the product or its ingredient. This is necessary to ensure the necessary reduction in the number of Listeria and other, less heat-resistant vegetative bacteria that cause food poisoning, that is, treatment at 70 ° C for 2 min or another combination of time and temperature, providing a similar lethality of microorganisms, based on D values ​​at 70 ° C for 0,14-0,27 min and z values ​​from 6 to 7,4 ° С [35].

Products prepared in the original original packaging (sous-vide and REPFED products)

Although the term “sous-vide” (packaging processing), strictly speaking, refers to vacuum packaging without any indication of heat treatment, it is used for pasteurized ingredients or products that are hermetically sealed before heat treatment, often in their original packaging ( risk class 4). Foods processed in this way include prepared dinners and their ingredients, soups and sauces; they all have a long shelf life in chilled form and are intended for use in the field of public catering, and more recently for retail sale. Sous-vide products are usually processed at relatively low temperatures (55 ° C or more). The heat treatment should be sufficient to maintain the safety and microbiological stability of the products at storage temperatures below 3 ° C (the minimum theoretical temperature of growth of Clostridium botulinum species growing in the cold). Depending on the severity of the thermal process and the microflora of the ingredients, the products may have a shelf life of up to about six weeks [22]. The outline of the HACCP methodology for these products is published in [2].

The most comprehensive of the early sous-vide treatment studies was done at Stockholm's Nacka Hospital. The finished products were packaged in vacuum, quickly cooled, and then stored under well-controlled cooling conditions for one or two months before subsequent heating for consumption [60]. The scope of the method then expanded to more or less centralized areas of public catering in a number of European countries, and recently (mainly in France) it has expanded to retail products.

Concern over possible microbiological safety problems is not due to doubts about the principles underlying the sous-vide processing process, but because of the difficulty in ensuring reliable maintenance of the required low temperature (max. 3 ° С) during transportation over long distances and especially at home (in [13, 55, 76] discusses the risks of botulism, and [10,44] discusses the effectiveness of sous-vide processes with respect to Listeria monocytogenes). [88] examined the effectiveness of this process with respect to Bacillus cereus and bacteria causing damage to chicken breasts, and some norms and rules were proposed in [11].

Other treatment options

Technological operations for the production of products processed in packaging {sous-vide and REPFED) basically follow the original concept and the traditional procedure of canning in cans, food packaging, sealing and subsequent heating. Another option is heating followed by filling and sealing - if these operations are not performed truly aseptically, there is a risk of introducing microorganisms after heating and before sealing, and therefore it is mainly used for products with a short shelf life. If the products are hot (> 80 ° C) when filling the container, an increase in shelf life at 3 ° C can be achieved. For such processing, special designs of packaging, filling and sealing equipment are used.

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