Chilled and frozen foods

Nemikrobiologicheskie factors affecting the quality and safety of food products

 Г. М. Браун и М. Н. Холл, Campden and Chorleywood Food Research Association


As the market for chilled products has expanded and become more competitive, the demands on the variety, quality and shelf life have increased. To meet these requirements with full responsibility, while respecting safety while maintaining profitability, it is necessary to understand and take into account factors affecting the safety of the product and its quality. If we rely on this understanding and knowledge, many problems can be avoided by using a formalized HACCP approach to determining critical control points that affect quality and safety, and realistic prediction of shelf life. Consideration of these issues at the initial stage of product development maximizes the likelihood of obtaining a product that meets consumer expectations and has the necessary competitiveness. Food is probably the most chemically complex substance that most people encounter. In the natural plant food there are more than half a million compounds, in addition, as a result of processing, preparing and storing food products, the number of compounds increases due to the formation of new ones. The appearance, flavor, texture and nutritional value of the food product (quality), as well as its physiological effects (safety), depend on these compounds.

Non-microbiological factors that influence the quality and safety of refrigerated products can be divided into chemical, biochemical, and physico-chemical. Each of them depends on the properties of the product (for example, pH and water activity) and the conditions in which it is stored (for example, temperature and gas environment).

In order to achieve high quality, attention to the choice of raw materials is of paramount importance, as subsequent processing cannot compensate for the poor quality of the raw materials. This is especially true for chilled foods, for which the feeling of “freshness” is one of the most important criteria when buying.

Chemical, biochemical, and physicochemical factors rarely act independently, but this division of factors into groups provides a convenient basis for discussion. These factors are not always harmful, and in some cases they are necessary for the development of the desired properties of the product. Below we present some characteristics of chemical, biochemical and physico-chemical reactions and give examples typical of chilled products.

Characteristics of chemical reactions

Chemical reactions proceed if there are reagents in the appropriate form and if the energy threshold for activation of the reaction is exceeded. The presence of inorganic catalysts reduces the energy threshold for activation and leads to a reaction that would not occur without them. The reaction rate depends on the concentration of the reactants and the temperature. An increase in temperature accelerates the random movement of reagent molecules, which increases the likelihood of their contact. It is usually assumed that for every 10 ° C temperature rise, the reaction rate doubles.

 Important chemical reactions in chilled products

lipid peroxidation

Lipid oxidation is one of the main causes of deterioration in the quality of meat and meat products. Boiled meat and poultry quickly acquire the characteristic oxidized taste / smell, called hot (WOF, Warmed Over Flavor) in [107]. It can best be described as a taste / smell associated with reheated meat, and it is described as such by organoleptic analysis experts with a free description of cooked meat that has been reheated after cold storage [19, 62]. A further description was made for WOF in pork [15] and chicken meat [16], dictionaries of terms for sensory analysis were created, containing 16 and 18 terms, respectively. In cooked meat stored at cold storage temperatures, this stale, oxidized taste becomes noticeable after a short time (within two days), which is different from the slower occurrence of rancid taste during storage frozen (within a week) [80]. Although it is believed that WOF appears only in cooked meat, there is evidence that it appears just as quickly in raw ground meat in air [35,92] and in restructured fresh meat products due to the destruction of tissue membranes and exposure to oxygen [34]. Nevertheless, the significance of the appearance of this taste for food producers has increased with the emergence and expansion of the markets for cooked and chilled products, such as ready-made meals from frozen semi-finished products (for example, in aluminum foil), meals for passengers and express trains. snack bars. In such cases, the consumer expects the taste of a freshly prepared product. The development and successful operation of the fast food industry and cooked and chilled food products will to some extent depend on the ability of manufacturers to overcome the emergence of WOF.

For a long time, the main cause of WOF is lipid oxidation. This is confirmed by studies showing the connection of WOF, determined organoleptically [61], with the results of measurements of the thiobarbituric number (TEA) (lipid oxidation index) [42,44,100] and the identification of volatile compounds released from the free space above the meat samples [3, 20,90]. As in other cases of oxidative rancidity, the process of lipid oxidation leads to the formation of many different compounds, some of which contribute differently to the formation of undesirable taste and odor associated with rancidity. Therefore, there is a discrepancy between the results of measurements of chemical indicators and the organoleptic assessment of rancidity.

The reactivity of dietary lipids depends on the degree of unsaturation of their constituent fatty acids, their availability and the presence of activators or inhibitors. The composition of fat in meat is determined by a number of factors, including the nutrition of the animal and the type of fat. Lipids are mainly found in fat reserves (adipose tissue) or in cell membranes in the form of phospholipids. When heat treated, unsaturated phospholipids, unlike stored triglycerides, become more susceptible to oxidation as a result of the destruction and dehydration of cell membranes. A higher level of fatty acid unsaturation in phospholipids contributes to their more rapid oxidation [43]. The role of phospholipids in the formation of WOF [41] and TEA-reactive substances [83,89] is shown.

It is generally recognized that lipid autooxidation involves a free radical chain reaction (Fig. 9.1), initiated by removing a mobile hydrogen atom from its position in the lipid (RH) to form lipid radicals (R *) (initiation). The reaction with oxygen gives peroxide radicals (ROO *), followed by the separation of another hydrogen atom from the lipid molecule. Hydroperoxide (ROOH) and another free radical (R *), capable of supporting the chain reaction (development), are formed. The decomposition of hydroperoxides includes other mechanisms associated with free radicals, and the formation of non-radical products, including volatile aromatic compounds.

Despite the efforts of researchers, start-up mechanism, leading to the formation of lipid radicals (alkyl or allyl) (R *) in meat, is still unclear.The chain reaction of free radicals

Fig. 9.1. The chain reaction of free radicals

The involvement of iron [73] has been established, but in addition to this, various other mechanisms have also been proposed that have not been confirmed by conclusive evidence [5].

The rate of free radical formation increases in the presence of metal catalysts. In the case of the development of the smell of heat in cooked meat, it has been shown that both free iron ions and hemoproteins, including metmyoglobin in the presence of hydrogen peroxide [5], accelerate oxidation. It is known that the presence of free iron increases as a result of heat treatment [42], since hemoproteins are destroyed and free iron is released. The amount of iron released depends on the heating rate and on the final temperature, and therefore on the heating method. Slow heat releases more free iron than fast - frying or leaning meat produces more of it than heating with ultra high frequency (microwave) [94].

WOF prevention measures were reviewed in [80]. The method used is often limited by the requirements for the final product. Phenolic antioxidants, such as BHTI BHA, do not work well in the case of whole pieces of meat [115], and are more suitable for minced meat products, since a more even distribution of antioxidant can be achieved in them. Overheating or sterilization in an autoclave leads to the formation of compounds in the meat that have antioxidant activity (Maillard reaction products). These compounds may be suitable for canned food, but they often result in the product acquiring characteristics that prevent it from being perceived as fresh, which is necessary for many foods. Such substances can be added to meat, but this is due to the same limitations as in the case of artificial antioxidants. A reduction in WOF is also achieved by using vitamin E (tocopherol). [53] showed that adding alpha-tocopherol to cooked pork reduces lipid oxidation and WOF. Problems with achieving an adequate distribution of antioxidant in meat could be overcome by introducing vitamin E supplements into animal feed. It has been shown that the addition of alpha-tocopherol acetate to the diet of rabbits [60] and meat chickens (broilers) [79] led to an increase in muscle tissue and a decrease in the development of WOF. Studies of the natural antioxidants present in vegetables have shown a certain effect of using extracts of green (unripe) pepper, onions and potato peels [84], herbs and spices, especially rosemary, sage, marjoram [37] and cloves [50]. The efficacy of rosemary essential oil as an oxidant in cooked meat is contradictory, although [75] indicated that rosemary essential oil and sodium tripolyphosphate effectively prevent WOF in roast beef. In pre-cooked rosemary-treated pork meatballs and stored at 4 ° C for 48 h, no oxidized flavors, similar to those in control samples [55], arose, and in beef steaks with rosemary essential oil, stored in a cooled form, there was a significant improvement in compared to control samples did not occur [102].

Nitrite added in the 50-200 ppm range is an effective inhibitor of the development of WOF [18,92]. Nitrite and hemoproteins form complexes of nitrosyl myochrome and nitrosyl hemochrome, in which iron is stabilized by the binding of nitric oxide to the porphyrin ring (Fig. 9.2). However, pink staining of meat may not beNitrozilmioglobin. Myoglobin with nitritnыm ligandom

Fig. 9.2. Nitrozilmioglobin. Myoglobin with nitrite ligand

lectern; The causes of pink in raw meat after heat treatment are discussed later in this chapter. The effectiveness of pyrophosphate, tripolyphosphate and hexametaphosphate, which form chelates with metal ions, especially iron ions that accelerate the oxidation, is shown in [107] for pork. This was then confirmed for shredded beef [92], beef steaks [68] and for chicken in dough and breaded [12]. Phosphates in combination with ascorbic acid can create a synergistic (mutually reinforcing) effect, so that the ground pork that has undergone heat treatment is protected from lipid oxidation to 35 days at 4 ° С [97].

An alternative approach is to protect the meat from oxidation. This can be achieved by creating an oxygen barrier using a sauce or gravy, which can be applied at the time of preparation or during subsequent storage. This principle has been demonstrated by comparing the shelf life of frozen meat with the shelf life of the same cooked meat without gravy coating [25]. Heat-treated and gravy-covered pork can be stored at -18 ° C for more than 100 weeks, and pork stored without gravy was unusable after only 22 weeks.

Packaging in a controlled atmosphere to reduce WOF was applied to pre-cooked turkey, pork and pork products. Although products stored in an atmosphere of nitrogen and carbon dioxide, were less “oxidized” than those stored in normal air, vacuum packaging [51,76] was most effective. [99] explores the potential benefits of using CGS packaging for cooked and chilled, ready-to-eat foods. Protection against oxidation during heat treatment is also helpful. Heat treatment and subsequent storage of chicken breasts in a nitrogen atmosphere reduced TEA values ​​and organoleptic evaluations (skor) for the intensity of WOF compared to a control product cooked in air and stored in nitrogen or air (Fig. 9.3).

Auto-oxidation or oxidative rancidity is not limited to meat and meat products. Dairy products and fatty fish are also highly susceptible. Migration of copper to cream when churning butter can cause a sequence of oxidation reactions that cause a rapid deterioration in taste. Buttermilk contains many unsaturated phospholipids, especially phosphatidyl ethanolamine, which can bind metal ions, accelerating oxidation, and the presence of a metal-phospholipid complex at the oil-water interface facilitates the formation of lipid hydroperoxides.

Fish oil contains a large number of n-W-polyunsaturated fatty acids, which are susceptible to oxidation with atmospheric oxygen, which leads to damage. Despite this, the taste of rancidity seems to affect only the acceptability of fatter species such as trout, sardine, herring, and mackerel; moreover, the trout and gutted mackerel are oxidized at temperatures above 0 ° C, and the herring remains relatively “unharmed”. In [17], it was suggested that in fish, oxidized lipids bind to lipid-protein complexes, rather than form carbonyl compounds, causing a rancid taste. Lipid-protein complexes are also one of the reasons for the rigidity of the texture that appears in poorly stored fish. The oxygen demand of microorganisms and enzymes (different depending on the species) can also determine the amount of oxygen going to auto-oxidation. Data on the activity of lipoxygenase in the skin tissue of trout give grounds to speak about the possibility of triggering lipid oxidation by providing a sourceThe impact of the gaseous medium in the preparation and storage of chicken breasts on the development ZVOF

Fig. 9.3. The impact of the gaseous medium in the preparation and storage of chicken breasts on the development of VOF

initiating radicals [31]. The assessment of the degree of influence of oxidation on the quality of fish is complicated by the fact that many products transported in the sales network chilled, are pre-frozen to eliminate the effect of the seasonality of supply (this applies especially to herring).

Staining meat products pink

Food color change is a common problem that can take on different forms and be associated with a wide range of chemical reactions. Biochemical or enzymatic darkening (acquisition of a golden brown shade) is discussed below. Coloring pink cooked meat is a long-standing and very common problem affecting production, retail, service, and household. Often this colored meat is perceived as undercooked. The problem is especially noticeable in the case of sliced ​​meat, roasted roasted foods, meat pies and casseroles. Identified various causes of the appearance of pink color, which are listed in Table. 9.1 indicating the type of pigment, which is believed to be associated with staining. In [67], the causes of the pink coloring of cooked white meat and the factors affecting this coloring are considered.

Myoglobin is a monomeric spherical heme protein found in all vertebrates, which, together with hemoglobin, gives the meat a red color. The amount of myoglobin varies depending on the type of animal and tissue, and, moreover, depends on many environmental factors. As shown in the table. 9.1, myoglobin can be present in several forms, some of which can give a red or pink residual color to meat, even after cooking. Recent work has shown that more than 80% of cases of pink staining are due to nitrosomioglobin due to nitrate impurities and their subsequent bacterial reduction to nitrites [13].

Features of biochemical reactions

Biochemical reactions are catalyzed by special proteins - enzymes. These are very specialized and effective catalysts that lower the activation threshold, so that the reaction rate of thermodynamically possible reactions increases dramatically. The specificity of enzymes for certain substances is indicated in their names usually by adding the suffix -az to the name of the substance on which the enzyme acts, for example, lipase acts on lipids, protease on proteins (proteins). The catalytic activity of enzymes strongly depends on the structure of the protein, and many features of the reactions catalyzed by enzymes are due to the influence of the local environment. Heat, extreme acidity or alkalinity, and high ionic strength can denature the enzyme, causing damage or loss of activity. Inhibitors and activators of enzymes that bind them reversibly or

9.1 Table. Kinds of pigments and causes coloration of meat

pink product (in [13])

Type of pigment Reason pink color



Karboksimioglobin Vosstanovlennыy denaturirovannыy myoglobin

Low-temperature processing

Pollution nitrites directly or reduction of nitrate; in furnaces oxides of nitrogen

Carbon monoxide in the furnace; gamma-irradiation

The high pH of, slow process, and the presence of a lot of salt


irreversibly, can act by causing changes in the structure or acting directly on the active site.

The temperature at which denaturation occurs often reflects the environmental conditions in which the enzyme usually acts. For most enzymes of warm-blooded animals, denaturation begins at temperatures around 45 ° C, and around 55 ° C, rapid denaturation deprives the enzyme of its catalytic function; fruit and vegetable enzymes usually denature at higher temperatures (70-80 ° C); Some microorganism enzymes (for example, lipases and proteases) can withstand temperatures in excess of 100 ° C [23].

In a living cell, enzymes catalyze a variety of reactions, which together represent a metabolism (metabolism). In the cellular environment, the control and coordination of enzyme activity is achieved through feedback mechanisms and compartmentalization. The destruction that occurs during slaughter or during harvesting may require action to prevent the subsequent action of enzymes (blanching vegetables) is a good example. Enzyme activity can be enhanced if they improve the quality of products, as in the case of meat conditioning, in which protease activity is used to destroy muscle fibers for a more complete manifestation of taste and softness.

The rate of reactions catalyzed by enzymes increases with increasing substrate concentration, but only to a certain limit (maximum activity) at which the enzyme is saturated with the substrate. A further increase in the concentration of the substrate does not increase the reaction rate, which increases with temperature (as in chemical reactions) to a temperature that is optimal for activity. At temperatures above the denaturation of the protein-enzyme occurs, and it loses activity. At refrigerated storage temperatures, enzyme activity in most products is low, but there are certain exceptions. Enzymes of cold-blooded species can adapt and remain active at low temperatures. In cod, lipase activity at 0 ° C shows a pronounced delay phase before reaching maximum activity, the level of activity decreases to 0 ° C and increases to a maximum at -4 ° C.

Enzymes from different sources, catalyzing the conversion of the same substrates into the same reaction products, can have different reaction rates, pH or optimal temperatures, depending on their origin. The shelf life of chilled pasta salad, consisting of ready-made pasta, onions, red and green peppers, cucumbers, sweet corn, mushrooms, and seasoning from vinegar and olive oil, is limited by the color change of the corn or mushrooms (which turn brown) depending on storage temperature [32]. The storage of lettuce at temperatures between 2 and 15 ° C showed that the temperature characteristics of browning reactions (probably catalyzed by the enzyme polyphenol oxidase) were completely different in mushrooms and maize (Fig. 9.4).

Mushrooms browning reaction rate was less dependent on temperature than the response in maize, so that at higher temperatures lettuce shelf life was limited darkening of corn, and at lower temperatures browning of mushrooms. To prevent such changes or predict shelf life as a function of temperature properties must be known reactions causing these changes appearance.

Enzymes in a food product can be endogenous, that is, naturally present in the tissues of a plant or animal. Hundreds of enzymes fall into this category, although not all of them significantly affect product quality. Exogenous enzymes can be added by the manufacturer to perform a specific function, for example, papain - for meat tenderization, protease - for cheese ripening or naringinase - for removing the bitterness of citrus juices, especially from grapefruit juice. Enzymes may be present as a result of contamination during migration from one product to another during their contact. An example is the migration of lipases from unblanched pepper in a pizza to cheese, which, with the presence of appropriate triglycerides, has a soapy taste as a result of lipolysis. Contamination with extracellular enzymes from microorganisms (for example, lipases and proteases) can also occur, and the microorganism can be destroyed by heat treatment, and the enzyme resistant to it can be preserved.Sensory changes in the salad of pasta

Fig. 9.4. Sensory changes in the salad of pasta seasoned with vinegar and olive oil, stored refrigerated. By [32]

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