Chilled and Frozen Food Products

Infobox on Chilled and Frozen Food Products
Example of Chilled and Frozen Food Products
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Chilled and Frozen Food Products

Description / Application / Shipment / Storage

Deep frozen meat (see also Meat, chilled or Meat, frozen)

Minimum requirements (raw materials) For meat, the main quality parameters are appearance (colour), texture, taste and juiciness. It is absolutely necessary that meat has an acceptable tenderness. Therefore, cold shortening during chilling must not occur, and appropriate ageing must be secured. However, aged meat should not be used for the manufacture of minced meat. Raw materials used in the manufacture of deep frozen foods must be of good and sound quality and be of the required degree of freshness. The freezing process should be started without undue delay, and be as rapid is practical. After thermal stabilization (equalization) the temperature of the product should be maintained at -18°C at all points.

Temperature During transport, a temperature of -12°C for 10 days will give a minor reduction of the remaining P.S.L., for minced beef only about 6%. Product temperatures up to -8°C can not cause microbiological problems, and as transport times generally are restricted to days or weeks, the quality degradation of frozen meat will normally be negligible. In practice, as long as the meat is still frozen i.e. the meat temperature is below -2°C, the quality will normally be only marginally affected by temperature abuses, on condition that the transport time is less than a few days.

Deep frozen poultry Minimum requirements (raw materials) The feeding of, for example, chickens with even small amounts of feeds containing unsaturated fatty acids (from fatty fish) may result in an early onset of rancidity. Slaughtering, including scalding, plucking, evisceration and chilling must be carried out hygienically and according to good manufacturing practice (GMP) and the relevant regulations. For whole turkeys an ageing period between chilling and freezing (12-24 h at about 0°C) or after freezer storage gives more tender meat. For smaller birds such as chickens, an ageing period (2-4 h) between chilling and freezing may give some improvement in tenderness. Broilers, turkeys and ducks are often cut into portions, resulting in additional surface area being exposed to air. PSL of cut-up poultry is somewhat shorter than that of the whole bird due to additional handling and contamination.

Temperature Deep frozen poultry, and especially chicken, is a rather robust product with long P.S.L. When the temperature is -8°C or colder, the influence on quality and remaining PSL is small for normal transport, lasting less than 1-3 weeks.

Fish (See also Fish, frozen) Minimum requirements (raw materials) The fish should be of good quality and of the desired degree of freshness, i.e. newly caught and/or chilled quickly to and maintained at 0°C, i.e. the temperature of melting ice. Frozen fish has a shorter PSL than frozen meat. Fish contain a higher proportion of polyunsaturated lipids which are susceptible to oxidation, and marine fish contain TMAO, which can be degraded to dimethylamine (DMA) and formaldehyde (FA). The degradation compounds from oxidized lipids and TMAO can give off-flavours and off-odours, but they can also cause breakdown of the proteins. The breakdown brings about loss of water and a change in texture. In fatty fish, a considerable amount of free fatty acids can be formed.

Temperature When the wrong temperature or a fluctuating temperature occurs, the quality loss is increased and the remaining storage life reduced. However, as long as the temperature is below about -8°C, no bacterial growth can take place. It must be stressed that most frozen fishery products have a rather short PSL which makes it essential to maintain the required temperatures throughout the freezer chain.

Fruits and concentrated juice Minimum requirements (raw materials) The raw materials, the fruits, should be clean and sound, and with no fungal growth. The desired properties of fruits differ considerably between the various industries that use these products. For production of jams it is preferable to use fruit with a firm consistency, which is retained after freezing and cooking. For fruit juices, fruit flavours, etc. it is preferable to use fruits with a full aroma and an intense colour. Especially for strawberries, the same variety seldom possess both firm consistency and good aroma. For direct consumption, only varieties with a firm flesh which retain as much of the original texture as possible should be frozen. Fruits for freezing must be clean and sound. The degree of ripeness at harvest has a marked influence on the quality of the frozen product. Prematurely harvested fruits lack flavour and colour. Fruits picked too late are soft and prone to crushing and fungal attack. Fruits should be cooled between harvest and freezing, unless the time can be kept very short. Processing (peeling, stoning, slicing etc.) is nearly always performed mechanically. Blanching is rarely applied to fruits. The freezing process should be as rapid as practicable. However, fruits when thawed are softer in consistency than the fresh equivalent, even using the fastest freezing method. The benefits of fast freezing are small when compared with the selection of the best varieties.

Temperature For retail packed deep frozen fruits in syrup it is absolutely essential to maintain the product temperature below about -15°C, in order to prevent melting which results in an unacceptable appearance of consumer packs. Due to the rather low pH of most fruits, microbiological growth is seldom a problem, even at temperatures rising to the freezing point of fruit products.

Vegetables Minimum requirements (raw materials) Only material which is clean, sound and of high quality should be frozen. Some varieties (cultivars) are better suited for freezing than others. The intensely coloured and highly flavoured varieties should be selected for freezing. The vegetables must be able to withstand mechanical harvesting. It is essential to harvest vegetables at the ideal moment of maturity. This optimum period may last a few hours for peas, but 2-3 days for French beans. The period between harvesting and freezing must be short, and the most susceptible varieties must be frozen 2-4 hours after harvesting.

Temperature Most deep frozen vegetables have a very long storage life, and in most cases temperature abuses during transport will only result in a minor decrease in quality and remaining PSL; loss of IQF (Individually Quick Frozen) condition may be disputed also. Microbiological problems are seldom seen in deep frozen vegetables when the temperature is kept below -2°C (for a limited period of time of course).

Miscellaneous deep frozen foods Minimum requirements (raw materials) The requirements depend on the product, but in any case the raw materials should be of normal and satisfactory quality, and the processing should be carried out according to good manufacturing practice (GMP).

Temperature Problems with microbiological growth do not exist as long as the product temperature is below -8°C, or as long as higher product temperatures (for example -2°C) are only experienced for a day or perhaps two. Some foods are very sensitive to temperature abuses. For ice cream, PSL becomes very short at temperatures warmer than -18°C, and it is often recommended to store and transport ice cream and similar products at -20°C or below, and to display these foodstuffs in special display cabinets in the supermarkets.

Frozen foods Minimum requirements (raw materials) The requirements for the raw materials are as for deep frozen foodstuffs. However, the freezing process for frozen foods does not have to be as rapid as for deep frozen foodstuffs. The cream for butter making must be pasteurized, preferably at temperatures above 90°C, to inactivate enzymes and to reduce the number of microorganisms which would otherwise cause quality loss. Only fresh butter, no older than 14 days, of high quality, is suitable for freezing. The storage life depends upon the butter type (sweet cream or cultured, salted or unsalted), the pH, the contents of copper, the water distribution through the butter, the salt contents and the storage temperature.

Temperature Frozen foods are very robust, and can withstand rather severe temperature abuses as long as the temperature violations are restricted to a few days. In practice there are no microbiological problems caused by transport temperatures when the temperature is maintained below -8°C. Beef, and especially butter, easily pick up odour from the surroundings resp. from other foodstuffs.

Chilled fresh meat, retail packed Minimum requirements (raw materials) The meat must be of good and sound quality prior to the chilling process. The chilling process must be started as quickly as practicable, and the chilling must be sufficiently rapid to ensure minimum weight loss and no growth of micro-organisms, but not so rapid as to result in toughening of the meat due to cold shortening. Temperatures above those recommended or prescribed will increase the growth rate of microorganism and lead to spoilage. Food poisoning is usually no problem in conventionally packed fresh meat as spoilage will turn the meat unacceptable long before food poisoning is possible. For vacuum-packed meat there seems to be a limited risk, but for MAP meat there is a greater risk and here it is absolutely necessary to maintain low temperatures throughout the chill chain. For MAP meat, it is often recommended or laid down in legislation that the temperature of the meat should be maintained below 2°C or 3°C. Fresh meat easily picks up odour from the surroundings. This is especially important for meat in conventional retail packs where the plastic overwrap has a high permeability.

Chilled meat products, retail packed Minimum requirements (raw materials) The meat must have a good and sound quality, and be of the desired degree of freshness. The processing must be carried out according to good manufacturing practice, and the temperature during processing should be below 20°C or above 50°C. The resulting PSL depends on the composition of the product (for instance the salt/water ratio), the initial bacteriological quality (total count and composition of the bacterial flora), the time and temperature of a possible heat processing, etc. The meat products should be chilled as quickly as practicable, preferably to 0°C or even colder. The initial freezing point for many cured meat products is about -3°C to -4°C, and such products would benefit from storage and transport at a temperature of around -2°C (deep chilling). Meat products are often sliced, and to achieve a good initial bacteriological quality, the slicing process must be carried out hygienically, which involved frequent and adequate cleaning and disinfection of machinery, conveyors, labels, etc.

Temperature The sensitivity to temperature depends on the product composition and on the packaging. Cured meats requiring chilling are usually vacuum-packed or packed in MAP, but the content of salt, and possibly nitrite, gives a certain degree of protection against pathogenic bacteria. Of course a higher storage temperature results in shorter storage life. Heat processed, uncured meat products are sometimes retail-packed in MAP or in vacuum-packs. However, it involves a significant risk of food poisoning if product temperatures are not maintained at the recommended or prescribed level, normally a maximum temperature of 5°C. For retail packed meat products, some countries prescribe lower maximum temperatures than 5°C, e.g. 3°C, while other countries allow 8°C.

Manufacturing meat Minimum requirements (raw materials) The meat must be of good and sound quality, and at the required degree of freshness. The chilling process must be started as quickly as practicable. In the EC, the meat should at all points be 7°C or below, before being taken out of the slaughterhouse.

Temperature For unwrapped meat, temperatures exceeding the prescribed level will reduce the quality and the PSL, but the spoilage bacteria will result in changes so pronounced that there is little risk of the meat being eaten and causing food poisoning. For meat packed in vacuum-packs or MAP, the growth of pathogenic bacteria are of much more concern, because most spoilage bacteria do not grow very well without oxygen in the package. The normal signs of spoilage (off-odour, discoloration) could be lacking, although the product could have experienced such a time-temperature history that there is a real hazard of food poisoning. For cured meat products the sensitivity to temperature depends on the PPP-factors (i.e. Product, Processing, Packaging). However, the content of salt, and possibly nitrite, gives a certain protection against pathogenic bacteria. The temperature should be kept at 5°C or below. Fresh meat, and especially unwrapped fresh meat, is highly susceptible to the uptake of foreign odours from the surroundings. The plastic materials used for vacuum-packs and MAP have low permeability, and this reduces (but does not prevent) the uptake of foreign odours.

Chilled poultry Minimum requirements (raw materials) The raw materials must be of a good and sound quality, and the contents of food poisoning bacteria must be as low as is possible. The initial microbial state, i.e. the number and type of microorganisms present on the poultry immediately after chilling and packaging has a pronounced influence on quality and storage life. The processes involved in slaughtering and chilling should be carried out according to good manufacturing practice, and in the EC countries or for export to the EC countries according to current EC directives.

Temperature In order to reduce the growth of spoilage and food poisoning bacteria it is absolutely essential to maintain low temperatures in the chill chain. Temperature abuses during transport will result in a significant reduction in quality and the remaining storage life. Warm temperatures in the initial stages of the chill chain promote growth of spoilage organisms, eventually leading to fast spoilage. Fresh poultry easily picks up odour from foodstuffs that give off strong odours, such as oranges, apples, onions and fish.

Chilled fish Minimum requirements (raw materials) The fish must be of good and sound quality prior to the chilling process. The chilling process should be started as quickly as possible in order to minimize growth of microorganisms. The best way of maintaining fresh fish at a temperature close to 0°C is to keep it in melting ice in a room with a temperature of 1°-3°C. If the thermostat in the room is set at 0°C, the air temperature may occasionally fall below 0°C which will prevent the ice from melting. This will inhibit cooling of the fish and cause some parts to freeze. Superchilling (also called deep chilling or partial freezing) to around -2°C may give a longer storage life, but this method is seldom used.

Temperature Storage without melting ice (or storage at temperatures above 0°C) will increase the activity of bacteria and lead to rapid spoilage. Bacteria able to cause food poisoning may develop in fish which are not well iced. Clostridium botulinum type E which is often found in fresh fish, can produce toxins at temperatures down to 3,3°C. Clostridium botulinum can only grow under anaerobic conditions, i.e. where the oxygen concentration is very low. Therefore, vacuum-packaging of fish products in plastic materials with low oxygen permeability and the use of MAP with no oxygen, necessitates product temperatures below 3,3°C. The toxin from Clostridium botulinum type E and harmful bacteria such as Vibrio will be destroyed under normal cooking. However, with fish products intended to be eaten raw such as sushi or oysters, or to be used as raw materials for the manufacture of lightly preserved fish products, such as gravid fish and cold smoked fish, it is very important that the fish is constantly well iced. Heat stable histamine may be formed, in fish of the tuna and mackerel type if they are not stored at low temperatures.

Lightly and semi-preserved fish products Minimum requirements (raw materials) The raw materials must be of good and sound quality and be of the required degree of freshness. The processing must be in accordance with good manufacturing practice, and should result in the desired characteristics, e.g. salt/water ratio, temperature, concentration of preservatives, pH, etc.

Temperature Most of the products in this group are consumed without further heat treatment. In lightly preserved fish products, there is the possibility of bacterial growth such as Salmonella, Listeria, Vibio and Clostridium botulinum type E and it is absolutely essential to keep these products at low temperatures and always below 5°C. In semi-preserved fish products there is no such risk, but high storage temperatures will reduce quality and storage life.

Live fish Minimum requirements (raw materials) Because of the risk of spreading diseases there are veterinary rules prohibiting import of fish from areas with certain diseases. For example, the import of live salmonids (i.e. salmon and trout) to United Kingdom is prohibited. This is to protect the salmonids in United Kingdom from the diseases Viral Haemorrhagic Septicaemia (VHS) and Infectious Haematopoietic Necrosis (IHN). The live fish must be healthy and sound.

Temperature The recommended temperature is 3-10°C. Live crustaceans and molluscs should not be kept at temperatures below 4°C, and during the winter live trout should not be suddenly transferred to water several degrees above the temperature of the water they came from. If the temperature of the water in which the fish live rises, the fish may suffer as the oxygen level falls below a critical. Oxygen is usually added to the water for transport of live trout and other fish.

Chilled dairy products Minimum requirements (raw materials) Milk is a very good medium for microbial growth. Many dairy products are highly perishable and demand chilled storage. On receipt from the farm the milk is subject to a variety of checks to ensure that the composition and bacteriological standards are met. The majority of milk is pasteurized, commonly at 72-78°C for 15 seconds. In many countries, there are legal requirements for pasteurization and for the subsequent chilling process. Heat treatment brings about a considerable reduction in bacterial flora and should ensure absence of pathogenic bacteria. This alone, however, is of no use if packaging etc. allows re-infection. Retail packed pasteurized milk has a short storage life and is seldom transported over long distances.

Temperature The temperature sensitivity varies widely, depending on the composition (salt content, pH, etc.), but most milk products are highly perishable. In order to market high quality products and to obtain the storage life indicated on the label, it is essential to maintain the required temperatures throughout the chill chain.

Cream Fresh cream is often pasteurized at 95-100°C for 15 seconds. UHT-milk and UHT-cream is of increasing importance; UHT products can be stored at ambient temperatures for some months.

Fermented (cultured) milk products Fermentation (culturing) of milk is a very old form of food preservation. There has been a dramatic increase in the consumption of these products which include several types, of which yoghurt is the best known. Milk is pasteurized at 90-95°C for 15-30 minutes (in some countries 5-10 minutes) and then cooled to 42-45°C. A starter culture, a solution of lactic acid bacteria, is added , and after 2-4 hours the product is cooled to 5°C. Fruit is often added.

Butter The cream for butter-making is pasteurized at above 90°C to inactivate enzymes and to reduce the number of microorganisms which would otherwise cause quality loss. Butter is an emulsion of water and oil (fat). The fat emulsion containing in solution sugar, albumen and salt, where as fats and casein are present in colloidal dispersion. There are several common types of butter; sweet cream unsalted butter (pH = 6,5-6,6), sweet cream salted butter (pH=6,5-6,6, 1,2% salt), cultured unsalted butter (pH = 4,6–5,1), cultured salted butter (pH=4,6-5,1, 1% salt), cultured butter is also called fermented or ripened butter. The storage life depends on the quality of the raw milk, salt content and pH, the size of the water droplets, etc. Butter is subject to becoming rancid due to oxidation, producing tallow oily flavours. Another common fault is undesirable flavours picked up from adjacent goods during storage and/or transport. The packaging for butter usually has a low permeability in order to reduce oxidation and dehydration. Butter is not normally transported over long distances together with other foodstuffs.

Cheese Milk is transformed into cheese by a process of partial dehydration and a coagulation of the casein. Cheese contains less water than milk. The amount of water in cheese greatly influences the storage life, and its sensory properties. Many types of cheese are stored for a certain period, known as the ripening period, which may last for weeks or months. During ripening the cheese acquires special organoleptic qualities (appearance, texture, flavour). Microorganisms play an essential role in the development of these characteristics. After ripening (at a temperature between 8°C and 25°C for many types of cheese), the cheese should be kept at a temperature between 0°C and the ripening temperature, depending on how long time it is to be stored.

Fresh cheeses Fresh cheeses have a high water content and should be kept at chill temperatures.

Processed cheeses Processed cheeses are produced from a blend of hard cheeses together with emulsifying salts which are cooled in the molten state and re-solidified. Other ingredients such as ham may be present, and the final product may be smoked before packing.

Desserts Fresh cream desserts consist typically of a flavoured base thickened by various types of starch, together with pieces of fruit, topped with a fresh, often whipped cream. The risk of contamination in the production together with the combination of ingredients make such products highly perishable, and also present a food poisoning hazard.

Temperature aspect There are several hundreds different types of cheese. The ripening process is reduced, especially for soft cheeses, but is not stopped at chill temperatures. The carriage temperature may vary with the type of cheese and whether or not it is required to ripen during the journey. The storage life of most cheeses is not particularly temperature dependent. However, PSL generally increases with decreasing temperature. Freezing, except in rare cases, is undesirable since it changes the texture and spoils the quality. For most soft processed cheeses and cheese spreads it is essential to maintain chill temperatures. Most dairy products easily pick up odour from their surroundings, for example from foodstuffs such as fish, citrus fruits, onions.

Miscellaneous chilled products The raw materials used must be of good and sound quality. The initial microbial state is important for most chilled foods as PSL is usually limited by microbial growth. Good hygienic practice (GHP) should always be followed. A description of some foodstuffs belonging to this group follows:

Margarine Margarine is now seen as a product in its own right rather than as a butter substitute. A significant factor is public awareness of the role of dietary fats in heart disease. A wide variety of different types of margarine are marketed today, differing in raw materials, proportion of polyunsaturated fats, spreading properties, packaging etc. Margarine is rarely subject to microbial spoilage, but may become rancid due to oxidation. Margarine can pick up taints if improperly stored.

Eggs Eggs should be collected frequently and chilled as rapidly as practicable. In some countries, eggs which are sound, clean and without cracks may be washed; after drying they may be treated with a suitable mineral oil to reduce weight loss during storage. The packaging must protect the eggs against shocks, vibrations etc. in the chill chain. During storage the temperature of eggs should be around -2°C. Eggs are normally tempered before they are removed from chill storage. This means that the temperature is raised sufficiently so as to avoid condensation of water on the shell.

Prepared meals Chilled prepared meals have a limited storage life, often just a few days at 3°-5°C, making such products unsuitable for long distance transport. Chilled meals may be given heat treatment after sealing the packaging, thus improving storage life considerably. This is used for example in the ‘sous-vide’ technique, whereby it is possible to achieve a storage life of 2-3 weeks at 3°C; but for ‘sous-vide’ products it is essential to maintain temperatures below 3°C. When very good hygienic practices (GHP) are followed and MAP (e.g. 50% nitrogen and 50% carbon dioxide) is used, the storage life of prepared meals can be increased to 1-2 weeks at temperatures below 3°C.

Prepared salads Prepared salads typically consist of chopped vegetables in either a mayonnaise or oil and vinegar base. The best known is coleslaw (basically cabbage in mayonnaise) which also forms the base for many variants including those containing meat or fish. The market for prepared salads has increased vastly in recent years. Care is needed when formulating salads because interaction between ingredients may create conditions suitable for growth of potentially pathogenic bacteria. The traditional coleslaw types are quite stable, but some types have a very short storage life and are fundamentally unsuitable to large scale retailing. Manufacturers of coleslaw and other salads should ensure that raw materials are obtained from suppliers whose practice preclude the possibility of contamination with Listeria mono-cytogenes or other pathogenic microorganisms.

Prepared raw vegetables Ready-to-eat raw vegetables consist of washed and peeled vegetables which have been diced, sliced, grated or otherwise size-reduced. They are becoming increasingly popular either for use as salad component or as ready-to-cook material for a main course. By preparing vegetables in this way the risks of bacterial infection and spoilage are increased, bacteria generally being unable to penetrate the intact surface of plant tissue. Consequently, packaging or storage conditions that would inhibit bacterial spoilage of prepared raw vegetables will increase the storage life and saleability of the product. Thus, packaging in plastic materials with low permeability, vacuum-packaging or MAP increases PSL; as with the other products using these systems it is still necessary to maintain cold product temperatures.

Temperature aspect Some products in this group are highly perishable, e.g. prepared meals which have not been treated so as to prevent the growth of Clostridium botulinum (especially type E). Thus it is essential to maintain vacuum-packed (also ‘sous-vide’) ready-to-eat products at 3°C or colder in order to eliminate the risk of food poisoning. For all foodstuffs in this group, low temperatures (i.e. around 0°C) during storage and transport will increase quality and storage life, and reduce the risk of food poisoning. Eggs and egg product easily pick up odour from surroundings, and this is also the matter for margarine and some other foodstuffs in this group.