Chilled Foods: Effects on Shelf-life and Sensory Quality D Bermu´dez-Aguirre and J Welti-Chanes, Tecnolo´gico de Monterrey, Monterrey, Nuevo Leon, Mexico ã 2016 Elsevier Ltd. All rights reserved.
Introduction The commercial cold chain has been evolved together with the progress on Food Science and Technology. In the past, chilled food was a term used for those items that need to be kept under refrigeration because of quick microbial spoilage. Sometimes, these products were seafood and fish caught in remote areas of the world and needed to be transported to specific markets. However, nowadays, the chilled food chain also includes some novel products that must be kept under refrigerated conditions; temperature abuse is not an option because of the microbial risk inherent in the product. These novel products are called cooked-chilled foods or ready-to-eat meals. During the storage of chilled foods, temperature must be kept at specific values and recorded to ensure the microbial safety of the product. Consumers must be aware of the importance of controlling the temperature of these products and make sure their responsibility to handle the product from the store to the final consumption. Even if the product is kept with ice on a boat or kept on a supermarket on the fridge, temperature must be carefully monitored to ensure that spoilage microorganisms are growing slowly or the microbial growth is delayed. Several microbial species have been identified in specific products and these are the ones that should be monitored during the storage; microorganisms such as Bacillus spp. are frequently found in chilled goods and it is recognized as one of the foodborne organisms, and high counts of this sporeformer microorganism can promote gastrointestinal diseases. Pathogens such as Listeria monocytogenes, or even spores of Clostridium botulinum, can be identified in some chilled foods, because of a poor pasteurization process, lack of hygienic measures, cross contamination, or underprocessing of the product. Furthermore, sensory quality of chilled foods can be drastically compromised because of some chemical reactions taking place during storage, such as rancidity, changes in color and flavor, or changes in texture. Refrigerated temperatures delay some of these chemical reactions but do not eliminate them completely. Even if the reaction rate is slow, the chemical processes are taking place, and if the product is stored for several weeks, noticeable changes on sensory properties could be observed. This article presents a brief discussion about some microbial and sensory changes in chilled foods. The specific microbial groups of representative food products are included as well as some of the novel approaches to minimize microbial risks and to improve the current technologies used to preserve chilled foods. Sensory changes in some refrigerated products and some novel strategies used by food technologists are also included.
Chilled Foods The term chilled foods includes a number of products: some of them are ready-to-eat, others require some quick preparation
by the consumer, and another category includes chilled products that will be used as ingredients for other products. In general, chilled foods must be kept at a temperature 5 C to ensure the microbial quality of the product through the chilled chain until they reach the consumer. Chilled foods include entre´es, pasta, vegetables, fresh soups, salad dressing, desserts, deli products, dips, ready-to-eat meals, different kinds of meat, fish and seafood, and poultry products. All of them, regardless of the product, must follow the food safety regulations in each country for processing, handling, transportation, storage, and final consumption. Often, a hazard analysis and critical control point (HACCP) program is followed to process, preserve, handle, prepare, transport, and package chilled foods. Some of the main concerns of chilled foods are related with chemical, sensory, and microbial changes of the product by the end of the shelf life when the consumer is eating the product. Chemical changes on the product can seriously compromise the nutritional quality of the product leading to a food item without the original nutrient content. Sensory changes are related more with the acceptability of the product by the consumer; even though these changes do not put at risk the health of the user, they might affect the perception and consumption of the product, making it unsuitable for eating. Finally, one of the most important characteristics of the chilled foods is the microbial quality; even though the growth of the microorganisms is delayed during storage of these products because of low temperature, there are some species such as psychrophilic microorganisms that can grow and represent a risk for the consumption. Besides, some of the emerging pathogens might stay alive on the product and grow if the temperature is not controlled, producing a high risk of foodborne illnesses.
Shelf Life The shelf life of the product depends on several factors such as initial microbial counts, the quality of the raw ingredients, the processing technology, the addition of antimicrobials, and the use of preservation factors such as decrease of pH or aw and storage temperature. The chemical composition of the product will also be an important factor to consider during the shelf life studies, as the richer in nutrients the product is, the faster its microbial growth. Furthermore, aw is an important fact to consider during the shelf life, especially for chilled foods that have high moisture content. Water is important not only for microbial growth but also for the promotion of chemical reactions on the product. Regardless of the chemical composition of the product and the initial microbial population, an additional aspect to consider during the shelf life is the packaging material and the packaging conditions of the product. Some packaging materials represent a strong barrier against moisture, oxygen, and temperature, which together will extend considerably the shelf
Encyclopedia of Food and Health
Chilled Foods: Effects on Shelf-life and Sensory Quality
life of the product. However, on the chilled food chain, some products are not packaged because they are sold ‘fresh and raw’ such as fish or produce or some products have a very weak barrier with very basic packaging materials. Some products might be packaged using some specific conditions such as vacuum and modified or controlled atmospheres, which also contribute to the extension of the shelf life. Also, several preservation factors have been tested together with low temperatures during food processing to extend the shelf life of different products; physical and chemical hurdles have been used as shown in Table 1.
Microbial Shelf Life Fish and seafood Several marine products such as seafood and fish have a short shelf life because of the chemical reactions taking place together with the postmortem changes. All of these chemical changes can promote faster microbial growth of the product. Spoilage microorganisms on fish include aerobic and proteolytic bacteria, coliforms, and lactic acid species. Pathogens on fish include Vibrio cholera, L. monocytogenes, Escherichia coli, and Salmonella spp., among others (Table 2). Numerous studies have been conducted to incorporate some compounds on the ice that keeps the temperature of chilled foods low. Some of the compounds have antimicrobial properties to delay the microbial growth and extend the shelf life of the product. Studies have been done with plants and herbal extracts, for example, thyme, oregano, clove, basil, and rosemary, among others. Other compounds that have been tested together with ice include some organic acids, alone and in combination. Some examples are citric, ascorbic, and lactic acids. The food products tested include marine species such as seafood (anchovies) and different kinds of fish (sardines and mackerel, among others). In most of the cases, the shelf life of the
product has been considerably extended when comparing with control samples kept only on ice; microbial loads reported for muscles are considerably lower for those treated products compared with control samples. Counts of aerobes, anaerobes, psychrotrophs, Enterobacteriaceae, and lipolytic and proteolytic microorganisms are affected because of the presence of acids and free radicals. This traditional method to preserve fish, using ice, is commonly used for transportation from the origin to the final market of the fish. Boxes with 30% of ice are used to keep the fish with low microbial growth and minimize the chemical reactions during transportation. In the last few years, a new approach has been studied to reduce the amount of ice required for transportation of fish. Superchilled fish has 10–15% of ice since the temperature of the product is reduced to about 1–2 C below its freezing point. The ice is surrounding the product, protecting it from microbial spoilage and enzymatic reactions, creating an ice shell within the product. Other studies on keeping chilling temperatures on fish include the use of ice with different shapes such as the traditional flake ice or the use of small spherical ice crystals known as slurry, flow, or fluid ice. The latest provides an extended shelf life of the fish rather than the flake ice (up to two times longer) because of the direct contact of the crystals within a bigger superficial area of the fish delaying microbial growth and preserving the texture of the product. Other approaches that have been researched to extend the shelf life of fish include the use of chilling slurry, edible films, Table 2 List of microorganisms found in chilled food according to the product Product
Fish and seafood
Lactic acid bacteria (LAB) Vibrio spp. Listeria monocytogenes Escherichia coli Pseudomonas spp. H2S-producing bacteria Pseudomonas spp. Lactic acid bacteria (LAB) Brochothrix thermosphacta Enterobacteria
Table 1 Examples of preservation factors used together with refrigerated conditions to extend the shelf life of chilled foods Food item Fish and seafood Ready-to-eat meals (meats and vegetables) Ready-to-eat meals (beef) Fish Shrimp Seafood Fish Fish Ready-to-cook meats and fish Fish Fish Sausages Fish
Chemical factors Herbal extracts
Irradiation Rabbit meat High hydrostatic pressure Essential oils Vacuum packing
Gas flushing Organic acids Modified atmosphere packaging (MAP) Chitosan
Yeast and molds Lactic acid bacteria (LAB) Salmonellaa E. coli O157:H7a L. monocytogenesa
E. coli Lactic acid bacteria (LAB) Pseudomonas spp. Yersinia enterocolitica
Clostridium botulinum Bacillus cereus Clostridium perfringens
Bacteriocins (nisin) Different shapes of ice crystals Olive oil Super chilling (partly freezing)
No common microorganisms but they might be present if the food has not been properly handled.
Chilled Foods: Effects on Shelf-life and Sensory Quality
and modified and/or controlled atmosphere. The use of chilling slurry has been tested in cod, basically using seawater slurry ( 2 C) that can reduce the temperature of the product faster than regular ice. During the shelf life, chilling slurry has shown better results on the fish because of the low microbial growth and the freshness of the product. However, some of the problems associated with the use of chilling slurry are associated with the weight gain and salt intake of the fish, both considered drawbacks for the product.
Meat products One of the meat that are highly valued is lamb; some of the most important markets are away from the highest consumers, for example, the lamb from New Zealand needs to travel to foreign markets that sometimes takes several weeks to reach its final destination. The vacuum-packed lamb needs to keep a temperature below 1.5 C to ensure a shelf life between 60 and 70 days. Some of the bacteria that can be found in lamb meat include E. coli, Pseudomonas spp., lactic acid bacteria, and even Yersinia enterocolitica (Table 2). It is essential to keep the product under strict temperature conditions to delay the microbial growth; besides, such as in other meat products, cross contamination can occur during the handling of the animal.
Miscellaneous foods As part of this category, there is a group of very complex foods that include all the cooked-chilled foods that have meat, poultry, fish, and vegetables as part of the list of ingredients. Sales on these products have been drastically increased in the last few years because of the variety of products, innovative concepts, and developed products that fit several lifestyles. The average shelf life of these products is about 5 days when the temperature has been 3 C. However, if the temperature is not well controlled, the products might represent a risk for consumption because of the kind of bacteria that can grow on them. These products are generally pasteurized and packaged such as mashed potatoes, pork chops, ratatouille, minced fish, spaghetti, and mac and cheese. A comprehensive list of these products is presented in Table 3. However, one of the main risks of these products is associated with the presence of some pathogenic microorganisms such as L. monocytogenes and spores of C. botulinum that can survive the pasteurization and be latent on the product. These cooked-chilled foods are also known as refrigerated processed foods of extended durability (REPFED) ready-to-eat meals. Depending on the thermal treatment applied to this kind of products, there are three categories of REPFEDs: (a) Mild thermal treatment at 70 C for 2 min. Basically, this treatment is applied to inactivate L. monocytogenes in at Table 3
least 6 log reduction. These products are pasteurized in the package used to sell them. However, this mild thermal treatment cannot inactivate spores in the product and represents a risk for the consumer if the food is not adequately handled by the consumer until the final consumption. (b) Medium thermal treatment at 90 C for 10 min. This treatment has the goal to inactivate the strains of nonproteolytic vegetative cells of C. botulinum, Bacillus cereus, and L. monocytogenes. The product is pasteurized in the package used to sell it, and because the thermal treatment is not really strong, some spores can resist the process and survive. However, the temperature used for this process can produce some thermal damage on the spores reducing the possibility of producing a foodborne problem. (c) Repackaged chilled foods. This kind of products is pasteurized in some packaging material and after is repackaged in the intended final package. Sometimes, these products are pasteurized in opened containers and after packaged to be sold. The big risks in these products are related with cross contamination after the pasteurization with pathogenic strains. Cold storage is one of the hurdles used on these products to extend the shelf life and delay the microbial growth; however, most of the time, a previous thermal treatment and some other preservation factors (such as reduction of pH, aw, and antimicrobials) are used together. One of the microorganisms that have been also associated with chilled foods is B. cereus. The spores of this microorganism can survive high temperature during conventional pasteurization but it can also survive refrigeration temperatures and grow during the storage of food, representing a microbiological concern. B. cereus is frequently associated with foodborne gastroenteritis. Several foodborne outbreaks have been reported in the food industry in the last decades because of the presence of spore-forming bacteria, mainly from the genera Bacillus. The vehicle of these microorganisms has been mainly associated with the vegetables that are part of most of these cookedchilled foods.
Chemical Shelf Life One of the main chemical reactions taking place on fish is oxidation of the lipids; these reactions known as rancidity are responsible of the generation of off-flavors and odors on the product. Marine products have a characteristic odor that can be easily noticeable. However, when there is lipid oxidation
Examples of cooked-chilled foods (REPFEDs) available on the market; classification is made based on the main ingredient
Pork chops Meatballs with tomato paste Veal stew Beef burgers Sliced ham Roast beef
Chicken and rice Curry chicken Chicken with vegetables Grilled chicken Turkey meat Poultry sausages
Salmon and rice Fish in sauce Minced fish Crab cakes Lobster cakes Surimi
Mashed potatoes Spinach mash Ratatouille Carrots Leak and potato mash Rice with vegetables
Spaghetti Cannelloni Penne with vegetables Mac and cheese Lasagna Fresh pasta salad
Chilled Foods: Effects on Shelf-life and Sensory Quality
because of the changes in pH during the shelf life, this odor changes to a very unpleasant and putrid smell because of the several biochemical reactions taking place, in addition to microbial growth. Also, there are chemical changes associated with proteins and microbial growth together with enzymatic activity, releasing some nitrogen that is associated with fish deterioration. Some metabolites coming from the microbial activity such as amino acids are related with the protein hydrolysis taking place on fish and are also responsible for changes on the texture of the product. On the other side, in some meat products, the chemical reactions taking place during the chilled storage are part of the natural process of tenderization of the product. Changes in pH, water-soluble compounds, and lipids on meat muscle during the storage will promote some desirable chemical reactions that provide the product with the characteristic flavor during the cooking process. For example, on beef, the biochemical changes during the postmortem period involving the adenosine triphosphate (ATP) degradation will produce specific flavor precursors on the product once it is cooked. These changes involve the reaction between sugars and amino acids producing specific volatile compounds. Nevertheless, the degree of production of these compounds is also influenced by other factors such as the diet and race of the animal, the season, the animal age, and the conditions of slaughtering, among others. Because the quality of the product depends on the control on the chemical reactions taking place during the storage, several efforts focus on how to optimize these chemical changes. Some examples are the reduction of temperature keeping the product in special chambers, the use of special packaging materials and conditions, the incorporation of some chemicals on the ice, and the use of antioxidants and radical scavengers, among others. The use of antioxidants has been extensively documented; several compounds have been tested in different food products to delay microbial growth, to inhibit enzymes or reduce the chemical reactions catalyzed by them, and to scavenge the free radicals in those products having rancidity problems during storage.
Sensory Quality The basic sensory evaluation of food products includes the assessment of odor, flavor, taste, texture, and appearance. During chilling of foods, some physicochemical changes take place; in some cases, these changes are part of the conditioning process of the product, for example, the postmortem changes mentioned earlier (such as ATP degradation) on fish, beef, pork, and poultry products. The flavor and aroma compounds in different kinds of meat are present on the water-soluble and lipid fractions of the product. Furthermore, these biochemical changes will impact the texture, general acceptability, and overall sensory quality of the product. The chemical reactions associated with the sensory characteristics of the food involve sugars, lipids, and amino acids. Several studies have shown that the conditioning process of beef and pork meat provides better products in terms of sensory characteristics when the product is kept at chilled conditions as long as 21 days. Important and significant changes are observed in aroma, flavor, taste, and texture characteristics
such as tenderness, juiciness, and chewiness when the meat is allowed for a long conditioning process. Another example is the use of CO2 snow and brine chilled ( 2 C) in ground beef; both techniques are able to delay the microbial growth on the meat up to 21 days. The use of CO2 snow produced also a better texture on the product having a more tender meat; meanwhile, the chilled option showed cooking losses on the product. Some specific products from beef, such as the heart and liver, which are used as by-products for other industries, are preserved as chilled foods. However, when the product is removed from the animal and immediately packaged under vacuum conditions, the meat is better preserved in terms of weight loss, off-flavor generation, and minimum microbial growth. Then, it is highly recommended for this kind of products to package the product under vacuum just as the postmortem period starts. The use of herbs has also been used in the meat industry to improve the sensory quality of some products such as chilled lamb. Some extracts from rosemary have antioxidant effects, and once applied to chilled lamb, the product can have a longer shelf life not only with minimum lipid oxidation but also with excellent sensory properties in terms of flavor, texture, color, and aroma. The main sensory issues observed on fish during the shelf life are related to the loss of freshness observed on the flesh, the changes on pigmentation, the presence of off-flavors, and the appearance of the skin. Regarding some of the novel applications in chilled foods, some of the previously mentioned studies using herbal extracts to extend the shelf life of fish have shown positive results regarding sensory characteristics. When the ice contains some herbs such as oregano, thyme, and rosemary, the product acquires a similar taste and flavor, making it more appealing to the consumer. Furthermore, when fish is stored under specific preservation factors, the shelf life can be extended and the quality improved. For example, studying the use of superchilled ( 2 C) and chilled (4 C) salmon and having the same fish under modified atmosphere, it is possible to extend the shelf life considerably. Products under superchilled conditions are able to show good quality and lower microbial counts (<1000 cfu g 1) after 24 days of storage (total aerobic plate, H2S-producing bacteria, and psychrotrophic counts). The superchilled salmon shows also acceptable sensory properties after 21 days of storage. The salmon kept under modified atmosphere and chilled was spoiled after 10 and 7 days, respectively.
Fruits and Vegetables In this group, there are not only some products highly consumed such as the ready-to-eat salad blends because of the practical approach for the consumer but also some other products with lower demand such as some fruit cuts sold as chilled food or just regular fresh produce. Chilled salads represent a very good example of vegetables that have a reduced shelf life because of the natural characteristics of the ingredients. Some brands are sold using modified atmospheres to provide additional hurdles for the microorganisms. In most of the cases, the
Chilled Foods: Effects on Shelf-life and Sensory Quality
microbial growth is delayed long enough to provide a safe product for the consumer but all the biochemical reactions affect considerably the sensory characteristics of the product, mainly the texture. Although chilled fruits and vegetables can be considered a safe product, it is well known that some foodborne outbreaks come from these products such as the salad blends involving pathogens such as Salmonella. Also, some consumers prefer to purchase fresh produce and disinfect and prepare them at home rather than use commercial chilled vegetables. Some common products kept under chilling conditions and sold by piece include leafy greens, mushrooms, cucumbers, green and red peppers, carrots, herbs, squash, green beans, turnips, and celery, among others. These products are kept under chilling conditions to preserve their freshness and delay the microbial growth. However, when these products remain on cold storage for longer periods of time, the known chilling injury is observed. Some of these products kept at 4–5 C and 95% of relative humidity can start to show some signs of injury such as tissue electrolyte leakage, stress because of ethylene production, and changes observed on appearance.
Conclusions The chilled food chain represents a high percentage of the food production around the world. Hundreds of food items are currently sold as final products or ingredients and hundreds are new in the market each year. However, one of the main concerns related with these goods is still the microbial quality because of the constant foodborne outbreaks and the possibility of microbial growth during storage. It is really important to
monitor the storage temperature of chilled foods throughout the production chain until the product reaches the consumer. Research has been done and is ongoing in the food technology field trying to identify new preservation factors to be used together with low temperatures not only to extend the shelf life of the product but also to ensure that pathogenic microorganisms are completely eliminated from the product after processing. The main challenge is to find these preservation factors strong against microorganisms but gentle with the nutritional and sensory quality of the product.
Further Reading Brown M (ed.) (2008) Chilled foods: a comprehensive guide, 3rd ed. Cambridge: Woodhead Publishing Ltd. CFA (2015). Chilled Food Association. http://www.chilledfood.org/. Daelman J, Jacxsens L, Lahou E, Devlieghere F, and Uyttendaele M (2013) Assessment of the microbial safety and quality of cooked chilled foods and their production process. International Journal of Food Microbiology 160: 193–200. ECFF (2015). European Chilled Food Federation. http://www.ecff.net/. ICFMS (International Commission on Microbiological Specifications for Foods) (2011) Microorganisms in foods 8: use of data for assessing process control and product acceptance. New York: Springer. Man CMD and Jones AA (eds.) (2000) Shelf-life evaluation of foods Gaithersburg, MD: Aspen Publishers. Mascheroni RH (ed.) (2012) Operations in food refrigeration Boca Raton, FL: CRC Press. Mills J, Donnison A, and Brightwell G (2014) Factors affecting microbial spoilage and shelf-life of chilled vacuum-packed lamb transported to distant markets: a review. Meat Science 98: 71–80. Peck MW and Stringer SC (2005) The safety of pasteurized in pack-chilled meat products with respect to the foodborne botulism hazard. Meat Science 70: 461–475. Sofos J (ed.) (2013) Advances in microbial food safety Cambridge: Woodhead Publishing Ltd.