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Frozen Food Storage Prof. Vinod Jindal 1 FST 151 FOOD FREEZING FOOD SCIENCE AND TECHNOLOGY 151 Food Freezing – Storage of Frozen Foods Lecture Notes Prof. Vinod K. Jindal (Formerly Professor, Asian Institute of Technology) Visiting Professor Chemical Engineering Department Mahidol University Salaya, Nakornpathom Thailand
Frozen Food Storage Prof. Vinod Jindal 2 STORAGE OF FROZEN FOODS The quality of a frozen-food is influenced by storage conditions. The changes in quality decrease as temperature is decreased, maintaining low storage temperatures increases the cost of frozen-food storage. Higher temperatures in frozen-food storage must be avoided due to the sensitivity of the frozen-food to temperature. Experience has established that a frozen-food storage temperature of -18 ℃ is accepted as a safe storage temperature for extended shelf life of a frozen food.
Frozen Food Storage Prof. Vinod Jindal 3 Fluctuation of Storage temp. on Product Quality An increase in the product temperature results in conversion of ice to liquid state, with the possibility of re-crystallization when the temperature decreases. Small ice crystals will tend to melt as the temperature rises and change back to ice when the temperature is lowered. The re- crystallization results in an increase in ice crystal size and the impacts on quality.
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Frozen Food Storage Prof. Vinod Jindal 13 Factors Affecting The Quality of Frozen Foods During Storage Freezer burn Freezer burn is caused by the sublimation of ice on the surface region of the product when the water pressure of ice is higher than the vapor pressure in the environment. Freezer burn produces changes in the appearance and texture on the product’s surface and may be the reason for off-odors and -flavors. Moisture migration causes weight losses during freezing and frozen storage, unless the product is packed using a material with low water vapor permeability.
Frozen Food Storage Prof. Vinod Jindal 14 Freezing Burn and Dehydration With the drop in the temperature air looses its capacity to hold moisture. The separated water vapors get converted into frost. The air when and where it has access to product, starts withdrawing water from the product and the spot or area from where water is withdrawn gets dehydrated and appears as white spot called freezer burn. Unpacked foods are more susceptible to freezer burns. However some moisture is lost from the product surface, which attributes to some loss in the weight of product called freezing loss. Longer freezing time will result in larger freezing losses – IQF type of freezing results in 2 to 4 %, however, slower methods may contribute to even 10% or more weight loss.
Frozen Food Storage Prof. Vinod Jindal 15 Change in the Color of Food Freezing may also affect natural color of a product due to difference in light diffraction of ice than water, however, this change in color is reversible when product is thawed. Improper freezing results in active enzymes and oxidative processes. It also denatures pigments and associated proteins. Proteins are part of pigment complexes and they get denatured by cooking, or by acids (change of pH either due to incorporation of acidic elements or by bio-chemical processes or by bacterial metabolism) or by chemical reactions. These changes are permanent and irreversible.
Frozen Food Storage Prof. Vinod Jindal 16 Change in the Color of Food (2) Mayoglobin, the red pigment in meats gets oxidized either during freezing or prolonged frozen storage if not packaged properly, or if packaging is of poor quality and pervious to atmospheric oxygen or if packaging is not sealed properly or it gets damaged during handling. Vegetables have very high level of enzymes and some of them like katalases and peroxydases are hard in nature and are active in slow speed in the frozen conditions too. It is therefore essential that vegetables are flash cooked or blanched before freezing in order to inactivate enzymes
Frozen Food Storage Prof. Vinod Jindal 17 Change in the Color of Food (3) Discoloration in some fruits and vegetables is more when they are chopped into portions or slices, as they have high level of enzymes called polyphenolase, which get converted into melanin when it comes in contact of oxygen i.e. blackening of potatoes, brinjals and apples. Proper packing and freezing will arrest this phenomenon.
Frozen Food Storage Prof. Vinod Jindal 18 Change in the Color of Food (4) Sea foods specially crustaceans also have high levels of this enzyme (polyphenol oxidase or PPO) and if they are not frozen and stored at right temperatures or subjected to temperature abuse, they undergo this process of MELANOSIS and render the product with black spots. Although melanin formation does not affect the edible qualities of food, it certainly impairs its visual quality. Some of the fish specially fatty fish develop a red/yellow discoloration or rust during prolonged storage and temperature abuse.
Frozen Food Storage Prof. Vinod Jindal 19 Water absorption and redistribution Temperature fluctuations lead to a net migration of moisture from interior towards the surface of the foodstuff or to the package. As temperature outside the packaging decreases, moisture on the warmer surface of food sublimes, diffuses and concentrates as ice crystals on colder surface of the packaging film. When ambient temperature increases, the ice on the warmer wrap tends to diffuse back to the colder surface of the food. However, re-absorption of water from the surface back to its original location in the food is impossible. This leads to a growing amount of ice crystals on the surfaces. The formation of ice crystals inside the package weakens the appearance of the product and indicates to the customers that the product has been stored improperly or for a long time.
Frozen Food Storage Prof. Vinod Jindal 20 Re-crystallization Physical changes to ice crystals known as re-crystallization are an important cause of quality loss in some foods. Small ice crystals, which have been formed in a fast freezing process, may thaw if the temperature around the product is increased. When temperature is decreased again, water molecules join the remaining ice crystals, which get larger instead of forming new ice crystals. In the end, the amount of small ice crystals is decreased and the amount of larger ice crystals is increased and the texture of the product becomes rough, crystalline and icy. Especially ice cream is sensitive to fluctuations in temperature.
Frozen Food Storage Prof. Vinod Jindal 21 Drip loss During freezing, frozen water is removed from the original location in the foodstuff to form ice crystals. During thawing, water may not be reabsorbed in the original region, leading to the formation of drip. Drip loss leads to the loss of nutrients, affects texture and juiciness and modifies the appearance of the product.
Frozen Food Storage Prof. Vinod Jindal 22 Re-crystallization of Ice Inside the Product Temperature fluctuations in the cold store and in the display freezer cabinets trigger one more process inside the product itself – re-crystallization of ice crystals. The small ice crystals that are formed during quick freezing change their shape and size once temperature of the product goes up. The core of the product tries to stabilize with out side air in the store and high temperature in the store subjects product to similar conditions as witnessed in the slow freezing - forming bigger ice crystals or re- crystallization of smaller ice particles into bigger ones, it damages tissue cells due to expansion, which ultimately results in loss of food value.
Frozen Food Storage Prof. Vinod Jindal 23 Biological and Chemical Changes The biological changes include reduction of micro flora on the surface and interior of food. Freezing has inhibiting effect on the metabolism and reproduction of microbes. As water gets converted into ice and it is not freely available to microbes for their metabolic and physiological activities, they starve and some of them either perish or go into dormant stage. However as and when they get right environment like temperature abuse anywhere during the cold chain, they get active and multiply. Due to this effect reduction in the bacterial load during freezing is witnessed however freezing is not a sterilization process.
Frozen Food Storage Prof. Vinod Jindal 24 Protein denaturation Freeze-induced protein denaturation and related functionality losses are commonly observed in frozen fish, meat, poultry, egg products and dough. Effects of the protein denaturation may be seen in water- holding capacity, viscosity, gelation, emulsification and whipping properties of the product. For example in fish, protein denaturation may be seen as changes in the texture (toughness). Protein denaturation may be reduced by keeping the storage temperature as low as possible.
Frozen Food Storage Prof. Vinod Jindal 25 Lipid oxidation Lipid oxidation is identified especially in products with polyunsaturated fatty acids, particularly phospholipids, like in fatty fish. Lipid oxidation causes off-odors and -flavors (rancidity), changes in the appearance (loss of pigment colors) and loss of nutritional value in the products.
Frozen Food Storage Prof. Vinod Jindal 26 Lipid oxidation (2) Lipid oxidation is best prevented by eliminating oxygen from the product by antioxidants or with vacuum or modified atmosphere packaging and by using packages with adequate oxygen barrier. The best way is to use packaging material with good oxygen barrier tight around the product so that there are no air pockets inside the package. Light, especially shortwave UV-light, increases oxidation, especially in fatty products. It also destroys vitamins and causes changes in the color. Therefore, packaging material should also function as a light barrier.
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Frozen Food Storage Prof. Vinod Jindal 36 Conclusion Understanding factors related to freezing should minimize loss in food quality
Frozen Food Storage Prof. Vinod Jindal 37 5 Frozen Food Components and Chemical Reactions Miang H.Lim, Janet E.McFetri dge, and Jens Liesebach University of Otago, Dunedin, New Zealand The effect of freezing on the food components is diverse, and some components are affected more than others. For example, protein can be irreversibly denatured by freezing, whereas carbohydrates are generally more stable. This chapter focuses on chemical and biochemical reactions that affect the quality of frozen food systems.
Frozen Food Storage Prof. Vinod Jindal 38 II. CHEMICAL AND BIOCHEMICAL REACTIONS IN FROZEN FOOD A. Protein Protein may undergo changes during freezing and frozen storage, primarily because of denaturation. Denaturation can be defined as a loss of functionality caused by changes in the protein structure due to the disruption of chemical bonds and by secondary interactions with other constituents B. Lipids Lipids can degrade in frozen systems by means of two well- known chemical processes: hydrolysis and oxidation. These processes lead to undesirable changes in the nutritional and sensory quality of foods, such as the production of rancid flavors and discoloration.
Frozen Food Storage Prof. Vinod Jindal 39 C. Carbohydrates Carbohydrates are susceptible to hydrolysis during frozen storage, as observed in frozen papaya (44). Sugar hydrolysis increases the number of moles of solutes in the food matrix, thus depressing the freezing temperature. D. Color Pigments The stability of color pigments during frozen storage is affected by treatment prior to processing and by processing and storage conditions (light, oxygen, heavy metals, temperature, water activity, pH, oxidizing, and reducing agents).
Frozen Food Storage Prof. Vinod Jindal 40 E. Flavor Compounds Food flavors are composed of volatile aroma compounds and taste components, such as organic acids and sugars. The effect of freezing and frozen storage on flavor compounds in food is variable; flavor changes are affected mainly by enzymatic activities and lipid oxidation.
Frozen Food Storage Prof. Vinod Jindal 41 F. Vitamins Freezing is considered as one of the best processing methods for preserving nutrients in food. In a comparison of different storage methods, the nutrient content of frozen beans, sweet corn, and peas was similar to that of fresh vegetables that had been cooked by boiling. Frozen vegetables were higher in vitamin C, riboflavin, and thiamin than canned vegetables. G. Minerals Minerals in food matrices may be present in many different forms such as chemical compounds, molecular complexes, and even free ions. Minerals present in any form can dramatically affect the color, texture, flavor, and stability of foods.
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Frozen Food Storage Prof. Vinod Jindal 46 4 Microbiology of Frozen Foods C.O. Gill Agriculture and Agri-Food Canada, Lacombe Research Centre, Lacombe, Alberta, Canada The effects of freezing on microorganisms in foods vary greatly with the type of microorganism, the physiological state or stage in the life cycle of the microorganism, the composition of the food, and the rates of freezing and thawing. In general, viruses, bacterial spores, and sexual spores of fungi are likely to be preserved by freezing, irrespective of the composition of the food and the rates of freezing and thawing. Other microorganisms are likely to be damaged by freezing, but the extent to which freezing and subsequent frozen storage reduces the numbers of any organism may be trivial.
Frozen Food Storage Prof. Vinod Jindal 47 8 Texture in Frozen Foods William L.Kerr University of Georgia, Athens, Georgia, U.S.A. One critical quality factor influenced by freezing is food texture. Texture can be defined as those properties of food determined by the rheological and structural nature of the food and determined by the tactile senses. This chapter discusses the issues regarding the effects of freezing on the texture of major food groups e.g. vegetables, fruits, meat, fish and seafood, poultry, eggs, dairy products and baked goods.
Frozen Food Storage Prof. Vinod Jindal 48 Due to the chemical and structural differences in different food groups, each has unique issues associated with changes in textural quality. With the exception of bread dough, most food groups suffer fewest changes in textural quality when frozen at a rapid rate. In addition, storage at low temperatures is preferential, and particular care should be taken to limit temperature fluctuations during frozen storage. Methods of thawing can also affect the texture of foods.
Frozen Food Storage Prof. Vinod Jindal 49 chapter fourteen Freezing preservation of fresh foods: quality aspects David Reid General discussion on technological basis of freezing. Storage stability of frozen vegetables
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Frozen Food Storage Prof. Vinod Jindal 54 From Tables 2–6 for storage stabilities, it can be seen that different tissue types exhibit both different absolute stabilities toward frozen storage and different temperature sensitivities. It is also clear that for many products, – 18°C is too high a temperature for effective long-term frozen storage. There is a trend within the frozen food industry to employ lower temperatures for the storage of many products. Most refrigerated warehouses operate at temperatures below –18°C, and indeed –30°C is not uncommon.
Frozen Food Storage Prof. Vinod Jindal 55 The effect of storage temperature is most marked for fish; though well-controlled low-temperature storage could contribute to a greatly improved product. There is growing evidence that the effects of temperature mishandling of products may be more solid than previously believed. A simple trial with frozen green beans that even a one-time fluctuation in temperature from –18°C to –15°C for a short time resulted in a detectable color change in the thawed product after only 3 more months of storage.