FREEZING INTRODUCTION Ferruh Erdogdu and Ziynet Boz University of Mersin Department of Food Engineering Mersin, Turkey
FREEZING OF FOODS Quality deterioration associated with chemical, bio-chemical and microbiological changes of foods accelerate with increasing temperature. Sufficient cooling provides a significant decrease in enzymatic activities leading to undesirable changes in quality and undesired microbiological growth rate. Based on this, freezing is a multi-purpose and convenient food preservation method to maintain the initial characteristics of food products.
FREEZING OF FOODS In a freezing process, temperature of the food products is reduced, via the removal of sensible and latent heat, below their freezing points (generally -18 C or lower), and they are stored at frozen storage conditions. Frozen storage generally refers to storage at temperatures below -18 C. In today`s world, in addition to the frozen storage, a major floor space is also reserved for the frozen food products in the supermarkets.
FREEZING OF FOODS Long-term preservation of foods by freezing has been used for thousand of years, and it has gained a widespread attention with the development of commercial refrigeration equipments. During freezing, water in the food product, required for the activities of microorganisms and enzymes and chemical and biochemical reactions, is frozen leading to longer storage time and stability of the product itself.
FREEZING OF FOODS Major groups of commercially frozen foods are (Fellows, 1997): Fruits as either whole or pureed (strawberries, raspberries, blackcurrants, etc.), Vegetables (peas, green beans, spinach, etc.), Fish fillets and sea-foods (cod, shrimp, crab meat, etc.),
Meats (beef, lamb, poultry, burgers, etc.), Baked foods (cakes, fruit and meat pies, bread, etc.), and Prepared foods (pizzas, desserts, complete meals, etc.).
FREEZING OF FOODS In a typical freezing process, three distinct zones representing the temperature change of the food product are seen. In the first zone, called pre-cooling or chilling period, temperature of the food product is reduced to its initial freezing point. Under carefully controlled freezing conditions, in the pre-cooling stage, a further decrease in the product temperature below to its initial freezing point, might be seen.
FREEZING OF FOODS The super-cooling occurs before the crystallization initiates the freezing process. This can be explained by the requirement of high activation energy for starting the nucleation (of the crystallization) as described in Nature of Freezing Then, due to the evolution of the latent heat of crystallization, temperature shows an abrupt rise back to the initial freezing point.
FREEZING OF FOODS In the second zone, called phase change period, temperature remains relatively steady as the latent heat (latent heat of fusion for water = 334.9 kj/kg or 80 cal/g) is removed from the product. In the third zone, called sub-cooling or tempering, with the completion of the removal of latent heat, temperature is further reduced to the storage temperature.
FREEZING OF FOODS Temperature ( o C) Freezing Diagram of a Potato 20 15 10 5 0-5 -10-15 -20 I II III 0 1000 2000 3000 4000 5000 6000 Pre-cooling Supercooling Phase change Tempering Time (s)
NATURE OF FREEZING Freezing is a crystallization process where the ice crystals are formed within the food product via the reduction of temperature below the initial freezing point. At this point, free water in the food product crystallizes to form the ice crystals. A typical crystallization process consists of two phases: Initial nucleation Subsequent crystal growth.
NATURE OF FREEZING Nucleation (homogeneous common for pure water and heterogeneous common for food products) is the association of water molecules into a nucleus or seed to serve as an active site for the crystal growth as a function of temperature. Crystal growth is the orderly addition of more water molecules to the initially formed nucleus. Number and size of the ice crystals affect the resulting quality of frozen-thawed product.
NATURE OF FREEZING As a general rule: Slow freezing results in large ice crystal formation in the extracellular locations while a rapid freezing process leads to the formation of numerous smaller sized crystals in both extracellular and intracellular locations. Increase in the size of the crystals might then tend to squeeze the cell structures. Therefore, rapid freezing processes are accepted to lead to the production of frozen food products with higher quality factors upon thawing.
EFFECT OF FREEZING ON MICROORGANISMS Freezing affects the microorganisms by: Converting the avaliable liquid water to ice thus decreasing the water activity, Reducing the temperature to suboptimal growth temperatures, Causing mechanical damage to cell membrane due to the formation of intra and extra-cellular ice crystals, and Denaturating the proteins via the loss of available water that leads to the formation of abnormal inter-protein bonding
PHYSICAL AND CHEMICAL CHANGES DURING A FREEZING PROCESS Freezing, especially the slow rated processes, might cause damages in tissues of the food products due to the growth of ice crystals. These damages might be irreversible and become apparent after thawing and before consumption of the product. Therefore, it is important to know the structural characteristics of the food products to determine the damage associated with the freezing process.
PHYSICAL CHANGES Physical changes of food products during freezing are: Increase and non-uniform volume change mainly due to the expansion of water by freezing (pure water expands 9% when transformed to ice) expansion of areas containing ice crystals and contraction of the remaining part lead to the mechanical damages
PHYSICAL CHANGES Mechanical damages More likely in plant tissues with their semi rigid cell structures, due to the expansion of areas containing ice crystals and contraction of the remaining parts Freeze-cracking Mainly due to the very high freezing rates and very low temperatures as in cryogenic fluids
PHYSICAL CHANGES Moisture migration Existing temperature gradients in the product might lead to moisture migration via water relocation as a consequence of temperature dependence of water vapor pressure. Maintaining the lower temperature fluctuations might prevent this problem.
PHYSICAL CHANGES Freezer burn surface dehydration identified by an opaque and dehydrated surface and produced by moisture losses, as a result of inadequate moisture barrier packaging. Ice re-crystallization increasing in the size of ice crystals as a result of temperature increases in the subfreezing temperatures - ice crystals with their higher surface area to volume ratios and higher surface energies tend to grow in size with temperature
CHEMICAL CHANGES Enzymatic activity decrease in the enzymatic activity at the lowest temperatures as a function of temperature and decrease in the amount of available water. Protein denaturation exposing the proteins to the increased concentration of salts in the unfrozen phase might lead to modify the protein structure.
CHEMICAL CHANGES DURING FREEZING PROCESS Lipid Oxidation Lipid oxidation can be caused by enzymatic (via the effect of lipoxygenase if it is not inactivated prior to freezing) or non-enzymatic pathways during frozen storage conditions
FREEZING METHODS AND EQIPMENTS Natural forced convection freezing Use of stagnant or dynamic cold air Liquid immersion freezing Use of cold liquid generally brine, glycol or sugar solution considerably higher heat transfer coefficients with reduced freezing times
FREEZING METHODS AND EQIPMENTS Cryogenic freezing spraying liquid nitrogen or carbon dioxide onto the surface of food products or immersing the food product into the liquid cryogen Contact freezing achieved by placing food products between the metal plates filled with a refrigerant
FREEZER TYPES Batch air blast freezers: A common form of using the forced convection in an isolated room consisting of fans forcing the air over the evaporator coils and food products Continuous air blast freezers: Use of a moving belt system at a high velocity through the cold air environment Cryogenic liquid freezers: Use of liquid cryogenic liquids, e.g., nitrogen, carbon dioxide, at a very low temperature
FREEZER TYPES Fluidized bed freezers: A continuous process forcing the cold air at a high velocity below the food products to create a fluidized medium Impingement freezers: Use a certain number of nozzles to increase the cold air rate through the food product to increase the heat transfer coefficient and produce a fast and efficient freezing process
FREEZER TYPES Liquid immersion freezers: A batch or continuous process include immersing the food products into a cold liquid bath sugar solutions for fruits and salt solutions for vegetables are generally preferred Plate freezers: Freezing process accomplished between vertical or horizontal plates generally preferred for thin and flat products, e.g. hamburger patties and fish fillets, with short freezing times
BIBLIOGRAPHY Fellows, P.J. 1997. Food Processing Technology. Chapter 19. Woodhead Publishing Limited. Abington, Cambridge, England. Ramaswamy, H. and Marcotte, M. 2006. Food Processing Principles and Applications. Taylor & Francis. Boca Raton, FL. Sun, D-W. 2006. Handbook of Frozen Food Processing and Packaging. Taylor & Francis. Boca Raton, Fl.