FREEZING

INTRODUCTIONFerruh Erdogdu and Ziynet BozUniversity of MersinDepartment of Food EngineeringMersin, Turkey

Quality deterioration associated with chemical,bio-chemical and microbiological changes offoods accelerate with increasing temperature.

Sufficient cooling provides a significant decreasein enzymatic activities leading to undesirablechanges in quality and undesired microbiologicalgrowth rate.

Based on this, freezing is a multi-purpose andconvenient food preservation method to maintainthe initial characteristics of food products.

FREEZING OF FOODS

In a freezing process, temperature of the foodproducts is reduced, via the removal of sensible andlatent heat, below their freezing points (generally -18°C or lower), and they are stored at frozen storageconditions.

Frozen storage generally refers to storage attemperatures below -18 °C.

In today`s world, in addition to the frozen storage, amajor floor space is also reserved for the frozen foodproducts in the supermarkets.

FREEZING OF FOODS

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 FOODSMajor 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 FOODSIn 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

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Freezing Diagram of a Potato

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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 FREEZINGAs 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.

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

EFFECT OF FREEZING ONMICROORGANISMS

PHYSICAL AND CHEMICAL CHANGESDURING 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 CHANGESPhysical 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 productmight lead to moisture migration via waterrelocation – as a consequence of temperaturedependence of water vapor pressure. Maintainingthe lower temperature fluctuations mightprevent 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-crystallizationincreasing 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 lowesttemperatures as a function of temperature anddecrease in the amount of available water.

Protein denaturationexposing the proteins to the increasedconcentration of salts in the unfrozen phasemight 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 freezingUse of cold liquid generally brine, glycol or sugar solution – considerably higher heat transfer coefficients with reduced freezing times

FREEZING METHODS AND EQIPMENTS

Cryogenic freezingspraying liquid nitrogen or carbon dioxide onto the surface of food products or immersing the food product into the liquid cryogen

Contact freezingachieved 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.