Introduction

The main objective of this chapter is to describe the evolution of the Food Freezing Processing Industry, its equipment improvement, and the review of freezing methods, technologies and concepts used in the Frozen Food Industry. The "Emerging Freezing Technologies" chapter is developed in two parts. The first part is a general overview of freezing methods and terminology used in this industry. The second part is focused on the description of new, evolving technology concepts and its applications.

The depletion of the ozone layer in the atmosphere caused by the use of chlorofluorocarbons, CFCs, and the concern for the world's environment, together with the high costs involved in freezing, high energy consumption, finished product quality, consumer convenience and other factors initiated the frozen food industry to investigate new alternatives and opportunities to improve the quality of frozen products, equipment and manufacturing procedures and efficiencies.

The twenty-first century for the Frozen Food Processing Industry will open new opportunities and challenges to the food freezing equipment manufacturers, product development scientists and engineers that will be based on the following points:

• Improved finished product quality and product development

• Environmental concerns/acceptance and safety

• Reduce processing costs

• Technological competitive advantage among food processors

• Maximum equipment efficiency and flexibility

Energy efficient systems, (CEP systems, automated computer control systems, self defrosting, etc.)

• Finished product distribution and storage

• Consumer acceptance

2. FREEZING TECHNOLOGY: AN OVERVIEW

Many factors are involved in the process of freezing food products that determine and affect the finished product quality and the freezing equipment performance. To achieve the desired freezing results, it is necessary to have the entire product mass at the same temperature throughout. This equilibrium condition in which the surface temperature and the center of the food mass are the same is dependent upon the freezing rate, heat transfer coefficient and the amount of heat removed from the food product.

This change is very important and it is directly related to ice crystallization, loss of moisture and microbial growth which becomes the basis for determining the quality of the frozen product. Some of the factors are:

• Freezing methods

• Product ice crystallization

• Freezing rate

• Specific heat

• Heat transfer coefficients

• Moisture losses and content

• Finished food product components and product shape

The freezing process time (dwell) is proportional to the freezing rate, packaging prior to freezing, the freezing method used, incoming product temperature to the freezer, final temperature, food product shape, specific heat, thickness and the food product components. The amount of heat removed and the rate at which it is removed by cooling depends on the chemical composition of each component. Larger amounts of sugars, salt and/or alcohol require longer freezing times and/or lower freezing temperatures. The finished product freezing point is related to the food components' molecular weights, their chemical structure and composition and the food product's low vapor pressure. It is known, that the lower the vapor pressure of sauces, liquid blends or solutions, caused by the dissolved components, the lower the freezing point. (Raoult's second law) (1). Large ice crystal formation and finished product damage could be reduced depending on the type of freezing system used, its efficiency, and freezing residence time. A simple definition of a freezing process is that it consists of lowering the product temperature to -182C (0®F), a generally accepted industry target temperature, at the thermal center of the product, resulting in crystallization of most of the water and some solutes (2).

The freezing rate is defined by the International Institute of Refrigeration as the difference between the product initial and final temperature divided by the freezing time (2). Currently, several numerical mathematical model studies exist on freezing processes for foodstuffs. Freezing time, rate of cooling, heat transfer methods, temperature distribution in regular and/or irregular food products are very important factors used in the design of freezers and freezing processes. Several assumptions are considered when developing a mathematical model that includes irregular shape, chemical composition, heat transfer coefficient and the type of freezing media to be used. In this chapter, no consideration to mathematical modeling for freezer and freezing foodstuffs will be covered. A few references (3, 4) of numerical mathematical modeling are recommended in the bibliography. The freezing methods can be classified depending on their refrigeration medium, processing system (batch or continuous system) and the heat transfer methods that are used. To simplify the description of the freezing methods, we will classify them regardless of their refrigeration medium and heat transfer method, as follows:

a. Mechanical methods b. Cryogenic methods and c. Combination of mechanical and cryogenic methods 2.1. Mechanical Freezing Methods

Worldwide environmental concerns have been raised regarding the use of CFC's products as cooling media in refrigeration systems. Due to the World environmental concerns, the Montreal Protocol and the EPA regulations, the use of chlorofluorocarbons (CFCs) will be banned by the year 2000. Consequently, it will no longer be used for refrigeration. An alternative for most of the mechanical freezers that used CFCs as a refrigerant may be to change to ammonia or to Hydrofluorocarbons (HFCs).

Currently various industries that use CFCs in their refrigeration systems are converting to ammonia. Ammonia is less expensive and its performance coefficient is better than other available refrigerants (5). Examples of the ammonia refrigeration system are water chiller generation, static ice storage system, ice-makers, and brine-chilling system (6). These refrigeration methods currently utilize ammonia, CFC's, air and brine solutions as a coolant medium.

The heat transfer method used in mechanical freezers is convection heat transfer. In freezing food products, a few factors influence the identification of the freezing method and equipment. Some of the factors to consider in choosing the right freezing method and equipment are: product dimensions, shape, specific heat, thickness, freezing rate, packaging and food components' chemical characteristics. These mechanical freezing equipments are produced by several US and International freezer manufacturing companies (7).

2.1.1. Blast Freezer. One of the oldest and most commonly used methods in the food industry to freeze food products is the blast freezer. The blast freezer consists of a insulated room in which high velocity air moves over cooled refrigerated coils and then the cold air is circulated in the room by fans which use convection heat transfer to freeze the product. The refrigeration medium used in this type of equipment can be ammonia or CFC's and the heat transfer method used is convection heat transfer.

Food products and materials can be frozen in batches or in trays. The blast freezing process is slow, with high moisture loss and large ice crystal formation. The freezing temperature generally is about -4(PF (-4(FC). The blast freezer is one of the most flexible methods used to freeze food products and it has been improved by increasing the cold air velocity.

2.1.2. Plate Freezer. Plate freezing equipment was invented by Clarence Birdseye (8). This method is still used in the frozen food industry. Major equipment improvements had occurred in this type of freezing method that have transformed a batch system into a continuous automatic freezing system. The plate freezer system can be classified as single or double plate freezers. Initially, food products were placed on the refrigerated plate and the heat transfer was transmitted from the bottom-up. The heat transfer principle used in this type of freezer is conduction heat transfer. Technology, energy concerns, freezing time and other factors forced the plate freezer manufacturing industry to develop and improve this freezing method.

The double-contact plate freezer sandwiched the food product between the two refrigerated plates, consequently, it accelerated the freezing process. Currently, automatic, continuous plate freezers with improved heat transfer capabilities and reduced energy consumption can be obtained in the freezing equipment market. Another type is the vertical plate freezer (9). The refrigerant medium used is ammonia and the refrigeration temperature could reach -409F, (-40aC)„ This type of freezer is mainly used in freezing flat package of foods, ice cream, whole fish and packaged vegetables.

The finished product moisture losses are minimum because the food product is usually frozen inside of the package. One disadvantage of this method is the layer of air that remain inside the package, which causes a longer freezing time.

2.1.3. Spiral Freezer. This mechanical freezing equipment is one of the most currently used in the freezing industry for large production needs because of its convenience, reduced floor space, flexibility and efficiency. The spiral freezer consists of a continuous moving stainless steel belt in spiral form that carries the food product to different heights within the chamber allowing the product to freeze very fast due to die high air velocity.

The spiral freezer is a versatile and flexible method that is used to freeze a wide variety of food products such as: chicken, fish, meat, pizza, bread-dough, bagels, prepared and processed foods on open trays or packaged containers. The heat transfer method used is convection heat transfer. The refrigeration medium is ammonia and the spiral freezer could be designed and equipped with a horizontal or vertical air flow circulation. Variable speed drive motors provide freezing flexibility and allows the freezer to be used for various food applications that require different dwell times.

Finished product moisture losses for cryogenic spirals could be less than 1% depending on the freezing time (dwell), product components and incoming product temperature, and the design of the spiral freezer. Higher moisture losses above 2% can be expected with mechanical spirals due to warmer operating temperatures.

2.1.4. Air Impingement Blast Freezer. The air impingement freezer is a type of blast-tunnel freezer that blast cold air at higher velocities directly on top and bottom of the foods. The high air velocity applied direcdy on the food breaks the insulating boundary air layer that covers the product, thereby allowing a very fast freezing (10).

This freezing technology is based on the maximization of the surface heat transfer coefficient and the high cold air velocity blasted on the food product. Dwell freezing time is reduced and is dependent upon the food product thickness and its mass surface area for heat convection. Surface moisture losses are low and freezing times are reduced. (This method is energy and cost efficient). This freezing method could freeze food products faster than any other mechanical method. The heat transfer method used is the convection heat transfer. The refrigeration medium is usually ammonia.

The air impingement freezing method is a versatile method used to freeze different types of food products such as: chicken, fish, meat, bread-dough, and compact processed foods which do not have small surface particles or toppings on open trays or packaging containers. Variable speed drive motors allow the freezer to be used for various applications. It is most commonly used for freezing whole or large particles of foods such as: Corn, meat patties, fruits, hamburger, vegetables, sea food, whole chicken and large pieces of meat, etc.

2.1.5. Belt Freezer. The belt freezer is a modification of a tunnel/blast freezer that consists of a long solid stainless steel moving conveyor belt, two large diameter drum wheels move the belt and a few pans are located under the belt. Cold brine solution -40^, (-40®C), is continually sprayed under the belt and the brine solution is then recirculated back to the ammonia refrigeration system. To increase the system efficiency and capacity, cold air or a spray of cryogenic gas could be used to add more refrigeration. The cold air or gas moves at high velocity in the opposite direction of the moving food. The liquid or solid food product is deposited on top of the belt and its is frozen by conduction-convection heat transfer.

The liquid product is frozen from the bottom and top. The frozen product is scraped from the belt by mechanical action, or the frozen product pops-up by itself because of its frozen-britde state, releasing it from the belt surface as the belt begins to turn. The belt freezer system is versatile and cost efficient depending on its application. This system could also be used for solid product such as: meat patties, shrimp, hamburgers, vegetables, pizza and semisolid food products.

2.1.6. Fluidized Bed Freezer. This freezing method uses the air blast concept to individually quick freeze, IQF, food products that are diced, loose or have a consistent shape and size. The system consists of a perforated bed in which cold air at high velocities is circulated and forced by the action of powerful fans through the holes in the perforated bed. Fresh or blanched food product is fed continually into a vibratory conveyor that releases the product into the fluid bed. The high air velocities, perforated bed, and the cold air allow the product to be kept in suspension and freeze the food product rapidly.

The turbulence and the agitation generated help to improve the heat transfer coefficient and reduces the overall freezing time. The suspended product continually moves along the fluidized bed. Convection heat transfer is applied in this freezing method. The production of frozen product varies depending on the air velocity, food thickness, food layer on the perforated fluidized bed, and the food product density. This is one of the best methods to IQF small food particles and vegetables and it is commonly used to freeze peas, corn kernels, small onions, diced food products, and other vegetables. The refrigeration medium is ammonia and the cold air temperature is -402C, (-4(FF), or lower depending on the air velocity and equipment efficiency (7,11).

2.1.7. Immersion Freezing - CFC's. The use of liquid CFC's in the 80's as the freezing media presented greater advantages compared to a blast or mechanical freezer. The heat transfer methods used are conduction heat transfer during the immersion and convection heat transfer in the equilibrium chamber. The heat transfer coefficient for the liquid refrigerant is higher than that for a gas. The CFC's immersion process is not considered a cryogenic process. The residual CFC's in the foods and the destruction of the ozone layer are concerns that have forced the Frozen Industry to reduce its use.

This freezing method may become extinct due to the ban on CFC's. This freezing system was used during the 80's to IQF vegetables, pasta, and other food products. The freezing process consisted of transferring the food product into a liquid CFC's bath. The frozen product is moved forward by action of a stainless steal mesh belt conveyor. To improve the equipment efficiency and freezing rate, liquid CFC's is sprayed on top of the food in the equilibrium freezing chamber with any excess liquid returning to the bath.

2.2. Cryogenic Freezing Methods

The introduction of cryogenic gases in the early 1960's as an alternative to improve freezing processes in the frozen industry was a major product quality and process improvement. Air, liquid nitrogen and carbon dioxide are the most common and safe cryogenic liquids and gases used in the food industry. The physical properties of the cryogenic gases provided an important tool to help the food and non-food Industry to improve their plant automation, versatility, efficiency and manufacturing costs. The application of cryogenic liquid/gases in the frozen industry presents many advantages over the mechanical freezing system that, certainly, have revolutionized the frozen industry and some of the benefits are:

• Reduction in the freezing time,

• Reduction in moisture and flavor losses,

• Reduction in ice crystal formation during the freezing process,

• Low maintenance equipment service,

• Elimination of the ammonia refrigeration system and its maintenance implications,

• Small space required for the frozen equipment,

• Minimum product cell damage,

• Low capital investment, electrical cost and risk factor,

• Quick installation, convenient and portable,

• Flexible and versatile system,

Cryogenic methods allow the food product to be frozen extremely fast, reduce the moisture losses to a minimum and increase the equipment flexibility and efficiency. The low temperatures of the cryogenics gases/liquids allow them to be used for different applications and for different food products.

The cryogenic low temperatures maintain the high quality of the finished product. However, if the freezing process is not properly controlled, the frozen product can be damaged.

The disadvantages of the cryogenic freezing methods are related to the cryogenic gas/liquid costs in the long term, the extended residence time in a liquid Nitrogen immersion bath that may cause product cracking, the need of pressurized tanks to keep the Carbon Dioxide as a liquid. Semisolid, diced and sliced products with high content of water such as sauces, sauce with particles, thin fresh pasta, etc. could become brittle if immersed for long periods of time in the liquid nitrogen. The cryogenic freezing optimization is related to the food product thickness and it is directly proportional to the freezing time, food physico-chemical structure and composition and its heat transfer efficiency. The cryogenic liquid/gas freezing method could be divided in two freezing processes:

2.2.1. Immersion and Spray Freezing. Liquid Nitrogen Immersion. This freezing system consists in allowing the food product to be in direct contact with the liquid nitrogen. The freezing equipment is comprised of a stainless steel tunnel-freezer with an immersion pan in which a conveyor belt travels through the bath. Level controls maintain the liquid nitrogen level. Variable speed drive motors provide the freezing system with a lot of flexibility in attaining different freezing times, temperatures, capacities and product freezing uniformity.

The food product is fed by a shuttle or vibratory conveyor to the freezing tunnel. The food product falls by gravity into the liquid nitrogen and within seconds its surface is hard frozen. To reach frozen temperatures inside die food an equilibrium time is needed. This step is achieved later in the last part of the freezer. Food freezing in liquid nitrogen will generate cold nitrogen gas because of the heat exchange that occur between the boiling liquid nitrogen and the food product. The cold nitrogen gas which is produced is then blasted on the food product while in the equilibrium chamber allowing the food product to continue to be frozen completely.

FAN SPRAY FAN

FAN SPRAY FAN

Figure 1. Schematics of immersion and spray freezing process.

The heat transfer methods used in these systems are conduction and convection heat transfer. Spray manifold systems and fans are located inside the equilibrium chamber to improve the heat transfer and the equipment freezing capability, (see Figure 1)

2.2.2. Spray Freezing. In this freezing method (see Figure 2) nitrogen and carbon dioxide are the most common cryogenic gases used to freeze food products.

The spray freezing system consists of a freezing tunnel that has spray nozzles that dispense the cryogenic gases on top of the food.

Cryogenic Supply

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