Learn how using freezer types that complement each other can help you maximize profits.

To minimize startup costs, new frozen food products can be processed cryogenically.


A typical cryogenic tunnel freezer is a simple countercurrent heat exchanger that uses liquid nitrogen or liquid carbon dioxide as the cooling medium. The gas exhaust temperature generally is -40°F (-40°C).
Food manufacturers today are locked in a battle to introduce new food types and make money from them before the competition catches up. Although a steady base market exists for staples such as burgers, fish and chicken fingers, today's consumers are buying new and different foods as fast as the manufacturers can bring them to market.

There also are food fashions over which manufacturers have little or no control. For example, the green movement helps to promote vegetarianism. The slimming and health lobby encourages consumption of white meat and fish at the expense of red meat.

Not all new products succeed, and those that do may be rapidly overtaken by others. In a business climate of frequent product launches and uncertain rewards, it may make sense to hold down capital costs - even if it means higher operating costs - until a new product has proven successful. In these situations, food manufacturers may want to consider a cryogenic freezer. Capable of processing many different food types with only minor adjustments, a cryogenic freezer has a lower startup cost and shorter on-stream capabilities than a mechanical freezer.

Once a food product has established itself, the food processor can reduce operating costs by switching to mechanical refrigeration. This combined freezing approach allows low startup costs when risks are high and capital is at a premium and low production costs for established products.

To minimize startup costs, new frozen food products can be processed cryogenically. Once a product has been established, the manufacturer can reduce operating costs by switching to mechanical refrigeration.

Putting the Approach to Work

One of the first cryogenic applications to produce a qualitatively different result was individually quick frozen (IQF) products such as shrimp. In the 1980s, IQF products typically were frozen by immersing them in liquid chlorofluorocarbon (CFC) refrigerants such as R12. When these refrigerants were legislated out of usage for this application, alternatives were needed.

To fill the gap, industrial gas companies developed freezers based on liquid nitrogen and carbon dioxide. Operating at lower liquid temperatures than were possible with CFCs, cryogenic freezers provide benefits in quality and throughput.

A typical cryogenic freezing tunnel uses liquid nitrogen at -320°F (-195.6°C) or carbon dioxide at -90°F (-67.8°C) as an expendable refrigerant. While differences in the thermodynamic properties of each gas influence tunnel design, the basic design principle is a simple countercurrent or cocurrent heat exchanger. Food is transported through an insulated tunnel on a conveyor belt. Liquid cryogen is introduced into the tunnel by sprays; it extracts heat from the food and, in doing so, is vaporized. The vapor then is exhausted to the atmosphere.

High latent heat of vaporization and large temperature differences between the cryogen and food product ensure rapid freezing even in compact equipment. A cryogenic freezer's refrigeration capacity is typically four times greater than that available from a similarly sized mechanical system. A cryogenic freezer typically costs one-third the cost of a mechanical freezer of equivalent capacity. Also, it often is possible to rent cryogenic freezers, further reducing capital outlays.

Other benefits of cryogenic freezing include its compact size and high production rate. Many food producers are constrained by limitations in plant floor space. The compact size of cryogenic freezers allows smaller production lines. High productivity can be expressed in terms of short residence time. Fast freezing and high throughput result in less product sitting around the factory waiting to be frozen and less unproductive time spent loading and unloading freezers.

In addition, rapid freezing can improve product quality. Because cryogenic systems freeze food products quickly, the ice crystals formed in the food remain small. As a result, the appearance, texture and flavor of the frozen food remain close to that of the original product. Fast freezing also helps avoid cell damage, which reduces drip loss when the product is thawed.

Cryogenic systems also minimize product dehydration during freezing. Most foods are sold by weight. Dehydration reduces product weight, translating to reduced profit margin. With conventional mechanical freezing, the extent of dehydration can be as high as 6 to 8%, most prevalently with hot-cooked products. In a cryogenic freezer, throughput is faster than in a mechanical freezer, so there is less time for moisture to be lost. Dehydration loss in cryogenic systems generally is less than 1%. Water retention also can lead to better end-product quality.

Seasonal availability and periodic marketing campaigns create large fluctuations in the quantities and types of food to be frozen. Cryogenic freezers have flexible turn up and turndown. As long as liquid cryogen is available in the storage tank, throughput can be increased or reduced with little effect on product quality. This flexibility allows engineers to compensate for changes in temperature, humidity and product type.

People's eating habits are changing rapidly, and the frozen food industry must be able to adapt to the ever-changing demands for new product. Mechanical and cryogenic freezers can be used to complement each other to meet consumer demands efficiently and economically.

Zero adhesion technology allows production of highly detailed, 3-D ice cream products because there is no melting or impact damage.

SIDEBAR:

Achieve Zero Adhesion Cryogenic cooling eliminates ice adhesion and allows detailed 3-D ice cream production.

How is a frozen product such as ice cream removed from a mold? The traditional answers are to hit it or heat it. Impact removal is noisy, not always successful and can damage the product. A more common method is to warm the mold slightly, but this also has disadvantages. As it thaws, the product's outer surface loses detail and edge definition. In addition, energy is wasted, both in warming the mold and cooling it again for the next batch. Further, the product melted during the removal process is hard to recover and may become a waste stream that requires treatment before disposal.

Often, it is assumed that adhesion between the mold and the product increases as the temperature falls. At temperatures down to -40°F (-40°C), generally this is true. However, at temperatures around -100°F (-73.3°C), adhesion no longer occurs (figure 1).

Zero adhesion technology is an ice cream production technique that involves cryogenically supercooling the mold instead of heating it. When the mold is cooled to a temperature of at least -112°F (-80°C) prior to filling, the temperature difference between the mold and the product eliminates adhesion and allows product removal. No warming is required, so the product does not melt. Removing the product from the mold causes no damage or loss of detail, and elimination of the thawing step results in the production line requiring fewer molds.

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