Requirements of the Raw Material
Freezing Methods
Freezing Costs
Product Quality Requirements
Selling Prices of the Frozen Product
Losses in the Process
Effect of Freezing Methods on Other Steps
Applications Unique to Freezing of Fish
Freezing Methods of the Future

The Western Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture, Albany, California 94710 produced this report. Copies are available on request.


Clyde L. Rasmussen
Anyone contemplating a new freezing plant has a problem with the type of freezer to install. When homework is carefully done, the most economic system may be, but is not always, the one that freezes water at the lowest cost per pound. Deliberations are akin to making a good stew. Freezing cost is a main ingredient, but not the only one. Other ingredients added include the physical requirements of the product, how it is packaged, its shape and size, and its form product quality requirements, selling price, or what the buyer believes the product and quality are worth to him, losses in processing and post-thaw handling, effects of freezing methods on other steps in the processing, effects of the different steps on freezing methods, and alternate methods for preserving the commodity. Those in the seafood business must add other factors, like whether the fish was previously frozen and by what method.

Anyone with a freezing operation and considering changing the procedures also has a special set of circumstances to evaluate. A word of caution is directed to those who may want to change the primary method of freezing to achieve a more efficient operation and improve quality. The new method, automated and streamlined as it will be, should not be compared with an antiquated system, with all the benefits being credited to the new freezing method. Suppose the old method involves air blasts in a largely hand-truck-and-tray setup. One need not abandon the air-blast method to modernize the product handling part of this population. A comparison of basic freezing methods (such as air blast, plate, drum, liquid, immersion, or spray) must involve only those steps unique to each particular process.

1 Presented at the Food and Agriculture Organization Technical Conference on the Freezing and Irradiation of Fish, Madrid, Spain, September 6, 1967.

2 Industrial Analyst, Western Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture, Albany, Calif 94710.
Application Unique to Freezing of Fish.
Two factors of unique importance to seafood are where the freezing was done and whether the product had been previously frozen. Many tons of fish are frozen aboard ships as they are caught. Most are frozen in bulk to be thawed and reprocessed later. Using a costly instant freezing method for later processing would seem questionable if the fish had previously been frozen by a slower method. The quality differences between the instant and slow processes in the second freezing may be minor compared with differences in freezing.
If fish freezing as fillets and other consumer-type products is done aboard the trawler, the choice of methods is quite restricted. If cryogens are to be used, the ship must have a holding capacity for a weight at least equal to the weight of fish to be frozen while the boat is at sea. An alternative would be to supply the cryogenic to the trawler from a supply ship. If the economics of cryogenic freezing are questionable ashore, it becomes even more doubtful at sea. Quality factors would have to be especially compelling.

Freezing Methods of the Future. Our growing affluence and increasing taste for the better things in life suggest that we might take a cost-be-damned attitude in our desire for top quality. But along with rising incomes and affluence, the industry is experiencing increased pressures from cost squeezes and consumer price strikes. Cost reduction will continue to be an essential, if not dominant, goal in the food business. I think that conventional freezing methods that can be applied at the least cost will continue producing most frozen foods. For specialized products and uses, however, I believe that some special freezing practices will find considerable room for growth, particularly as technology expands and the costs of cryogens decrease.

In reducing the cost of cryogenic materials, I was interested in a recent news item regarding manufacturing a system for making liquid air, apparently right in the plant where it is used. A cost of 11.25 cents a pound was mentioned for a machine producing 300 pounds of liquid air per hour. This is such a logical development that I don't understand why the idea wasn't suggested years ago. Separation of nitrogen from oxygen and transportation and storage of liquid nitrogen must account for a good part of the 2 1/2 to 3 cent cost of liquid nitrogen. Oxygen buildup is dangerous in an immersion-type freezer using liquid air, but in a spray-type unit, the liquid air should present no hazard.
I wonder where the cost curve for producing lower and lower temperatures in a conventional refrigeration system crosses the cost curve for producing liquid air. This relationship, plus the effect of freezing rates on quality, should be thoroughly studied to give us a better basis for selecting freezing methods in the future.

A factor that can nullify all efforts to improve quality and lower costs of frozen foods is poor handling in distribution and storage. It matters little how food is frozen initially if the product is allowed to thaw in an overloaded display case or on a loading platform. Much progress has been made in handling practices recently, but there is room for improvement. As we look for freezing methods of the future to give us better foods at reasonable costs, let us also be sure that our distribution system is geared to do as well.
I appreciate the help of the following individuals who provided cost data and information valuable to my analysis.

George L. Andrews, Amerio Contact Plate Freezers, Inc., Cliffside Park, N. J.

Walter H. Berg, Jr., A. F. Wentworth & Associates, Inc. Tampa, Fla

Howard Cheifetz, Pure Carbonic Co., Berkeley, Calif

Reese Lamb, Lamb-Weston, Inc., Portland, Oreg.

M. R. Overbye, Lewis Refrigeration Co., Woodinville, Wash.

Gilbert P owners, Norbest Turkey Growers Assoc., South Pasadena, Calif

A. W. Ruff, St. Onge, Ruff and Associates, Inc., York, Pa.

Thomas Woodson, J. W. Greer Co., Wilmington, Mass.

Trade names and commercial company names are used in this publication solely to provide specific information. Reference to a company or product name does not imply approval or recommendation of the product by the U.S. Department of Agriculture, excluding others that may be suitable.

Freezing Methods
Requirements of the Raw Material
Small, uniformly sized, free-flowing, unpackaged materials lend themselves to the widest range of freezing conditions. Thus, berries, green peas, cut corn, cross-cut green beans, Lima beans, and meat chunks can be frozen in bulk or in a box by several methods. Only one or two methods can freeze some products because of their shape and size and packaging requirements. Cauliflower, broccoli, and asparagus must be packaged first, so freezing is ordinarily limited to the plate and air-blast methods. Meat patties and raw shrimp are frozen by a method that permits them to remain stationary on a belt or tray or in a package. Packaged items could be frozen with cryogen, but the potential of instant freezing would be minimized because of the insulating properties of the box and wrapper.

To avoid becoming involved in all the intricacies of the many possible methods and procedures, this comparison is limited to only a few products that best illustrate economic differences among basic freezing methods. Thus, we shall attempt to stress principles, not just differences, in application to freezing products for the retail and institutional markets.

Freezing Methods
Making ice is probably the least costly example of commercial freezing and can thus serve as a base. Refrigeration is supplied by the least expensive method, and heat transfer is liquid to liquid through a metal container holding the water. Thermal losses are minor, and no cost is involved in moving air. Because rapid freezing is not essential, the least costly combination of CIL temperatures, compressor capacities, and brine circulation can be employed. Ice may sell at $6 a ton wholesale, or 0.3 cents per pound. The actual freezing cost is only a part of this; perhaps half

Plate freezing
has the similar advantage of heat transfer by conduction. However, food products conduct heat more slowly than water, most plate-frozen foods are in containers, and the plate freezer is more costly to build, install, and operate. Consequently, plate freezing is considerably more expensive than our reference process - water freezing. Labor costs are potentially high but can be reduced by automatic loading and unloading. Plate freezing is more limited in what it can handle than air-blast freezing, but new developments are widening its application.

Air-blast freezing
has several advantages. It can be applied to bulk or package freezing, is adaptable to most products, and operates under various conditions such as product handling, air temperature and flow rate, and fluidizing techniques. Labor costs can be pretty low depending on the product and its requirements. The main disadvantages are the longer time required if the product is packaged first and evaporation losses if the product is frozen in bulk. The latter can be reduced considerably under proper operating conditions.
A relatively new type of unit is the rotary freezer.
The rotary freezer is akin to a continuous plate freezer. In one version, the cylinder is a double-walled drum, with the refrigerant circulating between the walls. The product rides outside the cylinder, and refrigerant-to-metal-to-product contact is maintained. One manufacturer claims a refrigeration requirement of less than air-blast freezing and only a one-tenth-floor space area. Evaporation is, of course, virtually eliminated, and maintenance is minimal. Losses from the product sticking to the drum or from breaking when it is removed are claimed to be nil. This unit is not as flexible as air-blast freezing because it is limited to uniform, flat, or round materials easily spread on the belt feeder. It does, however, provide a method for essentially plate-freezing unpackaged foods. It is claimed that a 1-inch thick steak can be frozen in 20 minutes and small shrimp in 4 minutes.
Immersion and spray freezing
It can be done with liquids, including brine or alcohol, such as propylene glycol. The product must be perfectly protected. Turkeys are usually first wrapped in a plastic film. Contact with the liquid freezing medium gives fairly rapid freezing.

Cryogenic media include liquid nitrogen, liquid air, and liquid or solid carbon dioxide. Of these, liquid nitrogen has had the broadest commercial development. The extraordinarily rapid or instant freezing, which is possible by immersion or spray freezing, gives most products a quality closest to fresh. Evaporation is minimized, but the cost of cryogens is probably high. Installation and maintenance costs are relatively low.

A new method currently under development uses dichlorodifluoromethane (Freon 12) as the medium. This costly refrigerant is recycled in a closed system, and the refrigerant that cannot be recovered is a significant cost. Rapid freezing and minimum evaporation are major features. The U.S. Food and Drug Administration approved the use of Freon 12 for freezing foods on August 31, 1967.

Suppliers of various systems also claim other advantages, some real, others possibly doubtful. We cannot consider all of them. Each must be judged based on applicable facts.
Freezing Costs
Cost data were obtained from several commercial sources (see Acknowledgments at the end of the paper). To compare, all costs should be calculated based on equivalent assumptions. This was not possible because of the multiple sources used. I believe, however, that despite this reservation, the cost differences are valid. More extensive calculations would not change the primary data, but only refine them.

In the brief survey, I found freezing costs ranging from two-tenths of a cent to ten cents per pound of product for frozen common foods. This extreme difference represents radically different freezing methods and products and accounting methods. The lowest cost figures are generally for air-blast freezing of unpackaged materials for turkeys, usually tightly wrapped in plastic film. Typical costs are a half-cent a pound or less where volume is large, the season is long, and operation is efficient. Plate freezing of packages was found to be around half-cent a pound, with the automatic operation less costly than manual. Freezing costs over a cent a pound were found only in operations involving low capacity, unusual labor requirements, specialized products, or a high-cost freezing medium.

Comparative cost data supplied by one firm.
A summary of cost data supplied by one engineering firm. I adjusted the figures to approximately equal annual outputs. Costs are shown for freezing a vegetable such as peas at four thousand to five thousand pounds an hour in packages in plate freezers and in bulk on belts and trays.

Fluidized with a product value of about 8 cents a pound, losses would amount to 0.08 cent a pound for each I percent lost. If the loss were as high as 5 percent, the cost per pound would be 0.4 cent, about equal to freezing costs. For bulk freezing, the main cause of loss would be evaporation of water from the product. For freezing in the package, product loss would be from broken boxes and product spillage.

Plate freezing of freezing in bulk on belts in an air blast costs one-half cent a pound or less. The fluidizing technique apparently effects an important reduction in labor costs and also makes possible a lower evaporation loss. Freezing time is also much shorter in the fluidized systems. The additional investment cost in automatic plate freezing is more than offset by a saving in labor, as compared with the costs of the manual method. The automatic method is nearly two-tenths of a cent less per pound.

The costs of plate and air-blast freezing as shown are so close, there is little to choose between them on a cost basis only. These costs are doubtlessly lower than those experienced by most plants. The estimates assume less lost time than might be possible in most installations because of the many labor, raw-material, and operating problems a plant must experience.
Approximate freezing times
Table 1. Comparison of freezing times for small fruits and vegetables
Package freezing -- (10-oz. containers):
* air blast>>3 to 5 hr.
* Plate>>1/2 to 2 hr.
Bulk freezing -- air blast.
* Belt>>20 to 30 min.
* Fluidized belt or tray>>5 to 10 min.
* Cryogenic freezing>>1/2 to I minute.

Comparative costs supplied by an equipment manufacturer
Figure 2. Freezing fresh vegetables
For a general comparison, freezing times for various methods are roughly as shown in table I for small fruits and vegetables. Experience at any one plant may be significantly different, depending upon the circumstances involved.
Figure 2 shows freezing costs for breaded turkey pieces as supplied by a company that makes both air-blast and liquid-nitrogen (LN) freezers. Pertinent assumptions in the estimate are as follows* Product -- cut-up breaded 4 oz. portions.
* Output -- 2,500 pounds (1.13 ton) an hour; 5,000,000 pounds a year.
* Losses assumed -- 2 percent in air-blast freezing; I percent in LN freezing.
* Liquid nitrogen costs 2 1/2 cents a pound; 1.12 pounds required per pound of production.

Table 2.
Cost comparison of air-blast and liquid-nitrogen freezing
Air-blast freezing Liquid-nitrogen
freezing total cost of equipment installed $131,400
$59,000Annual Cost:
Operation and maintenance..................8,240.................. 1,200
Liquid nitrogen...................................... 50,000
Fixed costs (17% of equipment cost)......22,300................ 10,000
Product losses (at 46 cents a pound)......46,000................23.000
Total: 76.540 184,200
Cost per pound $0.0153 $0.0368Table 3.
Comparison of freezing costs for the first year
The difference in cost between these two methods is not minor -- it is 2.15 cents a pound, 140 percent more for LN than for air-blast freezing. To answer the argument that liquid-nitrogen freezing permits a much smaller initial outlay of cash (as needed for the plant and equipment), let us consider this fact: liquid nitrogen costs for the first year alone, a cost also to be reckoned in the initial capitalization, is more than the difference in plant costs plus difference in evaporation losses, as shown in table 3.
Table 3
Comparison of freezing costs in the first year
Air-blast freezing...................................... Liquid-nitrogen freezing. Plant investment ......................$131,400............... $59,000
Evaporation losses......................46,000..................23,000
Total: ..........................................177,400..................82,000
Liquid-nitrogen cost .................................................150,000

Comparative costs of freezing strawberries (IQF)
Figure 3 Compares costs of freezing IQF (individually quick frozen) strawberries with LN and with air blast in a Fluidized bed freezer. This comparison assumes LN at 3 cents a pound, used at a ratio of 1.5 pounds LN to I pound of strawberries. Despite the greater cost of operating the air-blast freezer, the only factors of significance in this comparison are the cost of liquid nitrogen and the evaporation losses in the air blast operation. If freezing costs and evaporation losses were the only two factors considered, the losses would have to be over 20 percent with strawberries at 20 cents a pound for the total costs to be equal
If I can conclude anything at this point about costs, it might be this: the differences in costs among the usual methods of freezing (air-blast bulk or package and plate) are likely to be minor. The decision on which method to use will probably be determined by other factors more important in the overall economic picture. If, however, we consider a radically different freezing method, such as the use of cryogenic materials, cost differences become large and important. Other factors that may overrule freezing cost considerations must indeed be important, or unavoidable, or both.

Product Quality Requirements. The profit motive requires that the least costly method of doing a satisfactory job must be used. But what is satisfactory or best varies by products. Some are easy to freeze because the rate of heat removal is not critical. Other products lose much quality by slow freezing.

An example of the former might be sliced strawberries mixed with sugar. Many in the industry feel that a fast freeze is not even wanted. Apparently, the main quality requirement is to avoid microbial growth. An example of a product suffering from slow freezing is tomatoes. The problem of freezing tomatoes is so great that so far they are not frozen commercially. Most other products lie between such extremes. Among the products subject to texture damage by excessively slow freezing are green beans, asparagus, melons, and whole strawberries. These have high moisture content and lack the physical structure to withstand freezing damage.

Milford S. Brown has reported studies on green beans, Q. Sci. Food AGR. 18: 7781, 1967)
The instantly frozen piece closely resembles the structure of a fresh bean, having little cellular damage. At the other extreme is a bean frozen slowly. Cell breakdown is extensive and large cavities develop between the cells. When cooked, beans such as this have a poor texture, in contrast to the instantly frozen beans which are more like the fresh.

If I can conclude anything at this point about costs, it might be this: the differences in costs among the usual methods of freezing (air-blast bulk or package and plate) are likely to be minor. The decision on which method to use will probably be determined by other factors more important in the overall economic picture. If, however, we consider a radically different freezing method, such as the use of cryogenic materials, cost differences become large and important. Other factors that may overrule freezing cost considerations must indeed be important, or unavoidable, or both.

Between these extremes is the bean frozen in a matter of minutes, 5 or 6 at the most. The physical condition is greatly improved over the slowly frozen one. When cooked, this bean is hardly distinguishable from the bean frozen instantly.

Freezing of fish and meat causes changes in product quality and the ability of the flesh to hold water. Thus, these products may drip upon thawing. The faster they freeze, the less, apparently, is the drip loss. The seriousness of drip depends partially upon how the product is handled during thawing.

Have no photomicrographs of fish similar to those I've shown for vegetables. But I imagine that such photographs would show some differences in cellular structure as a result of various speeds of freezing. Whether such differences are important to the ultimate user, however, is a key question.

Freezing with LN or other cryogenic materials produces a quality that is probably closer to fresh quality for most products than any other process now being used commercially, so far as physical appearance is concerned. The photomicrographs prove this statement for certain vegetables, at least. But application of this fact is subject to two qualifications:

1. For many, and perhaps most products, the physical difference between instantly and the quickly frozen is not so apparent, and in some cases is virtually nonexistent,
2. One may not need to go to instant freezing to obtain all the quality improvement practical in commercial operation. Sensory appraisals at the U.S. Department of Agriculture's Western Utilization Research and Development Division, have demonstrated that green beans frozen quickly in an icy air stream have a quality that is hardly distinguishable from LN frozen beans. At the present stage in our research program, it would appear that vegetables such as beans, asparagus, and peas frozen in 4 or 5 minutes may be good enough for even the most exacting demand.
Very little is generally known about how fast freezing must really be to obtain a desired quality. We have used cryogens because of their availability and because they do an excellent freezing job. But they maybe too costly, especially so if a less costly method can do an equally satisfactory job.

In fast freezing, it is the rate of heat removal that is important, not the temperature of the freezing medium. Extremely rapid freezing takes place in brine or liquid dichlorodifluoromethane (Freon 12) at minus 3 5 degree C (minus 3 1 degree F). The rate approaches that of freezing with LN or solid C02. Extremely high air velocity in air-blast freezing coupled with fluidization of the product can also greatly speed the rate of freezing. Coil temperature below those now accepted as the limit of economical operation might prove feasible. If air at minus 30' to 40 degrees C. freezes too slowly but air at minus 600, 700, or even 80 degrees C. would do the job. It might be less costly to provide air at these temperatures than to jump all the way to minus 196 degree C. (minus 325 degree F) at a cost of 3 cents -a pound. If the cost of air-blast freezing is now 0.3 or 0.4 cent a pound, we could double or triple that cost and still be way below the more costly methods.
I note that some new freezer developments in both Europe and the United States are using temperatures as low as minus 60 degree C. in an air-blast system and minus 65 degree C. in a drum system. In this fascinating and important matter of applying new freezing techniques; we may well find that the cliché that coil temperatures below minus 40 degree C are uneconomical is no longer true when the alternatives of instant freezing are considered.

Selling Prices of the Frozen Product.
This factor has two sides: (1) the relative value of the product, the higher the value, the more that can logically be spent on processing and (2) the amount of improvement the buyer is willing to pay for.

Most frozen fruits and vegetables sell for 5 to 10 cents a pound. High-cost freezing methods may be difficult to justify unless there is no alternative to get the desired quality. On the other hand, an improvement in quality might be easy to justify in fish and meat products, many of which are worth a dollar or more a pound, even though the process adds several cents of cost.

The amount of improvement a buyer is willing to pay for involves both real and imagined benefits. For example, the color of frozen turkeys within a fairly wide range has no apparent effect on product quality. Yet because of consumer preference, a bird with a certain color may be sold at a premium price. A somewhat more costly process to produce this color can be easily justified.

Someone defined quality as that for which a customer will pay more because of its presence. The key to that statement in the economic world concerns the customers' willingness to pay. To produce a better quality at a higher cost is futile unless the customer will pay that additional cost. Too often, however, buyers are not willing to pay for as good quality as processors can deliver. This may not even involve instant freezing. It may just be case-freezing to save a few cents a case instead of freezing in individual cartons or in bulk. In a market where price is an excessively dominant factor, little encouragement exists to improve the product.  About all the processor can reasonably do is to produce the best product the buyer is willing to pay for.

Losses in the Process.
Losses in freezing occur from evaporation and from actual loss of pieces dropping from conveyors and becoming enmeshed in or stuck to belts, conveyors, elevators, or other machinery. Freezing in packages should avoid most of the losses. I doubt that these losses, percentage wise, are huge in a well-run operation.

Evaporation losses have loomed large in the battle between cryogenic and air-blast freezing of products in bulk. Just how important these losses are seems to depend, in the propaganda stage, on what one is selling. But let us try to remain objective and see what the facts are.

Evaporation losses, according to one equipment manufacturer, are largely
determined by two factors:
(I) difference in vapor pressure between the product being frozen and the freezing medium, and
(2) length of time the product is subjected to the freezing medium.
After the product is frozen, its vapor pressure drops radically. In fact, this will occur after only the surface is frozen. The vapor pressure we are concerned with is that before freezing. It becomes obvious, that the colder the product is before it enters the freezing zone, the lower will be the evaporation losses. Pre-cooling in a moist environment is imperative to minimize evaporation losses.

The fact that the vapor pressure of virtually all freezing media is practically zero tends to wipe out an important difference among methods in this respect. What is left, then, is exposure time, which must certainly be the most critical factor in evaporation losses. This highlights the importance of fluidization in air-blast freezing so that all surfaces are equally exposed to freezing temperatures and quickly surface frozen. This might make a case for using cryogenics for surface freezing and finishing the job in air blast.

Whether a more costly freezing system should be used to avoid evaporation losses depends on the value of the product and the amount of loss. If a 10 percent evaporation loss is experienced on a 10cent-a-pound product, up to one cent a pound can be spent on a system to avoid the loss. A similar amount can be spent to avoid only a 1one percent lost on a dollar a pound product. Figure 6 shows the amounts that can be spent to avoid evaporation losses in products of various values. Only extremely low evaporation losses can be tolerated in high-value products. At the other end of the scale, substantial losses can be tolerated on low-value products before a more costly freezing system can be justified, as long as quality is not impaired!

Added costs that can be justified to eliminate evaporation losses.

While we might tend to use this chart for judging only cryogenic applications, we must remember that any bulk-freezing system which reduces exposure time will reduce evaporation losses.

Drip losses after thawing are also important in the overall economics. If a freezing system is available that will eliminate or reduce drip losses, it can be evaluated in figure 6 in the same manner as evaporation losses. The glazing of fish fillets seems to offer one way of minimizing evaporation losses. However, textural and other quality changes caused by a poor freezing method cannot be so modified.

Evaporation losses are critical to the processor as they affect the quantity of product he sells. But there is no loss of nutrients. The buyer gets all of them. Drip losses are of no immediate concern to the processor. They are essential, however, to the user. Only after the product is thawed do they appear. The method of thawing and cooking may help control drip losses, especially if cooking of a fish or meat product is begun before it thaws. If we look at these problems as a whole, without concern at any particular midpoint, what is important is the amount and quality of food, after preparation by the user, that is actually served.

Effect of Freezing Methods on Other Steps. Freezing in bulk, in contrast to freezing in packers, permits substantial changes in packaging and warehousing procedures. The essential feature is that packaging and labeling can be put off until the sales picture for the year becomes better defined.
Frozen peas, cut corn, green and Lima beans, berries, cherries, diced fruit and vegetables, and other small uniformly sized products are stored in large bins with plastic liners. Packaging and labeling are done as needed. Double handling into and out of storage is involved, but the savings in other respects probably more than offset it. Dehydration loss in storage is claimed to be nil.

Recently, a new storage method has developed. IQF fruits and vegetables are stored in bulk in large refrigerated silo-type rooms. Products moved pneumatically in and out. Claims made that the storage capacity of such rooms is double that of conventional methods and less refrigeration is needed. Dehydration is at a minimum because no air circulates in the rooms. The savings for this type of storage are presumably substantial.