Cold, hard facts

Freezing food has many advantages over other means of preservation, such as thermal processing, because it can provide better organoleptic quality and somewhat better retention of nutrients in the finished product. In addition, most food spoilage organisms cannot grow at frozen food storage temperatures and a reduction in their numbers may actually occur. To take advantage of these properties and to optimize quality, product designers should be aware of some freezing fundamentals.

Principles of freezing

One of the desired effects of freezing is that water is made unavailable for the growth of microorganisms by being in the form of ice. With the exception of water and fat, very little happens to the components of food as it is frozen. When water freezes, however, it expands by 9% in volume while forming ice crystals that vary in size depending on the rate of freezing (i.e., slow freezing gives large crystals, fast freezing smaller crystals). In products such as ice cream, lactose also may tend to crystallize out of solution. If such crystals are too large, they may damage the structure of cell walls which, upon thawing, causes juice loss, nutrient loss and unacceptable appearance.

The addition of a soluble component to water results in a depression of the freezing point. Whereas water alone will freeze at 0 degrees C (32 degrees Fahrenheit), the addition of a mole (molecular weight of a compound expressed in grams) of a substance will depress the freezing point of the solution by 1.86 degrees C under ideal conditions. A food can be thought of as a multi-component solution of various sugars, salts, carbohydrates, proteins, fibers, etc. in water.

Foods are complex systems containing many dissolved components and thus behave quite differently than pure water when frozen. As the temperature of a two-component system drops, the number of ice crystals increases while the concentration of the dissolved component, the solute, also increases. In the case of a food, as the concentration of various solutes increases, the system becomes more reactive. Some typical reactions include salt catalyzed oxidation and, in the case of meat products, oxidation catalyzed myoglobin pigment.

As the temperature drops further, the food reaches a point at which no unfrozen solution exists. This is called the eutectic point. For a sugar and water solution, the eutectic point is -9.5 degrees C.

Being complex systems, most foods have much lower eutectic points that often cannot be achieved with commercial freezing. For instance, ice cream has a eutectic point of -55 degrees C, meat -50 degrees to 60 degrees C, and bread -70 degrees C. At commercial freezing temperatures, a fraction of the water contained in foods remains unfrozen. For instance, at -200 degrees C, the percentage of ice in lamb is 88%, in fish 91% and in egg albumin 93%.

Because the eutectic point is limited as a tool for determining freezing effectiveness, many researchers look to the glass transition temperature, (Tg) which is the temperature at which a food undergoes a transition from the rubbery to the glassy state. When a food product passes from the rubbery state to the glassy state, the temperature is low enough so that the material between the ice crystals is extremely viscous and reactive substances cannot diffuse into the system. Consequently, most fast reactions stop at this point making the glassy state the point of greatest storage stability for a frozen product.

On the surface, the solution seems simple: a product's unique Tg can be determined analytically and product kept below that temperature to maintain optimum stability. A product's Tg, however, can be impractical to maintain. Solutions of sugars in water, for example, have Tg below -30 degrees C (-22 degrees Fahrenheit). Products that have a high sugar content will have a lower Tg than those with lower sugar content. However, adding a compatible copolymer with a higher Tg – such as maltodextrin – can raise the Tg of the mixture into the region of commercial frozen storage. At the very least, the Tg can be elevated enough to improve frozen storage.

Time and temperature

While freezing occurs, heat is conducted from the interior of a food to its surface where it is removed by the freezing medium. The rate of heat transfer is influenced by many factors such as: the thermal conductivity of the food, the surface area of food available for heat transfer, the distance that the heat must travel (thickness), the temperature difference between the food and the freezing medium, the insulating effect of air surrounding the food and the presence of packaging material.

Not only is the heat transfer rate variable, calculating the freezing time is further complicated by differences in initial temperature of the food; differences in size and shape of individual pieces; differences in freezing point and the rate of ice crystal formation within various regions of the same piece of food; and changes in density, thermal conductivity, specific heat and thermal diffusity that occur as the temperature is reduced.

As water changes to ice, it releases approximately 80 calories per gram of latent heat which must be removed. A formula developed by Plank is often used to calculate freezing times for food products. (See sidebar.)

Sub-zero storage effects

The quality of a frozen food depends on the treatment it receives prior to freezing, how it is frozen, subsequent frozen storage and thawing conditions.

A few fruits -- including apples, pears, peaches and apricots -- discolor because of oxidative enzymes such as polyphenol oxidase. For these and for certain vegetables, blanching is required. Blanching refers to the application of heat in order to clean, destroy enzyme systems, fix color, reduce microbial population and expel gases from tissue. If enzyme systems are not destroyed, flavor and texture deteriorate during long term storage. Bell peppers, cucumbers, herbs, Jerusalem artichokes, rutabagas and turnips do not require blanching if held at -18 degrees C or lower.

Three enzymes whose activities are often measured to determine the effectiveness of the blanching process are peroxidase, catalase and lipoxygenase. Peroxidase is commonly measured for vegetables because it is very heat stable and if its activity is destroyed, other enzymes will be inactivated as well. Lipoxygenase activity has been found to be an effective indicator for blanching of green beans.

A number of physical changes occur during frozen storage of foods. Among these are phenomena involving growth in the average size of ice crystals mostly due to temperature fluctuations during storage.

A phenomenon seen in ice cream and other frozen foods is known as Ostwald ripening. This is a recrystallization that results in the growth of large ice crystals at the expense of smaller ice crystals. As temperature rises, slight melting occurs, and when it drops again, the larger crystals capture the water as the solid phase is recreated.

During such temperature fluctuations, an accretion of crystals also may occur whereby crystals of ice in contact with one another meld together and form larger crystals. The effect is most noticeable when a large number of small crystals are in contact with each other.

Moisture migration also may be a problem during storage of frozen foods. Temperature gradients or differences will exist in a product due to temperature fluctuations. Water vapor pressure will be higher at higher temperatures than at lower temperatures, and moisture will relocate to the colder area(s) particularly at the surface or when there is a space or void. For this reason, moisture often will accumulate on the product surface. If, and when the temperature gradient reverses, the moisture will not migrate back to its original location.

This same mechanism is responsible for the "freezer burn" that can occur when frozen foods are poorly wrapped. Here, moisture migrates through the packaging material and disappears through sublimation leaving the product dried out.

Many of the physical changes described above can be minimized by storing product at temperatures below -18 degrees C.

Other changes that can occur in frozen foods are precipitation of solute from the unfrozen phase due to supersaturation, protein insolubilization due to cross-linking, polymer aggregation, lipid oxidation and pigment changes caused by oxidation or hydrolysis.

Vitamin losses during frozen storage generally parallel other undesirable changes, although considerable losses can occur during blanching -- particularly for Vitamin C. Considerable quantities of Vitamin E also may be lost at freezing temperatures.

During storage, chlorophyll can change to pheophytine which adversely affects the appearance of frozen peas and other vegetables.

Unmodified hydrated starch is affected by freezing and subsequent thawing. Gelatinized starch has a transition to the glassy state just below the freezing point. Thus a starch containing system would be expected to have a fairly high glass transition temperature and be fairly stable during frozen storage. Under low temperature and frozen conditions, however, a paste containing waxy maize (branched chain) starch will become cloudy and chunky and will weep like a paste made with regular corn starch. This is because the reduced kinetic motion at low temperatures prevents the outer branches of the waxy starch from forming hydrogen bonds.

Also, the concentration effect of freezing produces stable cross linkages which prevent rehydration of the starch on thawing. To produce a stable sauce or pie filling, it is necessary to chemically modify the starch by adding blockers such as acetyl or hydroxypropyl groups. The result of this treatment is a stabilized starch that will withstand several cycles of freeze-thaw before syneresis occurs. Proper cooking also is essential for starch stability.

Similar freezing effects occur with other colloids. A sol formed with locust bean gum will form a gel if frozen and thawed. Other products whose consistency depends on hydrocolloid content -- such as pectin in orange juice -- may lose consistency on subsequent freezing and thawing.

Bakery products offer special problems because of accelerated staling and moisture loss.
Staling rate increases as temperature decreases until the aqueous phase is frozen and starch can no longer crystallize. In order to prevent staling it is necessary to bring the product through the temperature zone of +10 degrees C (50 degrees Fahrenheit) to -5 degrees C as rapidly as possible during the freezing process itself.

Getting chilly

Broadly speaking, methods of freezing may be defined as either mechanical or cryogenic. Closed mechanical systems require a compressor, a condenser, an expansion valve and an evaporator. Cryogenic systems are open and use either liquid nitrogen, carbon dioxide or ambient air.

Further definitions of equipment under the mechanical category include "Sharp" type freezers which employ little or no air circulation and are usually used for storage rather than initial freezing although they may be used for freezing quarters of beef, butter or fish. Blast freezing involves moving cold air through the freezing area at a velocity of 100 to 400 meters/minute and is often used for institutional food service operations when complete meals are prepared for later use and delivery to other sites from a central kitchen.

A refrigerant commonly used for mechanical freezing systems is ammonia. Freon -12 is a fully halogenated chloroflurocarbon (CFC) and is being phased out of use due to its effect on the ozone layer. The Montreal international agreement of 1987 calls for cessation in use of CFCs by 1995. Substitutes such as CHClF2 and CF3CH2F do not appear to be as satisfactory.

Mechanical freezing has a great operational savings advantage over cryogenic freezing because no costly nitrogen or carbon dioxide gas is lost to the atmosphere. Proponents of mechanical freezing methods doubt that there is enough of these two gases in the United States to produce all of the french fried potatoes needed to satisfy the fast food requirements of the country.

While the initial costs for a mechanical system are high, the operating cost for a mechanical system can run from 1/4 to 1/2 cent per lb. of product processed. Mechanical freezing can be particularly economical for freezing cooked products with high heat loads.

IQF (individually quick frozen) and other systems involve intimate contact of the freezing medium with the product. Plate or contact freezing systems involve contact of the product on both sides with metal at freezing temperatures.

The IQF process is advantageous for small-sized particulate types of foods such as peas. One way in which IQF has been achieved is through the fluidized bed freezer which offers considerable saving in space requirements over tunnel or belt freezers. Fluidized belt freezing is particularly useful for products that tend to stick together such as French green beans or sliced carrots.

Fluidization is achieved by subjecting particles of uniform shape and size to an upwardly directed low temperature air stream. As a given air velocity is reached, the particles will be suspended in air and be free to move forward as more product is added. Thus a conveyor is not needed. This technique achieves very intimate contact between air and product and gives much better heat transfer than is achieved by tunnel or belt freezing.

For some products, such as ice cream, a scraped surface freezer is used. Freezing is accomplished by means of ammonia or brine. A rotor scrapes the product from the wall of the freezer and incorporates air or "overrun" into it.

Cryogenic freezing

Mechanical freezing systems can reach a temperature of only -40 degrees C. Liquid nitrogen, on the other hand, boils at -196 degrees C and carbon dioxide sublimes at -78.5 degrees C. In addition to being able to achieve colder temperatures, a cryogenic freezing system does not require a refrigeration plant as the compressed gas is received from suppliers.

In general, cryogenic freezing is used for high value, low volume products such as shrimp or berries. Nitrogen tends to be the gas of choice in the United States.

Both nitrogen and carbon dioxide tend to be relatively bacteriostatic. In addition, cryogenic freezing of raw product reduces dehydration (shrink) loss which may amount to as much as 3 to 6% with some mechanical air blast freezing systems.

One cryogenic method is to directly immerse product in liquid nitrogen. With this method, products may crack. Whether a food is prone to cracking depends on size, shape, porosity and density. Research has indicated that moisture content is not the primary indicator of cracking tendency.

The use of carbon dioxide for freezing to some extent depends on geographical availability. In some areas in the southern United States, carbon dioxide comes out of the ground from wells. In other areas it is available from an industrial feedstock, for instance, as a by-product of ammonia production for fertilizer. A large amount of power is required to produce liquid nitrogen.

Carbon dioxide at -18 degrees C extracts 135 BTU per lb., while nitrogen at -196 degrees C extracts 155 BTU per lb. There is a significant difference in the distribution of BTU between the states in which the freezing agents contact the product being frozen. Carbon dioxide exists at atmospheric pressure as either a solid or a gas. As a liquid, carbon dioxide is held under pressure. When the pressure is released the carbon dioxide comes out as a snow. This snow removes 85% of the BTUs while gas vapor removes 15%. With nitrogen, 48% of the BTUs are removed as the liquid expands and becomes a gas while the vapor removes 52% of the BTUs.

For this reason, a nitrogen freezing unit is set up so that the gas flows counter current to the product. The nitrogen is sprayed into the freezing unit with nozzles and evaporates on leaving the nozzles and contacting the product. Cold gas is circulated by means of fans toward the end of the tunnel or belt on which product is entering in order to pre-cool the product. The spent gas is exhausted at the front of the unit.

Carbon dioxide, on the other hand, is set up so that product and freezing medium flow in the same direction. Because the snow has to sublime, the point of injection is moved closer to where the product is to be frozen.

Carbon dioxide snow is often used for chilling as in manufacturing sausage type products. Because of the difference in physical properties and cooling mechanism, it is not possible to substitute either nitrogen or carbon dioxide for one another in a freezing system without making major modifications.

High heat transfers achieved at the surface and outer layers of a product through cryogenic means make the process ideal for foods that are sensitive to handling or are wet and sticky, such as late season strawberries. Often a product can be crust frozen by this means and then completely frozen by a mechanical system in order to cut costs.

Ambient air cryogenics

Newer developments have resulted in equipment and a process employing ambient air as the freezing medium. The air is delivered at -157 degrees C into a spiral belt freezing chamber. The air is brought to this temperature by means of compression, heat exchange, and expansion. This system is said to provide cryogenic quality at mechanical costs.

For raw poultry, freezing time using an ambient air system is about one-third of that for mechanical means and not quite as rapid as for a liquid nitrogen system. The percentage of dehydration is a little more than one-half of that experienced with mechanical freezing and very slightly more than for nitrogen freezing. Cost of the process in dollars per pound is slightly more than for mechanical freezing and about one half of the cost for freezing raw poultry with nitrogen. In addition to cost/quality advantages, ambient air systems present no environmental concerns regarding escape of ammonia or freon to the atmosphere.

Freezing food has often been considered an expensive process. This is actually not true when the cost of freezing and packaging in paperboard or plastic is compared with that of thermal processing and packaging in cans. In addition, long storage is not required for most food products due to rapid turnover caused by of consumer demand. An additional advantage is the high quality of most frozen foods.

Robert Dean, PhD, is the regional nutrition coordinator for the USDA's Midwest region of the Food and Nutrition Service. He has 35 years of experience in the food industry.

This table lists some approximate freezing points of foods together with their moisture content. In general, the higher the moisture content of a food, the closer its freezing point is to water.

Using Plank's Formula

The following formula calculates freezing time tf where ( (rho) is density. Tf is the initial freezing point, T8 (infinity) is the final freezing temperature, and L the heat of fusion. P and R are constants that depend on the geometry of the product, a is the characteristic dimension, h the surface heat transfer coefficient and k the thermal conductivity of the frozen product.

The advent of computers has made calculations of this type much easier to do. It should be noted that for small product sizes, surface area is important and surface heat transfer predominates. For larger products, k, thermal conductivity and the volume determination are most important.

tf = (L/Tf - T8 [Pa/h + Ra2/k]