January 1, 1997

23 Min Read
Designing Frozen Foods

 Designing Frozen Foods
January 1997 -- Cover Story

By: Lisa Kobs
Contributing Editor

  Historically speaking, freezing food by burying a carcass under a pile of snow and ice has been a safe method of preservation, ensuring our ancestors a meal in times of limited food supply. While this method may have eased the burden of expending valuable energy searching out the next meal, it probably was not very palatable.  The increasing pressure to serve quick and easy meals that are nutritious, economical and palatable, both in the home and in the foodservice arena, puts greater pressure on the food industry to find solutions. For the frozen food sector to maintain its place in the market, it is up to food scientists to fully understand the scientific principles, ingredient functionality, their relationship to processing methodologies, and the rigors placed on product during the food distribution system and through consumer handling and reconstitution.  Freezing animal and plant tissues causes many irreversible physical and chemical changes within the food matrix, often leaving the final product with lower eating quality than when it was in the fresh state. There is, of course, a trade-off in that freezing food allows us to take advantage of food in times of plenty, makes possible the distribution of food substances which otherwise could not be consumed due to seasonal and geographic location, and offers exceptional margins of food safety in return for extended storage time and costs.  With an understanding of the principles associated with the micromolecular structure of freezing a substance such as water, plant or muscle tissue, and their changes upon storage in varying environmental conditions, we have been able to improve upon the quality of the frozen foods available.During freezing  To combat quality losses in frozen food systems, the food scientist must understand the physical and chemical processes that take place during freezing. Freezing damage refers to irreversible tissue changes due to the freezing process that become apparent after thawing. It is not always apparent if the damage occurs during freezing or during thawing, as it may be the result of many separate processes. It is essential to be aware of these sources of quality loss so that they may be compensated for through formula modification and ingredient interaction or through processing modification and proper packaging.  Chill damage results from exposing plant tissue to low temperatures, including those above 0°C, so this should not be confused with damage resulting from freezing. The same mechanisms that can happen at temperatures below 0°C cause freezing damage in tissues that do not show signs of chill damage above 0°C.  Solute concentration damage occurs as ice forms in the frozen system, and the concentration of the solutes in the unfrozen medium increases. Increasing ionic concentrations and strengths leads to a decline in pH in the unfrozen phase - as much as a whole pH unit. This concentration of high ionic strength damages the food. The ionic strength of a solvent system affects the shape and function of many molecules. Charged molecules may react differently because of this increasing ionic strength. This can be devastating to emulsions and protein stability. Aggregation and precipitation may also occur. Many of these reactions are irreversible; the characteristics of the food system are permanently changed.  Dehydration damage occurs from an increase in solute concentration in the unfrozen medium. This leads to an osmotic transfer of water from a cell interior to the exterior environment. The cell interior will dehydrate, and its volume will shrink, potentially forcing the cell structures in the wall to change. This results in tearing of the cell wall, or cell membranes that rupture from pulling away from the cell wall.  Mechanical damage from ice crystals occurs when flexible cell components are stressed in areas where ice is present, causing mechanical damage to the structures. Ice crystals continue to grow in size, and they exert additional stresses on fragile cellular structures. As flexing of cellular tissues occurs, ice can grow into this newly created volume and prevent the structure from relaxing back into its original shape. The theory that ice crystals spear through structures is incorrect because ice crystals grow by adding water molecules to their surfaces.  Ice crystals can damage emulsions when the crystals that grow in the droplets of water penetrate through the fat. When the product is thawed, the emulsion has been broken.During storage  At subfreezing temperatures, foods are not completely frozen and will continue to deteriorate. Freezing is very destructive to tissue cells or anything else containing water because it expands when it freezes. Deterioration can occur due to either physical or chemical changes.Crystal size and shape. During frozen storage, crystals undergo metamorphic changes. Smaller ice crystals promote a better quality product. Crystal growth often inflicts damage during freezing. Recrystallization occurs because systems tend to move toward a state of equilibrium where free energy is minimized and the chemical potential is equalized among all phases.  During frozen storage, the number of ice crystals will be reduced, and their average size will increase as a result of the surface energy between the ice and the unfrozen matrix. Temperature fluctuations reduce the size of small crystals more than that of large crystals. During cooling cycles, crystals with larger cross-sections are more likely to capture water molecules that are transferring back to the solid phase. The combined effect of product structure, interfaces, and the different concentrations in moisture work together to move water toward the surface of the product.Moisture migration. Temperature gradients within a product during frozen storage may result in moisture migration. Water will migrate down a temperature gradient due to the temperature dependence of water vapor pressure. Temperature gradients will always exist in a food product due to the inability to eliminate temperature fluctuations. These temperature gradients will reverse in direction, but the moisture transfer does not. When there is a void space around a product in a package, moisture transfers into this space and accumulates on the product surface and on the internal package surface.Sublimation. This effect occurs as water passes directly from the solid state to the vapor state, or from the frozen food product into the atmosphere around the product. Moisture vapor in the atmosphere attempts to reach equilibrium with the materials within a room, as well as with the room itself. The temperature of the freezing coil is always lower than the air in the storage room, so ice will form and accumulate on the coil.  As moisture is removed from circulation, the relative humidity of the air drops. The moist materials within the chamber yield more water vapor, attempting to satisfy the vapor pressure deficit within the room. The water vapor pressure over the surface of the product is then higher than that of the surrounding air, producing a constant loss of water in the form of vapor from unprotected materials. It is difficult for moisture to transfer back to the initial location of the void. Thus, moisture has an overall tendency to move toward surfaces. Sublimation continues as long as this pressure difference continues.  Sublimation is a principal contribution to the formation of freezer burn. It increases oxygen contact with the food surface area. This increases oxidative reactions which irreversibly alter color, texture and flavor.  If a product is packed in tight-fitting, water- and vapor-proof material, evaporation can not take place. The temperature of the packaging material will follow the temperature fluctuations in the room faster than the product itself. As the temperature is lowered, evaporation from the product will form ice on the inside of the packing material, and when the temperature conditions are reversed, the ice will be deposited on the surface of the product.  Glazing, dipping or spraying a thin layer of ice on the surface of a frozen product helps to prevent drying. Sublimation is less pronounced since it takes place from an even surface that occupies a smaller area than the ice crystals. In addition, the loss of water comes from added water. The water loss can be calculated to prevent sublimation over prolonged storage periods.Solute crystallization and pH change. After freezing, many solutes may be supersaturated in the unfrozen phase. In time, these may crystallize or precipitate. This will change the relative amounts of solutes and the actual concentration of solutes. Therefore, the ionic strength can change, and pH can change due to changing ratios of buffer components. These factors also affect the stability of other molecules, and changes in characteristics of molecules in solution can occur.Freeze/thaw cycles. Repeated freezing and thawing is very damaging, and the food need not be completely thawed for damage to occur. Most frozen food distribution systems have measurable temperature cycles, and there is great variability in the temperature of consumer freezers. Whatever the fluctuations in storage temperature, there will be a lag effect on the food because heat transfer has a finite rate. However, large temperature variations over long storage times can cause noticeable damage.Freezing damage in tissue  Freezing damage in plant tissue systems includes disruption of metabolic systems, dislocation of enzyme systems, loss of turgor due to cell wall and cell membrane damage, and permanent transfer of intracellular water to the extracellular fluid through osmosis. This cannot be reversed upon thawing. The effects of frozen storage damage that are important in plant tissue systems are protein insolubilization, lipid oxidation, polymer aggregation and pigment oxidation or hydrolysis.  In unblanched tissues, freezing can disrupt normal enzyme-catalyzed processes. During storage, however, these reactions will proceed. Proper heat treatment inactivates the enzymes that cause undesired reactions. To properly control blanching, it is necessary to identify the enzymes that produce off-colors, off-flavors, and off-odors, and to inactivate these enzymes while minimizing deterioration to the plant tissue itself.  Animal cells do not posses a structurally strong cell wall and their cells are influenced by cell membrane dehydration. The animal cell membrane has a greater hydraulic permeability as compared to the membrane of plant cells; however, it is much less effective in preventing the propagation of ice. In most cases the rate of freezing animal tissue has very little influence on the properties of the frozen tissue and almost no effect on the quality of the thawed tissue. In the absence of rigid cell walls in animal cells, mechanical damage is less commonly found.  Drip loss is of concern with frozen meats as there may be financial considerations if the product is to be sold by weight. Slow freezing rates increase drip due to changes in the myofibrillar proteins that lead to a decrease in solubility. The amount of drip also depends on the size of the cut, method of cutting, end temperature to which the food is frozen, the length and temperature of freezer storage, and thawing method.  If the meat has been frozen in pre-rigor state and kept at a low temperature, the remaining enzymes cause a very fast reaction when the temperature is raised. This condition leads to a phenomenon known as thaw rigor which causes a sudden contraction of muscle fibers and a large drip loss.  There seems to be no correlation between drip and texture changes. Changes in fish muscle is generally more pronounced than in mammal meat. A characteristic of frozen fish and mammals is a decrease in the water-binding capacity of muscle tissue related to a loss of juice upon thawing, and an increase in firmness and dryness, resulting in a tougher, drier product.Making improvements  "Everyone is in such a hurry today, and with the fast-paced world that we live in, everyone is looking for instant gratification," says Gary Wine, vice president of AEP Colloids Inc., Ballston Spa, NY. "They are looking to immediately open the freezer, pull something out, and throw it in the microwave. That is probably the worst thing that you can do to food products."  Many factors influence storage stability. If products are held at elevated temperatures out on the floor, or are exposed to excessive sheer due to product rework, contrary to manufacturing process specifications, this must be acknowledged and compensated for.  An inherent level of safety is certainly built into frozen food systems due to their low temperature. Growth of microorganisms does not take place when the temperature is about 8°C or below. However, the microbiological quality of the product prior to freezing is important. A food product with a lower initial microbial count will have a longer shelf life than one with a higher initial count, even though the bacteriological quality may be acceptable. This difference is probably due to the activity of bacterial enzymes.  The freezing rate also has an impact on the finished product. Rapid freezing causes a large number of small ice crystals. Slow freezing allows initially formed ice crystals to grow in size. According to Dr. Gary Fennema, professor emeritus of Food Science, University of Wisconsin, fast freezing (1 minute) preserves the initial quality of the food better than slow freezing (90 minutes). Whether the consumer sees a benefit depends on storage time and temperature. If the temperatures are well controlled and short storage times are used, the chances of retaining the benefits of fast freezing are better.  Rapid freezing allows the product to get through the zone of solute concentration more quickly. This minimizes concentration effects by decreasing the time concentrated solutes are in contact with food tissues, colloids and individual constituents.  Increasing the concentrations of dissolved solids lowers the freezing point of a solution. As the water freezes, it concentrates the dissolved solids and reduces the freezing point of the unfrozen liquid. According to Fennema, lowering the freezing helps maintain desirable sensory properties by lessening the types of damage that occur during frozen storage. Solute concentration damage can be avoided during the freezing process if the freezing point is depressed so that no ice is formed. Lowering the freezing point can also make the product easier to thaw and prepare, and can create softer textures directly out of the freezer.  According to Fennema, lower storage temperature, shorter storage time, and tight fitting packaging improve frozen food quality. An additional approach is to use coatings that restrict moisture migration. There has been a lot of activity in this area in recent years. Protein and hydrocolloid barriers do not make very effective barriers, but they are better than nothing. The best barriers are either lipids or shellacs, natural exudates from plants.Glass transition  With processed foods, Fennema stresses the importance of glass transition temperature. Glass transition (Tg) is the temperature transition point where the matrix enters an amorphous, or glassy, state. The Tg varies greatly as a function of the ingredients, as well as their concentration. Elevating the glass transition temperature above the storage temperature can increase the shelf life of a frozen product. Storage at a temperature below the product's Tg reduces mobility in the system. Effects that are diffusion dependent - especially physical effects such as package ice formation, moisture migration and ice recrystallization, and some chemical reactions - will virtually stop. A Tg significantly below the storage temperature increases mobility, and increases the opportunity for ice crystal growth and movement of the water.   The greater the molecular weight of an ingredient, the greater its effect will be in raising the Tg. Longer length polymers promote a higher Tg point than shorter chain polymers. A soluble starch, a 5 DE maltodextrin or a 20 DE corn syrup solid, raises Tg levels more than a disaccharide. Proteins have some effect in raising Tg levels, however gums and polydextrose are not very effective.  Glass transition is a relatively new concept for the food industry. Fennema believes that it is the only one that currently exists to predict frozen food stability. Water activity works for many systems, but is useless as a predictor of frozen food stability. However, Marvin J. Rudolph, Ph.D., director at Arthur D. Little Inc., Cambridge, MA, believes this technique is more applicable to non-frozen polymeric foods that change from an amorphous to crystalline state, such as candy. He also points out that a differential scanning calorimeter is required to establish the glass transition temperature.  To illustrate the effect of glass transition temperature in relation to shelf life: a frozen dessert was formulated to promote spoonability directly out of the freezer. Substituting high fructose corn syrup in place of sugar reduced the amount of frozen water. However, the glass transition point was driven below that of the storage temperature. This increased water mobility and increased the size and growth rate of the ice crystals.  Starches, maltodextrin or low DE corn syrup solids alter the freezing point, but also drive the glass transition temperature up closer to that of the storage conditions. This produces less mobility in the system so that fewer things go wrong in storage.Negative effects  Color, flavor and texture can change for the worse during freezing. According to Fennema, almost any flavoring characteristic will change during frozen storage. Frozen foods are not stable, so flavor deteriorates. The only way to minimize problems is to decrease the storage temperature, shorten the storage time or do both.  Freezing can promote off flavors due to rancidity of fats. According to Fennema, oxidation is not effectively controlled by freezing. Oxidative changes of polyunsaturated fatty acids cause rancidity, and the more polyunsaturated fats, the greater the chance the fat will undergo oxidative changes. The presence of certain catalytic factors, such as special metal ions or salt and lipolytic enzymes, enhances oxidation. These enzymes are active at low temperatures and promote lypolitic changes, resulting in accumulation of free fatty acids. Interaction between these fatty acids and other components in the food can cause undesirable changes in product quality.  Antioxidants, if permitted, can often benefit the flavor characteristics of frozen foods. Using saturated fats instead of unsaturated fats will also decrease rancidity. Increasing the contact area between oxygen and the tissue - as in the case when ice crystals disappear due to drying - enhances oxidation. Close fitting or vacuum packaging in oxygen-proof material minimizes oxidative reactions and prolongs shelf life, as does flushing the package by an inert gas such as nitrogen.  Many food odor chemicals are volatile at frozen storage temperatures and evaporate from the surface of the product. Some aroma characteristics are diminished or lost upon prolonged storage. Products may pick up strong foreign aromas that can dissolve in either their aqueous or lipid phases. Foods with high fat contents are most susceptible to this as most flavor volatiles are oil-soluble. Physical separation of susceptible food items, barrier packaging, and reduced temperature will reduce, but not eliminate, the release of volatiles.  The greatest freezing damage to tissue foods is caused to texture. Some fruits and vegetables are best consumed raw, and are unsuitable in frozen foods. Increased firmness in meat or fish is often observed due to decreased water binding capacity of muscle tissue related to a loss of juice at thawing. Moisture migration and ice crystallization may create gritty textures due to increased ice crystal size or starch retrogradation. Batters may not adhere to a surface due to shrinkage or syneresis of the base. Products may be tough and dry due to water loss through sublimation.  Iciness is an important textural problem in frozen desserts, especially in reduced-fat systems, because of the increased level. Reducing the freezing point of the system and eliminating ice crystal growth is very important through the use of selected cryoprotectants to lower freezing point. Glycerol is one such ingredient, but it often creates undesirable flavors, and too much produces a laxative effect, as do all polyols.  According to Fennema, freezing rate can have a rather dramatic effect on the appearance of frozen food. Poultry products are frozen very rapidly on the surface, with little regard given to freezing rate of the interior. This produces a chalky white color, which the consumer has decided indicates good quality. However, upon thawing, there is no difference between the quality of the rapidly frozen product and one that has been frozen slowly.  Rapidly freezing red meat produces a very dull, unattractive surface color. One way to provide better color without changing quality is to cryogenically freeze meat and then use infrared heaters to slightly melt the surface and then refreeze the surface at a slower rate.  Oxidative deterioration can cause color changes in tissues due to reactions between proteins and the oxidation products. In other products, especially vegetables, enzymatically catalyzed reactions from improperly denatured enzymes can negatively affect color.Stabilizer solutions  Stabilizing a frozen food formula is a system of tradeoffs - the effectiveness of the formula is based on the variables for each and every system. Many key questions about components, production, pH, moisture and labeling restrictions need to be considered.  According to Wine, "You need to look at the entire stabilization picture and can not make a blanket statement stating that X is good for Y all the time. Stabilizers alone do a certain job, and together they have certain synergies that are phenomenal. But you have to look at each application, and what might be good in your application might not be for somebody else making the same exact product."Starch. According to Michael Augustine, manager of food ingredient applications at A. E. Staley, Decatur, IL, starch is probably the primary ingredient used to promote freeze/thaw stability. Starches have very high water-holding capacities, can modify texture, and are cost-effective. One part of starch can manage from 20 to 25 parts of water.  The same starch system can not be used for every frozen product. They have differing properties based on processing and will result in different end textures.  Of the native starches, waxy maize is inherently better for frozen food systems than tapioca, common cornstarch or potato. The waxy maize starches are the most freeze/thaw stable due to their highly branched nature. Freezer storage creates retrogradation (the recrystallization or reassociation of the starch branches), which leads to syneresis. Generally, waxy starches for thickening are most often cross-linked and rarely sold as native starch.  To make a native starch more freeze/thaw stable, the starch is substituted either by acetylation, or hydroxypropylation, according to Augustine.  "Acetylation is done at a lower level, up to 2%, whereas you can get up to 6% hydroxypropyl groups on starch," says Augustine. "Hydroxypropyl groups promote more freeze/ thaw stability and more viscosity because the starch is more open and gives you viscosity for a given cross-linking. While cross-linking is more related to stability of the starch to processing, at a given equal cross-link level, you get more viscosity, with the higher substitution of hydroxypropyl groups and more branching, therefore more freeze/thaw stability. A substituted starch is therefore better for freeze/thaw stability than a native waxy maize."  While a substituted waxy maize starch would be the most freeze/thaw stable, a substituted tapioca, corn or potato starch, with adequate substitution can also help. All of these starches also have inherent textures so the choice also depends on the finished product requirements. Waxy maize starches create more viscosity and clearer gels, cleaner flavor, and hold up to processing. Substituted dent starches help prevent syneresis, but have a shorter texture and may have slight gelling characteristics.  Augustine does not recommend that more than 15% of the stabilizing system in the formula be unmodified starch or flour. Wheat flour acts as a cheap thickener and source of opacity, but has poor freeze/thaw stability or viscosity. Formulators must be conscious of the amount of unmodified vs. modified starch to insure the right viscosity and freeze stability. High soluble solids slow down retrogradation, which may allow higher levels of unmodified starch.  Starches can be overcooked, which ruptures the granules and leaves a large percentage of fragments. This leads to a lower viscosity in the finished product. For products that undergo a rigorous reheating, it can be beneficial to slightly undercook the starch so it does not become overcooked upon reheating.  Starches can also handle uncontrolled water in a different way. Frozen vegetables and meats release water as they thaw. Adding a low-pasting temperature starch to a sauce or gravy can absorb the additional moisture. It cooks upon reheating and picks up water lost from the other ingredients.  Starch used in bakery products can improve dough crumb structure. Instant starches hold on to water, maintain and manage it through the storage life of the product. The starch hydrates without heat, and picks up that water immediately. This provides moistness for mouthfeel and helps retain crumb moisture in the finished product. Instant starches also help with batter viscosity to promote aeration.Other hydrocolloids. Gums minimize the freeze/thaw damage by tying up free water, controlling moisture migration and interacting with other components during thawing and reheating. Even with two separate systems with differing water levels, such as in a filled cake, gums help tie up water in a gel to prevent moisture migration. Ascertaining the proper gum level can be a challenge - the initial level may look appropriate. However, the burst cells created through freezing and thawing release additional water. The system almost has to be overstabilized a bit, anticipating that moisture when the product is used by the consumer.  Wine recommends locust bean gum, guar gum and xanthan gum for frozen systems. "Each gum will have its own properties, strong in some points, and weak in others," he says. "You have to look at the product, determine what process it's going to go through, how many freeze/thaw stable cycles it will incur, and what type of end product you desire. Ice cream may go in and out of the freezer 10 times. In this case, you need a system that is going to work for a long period of time without giving a gummy texture and mouthfeel."  During long periods of frozen storage, moisture is being slowly driven off and the solutes concentrate. If this results in an increased pH, the gum system must be stable under these altered conditions so that any extra moisture that comes out of the system can be managed.  Gums also reduce damage from sublimation. Because water is tied up so well, it will not be as quick to migrate. For example, if you have an ice cube in the freezer and leave it in there long enough, it will disappear. An ice cube made with a gum system ice would have much longer longevity because of the bound water molecules.Product reconstitution  The final concern in frozen foods is thawing. Thawed foods are subject to quality loss, especially if thawing is slow. Concentration effects can occur. Solutions that freeze last and are first to thaw are commonly concentrated eutectic mixtures, or ones that freeze as a mixture rather than becoming more concentrated. Slow thawing times give concentrated eutectic mixtures more time to be in contact with the food constituents and intensify the damage. Quick thawing techniques also prevent the opportunity for bacterial multiplication.  A problem found with microwaving a frozen product is that the thicker the product, the greater the disparity in reheating. Hot spots tend to form with products of higher viscosity. Ice is transparent to microwaves. Areas of high solids, syrups, fat or areas where water is not frozen, tend to absorb microwaves readily. This creates various levels of heating in a product. Therefore, the development goal is to create the most homogeneous system, with the appropriate viscosity to prevent this temperature disparity.  Technology has not advanced to the level where all of the problems encountered in designing frozen foods can be solved. However, understanding the forces at work and tailoring the formulation and process to account for these, goes a long way toward increasing the quality of the finished product.  Lisa Kobs is a senior food scientist specializing in product development of retail and foodservice goods for Food Perspectives Inc., Minneapolis. She has extensive experience in refrigerated and frozen foods, baked goods, dry mixes, beverages and nutritionally controlled foods.Back to top

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