Aassalamu alikum how are you I hope you are all well. All the praise to Allah subhanahu oatala.I am Abu Saeid Studying MS in Food Processing and Preservation in Hajee Muhammad Danesh Science and Technology University, Dinajpur. Stay in Zia hall room no#226# 1st floor in HSTU campus, Basher hat, Dinajpur.


Monday, April 20, 2015


Food Processing and Preservation Method- Freezing

Low temperature retards chemical reactions as well as the activity of food enzymes and slows down or stops the growth and activity of microorganisms in food. Low temperature processing and preservation methods may be sub classified into two types
  1. Refrigeration (not discussed this section)
  2. Freezing.
*      Freezing is the unit operation in which the temperature of a food is reduced below its freezing point and a proportion of the water undergoes a change in state to form ice crystals. The immobilization of water to ice and the resulting concentration of dissolved solutes in unfrozen water lower the water activity (aw) of the food.
*      Preservation is achieved by a combination of low temperatures, reduced water activity and, in some foods, pre-treatment by blanching.
*      Frozen food can be kept for a very long period of time. Usually about 3 months.
*      Deep freezing is the reduction of temperature in a food to a point where microbial activity cease.
*      A freezer should be kept at -18oC to -25oC
Goal of freezing
Ø  To prevent growth of microorganisms by
–Killing some bacteria (little effect)
–Reducing water activity
–Mechanical formation of ice crystals
–Osmotic changes in cell fluids
–Tying up some free water
Ø  To lower temperature enough to slow down chemical reactions
– (every 10°C decrease in temperature halves the reaction rate)
Basic Principles of Freezing:
         Does not sterilize food.
         Extreme cold (00F or -180C colder):
        Stops growth of microorganisms and
        Slows chemical changes, such as enzymatic reactions.

Advantages of Freezing:
         Many foods can be frozen.
         Natural color, flavor, and nutritive value retained.
         Texture usually better than other methods of food preservation.
         Foods can be frozen in less time than they can be dried or canned.
         Simple procedures.
         Adds convenience to food preparation.
         Proportions can be adapted to needs unlike other home preservation methods.
         Kitchen remains cool and comfortable
Disadvantages of Freezing:
·         Texture of some foods is undesirable because of freezing process.
·         Initial investment and cost of maintaining freezer is high.
·         Storage space limited by capacity of freezer.
Issues/defects with Frozen Foods:
1.      Chemical reactions can occur in unfrozen water.
A.    Some foods blanched or sulfited before freezing.
B.     Vacuum packaging to keep out oxygen
2.      Undesirable physical changes
A.    Fruits and vegetables lose crispness
B.     Drip loss in meats and colloidal type foods (starch, emulsions)
§  Freeze product faster
§  Control temperature fluctuations in storage.
§  Modify starch, egg systems, etc.
C.     Freezer burn
         Package properly
         Control temperature fluctuations in storage.
D.    Oxidation
         Vitamin loss
E.     Recrystallization

Freezing methods and equipment:
Three methods of freezing are employed in commercial practice. These are:
1.      Air freezing: In air freezing cold air is used with different velocities. It is done two way:
a.       Sharp freezing (or slow freezing) usually refers to freezing in air with only natural air circulation or at best with electric fans. The temperature is usually -23.30C or lower but may vary from -15 to -290C and freezing may take from 3 to 72 hrs.
b.      Quick freezing is variously defined but in general implies freezing air at -180C to -340C is blown across the food and freezing time of 30 min or less and usually the freezing of small packages or units of food. Quick freezing accomplished by one of three general methods:
                                            i.      Direct immersion of food or the packaged food in a refrigerant as in the  freezing of fish in brine or of berries in special syrups,
                                          ii.      Indirect contact with the refrigerant, where the food or package is in contact with the passes through which the refrigerant at -17.8 to -45.60C flows
                                        iii.      Air blast freezing, where frigid air at -17.8 to -34.40C is blown across the materials being frozen.
2.      Indirect contact freezing: the food or food package does not come into contact with the refrigerant. The food or food package is brought into contact with a cold surface maintained at temperatures in the range of -18 to -450C by a refrigerant. Following freezer are used:
                  -Plate freezing system
-Cabinet freezing system
-Scrapped surface freezing system
-Individual quick freezing (IQF) system
Fig: Indirect contact freezing
3.      Direct contact freezing: the food is immersed in the refrigerant (eg. Fish in brine or berries in special syrups) or sprayed with the refrigerant (e.g. cartons of fruits, vegetables, fish, shrimps and mushrooms). These foods can also be immersed in liquid nitrogen or sprayed with liquid nitrogen after packaging in cartons or aluminum cans. Following freezer are used:
-Air blast system
-Continuous spiral conveyor system
-Continuous fluidized bed system
-Continuous immersion freezing system
-Continuous cryogenic freezing systems
Fig: Direct contact freezing

Quick freezing vs Slow freezing
Slow freezing
Quick freezing
Ice crystals form in extracellular locations
Produces both extracellular and intracellular (mostly) locations of ice crystals
More time need for solidification
Shorter period of solidification
Large ice crystal
Small ice crystals
More disruption of cell due to large ice crystal
Less mechanical disruption of intact cell of food due to small ice crystal
Temp -15 to -290C for 3-72 hrs.
Temp-180C to -340C for 30 min
Comparatively slow prevention of microbial growth
More prompt prevention of microbial growth
Comparatively less
More rapid slowing enzyme action
Maximum dislocation of water
Minimum dislocation of ice crystals
Shrinkage (shrunk appearance of cells in
frozen state)
Frozen appearance similar to the unfrozen state
Less than maximum attainable food quality
Food quality usually superior to that
attained by slow freezing

Novel Freezing Techniques:
Novel methods for freezing and thawing have been investigated on a laboratory scale; however, these are generally more expensive than their conventional counterparts (reviewed by Bing & Sun, 2002). Such novel method is

Ø  High-Pressure Freezing:
Which result in instantaneous and homogenous ice crystal formation throughout the product due to the high supercooling effect achieved on pressure release. The result of the increased pressure causes a shift in the type of ice crystals that are formed from type I (lower density than liquid water) to type IV ice crystals. Type IV ice crystals are smaller and denser than water and do not cause the product to swell by 9–13%, the normal expansion that occurs with type I crystals. The theory is that, with type IV ice crystals; there is less mechanical damage to the cell structures, which results in a superior quality product. The drawback of this method is the capital layout and the product size limitation.
Ø  Ultrasound assisted freezing:
Ultrasound has several different effects, often contradictory
Ø  Agitation, leading to enhanced heat and mass transfer and faster freezing near surfaces of food.
Ø  Heating, leading to slower freezing
Ø  Cavitation (formation of gas bubbles) near surfaces, leading slower freezing
Ø  Triggering of nucleation, as long as the food is below nucleation temperature, leading to more  and smaller crystals
Ø  It has been surmised that ultrasound can even cause intracellular nucleation, which can normally happen only at very high freezing rates.
Ø  Enhancing crystal growth when the food is above nucleation temperature but below freezing point, leading to bigger crystals
Ø  Fragmentation of crystals, leading to smaller crystals Thus, by tuning the power and timing of ultrasound, it can be used to accelerate freezing by increasing the rate of heat transfer at the surface
Ø  Reduce crystal size during food freezing, leading to better quality. The food is super cooled then nucleation is initiated by a short pulse of ultrasound. This has an effect somewhat similar to pressure-shift freezing.
Ø  Increase crystal size during freeze concentration by repeated US pulses as soon as the food is below freezing point.
Ø  Dehydrofreezing:
Dehydrofreezing is a variant of freezing in which a food is dehydrated to a desirable moisture and then frozen. The processes which reduce free water in foods do not damage the quality of the frozen product providing the removal of water itself does not cause deleterious changes in the food substrate. A reduction in moisture content would reduce the amount of water to be frozen, thus lowering refrigeration load during freezing. In addition, dehydrofrozen products could lower cost of packaging, distribution and storage, and maintain product quality comparable to conventional products. Successful applications of dehydrofreezing on fruits and vegetables have been reported.
Ø  Antifreeze protein and ice nucleation protein:
Controlling the growth of ice crystals in frozen foods is a primary concern to food technologists. Antifreeze protein and ice-nucleation protein (INP) can be directly added to food and interact with ice, therefore influencing ice crystal size and crystal structure within the food, which are two functionally distinct and opposite classes of proteins. Antifreeze proteins can lower the freezing temperature and retard recrystallization on frozen storage, while ice-nucleation proteins raise the temperatures of ice nucleation and reduce the degree of supercooling. Although these two proteins show opposite effects on ice crystals, they are potentially added in food, attracting more attention of food technologists
Ø  Progressive freeze concentration(PFC):
Freeze concentration is the concentration of a solution by freezing out the ice and removing it. Compared to other forms of concentration such as evaporation, drying or membrane processes, it has the great advantage of low temperature and very gentle processing, and is thus excellent at preserving quality and avoiding thermal damage. It has been therefore use on very sensitive products such as coffee, dairy products and fruit juices.
Ø  Immersion freezing and freezing in ice slurry:
Conventional immersion freezing used brines to lower the freezing point, or some refrigerant. The product is usually wrapped to prevent absorption of refrigerant. However, absorption may be an advantage for some processed foods such as desserts. Thus, ice slurries based on sugar-ethanol aqueous solutions have been used to freeze fruit for dessert. The advantages of this process are:
- Short freezing time due to high heat transfer rate from ice slurry
 (Best results are obtained with low Biot numbers!e.g. small product such as peas.
- High quality due to small crystal size
- Absorption of food additives (antioxidants, flavorings, aromas and micronutrients)
- Improved quality and shelf life
Plate freezing system:
In plate freezing system, between product and refrigerant will include both the plate and package material. The heat transfer through the barrier (plate and package) can be enhanced by using pressure to reduce resistance to heat transfer across the barrier. In some cases, plate systems may use single plates in contact with the product and accomplish freezing with heat transfer across a single package surface. As would be expected, these systems are less efficient, and they are costly to acquire and operate.
Fig: plate freezing

Cabinet freezing system:
The product is placed in a package prior to freezing, and the packages are placed on trays before they are moved into the freezing system. These types of freezing systems operate as batch systems, with ti1e freezing time established by the length of time that the product remains in the cabinet. The environment in the room is maintained at a low temperature, and air movement is established by fans within the cabinet.

 Fig: cabinetfrezzing system

Scrapped surface freezing system:
These types of freezing systems utilize a scraped surface heat exchanger as a primary component of the continuous system used to convert liquid product into frozen slurry. In these systems, the outer wall of the heat exchanger barrel represents the barrier between the product and the low-temperature refrigerant used for product freezing.
For freezing liquid foods, the residence time in the freezing compartment is sufficient to decrease the product temperature by several degrees below the temperature of initial ice-crystal formation. At these temperatures, between 60 and 80% of the latent heat has been removed from the product, and the product is in the form of a frozen slurry. In this condition, the product flows quite readily and can be placed in a package for final freezing in a low-temperature refrigerated space. The scraped-surface heat exchanger ensures efficient heat exchange between the slurry and the cold surface.
Application: scrapped surface heat exchanger used in ice-cream hardening.

     Fig:scrap surface freezing system

Individual quick freezing (IQF) system:
Individual Quick Freezing (I.Q.F.) is the latest technology available in freezing and with the advent of the same, it is now possible to preserve and store raw fruit and vegetables in the same farm-fresh condition for more than a year, with the colour, flavour and texture of produce remaining as good as fresh from the farm. In individual quick freezing, very low temperatures (-30°C to - 40°C) are maintained to halt the activities of the microorganisms that cause decay and deteriorate foodstuffs.

Fig: Individual quick freezing
Air blast system:
This refers to vigorous circulation of cold air in order to freeze the product. Freezing is done by placing the foodstuffs on trays or on a belt which are then passed slowly through an insulated tunnel containing air in it. Here the air temperature is approximately -18 to -340C or even lower.
For food products with unusual shapes, air-blast freezers are used.
Fig: air blast freezing
Continuous spiral conveyor system:
In this type of system the product is carried on a spiral conveyor from the time it enters the low-temperature environment until it leaves the system. For these types of systems, the freezing time is established by the rate of movement of the conveyor systems through the low-temperature environment.

Description: http://www.genemco.com/catalog/imgpop/MDFP122ijwhitespiralfreezer_pop.jpg  Fig: spiral conveyor freezing system

Continuous fluidized bed system:
Vertical jets of refrigerated air are blown up through the product, causing it to float and remain separated. This is a continuous process which takes up to 10 minutes. The product, e.g. peas, beans, chopped vegetables or prawns, move along a conveyor belt.

Continuous immersion freezing:
For products where rapid freezing is appropriate, direct contact between a liquid refrigerant such as nitrogen or carbon dioxide may be used. The product is carried on a conveyor through a bath of liquid refrigerant to establish direct and intimate contact with the liquid refrigerant.
Fig: Immersion freezing

Continuous cryogenic freezing systems:
Cryogenic freezing is defined as freezing at very low temperature basically below -60 0C. The refrigerant used at present in cryogenic freezing is liquid nitrogen and liquid carbon dioxide. Foods like mushrooms, sliced tomatoes, whole strawberries and raspberries are cryogenic frozen because they require ultrafast freezing.

Fig:Cryogenic freezing

Freezing characteristic of foods:
*      If the freezing not properly controlled can disrupt food texture, break emulsion, denature protein and cause undesirable physical and chemical changes.
*      Different food will have different freezing temperatures depending on the different composition and solid contents.
*      A given units of food  whether it has a bottle of milk, a cut of beef or a can sliced apples in sugar syrup will not freeze uniformly that is it will  not suddenly change from liquid to solid state.
*      A progressive freezing occurs starting from the outer surface and gradually progresses to the inner core.
*      As water in the outer, sections freeze the water in the inner portion of the food becomes more concentrated in dissolved solid and requires longer time to freeze.

Properties of Frozen Foods:
The food product properties of interest when considering the freezing process include density, specific heat, thermal conductivity, enthalpy, and latent heat. These properties must be considered in the estimation of the refrigeration capacity for the freezing system and the computation of freezing times needed to assure adequate residence times. The approach to prediction of property magnitudes during the freezing process depends directly on the relationship between unfrozen water fraction and temperature.

Factor affecting the quality of frozen foods:
There are five factors are important in the maintenance of the quality of foods in frozen storage. These include:
1.      Solute concentration: the food must be a solid or nearly so to maintain good quality during frozen storage. A partially frozen zone or an unfrozen core will deteriorated with respect to texture, color, flavor and other properties. This is not only because of the possible growth of the microorganisms but also due to high concentration of solutes in the remaining unfrozen water.
2.      Ice crystal size: when water inside the cells of living tissues such as meat, fish, fruits and vegetables freezes rapidly it forms minute crystals of ice. If the rate of freezing is slow the ice crystal size is large and clusters are also formed leading to physical rupture of cells. Large ice crystal can disrupt emulsions such as butter, frozen foam like ice cream, gel such as puddings and pie filling. Large ice crystals can puncture frozen foam bubbles of ice cream, resulting in the loss of volume on storage and often exhibiting synersis or water separation.
3.      Rate of freezing: it is important because rapid or instantaneous freezing produces small size ice crystal and also minimizes concentration effects of solutes by decreasing the time of contact between solutes and food tissues and other constituents.
4.      Final temperature: the final temperature to accuracy of ± 10 C is important. The temperature -180C or below is recommended for maintaining quality and cost. Microbiologically, -180C storage is not necessary since most pathogens do not grow below +3.30C and food spoilage organisms do not grow below -9.40C. but enzymatic reactions are not stopped by only slowed down because enzymes retained their activity even -730C.
5.      Intermittent thawing: repeated thawing and freezing during storage due to temperature fluctuation are more detrimental. As little as a 30C fluctuation at -180C can be damaging. In general, a quick final thawing is superior to slow thawing.

Packaging requirements for frozen food:
Unprotected food is subjected to oxidation and contamination from the atmosphere of the chamber leading to substantial deterioration in quality of such foods. Hence it is necessary to package the food to maintain the quality. The package must be functional, lend to mechanical handling, economical in space and cost. Wood, metal, glass, paper and plastic materials are used successfully as frozen food containers. The simplest protective coating possible for frozen foods is a glaze (coating of ice) which has been used in fishing industry. The glaze must be replaced.

Effect of Freezing on Food:
         Low temperatures do not significantly affect the nutritional value of food, but thiamine and vitamin C may be destroyed when vegetables are blanched (briefly immersed in boiling water) before freezing.
         Freezing does not affect the nutritive value of protein but it is possible to denature proteins. Though biological value of denatured protein need not differ from native protein, the appearance and quality of the food may be seriously altered.
         Enzyme activity is only retard by freezing temperature hence control of enzyme activity is archived by heat treatment (blanching) prior to freezing and storage in case of fruits and vegetables.
         Frozen storage even at -90C permits serve damage to the quality of the food both in the loss of nutrients and appearance. Long term storage at -60C   yield unacceptable food.
         Moisture loss, or ice crystals evaporating from the surface area of a product, produces freezer burn—a grainy, brownish spot where the tissues become dry and tough in frozen storage. This surface freeze-dried area is very likely to develop off flavors. Packaging in heavyweight, moisture proof wrap will prevent freezer burn.
         If fish is frozen too slowly, some of its cells may rupture and release nutrients into the liquid that drips from the fish when it thaws.
         Some flavours become weaker and some become stronger when food is frozen.
         Freezing may be destabilizing emulsion of oil-in-water or water-in-oil; this is serious in prepared precooked frozen foods and food products.
         Recrystallization is frequently spoils ice cream.

Effect of Freezing on Microorganism:

Ø  The growth of microorganisms in foods at temperatures below about –120C has been confirmed. Thus storage of frozen foods at about –180C and below prevents microbiological spoilage.
Ø  Although microbial numbers are usually reduced during freezing and frozen storage (except for spores), frozen foods are not sterile and can spoil as rapidly as the unfrozen product if temperature are sufficiently high and storage times at these temperatures are excessive.
Reverse process of freezing. More difficult than freezing since:
1.      Transfer of heat of fusion has to be made through  a layer of already thawed product which has a much lower thermal conductivity (about 1/4th), and thermal diffusivity (about 1/8th), as compared to frozen product.
  1. The driving force for heat transfer, the temperature difference between the surrounding and the thermal center is limited due to heat damage on food.
Thawing is done by using hot air blasts, by immersion in hot water, by microwave, dielectric and resistance heating.

Freezing Rate and Freezing Point:
The freezing rate (°C/h) for a product or a package is defined as the difference between the initial and the final temperature divided by the freezing time (IIR, 1986). Since the temperature at different locations of a product may vary during freezing, a local freezing rate is defined for a given location in a product as the difference between the initial temperature and the desired temperature, divided by the time elapsed until the moment at which the desired temperature is achieved at that location. The freezing point is usually defined as the highest temperature at which ice crystals are found to be stable.
Freezing point depression:
Probably one of the more revealing properties of water in food is the freezing point depression. Since all food products contain relatively large amounts of moisture or water in which various solutes are present, the actual or initial freezing point of the water in the product will be depressed to some level below that expected for pure water. The magnitude of this freezing point depression is due to the molecular weight and concentration of the solute in the food product and in solution in the water.

 The freezing point depression equation for an ideal solution can be expressed as-
            Where, λ= molar latent heat of fusion, TA = is the equilibrium freezing temp. Depression; TAo= Absolute temperature; XA= mole fraction of water in solution. Rg= universal gas constant.
The commonly used expression for freezing point depression in dilute solutions is obtained:
Where L= latent heat of fusion,m=molality

Freezing curve:
Point AB
–Food cooled below freezing point (less than 0)
–At point B water remains liquid although the temperature is below 0°C.
–This phenomenon is called super cooling
-           Going below freezing point without the formation of ice crystals (crystallization)
-          It yields better quality food than if not present
-          This shows that the undesirable effects of freezing are due to ice formation rather than reduction of temperature

Point BC
–Temperature rises rapidly to the freezing point (giving off heat of fusion)
–Ice crystals begin to form
–Latent heat of crystallization is release
(Ice crystals forming-Crystallization)
• Consists of
  1. Nucleation (site for crystal formation and growth)
• Association of molecules into a tiny ordered particles sufficient to survive and serve as a site for crystal growth. It can be:
– Homogenous (pure water)
– Heterogeneous (most foods)
– Dynamic (spontaneous)
  1. Crystal growth (where it is formed)
-          Is the enlargement of the nucleus by the orderly addition of molecules. Crystal growth can occur at temperatures just below melting point while nucleation starts at lower temperature (supercooling)
-          Heat transfer is most responsible for limiting the rate of crystallization due to the large amount of latent heat needed
Point CD
–Heat is removed as latent heat so the T=constant
–Major part of ice is formed
–In unfrozen liquid there is an increase in solute concentration and that is why the temperature falls slightly
Point DE
–One of the solutes becomes supersaturated and crystallizes out.
–Latent heat of crystallization is realized and the temperature rises to EUTECTIC point for that solute.
         Temperature where there is no further concentration of solutes due to freezing, thus the solution freezes.
         Temperature at which a crystals of individual solute exists in equilibrium with the unfrozen liquor and ice
         Difficult to determine individual eutectic points in the complex mixtures of solutes in foods so term FINAL EUTECTIC POINT is used.
         Lowest EUTECTIC temperature of the solutes in the food

Point FG
–Temperature of the ice water mixture falls to the temperature of the freezer
–Percentage of water remains unfrozen
• Food frozen below point E forms a glass which encompasses the ice crystals.

Freezing time calculation:

*        Plank’s model is one of the most widely used equations to predict freezing time and is one of the most simple to use. The derivation makes a number of assumptions which may be summarized as follows:
(i)     Initially all food is at a distinct freezing point, which remains constant for both the frozen and the unfrozen layers;
(ii)      The thermal conductivity of the food is constant and the frozen and unfrozen layers have equal density;
(iii)    There is a distinct interface, or ice front, which moves from the freezing medium into the block at a uniform rate;
(iv) The heat capacity of the frozen layer is negligible, that is the latent heat changes are far greater than the sensible heat changes to the frozen layer; and
(v)               Heat transfer is sufficiently slow that it approximates to a steady-state process
A more generalized form of Plank’s equation can be written as
Where the parameters P and R assume different values for different geometries. These are summarized in table below:
Table: Values of parameters in Plank’s equation

*     Other Freezing-Time Prediction Methods:(Not discussed at this section)
§ Pham’s Model.
§ Stefan’s Model
§ Nagaoka’s model

The major groups of commercially frozen foods are:
Ø  Fruits (strawberries, oranges, raspberries) either whole or pureed, or as juice concentrates
Ø  Vegetables (peas, green beans, sweet corn, spinach, and potatoes)
Ø  Fish fillets and sea foods (cod, plaice, shrimps and crab meat) including fish fingers, fish cakes or prepared dishes with an accompanying sauce
Ø  Meats (beef, lamb, poultry) as carcasses, boxed joints or cubes, and meat products (sausages, beef burgers, reformed steaks)
Ø  Baked goods (bread, cakes, fruit and meat pies)
Ø  Prepared foods (pizzas, desserts, ice cream, complete meals and cook–freeze dishes).

Sivasankar,B. Food Processing and  Preservation,Ch-17; Low temperature Food Processing and preservation;pp-231-243.
William C. Frazier and Dennis C. Westhoff, Food Microbiology,4th Edition,ch-7,Preservation By Use of Low Temperatures,pp-125-131.
Paul  Singh, R.  And Dennis  R.  Heldman ; Introduction to Food Engineering 4th   Edition,Ch-7,Food Freezing pp-501-531.
Smith, P.G. Introduction to Food Process Engineering 2nd Edition Ch- 11, Low-Temperature Preservation, pp-276-294.
Desrosier and desrosier;The technology of food preservation;4th Edition;ch-5,Principles of food freezing,pp-110-151
Jinnat Are Begum,Food Technology,ch-7,Freezing;p-52.
Dennish R. Heldman and Daryl B. Lund; Handbook of Food Engineering;2nd Edition,Ch-6,Food Freezing,pp-438-444.
Bing, L., & Sun, D. -W. (2002). Novel methods of rapid freezing and thawing of foods—A review.Journal of Food Engineering, 54, 175–182.
Quang Tuan PHAM.(2008). Advances in Food Freezing/Thawing/Freeze Concentration
Modelling and Techniques; Japan Journal of Food Engineering, Vol. 9, No. 1, pp. 21 -32,

Garrote, R. L., & Bertone, R. A. (1989). Osmotic concentration at lowtemperature of frozen strawberry halves. Effect of glycerol glucose,and sucrose solution on exudate loss during thawing.Food Scienceand Technology, 22, 264–267.

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