Ice cream manufacturing

The details of the process vary from factory to factory according to the type of equipment and scale of manufacturer.

These are: mix preparation, which consists of dosing and mixing of the ingredients; pasteurization and homogenization; ageing; freezing and hardening.

Knowing a mix specification, mix calculations are performed to determine the amounts of desired ingredients needed to formulate the mix.

Mix processing begins with the assembly of the necessary ingredients in the desired amount.

In the next step is blending the ingredients. Mix blending can be performed at refrigeration temperatures 4 °C or at warmer temperatures 45 °C.

Freezing the mix is one of the most important operations in making ice cream, since the quality, palatability and yield of the finished product depend on proper freezing.

In the dynamic freezing step, cold, flavored ice cream mix enters the cylindrical freezer barrel and is chilled with a liquid refrigerant. The temperature of the mix entering the freezer is very important, 0 to -1 °C optimizing performance. The entry temperature of the mix should be constant to facilitate control of overrun and freezing rate.

At the end of freezing process, the ice cream is only partially frozen. The ice cream exiting the scrap-surface freezer is filled into packages, which are then sent through gardening step where additional ice is frozen and the cream becomes hard.
Ice cream manufacturing

Ice crystal formation

Freezing is the crystallisation of water, and the freezing point is the temperature at which ice crystals can be sustained in equilibrium with water. The freezing point of water is depressed by the addition of solute.

Unlike pure water ice crystal formation and growth in aqueous solutions occurs over a temperature range. The process begins with a decrease of the temperature below the freezing point of the solution. Once it has arrived at this point, the nucleation or formation of the ice glass nuclei begins. The nuclei are formed for a mechanism of homoemous nucleation or molecular addition.

Once ice crystal nuclei have been formed, the following step is crystal growth. In the stage of crystal growth, different process can be present that include the diffusion of water molecules from the bulk solution toward the ice crystal surface, with subsequent incorporation of these molecules into the crystalline reticule that also include the transfer of of the formation of latent heat from the crystal surface toward the bulk solution.

As ice forms during cooling, only water molecules comprise the ice crystals. As a result, all other components (salts, etc) become concentrated in the remaining solution. As the solution concentration increases, the chemical potential of the water in the solution decreases.

Water will continue to crystallise until the chemical potential of the water in the liquid phase equals the chemical potential of the water in the solid phase. In other words, the remaining solons will reach it equilibrium freezing point.

In meat processing, ice crystals may shorten shelf life by puncturing membranes and causing fluid loss from muscles. They also lower metmyoglobin reducing power, and induce off-colour development and inferior bloom.
Ice crystal formation

Processing of green and wax beans

At the processing plant, beans are sorted out for size, washed and their ends are snipped off mechanically. They are then cut transversely or longitudinal and blanched.

Some beans are frozen individually and placed in plastic pouches alone or as a component of mixed vegetables or they may be first packaged and then frozen in a blast freezer. Some beans are canned in brine and processed.

Canning and freezing are the two most commonly used commercial methods of processing the beans. Other products such as purees for the beans are marketed as baby foods or specialty applications.
Processing of green and wax beans

Brine freezing of fish

Brine freezing was valued in the first half of this century as an inexpensive way of quick freezing. It is widely used since it allows relatively fast freezing.

In freezing with brine, the fish is frozen by contact with a refrigerated liquid suitable for contact with the fish. Various types of brine are used but those with sodium chloride are most important and most economical for fish.

The freezing temperatures are then limited to -18 °C (NACl) to around – 25 °C due to their eutectic points, and the fish is frozen by spray freezing or by immersion in the brine.

In tuna fisheries, such brine has been used for quick freezing of fish on board directly in the wells.

Spray brine has been used in smaller vessels on order to decreases the weight of brine on board. A small tank of brine can be kept as a reservoir for the spray, thus eliminating the full flooded holds.

Freezing by sprinkling is used for freezing herring and mackerel in Scandinavia particularly Norway. The boxes of fish are placed in stacks.

On top there is a box with a perforated base in order to distribute the brine evenly. The brine passes from one box to other and down into a tank where it again passes over cooling coil and is then pumped up and sprinkled over the boxes of fish until the fish is frozen (1-2 hours).

Using modern blast freezers operating at temperatures as low as -50 to -60 C, the fish may be frozen less than 24 hr.

Brine freezing, once popular in the tuna fishing industry, is begin replaced by air-blast freezers for preservation of the catch.
Brine freezing of fish 

High Pressure shift freezing (HPSF)

In pressure shift freezing, the food is cooled under high pressure to sub-zero temperatures.  If water is subjected to very high pressure and cooled, it may remain liquid at a temperature much lower than its freezing temperature at atmospheric pressure.

If now the pressure is suddenly released, rapid freezing will occur. This method of freezing is called high pressure-shift freezing. Because the freezing point decreases with pressure, phase change does not take place.

Application of high pressure during freezing can avoid products damage due to the instantaneous and homogenous formation of ice through the product.

High pressure shift freezing, are gaining attentions a freezing method for high quality or freezing sensitive foods. The high level of supercooling during high pressure shift freezing of peach and mango led to uniform and rapid ice nucleation through sample volume, which largely maintained the original tissue microstructure.

In case of salmon, high pressure shift freezing produced a large amount of homogenously distributed fine and regular intracellular ice crystals, which helped in the maintenance of muscle fibers in comparison to the frozen muscle structure.
High Pressure shift freezing (HPSF)

High Pressure assisted Freezing (HPAF)

High pressure freezing processes operate in pressure-resistant vessels with thermally isolated thermostatic circuits to reach temperatures below 0 ° C.

Packed foods are immersed in the pressure/cooling medium and frozen.

In high pressure assisted freezing process, water solidification is produced while pressure around the product is high.  During freezing, use of high pressure facilities supercooling and promotes uniform and rapid ice nucleation throughout the product on pressure release, producing smaller ice crystals.

Phase transition occurs under constant pressure, higher than atmospheric pressure while the temperature is lowered to below the corresponding freezing point.

HPAF is based on the fact that freezing point changes with pressure and the product can be stored at normal frozen storage conditions.

High pressure assisted freezing was particularly useful for freezing large pieces of food when uniform ice crystals are requires.

In high pressure assisted freezing, product are cooled under 200 MPa to -20  ° C without ice formation, then the pressure is released and the high supercooling reached ( approx. 20 °C) promotes uniform and rapid nucleation.
High Pressure assisted Freezing (HPAF)

Cryogenic freezing processing of food

Cryogenics is a term defined as the ‘science of low temperature’ and their effect on the properties of the matter.

The introduction of cryogenic gases in the early 1960’s as an alternative to improve freezing processes in the frozen industry was a major product quality and process improvement.

In food processing, cryogenics implies use of very low temperature materials for chilling and freezing.

This type of refrigeration differs from other procedure because it does not depend on external the low temperature production systems.

Cryogenic freezing with nitrogen is carried out by first passing the food through nitrogen vapor at about -50° C and then freezing the food by spraying the refrigerant directly onto the food.

Fish, meat, poultry, fruit, vegetables and bakery products can all be frozen in this way.

The extremely low temperature characteristics of cryogenic food provide the ultimate in chilling and freezing rates.

The physical properties of the cryogenic gases provided an important tool to help the food industry to improve the plant automation, versatility, efficiency and manufacturing cost.

Cryogenic freezing processing of food

The Principles of Freezing Seafood

The Principles of Freezing Seafood
Freezing of fish takes place in three stages. The first stage involves the removal of heat and the lowering of temperature of the product to the freezing point. The third stage involves the subcooling of the food from freezing point to its terminal temperature.

The process of freezing is actually the transfer of heat from one material to another. The heat transfer is affected by the different factors such as:

*The difference in temperature between the fish and the cooling medium
*The method of heat transfer
*The type and technique of packing
*The thermal properties of the fish to be frozen.

In freezing , the transfer of heat, is primarily achieved by conduction or convection. The movement of heat from one center section of the product to its outer surface is a function of temperature difference and product thickness.

In the process, the heat from the center moves to the surface then to the cooling medium be it brine, air or refrigerant circulating in contact plates by forced convection of radiation.

The transfer of heat to the cooling medium varies with the velocity of the gas or liquid, its thermal properties, and its temperature.

The time required for freezing is influenced by different factors, such as thickness of the product type of package and method of packaging.

Packaging materials having low conductivity slow down the removal of heat from the product. A suitable material for packaging should have high thermal conductivity, which should be thin enough to allow rapid transfer of heat from the surface of the product to the cooling medium.

The fit of the package will also affect the freezing time and low freezing efficiency.
The Principles of Freezing Seafood