Freezing of Mix

When the mix has been properly aged it is ready for next step, that is, freezing.Freezing is one of the most important operations in the preparation of ice cream,because, upon this depends the quality, palatability and yield of the finished product.The freezing process may be divided as follows into two parts:

a) The mix with colour and flavouring materials added is quickly frozen in the ice cream freezer. During freezing, the mix is agitated to incorporate air in such a way as to produce and control the formation of small ice crystals so necessary to give smoothness in body and texture, palatability and satisfactory overrun in the ice cream.

b) The mix partially frozen to a certain consistency in the ice cream freezer is drawn out from freezer into packages and quickly transferred to cold storage rooms where the freezing and hardening process is completed without agitation.Fast freezing is essential for smooth textured ice cream because ice crystals that are formed quickly are smaller than those formed slowly. Therefore it is,desirable to freeze and draw out from the freezer in a short time as possible. A continuous freezer accomplishes this in few seconds, while batch freezer takes 6 to 10 minutes or more time depending upon several factors. Also, since freezing continues after ice cream is placed in the hardening rooms, the ice crystals formed during the hardening period are larger because they form more slowly than in the freezer. For this reason, it is desirable to freeze the ice cream as stiff as possible and yet have it liquid enough to draw out of the freezer.

i. Types of Ice Cream Freezers

The equipment used for partial freezing of ice cream mix is called ice cream freezer.Three types of ice cream freezer are identified

i) Batch freezer : It consists of a refrigerated drum or cylinder provided with a dasher and scrapper fitted in to it. An electric motor drive turns the dasher assembly within the cylinder. The scrapper removes the frozen ice cream from the refrigerated surface and dasher whips the ice cream to introduce air into it.Freezer is also filled with a hopper to introduce fruits and flavour. Commercial freezers vary from 5 litres to 50 litres capacity.

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The batch freezer consists of freezing chamber and a dasher. The dasher assembly comprises of two parts, viz., the scrapper blades and the beater.

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The dasher performs the following functions: 

•  Aids in transmission of refrigeration by keeping mix in continuous contact with freezer walls.
•  Scraps freezer walls free from ice crystal.
•  Incorporates air.
•  Pushes mix continuously forward (this helps to unload ice cream from freezer).
•  The temperature of the refrigerant circulating in the freezer should be below – 23°C to get rapid formation of ice crystals.

 Freezing procedure in the batch freezer: The mix is run into the freezer, the dasher started and then the refrigerant is turned on. This sequence must be observed at all times to avoid damage to the machine. The flavouring and colouring materials are added to the mix. The total volume of the mix, flavour and colour should be about half the total volume of the freezing chamber. To avoid variation from batch to batch, each batch of mix must be measured carefully before putting in to the freezer.

The freezing operation must be closely watched to know the correct time to shut-off refrigeration. This comes mostly by experience of the operator. If the refrigerant is allowed to run for a longer period the following problems may arise.

•  Difficulty in obtaining the desired overrun
•  Mix becomes stiff
•  Whipping becomes difficult

 After refrigerant is shut-off, the freezer without certain limits continue to operate,incorporate more air, freezing more of water until the product attains both the desired overrun and desired consistency. The ice cream from the freezer should be emptied into the container/package as rapidly as possible to avoid fluctuations in overrun in packages. When the ice cream is drawn from the freezer it should be stiff enough to hold its shape and yet soft enough to loose its shape within a short time.

Cleaning of batch freezer: After the last batch of ice cream is drawn, the freezer is first flushed with water. The rinse water should not be very hot. During rinsing operation the dasher should be turned only a few revolutions. The dasher and other removable parts should be removed to a sink and thoroughly scrubbed with a hot (about 50°C) detergent solution, rinsed, sanitized and stored where they may dry.The freezer chamber is also scrubbed with hot detergent solution, rinsed and left open to dry.

ii) Continuous Freezer: The process consists of continually feeding into end of the freezing chamber a metered amount of ice cream mix and air. As the mixture passes through the freezing chamber it is agitated, partially frozen and then discharged in a continuous stream of about same consistency usually obtained from a batch freezer.

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The continuous freezers are of two types: One type has two pumps at the back of the freezer barrel. One pump pulls mix from the supply tank and pumps into second pump. The second pump operates at a speed almost twice that of the first pump. This has the effect of creating a partial vacuum between the two pumps. A valve device in the piping between the pumps allows air to be sucked in. The amount of air incorporated, in the mix can be regulated by a control valve. The mix and the air both are pumped into the freezer’s barrel by the second pump. As mix freezes, semi-frozen ice cream is forced out of the front end.

The second type of continuous freezer operates in a similar fashion. However the two pumps are at the front end. One pump pushes the mix into the back end of the freezer and the other helps to pull the semi frozen ice cream from the freezer barrel. There is a separate pump that forces air directly into the freezer barrel.

In the continuous freezer, the mix is continuously pumped into the machine and the frozen product is continuously discharged from the machine. Initially, the amount of air introduced is regulated to give the desired overrun in the ice cream. The temperature of refrigerant on the freezing chamber is adjusted to give the desired consistency when the product leaves the machine.

Cleaning of continuous freezer: Method of cleaning continuous freezer is similar to the methods of cleaning most of the dairy equipments. Remove all pipelines, which carry ice cream mix to the freezer. Remove the front of freezer unit and pull out the freezer dasher. Rinse all parts of machine, which come in contact with ice cream mix, with warm water (35°C-40°C). Flush the freezing tube with cold water followed by warm water. Thoroughly wash and clean all parts of freezer with hot detergent solution. Rinse with hot water to remove all traces of detergent. Finally rinse with scalding hot water and allow it to dry.

iii) Soft serve freezer: This is similar to batch the freezer in construction and operation with lesser capacity. The overrun obtained in this type of freezer is less than that is obtained in batch freezer. Time taken for freezing ice cream in the freezer will dependent on many factors. Some factors are mechanical while some others due to the property of ice cream mix.

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  ii. Factors Influencing Freezing

These factors may be mechanical factors and the factors related with the properties of the mix as listed below.

A) Mechanical factors:

•  Type and make of ice cream freezer.
•  Condition of freezer wall and blades.
•  Speed of dasher.
•  Temperature of refrigerant.
•  Velocity of refrigerant passing around freezing chamber.
•  Overrun desired.
•  Temperature at which ice cream is drawn.
•  Rate of unloading of freezer.

B) Characteristics of mix or property of mix:

•  Composition of mix.
•  Freezing point of mix.
•  Acidity content of mix.
•  Kind of ingredients, particularly those carrying fat.
•  Methods of processing mix.
•  Kind and amount of flavouring materials used.

 Freezing time and temperature: The freezing time and temperature of different types of ice cream freezers are given below in Table

Freezing time temperature for different freezers

Changes occurring during freezing: The functions of the freezing process are listed below.

•  To freeze a portion of water of the mix
•  To incorporate air into the mix.

 This involves:

•  Lowering of the temperature of the mix from ageing temperature to the freezing point.
•  Freezing a portion of water of the mix.
•  In-corporation of air into the mix.
•  Cooling the ice cream from the temperature it is drawn from the freezer to hardening room temperature. 

The temperature of the mix in the freezer drops rapidly while the sensible heat is being removed and before the ice crystals are formed. This process takes less than a minute or two. Rapid agitation of the mix reduces the viscosity of the mix. Also rapid agitation hastens incorporation of air into the mix. When freezing point is reached, water in the mix starts freezing and ice crystals are formed. As part of water is converted into ice crystals, the concentration of sugar in the remaining water increases causing the freezing point of the mix to be lowered. So the temperature of the mix must be lowered before some more ice crystals could be formed. Thus, the freezing point is continuously being lowered by the formation of more ice crystals while temperature drops. The first phase of the freezing process accounts for freezing 33 to 67% of water depending on the drawing temperature. The hardening process then may account for the freezing of remaining portion of water up to 90-95%.

During this period when the temperature of ice cream mix is being lowered and ice crystals are being formed, ingredients such as acid fruit juices fruits and nuts may be added without any danger of coagulation of the mix. During this process more air is also incorporated into the mix due to agitation. Freezing process gives a definite texture to the ice cream. The texture of ice cream depends on the size and no of air cells and ice crystals and unfrozen materials in the ice cream. Air cells are dispersed in a continuous liquid phase with embedded ice crystals. The liquid phase contains solidified fat particles, milk proteins, in-soluble salts, stabilizers and some lactose crystal. Sugar, part of lactose and soluble salts are in true solution.