Basic Principles of Ice Cream Manufacture in Industry

The basic steps in the manufacturing of ice cream are generally as follows:

Blending of the mix ingredients
Homogenisation
Pasteurisation
Aging the mix
Freezing
Packaging
Hardening

Blending of the mix ingredients
First the ingredients are selected based on the desired formulation and the calculation of the recipe from the formulation and the ingredients chosen, then the ingredients are weighed and blended together to produce what is known as the "ice cream mix". Blending requires rapid agitation to incorporate powders, and often high speed blenders are used

Homogenisation
The mix is also homogenised which forms the fat emulsion by breaking down or reducing the size of the fat globules found in milk or cream to less than 1 µ m. Two stage homogenisation is usually preferred for ice cream mix. Clumping or clustering of the fat is reduced thereby producing a thinner, more rapidly whipped mix. Melt-down is also improved. homogenisation provides the following functions in ice cream manufacture:

Reduces size of fat globules
Increases surface area
Forms membrane
makes the use of butter, frozen cream, etc. possible

By helping to form the fat structure, it also has the following indirect effects:

Makes a smoother ice cream
Gives a greater apparent richness and palatability
Better air stability
Increases resistance to melting
Homogenisation of the mix should take place at the pasteurising temperature. The high temperature produces more efficient breaking up of the fat globules at any given pressure and also reduces fat clumping and the tendency to thick, heavy bodied mixes. No one pressure can be recommended that will give satisfactory results under all conditions. The higher the fat and total solids in the mix, the lower the pressure should be. If a two stage homogeniser is used, a pressure of 2000 - 2500 psi on the first stage and 500 - 1000 psi on the second stage should be satisfactory under most conditions. Two stage homogenisation is usually preferred for ice cream mix. Clumping or clustering of the fat is reduced thereby producing a thinner, more rapidly whipped mix. Melt-down is also improved

Pasteurisation
The mix is then pasteurised. Pasteurisation is the biological control point in the system, designed for the destruction of pathogenic bacteria. In addition to this very important function, pasteurisation also reduces the number of spoilage organisms such as psychrotrophs, and helps to hydrate some of the components

Both batch pasteurisers and continuous (HTST) methods are used

Batch pasteurisers lead to more whey protein denaturisation which some people feel gives a better body to the ice cream. In a batch pasteurisation system, blending of the proper ingredient amounts is done in large jacketed vats equipped with some means of heating, usually steam or hot water. The product is then heated in the vat to at least 68º C and held for 30 minutes to satisfy legal requirements for pasteurisation, necessary for the destruction of pathogenic bacteria. Various time temperature combinations can be used. The heat treatment must be severe enough to ensure destruction of pathogens. Following pasteurisation, the mix is homogenised by means of high pressure homogeniser. Batch tanks are usually operated in tandem so that one is holding while the other is being prepared. Automatic timers are recommended to ensure the proper holding times are adhered to

Continuous pasteurisation is usually performed in a high temperature short time (HTST) heat exchanger following blending of ingredients in a large, insulated feed tank. Some preheating from 30ºC to 40º C, is necessary for solubilisation of the components. The HTST system is equipped with a heating section, a cooling section, and a regeneration section. Cooling sections of ice cream mix HTST presses are usually larger than milk HTST presses. Due to the preheating of the mix, regeneration is lost and mix entering the cooling section is still quite warm.

Ageing

The mix is then aged for at least four hours and usually overnight. This allows time for the fat to cool down and crystallize, and for the proteins and polysaccharides to fully hydrate. Aging provides the following functions:

Improves whipping qualities of mix and body and texture of ice cream
.
It does so by:
Providing time for fat crystallization, so the fat can partially coalesce
Allowing time for full protein and stabilizer hydration and a resulting slight viscosity increase
Allowing time for membrane rearrangement and protein/emulsifier interaction, as emulsifiers displace proteins from the fat globule surface, which allows for a reduction in stabilization of the fat globules and enhanced partial coalescence
Aging is performed in insulated or refrigerated storage tanks or silos. Mix temperature should be maintained as low as possible without freezing, at or below 5º C. An aging time of overnight is likely to give best results under average plant conditions

Freezing and Hardening

Following mix processing, the mix is drawn into a flavour tank where any liquid flavours, fruit purees, or colours are added. The mix then enters the dynamic freezing process which both freezes a portion of the water and whips air into the frozen mix. The "barrel" freezer is a scraped-surface, tubular heat exchanger, which is jacketed with a boiling refrigerant such as ammonia or freon. Mix is pumped through this freezer and is drawn off the other end in a matter of 30 seconds, (or 10 to 15 minutes in the case of batch freezers) with about 50% of its water frozen. There are rotating blades inside the barrel that keep the ice scraped off the surface of the freezer and also dashers inside the machine which help to whip the mix and incorporate air.

Ice cream contains a considerable quantity of air, up to half of its volume. This gives the product its characteristic lightness. Without air, ice cream would be similar to a frozen ice cube. The air content is termed its overrun, which can be calculated mathematically

As the ice cream is drawn with about half of its water frozen, particulate matter such as fruits, nuts, candy, cookies, or whatever you like, is added to the semi-frozen slurry which has a consistency similar to soft-serve ice cream. In fact, almost the only thing which differentiates hard frozen ice cream from soft-serve, is the fact that soft serve is drawn into cones at this point in the process rather than into packages for subsequent hardening

Hardening

After the particulates have been added, the ice cream is packaged and is placed into a blast freezer at 30º C to - 40º C where most of the remainder of the water is frozen. Below about -25º C, ice cream is stable for indefinite periods without danger of ice crystal growth; however, above this temperature, ice crystal growth is possible and the rate of crystal growth is dependant upon the temperature of storage. This limits the shelf life of the ice cream

A primer on the theoretical aspects of freezing will help you to fully understand the freezing and recrystallisation process.

Hardening involves static freezing of the packaged products in blast freezers. Freezing rate must still be rapid, so freezing techniques involve low temperature (-40ºC) with either enhanced convection (freezing tunnels with forced air fans) or enhanced conduction (plate freezers)