The basic steps in the manufacturing of ice cream are generally as follows:
- blending of the mix ingredients
- aging the mix
First the ingredients are selected, weighed and then blended together to produce what is known as the "ice cream mix". The mix is then pasteurized. Pasteurization is the biological control point in the system, designed for the destruction of pathogenic bacteria. In addition to this very important function, pasteurization also reduces the number of spoilage organisms such as psychrotrophs, and helps to hydrate some of the components (proteins, stabilizers).
Both batch pasteurizes and continuous (HTST) methods are used. The mix is also homogenized 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 µm. Two stage homogenization 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. Homogenization provides the following functions in ice cream manufacture:
- Reduces size of fat globules
- Increases surface area
- Forms membrane
- makes a smoother ice cream
- gives a greater apparent richness and palatability
- better whipping ability
- decreases danger of churning the fat
- makes possible the use of butter, frozen cream, etc.
- increases resistance to melting
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 * Fat crystallization * Protein and stabilizer hydration viscosity increase * Membrane rearrangement protein/emulsifier interaction
Freezing and Hardening
Following mix processing, the mix is drawn into a flavor tank where any liquid flavors, fruit purees, or colors 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 (see refrigeration section). 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 air is referred to as overrun, the calculations of which are explained in detail elsewhere. This gives the product its characteristic lightness. Without air, ice cream would be similar to a frozen ice cube.
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. After the particulates have been added, the ice cream is packaged and is placed into a blast freezer at -30° to -40°C where most of the remainder of the water is frozen. A primer on the fundamental aspects of freezing will provide more details of this complex process.
Below about -25°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 dependent upon the temperature of storage. This limits the shelf life of the ice cream.