Ingredients are chosen by the manufacturer on the basis of desired quality, availability, and cost. The ingredients are blended together and produce what is known as the "ice cream mix". The mix is first pasteurized. Pasteurization is a process which is designed to kill all of the possible pathogens (disease causing organisms) that may be present. Organisms such as Mycobacterium tuberculosis, Salmonella, Staphylococcus, Listeria, and others that cause human disease can be found associated with farm animals and thus raw milk products must be pasteurized. In addition to this very important function, pasteurization also reduces the number of spoilage organisms such as psychrotrophs, and helps to "cook" the mix. 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. Homogenization helps to produce a smooth product when frozen. 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.
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.
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 -30o to -40oC where most of the remainder of the water is frozen. Below about -25oC, 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.
Salt and ice
Making ice cream at home requires the use of an ice cream machine. The "homemade" or hand-crank freezer used was the forerunner to today's modern equipment. Many people enjoy fond memories of hot summer days spent preparing the ice cream mix, loading the bucket with ice and salt, and cranking the freezer for a half hour until it was considered too stiff to continue or until one's hunger got the best of them. All of the various steps in making ice cream via the bucket are similar to the commercial processing stages. The mix is prepared and pasteurized, aged, dynamically whipped and frozen in a freezer equipped with blades and dashers, and then hardened prior to consumption. Ice and salt are used, however, rather than the ammonia or Freon jacket in the commercial freezer above.
The concept of melting ice with salt is not new to anyone in this latitude. Indeed, our roads, driveways, and sidewalks are kept bare in the winter by such a process. As salt is applied to ice, a concentrated brine solution forms on the ice, which has a very low freezing point. The freezing point of a 20% solution of salt is -16.6oC. As a result, more ice melts to dilute this solution, until the freezing point of the solution matches the outside temperature (equilibrium is established). The same phenomenon is occuring in the brine solution in the ice cream freezer. As the salt continues to dissolve more ice melts to accommodate this concentrated salt solution with its very low melting point. At the same time, both the heat of solution of the dissolving salt, and the latent heat of fusion of the melting ice are adsorbed from the ice itself, thereby lowering the temperature of the salt, ice and brine mixture. The temperature of this mixture can be controlled by the amount and ratio of salt and ice present. As examples, consider the following data: a 2% NaCl (salt) solution has a freezing point of -1.4oC, 5% salt conc. = -3.5oC, 10% salt = -7.4oC, 15% salt = -11.7oC and 20% salt = -16.6oC. The lowest temperature which can be achieved with a sodium chloride brine is -20oC, at a concentration of 23% salt. Higher concentrations result in salt crystallization.
This brine, in turn, is adsorbing heat from the freezing ice cream inside the can, and thus ice and salt need to be continually added to keep the ice temperature low enough to freeze the ice cream. (Bear in mind that the freezing temperature of the ice cream is depressed below 0o due to the presence of dissolved sugars.) This process is a lesson in heat transfer in itself!
Prof. Douglas Goff, Ph.D.