A frozen mixture of milk and rice was used in China around 200 BC. The Chinese may be credited with inventing a device to make sorbets and ice cream. They poured a mixture of snow and saltpetre over the exteriors of containers filled with syrup, for, in the same way as salt raises the boiling-point of water, it lowers the freezing-point to below zero.
Before the development of modern refrigeration, ice cream was a luxury reserved for special occasions. It was made by hand in a large bowl placed inside a tub filled with ice and salt. This was called the pot-freezer method. In the pot-freezer method, the temperature of the ingredients is reduced by the mixture of crushed ice and salt. The salt water is cooled by the ice, and the action of the salt on the ice causes it to (partially) melt, absorbing latent heat and bringing the mixture below the freezing point of pure water. The immersed container can also make better thermal contact with the salty water and ice mixture than it could with ice alone.
The development of industrial refrigeration by German engineer Carl von Linde during the 1870s eliminated the need to cut and store natural ice, and, when the continuous-process freezer was perfected in 1926, commercial mass production of ice cream and the birth of the modern ice cream industry was underway.
In ice cream plant, general ice cream processing steps includes: Blend ingredients, Pasteurize mix, Homogenize, Age the mix, Add colors and liquid flavors, Freeze, Add nuts, fruits and bulky flavorings, Package, and Harden.
1. Blend Ingredients
The milk fat source, nonfat solids, stabilizers and emulsifiers are blended to ensure complete mixing of liquid and dry ingredients.
2. Pasteurize Mix
Ice cream mix is pasteurized at 68.3°C (155°F) for 30 minutes or 79.4C (175°F) for 25 Seconds. The conditions used to pasteurize ice cream mix are greater than those used for fluid milk because of increased viscosity from the higher fat, solids, and sweetener content, and the addition of egg yolks in custard products.
Ice cream mix is homogenized (2500 to 3000 psi) to decrease the milk fat globule size to form a better emulsion and contribute to a smoother, creamier ice cream. Homogenization also ensures that the emulsifiers and stabilizers are well blended and evenly distributed in the ice cream mix before it is frozen.
4. Age the Mix
Ice cream mix is aged at 5°C (40°F) for at least 4 hours or overnight. Aging the mix cools it down before freezing, allows the milk fat to partially crystallize and the gives the proteins stabilizers time to hydrate. This improves the whipping properties of the mix.
5. Add Colors and Liquid Flavors
Colors and Liquid Flavors may be added to the mix before freezing. Only ingredients that are liquid can be added before the freezing, to make sure the mix flows properly through the freezing machines.
The process involves freezing the mix and incorporating air. Ice cream mix can be frozen in batch or continuous freezers and the conditions used will depend on the type of freezer. Batch freezers consist of a rotating barrel that is usually filled one-third to one-half full with ice cream mix. As the barrel turns, the air in the barrel is incorporated into the ice cream mix. Ice cream freezers designed for home use are batch freezers. Continuous freezers consist of a fixed barrel that has a blade inside that constantly scrapes the surface of freezing barrel. The ice cream mix is pumped from a bulk tank to the freezing barrel and the air is incorporated with another pump just before it enters the freezing barrel. The continuous freezing process is much faster than the batch freezing process. The addition of air is called overrun and contributes to the lightness or denseness of ice cream. Up to 50% of the volume of the finished ice cream (100% overrun) can be air that is incorporated during freezing. The overrun level can be set as desired to adjust the denseness of the finished product. Premium ice creams have less overrun (approximately 80%) and are more dense than regular ice cream. At the point of discharge from the freezer (draw temperature), only about 50% of the water in ice cream is frozen. Soft serve ice cream is generated at this point in the freezing process.
7. Add Nuts, Fruits and Bulky Flavorings
Fruits, swirls, and any bulky type of flavorings (candy pieces, nuts, and etc.) are added at this point. These ingredients can not be added before freezing or they would interfere with the smooth flow of the mix through the freezer. The ice cream at this point is soft and it is easy to mix in the bulky flavorings so they are uniformly distributed throughout the ice cream. Mixing in bulky flavorings after freezing also prevents damage to the pieces and allows them to remain whole or in large chunks.
As desired, depending on the product.
The ice cream is cooled as quickly as possible down to a holding temperature of less than -25°C(-13°F). The temperatures and times of cooling will depend on the type of storage freezer. Rapid cooling will promote quick freezing of water and create small ice crystals. Storage at -25°C(-13°F) will help to stabilize the ice crystals and maintain product quality. At this temperature there is still a small portion of liquid water. If all the water present in the ice cream were frozen, the ice cream would be as hard as an ice cube.