To understand and appreciate the scientific principles involved in making ice-cream
Ice-cream is a foam. A foam is a dispersion of gas bubbles in a liquid. Some ice-creams may have more air by volume compared to actual solids. The amount of air present in ice-cream is referred to as its overrun. Apart from air, ice-cream ingredients include ice crystals, fat, sugars, proteins and emulsifiers.
- Ice crystals: Comes from the water present in the ice-cream mix. It contributes to the firm texture associated with ice-cream
- Milk fat: Provides flavor since flavor compounds in foods are generally absorbed in fat. It also contributes a soft texture
- MSNF: Provides additional protein which produces a firm texture. The firmer texture is likely due to the water-absorbing properties of the milk proteins
- Stabilizer: These are generally gums and pectins, although gelatin is sometimes used. Stabilizers act like a mop in the food system. Whenever the ice-cream melts, for example when freezer temperature drops, a portion of the ice is thawed which, if not refrozen quickly will grow into large crystals causing a grainy texture. Stabilizers prevent this by absorbing the melted ice.
- Emulsifier: Emulsifiers such as lecithin and egg yolk, helps to destabilize the natural protein emulsifiers in the ice-cream, causing them to partially coalesce (or stick together), and trap air bubbles to create a more stable foam.
- Air: Air is the cheapest ingredient added. Ice-cream volume is dependent on the amount of air that is whipped in.
The ice-cream manufacturing process includes the following basic steps:
- Blending of Ingredients: All liquid and dry ingredients are blended in.
- Pasteurization: The ice-cream mix is pasteurized either by batch (71oC, 30 min) or continuous method (82oC, 25 sec). Pasteurization temperature for ice-cream is higher than for milk since the higher fat and protein content act as a protective shield for bacteria.
- Homogenization: The ice-cream mix is homogenized to disperse fat throughout the mix. This helps to improve texture and smoothness of the ice-cream.
- Aging: The temperature of the mix is dropped to 4.4oC and held overnight. Aging gives time for hydration of proteins and stabilizers which helps in improving viscosity. It also allows time for fats to coalesce and trap air.
- Freezing: The ice-cream mix is frozen to -5.5oC using a scraped surface freezer. The ice-cream mix comes in contact with the cylindrical wall of the freezer which is cooled by a coolant (usually ammonia) in its double-jacketed wall. As it makes contact with the wall, heat is removed rapidly. The rotating blades in the cylinder not only brings the mix in contact with the wall, but also mixes in air to create the foamy texture. Rapid cooling is necessary to prevent the formation of large ice crystals that would cause the ice-cream to be grainy. At the end of freezing, the ice-cream has a soft-serve consistency.
- Packaging: The ice-cream is nozzled into packages.
- Hardening: The soft-serve consistency of the ice-cream is not suitable for distribution. Therefore, the ice-cream is hardened by placing the packages into a blast freezer that is capable of rapid freezing to -34oC.
Materials and Equipment
- Ice-cream ingredients (See formulation table below)
- Ice-cream maker
- Stainless steel pot
- Mixing spoon
Product Formulation Table
|Ingredients||Weight (g)||Percentage (%)|
|Heavy whipping cream||397.0||40.5|
|Dry milk powder||35.0||3.6|
- Add half the sugar in the formula to the egg yolk
- Collect milk in pot
- Add the remaining cream, dry milk powder, salt, and remaining sugar, and combine in a homogeneous mix
- Heat to 88oC with continuous stirring to prevent burning at bottom
- Pour hot milk slowly onto the egg/sugar mixture with continuous stirring. Note: This process is called tempering. It is done to prevent the eggs from coagulating as sugar increases the egg coagulation temperature
- Gradually transfer the egg to the remaining whole milk with continuous mixing. (Note: The temperature will drop)
- Reheat the milk to bring temperature up to 82oC and dissolve vanilla extract
- Cool to 4oC (placing the pot in crushed ice can speed up the process)
- Freeze in ice-cream maker for 20 minutes
- Store in freezer to harden for at least 12 hours
- Draw a flow diagram for making ice-cream (5 points)
- What is a foam? (2 points)
- Name the type of emulsifier found in egg (1 point)
- A gum called carrageenan is often used in ice-cream. What is its function? (2 points)
- Add the missing word: “Ice-cream overrun is high if it has a large amount of ________ incorporated in it” (1 point)
- Home-made ice-cream that is made in a bag and frozen in the refrigerator is generally very icy and lacks smoothness. Explain why it has this texture and not the typical smooth mouthfeel you get when an ice-cream maker is used? (5 points)