Milkfat (or "Butterfat") / Fat
Milkfat, or fat in general, including that from non0dairy sources, is important to ice cream for the following reasons:
- increases the richness of flavour in ice cream (milkfat more so than non-dairy fats)
- produces a characteristic smooth texture by lubricating the palate
- helps to give body to the ice cream, due to its role in fat destabilization
- aids in good melting properties, also due to its role in fat destabilization
- aids in lubricating the freezer barrel during manufacturing (Non-fat mixes are extremely hard on the freezing equipment)
The limitations of excessive use of butterfat in a mix include:
- hindered whipping ability
- decreased consumption due to excessive richness
- high caloric value
The best source of butterfat in ice cream for high quality flavour and convenience is fresh sweet cream from fresh sweet milk. Other sources include butter or anhydrous milkfat.
During freezing of ice cream, the fat emulsion which exists in the mix will partially destabilize or churn as a result of the air incorporation, ice crystallization and high shear forces of the blades. This partial churning is necessary to set up the structure and texture in ice cream, which is very similar to the structure in whipped cream. Emulsifiers help to promote this destabilization process, which will be discussed below.
The triglycerides in milkfat have a wide melting range, +40° C to -40° C, and thus there is always a combination of liquid and crystalline fat. Alteration of this solid: liquid ratio can affect the amount of fat destabilization that occurs. Duplicating this structure with other sources of fat is difficult.
Vegetable (non-dairy) fats are used extensively as fat sources in ice cream in the United Kingdom, parts of Europe, the Far East, and Latin America but only to a very limited extent in North America. Five factors of great interest in selection of fat source are the crystal structure of the fat, the rate at which the fat crystallizes during dynamic temperature conditions, the temperature-dependent melting profile of the fat, especially at chilled and freezer temperatures, the content of high melting triglycerides (which can produce a waxy, greasy mouthfeel) and the flavor and purity of the oil. It is important that the fat droplet contain an intermediate ratio of liquid:solid fat at the time of freezing. It is difficult to quantify this ratio as it is dependent on a number of composition and manufacturing factors, however, 1/2 to 2/3 crystalline fat at 4-5oC is a good, working rule. Crystallization of fat occurs in three steps: undercooling to induce nucleation, heterogeneous or homogeneous nucleation (or both), and crystal propagation. In bulk fat, nucleation is predominantly heterogeneous, with crystals themselves acting as nucleating agents for further crystallization, and undercooling is usually minimal. However, in an emulsion, each droplet must crystallize independently of the next. For heterogeneous nucleation to predominate, there must be a nucleating agent available in every droplet, which is often not the case. Thus in emulsions, homogeneous nucleation and extensive undercooling may be common. Blends of oils are often used in ice cream manufacture, selected to take into account physical characteristics, flavor, availability, stability during storage and cost.
We have recently completed a study on the use of non-dairy fats in frozen desserts. A blend of 75% of either fractionated palm kernel oil or coconut oil and 25% of an unsaturated oil, like high oleic sunflower oil, was shown to produce optimal levels of fat destabilization, meltdown and flavour, although coconut oil may take longer to crystallize during aging. Blends of 50% milkfat, 37.5% fractionated palm kernel or coconut oil, and 12.5% high oleic sunflower oil were also shown to be very acceptable.