Extrusion: a novel technology for the manufacture of ice cream

Conventionally ice cream is continuously frozen in so-called wall-scraped freezer devices. Primary heterogeneous nucleation takes place in such processes at the cooled wall and are scraped off and mixed. In general, the watery phase contains solved components such as sugars or colloidally dispersed hydro-colloids (polysaccharides) and proteins. These ingredients lead to a freezing point depression. If water is crystallised, the remaining solution is concentrated, thus increasing the freezing point depression further. At the outlet of a continuous freezer system, temperatures of approx. -6 to -5°C are conventionally reached for ice cream and, related to this about 40-50 percent of water in a typical ice cream recipe are frozen. If such a system is further deep frozen (-20°C) in the state of rest, e.g. in a cooling-/hardening tunnel, the liquid water crystallises mainly at the surface of the existing crystals and consequently increases their size significantly. This can lead to quality problems due to iciness, roughness or related reduced shelf life of the product. Higher structuring forces can be applied to such watery systems if the mechanical treatment is carried out "dissipation controlled" at lower temperatures. This can be reached down to outlet temperatures of -15 to -18°C in a newly developed low-temperature extrusion device in which a homogeneous shear treatment due to locally narrowly distributed shear stresses, pressures and residence times is applied. Under low temperature conditions additional crystal nuclei are formed by secondary nucleation and de-aggregation of ice crystal aggregates. Low temperature extrusion allows us to get more finely dispersed ice crystals by a factor of more than 2-3, compared to the conventional process. Furthermore air cells and fat globules, the other disperse phases in ice cream are also more finely structured by the increased viscous forces under low temperature extrusion conditions. For low temperature extruded ice cream a hardening tunnel is no longer needed. The perception properties and shelf life are significantly improved.