The Effect of Processing Conditions on the Physical Properties of Ice Cream

Maya Warren and Richard Hartel

Ice cream continues to be one of the most popular frozen desserts across the world.  Whether on a hot summer day or a brisk winter evening, consumers are always ready for the sweet creamy, often indulgent, treat. 

Although ice cream seems like a simple treat, the creation of ice cream, particularly its microstructure, is quite complex. Both ingredients and processing conditions play a large role in the final microstructure and melting properties of ice cream products.  The microstructure components, such as ice crystals, air cells, and partially-coalesced fat, are all influenced due to the processing conditions under which the ice cream is made.  The processing conditions, such as dasher speed, overrun, and draw temperature, greatly influence the overall quality of ice cream as well as the final microstructure and other physical properties. Below, the various processing conditions and their influences on the microstructure properties of ice cream are discussed. 

Formation of Ice Cream

Ice cream is formed when mix is pumped into a scraped surface heat exchanger and undergoes shear while freezing.  The liquid refrigerant (i.e. Freon or ammonia) surrounding the barrel vaporises under pressure at -30oC and allows for the ice cream mix to begin cooling.  To begin freezing, the refrigerant removes heat to cause the mix to fall below its freezing point.  This causes nucleation to occur at the barrel wall, where latent heat is released as ice begins to crystallise.  An ice layer is scraped from the sides of the barrel wall by the blades of the dasher.  As the ice crystals are removed, they are displaced to the middle of the product where they ripen to form disc shaped particles (Cook and Hartel 2010).  As this is occurring, the fat globules in the mix are also undergoing physical changes.  The rotation of the dasher and formation of ice crystals provide shear stresses to the system under which the fat globules become destabilised, resulting in the clustering of partially-crystalline fat globules, with sizes over 100 μm. In general, the more shear added to the system whether through an increase in dasher speed, ice crystal content, and/or residence time, the larger the partially-coalesced clusters will become.  

During the freezing and formation of the ice cream, air is directly entrained into the freezer (batch freezing) or injected (continuous freezing).  Large air cells are formed first and are then broken into smaller sizes by the shear forces the longer the product is in the freezer (Goff and Hartel 2013).  The formation and the size of the air cells are greatly dependent on the viscosity of the ice cream as it freezes.  

Processing Parameters and their Influence on the Microstructure Properties of Ice Cream

During the freezing process, the speed of the dasher, amount of overrun, and draw temperature all play an important role in the development of the final microstructure.

Dasher Speed

The speed and design of the dasher during processing has great influence on the properties of ice cream products.  In general, an increase in dasher speed tends to lead to an increase in partial coalescence (Gelin et al. 1994, 1996a,b, Goff et al. 1999; Goff and Jordan 1989, Pelan et al. 1997, Tharp et al. 1998, Thomsen and Holtsborg 1998, Walstra 2003) .  With an increase in shear rate, there is an increase in the encounter rate among the fat globules, which results in an increase in partial coalescence (Walstra 2003).

As dasher speed increases, air cell size has been known to decrease (Kroezen 1988, Goff and Hartel 2013). This occurrence greatly depends on the viscosity of the slurry phase and the applied force in the system during freezing (Chang and Hartel 2002a).  If the slurry phase has the same viscosity across dasher speeds, air cells produced should be similar in size (Chang and Hartel 2002a). 

 The size and amount of ice crystals can greatly be affected by the processing conditions.  For example, as dasher speed increased, it was expected that mean ice crystal size would increase as the temperature inside the freezer increased due to greater frictional heat released, resulting in dissolution and recrystallisation (Donhowe and Hartel 1996).

Overrun 

Aeration is an important part of making ice cream.  Without air incorporated, ice cream would be a densely-packed solid mass, resulting in little to no scoopability properties. Overrun, which is the amount of air in ice cream, is controlled and injected via an inline system.  Although the amount of air can be controlled (continuous freezing), the viscosity of the system affects the entrapment of the air as well as the size of the air cells.   Sofjan and Hartel (2004) noted a decrease in air cell size with an increase in overrun, which in turn, increases the apparent viscosity.  Furthermore, Goff and Hartel (2013) indicate that the sizes of air cells in ice cream are governed by the shear stress caused by the rotation of the dasher (whipping) and the development of ice crystals, which leads to increased viscosity of the mix.  In turn, during the batch freezing process, where the air is whipped in and entrapped into the ice cream system, the amount of overrun in the finished product is greatly dependent on the viscosity (amount of ice formed, stabilisers, etc.) of the ice cream.  

Sofjan and Hartel (2004) determined that with an increase in overrun, air cell sizes decreased at draw.  This was attributed to the increase in the apparent viscosity of the slurry as overrun increased (Chang and Hartel 2002b).

As overrun is increased in the system, if affects other physical properties of ice cream and not just air cells.  Partial coalescence increases as overrun is increased, as an increase in air content causes the repeated adsorption / desorption of fat to the air interface, which in turn increases the collision rate of the fat globules (Goff and Vega 2007).  As a result, the introduction of more air along with shear causes an increase in partial coalescence. 

In addition, Flores and Goff (1999) indicated that an increase in air lead to a decrease in overall size of ice crystals.  This was attributed to ice creams with higher amounts of air having a thinner serum phase dispersed around the air cells and as a result, the collision rate of ice crystals is decreased.  Arbuckle (1986) and Sofjan and Hartel (2004) both indicated that the incorporation of air during freezing influences the ice crystal size distribution, with larger ice crystals observed at lower overruns.  Large ice crystals are known to lead to coarse ice cream while too much air can cause a fluffy and crumbly product, which are all defects in ice cream.  

Draw Temperature

The effect of draw temperature, the temperature at the outlet of the freezer, can influence the final viscosities as well as amount of total ice in the system.  With warmer draw temperatures, less ice is formed, which results in lower viscosities, larger air cells, and lower amounts of partially-coalesced fat. Ice cream is typically drawn at -6oC but this can vary depending on the freezing point depression as well as other freezing parameters.  

Drip-through Rate

Drip-through rate (g/min) is determined by allowing ice cream to drip-through a wire mesh screen at ambient temperature and measuring the dripped weight every 5 minutes. The weight of the dripped portion can then be plotted versus time and the slope of the drip-through curve is determined as the drip-through rate. In general, ice creams with faster drip-through rates have less remnant foam left atop the screen at the end of the drip-through test while more remnant foam is generally seen in ice creams with slower drip-through rates (Warren and Hartel 2014). 

In general, as the dasher speed increases, drip-through rate is expected to decrease as there would be more partially-coalesced fat in the system.  An increase in partially-coalesced fat has been known to lead to a decrease in drip-through rate (Berger and White 1971, Berger et al. 1972, Segall and Goff 2002, Muse and Hartel 2004).  

With an increase in overrun, a decrease in drip-through rate is generally seen.  This is attributed to thermal diffusivity of heat into the ice cream during melting. Goff and Hartel (2013) have indicated that an increase in both the number and dispersion of ice crystals and air cells are expected to reduce thermal diffusivity, and therefore, reduce the onset of drip and the drip-through rate. Typically, lower thermal diffusivity (rate of heat transfer) results in a slower onset of melting and slow drip-through rate.   

Conclusion

The ice cream making process results in many complex interactions within the microstructure.  Different microstructures can be generated depending on the processing parameters and the composition of the mix. The ice cream manufacturer must completely understand the complex processes taking place in the ice cream freezer in order to create products with the desired microstructure and physical properties. 

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Dr Maya Warren was a postgraduate student and Dr Richard Hartel is Professor of Food Engineering in the Department of Food Science, University of Wisconsin-Madison, Madison, WI 53706, USA; e-mail: rwhartel@wisc.edu