Goff et al. (1999) describe ice cream as a complex food colloid, containing fat globules, air bubbles, and ice crystals dispersed in a freeze-concentrated solution of proteins, salts, polysaccharides and sugars. In this post, we will be looking at the role of air bubbles in ice cream.
THIS POST WAS FIRST PUBLISHED ON 13TH FEBRUARY 2012 AND UPDATED ON 12th FEBRUARY 2016
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The production of ice cream starts with formulating, pasteurising, homogenising, and ageing an ice cream mix, followed by aeration, freezing, hardening, and storage. During aeration and freezing (also known as dynamic freezing), air is incorporated to about 50% of the volume (Goff & Hartel, 2013) through the folding and mixing action of the rotating dasher and scraper blades. Careful control of the amount of air incorporated into ice cream, or overrun, and the air cell size distribution is critical for ice cream texture, meltdown, and hardness (Sofjan & Hartel, 2003; Xinyi et al., 2010).
1. AIR CELL SIZE DISTRIBUTION
During dynamic freezing, air cells start out as large entities but are continually reduced in size by the shear stress, or the imposed force, of the rotating dasher and scraper blades (Goff & Hartel, 2013). Smaller dispersed air cells produce a creamier mouthfeel during consumption (Eisner et al., 2005).
To break air cells down into smaller ones, a high local shear stress is required. This shear stress is governed by the rotational speed of the dasher and scraper blades and by the viscosity, or the resistance of a liquid to flow, of the ice cream slurry as it is forming (Sofjan & Hartel, 2003).
1.1. ROTOR SPEED
An increase in the rotational speed of the dasher and scraper blades results in a smaller mean bubble diameter (Den Engelsen et al., 2002). In a study on the size of bubbles in foam in a dynamic mixer, Kroezen (1988) found that the mean bubble diameter decreased with increasing rotational speed of the mixer. This was ascribed to an increase of the shear stress in the mixer at higher rotational speeds. Similarly, Gido et al. (1989) found that increasing the rotational speed of a dynamic mixer caused a decrease in the average bubble diameter.
Increased rotor speed, however, appears to have a detrimental effect on ice crystal size, although there appears to be conflicting research on the extent of this effect. Ice crystal size is a critical factor in the development of smooth and creamy ice cream (Donhowe et al., 1991). Smooth and creamy ice cream requires the majority of ice crystals to be small, around 10 to 20 µm in size. If many crystals are larger than this, the ice cream will be perceived as being coarse or icy (Drewett & Hartel, 2007; Goff & Hartel, 2013).
Hartel (1996) argues that increasing the agitation speed or the number of scraper blades has significant effect on ice crystal formation during the freezing of ice cream. This is because the increased agitation speed causes an increase in the input of heat, which may result in larger ice crystals.
Russell et al. (1999) also found that increasing dasher speeds resulted in an increase in ice crystal size. Drewett & Hartel (2007), however, found only a slight increase, Koxholt et al. (2000) found no effect, and Inoue et al. (2008) found mixed effects on ice crystal size.
Cook & Hartel (2010) argue that it is possible that the dasher speed itself is not a direct predictor of ice crystal size. Instead, heat generation by the dasher may give a better correlation with ice crystal size.
Hirt et al. (1987) found that an increase of the viscosity of the liquid phase, or a thicker ice cream mix, resulted in a smaller mean bubble size. Similarly, Den Engelsen et al. (2002) found that a small increase of the viscosity of the liquid ice cream mix yields a decrease of the bubble size. They found, however, that above a certain viscosity, an increase of the large bubbles is observed and the process of bubble formation is apparently retarded.
Stabilisers are known to increase the viscosity of the aqueous phase. Chang & Hartel (2002b) found that the addition of stabiliser caused an increase in viscosity and this led to smaller air cells during the early stages of freezing. However, after about 10 minutes of freezing, slurry viscosities were approximately the same regardless of stabiliser level and air cell size distributions were smaller.
The formation of ice during dynamic freezing is also necessary for air incorporation. This is because during freezing, a slurry of ice crystals is formed, which, together with the freeze-concentrated continuous phase, causes the viscosity of the ice cream to increase dramatically (Goff & Hartel 2013). This increased viscosity enhances stabilisation of air cells and allows air cells to be reduced to smaller sizes. Similarly, Chang & Hartel (2002b) found that freezing was required to break down air cells incorporated during mixing, since whipping alone did not lead to small air bubbles. As freezing commenced, the apparent viscosity increased, which caused a reduction in maximum air cell size due to the increased shear stress applied to disrupt the air cells.
1.3. RESIDENCE TIME
The amount of time a mix spends in a machine, or residence time, during dynamic freezing affects air bubble size, with longer residence times resulting in smaller air bubbles. This is because extra whipping, provided by the increased residence time, breaks the air cells into smaller bubbles (Chang & Hartel, 2002b; Thakur & others, 2005). Kroezen (1988) found that a shorter residence time increased the mean bubble diameter.
However, Koxholt et al. (2000) note that the dynamic freezing step must account for competing phenomena as shorter freezing times are needed to produce small ice crystals, but longer freezing times give smaller air cells. For this reason, they suggest that pre-aeration may be a good choice to better control ice cream structures.
Pre-aeration involves first whipping the ice cream mix to incorporate air and begin destabilising the fat before the mix is frozen in an ice cream machine. The effects of pre-aeration are to produce slightly smaller air bubbles and improve perceptions of creaminess and smoothness, especially in low fat ice cream (Tharp & Young, 2007; Burmester & others, 2005).
However, Kusumaatmaja (2009) found little correlation between pre-aeration and air cell size. Air cell size likely did not decrease significantly with pre-aeration due to relatively low shear before the mix was frozen. Windhab & Wildmoser (2002) also note that without the ice crystals present to provide viscosity and stability to the ice cream structure, air bubbles are prone to coalescence, which reduces the impact of pre-aeration.
2. STABILISING AIR BUBBLES IN ICE CREAM
During aeration and freezing, the small, newly formed air bubbles are not stable and need to be stabilised to prevent coalescence. Coalescence involves the coming together of two or more bubbles and results in larger air bubble sizes (Ronteltap & Prins, 1989).
Dynamic freezing involves numerous physical changes which affect air bubble stability. Goff (2002) summarised these as: the partial coalescence of the fat emulsion causing both adsorption of fat at the air interface and formation of fat globules clusters that stabilise the lamellae between air bubbles; the action of proteins and surfactants in forming and stabilising the foam phase; and freeze concentration of the premise by the removal of water from solution in the form of ice.
2.1. THE PARTIAL COALESCENCE OF THE FAT EMULSION
During aeration and freezing, the ice cream mix undergoes partial coalescence, where clumps and clusters of the fat globules form and build an internal fat structure or network into the frozen product by trapping air within the coalesced fat. These fat globule clusters are responsible for stabilising the air cells, thus preventing them from recombining (Walstra, 1989; Chang & Hartel, 2002a, b). This results in the beneficial properties of dryness, smooth texture, and resistance to meltdown (Lin & Leeder, 1974; Buchheiim et al., 1985; Berger, 1990).
Proteins play an important role in forming and stabilising the foam in ice cream (Turan et al., 1999; Zhang & Goff, 2004; Patel et al. 2006). Proteins adsorb to and help to stabilise the air bubble interface, along with adsorbed fat globules, during dynamic freezing.
Emulsifiers enhance fat destabilisation, incorporate more and smaller air bubbles, and form thinner lamellae between air bubbles (Marshall & others, 2003). If too much emulsifier is present, however, or if an ice cream mix is subjected to excessive shearing action, the formation of objectionable butter particles can occur as the emulsion is churned beyond the optimum level (Goff, 1997).
Flores & Goff (1999) found that added emulsifier resulted in more air incorporation and air was more finely dispersed. Zhang & Goff (2005) found that added emulsifiers increased air bubble stability through promoting partial coalescence of fat, but too much partially coalesced fat led to unstable air bubbles.
3. OVERRUN IN ICE CREAM
The amount of air incorporated during freezing, or overrun, affects the size of the ice crystals, with larger ice crystals observed at lower overrun (Arbuckle, 1977). Flores and Goff (1999) suggested that overrun below 50% does not influence ice crystal size because it does not affect overall microstructure. They held that the amount of air cells at 70% overrun is just enough to prevent collisions among ice crystals and to disperse the serum phase around each crystal. Sofjan & Hartel (2003) found that increasing the overrun in ice cream (from 80% to 100% or 120% led to formation of slightly smaller air cells and ice crystals, probably due to the higher shear stresses exerted in the freezer barrel due to the higher air content. They also found that ice cream with 80% overrun had larger air cells and ice crystals after hardening than ice creams made with 100% and 120% overrun.
Thomas (1981) notes that an increase in air-cell dispersion results in limiting the size of ice crystals. Smaller air cells pack more tightly, which leaves smaller spaces between bubbles for ice to grow in, and the ice crystals, consequently, are smaller in size (Barfod, 2001)
4. CHANGES DURING HARDENING
Changes in air cell size distribution also occur during the hardening stage, where ice cream is hardened in a freezer without agitation to -18°C (0.4°F), preferably -25°C (-13°F). Once hardening is complete, changes in air bubble size are greatly reduced.
Goff & Hartel (2013) summarise the changes that take place during hardening as disproportionation (Ostwald ripening), coalescence (fusion of neighbouring bubbles), drainage (leading to an uneven distribution of air as bubbles rise, especially at warmer temperatures when ice cream is still soft), and distortion of air bubbles by growing ice crystals.
Changes in air cell size distribution during hardening can be minimised by reducing temperature as quickly as possible and ensuring that ice cream does not remain at elevated temperatures for an extended period of time (Goff & Hartel 2013). Increasing serum viscosity through addition of stabilisers decreases rates of drainage and slows air cell growth. Addition of emulsifiers also reduces air cell changes during hardening, most likely by increasing the extent of fat destabilisation.
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