Elaboración de la geometría (dominio computacional)

In ice cream or similar frozen dessert production, homogenisation, ageing and freezing are the main processing steps that determine the quality and properties of finish frozen products. Blending Pasteurisation Packaging Homogenisation Ageing Hardening Liquid ingredients

(e.g., fat, corn syrup and water)

Dry ingredients

(e.g., solids-not-fat, i.e., proteins, skim milk powder, emulsifiers, stabilisers and sugars)

Cooling

Frozen ice cream

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2.9.1 Homogenisation

In ice cream making process, homogenisation is a process by which an ice cream premix is forced by pressures through a very small orifice in the homogeniser to breakdown bulk oil in small droplets and/or reduce the size of existing fat globules from milk or cream formulated in the ice cream mix (Marshall et al., 2003, Wilbey, 2003). The size of oil droplets is normally reduced to smaller than 2 μm in diameter, which makes an ice cream mix emulsion uniform and stable without phase separation (Marshall et al., 2003). As the fat droplets reduce in their size, the surface active materials available in the ice cream mix such as proteins and small molecule surfactants tend to competitively adsorb at the newly formed droplet surface to reduce the interfacial tension at the oil-water interface (Dickinson and Tanai, 1992, Marshall et al., 2003).

Homogenisation pressure has a significant influence on the number, size and particle size distribution of fat globules formed in oil-in-water emulsions. In ice cream making, it is ideal to have the large number of small fat droplets be formed by homogenisation in order to enable its contribution to the body and structure of ice cream through their partial coalescence (Walstra and Jonkman, 1998). As a result of homogenisation, the homogenised small fat droplets increase their ability to cover air bubbles and crystals from 6 m2 to 150 m2 per litre at a later stage of the ice cream making process (Walstra and Jonkman, 1998). According to the Kolmogorov's theory of isotropic turbulence, during homogenisation, large molecules, such as proteins, are more preferably adsorbed to the newly created fat droplets than small molecule surfactants added in the ice cream formulations (Gelin et al., 1994).

Currently, the homogenisation pressure up to 350 MPa (3,500 bar) can be applied using a high-pressure homogeniser in order to prepare emulsions (Floury et al., 2000). In general, an increase in homogenisation pressure decreases the size of fat globules being formed in the homogeniser (Floury et al., 2000). The homogenisation pressures normally used for the ice cream mix containing 10-14 wt% fat are 130-180 bar and 34 bar for the 1st and 2nd stage of homogenisation, respectively (Marshall et al., 2003).

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Koxholt et al. (2001) showed the homogenising pressures at 100/20 bar (1st/2nd stage) were high enough to prepare a stable ice cream containing 10 wt% fat with acceptable meltdown properties. Innocente et al. (2009) also showed the effect of different homogenisation pressures (1st_/2nd _{stage) at 970/30 bar and 150/30 bar on the properties}

of ice creams containing 5-8 wt% fat. The higher pressure resulted in a change in the particle size distribution of oil droplets in the ice cream mix from bimodal to monomodal with smaller droplet diameter, which in turn led to an increase in viscosity and more pronounced solid-like ice cream.

2.9.2 Ageing

Ageing is one of the crucial steps in an ice cream production. It is a process by which a homogenised ice cream mix is kept undisturbed at about 4°C for at least 4 hours or a longer time, normally 24 hours (Marshall et al., 2003). During the ageing step, proteins and other hydrophilic colloidal materials become fully hydrated fat becomes crystallised and the rearrangement of oil droplet membranes occurs (Marshall et al., 2003). The fully hydrated proteins and stabilisers lead to a marked increase in the viscosity of the aged mix after at least 4 hour ageing (Marshall et al., 2003, Minhas et al., 2000a). The minimum ageing time for at least 4 hours is required to ensure that the fat droplets in ice cream mix containing 10 wt% fat almost become crystallised because the homogenised fat droplets undergo crystallisation at slower rates than the droplets in the state before homogenisation (Marshall et al., 2003).

Another important phenomenon occurring in the ice cream mix during the ageing is that in this quiescent step, small molecule surfactants become more active than proteins and migrate from the serum phase to the oil-water interface, displacing a substantial amount of adsorbed proteins at the fat droplet surface (Bolliger et al., 2000a, Davies et al., 2000, Gelin et al., 1994). This phenomenon makes the resulting membrane thinner and more fragile which in turn facilitates the formation of partially coalesced fat droplets during the subsequent treatment of the aged ice cream mix with whipping and freezing. It was reported that ageing of ice cream mix did not change the droplet size distribution of particles in aged ice cream mix (measured in terms of volume surface mean diameter)

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(Gelin et al., 1994), indicating the droplet aggregation and coalescence do not take place during ageing. The properly aged mix provides the resulting ice cream with an optimal extent of fat destabilisation (partial coalescence) which is important for the proper melting rate and shape retention of ice creams (Marshall et al., 2003).

2.9.3 Whipping and freezing

At a temperature above the melting point of fat, fat droplets are in a liquid form. When an oil-in-water emulsion in the aged ice cream mix is subjected to shear forces introduced by agitation during churning (whipping) and freezing stage, the fat globule membrane of two colliding droplets is disrupted by the penetration of fat crystals from one partly crystalline fat droplet into approaching another fat droplet (Fredrick et al., 2010, Vanapalli and Coupland, 2001). This leads to the joining of two droplets through the protruding fat crystals piecing into neighbouring droplet interface which are partly enclosed by some liquid fat flowing to the fat crystals from the droplets, resulting in the partially coalesced droplets (Fredrick et al., 2010, Vanapalli and Coupland, 2001). In an ice cream making freezer, the concurrent churning and freezing are introduced to ice cream mixes. During freezing, temperature of the system employed is lower than the melting temperature of fat used, in this case, fat droplets that start crystallisation from the ageing step become more crystallised in the freezer (Eisner et al., 2005).

During the freezing and whipping of ice cream mix at this low temperature range about -28°C, full coalescence (true coalescence) of fat droplets by their collision is obstructed by the presence of a network of fat crystals inside both of the colliding droplets (Davies et al., 2000). As a result, the collision of crystallised droplets instead leads to the formation of irregularly aggregated droplets or partial coalescence of fat droplets throughout the whole ice cream structure (Goff, 1997b). It is worthy to note that for partially coalesced droplets, the identity of individual droplets is still retained in the aggregates (Davies et al., 2000, Goff, 1997b). These aggregated droplets are reportedly important to stabilise air cells generated via whipping in ice creams by adsorbing and building up a network structure (indicated by an arrow) at the surface of air cells as

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illustrated in Figure 2.4 (Eisner et al., 2005). In the droplets, the size of fat crystals is governed by the crystal form of fat, e.g., β form leads to granular crystals and βʹ form

leads to smaller crystals when fat becomes crystallised (Harada and Yokomizo, 2000, Kawamura, 1979, 1980).

Figure 2.4 Micrograph obtained from the low-temperature scanning electron microscopy (LT-SEM) of an air cell in ice cream with fat droplets adhered at the surface. Source: Eisner et al. (2005).

During the freezing stage, air is incorporated into the ice cream structure by direct entrainment driven by scraper blades of the manual ice cream freezer or direct injection through orifices into ice cream in the continuous ice cream freezer (Chang and Hartel, 2002, Hartel, 1996). In normal standard ice creams, air cells account for 50% of the whole volume of ice cream (Thomas, 1981, Walstra and Jonkman, 1998). Chang and Hartel (2002) demonstrated in their study that the size of air cells decreased as the churning time of ice cream mix in the ice cream freezer increased but the size remained stable without changing after a certain time. From their study, the size of air cells progressively decreased from the first 1 minute to about 10 minutes before ice cream was drawn from the freezer and remained constant until 22 minutes of drawing time. They also reported no dependency of air cell size on the fat level and small molecule

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surfactant content but instead the size of air cells depended largely on the content of stabiliser which was attributed to its effect for an increase in the apparent viscosity of the mix as it was frozen. Eisner et al. (2005) underlined the importance of ice cream mix viscosity for its influence on air cell size by demonstrating a substantial reduction in the size of air cells in the ice cream with a high viscous matrix.