Factory freezer

Mask the detection of ice crystals in the mouth during eating. Allow easier pumping and more accurate filling during processing. Facilitate the controlled incorporation of air in the factory freezer and help produce a stable foam. 

Ice cream mix at approximately 4 °C is pumped from the ageing tank into the barrel, where it is aerated and frozen, before being pumped out from the other end. The operation of the factory freezer is controlled by several parameters. The pressure of the refrigerant sets the temperature at which it evaporates, and hence the wall temperature... 

Whilst the mix is aerated, it is simultaneously frozen. A significant difference between Victorian freezers and modern ones is the refrigerant. The best refrigerant available to the Victorians was a eutectic mixture of ice and salt, and the lowest temperature that could be achieved was —21.1 °C. Modern factory freezers commonly use liquid ammonia at about —30 °C. As it absorbs heat from the ice cream mix, it boils and evaporates. It is then sucked out, reliquefied, cooled and returned to the jacket. According to Newton s law of cooling (equation 2.9), the colder the refrigerant the faster the heat removal from the mix, and hence the faster the rate at which ice cream can be made. 

Heat must be extracted from the mix both to cool it down (the sensible heat) and to freeze water into ice (the latent heat). We can estimate their relative contributions as follows. Consider a 1 kg mass (m) of a 20% sucrose solution as a simple model for ice cream. This enters the factory freezer at +4.0 X (Tj), and leaves at —5.0 The specific... 

As the mix passes along the barrel, its temperature decreases, and its ice content increases. As we have seen in Chapter 2, both of these cause the viscosity of the mix to increase the viscosity of the sugar solution increases as the temperature decreases and the solute concentration and the viscosity of the suspension increase as the volume fraction of ice increases. Figure 4.13 shows the ice content (i.e. the mass of ice as a percentage of the total mass) of a typical ice cream as a function of temperature between 0 and — 20 °C. This is known as the ice curve. The ice content of this formulation is 35% when it leaves the factory freezer at — 5 °C and 54% at a typical storage temperature of —18 °C. 

The outlet temperature controls the total amount of ice (and hence the mean crystal size for a given total number). This determines the viscosity of the ice cream (typically 10 Pa s on exit from the factory freezer) and its ability to retain its shape. A low outlet temperature is required for ice cream that will be shaped by extrusion because it needs to be quite stiff, whereas a warmer temperature may be preferred for ice cream that is dispensed directly into tubs. 

As ice cream leaves the factory freezer there is a pressure drop of typically 4 atm (the difference between the pressure inside the barrel and atmospheric pressure), so the air bubbles expand (Figure 4.14). 

Figure 4.14 Expansion of air bubbles on leaving the factory freezer...

The mix density is simply determined by weighing a known volume of mix. The density, and hence the overrun of ice cream as it leaves the factory freezer, is measured using an overrun cup. The cup, whose volume is known, is carefully filled to over-flowing with ice cream, ensuring that no air gaps are left, and that the ice cream is not compressed. The ice cream is then levelled off at the top with a knife. The mass of the ice cream is determined by weighing the full cup, and subtracting the known mass of the empty cup. 

When ice cream leaves the factory freezer at about — 5 °C, its ice content is only about half that at a typical serving temperature of —18 °C, so it is very soft. As shown in Chapter 2, the microstructure of dispersed ice crystals and air bubbles is thermodynamically unstable -the system tends towards a state in which the phases are less dispersed. If the ice cream were simply stored at the factory freezer outlet temperature it would deteriorate very quickly. The ice crystals and air bubbles would coarsen their mean size would increase and their total number would decrease. Since it is not possible to stabilize the microstructure thermodynamically, the best that can be achieved is to trap it kinetically, i.e. to slow down the rate at which coarsening occurs, so that significant... 

Ice cream is hardened in a hardening tunnel, an enclosed chamber into which the ice cream passes on a conveyor belt from the factory freezer. Inside, cold air (typically —30 °C to —45 °C) is blown over the ice cream. The lower the air temperature, and the faster the air flow, the faster heat is removed from the ice cream. Air turbulence also increases the rate of heat transfer. The chamber is enclosed to minimize exchange of cold air inside the system with warm ambient air, and so to reduce the build up of frost that would reduce the efficiency. Cold stores, which are typically about — 25°C, are not suitable for hardening because they are not cold enough and have still air, so they cannot cool the ice cream rapidly enough to minimize recrystallization. 

The air cells also coarsen after leaving the factory freezer, by disproportionation and coalescence. Figure 4.16 shows the air bubble... 

Recently a new freezing method, low temperature extrusion, has been developed to overcome the self-limiting nature of the factory freezer, and to avoid the need for hardening. Ice cream from the factory freezer at about — 5 °C is passed through a screw extruder with refrigerated walls and cooled to about —15 °C (Figure 4.18). The extruder applies a higher shear stress (and lower shear rate) to the ice cream than a... 

Tubs and soft ice creams do not require substantial product assembly. Tubs are filled with ice cream straight from the factory freezer and then hardened. Inclusions may be added at the point of filling, using a fruit feeder. This consists of a hopper for the inclusions a means for... 

There are many different kinds of dessert product and processes to produce them, so manufacturing lines must be flexible. Desserts usually contain more than one flavour of ice cream and/or water ice, so they are commonly made by co-extrusion of the components. The co-extruded components e.g. ice cream and sauce) should have similar viscosities so that their flows can be balanced and the correct product shape produced. If necessary, the viscosity of a sauce can be increased by first passing it through a factory freezer. A closed dasher is used as the throughput is very low and a short residence time is required. This also has the advantage that the sauce then has approximately the same... 

Careful temperature control is necessary so that ice crystals neither melt nor grow while they are imaged. This can be achieved by performing the sample preparation and microscopy inside temperature-controlled chambers or by carrying out the whole experiment in a cold room. The temperature of the cold chamber is set to the required imaging temperature, for example — 5 °C for ice cream that has just come out of the factory freezer, or —18 °C for ice cream that has been hardened and stored. A small sample of ice cream is smeared onto... 

Figure 6.9 Series of simulated microstructures of a slush-frozen water ice (a) on exit from the factory freezer (b) part way through hardening and (c) at the end of hardening...

Figure 7.14 shows air bubble size distributions at three different stages on leaving the factory freezer, at the end of hardening and after being thermally abused by being held at —10 °C for five days. The mean air bubble size is initially 23 pm. The dispersion of small air bubbles (like other dispersions) has an inherent tendency to coarsen. After hardening the distribution is broader and the mean size is 43 pm. (This is very similar to the increase in size on hardening in a different sample that was shown in Figure 4.16.) On abuse, this increases to 84 pm and the distribution becomes very broad, with a small number of crystals larger than 100 pm.

Figure 7.14 Air bubble size distribution in ice cream on exit from the factory freezer, after hardening and after being abused...

At — 5 C (the temperature at which it normally leaves the factory freezer) ice cream is a viscoelastic, shear-thinning fluid. Like the mix, it obeys the power-law equation, but with different values of b and n. As its temperature is lowered, it becomes more solid-like. Below about — 12°C it displays a yield stress whose value increases as the temperature decreases further. 

To see a scraped surface heat exchanger in operation. Domestic ice cream makers work on the same principle as factory freezers, but since they do not operate under pressure you can see what happens inside. 

Factory freezer A scraped surface heat exchanger in which the first stage of ice cream freezing takes place. 

Hardening The second freezing step in the manufacturing process in which partly frozen ice cream from the factory freezer is placed in a... 

Residence time The length of time that ice cream mix spends inside the barrel of the factory freezer. 

Slush freezing Freezing with agitation, for example in a factory freezer. 

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