An knowledge of the physics chemistry of ice cream cream is the course to a smooth, soft, creamy dessert

Simply mixing cream, sugar and egg yolks, with a flavour such together fruit or chocolate, and also putting them in the freezer won"t offer you ice cream cream. Exactly how these ingredients space processed will impact the texture of the product and also this comes down to understanding the physical chemistry of ice cream.

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Look at ice cream cream intensified several hundred times in a scanning electron microscope and you will check out that it has actually a complex structure on size scales that 1 μm come 1 mm(Fig1). There room ice crystals(ca 30 per cent through volume), air bubbles (50 per cent) and fat droplets (5 every cent) from the cream, hosted together by a viscous sugar solution (15 every cent). Ice cream thus has all three states of matter simultaneously and also is both a foam and also an oil-in-water emulsion. The quality of ice cream counts on that microstructure: tiny ice crystals and also air bubbles provide the ice cream cream a smooth, soft texture. If the ice cream crystals are too large, the ice cream cream becomes gritty and also unpleasant come eat. Developing this microstructure is the crucial to making great ice cream - and to understand just how to execute this, you need to recognize some physical chemistry.

Creating a microstructure

The very first step is come know exactly how much of each ingredient to use. Lce cream commonly containsca60 per cent ice cream by weight. Much an ext than this and also the product would certainly be as well hard; much less it would certainly be favor custard and would no cool you down on a warm day. The lot of street (sucrose, C12H22O11) determines the amount of ice cream in the ice cream due to the fact that this will impact the freezing point.

Enter François-Marie Raoult

Pure water freezes in ~ 0°C, but when a solute(eg sugar) is added, the freezing point of the solution is lowered ("depressed"), since the visibility of solute molecules renders it harder because that water molecules to order onto a decision lattice,ie come freeze. (Salt is put on roadways in winter to remove ice.) Our expertise of systems phenomena such as freezing suggest depression owes lot to François-Marie Raoult (1830-1901), that measured the freeze points and other physical properties of plenty of solutions. He showed that there to be a direct relationship in between the freezing point and the alcohol contents of drinks. The freezing point depression (ΔT) counts on the concentration of solute molecules, however not their type, and also is given by:

ΔT= Tw- Ts =(RTw2Hf) x

whereTw is the freezing allude of pure water (273 K),Ts is the freezing allude of the solution,Ris the gas constant,ΔHf is the latent warmth of combination of water (6.01 kJ mol-I) andx is the mole portion of solute. (The related monitoring that the vapour pressure of a solvent over a equipment is proportional to the mole portion of the solvent in equipment is Raoult"s Law.) most solutions follow this equation as soon as the solute concentration is low, however may diverge native it in ~ high concentrations.

Making useofthe freezing point curve

Figure2 mirrors the freezing allude of sucrose options as a function of concentration. The freezing allude depression deviates indigenous linearity over mole fractions ofca0.02. If you cool a sucrose solution listed below its freezing point, ice cream forms and removes water indigenous the solution; the sucrose concentration because of this increases and the freezing point decreases further. Together the temperature is rusmam.orgced, an ext ice forms, and the equipment concentration complies with the curve. At any kind of temperature below the freezing point, over there is a offered amount of ice in equilibrium v the freeze-concentrated sucrose solution.

Ice cream researchers use the freezing suggest curve once formulating ice cream cream recipes. Suppose, for example, they desire to make 1 kg of ice cream cream that includes 50 per cent ice by weight at a typical freezer temperature the -18°C. Just how much sucrose should they use? native the curve, the sucrose mole portion in equilibrium v ice at -18°C is 0.083 or 63 every centw/w, ie:

Msucrose/(Msucrose +Mwater) = 0.63

The total mass is 1 kg, and 50 per cent the this will be ice, so the remainder, have to be sucrose and also unfrozen water.

Msucrose +Mwater = 500 g

Solving these 2 equations givesMsucrose = 320 g. In practice, they additionally have to enable for other solutes,eg indigenous milk, when formulating ice cream recipes.

Fig 3(a) Schematic diagram of homgenisation mirroring the break up of huge fat globules into small droplets

First, the intrinsic viscosity of the sugar equipment decreases together it it s okay colder.Secondly, as Einstein originally pointed out, the viscosity of a suspension of hard particles increases as the volume portion of solid(ie ice) increases. This way that the mix it s okay harder come beat, and the energy input necessary to turn the dasher is greater. This energy is dissipated in the ice cream cream together heat. Eventually, when the temperature the the ice cream cream reachesca -5°C, the energy input with the dasher equals the energy removed as heat by the refrigerant, and also it is not feasible to cool the ice cream any further: the process becomes self-limiting. In ~ -5°C, the ice cream is too soft to process, by spanning it in cacao for example, and the microstructure is unstable. As such the ice cream is extruded native the freezer and also cooled rapidly ("hardened") by blowing air atca -40°C end it in an enclosed chamber. The dimension of the ice cream crystals (and hence the quality of the last product) depends on the problems inside the freezer, such as the wall temperature, and also the quantity of time the ice cream cream spends in the barrel, and the speed at which the is hardened.

The trip home

After hardening, the ice cream is packaged and also transported native the factory to the shop. That is crucial that the microstructure the the ice cream is preserved throughout its journey. Ice cream cream is temperature sensitive: if it warms up also much, that melts. However, also at temperatures listed below its melting point, the quality deserve to deteriorate, owing to changes in the microstructure (seeFig 4).

Fig 4(a) ice cream crystals at -10°C

Fig 4(b) ice cream crystals warmed come -7°C

We have already seen the dispersions of one step in one more are unstable and have a propensity to lower their energy by developing fewer, larger particles. In emulsions this usually occurs by the coalescence of two drops. However, as soon as the dispersed phase is dissolve in the constant phase, an additional mecha?nism operates. This is recognized as Ostwald ripening, found by Wilhelm Ostwald, that won the 1909 Nobel compensation in chemistry because that this work. The solubility of a fragment in?creases together the particle gets smaller,ie little particles are more likely come dissolve than big ones. The solubility that a particle of radiusr,s(r) is offered by:

In<s (r)/s (∞)> =<2γV/RT> <1/r>

whereV is the molar volume ands (∞) is the solubility of one infinitely huge particle. This is known as the Gibbs-Thomson equation. (William Thomson, later Lord Kelvin, is finest known for proposing the absolute scale of temperature.)

When you take ice cream cream the end of a supermarketfreezer its temperature may rise by a couple of degrees throughout your journey home, for this reason that several of the ice cream melts.Crystals that space initially huge become smaller, and also those that space initially little disappear completely. As soon as you placed the ice cream cream in her freezer at home, that is cooled down again, and the ice cream that had melted freezes again(Fig 4). However, the ice cream cannot type on the crystals that have disap?peared. Rather it freezes top top the crystals that have survived. The net effect is the the total number of crystals is rusmam.orgced, and their mean size increases, when the full amount of ice cream is unchanged. If the temperature fluctuation is too big, the ice crystals end up being very large (> ~100μm), producing an icy, gritty texture.

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So having created and preserved the microstructure, the ice cream finally reaches the consumer in perfect condition. The next time girlfriend eat ice cream, think of Raoult, Gibbs,Newton, Kelvin and Ostwald - and all the physics chemistry the has gone into making it!

Dr kris Clarke isa research study scientist at unilever R &D Colworth, Sharnbrook,Bed?ford MK44 lLQ.