The Science of Ice Cream

Is cookery an Art or a Science? Ice cream making is very much at the Science end of the spectrum as a new generation of gelato makers attest.


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Whether cookery owes more to art than science is often debated, and whilst both elements are present in any cooked dish, the balance swings between the two depending on what is being made.

Part of teaching cookery is to recognise how people learn, and one indicator is where they stand on the axis between art and science.  If I know I will be teaching someone who is high on the science axis I have to do some research in advance, because usually, as long as I have learnt how to make something work, I forget about the science behind it. But at the very least, for reasons of food safety, you need to know what rules must be followed and, at the next level, experimentation becomes more successful if you have understood the science behind what you are cooking.

A spate of new gelaterie being set up in this part of the West Country reminded me that making ice cream is very much on the science end of the cooking spectrum. Despite growing up in the famous Forte ice cream parlour family, Bruno Forte still undertook at month long course at Carpigiani Gelato University in Bologna before setting up Swoon.  He hopes that swooning is exactly what people will do on tasting his artisan gelato, and so come to understand the vast difference between an artisan product and the mass-produced ice cream that is made commercially. Every different flavour requires a reassessment of the proportions of fat to sugar and solids to water introduced by the new ingredient. Getting this balance right is crucial to the texture of the ice cream, the mouth feel in turn affecting how we perceive the flavour.

The term “Gelato” has no legal definition and, according to Elizabeth David, when first coined meant “gellied” rather than frozen.  Now a new generation of artisanal ice cream makers are using the term to distinguish their product from mass produced ice cream.  For most people, the word Gelato is simply the Italian for Ice Cream, so perhaps a good starting point is to understand the difference between Italian ice creams and the French ice cream on which most British recipes are based.  French ice cream contains cream, making it richer than Italian ice cream which is made with milk.  Cream melts more easily than milk, so the Italian milk-based ice can be served at a slightly warmer temperature (-12˚ C rather than -18˚ C) that enables the flavours to be tasted more clearly.  Another characteristic of Italian gelato is that it contains less air than commercial ice creams so that whilst the milk gives a lower fat content, the quantity produced is less, so that the flavour is comparatively intense but at the same time clean and light.  Freshness is the watchword for gelato, in Italy I have waited patiently for that day’s mix to finish churning and it’s a world away from an ice cream that has sat in the freezer for months.  At Gelato Village in Leicester, they pledge never to serve a product older than 72 hours.

Gelato is a way of life in Italy, an integral part of the passegiata – the afternoon stroll during which to see and be seen.  There are relatively few recipes for it in Italian cookbooks, probably because it is always eaten outside.  In France, Ice Cream frequently accompanies a dessert in a restaurant (which gave rise to the American’s describing this practice as À la Mode), and that is not the way you will usually encounter it in Italy. 

Of course, making ice cream at home is a relatively recent concept in Britain.   The availability of home freezing and churning units has brought the possibility to the home cook, and the pairing of a hot fruit dessert with the perfect companion ice cream might be described as my “signature dish”.  Molecular gastronomist Heston Blumenthal uses liquid nitrogen to freeze a mixture before the diners’ eyes, and back in 2012 predicted that in time this method will replace the electric ice cream maker at home, although we are yet to see that happen.

Italian chef Giorgio Locatelli was taught to take ice cream seriously from childhood – the remembers being told “no – those two flavours do not go together” when he made his selection.  In his restaurant, ice cream making involves scientific precision, for example by weighing rather than measuring liquids.  It is, he says, one area of the kitchen where we have to forget about spontaneity because accuracy really makes a difference. Using varying ratios of different sugars (sucrose, dextrose and glucose syrup) he can alter the freezing point of the mixture and therefore the texture.  In addition to full-fat milk and cream he uses some milk powder (0% fat) to give a more pronounced milk flavour.  Some recipes include eggs, which contain lecithin, a natural emulsifier; and when a stabiliser is deemed necessary it comes from natural ingredients such as algae.  Through these methods he claims he can keep an ice cream in the freezer for two weeks in a perfectly soft and smooth state.  Two weeks is, incidentally, the same time period recommended for keeping home-made ice cream. 

If all of this has scared you off making your own ice cream, it needn’t.  I have made many successful ice creams at home simply by following a reliable recipe where someone else has done all the science!  However, it should also sound a note of caution against having a gung-ho approach which assumes you can simply swop ingredients willy-nilly.  Yes, you can make an ice cream using yoghurt rather than cream, but remember that water freezes harder than fat, so if you try to reduce the fat content (or even the sugar) it will affect the texture.  It is now estimated that over half of what Briton’s eat is “ultra-processed” and making foods more “healthy” is a large part of this.  Before you buy ice cream have a look at the list of ingredients to see how many you don’t recognise.  Perhaps also try one of the new generation artisan ice cream or gelato makers to fully appreciate what an artisanal product can taste like.

If you would like to understand more about the chemistry of ices there is a wonderful chapter on the subject within Caroline Liddell and Robin Weir’s Ices: The Definitive Guide alongside a large number of recipes made using different ingredients and methods.  Giorgio Locatelli’s Made in Italy also has recipes suitable for making at home which benefit from his chef’s know-how.

What follows is just a few of the key scientific points to keep in mind, followed by two seasonal recipes using blackberries to encourage you to have a go.  The second recipe uses Meringue Italienne to introduce air into the mixture so that it can be made without churning in a machine.  The result is something perhaps part way between an ice cream and a sorbet, probably technically a Parfait although this also has no legal definition. It would be my choice for eating from a cone, whilst the richer French custard version is ideal to accompany a dessert.

Key Points to Remember

  • Bacteria reproduce most quickly between 45 and 15˚ C so heated custards must be cooled quickly, ideally over a bowl of ice (or in a sink of ice-cold water), stirring until cool enough to transfer to the fridge.  Cover the bowl and leave to chill for 4 hours (and definitely no more than 12) before churning.  Add whipped cream only at the point of churning.
  • Once frozen the flavour may continue to develop over the next day or two but the texture will start to diminish.  Store for no more than 2 weeks.
  • It is usually necessary to allow a homemade ice cream to soften before serving but return it to the freezer as quickly as possible and never refreeze a mixture that has thawed rather than just softened.
  • Overrun – this is the term used in commercial ice cream production to describe the finished quantity of ice cream as a percentage of the volume of mix, i.e., if you put a litre of mixture into your ice cream maker at home you might get 1.25 litres of ice cream (25%).  Effectively this increase is measuring the air, which can be 100% or more in cheap bulk ice creams.
  • Adding alcohol to your mix is a simple way to prevent it freezing solid but add too much and it won’t freeze at all.  How much you can add depends on the strength of the alcohol, as a rough guide, one tablespoon of 20% alcohol (e.g., fortified wines, sherries and port) added to a litre of ice cream mixture would reduce the freezing point by 0.6˚ (but only 0.3˚ in a sorbet).  I normally add 2 tablespoons per litre of ice cream at this strength of alcohol.  A tablespoon of 40% spirit would decrease the setting point by 1˚ in ice cream.

Rich Blackberry Ice Cream

The following recipe produces a rich smooth French style ice cream that makes a perfect accompaniment to a Blackberry and Apple pie. 

1lb/450g blackberries

4oz/115g light muscovado sugar

2 tbsps Crème de Mûre

lemon juice

6 fl oz/175 ml double cream

6 fl oz/175 ml milk

vanilla pod

5 egg yolks

3oz/85g caster sugar

Place the blackberries in saucepan with the muscovado sugar, cover with a lid and heat until the sugar has dissolved, and the blackberries have begun to release their juice.  Press through a nylon sieve to remove the pips.  Add the Crème de Mûre and sharpen to taste with lemon juice.

Meanwhile heat the milk and cream in another pan with a vanilla pod until just below simmering point.  Whisk together the egg yolks and caster sugar until pale and fluffy. Pour on the hot milk and cream, whisking as you do so, and remove the vanilla pod, which can be washed and kept to use again.

Return the custard mixture to the pan and heat to 82˚ C and thickens slightly.  Pour this into the blackberry purée and leave the mixture to cool.  Transfer to the fridge when the mixture has sufficiently cooled and leave until thoroughly chilled before transfer to and ice cream maker or the freezer.  If you do not have an ice cream maker, remove the ice cream from the freezer as soon as the edges begin to freeze and beat well.  Repeat this process again before the mixture is fully frozen.

Blackberry Parfait

500g/a good pound blackberries

approx. 3 tablespoons caster sugar

lemon juice

2 tbsps. Crème de Mûre liqueur

284ml/½ pint double cream

Meringue Italienne:

225g/7½ oz caster sugar

100ml/3½ fl oz water

2 egg whites

Liquidise the blackberries with 3 tablespoons of caster sugar then pass through a nylon sieve to remove the pips.  Add a good squeeze of lemon juice and the Crème de Mûre then taste and add more sugar if necessary.

Lightly whip the cream and fold through the blackberry purée.  Churn in an ice cream maker if you have one or pour into a shallow container and freeze until edges of the mixture are solid.  Stir the edges into the centre with a fork and then proceed to making the Meringue Italienne.

At the stage when you have a semi- frozen ice cream make the Meringue Italienne.  Dissolve the sugar in the water, stirring carefully and using a pastry brush to wash down any sugar crystals from the sides of the pan.  When the sugar has dissolved turn up the heat and boil the liquid until it has thickened but not begun to caramelise (about 5 minutes or 120˚ C or soft ball on a sugar thermometer).  Whilst the sugar is boiling whisk the egg whites to soft peaks and then pour on the sugar syrup once it reaches the correct point, taking care it does not hit the turning blades of the mixer.  Keep whisking until cold, then fold into the ice cream mixture and return to the freezer.

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