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How to use emulsifiers in ice cream

Emulsifiers are probably one of the most confusing parts of ice cream science. So let's start right at the beginning with the basics.

What's an emulsion?​

An emulsion is a mix of two (or more) liquids that ​won't usually mix together. Why won't they usually mix? Because the molecules of each respective liquid prefer to stick together. 

For example, an oil and vinegar salad dressing is an emulsion. Think about how we make it. When we add the vinegar to the oil, they don't mix properly. In fact, they form two separate puddles!

It's only when we stir them together vigorously, that the oil breaks into smaller and smaller globules and is dispersed throughout the vinegar to make a consistent mixture. This mixture is an emulsion.

Milk is another example of an emulsion. In milk, tiny globules of liquid fat are dispersed in water.

But the thing about emulsions is that because the liquids don't want to mix together, they're unstable. And over time, they'll separate. If you leave an oil and vinegar dressing in the fridge, eventually it will separate into two layers, one of oil and one of vinegar.

And if you leave milk that's come straight from the cow, it will also separate into two layers: on top, fatty cream and underneath watery milk.

Homogenized milk​

However, these days, the milk we buy in the supermarket has been "homogenized"​ so that it doesn't separate. "Homogenized"? Basically, this means that the milk has been mixed at high pressure until the fat globules are so small, they're unable to separate from the water.

Why are they unable to separate? Well, milk contains protein molecules with two "heads". One head is attracted to water while the other is attracted to fat. So these proteins end up at the interface between the two, with one head stuck in the fat globule and the other in the water.

By attaching to the fat globules and anchoring them in the water in this way, the proteins naturally discourage the globules from clumping together and separating from the water.​ Under normal conditions, the action of the proteins is not strong enough to stop the fat and the water separating for very long.

But homogenization creates a greater number of smaller fat globules, which in turn means there's a larger surface area for those proteins to attach themselves to.

So after homogenization, there are even more proteins attached to smaller, more widely distributed fat globules. To the extent that even over time, the fat globules are unable to separate from the water.

And that's why modern milk doesn't separate into two layers in our fridges! Homogenization has created a relatively stable emulsion.

But what's this got to do with ice cream?​

Good question. It's all about the air bubbles in ice cream. Ice cream contains lots of tiny air bubbles. And these bubbles are really important because they contribute to the texture and body of the ice cream and how fast it melts.

An ice cream with lots of tiny, stable air bubbles is smoother and melts more slowly than one with less and/or larger bubbles.

So part of the ice cream making process involves getting air into the mixture and making sure the bubbles are as small as possible. And that's the job of the paddle (or dasher) that stirs the mixture in the ice cream machine as it's being frozen.  

The motion of the dasher does two things. Firstly, it adds air bubbles to the mixture. And secondly, it causes the fat globules in the mixture to collide and start to clump together to form long strings in a structure that surrounds and supports those air bubbles.

This process, in which the fat globules separate from the water and start to clump together is called "partial coalescence". And the "scaffolding" structure it forms is essential for holding the air bubbles in the ice cream and keeping them small.

OK but what's this got to do with emulsifiers?!​

Bear with me, this is where it all comes together...

So on one hand, we have milk. The milk is a pretty stable emulsion in which the fat globules are very small and prevented from clumping together by proteins that anchor them in the water.

But on the other hand, we want to make ice cream with that milk. And to do so, we know we need to de-stabilize the milk emulsion so that the fat globules do clump together to form the scaffolding that supports the air bubbles.

Hold on, we also know that the motion of the dasher in the ice cream maker will cause the fat globules to collide and clump together. So what's the problem?

Well, the problem is that in most cases, the motion of the dasher alone is not enough to de-stabilize the emulsion. There's just not enough fat in an average ice cream mixture for it to be de-stabilized by just agitation. (That's why you can whip high fat whipping cream but you can't whip milk).​

No, the dasher needs help. And this is where the emulsifiers come in. So bizarrely, we add emulsifiers to the ice cream mixture to de-stabilize (or de-emulsify) the fat emulsion in the milk.

​How do emulsifiers de-stabilize the emulsion?

We already know that in homogenized milk, the emulsion is stabilized by protein molecules that attach themselves to the surface of the fat globules and prevent them from clustering together.

So to encourage "partial coalescence" in which the fat globules do clump together, we need to remove those protein molecules. And this is the job of the emulsifiers.

Emulsifiers are similar to milk proteins in that they have two heads, one attracted to fat and another attracted to water. And when we add emulsifiers to an ice cream mix, they ​behave in the same way as the proteins: they migrate to the interface between the fat and the water where they attach themselves to the surface of the fat globules.

In doing so, they actually displace the protein molecules. And since ​emulsifiers lower the tension between the fat and the water, they don't interfere with the inclination of the fat globules to cluster together.

So in this way, they de-stabilize the emulsion. And later on, when the motion of the dasher starts to cause those fat globules to collide, they're much more likely to separate from the water and start clumping together.

​But what are these emulsifiers?

There are several different types of emulsifiers. Let's have a look at what's available...

Egg yolks

Yes, the most traditional ice cream emulsifier is egg yolk! As well as other proteins and fats, egg yolks contain a chemical called Lecithin which has very good emulsifying properties.

In fact, when cooked, egg yolks work as both a stabilizer that thickens the mixture and as an emulsifier which encourages partial coalescence. This is why they're so useful. And for home made ice cream they'll nearly always be the number one choice.

How many egg yolks you use depends on what type of ice cream you're trying to make.​ To just make use of their emulsifying properties, you'll need 0.5 - 1% of the mixture to be egg yolk. To use their stabilizing (thickening) properties as well, you'll need to increase this proportion to 3 - 4%. But some frozen custard style ice creams might be over 8% egg yolk!

However, just as egg yolks are not the very best stabilizer, neither are they the very best emulsifier. They can also add a distinctive eggy taste to our ice cream which may or may not be desirable.

Eggless Emulsifiers

So while for most home made ice cream, egg yolks will be the best emulsifier choice, there may be times when you want to avoid them.

For example, if you want to make a particularly light and clean tasting ice cream. Or if you're making an ice cream that already has a high fat content from chocolate or nut pastes. In these cases you may want to avoid the flavor dampening richness or extra fat content of egg yolks.

Powdered, eggless emulsifiers are usually one of the ingredients in the generic ice cream stabilizer mixtures of you can now buy. ​But it's worth looking at them individually...

​Soy Lecithin

It's the Lecithin in egg yolks that makes them such good emulsifiers. However, Lecithin can also be extracted from soybeans, sunflowers and rapeseed. And this plant based Lecithin emulsifies just as well as the stuff in egg yolks without any of the eggy flavor and extra fat.

One large egg yolk contains about 1.5 g of Lecithin​. So with mixtures that are between 0.2 - 0.5 % Lecithin by weight, you should be able to make an eggless ice cream that's emulsified as well as it would be with egg yolks. Just mix it well with the rest of the dry ingredients.

Soy Lecithin has a pretty neutral taste. So should be undetectable when used in these small quantities. However different brands may vary so it's worth trying a few if you have issues. 

​Buttermilk

​Buttermilk, either condensed or dried can be used as a source of Milk Solids Non Fat (MSNF) in ice cream and will provide extra emulsifying properties because the protein-phospholipids that do some of the emulsifying are concentrated during buttermilk churning.

Polysorbate 80 and ​Mono- and Diglycerides

Yes, they're a bit of a mouth full! They're actually two different types of emulsifiers, but since they're complimentary and often used together, I'll cover them both at once.

Polysorbate 80 is a type of sorbitan that's derived from oleic acid. It's most effective at the interface between fat and water. This means it's good at de-stabilizing the emulsion so that the fat globules clump together in partial coalescence.

​Mono- and Diglycerides are derived from the partial hydrolysis of animal or vegetable fats. They are most effective at the interface between fat and air, so they're good at stabilizing the air bubbles and keeping them small.

​Since one acts at the fat-water interface and the other acts at the fat-air interface, these emulsifiers complement each other and used together they're probably the most effective available.

And because of their effectiveness (and cheapness!), this combination is often used in commercial ice cream manufacture.

However, their synthetic nature tends to make them less attractive to home made ice cream enthusiasts. And to be honest, since home made ice cream is consumed much more quickly than commercial ice creams, we don't really need the very best emulsifiers!

If you do want to experiment though, you can try Polysorbate 80 at 0.02 - 0.04 % by weight and Mono- and Diglycerides at 0.1 - 0.2 % by weight.

​Getting the balalnce right

Whether your using egg yolks, soy lecithin or some of the synthetic emulsifiers, you need to make sure you get the balance right.

Too little emulsifier and there won't be enough partial coalescence while the ice cream is being churned and frozen. This will result in an unstable foam and a wet, coarse ice cream that melts very quickly.

Too much emulsifier and there may be too much partial coalescence. This is where so much of the fat separates from the emulsion and clumps together, that it becomes detectable on the tongue as small, buttery lumps.

The quantities above should give you a rough idea. You don't need much. But as always the best way to get a feel for this is to experiment! My ice cream calculator may help.

Wrapping up

​I hope that's made the role of emulsifiers in ice cream a little clearer. They are often over shadowed by stabilizers. But emulsifiers have a huge influence on the size and stability of the air bubbles in ice cream.

And since the air bubbles have a huge influence on the texture, body and meltdown characteristics of ice cream we definitely shouldn't ignore them.

For most home made ice cream enthusiasts, egg yolks will be enough. But if you interested in lighter, cleaner tasting, eggless ice creams then the other options are certainly worth investigating. 

How to use stabilizers in ice cream

Stabilizers are probably the most controversial area of ice cream science. They're denounced by traditionalists who think everything should be “natural”. And lauded by molecular gastronomers that think science solves everything.

​They're a complicated subject. But there's nothing to be scared of here. Most of them are natural. When you use them correctly they'll almost certain improve your ice cream. And in fact, if you've made ice cream then you've almost certainly used them in some form already.

​So what are stabilizers?

Put simply: stabilizers are ingredients that thicken water. This is also called adding “viscosity”. The more viscous a liquid is, the “thicker” it is.

In scientific terms, they're hydrocolloids. When hydrocolloids disperse in a liquid they bind to the water molecules, so reducing their movement. This reduced movement appears to us as increased viscosity or thickening.

The most obvious example of using a stabilizer in cooking is when you thicken gravy with flour.

​Most stabilizers are natural in origin. They generally come from plant, animal or bacterial origins. However under European Law they are considered food additives and must be represented by E numbers in ingredient lists.

Why do we use stabilizers in ice cream?​

Stabilizers can improve ice cream in several significant ways:

  • reduce ice crystal growth
  • reduce air bubble size
  • slowdown melting
  • increase smoothness, body and creaminess
  • deliver flavour cleanly

Let's have a look at each of these contributions in turn...

​How do stabilizers reduce ice crystal size in ice cream?

First of all, why do we want to reduce ice crystal growth in our ice cream? Well, ice crystals are an important part of ice cream. But if the crystals get too big, they are detectable by the tongue and give the ice cream a coarse, grainy texture.

Ice crystals are formed only during the initial freezing and churning stage in the ice cream maker.

​When the ice cream is subsequently stored in a freezer, the existing ice crystals may get bigger, but no new ones are formed.

​But ice crystals can grow when there are temperature fluctuations that cause existing crystals to melt and then re-freeze. Because when they re-freeze, rather than creating new crystals, the water migrates to join existing crystals, so increasing their size.

​Such temperature fluctuations can actually occur both during the batch freezing process (in the ice cream maker) and during storage (in the freezer).

Ice crystal growth in the ice cream maker​

​During batch freezing, ice crystals are initially formed against the super cooled sides of the ice cream machine. The rotating dasher then scrapes them from the sides and moves them into the center of the mix where temperatures are warmer and where they may melt and then re-freeze later as the temperature of the whole mix decreases.

Ice crystal growth in the freezer​

​During storage there are many situations that could cause these temperature fluctuations. For example when the freezer door is opened and then closed. Or if the ice cream is taken out to soften before serving and then replaced in the freezer.

​So the challenge for all ice cream makers, is to try to make the ice crystals as small as possible during the freezing and churning stage and to stop them getting bigger while the ice cream is being stored.

​While there's plenty of material that suggests stabilizers only restrict ice crystal growth during storage, there are other studies that show that the initial size of the ice crystals formed during batch freezing are smaller in mixes that use stabilizers.

How do they do this? The science around this is not clear. But it seems likely that by restricting the free movement of water, stabilizers prevent melted ice crystals from finding and joining existing ice crystals when they re-freeze.

How do stabilizers reduce air bubble size in ice cream​?

Why do we want small air bubbles in ice cream? Because lots of small air bubbles make the ice cream smoother.

​How do stabilizers keep the air bubbles in ice cream small? Again, the science is not totally clear here. We know that stabilizers make base mixes more viscous. And more viscous base mixes produce smaller air bubbles.

​But why do viscous mixes produce smaller bubbles? One theory is that the greater shear stress that's applied to more viscous liquids when they are being churned in the ice cream maker reduces the size of the bubbles more.

Just like ice crystals, air bubbles can grow in size and reduce in number during storage. This happens in two ways:

  1. Disproportionation occurs when air transfers from smaller to larger bubbles.
  2. Coalescence happens when two bubbles come into contact and join.

​The increased viscosity that stabilizers produce also protects against these processes by thickening the films around the air bubbles and keeping neighbouring bubbles away from each other.

​How do stabilizers slow down melting?

Ice cream that melts too quickly is no fun to eat! Stabilizers can help here too by both slowing the rate at which ice cream melts and maintaining it's shape better as it does melt.

​This is partly due to the water binding qualities of stabilizers. Viscous mixtures simply melt slower.

​But it's also due to the smaller air bubbles. Ice cream with many small air bubbles melts significantly slower and retains it's shape better that ice cream with fewer, larger bubbles. And we already know that stabilizers promote smaller bubbles!

​How do stabilizers increase smoothness, body and creaminess?

So we've seen that by reducing the size of the ice crystals and the air bubbles in ice cream, stabilizers make smoother ice cream and slow meltdown.

But they also add body, produce a creamy mouth-feel and a silky finish. These qualities are largely a result of the added viscosity that stabilizers produce.

​Less free flowing water produces a more solid, ice cream that tastes creamier and silkier because it's less watery.

​Why are people suspicious of stabilizers?

Despite the benefits of stabilizers, many people are suspicious of or outright hostile to their use in ice cream. I think there are two main reasons for this...

  • ​bad experiences of over stabilized ice creams
  • the strange names, E numbers, and their chemically appearance make them seem "unnatural"

Over stabilized ice creams are horrible. They may have a gummy or excessively chewy texture. They can exhibit extremely unnatural melting (they don't seem to actually melt!). And they often leave a pasty after-taste in your mouth.

​But this is stabilizers used badly. When stabilizers are used well, you don't even realize they're being used at all. You're just amazed by how good the ice cream is!

​However, the very idea of using stabilizers is too much for some people. This tends to be down to the idea that they are somehow unnatural, that they're chemicals added to reduce costs rather than improve quality, and that they're unhealthy or even unsafe.

​But as I mention above, most of stabilizers come from natural sources. And most of them have been used in cooking for hundreds of years.

​It's true, they are often used in cheap, commercial ice creams to cut corners and save money. But it's all a matter of intent. If people are using them to save money we should be wary. If they're using them to make better ice cream then we should be curious.

Besides, if you've already made ice cream at home, you've probably already used stabilizers.

​Stabilizers you're already using

​Egg custard

​Yep, egg yolks act as a stabilizer. So if you're making egg custard bases, you're already stabilizing your ice cream. The stabilizing chemical is egg yolk is called Lecithin and even has it's own E number: E322.

​Egg yolks will give your ice cream fantastic texture and body. They'll emulsify your mix. And they'll also reduce the growth of ice crystals and air bubbles.

​So why not use egg yolks all the time? Well, they're good, but they're not great. They're certainly not as good at reducing ice crystal growth as other stabilizers. And over time they let water escape, which re-freezes and makes the ice cream icy.

​They subdue other flavours (especially lighter flavours like herbs and water based flavours like fruit). And add their own eggy flavour, the strength of which depends on how many eggs you use and how long and how hot you cook the base for.

So yes egg yolks will stabilize your ice cream but they're not the best performing stabilizers available.

Starches​

If you've ever tried to make a Sciilain Gelato then you may have used corn flour or tapioca flour. In southern Italy they don't use eggs (or much cream) in ice cream. Instead they use these starches to stabilize their milky gelatos.

​Corn flour and tapioca flour work quite well as stabilizers. They don't subdue other flavours like eggs and impart much less flavour themselves.

​However, I can still detect them, whether it's through a light flavour trace or a slightly pasty texture. And again, there are other stabilizers that perform better.

​Gums

​While you almost certainly have some experience of the thickening properties of egg yolks and corn flour, you're less likely to have used gums before. But when people talk about the stabilizers used in ice cream it's gums that people are usually thinking about.

​Gums are the most powerful, flexible and most useful stabilizers that are available to us. They suppress the growth of ice crystals better than any other ingredient. They can be used to alter the texture of ice cream in many different ways. They don't suppress other flavours and are almost flavourless themselves.

​What's more, they are so powerful that we only need to use them in tiny amounts. Typically gum stabilizers would only make up 0.1 – 0.5 % of the base mix!

Most gums appear as off white powders. In fact, they're all just complex sugars, also known as polysaccharides. And they're almost all derived from natural products.

​Different gums have subtlety different chemical structures that will have very different effects on the texture, body and sensory qualities of ice cream. And even used alone they are very useful.

However, if you combine one or more gums together, the effects of each can be amplified. Or nullified. Or you might get a whole new set of effects! So it is worth experimenting with different combinations of gums to see what effects you find pleasing.

​Thickeners vs Gels

​While all gums will thicken a liquid, some of them also form gels. Gel are substances that exhibit the characteristics of both a liquid and a solid. Food technologists define a gel as a high moisture food that more or less retains it's shape when released from it's container. And that definition's good enough for us here!

While some gums always form gels, some will only form gels in dairy based mixtures. And others will form gels only when mixed with other gums!

And different gels have different characteristics. For example they might be strong or weak, brittle or elastic etc.

When we're making ice cream, gels are generally harder to work with than mixes that are simply viscous. They can be difficult to get in the ice cream maker cleanly, so you often have to attack them with a blender them to break up the gel.

However, if you're making low fat ice creams or sorbets they're really useful because they add a creamy texture and substantial body that you wouldn't get otherwise.​

​Gums that form gels by themselves

Gum

Gel qualities

Locust bean gum

Weak

Iota Carrageenan

Soft, elastic gel with dairy 

Kappa Carrageenan

Stiff, brittle gel with dairy

Sodium Alginate

Rigid, brittle gel with dairy

Gelatin

Brittle, unstable

Pectin

Brittle, slightly sticky gel

Gums that form gels with other gums

Gum combinations

Locust bean gum + Xantham gum

Locus bean gum + Kappa Carrageenan

​Carboxymethyl cellulose + Guar gum

​Carboxymethyl cellulose + All Carrageenans

Plant based gums​

​Most of the gums we use in ice cream are derived from plants. What sorts of plant? Well, generally they're extracted from seeds or seaweeds!

​Locust bean gum (E410)

​Locust bean gum (LBG), is also known as Carob Bean Flour and is made from the seeds of the Carob Tree. This tree is very common in Mediterranean countries and LBG has been used as a thickener in cooking for thousands of years.

​LBG is a very popular stabilizer in ice cream. It has one of the best ice crystal size reducing powers of all the gums. And it produces, a smooth texture, a creamy mouth-feel and a silky finish. It also works well with other gums, especially Guar and Carrageenan.

​The great thing about ice creams stabilized with LBG is that usually they don't seem like they've been stabilized at all. It gives ice cream a very natural feel. This is because although it forms a weak gel when frozen, that gel disappears when the ice cream melts.

​However, LBG is not without it's disadvantages. It needs to be heated to fully hydrate and different types of LBG hydrate at different temperatures. But it's typically around 185°F  (85°C), which is higher than ideal when making ice cream.

​Used alone it can also cause wheying off, which is when milk proteins come out of solution to form crystals that are detectable by the tongue and give the ice cream a grainy texture.

​Guar gum (E412)

​Guar gum is also derived from a seed, in this case the seeds of the guar plant which is a legume, like a bean. Guar beans have been eaten in India for thousands of years but guar gum has only been used as a stabilizer since the 1950's.

​Guar gum doesn't reduce ice crystal size as well as LBG, but it adds much more viscosity to the mix which gives more body to the final ice cream. Unlike LBG, it also hydrates at low temperatures.

​But Guar works well with LBG, with each amplifying the powers of the other. So they are often used in combination.

​Used in high quantities, Guar can give ice cream a chewy texture like toffee, which may be desirable or not depending on how you like your ice cream!

​Carrageenans (E407)

​Carrageenans are extracted from seaweeds. Originally these were Irish Moss seaweeds and carrageenans have been used as thickeners in Irish cooking for centuries.

​However, nowadays they're extracted from other types of red seaweeds that are grown in the Philippines, Tanzania and Indonesia.

​Carrageenans perform pretty averagely at reducing the size of ice crystals. But they have a strong effect on texture, producing a rich and creamy mouth-feel that's similar to egg custard ice creams.

​They also help prevent wheying-off (see above) so are often used in conjunction with LBG which can cause this defect.

​There are three different types of Carrageenans that are used in cooking, each of which varies slightly in their molecular structure:

  1. Lambda
  2. Iota
  3. Kappa

Iota and Kappa Carrageenans form gels with milk so are more commonly used in sorbets and low fat ice creams. While Lambada is used in ice creams with sufficient fat to stabilize without gelling.

​Sodium Alginate (E412)

​Sodium Alginate is also extracted from seaweed, this time the brown ocean kelp that's found in cold water areas.

​It dissolves in cold water but hydrates best at temperatures between 155 and 160°F (68 - 71°C). It's pretty good at keeping ice crystals small. Add contributes a texture and body to ice cream that other gums can't replicate.

​Sodium Alginate forms a gel with milk, so it's popular in low fat ice creams. And it's the way in which the rigid gel breaks into a fluid gel when it's being churned that gives the finished ice cream it's unique sensory qualities.

​Carboxymethyl cellulose (E466)

​Carboxymethyl cellulose (CMC) is also know as cellulose gum and is synthesized from plant cellulose.

​It's probably better at suppressing the growth of ice crystals than LBG. It adds body and chewiness to ice cream to the same degree as Guar. But it forms a gel when combined with LBG, Guar and Carrageenans, which may or may not be desirable.

​Since Carboxymethyl cellulose is a synthesised product that is extracted from cotton and wood pulp it pushes the boundaries of what many people would call “natural”. However it's perfectly safe and is commonly used in ice cream production.

Fermented gums

Xantham gum (E415)

​Xantham gum is a product of fermentation and is created when the bacteria Xanthomonas campestris feeds on sugar. This might sound weird. But it's just like yeast in beer!

​It's an extremely versatile stabilizer. It dissolves in (and thickens) hot or cold water. Viscosity doesn't vary with temperature. It's highly resistant to freeze/thaw cyles. It works at a wide range of acidities. And it works well with other gums.

​It's not the best gum at suppressing ice crystal growth. But it's really easy to get hold of in health food stores (because vegans use it as an egg substitute). And it's ready availability, it's ease of use and it's versatility make it a great gum to experiment with.

Other stabilizers​

​Gelatin (E441)

​Gelatin is derived from animal collagen, usually pork or beef. And this is what they used to stabilize ice cream in the old days.

​It suppresses ice crystal growth really well and gives ice cream a very pleasing smooth texture. It's also very easy to get hold of.

​However, it has largely fallen out of favour, because it's expensive and because it's an animal product.

​Pectin (E440)

​Pectin is extracted form citrus feel and apple pomace. Pectin has been used for many years as the gelling agent in jam.

There are two types: “low methoxy”, which requires calcium to gel and “high methoxy” which will gel at low pH with loads of sugar.

​Denatured Whey Proteins

​When we heat the ice cream mix, some of the whey proteins in the milk undergo partial unfolding and begin to form a network similar to those formed by hydrocolloids. This process in called “denaturing” and will help stabilize the ice cream in a similar way too.

However, the stabilization is not nearly as powerful as the hydrocolloid's and should be seen as an addition to rather than instead of.

​How to use stabilizers

So if you're thinking you might like to experiment with stabilizers (and specifically gums) there are three steps that you need to get right:

  1. ​measuring
  2. dispersion
  3. hydration

Measuring

​Gums are so powerful you only need to use a tiny amount. Typically they make up between 0.1 and 0.5% of the weight of the base mixture. And if you go just slightly above these proportions you'll start to get over stabilized ice cream which can be quite unpleasant.

​So in order to get your weight measurements right you'll need some scales that are accurate to 0.01g. You might be able to get away with experimenting with ¼ teaspoon measures, but it'll be hit and miss. And good quality scales aren't expensive.

​Dispersion

Once you've accurately measured out your stabilizer, you need to mix it with the rest of the ingredients. Gums tend to clump together and won't disperse properly if you just dump them into a liquid.​ And if they're not dispersed, they don't work!

The best way to get an even dispersion is to add the stabilizer to all the other dry ingredients and then mix them thoroughly with a fork or a whisk. Spend a good 5 minutes on this to make sure it's properly mixed.​

Once it is mixed, add the liquid and give it a proper going over with a hand blender. Again spend a good few minutes on this.

​Some people suggest using the blender to form a vortex in the centre of the liquid and then pouring the dry ingredients into the middle of the vortex for the best dispersion. I haven't found this necessary but it might help.

​But I can't stress the importance of this step enough. If you don't disperse the stabilizer it will clump together and won't fully hydrate. Which means your mix won't thicken properly. And your ice cream will be poorer.

​Hydration

​In order to be effective, a stabilizer must be hydrated: it must absorb water. Some need to be heated, others hydrate in cold water. Some stabilizers hydrate faster than others. You need to know how to get the best hydration from the stabilizer you're using.

​For example, Locust Bean Gum needs to be heated to about 185°F (85°C). While Guar and Xantham gum hydrate at room temperature. And Guar needs up to an hour to fully hydrate. But Xantham gum hydrates much quicker.

Measuring, Dispersion and Hydration: A summary

So remember. Make sure you're working with the right amount of stabilizer by weighing it accurately. Properly disperse it by mixing it thoroughly with the other dry ingredients before you add liquid. And treat it with the appropriate amount of heat and time to fully hydrate it. Get these three steps right and you should have great success!​

​Wrapping up

​Stabilizers are often treated with great suspicion and even hostility. Generally, I think this is because people are ignorant of what they are and why we might want to use them.

​They're natural and they're safe. Of course some people may be allergic to them. Just as some people are allergic to eggs. If that's the case, they should be avoided.

​However, they can help us make much, much better ice cream. And that's the most important point here. We're not using them to save money. We're not using them to cut corners.

When we use them, we use them to deliver better texture, better body, a more creamy, luxurious finish. And more stability!

As home-made ice cream enthusiasts we're already hampered by shitty machines that take ages to freeze our mixtures and inflexible freezers that are never at the right temperature.​

Stabilizers can help us overcome these disadvantages to make ice creams that rival the professionals. I urge you to at least experiment!​

Glossary

Agar Agar
A stabilizer. Comes from seaweed. Often used as a vegetarian alternative to gelatin.

Ageing
Occurs when we leave the ice cream mixture in the fridge for around 6 hours. During this time the fat globules shed their coating of milk proteins and develop spiky needles. This makes it easier for them to clump together to form the structures that support the air bubbles when we start to churn it.

Air
Gives ice cream its softness. Ice creams with more air seem fluffy. Ice creams with less air seem more creamy. The amount of air in an ice cream is measured by something called “overrun”. It can vary between 20 and 100%.

Batch Freezer
This is just what the professionals call their ice cream machines! It churns and freezes the base mixture into ice cream.

Blast Freezer
This is a a very powerful freezer that the professionals use to chill the churned ice cream very quickly to the desired storage temperature. The faster it’s frozen in the blast freezer, the less chance of ice crystal growth.

Butterfat
Is the proper name for the fat in milk and cream.

Carrageenan
A stabilizer. Extracted from Irish Moss seaweed. There’s three different types: Kappa, Iota, and Lambda each offering different properties.

Cream
This the source of most of the fat in ice cream. The proportion of fat in cream can vary between 12% and 55%. I tend to use 32%, just because it’s the easiest to get hold of.

Dextrose
A sugar. 70% as sweet as table sugar (sucrose). But lowers the freezing point of water by nearly twice as much. So it’s useful when you want softer, less sweet ice cream.

Emulsifiers
Encourage the fat globules in the milk and cream to cluster together to form the long strands that support the air bubbles. Lecithin, found in egg yolks is the most common traditional emulsifier. Commercial ice cream often uses the synthetic Polysorbate 80.

Extraction
Where the churned ice cream is removed from the machine (and usually moved to the freezer). You should try to keep the extraction time as short as possible to reduce the chances of melting and subsequent ice crystal growth.

Gelato
An Italian ice cream. Usually contains less fat, less air and is served at warmer temperatures than other ice creams.

Guar Gum
A stabilizer. Comes from the milled seeds of the Guar plant which is a type of bean. Suppresses ice crystal development. Also thickens and adds body to the ice cream. In higher concentrations it will make the ice cream elastic and chewy.

Hardening
When we remove the mixture from the ice cream maker it usually has the consistency of soft serve and will melt very quickly. So we put in the freezer to harden for a certain amount of time.

Hydrocolloids
The scientific name for stabilizers. Hydrocolloids are simply tiny insoluble particles of a substance dispersed in water. They form a network that restricts the flow of water to increase viscosity (thickness).

Ice
Gives ice cream its hardness and body. Around 30% of ice cream is made up ice crystals. For smooth ice cream, we want to keep those ice crystals as small as possible.

Lactose
A sugar. Found naturally in milk. It contributes very little to sweetness or freezing point suppression. Too much Skimmed Milk Powder in the mix can raise the levels Lactose beyond its solubility limit so that it forms small crystals that give the ice cream a sandy or grainy texture.

Locust Bean Gum (LBG)
A stabilizer. Comes from the seeds of the Carob tree. One of the most effective suppressors of ice crystals. And will also thicken the mixture. Usually needs to be heated to high temperatures (up to 85 °C / 185 °F) to get the best out of it.

Maltodextrin
A sugar. Not very sweet at all. And has very little affect on freezing point. Great for soaking up liquid and adding bulk to ice cream.

Milk
Contributes water, protein and some fat to the ice cream. Important for that distinctive dairy taste.

Milk Solids Non Fat (MSNF)
All the solids that aren’t fat in the milk! So that’s the proteins, the lactose and various minerals. MSNF make up between 6 and 11% of ice cream. You can increase the proportion of MSNF in a mix either by reduction through heating or by adding Skimmed Milk Powder.

Partial Coalescence
Occurs during churning. This is where the fat globules in the milk start to clump together to form long strings that support the air bubbles in the final ice cream. Can be encouraged by adding emulsifiers to the mix and ageing it in the fridge.

Pasteurization
Heating the mixture to kill harmful bacteria. With home made ice creams, this is important if you’re using eggs.

Skimmed Milk Powder (SMP)
Skimmed milk with all the water removed! Useful for adding protein and absorbing water. And will intensify the dairy flavors. Make sure you use spray dried SMP, as the barrel dried stuff can taste caramelized.

Stabilizers
Soak up water that might otherwise make the ice cream icy, coarse, thin or watery. They thicken the mixture, add body and smoothness to the ice cream and slow melt down. Popular stabilizers include egg yolks, cornstarch, Locust Bean Gum, Guar Gum.

Sorbet (Sorbetto)
Ice cream without any dairy. No milk. No cream. So sorbets contains more water and more sugar. Sorbetto is just Italian for sorbet.

Sugar
Sugar adds sweetness to ice cream. But it also keeps it soft by lowering the freezing point of the water in the ice cream. As more water freezes, the sugar in the remaining water becomes more concentrated. At some point the concentration will be so high that remaining water will not freeze. And it’s liquid solution that keeps the ice cream soft.

Water
Milk is 90% water. Cream is 60% water. It’s this water that freezes into the ice crystals that give ice cream it’s body. Too much unaccounted for water in the mix will result in thin, icy ice cream.

Xanthan Gum
A stabilizer. Comes from the fermentation of glucose, sucrose or lactose by the bacteria Xanthomonas campestris! It’s acid resistant so is good for fruit sorbets. And it also dissolves and works at any temperature. Be careful when using it with Locust Bean Gum as together they’ll form a gel.

Ice cream Science

The Science of Ice Cream. Sounds a bit heavy doesn’t it? Maybe a bit boring? You just want to get on with inventing crazy flavor combinations don’t you?

Hold on though. Not only is it actually really interesting, a basic scientific understanding of the ingredients and the way they work together will also help you make much better ice cream.​

Science will help you in two important ways:

  1. When something’s not working, it will help you fix it
  2. ​When you want to start inventing, it will help you do so successfully.

​So in this post, I’ll give you a fuller understanding of what ice cream is. I’ll introduce the individual components, highlight the special contribution of each and explain how they all work together in the final product.

Finally, I’ll describe the five stages that we go through to make ice cream and why each one is important.

I’ve tried to explain everything in as clear and straightforward way as possible. So there shouldn’t be any parts where you’re scratching your head in confusion.​

But I’ve also tried to go into as much depth as possible to give you a complete understanding of what’s going on. This means it’s quite a long post! If there’s anything that’s not clear let me know and I’ll try to expand and improve.​

So, make yourself comfortable and lets get started…

What is ice cream?​

Ice cream is an very complex, intricate and and delicate substance. It includes all three states of matter at once: solid (ice and fat), liquid (sugar solution) and gas (air bubbles).

These states exist in a precarious balance. And it’s in that balance that we find the unique sensory qualities that so enchant us!​

Essentially, tiny particles of sold ice and fat surround and support air bubbles in a thickened sugar solution.

The basic structure of ice cream

Let’s look at each of these four components in more detail...

Component #1: Ice

Ice crystals give ice cream it’s firmness. They give it body and solidity. That resistance to your spoon and your tongue: that comes from the ice crystals. About 30% of ice cream is made up of ice crystals.

Ice crystals are formed from the water in the mixture as it starts to freeze. You might think that there's no water in ice cream. We don’t usually add any directly. But don’t forget milk and cream are mostly water. Milk is 90% water. And cream is around 60% water.

The size of these ice crystals is very, very important! Small ice crystals will make the ice cream smooth and less cold in the mouth. While large ice crystals will give the ice cream a grainy, coarse texture and a cold, icy mouth feel.​

(This apparent difference in temperature is because larger ice crystals require more heat to melt. Since this takes heat away from your mouth, it makes the ice cream seem colder.)

Different people like different types of ice cream but one thing's for sure: good ice cream should be smooth. So keeping those ice crystals small is one of the most important parts of making quality ice cream.​

​Component #2: Air

​Air gives ice cream it’s softness. It keeps the ice cream pliable and makes it easy to scoop. The air also contributes greatly to texture and consistency. Ice creams with more air are lighter, fluffier and less "creamy". While ice creams with less air are heavier, more dense and more "creamy".

Air bubbles are added to the ice cream by the paddle (also known as the dasher) that churns the mixture as it freezes. The faster the blades of the dasher move through the mixture, the more air they add. And different shaped dashers will also affect how much air is pushed into the mix.

The air bubbles also give ice cream most of it’s volume. The amount of air in ice cream is measured by something called "overrun". This is simply the increase in volume that the air contributes to the ice cream (measured as a percentage).

So, if you start off with 1 litre of ice cream mix and once churned it’s 1.5 litres, the volume has increased by 50%. And so the over-run would be 50%.

So called “premium” ice creams tend to have lower overrun (around 25%). While cheap ice creams can have as much as 100% overrun. Why? Since air is free, it’s a very efficient way to increase the volume of your product without increasing the manufacturing cost!

Different types of ice cream also have different levels of overrun. Italian gelato for instance can have an overrun as low as 20%.

Brooklyn Brainery did a great study on the different levels of overrun in a variety of popular American ice creams. ​I'm providing a summary in the table below...

Ice cream Brand

Overrun %

America's Choice Tub o' Vanilla

102

America's Choice Premium "Vanilla Bean"

97

Breyer's "Homemade Vanilla"

98

Turkey Hill "Vanilla Bean"

104

Haagen Dazs "All Natural Vanilla Ice Cream"

31

Stonyfield Farm "Gotta Have Vanilla"

31

Ronnybrook "Hudson Valley Vanilla"

22

"Artisan" gelato

20-25

Is it best to have a higher or lower over-run? Ultimately it’s all down to personal preference: if you like light, fluffy ice cream, you’ll want to make it with a high over-run, if you like dense, creamy ice cream you’ll be looking to make it with low over-run. For home-made ice cream this can be influenced by which machine your buy.

Component #3: Fat​

Fat contributes to ice cream in four important ways: 1) it helps to stabilize the final structure by trapping air bubbles 2) it thickens the mixture which slows melting, 3) it delivers flavour and 4) it gives that lovely creamy texture and mouth-feel.​

The fat in ice cream mostly comes from the milk and cream and is called butterfat. Around 3.4% of whole milk is fat. While cream can vary between 30 and 48% fat, depending on what type is being used. So mostly it comes from the cream!

This fat exists as tiny, solid globules suspended in the milk and cream. But how do these fats globules stabilize the ice cream?

Well, before the mixture is churned the fat globules are very small and are kept apart from each other (more on this later). However, while the ice cream mixture is being churned, the fat globules bang together and join up to form long, pearl like strings that wrap around the air bubbles.

​These strings hold the air bubbles in place, keeping the “foam” stable. This is how the ice cream maintains it’s volume, light texture and soft consistency: all the qualities that the air bubbles contribute to ice cream.

The fats also give ice cream it’s creamy texture and richness. Higher fat ice creams are rich and creamy with a long lingering after taste. Lower fat ice creams have a much lighter, cleaner flavour with a short lived after taste.

Interestingly, any additional flavours (fruits, chocolates, nuts etc) in the ice cream are also affected by this. This is because the fat tends to hang onto these flavours.

So the fruit flavour in a strawberry ice cream will be delivered more slowly and subtly (but more long lastingly) in a higher fat ice cream. And will usually be more clearer and prominent (but relatively short lived) in a lower fat ice cream.

Whether you prefer higher fat, rich and creamy ice creams or lighter, cleaner lower fat ice creams is a matter of personal taste. American and French ice creams tend to be higher in fat. While Italian ice creams are usually a bit leaner.

You can alter the fat content of your own ice cream by playing around with the proportions of milk and cream in your base mixture. Higher fat ice creams have more cream, while lower fat ice creams have more milk. You need to be careful though: too much or too little fat will can ruin your ice cream...

Too much will give an unpleasant, cloying flavour, a grainy texture from the crystallisation of the fat particles and it will probably freeze into a hard solid block because of the excess fat solids.

Too little and there wont be enough fat globules to form the strands that stabilize the air bubbles so the ice cream will be wet and coarse and melt easily.

Component #4: Sugar Solution​

This is the liquid part of ice cream. It’s in this solution (also called the matrix) that the ice crystals and fat globules (solids) and the air bubbles (gas) are suspended.

But what does it consist of? Well essentially there are 3 elements: water, sugars and proteins. Let’s look at each in turn...

Water​ in the sugar solution

As we already know, the water in ice cream comes from the milk and cream. Some of this water freezes into solid ice crystals. But some of it will remain in a liquid state even at 0°F / -18 °C. 

Hold on, water freezes at 32°F / 0 °C, so how can this be? Well, the sugar that’s dissolved in the water lowers the freezing point of water which prevents ice crystals from forming.

The initial concentration of sugar in the water does allow ice crystals to form. But as more ice crystals grow, there’s less free water so the concentration of sugar in the remaining water increases. This further lowers the freezing temperature until a point at which the remaining super sweet water will not freeze, even at 0°F / -18 °C.

In this way a proportion of ice cream always remains liquid.​

Sugars​ in the sugar solution

Some sugar (lactose) occurs naturally in milk and cream. But the vast majority of the sugar in ice cream is added separately by us.

We can add loads of different types of sugar to ice cream. And they all play the same role: they make it sweet and they keep it soft. But different sugars will do each of these to different extents.

Sugar provides sweetness​...

All sugars obviously add sweetness. But different types of sugar are less or more sweet. The sweetness of different sugars is measured against that of table sugar (sucrose).

... and ice crystal retardation​

All sugars lower the freezing point of water which stops ice crystals from forming. More sugar means less ice crystals. And less ice means a softer ice cream. But different types of sugars lower the freezing point to different degrees.

This is why you’ll often see ice cream recipes that include several different types of sugar. By mixing them up we can fine tune the sweetness and softness of the final ice cream.

Milk Solids Non Fat​ in the sugar solution

It sounds very technical but Milk Solids Non Fat (MSNF) are just the proteins, lactose and minerals found naturally in milk and cream. Cow’s milk is around 87% water, 4% proteins and 4.8% lactose with the remainder being salts and minerals.​

The proteins have two very important functions in ice cream: 1) they help the fat globules trap the air bubbles and stabilize the final product and 2) they contribute to the characteristic dairy flavour.

In some recipes you will see the addition of Skimmed Milk Powder (SMP). Since skimmed milk powder is essentially milk protein and lactose, these recipes are simply increasing the MSNF component of the mixture.


So that was a quick look at the basic components that make up all ice creams. Their chemical structure and the way they interact under different conditions are what makes ice cream, well, ice cream!

However, these relationships are fragile and we can add two other components to help strengthen them and improve the quality and stability of the final product. They go by the rather scientific names: Stabilizers and Emulsifiers.

​Emulsifiers

The role of emulsifiers in ice cream can be a little bit confusing. So let’s start with the basics…

​What is an emulsion?

An emulsion is a mixture of two or more liquids that are not normally mixable! By not normally mixable, I mean that one won’t dissolve in the other.

The most obvious example of an emulsion is an oil and vinegar salad dressing. Neither the oil nor the water will dissolve in the other. But when we combine the two and stir vigorously, the oil is broken up into tiny particles which are dispersed evenly throughout the water to create a consistent mixture. This is an emulsion.

A less obvious example of an emulsion is milk! Milk is basically an emulsion of liquid fat globules in water.

Now emulsions are by their nature unstable: remember they consist of liquids that are not normally mixable. And left to their own devices they will separate. A salad dressing left on the shelf will separate into two layers of oil and water.

​And milk straight from the cow will quickly separate into two layers: fatty cream and watery milk. This happens when the fat globules in the milk cluster together, separating from the water.

However, the milk we buy in the supermarket is “homogenised”. When milk is homogenised it is essentially mixed vigorously under high pressure. This breaks up the fat globules into much smaller particles.

These smaller, weaker particles attract proteins in the milk which interfere with the natural inclination of the fat globules to cluster together. And if they can’t cluster together, the milk can’t separate into cream and milk.

​So when milk is homogenised the natural emulsion is strengthened, making it much more difficult for it to separate into milk and cream.

​What’s this got to do with ice cream?

The purpose of milk homogenisation is to create a stronger emulsion where the fat globules are unable to cluster together. But remember: for ice cream we need the fat globules to cluster to form the long strands that will hold the air bubbles in place.

So when we make ice cream we actually need to de-emulsify the milk to emulsify the ice cream!

How do we de-emulsify the milk?

It’s the proteins attached to the fat globules that are preventing those globules clustering together. So we need to remove the protein molecules from the fat globules. And this is where emulsifiers come in.

Any emulsifiers in the mixture will attach themselves to the surface of the fat globules, displacing many of the proteins. And the emulsifiers don’t interfere with the natural inclination of the fat globules to cluster together in the same way as the proteins do.

But what are these magic emulsifiers? And where do them come from? Well, in home-made ice cream they often come from eggs! Or more specifically egg yolks.

Egg yolks contribute three important things to an ice cream mixture: fat, protein and a chemical called Lecithin. And it’s the Lecithin in eggs that acts like an emulsifier: displacing the proteins on the membranes of the flat globules and allowing those globules to cluster again.

Of course you can make ice cream without eggs. Some Italian gelatos use cornstarch or tapioca starch instead, which amongst other things fill the emulsifying role of eggs. And of course commercial ice cream manufacturers prefer synthetic emulsifiers such as Polysorbate 80.

But if you don’t use something as an emulsifier, your ice cream wont have the same smooth texture and solidity as those made with emulsifiers. This is why Philly style ice creams tend to be coarser and more fast melting with less body: they don’t use eggs or any other emulsifiers.

Check out my emulsifier page for lots more information about these magic ingredients and how to use them properly.

Stabilizers

Just like emulsifiers, stabilizers can also improve the texture and structure of ice cream. But they do it in a different way.

Stabilizers act a bit like sponges, soaking up any excess water in the ice cream mixture. This will obviously thicken the ice cream. By holding onto that water it also slows melting. And it also helps prevent the growth of ice crystals during storage, so the ice cream maintains a nicer texture for longer.

Most stabilizers are derived from plants. However, some come from bacteria or animal origins.

Stabilizers are used in pretty much all commercial ice creams. This is largely to guarantee a long shelf life for their products. But in home-made ice creams they are less common. And in fact many purists are wary of them.

I don’t take this line at all. Making the best possible ice cream is my main priority. As home enthusiasts on small budgets we’re already handicapped by limited machines and freezers. So if I can use a perfectly natural and safe ingredient to improve the quality of my ice cream I want to at least try it out.

Having said that some people have intolerance's to the stabilizers. So you’d be wise to check that out if you plan on using them!

For more information on the different stabilizers​, how they can improve our ice cream and the best ways to use them, check out the stabilizer page!


OK, so we’ve looked at all the four key components that make up ice cream. And we should have a pretty good idea how they work together in the final product. But how do these raw ingredients come together to make ice cream?

Let’s look at the five stages of ice cream production and see how each stage contributes to the final product.

The Five Stages of Ice cream Production

  1. Preparing the mix
  2. Pre-chilling
  3. Ageing
  4. Freezing
  5. Hardening​

Stage 1: Preparing the mix

This is all about the recipe. And it’s probably the most important stage in the whole process. Because if we get this wrong it doesn’t matter what happens in the next four stages: our ice cream will be rubbish.

This means we’ve got to make sure we’ve got the right amounts of each ingredient so that we have the ideal proportions of fats, sugars and solids.

Once we're sure the recipe is balanced, it's time to heat the mix. This serves two purposes. Firstly, it pasteurizes the mixture, which is important for health reasons. And secondly, it encourages the "denaturation" of the milk proteins, which will help produce a more stable end product.

i) Pasteurization​

Home-made ice cream generally uses pre-pasteurized milk. But if the mixture contains eggs, they need to be pasteurized by us. This is to kill any dangerous bacteria such as Salmonella which may be present.

​Traditionally we would heat the mixture to 85°C (185 °F). However, if we want to reduce the eggy flavour in the final ice cream we could instead keep the mixture at 69 °C (156 °F) for 20 minutes.

This will also pasteurize the eggs and thicken the mixture. But it keeps the mixture beneath the 72 °C (162 °F) at which egg proteins start to denature, releasing that characteristic eggy flavour.

Sometimes that flavour is desirable. But if it’s not and we still want to take advantage of the thickening and emulsifying properties of egg yolks, this is an option!

​If there are no eggs in the mixture, then there’s no need to pasteurize it. However, there are a couple of good reason why it’s a good idea to heat it up anyway:

  1. It can help infuse any added flavours into the mix
  2. It can help with milk protein "denaturation"

ii) Protein denaturation

When we heat the mixture the whey proteins in the milk undergo partial protein unfolding which is also know as denaturation. Essentially this just means that their structure changes under the stress of the heat.

However, this new structure is more inclined to stabilize the air bubbles in the ice cream. So what it means for us is a smoother, more stable final product.

​Stage 2: Pre-chilling

​Once the mixture has been pasteurized, it should be cooled down as quickly as possible. This is to minimize the time it remains between 45 °C (113 °F) and 15 °C (59 °F) which is when harmful bacteria can re-develop.

​Ideally, we should place the mixture in an ice bath until it reaches room temperature and then transfer it to the fridge where it will continue to cool down to 4 °C.

​This pre-chilling also contributes to a smoother texture in the finished ice cream. There's a close relationship between the amount of time the mixture spends in the ice cream machine and ice crystal size in the final product. Essentially: the less time in the machine, the smaller the crystals.

So clearly we should do all we can to reduce the time the mixture spends in the machine. And by adding a mixture that's already chilled, we help the machine do it's job more quickly, which means smaller crystals and a smoother final ice cream.

If you pre-chill the mixture in the fridge overnight it will also benefit from "ageing". But what is ageing?

Stage 3: Ageing​

​Keeping it in the fridge overnight will obviously allow any flavors we’ve added to the mixture to develop more depth.

But it also encourages two chemical changes which will encourage the mixture to hold air better once it’s being churned and produce a more stable final product:

​Firstly, the fat globules start to crystallize. This is where small, spiky crystals emerge from the surface of the globules. Once the mixture is being churned, these pointy crystals will help globules stick to other globules to form the long stings that hold the air bubbles in place.

Secondly, any emulsifiers in the mixture (either from the eggs yolks or other sources) will start to displace the milk proteins on the surface of the fat globules. Remember, it’s these proteins that were helping keep the milk homogenized by discouraging the globules from clumping together.

​So, with the milk proteins gone and these spiky needles protruding from their surface, the fat globules are now primed for "partial coalescence". This is when they will start to clump together to form the scaffolding that supports the structure of the final ice cream!

Stage 4: Freezing​

​The ice cream mixture is then added to a machine which simultaneously chills and churns it. In doing so it makes three important changes to the mixture which will transform it into ice cream...

​1. Creates ice crystals in the mixture

The ice cream mix sits in a container within the machine. The outside of this container is cooled rapidly and the mixture that's touching the sides of the container begins to freeze, forming ice crystals.

In the middle the container is a rotating “dasher” with blades that scrape against the sides. As it rotates, the blades scrape the ice crystals off the sides and moves them into the middle of the mixture.

The displaced ice crystals further cool the rest of the mixture. And the space they leave on the sides is rapidly filled by new ice crystals.

Then as the dasher continues to rotate, the new ice crystals are scraped away again. And so it goes on, with more ice being distributed throughout the mix as it cools down.

​2. Adds air to the mixture

​The blades of the dasher also push pockets of air into the mixture. Then as they turn, the blades break these pockets into smaller and smaller air bubbles which are evenly distributed throughout the mix.

These small, evenly distributed air bubbles are essential for a smooth, stable final product.

3. Encourages partial coalescence​

​The dasher blades fulfill one other very important function. By churning the mixture they cause some of the fat droplets to collide and join together or “coalesce”. This is where we see the benefits of the ageing stage...

​The emulsifiers that replaced the proteins on the surface of the fat globules during ageing have already weakened the stability of the emulsion. And now as the fat globules are mixed, the spiky crystals (which also formed during ageing), pierce the layers of other fat globules as they collide, allowing them to stick together more easily.

This process is called partial coalescence. And the partially coalesced fat droplets are known as de-emulsified or destabilized fat.

More importantly, this coalescence creates the strings of fat globules that wrap around the air bubbles and hold them in place. So ironically, it is the destabilized fats which stabilize the air bubbles in the final ice cream!

There is an important balance to be maintained here. If there’s too much protein or not enough emulsifiers in the mix, the emulsion will be too stable, and the fat globules won’t coalesce. While if there’s not enough protein or too many emulsifiers, the emulsion will de-stabilize and too much of the fat will coalesce.

Too little partial coalescence and there may not be enough fat to hold the air bubbles in place. This will result in an unstable foam that’s wet and coarse.

Too much partial coalescence and the coalesced fat droplets become so large you can feel them in your mouth. Known as “buttering”, this isn’t pleasant either!

​The final ice cream starts to come together

As more and more water starts to freeze into ice crystals, the mixture starts to thicken. Since there's less water, the sugars in the remaining water become more and more concentrated.

This further reduces the freezing point of the mixture until at last the remaining liquid contains too much sugar to freeze further.

​At the same time, the beating of the blades and the emulsifiers in the mixture encourage the fat globules to group together and coalesce. They form strings which together with the proteins trap and stabilize the air bubbles that are introduced to the mix by the rotating blades.

And so here we have the three states in a delicate balance. The solid ice crystals, the air bubbles and the super sweetened liquid cream.

Commercial vs Domestic set ups​

Commercial ice cream machines (also called batch freezers) have two significant advantages over the ice cream machines we use at home: 1) they can freeze the mixture much faster and 2) they can add more air to the mixture.

​We already know that smaller ice crystals mean a smoother ice cream. The faster the mixture is frozen, the less time the small, newly formed ice crystals will have to grow into large, coarser crystals.

The mixture should leave your fridge and enter the machine at about 4 °C. The machines job is to cool it to between -5 °C and -7 °C (23 °F and 19.4 °F). Commercial machines can do this in less that 15 min. Domestic machines may take as long as 30 min. And it’s during this extra time that the small crystals can grow.

​There’s not a lot we can do about this. Except get the mixture as cold as possible before we put in in the machine.

Stage 5: ​Hardening

When it reaches about -6 °C (21 °F) the ice cream should be removed from the machine. At this point it is still very soft with a consistency much like soft serve ice cream. So we transfer it to a freezer to harden.

​Commercial vs Domestic set ups

This is another area where the domestic set up can’t match up to the commercial operations. Ideally the ice cream should be cooled to -35 ° C (-31 °F). At this temperature, further ice crystallisation is impossible so the ice cream will remain smooth.​

​Unfortunately, domestic freezers won’t reach these temperatures so we just need to content ourselves with cooling the ice cream as quickly as possible.

​If you want to eat it all the same day (and why not?), you should cool it to around -12 ° C (10 °F) which is its ideal serving temperature.

​But if want it to last more than a couple of days you need to get it down to -18 ° C (10 °F) as quickly as you can. However home made ice cream is not meant to be stored for long periods of time and you won’t get much more than a week before the quality really starts to deteriorate.

Wrapping Up​

​I hope that this post has given you a solid understanding of the science behind ice cream. Not only is it pretty interesting (I hope!). It should also help you understand how to fix things when they go wrong and how to experiment within the bounds of what is scientifically possible!

As always, if you have any questions or comments, please let me know below…​