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:
- When something’s not working, it will help you fix it
- 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.
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
America's Choice Tub o' Vanilla
America's Choice Premium "Vanilla Bean"
Breyer's "Homemade Vanilla"
Turkey Hill "Vanilla Bean"
Haagen Dazs "All Natural Vanilla Ice Cream"
Stonyfield Farm "Gotta Have Vanilla"
Ronnybrook "Hudson Valley Vanilla"
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.
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.
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
- Preparing the mix
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.
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:
- It can help infuse any added flavours into the mix
- 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.
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…