Apart from being a delicious dessert which can cool us down on a hot summer’s day and satisfy our sweet desires, ice cream is also a fascinating food to examine scientifically. Ice Cream is both an emulsion of fat and water and a partially frozen foam with pockets of air a tenth of a millimeter across. Read on to discover all there is to know about ice cream science and get some simple tips for making your homemade ice cream better.
I’ve split the article into the four stages of ice cream preparation so we can discuss the chemical and physical changes that are happening at each stage to take the raw ingredients to the final product we love so much. Those stages are;
- Heating the base
- Chilling & Ageing
- Hardening & Storing
Heating the Base
The stage includes both pasteurization and homogenization. The purpose of pasteurization is to kill off bacteria in the mixture to make it safe (or safer) for consumption. The factors that go into this are heating the base to a particular temperature and the time it is kept at that temperature.
In recipes with eggs, the heating process also creates the egg custard where the egg proteins break down and catch on each other to form a gel, hence thickening the mixture into a custard.
Homogenization is the process of combining two mixtures that don’t normally go together, such as oil and water. When we’re talking about two liquids, the resulting mixture is called an ‘emulsion.’ If you have a salad dressing that includes both olive oil and balsamic vinegar, for example, you’ll probably find that once its been sitting still for a while the oil and vinegar/water will separate. Before you use it you need to make sure the lid is on properly then shake it vigorously to redistribute the two liquids amongst each other, thereby creating a homogeneous emulsion.
Another example is the fat and water elements in milk. The milk you buy at the supermarket is probably already homogenised and it should say as much somewhere on the bottle. However, if you’ve ever used a home cheese making kit then you’ll probably have had to seek out unhomogenised milk where you can see the cream/fat in the milk rises to the top.
For ice cream making we’re also talking about creating a fat and water emulsion. In this process, the size of the fat globules is reduced which increases their surface area and they’re evenly distributed throughout the mixture where the proteins can cling to them and prevent them from coming back together (as happens with the salad dressing emulsion settles and separates). This helps to make the ice cream smoother and richer and increases the air stability and resistance to melting.
Commercially, homogenisation is done at a high pressure and temperature.
- Always heat and pasteurize your ice cream base
- Use already homogenized cream and milk as you won’t have the equipment necessary to do it yourself
- Thoroughly mix the base during this process to create an emulsion of the various ingredients
Chilling & Ageing
Ageing allows crystals from the milk fat to start protruding from the surface of the globule. These protrusions are important for trapping air in the churning process so allowing them time to form helps to improve the whipping qualities of the mix as well as the body and texture of the ice cream. Overnight is usually a good amount of time for ageing but it should be at least 4 hours. It’s even more important for recipes that contain eggs.
Chilling is also an important step and can conveniently be done at the same time as ageing. When the ice cream is churned, the quicker it is frozen the better as quicker freezing means smaller ice crystals and therefore a smoother and creamier ice cream. The chilling step ensures that the base is already quite cold when it is churned which means the freezing will be faster.
- After pasteurization and homogenization of your base, cover and put in the fridge overnight to chill and age
The churning or whipping process incorporates air into the ice cream while freezing it, hence turning the stable emulsion into a stabilized foam (a foam is a mass of small bubbles formed on or in a liquid). The foam is ‘stabilized’ by the protrusions around the fat globules piercing each other and forming a network which is able to trap air bubbles.
As mentioned in the last section, it’s important that the base is as cold as possible before churning starts to make the freezing process faster and produce smaller ice crystals and a smoother final product. It’s said that ice cream made with liquid nitrogen is some of the smoothest you will find because it’s frozen almost instantly with the incredibly could liquid nitrogen.
The base is frozen by coming into contact with the sides of the ice cream maker that are cooled either by pre-freezing or a compressor device. The churning arm (or dasher) moves the base against the sides at its churning. To ensure this works properly, the cooling mechanism of the machine needs to pull more heat energy out of the base than the kinetic energy being added to the base by the churning arm. This in/out energy equilibrium happens when the churning temperature is -5 to -7 degrees C (23 - 45 F).
The amount of air that’s incorporated into the ice cream is called the ‘overrun.’ If the base doubles in volume during the churning process then that would be described as an overrun of 100%. The overrun is controlled by how fast the churning arm moves with a faster churn resulting in a higher overrun.
Stabilizers also affect the churning process. The protein in eggs have stabilizing properties so if the recipe includes eggs then this section will be relevant. There are also other stabilizers that can be added to improve the shelf life and texture of the ice cream. Egg proteins and other stabilizers reduce the formation of ice crystals, slow down the melting process and stabilize the air bubbles in the foam.
- Along with the base being chilled and aged overnight in the fridge, make sure you do your churning in a cold room without direct sun and make sure your machine or pre-frozen canister is as cold as possible before beginning
- If you’re adding mix-ins, make sure they are also pre-cooled
- Use stabilizers and/or eggs in your recipes to improve the texture
Hardening & Storing
The final stage of ice cream production is the hardening stage. If you were making soft serve then you’d already be finished before this stage but for all other ice cream, hardening is required. During the churning process, only about 40-50% of the water in the base freezes and then hardening freezes most of the rest. The reason it’s ‘most’ and not ‘all’ is that the sugar in the ice cream is dissolved into the water and this lowers the freezing point of the water. As more and more of the water freezes, the remaining water gets more and more sugary and at some point the sugar content is so high that the water won’t freeze. However, once the ice cream gets down to a temperature of -35C (-31F), the remaining unfrozen syrup turns into a ‘glass.’
To achieve these steps, the ice cream should be placed in a very cold and consistent freezer, ideally at -35C (-31F). Any temperature below -25C will keep the ice cream stable for an indefinite period of time. If it goes above -25C, the previously formed ice crystals will start to grow in size and negatively affect the texture of the ice cream. This is where stabilizers are important for maining a longer shelf life. The typical serving temperature of ice cream is -16C at which point around 72% of the water is frozen.
Home freezers are less powerful, less cold and have more temperature fluctuations than commercial ice cream freezers which are all factors that will make the hardening and storing process more difficult and less effective. To combat this, use these tips;
- Store your ice cream in small, flat containers that maximize the surface area and therefore will make freezing faster
- Store your containers as close as you can to the back and bottom of the freezer where it’s coldest and minimize door opening to maintain a consistent temperature
- Use eggs and stabilizers in your recipes to improve shelf life
- Eat your ice cream sooner rather than later before the texture has a chance to deteriorate
Let’s solidify our understanding by looking at the individual paths of each of the key elements;
Fat (including milkfat/butterfat) is introduced into the ice cream mix through cream (~30% fat), milk (~1%), egg yolks (~27%) and sometimes non-dairy sources and comprises 10 - 16% of the final product by weight.
The first process that affects the fat is homogenization where the milk and cream are agitated at a high pressure and temperature to break down the fat globules to a size of 1 micron and evenly distribute them throughout the rest of the mixture.
During the ageing and churning processes, the fat globules come back together a little, known as ‘partial coalescence,’ ‘destabilization’ and ‘flocculation.’ This partial coalescence allows the fat globules to capture bubbles of air to form a foam.
The partially coalesced fat molecules capture and hold the air bubbles that are whipped into it during the churning process. At this stage the mixture is also frozen, locking the fat molecules in place.
Protein is introduced into the mix through cream (~2%), milk (~3.4%) and egg yolks (~16%) and will comprise around 5% of the final produce by weight.
During the homogenization process, the protein particles stick to the outside of the fat globules creating a membrane which helps to stabilize the emulsion and prevent the fat globules from coalescing.
Milk proteins have good foaming properties and therefore contribute to the aeration of the ice cream during the churning stage. The proteins also interact with the added stabilizers to stabilize the foam and prevent it from collapsing as it freezes. Also during the churning stage, the unfrozen water and those molecules dissolved into it, including proteins, keep the entire structure together. The proteins help to control ice crystal growth as well.
At the hardening and storing stage, proteins help to maintain the texture over time.
Emulsifiers are added ingredients that comprise less than 1% of the final ice cream product. The original ice cream emulsifier was egg yolk but these days, the fat and/or carbohydrate compounds of polysorbate 80 and mono- and di-glycerides are most commonly used.
The role of emulsifiers is as a ‘surface active molecule.’ Proteins such as those found in milk are also surface active molecules but they perform slightly different tasks in the ice cream making process. Proteins help to stabilize the fat emulsion at the homogenization stage and then emulsifiers destabilize the fat emulsion at the ageing and churning stages. The destabilization of of the fat emulsion allows some of the fat globules to coalesce so that they can capture pockets of air during the churning process.
Water is introduced to the mix in the cream (60%) and milk (90%) and will comprise about 55 - 64% of the final product by weight. The size of the water ice crystals play an important role in the texture. The smaller they are (ideally 1-2 tenths of a millimeter across), the smoother the ice cream.
The first process that affects the water in the mixture is homogenization where the fat globules are distributed throughout the rest of the mixture through agitation and heating. This creates an emulsion between the fat and water.
The next important step as far as the water is concerned is chilling. The mixture is chilled so that it will freeze as quickly as possible at the churning stage.
Churning is all about freezing the mixture whilst incorporating air into it at the same time. During churning, about 40-50% of the water is frozen. The faster the water is frozen, the smaller the ice crystals will be and the smoother the ice cream.
During the churning and hardening phases, the water is involved in another important process along with the sugar. As the water begins to freeze, the sugar content of the unfrozen water gets higher and higher and this in turn lowers the freezing point of that water, known as ‘freezing point depression.’ Due to this effect, only around 70% off the water is frozen at ice cream serving temperature of -16C. This helps the ice cream to be smooth and scoopable.
In commercial ice cream production, the hardening stage will quickly bring the ice cream down to a temperature of -35C at which point the remaining water is either frozen or has become a ‘glass’ because the sugar content is too high to allow it to freeze.
Air is introduced to the mix during the churning process and will comprise 10 - 50% of the volume of the end product (0% of the weight). The air in ice cream contributes to the smoothness of the texture.
The amount of air incorporated is known as the ‘overrun.’ Cheaper ice creams have a higher overrun, up to 100%, and more expensive ‘premium’ brands have an overrun as low as 20%.
During churning, the air bubbles start our large but are cut down in size by the mixing and scraper blades of the ice cream machine until they are around 1/10 of a millimeter in diameter.
Stabilizers are added to the mix individually at the blending stage (before heating) and comprise less than 0.5% of the final product but contribute significantly to the texture and shelf life of the ice cream. They do this primarily by enhancing ‘mix viscosity’ which reduces the mobility of the water molecules.
The ageing stage allows stabilizers to hydrate so that can start to work their magic. At the churning and hardening stages, they help to keep the ice crystal size small. During storage and distribution, stabilizers help to prevent melting and ice crystal growth (recrystallization) in instances of heat shock (temperature fluctuations where ice cream is left at room temperature and then refrozen) which can happen in various situations as the ice cream is transported.
There are a range of sweeteners used in ice cream from regular table sugar (sucrose) to syrups such as Corn Starch Hydrolysate Syrup (CSS) and maple syrup. These are added to the mix as individual ingredients as well as part of other ingredients. Lactose is a sweetener and it is introduced in the milk (4%) and cream (3%). Sweeteners comprise around 15% of the end product by weight.
Aside from providing sweetness (obviously), sweeteners also contribute to the texture and scoopability of the ice cream. At the churning and hardening stages the sweeteners lower the freezing point of the water into which they have dissolved. This results in some of the water remaining unfrozen at serving temperature which makes the ice cream scoopable and easy to eat. They also contribute to smaller ice crystal growth size which, as we know by now, is instrumental to producing a creamy texture.
Take these tips and use them wisely to make your next batch as creamy, flavorsome and long lasting (until you’ve eaten it all of course) as possible. Next time you’re eating our favorite frozen treat with family or friends, don’t forget to drop some ice cream science in between bites and brain freezes. Don’t leave them in suspense, let them that SpoonReport.com is where you gained this impressive knowledge! 😉