Whether you make one bowl of ice cream at home, or 1000 liter in a factory, the science of ice cream stays the same. You’d still want small ice crystals, to make a super smooth ice cream and you’d like to incorporate enough air for a pleasant eating experience.
What changes though, is the type of equipment and process steps you’d have to go through to make a 1000l of ice cream instead of that one bowl. When doing everything at a larger scale, all of a sudden new concerns pop up that a home ice cream maker might not have thought of yet! We’ll dive into the world of ice cream manufacturing, at scale.
Ice cream making re-cap
When making ice cream you have two main goals:
- Include air in the ice cream to make it light and soft.
- Freeze part of the water (but not all!) to solidify the ice cream mixture. Ideally, you’d want to create a lot of small ice crystals when doing so.
At home, there are plenty of ways to do this. The simplest way is to whip up some cream and mix it with a sweet liquid (as we did here). Alternatively you can make a custard (or other flavourful mixture) and freeze this in an ice cream machine. The ice cream machine will freeze and incorporate air by continuously mixing it.
Whipping up less than a liter of cream and mixing it in with your flavors is pretty easy at home. You just take a regular kitchen mixer to do so and then leave it t cool in the fridge or freeze it in the freezer. But, imagine having to do so for 1000 liter of ice cream a day. Whipping some cream because a lot bigger challenge then. Cooling won’t be as easy either, how are you going to get rid of all that heat!
Factory ice cream & pasteurization
All these factors have to be taken into account when looking into making ice cream on a factory scale. But there’s more, the first one being food safety of your raw materials. If you make ice cream at home you most probably use milk and cream that have been pasteurized already. However, large scale producers will generally have to do this step themselves since there’s a risk of micro organisms coming back in after pasteurization if it’s not a well controlled and enclosed system.
Pasteurization & preservation
Pasteurization is a preservation method. By heating the dairy ingredients (milk and cream) to a specific temperature for a pre-defined duration you kill off a lot of micro organisms. By reducing the amount of micro organisms (pasteurization never removes all of them), you increase the shelf life of the product. Also, pasteurization will kill off most of the pathogenic micro organisms, those that make you sick, making the ice cream safe for consumption.
Pasteurization & ice cream quality
Apart from the safety aspect, pasteurization is done for quality purposes as well. Pasteurization of the ice cream also prevents milk fat in the cream from becoming rancid through oxidation by breaking off enzymes which catalyze the reaction.
And there’s another benefit of pasteurization. The heat treatment will help the proteins in the milk to hydrate. The heat will partially unfold the proteins which makes it easier for the protein to hydrate and bind to water.
Not all is pasteurized
A heat treatment is not suitable for all ice cream ingredients though. Whereas the cream and milk benefit from the heat treatment, flavours for instance can break down because of the heat treatment. Therefore, often just the dairy ingredients and other ingredients are added after this first step.
Homogenization of factory ice cream
Immediately after pasteurization the ice cream mix can be homogenized. Homogenization is a process in which the fat globules from the milk and cream are made into an identical size. All fat particles will have the same size and are smaller than 1 micron.
If the particles are the same size this will give a more stable ice cream. These very small fat bubbles serve as a pickering emulsifier. They sit as small particles around the air bubbles in the ice cream preventing the bubbles from coming together and escaping out of the ice cream.
Also, the smoothness of the ice cream improves because of all these small fat particles. Instead of large clumps the fat is spread out more.
Cooling, aging & freezing
The pasteurized mixture then has to cool down. It is cooled down to 0-4ºC, so above freezing temperature. At this point, no air is yet introduced, it is really all about cooling down and getting ready for the next step of freezing and aeration. The mix should be cooled fast enough to make sure no growth of some left over micro organisms will take place. With pasteurization not all micro organisms are killed off, so it is still important to treat the mix carefully.
If an ice cream manufacturer wants to add ingredients which are sensitive to heat (e.g. fruits) this will be done once the ice cream has cooled down.
Aging to prepare for aeration
Once the ice cream has cooled down it will be left to age for a while. Aging helps to get the mix ready for being transformed into an ice cream. Several things happen here which will help make a good ice cream.
First of all, the fat solidifies. Once the ice cream has cooled down the fat (mostly butter fat) will become solid. The solid fat will help to form the ice cream structure once you start freezing the mixture and forming ice crystals. Also, if emulsifiers are used, these will sit onto the fat surface during this surface which will make it easier to break the fat particles later on.
Also, some ingredients may still be hydrating, thus binding to water during this phase, proteins and stabilizers tend to do so.
How long you should age your ice cream for really depends on the process you’ll be using in a next step as well as the ingredients you use. If your fats crystallize quickly you probably need less aging time for instance. If you use extrusion to make the final ice cream though, it may need somewhat longer.
Freezing large ice cream batches
Now that the ice cream has been aged and readied for being made into ice cream it is time to transfer it into ice cream. The main challenge when doing this is to cool it down quick enough. This is hard with large quantities of ice cream since a lot of heat has to be taken out of the mix.
In factories this is often done using scraped surface heat exchangers. These are large pieces of equipment through which the ice cream mixture flows in a continuous manner and is scraped alongside the surface which is kept very cool. It’s a bit like an ice cream machine, but with material flowing in and out continuously. The temperature of the wall is very cold, down to -30ºC which helps in forming a lot of small crystals. This is the only place in the process where new ice cream crystals are formed. Later on in the process the existing crystals may grow, but no new ones will be formed.
But it’s not just freezing what happens at this stage. It is also important to incorporate air into the ice cream. The air will make the ice cream lighter and easier to scoop. Air is injected into the ice cream. This is done under pressure. As a result, the air bubbles are very small, but when the ice cream leaves the pressurized mixer it will expand. Before the pressure is released the scraper breaks down the air bubbles into smaller ones to create a nice dispersion of small bubbles all throughout the ice cream.
Ice crystals are formed and air is incorporated, but that’s not all that happens. Those fat crystals that were prepared during the ageing process will sit together partially. This helps in creating a network to stabilize the ice cream. These fat particles can especially stabilize the air bubbles using a phenomenon called ‘pickering’ stabilization. The partially coalesced fat bubbles help to incorporate more air into the ice cream.
When the ice cream comes out of the freezer it isn’t yet as hard as your ice cream in a tub. Instead, it will only be around -5ºC. The exact temperature though will depend on the process. The ice cream will have to harden out further. This again has to be done efficiently and fast so it is often done in a tunnel with blowing ice cold air to rapidly remove the heat.
To what extent the ice cream has to be cooled at this point really depends on the way it is processed further. If the ice cream still has to be shaped into all sorts of intricate forms it will have to be hard enough to keep its shape, but soft enough to be shaped. On the other hand, if the ice cream is dosed in a tub, it will have to be flexible enough to still flatten out in the tub.
After the ice cream has been put in its final packaging it will continue to harden out. If you’ve made ice cream yourself you will recognize that ice cream coming straight from the ice cream machine is still very soft and needs to harden out in the freezer.
Bringing it all together
The video below gives you some examples of how ice cream are made in a factory. It really focuses on the packaging step (which is often most fun to watch in any food factory), but just realize how the ice cream has to be made in such a way that it can actually be packaged in the way it is!
Clarke, C., The Science of ice cream (2012), link
IJsbereiding, ILVO TAD Zuivel, link
Ice Cream Manufacture, University of Guelph, link