This recipe will be split into two sections: SECTION 1 will cover the science and preparation and SECTION 2 the recipe itself. I recommend reading section 1 before you start making your ice cream.
SECTION 1: THE SCIENCE AND PREPARATION
1. Freezing the bowl
For this recipe, I use the Cuisinart ICE-30BC, which comes with a removable bowl that needs to be frozen overnight before it can be used. If you’re using the Cuisinart ICE-100, Breville BCI600XL, Lello Musso Pola 5030, or any other machine with an in-built compressor, you can jump to part 2 on the formation of ice crystals.
The day before you start making your ice cream, take the bowl and cover the top with cling film; use an elastic band to help keep it in place. Put the bowl in a plastic bag and tie the ends.
The plastic bag and cling film will help prevent water from freezing to the inside of the bowl whilst it’s in the freezer. Any ice that freezes to the side of your bowl will act as an insulator and slow heat transfer from the ice cream to the bowl.
As we will see in part 4 on residence time, slow heat transfer is likely to increase the time your ice cream spends in the machine. The longer your ice cream spends in the machine, the sandier the texture is likely to be.
Take a 1 litre plastic container, the ice cream dasher, and the freezer bowl and place them in your freezer overnight. Freezing the dasher and plastic container will remove any heat stored in them. This will help prevent ice cream that comes in contact with the side of the relatively warm container during the extraction stage from melting, which is likely to result in coarse texture.
It’s also important that you freeze enough water to be able to make an ice bath. We will be using an ice bath to quickly cool the ice cream mix after it’s been heated and before it goes in the fridge to age.
2. The formation of ice crystals
When making ice cream, the two salient points you should always consider are flavour and texture; the best ice creams in the world are bursting with flavour and have a smooth and creamy texture.
Ice crystal size plays a significant role in promoting this smooth and creamy texture: small ice crystals contribute significantly to smooth and creamy texture, whilst large ice crystals produce ice cream that is coarse (Goff and Hartel (2013)).
Ice crystal size is affected by the recipe and by freezing. Freezing is done in two stages : 1. dynamic freezing, where the ice cream mix is frozen in a machine to incorporate air and to limit the size of the ice crystals that form; and 2. static freezing where the ice cream is hardened in the freezer.
In this post, we will look at what we can do to promote the development of small ice crystals during both the dynamic and static freezing stages.
3. Setting the fridge temperature
It’s very important that set your fridge to between 0 and 2°C to increase the rate of crystallisation of the fat globules when you age your mix overnight. Crystallisation of fat during the ageing process helps maintain the shape of ice cream when it is served and also helps minimise the rate at which the ice cream melts (Goff and Hartel (2013)).
If you don’t allow the fat globules sufficient time to crystallise, it is likely that your ice cream will suffer from relatively fast meltdown and less retention of shape.
4. Setting the freezer temperature
It’s also very important that you get your freezer as cold as it will go, ideally around -25 to -30°C. This is because when you finish churning your ice cream and extract it from your machine at around -5°C, significant changes to the ice crystals continue to take place until the temperature decreases to -18°C, preferably -25°C to -30°C (Goff and Hartel (2013)).
The longer it takes for your ice cream to reach -18°C (-0.4°F), the larger the ice crystals will grow and the sandier the texture is likely to be. Donhowe (1993) showed that faster cooling of ice cream during hardening resulted in smaller mean ice crystal size.
So to promote faster cooling of your ice cream during the hardening stage, it’s important to ensure that your freezer is set to its coldest temperature. If you have a super chill button, or something similar, it’s a good idea to switch this on. Also, try and place your ice cream in the back of your freezer where the temperature is coldest.
5. Freezer temperature effect on residence time
If you’re using the Cuisinart ICE-30BC, or any other machine that requires you to freeze the bowl before it can be used, your freezer’s temperature will also have a considerable effect on residence time.
Residence time is the time a mix spends in the machine and has a significant effect on ice crystal size. In their study of the effects of sweetener type, draw temperature, dasher speed, and throughput rate on ice crystal size during freezing of ice cream, Drewett and Hartel (2007) found that residence time had the most pronounced effect on mean ice crystal size.
So to obtain the smallest ice crystals, it is necessary to have the shortest residence time possible (Goff and Hartel (2013)).
I’ve found that my freezer’s temperature has a noticeable effect on residence time when I use my Cuisinart ICE-3OBC to churn my ice cream. When I set my freezer to ‘super freeze’, which gets the temperature down to about -27°C (-16.6°F), it takes about 18 minutes to churn an 800g batch of ice cream. When I set it to -18°C (-0.4°F), it takes about 24 minutes to churn the same amount. This is a considerable difference in residence time that should not be overlooked.
So, to reduce residence time and promote the development of small ice crystals, get your freezer as cold as it will go when freezing your bowl and when hardening your ice cream.
6. The Size of Your Pan
The size of the pan you use plays an important role in formulating the final mix composition as it affects the amount of water that is evaporated during the heating stage.
In this recipe, we will be aiming for a 15% reduction after 25 minutes of heating at 72°C (162°F). This will produce a mix consisting of around 50.02% total solids after heating, which I’ve found to be ideal for the promotion of smooth and creamy texture in homemade ice cream. If your pan is too small, you are unlikely to achieve a 15% reduction after 25 minutes of heating.
I will be using a large pan with a 23cm diameter for this recipe and recommend you use the same. If your pan is smaller than 23cm, you will need to continue heating your mix for a further 5 minutes. Using a pan with a diameter greater than 23cm is not a problem.
7. The weight of your pan
Before you start preparing your mix, it’s important to first weigh your pan. This is necessary so that you can check the level of reduction after 25 minutes heating.
The starting weight of our mix will be 1000g. After 25 minutes of heating and a 15% reduction, you should have a mix weight of 850g. If your mix weighs more than 850g, put it back on the heat and continue heating.
Here is how to check the level of reduction after 25 minutes heating:
My 23cm diameter pan weighs 1606g.
1606g pan + 1000g starting mix = 2606g starting weight.
After 25 minutes of heating, my total weight (1606g pan + 850g 15% reduced mix) should be 2456g.
If my total weight after 25 minutes heating is greater than 2456g, I will continue heating until the weight drops below 2456g.
8. Why 72°C (162°F) for 25 minutes?
When I first started making ice cream, I would always see the ubiquitous ‘heat the mix until it coats the back of a spoon and holds a clear path when you run your finger across it’ in recipe books and always found it frustrating that no one ever explained why this had to be done.
There are three principal reasons why we are going to heat our mix to 72°C (162°F) and hold it there for 25 minutes: 1. to pasteurise the mix, 2. to improve foaming and emulsification, and 3. to improve body and texture. Let’s take a look at each of these three points.
If you’re running a business and making ice cream to sell, you need to ensure that you are in compliance with food safety legislation. Here in the U.K, the Dairy Products (Hygiene) Regulations 1995, Schedule 6, part v 1 (a) states:
1. Pasteurised ice-cream shall be obtained by the mixture being heated—
to a temperature of not less than 65.6°C (150.1°F) and retained at that temperature for not less than 30 minutes;
to a temperature of not less than 71.1°C (160°F) and retained at that temperature for not less than 10 minutes; or
to a temperature of not less than 79.4°C (174.9°F) and retained at that temperature for not less than 15 seconds.
Ice cream needs to be pasteurised in order to destroy all pathogens and the enzyme phosphatase that may be harmful to health. This is just as important for you guys making ice cream to sell as it is for you guys making ice cream at home.
8.2 To improve foaming and emulsification
The second reason we are going to heat our mix to 72°C (162°F) for 25 minutes is to improve whey protein foaming and emulsification.
Foam is a volume of gas dispersed in liquid. Foams are unstable and will eventually break down. Therefore, a surface active agent such as protein is used to provide stability (Indrawati et al. (2008)).
Foam formation and its stability is important for texture and for the retention of air that is incorporated into the ice cream in the machine. Heating milk so that the whey proteins undergo partial protein unfolding yields more voluminous and more stable foam (Goff and Hartel (2013)) and improves the emulsifying characteristics of milk protein (Philips et al., 1990).
However, at high temperatures, foaming and emulsifying characteristics may be impaired due to protein aggregation (phillips et al., 1990).
So, heating whey proteins so that they undergo partial unfolding improves foaming and emulsification, whereas whey proteins that undergo irreversible aggregation due to excessive heat may impair foaming and emulsification.
At what temperatures, then, do the whey proteins start to undergo partial protein unfolding? Sava et al. (2005) held that thermal denaturation involves 2 steps: an unfolding step at 70 to 75°C, and an aggregation step at 78 to 82.5°C, that mostly follows unfolding.
So to promote reversible unfolding and prevent irreversible protein denaturation during prolonged heating, it’s prudent to keep the temperature of our mix below 75°C (167°F).
Surface hydrophobicity is also important in determining emulsifying activity (Monahan et al., 1993). Denatured proteins have been found to have better foaming properties, attributed to increased hydrophobicity, and greater interfacial contact (Damodaran, 1996).
Save et al found that surface hydrophobicity increased considerably at temperatures between 70 and 77.5°C (171.5°F) when whey protein was heated for 45 minutes, with greater increases noted after longer heating times.
Functionality of whey protein also depends on its solubility. Heating at a temperature between 70 (158) and 75°C (167°F) results in a minimal loss of solubility. A decrease in solubility of only 10 to 20% compared with the native protein was observed after 45 minutes by Sava et al.
So, research points to an optimal heating temperature for whey protein at somewhere between 70 (158) and 75°C (167°F). In this temperature range, whey proteins undergo reversible unfolding, which improves foaming and emulsification.
Holding whey protein at between 70 (158) and 75°C (167°F) for a prolonged period of time significantly increases surface hydrophobicity with only a minimal loss of solubility, which improves foaming. Greater increases in surface hydrophobicity were also noted after longer heating times.
During testing, I found that when I held my ice cream mix at temperatures above 72°C (162°F), the unpleasant ‘eggy’ hydrogen sulphide taste began to form and was noticeable on eating. I have found that heating an ice cream mix to 72°C (162°F) and holding it at this temperature for 25 minutes considerably improves body and texture. This is compared to a mix that is heated to 72°C (162°F) and held for 10 and 15 minutes only.
8.3 To improve body and texture
The third reason we are going to heat our ice cream mix to 72°C (162°F) and hold it there for 25 minutes is that heating milk also improves body and texture because of the denaturation of proteins and the consequent increase in their water-holding capacity (Goff and Hartel (2013)).
Goff and Hartel (2013) noted that denatured whey proteins can improve smoothness by helping to minimise the size of ice crystals.
9. Why is skimmed milk powder added to ice cream?
The use of skimmed milk powder in commercial ice cream making is usually associated with economy-style ice cream as it is a cost effective way of reducing the more expensive cream. In homemade ice cream, however, I’ve found that it is essential for the promotion of smooth and creamy texture.
Skimmed milk powder’s primary role in homemade ice cream is to increase the non-fat milk solids (NMS), namely the whey protein. Flores and Goff (1999) demonstrated that milk proteins had a large impact on texture by limiting ice crystal size and enhancing their stability.
I have not been able to achieve the same smooth and creamy texture in my homemade ice cream after 25 minutes of heating without the addition of skimmed milk powder.
SECTION 2: THE RECIPE
Ice cream mix: 25 minutes
Roasted bananas: 30 minutes
Ice cream maker
A zip-lock bag
1 litre plastic container
Makes just over 1 quart (1 litre) of ice cream
1. The importance of butterfat
It’s important that the fat percentage of the cream and milk you use match that stated in the recipe. I’ve included a recipe for 35% fat cream, 36% fat cream, and 47.5 fat cream. All recipes use skimmed milk at 0% fat. Get in touch if your cream does not match the fat percentages listed.
For readers in the United States and Canada, use ‘heavy’ or ‘whipping’ cream and non-fat milk. For readers in the U.K, use double cream and skimmed milk from Sainsburys as they have a 47.5% and <0.5% fat content respectively.
Double Cream at 35% fat
Double cream 566g
Skimmed milk 170g
Egg Yolks 78g
Skim Milk Powder 46g
Double Cream at 36% fat
Double cream 550g
Skimmed milk 186g
Egg Yolks 78g
Skim Milk Powder 46g
Double Cream at 47.5% fat
Double cream 417g
Skimmed milk 319g
Egg Yolks 78g
Skim Milk Powder 46g
Very ripe bananas 450g
Brown sugar 20g
Sea salt 1/4 teaspoon
I strongly recommend using organic milk and cream and organic free-range eggs whenever possible. I use organic milk and cream and organic free range eggs from farms in Wiltshire when I make ice cream for my business and find that I get a much richer flavour from organic milk and cream and a much deeper colour from free range egg yolks.
The table below shows the composition of our 1000g ice cream mix before heating.
After 25 minutes heating and a 15% reduction, our 850g mix will have the following composition:
Total Solids 50.9%
Non-fat Milk Solids (NMS) 10.11%
Egg Yolks 4.07%
2. Preparing an ice bath
Before you start preparing your mix, take a large bowl and fill it with enough ice to make an ice bath. Have a large zip-lock freezer bag ready next to the bowl, along with some table salt.
The zip lock bag and water bath is to ensure that the mix is cooled as quickly as possible once it has been cooked, minimising the time the mix spends in the ‘danger zone’, between 5 (41) and 65°C (149°F), where bacteria likes to multiply.
The longer your mix spends in the ‘danger zone’, the more bacteria is likely to multiply, imparting an undesirable taste and smell.
3. Heating the mix
Weigh your pan and record its weight. This is so we can check whether we have achieved the desired 15% reduction after 25 minutes of heating.
Once you’ve prepared the ice bath and weighed your pan, add the sugar and egg yolks to your pan. Mix the yolks and sugar; the sugar will help prevent the yolks from curdling. Add the skimmed milk powder and mix into the yolks and sugar.
Add the cream and milk and spend a good minute mixing all the ingredients together before you switch on the heat.
Over a medium heat, heat the mixture until the temperature reaches 71°C (160°F), making sure that you are constantly stirring. You will risk burning the proteins and curdling the egg yolks if you do not constantly stir the mix.
Once the temperature reaches 71°C (160°F), turn the heat down to low, move your pan slightly off the heat, and continue heating until the temperature reaches 72°C (162°F). Use your thermometer to keep your mix at 72°C for 25 minutes, adjusting the position of your pan to help regulate the temperature.
It’s important that you keep your mix at 72°C(162°F) for the full 25 minutes to obtain optimum foaming and emulsifying characteristics.
4. Cooling the mix
After 25 minutes, take the pan off the heat and weigh it. If the weight of the pan and the mix is more than 850g + the weight of the pan, place it back on the heat and continue heating for another 2-3 minutes or until you get the weight down sufficiently.
Carefully pour the mix into the zip-lock bag, seal and place it in the ice bath. Pour about a tablespoon of salt onto the salt to lower the temperature and cool the mix faster.
Once the mix has cooled to about 10°C (50°F), place it in the fridge and leave it overnight to age. Remember that ageing your mix is important as crystallisation of fat during the ageing process helps maintain the shape of ice cream when it is served and also helps minimise the rate at which it melts.
5. Roasting the bananas
Once you’ve allowed your mix to age overnight, you can start roasting your bananas. It’s important to use very ripe bananas; the riper your bananas, the more intense the flavour will be.
Pre-heat your oven to 180°C (356°F). Slice the bananas into small pieces and combine with the brown sugar and sea salt on a baking tray.
Bake for around 30 minutes or until the bananas are nice and brown. Don’t stir the bananas during baking to allow them to develop a nice caramelised layer.
Remove the bananas from the oven and use a fork to break them down into a pulp. Don’t worry if you have some large bits remaining as this will produce nice chunky bits of roasted banana in the ice cream.
Allow the bananas to cool to room temperature.
Once your bananas have cooled to room temperature, place the freezer bowl in your machine and add the dasher. Put the lid on and, with the machine switched on, pour in the mix followed by the roasted bananas.
If you’re using a machine with an in-built compressor, with the bowl in, switch on the compressor and leave it running for 10-15 minutes before adding the mix. This will ensure that the freezer bowl is as cold as possible when the mix is added, which will contribute to a reduction in residence time.
If you’re using the Cuisinart ICE-30BC, use your thumb to push the dasher against the side of the bowl as soon as you pour in the mix. This will ensure that the dasher scrapes off the layer of ice that freezes to the side of the bowl, improving heat transfer.
Any ice that is frozen to the side of the bowl will act as an insulator, slowing the release of heat from the ice cream to the bowl and increasing the residence time. Remember that the longer the residence time, the larger the ice crystals will grow and the sandier the texture is likely to be.
Goff and Hartel (2013) held that even a very thin layer of ice remaining on the bowl wall can cause a dramatic reduction in heat transfer so do try and push the dasher against the bowl.
Use a spoon to push along any static lumps of ice cream and ensure that the mix is constantly moving whilst in the machine.
7. Extraction time
Your mix will be ready when it develops a nice dry, stiff texture, and starts forming ribbon-like swirls. When you remove the dasher, your ice cream should stick firmly to it.
The point at which your mix is ready for extraction will vary from 15-45 minutes depending on the machine you use. For the Cuisinart ICE-30BC, your ice cream should be ready at 35 minutes of churning. For the Cuisinart ICE-100 and the Breville BCI600XL, this should be after 32 and 33 minutes respectively.
Just before your mix is ready, quickly take the plastic container out of the freezer and have a large and a small spoon handy.
When you finish churning a batch, it will be extracted from your machine at around -5°C (23°F) and will have a consistency very similar to that of soft serve ice cream. Ice cream is usually served in its scoopbable state at around -12°C (10.4°F) and so you will need to get your ice cream into your freezer to harden.
The extraction time, that is the time it takes to empty the ice cream from the machine and get into into your freezer, is another factor that has a considerable effect on ice crystal size.
This is because as you extract your ice cream from the bowl and into a plastic container, it spends time at room temperature. During this time at the relatively warm room temperature, some of the ice melts from the large ice crystals and the crystals that were initially small melt completely. This is known as ripening and occurs when ice cream is held at elevated temperatures (Goff and Hartel (2013)).
When you then get your ice cream into your freezer for the static freezing stage, the melted ice re-freezes onto the large ice crystals that survived. The result is that the total number of ice crystals is reduced and their size increases, the perfect formula for coarse texture.
So, just holding your ice cream at room temperature results in an increase in mean ice crystal size, which, in turn, contributes to coarse texture (Goff and Hartel (2013)).
It is therefore imperative that you extract the ice cream from the freezer bowl and get it into your freezer as quickly as humanly possible.
After about 4 hours, depending on your freezer, the ice cream will have a nice firm scoopable consistency, somewhere around -15°C (5°F), and be ready to serve.
8. Serving the ice cream
Serve your the ice cream at around -15°C (5°F). If you can wait, allow the ice cream to warm to below -12°C (10.4°F) before eating. As the serving temperature is increased from -14.4 (6.1) to -7.8°C (18°F), flavour and sweetness become more pronounced.
9. Storing your ice cream
At -18°C (-0.4°F), it is recommended that homemade ice cream be kept for about a week. Ice cream can be stored for several weeks at -25°C (-13°F), and several months at -30°C (-22°F) (Goff 2012). Even at these low temperatures, ice crystals will eventually start growing in size.
The longer you store your ice cream in the freezer, the larger the ice crystals will grow and the sandier the texture is likely to be.
I’d be happy to answer any questions so do get in touch!
All the best,
Damodaran, S., (1996) Functional properties. In: Nakai, S., Modler, H.W. (Eds.), Food Proteins – Properties and Characterization. VCH Publisher, New York, pp. 167–234.
Donhowe, D. P. (1993) Ice Recrystallization in Ice Cream and Ice Milk. PhD thesis, University of Wisconsm-Madison.
Drewett, E. M. & Hartel, R. W. (2007) Ice Crystallization in a Scraped Surface Freezer. J Food Eng 78(3):1060-1066
Flores, A. A. & Goff, H. D. Ice Crystal Size distribution in Dynamically Frozen Model Solutions and Ice Cream as Affected by Stabilzers. J Dairy Sci. Volume 82, Issue 7, Pages 1399–1407
Goff, H. D. and Hartel R. W. (2013) Ice Cream. Seventh Edition. New York: Springer
Goff, H.D. (2012) RCI 720, -Finding Science in Ice Cream. Presentation – Royal Canadian Institute for the Advancement of Science.
Indrawati, L. et al. (2008) Effect of processing parameters on foam formation using a continuous system with a mechanical whipper. J Food Eng 88 65–74
Monahan, F. J.,McClements, D. J. & Kinsella, J. E. (1993) Polymerization of whey proteins in whey protein-stabilized emulsions. J. Agric. Food Chem. 41:1826–1829.
Phillips, L. G., Schulman, W. and Kinsella, J. E. (1990) pH and heat treatment effects on foaming of whey protein isolate. J. Food Sci. 55:1116–1119.
Sava, N et al, (2004) The Kinetics of Heat-Induced Structural Changes of B-Lactoglobulin. J. Dairy Sci. 88:1646-1653
Sava, N., Rotaru, G. & Hendrickx, M. (2005) Heat-induced changes in solubility and surface hydrophobicity of β-Lactoglobulin. Agroalimentary Processes and Technologies, Volume XI, No. 1, 41-48