Roasted Pistachio Ice Cream – Recipe

Roasted pistachio is, undoubtedly, my favourite ice cream flavour. During my time in South Korea, I spent months and months grinding, crushing, and steeping pistachios to try to extract both the flavour and colour but was never satisfied with the results. I learnt that I could steep chopped pistachios for 24 hours in an ice cream mix (during the ageing process) and get a nice subtle pistachio flavour. However, this method failed to impart the lush green pistachio colour onto the ice cream. Using a pestle and motor, or food processor, to crush whole pistachios into a paste did impart a nice green colour but caused a grainy texture.

After much crying and then research, I finally stumbled across what I needed; an industrial colloid mill that can be used to create extremely fine nut pastes. At around £20,000, this was, and remains, out of my price range and so I resigned myself to looking for a company that makes extremely fine nut pastes.

I have found two companies that produce fine pistachio paste: whynut, based in the UK, and callebaut, who are international. Both companies have their advantages and disadvantages.

The whynut paste is made from early harvest roasted green kernels and so has a more pronounced, and slightly bitter, taste and colour. Whynut does offer 110g pots for sale, which is a good amount for the home cook. If you do decide to use the whynut paste, I would recommend using less than what I state in this recipe below as it does have that deeper, slightly bitter, flavour.

For my recipe below, I use the callebaut paste, which has more of a roasted nut flavour, great colour, and isn’t quite as bitter as the whynut paste. The downside is that I believe they only ship in 1kg buckets, which is expensive for the home cook.

Ingredients

organic cream, organic semi-skimmed milk, organic un-refined sugar, organic free-range eggs, roasted pistachio paste, organic sea salt.

If you are using cream at 36% fat:
Cream 474g
Semi-skimmed milk 413g
Sugar 141g
Egg yolks 72g
Roasted Pistachio Paste 100g
Sea salt 1/4 tsp

If you are using cream at 38% fat:
Cream 448g
Semi-skimmed milk 440g
Sugar 141g
Egg yolks 72g
Roasted Pistachio Paste 100g
Sea salt 1/4 tsp

If you are using cream at 50.5% fat:
Cream 333g
Semi-skimmed milk 555g
Sugar 141g
Egg yolks 72g
Roasted Pistachio Paste 100g
Sea salt 1/4 tsp

The tables below show the composition of the ice cream mix before and after heating. The totals are expressed as a percentage of the mix.

Composition of the mix before it has been heated

Composition of the mix after  it has been heated

1. Freezing the freezer bowl

For this recipe, I use the Cuisinart ICE-30 which comes with a large 2 litre bowl. I have used this machine for a very long time and would highly recommend it. The day before you start making your ice cream, take the removable 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 in the freezer. Any water that is frozen to the inside of the bowl will melt into the mix and is likely to have an adverse effect on ice cream texture.

Take a 1 litre plastic container, the freezer bowl, and the ice cream dasher and place them in your freezer overnight. Freezing will remove any heat stored in the dasher and container. Heat that is transferred to the ice cream during freezing from the container or dasher increases ice crystal size, by melting the crystals, contributing to a sandy texture. If you add ice cream to a plastic container that is at room temperature, you will notice the ice cream that comes in contact with the side of the container quickly start to melt, which is a big no no as is likely to cause a sandy texture.

It’s also important that you freeze enough water to be able to make an ice bath. Freeze water in several ice trays or small plastic containers.

2. Setting the fridge and freezer temperature

It is 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. If you don’t allow these fat globules sufficient time to crystallise, it is likely that your ice cream will suffer from relatively fast meltdown and less retention of shape.

Set your freezer to around -25°C, or as cold as you can get it.  Once churned in the ice cream maker, the quicker you can get your ice cream to below -18°C, the smaller the ice crystal are likely to be, and the creamier the texture. The temperature and rate of hardening determine the final ice crystal size . Hardening is complete when the temperature at the centre of the ice cream container drops to -18°C or lower, preferably -25 to -30°C. The longer it takes for the ice cream to reach these temperatures, the larger the ice crystals will grow and, subsequently, the sandier the texture will be.

The importance of a really cold freezer

I cannot stress enough the importance of setting your freezer as cold as you can get it. An extremely cold freezer is not only important when storing your ice cream, but is also extremely important for your machine’s removable bowl.

I’ve noticed that a bowl frozen to around -14°C takes longer to churn a batch of ice cream, compared to a bowl frozen to around -18°C or below. Because a bowl frozen at around -14°C will take more time to churn a batch of ice cream, it is more likely to produce a colder, sandier, and wetter texture. I set my freezer to around -25°C and the resulting ice cream is much drier, smoother, and holds its shape better, compared to when I use a bowl that has been frozen to around -14°C.

At about -25°C, it takes me about 16 minutes to churn a batch of ice cream, which is very good for a domestic ice cream maker (professional ice cream makers take about 8 minutes). If I set my freezer to around -14°C, it takes between 20-25 minutes for the bowl to churn a batch. Remember, the quicker you can churn your mix, the smaller the ice crystals are likely to be and the creamier the texture of your ice cream.

3. Preparing an ice bath

The point of using a 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’ where bacteria likes to multiply; between 5 and 65°C. The longer your mix spends in this temperature range, the more bacteria is likely to multiply, imparting an undesirable taste and smell.

4. Size of pan

As strange as it sounds, the size of the pan in which you heat your mix is extremely important. The larger the diameter of your pan, the more water will evaporate during heating. This is important because the fundamental reason for heating your mix for 60 minutes is to concentrate the milk solids non fat, more specifically the protein.

By concentrating the mix, you increase the percentage of protein. Protein plays a significant role in limiting the size of ice crystals, thereby improving texture. The higher the percentage of protein in your mix, the creamier the texture is likely to be.

I recommend using a large pan with a 23cm diameter. This will allow you to concentrate your mix by about 32%. If you use a pan with a smaller diameter, you won’t be able to reduce the mix by the same amount in 60 minutes. The texture won’t be as smooth and creamy as a mix prepared in pan with a larger diameter.

Use a large pan with a 23cm diameter, similar to the one on the left

5. Heating your mix

Once you have prepared the ice bath, add the sugar and the egg yolks to the pan. Mix the eggs and sugar together until both ingredients have combined. Mixing the sugar with the yolks will help prevent the yolks from curdling. Add the cream, milk, and the sea salt and gently stir the mix before you switch on the heat.

Over a medium heat, heat the mixture until the temperature reaches 70°C, 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 70°C, turn the heat down to low and continue heating until the temperature reaches 71.4°C. Use a kitchen thermometer to keep the temperature at 71.4°C for 60 minutes. The aim is to promote reversible unfolding of the whey proteins but not aggregation, which will significantly improve the texture of the finished ice cream.

Try not to let the temperature rise above 71.4°C. It will be difficult to keep the temperature at 71.4°C and you will find it quickly fluctuating between about 71.2°C and 71.8°C. It’s not the end of the world if you do briefly go above 71.4°C but do quickly bring the temperature down if you notice it creeping up. You will risk aggregating the proteins, which is detrimental to texture, and developing the unpleasant and eggy hydrogen sulphide taste if you heat the mixture above 71.4°C.

Heating the mix for 60 minutes may sound like a long time but believe me it is a small price to pay for extremely smooth and creamy ice cream.  Holding the mix at 71.4°C for 60 minutes will concentrate the mix by about 32%, thereby increasing the percentage of protein. After trying different variations, I have found that a mix heated for 60 minutes produces a much creamier  and smoother ice cream than one heated for 15, 30, and 45 minutes.

6. Cooling the mix

After 60 minutes, take the pan off the heat and carefully pour the mix into the zip lock bag. Pour about 2 tablespoons of salt onto the ice to lower its temperature. This will cool your mix faster.

Once the mix has cooled to about 10°C, place it in the fridge and leave overnight to age. Remember that 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.

7. Churning the mix

Once you have allowed the mix to age overnight, place the freezer bowl in the machine and add the dasher. Put the lid on and, with the machine switched on, pour in the mix followed by the pistachio paste.

If you’re using a machine with an in-built compressor, switch the machine on and leave it for about 20 minutes before you add the mix. This will ensure that the bowl gets as cold as possible.

As soon as you pour in the mix, use your thumb to push the dasher against the side of the bowl. Pushing the dasher against the bowl will prevent any ice from freezing to the side. 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.

If the transfer of heat from the ice cream to the freezer bowl is reduced, the ice cream will take longer to freeze. The longer the ice cream takes to freeze, the larger the ice crystals will grow and the sandier the texture is likely to be. Keep the dasher pushed against the side of the bowl until you have finished churning the mix (a sore thumbs is yet another price to pay for smooth creamy ice cream).

Use a spoon to push along any static lumps of ice cream. Any static lumps will start to melt, causing a sandy texture. Ensure that the mix is constantly moving whilst in the machine.

After about 20 minutes (or about 16 minutes if your freezer was set to around -27°C), the mix should have a nice dry, stiff texture and should stick easily to the dasher when you take it out of the machine. If the mix drips too easily off of the dasher and has a wet shine, churn the ice cream for another 5-10 minutes.

Quickly empty the mix into the pre-chilled container. The longer you take to empty the ice cream into the container, the longer the ice cream will spend at room temperature. At room temperature, ice crystals will start to quickly melt. If ice crystals melt, they will grow bigger in size when the ice cream is placed in the freezer. The longer the ice cream spends at room temperature, the sandier the texture is likely to be. Be as quick as you can when emptying the ice cream into the plastic container.

8. Hardening the ice cream

Once the mix has been churned, it will have a consistency similar to soft-serve ice cream (Mr Whippy). Although you can eat the ice cream straight after it has been churned in the machine, I recommend hardening it in a plastic continaer in the freezer for at least 4 hours before serving. After about 4 hours, depending on your freezer, the ice cream will have a nice firm scoopable consistency, somewhere around -15°C, and be ready to serve.

9. Serving the ice cream