Ever seen and eaten a bright green pistachio ice cream which didn’t taste like pistachio at all? I surely have. Ever tried making pistachio ice cream and had it turn out brownish instead of the nice bright green? Again, I surely have. I’ve seen pistachio ice creams varying from bright green (Granny Smith apple type of green) to slightly brownish ones.
Those big colour differences did get me wondering what happens in those ice creams. Have some simply not been processed well, causing them to lose their colour? Or has colouring been added to others?
To find out, we’ll be looking into the green colour of pistachios. Even better, we’ll be discussing how to make pistachio ice cream, one of the my favorite ice cream flavours!
How I started thinking about pistachio ice cream? Well, I made pistachio ice cream and it just didn’t look as green as I had expected/hoped. Also, I’d seen quite a lot of differently colored pistachio ice creams over time, so I started wondering, where does that green come from?
Introducing pistachio nuts
Introducing the pistachio nut: a green/yellowish nut, which is related to the cashew. Pistachio nuts are native to the Middle East and Asia. Since they stem from this region, they can be found in a lot of Arabian sweets and dishes, for example baklava. Nowadays California is also a huge producer of the nuts though and they can be found in countries such as Italy.
The vibrant green colour of pistachio nuts is what makes them special, even though this colour is generally hidden beneath an outer shell. Most other nuts are brownish/yellowish. This unique colour makes pistachios so desired for pastries, ice cream and other snacks. The flavour matches well with sweet and it has a colour that otherwise couldn’t be found easily for sweet ingredients.
The green colour of pistachio nuts
So why is a pistachio coloured green? As usual, the colour of foods can be explained through food chemistry. It’s a specific type of molecule that causes the colour. In this case the main molecules at play are chlorophyll molecules.
Chlorophyll is a very important group of molecules with a green colour. They do not only give pistachios their green colour. Most leaves are actually also coloured green because of chlorophyll. In this role chlorophyll plays an important role in the photosynthesis process. This process is the fuel plant for plants, producing energy for them to grow and live.
Green, greener, greenest
Just like for just about any natural product the quality and appearance of the product depends on a lot of factors and is seldom identical between batches. The concentration of chlorophyll (and other colour molecules) in the pistachio depends on a lot of variables. Important parameters are the region they’re grown in, the weather they’ve experienced, etc. Because of this, the colour of pistachio nuts of different regions can differ quite a lot.
But it’s not just the location and growing of the pistachio that influences its colour. Chlorophyll isn’t a very stable molecule. Upon ripening a lot of fruits lose their green colour (bananas for example). This is due to chlorophyll which is being transformed. Pistachio nuts do so as well. The more ripe pistachio nuts are, the less green they tend to be.
Maintaining the green colour, thus chlorophyll
In a lot of cases chefs and food producers want to maintain a green colour of the product they’re preparing. Broccoli simply looks more appealing when it’s green than when it has turned a mushy brown, the same goes up for a lot of other vegetables, and, pistachio nuts. The trick here is to make sure the chlorophyll molecules stay in tact in its green version.
Chlorophyll is green as long as a magnesium ion sits in the center of the large complex molecule. Once this is gone it will become grey or yellowish. There’s some great chemistry behind this, but we’ll leave that for another time.
Chlorophyll can lose the magnesium ion either through heat or through the influence of an enzyme. In our case especially the heat is of importance. As I mentioned above, the pistachio ice cream recipe involves heating the cream with the pistachio nut paste. However, prolonged heating of the pistachios in the ice cream will dull out the green colour because of the loss of the green chlorophyll molecule. (Think of that broccoli again, the same happens here.)
A trick to prevent chlorophyll from losing the green colour is to make the conditions in which you boil the pistachios slightly alkaline (= the opposite of acidic). Acids speed up the loss of green colour quite a bit. An example of an alkaline ingredient would be baking soda. However, in the case of ice cream, baking soda can also affect the flavour of your ice cream.
Making a bright green pistachio ice cream
Making a super bright green pistachio ice cream is hard to do when all you have for colourant is the pistachios themselves. The pistachios become quite diluted with cream and milk and thus the colour is a lot less bright than the pure pistachios are. Adding more pistachios might make them too overpowering and will affect the texture of the ice cream.
But, no worries, by stabilizing the chlorophyll in the pistachios, there are a few things you can do to make the ice cream as bright green as possible:
- Buy pistachios with a bright green colour. Some pistachios simply are less green than others and have less chlorophyll.
- Heat the pistachio/cream mixture from the recipe for as short as possible. My advice would be to first heat the cream and only when it’s boiling add the pistachio paste. Mix it in quick and turn off the heat. The high heat has the advantage that enzymes are broken down immediately and these could otherwise speed up the loss of green colour.
- Add a little tidbit of baking soda to make the cream less acidic. I don’t have experience with this method in ice cream, but if you’re desperate, give it a try. Take care and don’t add more than 1 tsp to the recipe mentioned below, else you’ll get a strange metalic taste.
- Last but not least, if you really want a bright green pistachio ice cream, add some food colourant. Artifical food colouring generally is a lot more resistant towards heat and processing and doesn’t fall apart as easily as natural one. What’s more, it’s often a lot stronger, so only a few drops might be enough.
Pistachio ice cream recipe
As most of the other ice cream recipes I’ve shared before (e.g. semifreddo or a vanilla ice cream) pistachio ice cream is not too hard and can be made with and without eggs (substitute eggs with custard powder).
- 2 egg yolks (prefer to make ice cream without any eggs? leave out the eggs and double the amount of corn starch)
- 75g granulated sugar
- 1,5 tbsp corn starch (maizena in Dutch)
- 300ml milk (I use semi-skimmed, but other types are also ok)
- 115g pistachio nuts (not yet peeled) (I used pistachios that had been roasted and salted, I did wash away some of the salt by shortly placing them under tap water. You can probably better take unsalted ones.)
- 300ml cream
- Mix the egg yolks, sugar and corn starch in a small pot. Do not skip this step! Corn starch makes lumps very easily in liquids, so by mixing it in with only little liquid (the egg yolk) and sugar you disperse it and prevent lumps from forming.
- Mix in the milk and whisk through. (If you want you can add some vanilla essence or fresh vanilla pods to create some more depth of flavour.)
- Place the pot on the stove and heat slowly while continuously whisking. (You can leave out whisking at the start, but once it starts getting warmer, keep an eye on it because the egg can start clumping up before you know. So I stay on the safe side and will continuously whisk once it starts getting warmer.)
- Take from the heat once it has become thicker. Leave to cool down to room temperature.
- In the meantime, peel the pistachios. Place them in a chopper and add approx. 30 ml of cream. Chop until you've created a thick greenish paste.
- Add the pistachio paste and the remaining cream in a pot and bring to the boil. Since the pistachio paste is so thick it will not mix well with the cream until it becomes warm. Once you can mix it through to a homogeneous mix, turn off the heat and leave to cool (continue reading the article for some more tips on this step that will help making a greener ice cream!).
- Mix the two (the thick egg paste and the cream) and place in your ice cream machine. Leave to churn according to your machine instructions and place in th freezer once it's finished.
- You can enjoy your ice cream immediately, or leave it to harden somewhat more in the freezer, it will melt less easily once it's hardened out a little.
Scientific research done on the composition of pistachio nuts from various origins (very insightful!): Anthocyanins, chlorophylls and xantophylls in pistachio nuts of varying origins.
Analysis of Sicilian pistachio nuts: Carotenoid, chlorophyll and chlorophyll-derived compounds in pistachio kernels from Sicily.
Chlorophyll in pistachios from the Kitchn.
On Food and Cooking, from Harold McGee, version 2004
Two other articles I didn’t use as much are about the effect of roasting on the colour and presence of chlorophyll and an article describing the effects of ripeness of a pistachio on its properties.