I tend not to buy completely unknown ingredients – my cupboards are too full for that. But if something rings little bells of recipes I have seen before, then I’ll pick it up, and work out what to do with it later.

I like discovering new ingredients in this way. They present a new frontier of challenges, a new coastline to be explored. They give me a starting point to wander through my cookbooks and discover new things. (And have I mentioned how brilliant Eat Your Books is for this? This indexing service does what I have never had time to do: provides a list of all the recipes in cookbooks and magazines, along with a listing of the ingredients in them, so you can search through all your book indexes at once).

So when I was browsing for Christmas presents in Gelupo, a gelateria in Soho that is the sister location to Italian restaurant Bocca di Lupo, and spotted pistachio paste, I bought it without knowing how I would use it.

I’ve come across recipes asking for pistachio paste in the past. It’s like a pistachio version of smooth peanut butter, and has that nutty flavour and sticks to your palate in the same way. It is often used in professional recipes because it is so smooth, and because it provides a concentrated pistachio flavour.

Although the Gelupo staff seemed convinced that the pistachio paste wouldnt go off, I know that nut oils can go rancid quite fast, so I opted to store it in the fridge. And there it sat over Christmas while I wondered what to do with it. I thought about pistachio cakes and biscuits, where that bright green colour could really be shown off.

In the meantime, I got the Bocca cookbook for Christmas. And, of course, the answer I was looking for was pistachio gelato.

Pistachio has been my favourite ice cream flavour for ages. When on holiday in France as a child, I would usually order a double cone with one scoop of chocolate and one of pistachio (either that or coffee and chocolate). I even tried to make pistachio ice cream at home when I was 12 or 13. The results, using nuts whirled in a not particularly powerful food processor, were predictably grainy. There was still a whisper of the right flavour there though.

Gelato is something that you mainly see in Italy and the US. Whereas ice cream is properly made with cream and thickened into a custard with egg yolks, gelato is made with milk, and instead of being thickened with eggs, it uses either gelatin, or cornflour or both to thicken the mixture. Gelato fans say that the lower fat content and no eggs allows the flavour of the gelato to be less obscured than that of ice cream. Explaining its appeal in the Bocca cookbook, Jacob Kenedy says: “a flawless representation, like that of Dorian Grey, the picture is so perfect it steals its subject’s soul”.

What’s missing from the recipe

Following the theme of my last post, I wanted to look in more detail at this recipe, and why it works the way it does. The recipe asks you to make a ‘base bianca’ first, with milk, cream, sugar, honey or glucose, and milk powder. This is then heated and gelatin stirred in to dissolve before chilling it. Finally, more sugar and the nut paste are whisked in before churning it in an ice-cream maker.

It took me a while to understand the reason for some of the instructions in the recipe. It instructs you to remove it from the heat “when the mixture approaches a simmer”. Apart from being a very unhelpful instruction (how do you know it’s approaching a simmer until after it’s simmering?), was there a reason for needing to heat to a specific (unnamed) temperature, rather than just enough to dissolve the sugar and milk powder?

I remembered that I had read about the need to scald milk for bread making, to destroy proteins, and wondered if the purpose was similar. In Shirley Corriher’s food science book ‘Cookwise’, she explains in the Ice Cream section that:

“one step is essential for optimum smoothness if using any milk or half-and-half in the recipe. The milk or half-and-half should be heated to 175F (79C), just below scalding. I do not know the exact nature of the changes that this heating causes – perhaps denaturing or partial coagulation of some of the proteins. Whatever it is, the effect is a noticeably smoother texture in the ice-cream.”

Further clues are given by Harold McGee when explaining commercial ice-cream making:

“If carried out at a high enough temperature (above 170F/76C), cooking can improve the body and smoothness of the ice cream by denaturing the whey proteins, which helps minimize the size of the ice crystals.”

This recipe in fact contains three magic elements to make it smooth and creamy:

1. the heated milk denatures the whey proteins, adding proteins to interfere with ice crystals
2. the gelatin acts as a stabiliser, adding yet more protein, both thickening the final result, and interfering with ice crystals forming
3. the honey or liquid glucose, which lowers the freezing point, and produces a softer texture at a given temperature. All sugars lower the freezing point of water below 0C. This site explains that the smaller the sugar molecule, the better it is at lowering the freezing point. Little glucose is much better than sucrose (table sugar) because it is half the size, sucrose being made of one glucose and one fructose molecule stuck together. Azelia has written much more about using liquid glucose in ice-cream.

The whole mixture is much more liquid than a normal ice cream base when chilled, with barely a hint of thickening from the gelatin.  But the final result is silky smooth, and much more scoopable than a traditional custard ice-cream.

Recipe: Pistachio Gelato

(adapted from Bocca by Jacob Kenedy)

I have tweaked this a little to suit the ingredients I had on hand, using semi-skimmed milk and double cream in place of whole milk with whipping cream. Gelatin can usually be found in the baking aisles – I used a Supercook brand, which comes in packets of 12 small sheets. I have also seen agar agar in some supermarkets – this is the one to choose if you are vegetarian.

Base:

• 480ml semi-skimmed milk
• 160g double cream
• 40g glucose syrup or light honey (I used orange blossom)
• 130g caster sugar
• 40g skimmed milk powder
• 3g leaf gelatine (2 small sheets) or 4 teaspoons agar-agar

Gelato:

• 85g pistachio paste
• 45g icing sugar

To make the base: heat the milk, cream and honey in a heavy pan until steaming. Meanwhile, mix the caster sugar with the milk powder in a small bowl. Pour into the warm milk in a stream, while whisking to prevent lumps forming. Bloom the gelatin sheets in a bowl of cold water, and continue to heat the milk. When the mixture is almost simmering, with bubbles around the edge of the pan, remove from the heat and stir in the gelatin. If using agar-agar, sprinkle it over the surface and leave for 5 minutes to swell and ‘bloom’ before stirring in.

Remove the base to a bowl to cool, then chill. When you’re ready to make the ice cream, whisk in the pistachio paste and icing sugar (I did this by hand, but a blender or hand-blender would probably do a more thorough job). Churn in an ice-cream machine, then turn into a container and freeze.

Gelupo

7 Archer Street, Soho, London, W1D 7AU

If you’re ever in London, near Piccadilly Circus, I highly recommend a visit to Gelupo. They serve Italian gelato, as well as sorbets and granitas. They always have new seasonal flavours in, as well as classic Italian flavours that you don’t often see here, like Stracciatella and coffee granita. The fruit flavours are vibrant and clear. You can also pick up little treats like handmade almond biscuits.