You don’t really understand food and recipes, until you’ve tried them. At least, that is my opinion. Once you’ve tried something a few times, it starts making more sense.
This strawberry mousse mirror cake covers several learning points which we’ll walk through briefly today. It sort of is a science project, although what you can make is really there to eat! The cake is really a more advanced cake to make, but tastes as if it comes fresh from a bakery, so definitely worth your effort!
This cake is made up of three components:
- A light airy cake, pretty similar to one used for Swiss rolls
- A strawberry mousse
- A translucent strawberry mirror glaze
Each of them is full of science. How you make them involves a lot of chemistry & physics: gelling of a glaze, denaturation of proteins in a cake, aeration of a mousse. And don’t forget the ingredients themselves, the red in strawberry and the proteins in egg! If you just want to make the cake you might as well start scrolling down to get the recipes. If you would like to see some great cake science, continue reading!
A light cake
This cake will be covered with a strawberry mousse that is full of fat and cream. Therefore, the cake itself is actually quite light. It doesn’t contain any fat, apart from the fat that is present in the eggs you use.
By not using as much fat you are able to create a very light and airy cake. Normally, fat tends to push down a cake and make it more dense (for example in a pound cake). Now that we don’t have much fat we don’t have to use baking powder to help it puff somewhat. Instead, we whip up egg whites and fold those into the batter. The egg whites lighten up the cake even before it goes into the oven.
A rich & airy strawberry mousse
Strawberries are very different fruits than let’s say a banana. It would be pretty hard to make a light and airy banana foam after you’ve removed most of its loose moisture (as we do in this recipe). Not so for stawberries. Bananas contain a lot of fiber which thickens up a mixture quite easily. Strawberries on the other hand have a lot less fiber. It is more sugar & water. What’s more, once you puree a strawberry the remainders turn liquid quite easily as well. Making them perfect for this mousse!
To make a mousse we use two other components: an anglaise & a whipped cream. The anglaise provides richness to the foam and helps thicken the strawberry puree. You will often see anglaises used for making ice cream!
An anglaise is not airy though! That is why you add in a whipped cream. Cream with a high fat content is exceptionally suited for aeration. All the fat in the cream will hold on to all the air bubbles as you whisk the cream. Heat on the other hand can destroy the foam again, making the whipped cream collapse. That is why in the recipe you will need to cool down the anglaise first, before gently mixing it with the whipped cream!
A bright red strawberry mirror glaze
The cake has a translucent, bright red strawberry mirror glaze. This cake though does not contain any added colouring. All the colours come from frozen strawberries!
Remember our extensive analysis of the colour red and the colour of red cabbage? Remember that group of molecules called anthocyanins that are responsible for a whole array of purples, reds and blues? It’s those molecules that give the strawberry it’s bright red colour. In strawberries as much as 25 different anthocyanins have been found. Together creating that beautiful colour.
Acidic strawberries & lemon juice
The colour of anthocyanins depends a lot on the pH-value (a measure for acidity) of their environment. They are red at lower pH-values (more acidic). You might think strawberries aren’t sour, however, take away their sugar and you will realize they are indeed acidic. Once you transform that strawberry into strawberry puree & liquid, you will notice that the liquid actually tastes sour!
To help the red colour a little more, we also add some lemon juice to the glaze. Lemon juice is very acidic and will help bring that pH-value down and brighten the strawberry red!
Understanding mirror glazes
We’ve discussed mirror glazes quite extensively before. Mirror glazes are made with gelatin which helps them to set and helps make them beautifully nice and shiny. If you read those articles, you should be all set to go for this strawberry glaze!
Strawberry puree & liquid
- approx. 500g frozen strawberries
- 2 eggs (not split)
- 2 egg yolks
- 115g sugar
- 60g flour
- 2 egg whites
- 20g sugar (approx. 4 tsp)
- 7g of gelatin powder (about 2,5 tsp)
- 180g strawberry puree (made in a previous step)
- 3 egg yolks*
- 75 sugar
- 210g milk
- 240g cream (high fat content)
- 7g of gelatin powder
- 1 tbsp lemon juice
- 2 tbsp water
- 150g strawberry liquid***
- 100g sugar
- 100g water
Strawberry puree & liquid
- Place your frozen strawberries in a small saucepan, cover with the lid (to prevent moisture loss) and slowly defrost on a low heat. You don't want to bring it to boil, just defrost the strawberries. Heat until the strawberries are soft (but still maintain their shape), this will take about 20-30 minutes. Your strawberries will lie within some strawberry liquid.
- Strain the mixture into a bowl to separate the liquid from the strawberries. Gently push on the strawberries to push out some more liquid, but don't mush them together. You will get about 100-150g of strawberry liquid.
- Add the remaining strawberries back in your pan and, using a hand held mixer, puree the strawberries until smooth, this will be your strawberry puree.
- Pre-heat an oven to 180C (350F) and line your 20cm springform with parchment paper on the bottom. Lightly butter the sides of the tin.
- Whisk together the whole eggs, egg yolks and first portion of sugar (115g) using an electric mixer. It is important to incorporate quite a bit of air. You will notice the mixture getting paler in colour, that is a sign that air is incorporated.
- Gently fold in the flour with a spatula.
- Place the egg whites in a clean bowl of your stand mixer. Whisk the egg whites until they start foaming up. Add the second portion (20g) of sugar and continue whisking until the foam is glossy, firm and holds it shape. You can test this (carefully!) by holding the bowl upside down. It should stay in place.
- Gently fold about a third of the egg whites with your egg & flour mixture. Using a whisk makes this very easy, just make sure you mix very gently. Otherwise use a spatula, just take care you break down all lumps.
- Once the first portion is mixed in, add the rest in 2 more batches, mixing in the same way. The mixture becomes lighter with every portion, making it easier to add in more egg whites.
- Pour into your baking tin.
- Bake immediately, for 40-50 minutes. A cake tester should come out clean.
- Take the cake out of the oven and leave to cool for about 10 minutes before attempting to release the outside ring.
Note: This cake gets all its airiness from the whipped eggs and especially egg whites. Therefore, do not skip over this step and try to preserve as much air as you can.
- Sprinkle the gelatin powder over the strawberry puree and mix it through. Leave to sit for at least 10 minutes to allow the gelatin to hydrate.
- Add the eggs & sugar to a bowl and whisk together.
- Bring the milk to the boil in a separate pan.
- While continuously whisking, slowly pour in the milk into the eggs & sugar mixture. This ensures you don't accidentally cook the eggs.
- Pour the mixture back into the pan and slowly heat to 80-85C to just set the eggs.
An alternative way to test whether it is done, place some of the mix on a spatula and hold the spatula vertically. Pull a line over the spatula with your finger. If the line remains visible, the mixture is done.
- Take the mixture from the heat and mix in the strawberry puree with gelatin.
- Leave to cool down to room temperature.
- Once the mixture has cooled down to room temperature (and not sooner!) whip the cream using an electric mixer until you've got firm peaks.
- Fold the cream into the egg mixture. Start with a third of the mixture, add the next 1/3 portion once you no longer see white lumps, same for the last one.
Assembly cake + mousse
- Cut you cooled down cake and in half horizontally. Place the bottom half back inside the tin cover the top half so it doesn't dry out.
- Add about a third of the mousse on top of the bottom cake layer. The layer should be about the same thickness as the cake.
- Place the cake + mousse in the fridge to set. The gelatin will now get to work and set the mousse.
- Once your mousse is firm enough (this will take 1-2 hours depending on your gelatin and fridge temperature) take the cake + mousse from the fridge and add the top half of the cake.
- Using the remainder of the strawberry mousse, cover the top and sides of the cake. The mousse should at this point be firm enough to hold its shape and not run off the cake. If it is still too liquid, place it in the fridge to set a little further.
- Place the cake back in the fridge to continue to set and firm up.
Strawberry mirror glaze
- Mix the water, lemon juice, strawberry juice and gelatin powder in a small bowl and leave to hydrate on the side.
- Add the sugar and water in a pan and bring to the boil. Once it is boiling, take from the heat. Mix in the gelatin/strawberry mixture.
- Leave to cool to room temperature, it should be viscous enough so it won't run off the cake immediately after application.
- Once it starts to thicken just slightly, take the cold cake from the fridge and pour the glaze over.
- Place the cake back in the fridge to leave the glaze to set further.
*You won't need the whites, use them for marshmallows, or [meringue](https://foodcrumbles.com/a-blog-on-meringue-a-literature-study/)!
**You will most likely make too much glaze. However, when it comes to glazing it is better to have a little too much since some will always flow of the cake again. If you have a lot of left overs you can set them in little moulds or on a tray and cut them in pieces. It makes a very tasty strawberry jelly! Alternatively you can store it in the fridge for at least a week. Just re-heat it when you need it again to glaze another cake.
***Don't have enough strawberry liquid? Replace what you miss with a 50:50 sugar:water syrup.
Lopes da Silva, F., et. al., Anthocyanin pigments in strawberries, 2007, link ; great article if you want to know exactly which anthocyanin molecules sit in strawberries