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!
This cake gets all its airiness from the whipped eggs and especially egg whites. Therefore, do not skip over that step and try to preserve as much air as you can.
- 2 eggs
- 2 eggs yolks (you will also use the whites, but keep them separate!)
- 115g sugar
- 60g flour
- 2 egg whites
- 4 tsp sugar
- 7g of gelatin powder (is about 2,5 tsp)
- Your cooled down strawberry puree from earlier, you need 180g
- 3 egg yolks (you won't need the whites, use them for marshmallows, or meringue!)
- 75 sugar
- 210g milk
- 240g cream (high fat content)
You need a total amount of 350g of strawberry liquid + sugar syrup, if you have less strawberry liquid, replace it with an equal amount of sugar syrup.
- 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 and place on a very low heat on the stove. You don't want to bring it to boil, you just want to defrost the strawberries and want them to fall apart only slightly to release their juices. Place a lid on top to not loose any moisture.
- Leave it on for about 30 minutes or until the strawberries are soft (but still maintain their shape) and there is a puddle of red strawberry juice at the bottom.
- Strain the mixture into a second bowl. Gently push on the strawberries to push out some more liquid, but don't mush them together. You will get about 100-150g of liquid.
- Add the remaining strawberries back in your pan and, using a hand held mixer, puree the strawberries until it is pretty smooth, it is no problem if there are still a few little pieces in there.
- Pre-heat an oven to 180C (350F) and line your 20cm springform with parchment paper on the bottom. Slightly butter the sides of the tin.
- Whisk together the whole eggs, egg yolks and first portion of sugar. You can either do this by hand or using an electric mixer. You will notice the mixture getting paler in colour.
- Gently mix in the flour, you can do this with a whisk, just don't whisk vigorously anymore.
- Place the egg whites in a separate clean bowl of your stand mixer (or a regular bowl). Whisk the egg whites until they start foaming up. Add the 4 tsp of sugar to the eggs and continue whisking. You want to whisk until the foam is firm. If you would hold it upside down it should stay in place, it will be glossy as well. Don't whip too long or it will start looking grainy.
- Gently mix 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.
- Bake immediately for 40-50 minutes, this will depend a lot on your oven. A cake tester or other thin long tested should come out clean if it insert it into the center of the cake.
- Take the cake out of the oven and leave to cool for about 10 minutes. It should then be very easy to remove the spring form to continue cooling it down. Taking the outside off will prevent the cake from becoming soggy.
- 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. You don't want to cook the eggs which is why you should pour slowly and continue whisking, to distribute the heat.
- Pour the mixture back into the pan and heat back up until 80-85C. You don't want to bring it to the boil and you will need to whisk continuously, especially when it is almost at target temperature. If you don't have a thermometer, the mixture will thicken up slightly during heating (but not that much): when you take some of the mixture on a spatula, hold it vertically and can mark a line across with your finger, without it being overflown again, that's when it is ready.
- Take the egg mixture from the heat and mix in strawberry puree. Since the egg mixture is hot, the gelatin will dissolve.
- Leave to cool down to room temperature, you can place it in the fridge if you want.
- Once the mixture has cooled down to room temperature (and not sooner!) whip up the cream until you've got firm peaks.
- Fold the cream in with the egg mixture. Start with a third of the mixture, mixing it in until you see no more white lumps. Then add the rest of the cream in two other portions.
- Now take your earlier baked, cooled down cake and cut it in half horizontally. Place it back inside the springform, this will help with assembly at this first step since your mousse is still quite liquid.
- Add about a third of the mousse on top of the cake. The layer should be about the same thickness as the cake.
- Place the cake + mousse in the fridge as well as the remainder of the mousse. The gelatin will start doing its job now so it will thicken over time. You want to make sure that first layer is firm enough to hold the second layer of cake, without collapsing. You want the mousse to firm up enough such that it won't flow off the sides of the cake when you apply it. This might take 1-2 hours, depending on the temperature of your fridge and type of gelatin you use.
- Once it looks firm enough, take the cake from the fridge and add the second half of the cake on top.
- Cover the top and sides of the cake with the remainder of the strawberry mousse. If it is still too liquid, place it back in the fridge.
- Place the whole covered cake back in the fridge too continue to set.
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. You want to bring it down 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 will most likely have too much glaze. However, when it comes to glazing it is better to have a little too much since part of it will always flow of the cake again, even it you cooled it perfectly. If you've got a lot left over you can set them in little moulds or on a tray and cut them in pieces. Because of the strawberry juice this jelly is also very taste without cake! 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.
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