Add the raspberries to a bowl and using a spoon, press down to form a puree. Press the puree through a sieve to form a raspberry liquid, discard the pips and set aside.
Preheat oven to 120°C and line a large baking tray with baking paper.
Add egg whites to a large clean bowl and using an electric whisk, whisk on low until the mixture starts to foam, turn to medium and whisk to stiff peaks. (Stiff peaks stand up straight when the electric whisk is lifted, you should be able to hold the bowl upside down with no mixture falling out).
Add caster sugar 1 tablespoon at a time and continue to whisk with your electric whisk until the mixture is stiff and glossy. Adding too much sugar at once can cause the mixture to become runny.
Take a piping bag and spoon in the raspberry juice. Swirl the bag so the juice covers the entire inside of the bag. Spoon the meringue mixture into the bag. Pipe the meringue kisses onto the baking sheets and bake in the oven for around 1 hour, until crisp on the outside. Turn the oven off and leave them to stand with the door open for 1 hour.