Defrost the raspberries. For the sugar syrup (refined sugar), boil 150 ml water, sugar and lemon peel until the sugar is dissolved. Continue cooking for about 6 minutes at medium to low heat.
Mash the raspberries. Allow the syrup to cool completely, pour it through a sieve into the raspberry puree and mix everything. Then pass everything through a sieve again.
Put the raspberry puree in a wide bowl and let it freeze slightly for about 1 hour. Stir vigorously with a whisk.
Beat the egg whites until stiff and carefully fold into the frozen mixture. Cover and freeze for about 4 hours. Stir again about every 30 minutes until a portionable sorbet is formed.
Cut raspberry sorbet into balls with an ice cream scoop and serve immediately in glasses.