I notice that Nigella used the Two Chicks egg whites to make the meringue base in her Cappucino Pavlova, but the recipe doesn't give a measurement for these. I really want to give them a go, especially after picking up a coupon recently! Thanks!
There are a few advantages to using carton egg whites in certain recipes. If a large number of egg whites are needed then it can save having to separate lots of eggs and potentially having to discard the yolks. Also carton egg whites are pasteurized so they are good to use in recipes that may contain uncooked or undercooked eggs, especially for people who should normally avoid raw eggs.
Usually a large egg white has a volume of 30mls, or 2 tablespoons. The Two Chicks carton suggests using 2 tablespoons to replace one egg white, or 3 tablespoons to replace one whole egg. You should however always check the packaging information for specific instructions on how much to use to replace one egg white.
The Two Chicks whites will usually last for a week in the fridge, once opened (as long as this is before the "use by" date on the carton). You can freeze the whites but it does mention on the packaging that these whites do not whisk well once thawed. It is also worth noting that raw egg whites freeze very well and raw whites whisk up fine once thawed. If you have leftover whites from a recipe then it is worth freezing them for a time when you would like to make meringues or a pavlova. Raw egg whites will usually last for up to one year in the freezer.