I'm hoping to make Nigella's Lemon Pavlova and wanted to pipe the meringue base into a shape other than a circle. Would the chemistry of the meringue and the air that it holds be compromised in any way?
The meringue base in Nigella's Lemon Pavlova (from SIMPLY NIGELLA) can pe piped into a shape and it will not squeeze out too much air from the mixture as the whisked egg whites are made more stable when the sugar is added. However you should bear in mind that the baked meringue is fairly fragile so we would suggest that the shape is fairly compact, without thin appendages. You will need a large piping bag and we suggest using a nozzle (piping tip) that has a fairly wide opening.
Draw the shape on a piece of baking parchment (parchment paper) and then flip the paper over and put it on a large baking sheet, so that you can see the design through the parchment. Use this as a guide and start by piping the outline of the shape, then fill in the centre. The baking time and oven temperature will remain the same. Once the meringue is cold and you are ready to serve, invert it onto your serving plate and gently peel off the baking parchment.