|1||tablespoon||potato starch||cornstarch, or arrowroot|
|1/2||each||lemon||grated zest only|
Lightly coat a nonstick 9-inch round cake pan with vegetable oil (if not nonstick, lightly butter pan and line the bottom with a round of buttered parchment or waxed paper).
Put the pumpkin puree in the bowl of a food processor.
Add the ricotta cheese, potato starch, sugar, and lemon zest and process until smooth.
Scrape into a large bowl.
In another large bowl, beat the egg whites to stiff peaks.
Stir about one quarter of the whites into the pumpkin mixture to lighten it.
Then gently fold in the remaining whtes.
Scrape the mixture into the pan with a rubber spatula and smooth the top.
Cover with aluminum foil and press the foil around the rim to seal.
Put the pan on the steamer rack, place over simmering water, cover, and steam 20 minutes.
Then set the lid ajar and steam 5 minutes more, or until the cake is set and a knife inserted into the center comes out clean.
Let the cake cool, then refrigerate until chilled.
To serve, invert a serving plate over the pan and quickly reverse the two to unmold the cake.
Remove the parchment or waxed paper, if used.
Cut the cake into wedges.
P.s. If you don't want to buy canned pumpkin, you can steam the fresh pumpkin and cut in into small cube.