Historically, why were egg yolks crucial in Braga's conventual sweets tradition?

Answer

They were often leftover from starching nuns' habits

The tradition of conventual sweets in Braga is deeply linked to the practical resource management within the convents centuries ago. Nuns needed starch to stiffen and press their habits, a process that required a significant amount of egg whites. Consequently, large quantities of egg yolks were left over. Instead of discarding this rich resource, convent bakers ingeniously utilized the yolks, combining them with sugar and other ingredients like fat or flavorings to create the dense, decadent custards and cakes for which Braga became famous, such as the Pudim Abade de Priscos.

Historically, why were egg yolks crucial in Braga's conventual sweets tradition?
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