We have been discussing the melocha of borer. Borer is permissible when the piece you are removing from the mixture will be eaten right away. Does the food taken need to be eaten immediately? The Rama in 319:1 says anything taken to be eaten at any point during the seuda is considered as if you are eating it right away. Therefore, it is permissible to prepare for a seuda prior to the start of the seuda even though what you are taking will not be eaten until the end of the seuda. For example, one may peel oranges before the start of the meal to be eaten for desert, even though the oranges will not be eaten until the end of the meal.
How long before the meal can one begin doing borer? The Magen Abraham 321:15 says from the time shul finishes you can start doing borer in preparation for the seuda. Most poskim agree that 30 minutes before the seuda is permissible. More time is permitted for large seudas when 30 minutes will not suffice.  

Rabbi Tzvi Aryeh Hyman Mir Yeshiva, Jerusalem- If you have any questions or comments please send them to: [email protected]