Advice from Joy in Hanoi where brownouts happen once in a while:
“To keep things fresh in the fridge, make some packets of ice in vinyl bags and make sure water won’t leak when ice melt.
Spread them throughout the freezer and even chiller if you want. Most frozen stuff stayed frozen after a long time. But fatty fish like cod fish and flat fish like promfet get abit soft. Place fresh milk, yoghurt, cheese and butter in the freezer during the blackout.
That’s why over here, we only get fresh stuff from supermarkets with back up generators.”
From Asaichi Programme on NHK1: Use coolants (that are used in BBQ cooler boxes) and pack all frozen foods together tightly, and some refrigerated foods like dairy products and milk can be moved into the freezer compartments during brownout. Then make sure the refrigerator isn’t opened at all during the brownout period to keep the cool air from escaping.