There are, of course, tons of these, depending on how far back you want to go. To me, the most important technological advance affecting food preparation is the electric stove and oven. I cannot even imagine how much harder it was to cook back in the days of wood stoves or even before that.
This is especially true of the oven. Nowadays, our recipes are really specific about temperatures. For example, when I make bread, I bake it at 400 degrees for a while and then turn it down to 325. If all I had was a wood oven, I can't imagine how I would even know what 400 was, let alone trying to turn it down to 325. It must have been so much harder to bake back then...
Food preparation has come a long way since the ancient days of salting everything and hanging it up to dry. Even today our word for salary comes from word "salt", harking back to ancient times where people's remuneration was salt to preserve and cook with. Three technological advances affecting food preparation are: 1) cast-iron stoves: the heat source could be approached and the heat more easily controlled, this is in stark contrast to wood burning ovens and other burning devices in which the temperature had to be controlled by the source rather than the modality, 2) canned foods: preservation and extended storage of items proved an immense advancement as foods could be kept for years without spoilage and food portions can be varied with the size of the can, and 3) transportation: increased availability, freedom from using only locally produced ingredients and refrigerated and iced trains, planes, and cargo carriers allowed people all around the world the ability to experience foods from other regions.