These things don't contain any reishii, so there's not really any power to consume when it's killed. All the power comes from the chemical reactions in your body when you eat it. Because as you digest the food, your body turns it into energy.

[Ichigo starts with the toughest veggies, knowing they need plenty time.]

Ah, I suppose that's a good point. So cooking as a whole encompasses everything I'm doing, but it can also refer to just the part of the process when you're using heat to basically transform the different ingredients. Because heat can fundamentally change how things taste, their texture, sometimes even the structure of the food because it causes chemical reactions.