How to collect yeast?

There will have three layers for yeast at the bottom of fermentation tank.
The top layer is light yeast, including bubbling cap, yeast sedimentation, some protein and hop resin precipitate and other impurities. It can be used as fodder and other comprehensive utilization.
The middle layer is core yeast, it is combined by strong, health and high fermentive yeast. The ratio is about 65-70%. This part should be recycled and use for next batches fermenting. The color is light.
The bottom layer is dead and weak cell, it has many impurities, so it can be used as fodder or just throw it.