Please note that isolating genomic DNA requires gentle mixing because the DNA can be sheared by vortexing in phenol solutions. Carefully remove the aqueous or top layer to a new tube, avoiding any flocculent material at the interface. Even though it is sometimes present in the aqueous layer, this flocculent material contains proteins and other debris which should be avoided. For optimum recovery, the spent phenol can be back extracted to obtain residual aqueous material left at the interface.
Back extraction involves adding an equal volume of aqueous buffer to the spent phenol solution.
The solution is vortexed, centrifuged and the aqueous layer removed again as described above. The two aqueous solutions are then pooled and precipitated to concentrate the nucleic acids. Don't Don't simply invert the extraction a few times, because inadequate mixing results in insufficient removal of contaminating proteins. Don't be tempted to completely remove the aqueous layer. The purpose of the extraction is to remove the contaminating proteins, lipids and carbohydrates some of which can be in the aqueous layer near the interface.