We normally use BMGY for yeast culture on suspension. The buffering solution of this media is pottasium phosphate, have you tried it?

BMGY medium, per litre （final work solution）

1%            Yeast extract

2%            Peptone

0.1M         Potassium phosphate (pH6.0)

1.34%       YNB (Yeast Nitrogen Bases)

0.0004%  Biotin

1%            Glycerol

To maintain a high pH (pH 9) for yeast cell suspensions, you can use a buffer that is effective in this pH range. Tris buffer is commonly used for maintaining alkaline pH and may be suitable for your needs. Here's how you can prepare Tris buffer at pH 9:

Tris buffer is widely used in biological research and is generally compatible with yeast cell suspensions.

Potassium phosphate. This helps to maintain an alkaline pH.

Potassium phosphate