For those not familiar with expression with p. pastoris yeast: proteins are secreted into the growth medium.

This is the medium:  1% yeast extract, 2% peptone, 100 mM potassium phosphate, pH 6.0, 1.34% Yeast Nitrogen Base, 4 × 10-5% biotin,  0.5-1% methanol

Lately I am experience loss of the nickel beads as they turn grey or white after incubating the beads with the medium. The beads are added to the medium and mixed with a magnetic stirrer.

I have done this type of purification about 15 times over the past 3 or 4 years and only lately I am experiencing this problem.

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