β-galactosidase assay: β-galactosidase assay was performed using a previously described protocol (5).  Briefly, cells were lysed by repeated freeze/thaw cycles.  Cell extracts were then tested for β-galactosidase using ONPG (o-nitrophenyl-B-D-galactopyranoside) and read at 405nm.

Biochemical/physical assays: Amino acids were quantified using the Waters AccQ-Tag precolumn derivatizing agent followed by a reverse-phase HPLC separation. Vitamins were assayed using a reverse-phase HPLC separation followed by UV detection. Osmolality was determined using an Osmette A unit (Precision Systems). Media pH values were obtained on a Mettler Toledo pH unit, model MP220. Moisture content was determined using a Karl Fischer moisture apparatus following current USP procedure.