Culture medium pH is influenced by basal medium, carbohydrate source, gelling agent, activated charcoal, and medium storage method.

When four carbohydrates were tested against six commonly cited inorganic basal media, post-autoclave pH was highest for carbohydrate-free and sucrose containing media, and progressively lower for maltoseglucose and fructose-containing media, respectively. Post-autoclave pH for these media without carbohydrates was related to medium buffering capacity. Addition of gelling agents (10 of 11 tested) increased the postautoclave pH of MS medium containing sucrose. Neutralized and acid-washed activated charcoal also increased the post-autoclave pH of liquid and agarsolidified MS medium, and the pH changed further during 8 weeks of storage. Changes in medium pH caused by gelling agents, but not charcoal, could be alleviated by adjusting the pH after their addition but prior to autoclaving.