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Powder media are typically more stable than liquid media and therefore tend to have a longer shelf life. The salts in liquid media exist in ionized forms and are subject to reaction and degradation. Another common stability issue for liquid media is precipitation of components that are less soluble or formulated at high concentrations. In contrast, stability issues of powder media can be caused by post-manufacturing physical changes such as particle settling and moisture absorption, both contributing to product heterogeneity. Compositions can be very similar between powder and liquid media except that a few compounds such as bicarbonate are typically not included in bulk powder formulations.