Sodium propionate is used in bakery products as a mold inhibitor.
Propionates prevent microbes from producing the energy they need,
like benzoates do. However,
unlike benzoates, propionates do not require an acidic environment.
Calcium propionate is often
preferred as an anti-fungal agent,
to reduce sodium levels in the diet, but also because
calcium ions are necessary for the enzyme α-amylase
to act on the starches in bread, making them available for the
yeast, and improving the texture of the bread. Stale bread is
caused by the starch amylose recrystallizing.
The enzyme α-amylase converts some of this starch
to sugars, preventing the recrystallization.