A jar of ground spice bought a year ago will not season a dish the way a fresh one does. The loss is physical and predictable rather than a matter of the spice going bad.
Grinding multiplies the exposed surface
A whole seed keeps its aromatic oils inside intact cells behind a hard coat. Grinding ruptures those cells and turns a small outer surface into an enormous one.
Every particle then has its oils in direct contact with air. Evaporation and oxidation start immediately and continue for as long as the powder is stored.
This is why the aroma of a freshly opened jar is strongest at the moment of opening and diminishes each time it is used.
Oxidation changes flavor as well as strength
Some of the loss is simple evaporation of the most volatile compounds, which removes the top notes first and leaves a duller, flatter profile behind.
The rest is oxidation, where compounds react with air into different ones. Old ground spice does not just taste weaker, it tastes different and slightly stale.
Spices rich in fats, such as ground nutmeg and mustard, can also turn rancid, adding a note that no amount of extra quantity will cover.
Heat, light and moisture accelerate all of it
A rack above the stove is the worst place in a kitchen, since it combines heat with steam every time something cooks below it.
Clear jars on a sunny counter suffer from light-driven degradation, particularly for the pigments in chile and turmeric that fade visibly over months.
Moisture is the fastest route to trouble, because a damp spoon dipped into a jar introduces water that cakes the powder and lets microbes take hold.
Whole spice is the storage form
Whole seed keeps its oils sealed and holds usable strength for a year or more in a closed container, which is why Indian kitchens buy whole and grind small.
Grinding to order is not a purist gesture but a storage strategy. The pantry holds the durable form and the powder exists only for as long as it is needed.
Not everything can be handled this way, since turmeric and dried ginger are hard rhizomes that resist a domestic grinder, which is why those two are usually bought ground.
Buying quantities to match use
Bulk bags from an Indian grocery are cheaper per ounce and are false economy for a powder that will be half-strength before it is finished.
The reasonable split is to buy whole spices in bulk and ground spices in the smallest package sold, then repackage the rest into a sealed container in the freezer.
Labeling jars with the month they were opened gives a household the information it needs, since a faded spice looks identical to a fresh one on a shelf.