Apple pastille is a dried fruit product, so moisture is its main enemy. Once the package is opened, the slices can absorb moisture from the air and become stickier, softer, or lose their characteristic chewy texture. To keep them in good condition, close the package tightly after each use and store it in a cool, dry place away from direct sunlight and strong odors. If you live in a humid climate, transferring the slices to an airtight container can help.
Because the product is often used as a tea snack or lunchbox addition, it travels well — but in warm conditions the citrus coating may soften or become slightly tacky. That is a texture change, not necessarily a sign of spoilage. The product information does not specify a shelf life after opening, so follow the best-before date printed on the package and use common sense: if the slices develop an off smell, an unusual taste, or visible mold, discard them. For a softer alternative to standard sweets, this product is intended for children over 3 years old and adults; as with any chewy food, supervise young children while they eat.