Sugar should be kept in an airtight container in a cool dry place, and once opened, packets should be resealed between use. Moist light and dark brown sugars are the ones that tend to go hard and lumpy, as exposure to air dries them out. Adding a slice of apple or bread to the container in which the pack is stored for a couple of days should restore moisture. If you want to use the hardened sugar immediately, blend it in a food processor and then sieve it, or heat in a microwave oven. Wrap the pack in a damp tea towel and microwave it on Medium for a minute or two or until the sugar begins to soften. You can then use it as normal.
** Asian Recipes