Thanks to all those who provided information re this post. Here's a few tips for those who have problems keeping spices fresh at home.
The key with spices is to always try and buy them in their seed/ nut form and then produce your own powder at home. Take nutmeg for example, by buying a packet of the nuts in their whole state (each about the size of a small chestnut), they will keep in a spice jar for literally years, no matter how many times the jar is opened. When you need nutmeg, you simply grind it into powder. For the uninitiated, special nutmeg graters can be purchased but they aren’t necessary. I’ve always used a simple, hand-held, fine-toothed grater, similar to that used for grating cheese, only finer. It should take no more than 30-60 seconds to grate a teaspoon of nutmeg powder. Once you get a whiff of the fresh nutmeg smell, you just know that it beats anything that you buy from the shop in powder form. After that, there’s no going back! Check out the following URL for photos of nutmeg grinders.
Similarly, when it comes to cumin, coriander (cilantro) and fennel etc, if you only buy the seeds, you can make your own powder as and when needed. For this, all you need is a basic coffee grinder. Plonk in the seeds and away you go. It’s preferable to keep a separate grinder for spices only as the strong spice flavours can linger in the grinder and spoil the flavour of your coffee. Again, for those with a love of cooking and all things spices, I doubt if you’ll ever go back to powders once you’ve tried the grounded seeds version. Even cinnamon, we often buy that in powder form because we use a lot of it and therefore it doesn’t get a chance to lose it’s flavour. However, I still keep cinnamon bark
in my cupboard. As well as using it whole in Indian dishes, I sometimes grind it down the same as for seeds to get a great, fresh tasting cinnamon powder. Just remember to put the ground cinnamon through a sieve to avoid those tiny bits of bark that didn’t break down fully, first time round.
By purchasing these spices in their original form, (a) it's usually much cheaper, (b) they keep for much longer in your cupboard and (c) the fresh powder that you get after grating or grinding is simply incomparable to that purchased in powder form. Do give it a try, you won’t be disappointed. Happy cooking.