I once read "a kumquat is ready to eat whenever you can bear to partake of it". They are rather sour little nuggets so you need to use them accordingly as you would other sour Citrus. With this in mind i would probably avoid eating them fresh off the tree as they will definitely make your mouth pucker. Here is my post on Kumquats in general that i previously wrote. At home i have the following Kumquats planted in ground: Meiwa, Nagami, Indiomandarinquat & a Variagated Calamondin. The only trees that have fruited so far are the Meiwa & the Calamondin. The Meiwa seems very prolific and within a few months of coming home from the garden centre this young tree is fruiting abundantly. So with its recent round of fruit harvested, i thought i would do a Meiwa Kumquat taste test.
Eaten fresh off the tree the skin is fairly sweet on the outside. However, when combined with the tart interior flesh the experience goes sour very quickly and usually results in a spitting out. Meiwa is apparently the sweetest of all Kumquats so bear this in mind before attempting a Nagami or Mandarinquat off the tree. The flavour i would describe as Mandarin like and if made into a syrup or used in a dressing is very pleasant so don't let its initial sourness put you off.
After trying fresh i decided to make my own kumquat syrup to add into some sparkling water. To do this i muddled about 6-8 kumquats in glass with some caster sugar or simple syrup until combined. Strain out the skins & seeds, then pour the syrup in a glass with some sparkling water & ice for a beautiful refreshing Kumquat Lemonade.
I have also used some in a salads or salsas as part of a dressing which works really well as the acid component in a dish. You can also add in sliced kumquats to salads. Their intense flavor makes kumquat a good pairing for bitter or peppery greens, such as rocket. Slice into thin rounds with a sharp knife. Remove the seeds, then layer the slices on top of the salad to show off the color.
Make kumquat marmalade. Kumquat marmalade is much sweeter and less bitter than regular marmalade. The recipe is similar to most marmalades or jams. Since the kumquat seeds contain pectin, you can boil them along with the fruit to thicken your preserves. Keep the seeds in a cheesecloth bag while boiling, so they don't end up in your jar.
Make kumquat-infused vodka. Wash plenty of kumquats and cut them in half — at least 10 fruits per cup (250mL) of vodka. Cover with vodka and let sit in a cool, dark place, shaking once a day. It should pick up a faint taste after a couple days, a strong taste after a week or two, and infuse for about a month. Once infused you can add some sugar syrup got taste to make your own Kumquatcello. Here's my recipe for Limoncello.