Cranberries.

Cranberries are one of those fruits most people will avoid in their raw state – as they are both bitter and sour. However, they are at their most potent in their raw state and researchers are making some remarkable claims for them (see here for details).

You can use raw cranberries in smoothies, but you will have to balance the sourness with something sweet. Either sweet fruits, or something like honey.

Dried cranberries nearly always have lots of added sugar. So it’s probably best to use the raw variety and add the sweetness yourself.

I, for one, like the occasional sour smoothie. I know it’s personal taste – most people don’t like to go there. Your mileage will vary.