Apples & Pears.
There are a wide range of apple varieties available – some of which are very sweet and some exceedingly sour. So just saying … “add an apple” … is not going to help very much.
In short – you have to treat sour apples with a bit of caution, always wash and core them, leave the skins on and chop them up into cubes to help the blending process.
Pears are similar – although you don’t get them quite as sour. You can use them hard or soft – and just need to be cored.
Both apples and pears are quite fibrous – but on the whole blend well. Apples will completely disappear. Pears however do leave a slightly gritty texture – especially when unripe. This is not normally an issue.