Which is sourer: lime, lemon or rhubarb?

The lower pH is, the sourer the taste is.

lime – citric and ascorbic acid, pH around 1.8

lemon – citric acid, pH around 2.3

vinegar - pH 2-3

rhubarb - pH around 3.3

Cheap and satisfying food for losing weight

1 Plain corn flakes with no added sugar or honey
2 Salad vegetables: cucumber, tomatoes, radishes
3 Oats porridge
4 Plain yogurt with no added sugar or flavour
5 Low-sugar fruits: apple, berries, grapefruit, orange, plum