From the Super Mario Wiki
The Lucky Fruit is a gameplay mechanic in Yoshi's Story. Before each playthrough in Story Mode and Trial Mode, the game will randomly selected a Lucky Fruit among the four regular fruits (Apples, Bananas, Watermelons, and Grapes) to serve as the Baby Yoshi's "yummiest" fruit until the game is completed. The Lucky Fruit rewards the Yoshi with eight ♥s, (versus three for the Favorite Fruit and a hundred ♥s for Melons) and recover all of the petals on a Yoshi's Smile Meter, even if the selected Yoshi colour does not like the fruit.
In each level, there will be twelve of the selected Lucky Fruit to find, while there will be six of each of the other three "ordinary" fruits. When doing a Ground Pound in Super Happy Yoshi mode, every enemy onscreen is turned into the Lucky Fruit.