Many years ago scientists classified fruits into two categories:  climacteric and non-climacteric.  Climacteric fruit usually undergo dramatic changes during "ripening" and these changes have often been associated with a burst of respiration and ethylene (a natural plant hormone) production.

The avocado and banana are classified as climacteric fruits whereas the pineapple is classified as a nonclimacteric fruit.  Nonclimacteric fruit do not exhibit the increase in respiration nor the rise in ethylene production nor do they normally undergo dramatic changes (such as softening) after harvest.

Environmental conditions (i.e. cool weather) can influence the peel color of pineapples as well as fertilization practices in the field.

For avocado, oil content increases as the fruit matures on the tree. Oil content once was used as an index of horticultural maturity for avocado.  In California we shifted to measuring the dry matter content of the fruit for maturity determination.  Researchers at the University of California, Riverside and elsewhere demonstrated that there is a very close correlation between dry matter and oil content. The oil content (and dry matter) of the fruit during ripening remains fairly constant or slightly declines during ripening.  Avocado fruit are unusual since they do not normally ripen on the tree.  Many years ago, it was hypothesized that there is a ripening inhibitor in the avocado tree that prevents on-tree ripening.  So far, no one has ever identified the ripening inhibitor.

Bananas when harvested mature have very high levels of starch (like a potato).  During ripening this starch is converted to soluble sugars and that is why they taste sweeter when they are ripe.  The hormone ethylene is normally used to trigger ripening in bananas (and also avocados) by applying ethylene gas into a ripening room for a period of time.  The application of ethylene gas helps to reduce fruit to fruit variability in ripening time as well shortening the overall ripening time.  Hope this helps.