DOES a banana's weight change between the under-and-over ripe stages? As bananas ripen, enzymes in the fruit break the fruit's starch into sugars. An unripe banana is roughly one per cent sugar and 20 per cent starch; a ripe banana is about 20 per cent sugar and one per cent starch. They are at their peak of sweetness, without starting to rot, when they are yellow with brown dots. This is one of two processes that make the fruit softer as it ripens; second is the breakdown of pectins, the ''glue'' that holds fruit cells together, caused by another enzyme reaction. This pectin breakdown is largely responsible for the softening of the banana's peel, which contains little sugar. Unripe bananas on the plant are usually triangular, gradually becoming five-sided and ultimately round as they ripen and soften. Growers generally pick the bananas when they become five-sided. Bananas probably lose a little water as they ripen, and thus become a little lighter, but that difference should not affect your buying habits. The best way to buy the fruit is to buy a few that are yellow and a few that are green, so they will not all be ripe at once. Incidentally, individual bananas are known in the trade as ''fingers'', three or four together are a ''cluster'' and a larger number - 10-20 fingers - are called a ''hand''. There are 10-12 hands on a stem known as a bunch. Hence the line in the song: ''Six hand, seven hand, eight hand - Bunch!''
