When the stem end of a tomato remains green or yellowish, it is called green shoulders. The green area is also noticably harder than the ripened fruit. This can occur when temperatures remain high for a prolonged period or when the fruit receives too much direct sun exposure. The chlorophyll in the fruit that would normally break down as the fruit ripens, doesn't or does so too slowly.
This problem is more prevalent in heirloom varieties, since many hybrid tomatoes have been bred not to do this. If you find this happening to your tomatoes, make sure the fruits have good foliage cover. If the problem persists, try picking the tomatoes while they are still green and allowing them to ripen away from the sun. As with tomatoes that crack, tomatoes with green shoulders are still edible. Just cut around the shoulders.
