Answer: There are a few explanations for your problem. During the ripening process, chlorophyll breaks down and the fruit softens and turns red. However, in some varieties, during periods of high temperatures and/or in direct sun exposure, the chlorophyll doesn't break down normally and the exposed side stays hard and green.
You can try picking the tomatoes when they are still quite green, and ripen them indoors out of direct sunlight. Also, try to maintain good foliage cover--the problem will be worse on plants that have been defoliated by disease or insects.
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