Fruit ripeness, seasonality, storage, safety, nutrition and practical signs of quality.
Botanically, a tomato is a fruit because it develops from a flower and contains seeds. In the kitchen, it is treated as a vegetable because of its flavor and how it is used.
A green plantain is much starchier and less sweet than a ripe banana; as plantains ripen, part of that starch is converted to sugars and their flavor changes.
A ripe pear should be tender, but the flesh around its core should not become a watery or brown mass. If that area collapses, leaks, smells fermented, or has mold, the pear has moved beyond useful ripeness and should be discarded.
The gritty sensation in pear flesh comes mainly from clusters of thick-walled cells called stone cells or sclereids. They are a normal part of the fruit, not seeds or crystals, and their presence by itself does not mean the pear is spoiled.
An apple can develop brown flesh inside without any bruise showing on the skin. Internal browning can result from maturity and storage conditions; if the brown tissue is wet, mushy, moldy, or fermented-smelling, it should no longer be treated as a simple quality defect.
An apple can develop mold in the seed cavity while its skin still looks firm and healthy. If cutting the fruit reveals fuzzy or powdery fungal growth in the core, discard the whole apple rather than simply scooping out the seeds and eating around it.
A pear can begin browning around the core after it has ripened too far or exceeded its useful storage life. A small firm-to-soft brown area may reflect overripeness, but watery, collapsing, moldy, or fermented-smelling tissue is a reason to discard the fruit.
A mealy apple can still be sound. As apples age and their cell structure changes, the cells separate more easily instead of bursting and releasing juice, creating a dry, grainy bite. If there is no mold, rot, or off odor, the problem is mainly quality.
Firm, glassy, water-soaked-looking flesh around an apple core can be watercore, a maturity-related physiological disorder. It is not rot by itself, although severe watercore shortens storage life and can progress to internal browning and breakdown.
A watermelon cannot be made much sweeter by leaving it on the counter after purchase. The best strategy is to choose fruit that was already mature when harvested, using the ground spot, weight, rind condition and overall shape together rather than trusting one folk test.
Apple browning is an enzyme-and-oxygen reaction that starts when cutting breaks cells. Lemon juice or another acidic solution, tight wrapping, and refrigeration can slow it. A browned apple is not automatically unsafe, although normal food-safety and spoilage checks still apply.
You do not have to refrigerate an apple that will be eaten in the next few days, but cold storage clearly preserves quality longer. At room temperature apples continue respiring and ripening much faster; in the refrigerator they hold firmness and acidity longer.