How do you know when fruit or veggies are perfectly ripe? Sometimes pressing on them makes them bruised, and I know of know other way, other than color, which can sometimes be deceiving, to make sure my fruit and vegetables are ripe!
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Every vegetable and fruit is different. For fruit, if it is in season and hasn't been hauled thousands of miles they are usually ripe. Just smell them. Strawberries, raspberries and blueberries you can tell by looking at them. And buy organic, then you know they aren't fake fruits being raised so it can stand shipping. Can you be more specific what you are looking for?
Agreed -- nearly all fruits are different, and even varieties (like pears) can show ripeness in different ways. What I would recommend is going to a really good grocer and having a conversation with the folks who are in charge of the fruits and vegetables. You want a place that has pro's in charge, not just random teenage kids. These pro's know their fruits (and veggies), and are always happy to share their information. They'll help you to understand that with citrus, you are looking for fruit that is heavy for its size (so still has lots of juice inside -- not dried out), that with Tuscan melons, you want fruit that has light or yellow streaks between the sides, not green and so on. Some fruit shows ripeness by color, some by smell and some by weight (or a combination of all these). Eating in season provides a much higher likelihood of that wonderful, juicy, full-flavor mouth pleasure that only a truly ripe fruit can provide. Your green grocer (slap to the forehead as I just remembered that is the proper term) can also help you understand which fruit will ripen best on your counter (and how to know it has achieved that), in a brown paper bag, or has to be purchased ripe because it doesn't continue to ripen once picked.
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Usually I go by smell. Stone fruits are in season now and the bins holding those should be filling the air with their perfume. I'm still waiting for that sensuous aroma that comes from the blossom end of a summer melon.