Editor's note: This is an occasional column focusing on our sources of food.

With Northern California's summer produce season in high gear, many shoppers are choosing to buy from organic farmers. But organic produce is generally more expensive than conventional, so when budgets are limited choices often have to be made.

Hardier fruits and vegetables, such as those with tougher peels, are often less likely to carry traces of pesticides. "If you're dealing with a fruit that has a hard shell that you're going to peel off, it doesn't really matter if it's organic or not," says San Francisco registered dietitian Nancy Bennett.

This applies to items like papayas, mangoes, bananas, kiwi, cantaloupe, avocado, eggplant, watermelon, grapefruit, onions, corn, cabbage and pineapple. Broccoli, sweet peas, asparagus, cauliflower and sweet potatoes are also known to have low pesticide levels.

On the other hand, conventional peaches, nectarines, strawberries, apples, spinach, celery, pears, bell peppers, cherries, potatoes, carrots, lettuce and grapes tend to have high pesticide levels.

Washing produce carefully under running water can significantly reduce pesticide residue and bacteria. "If (organic) is too expensive, wash the heck out of it," Bennett says.

Some dietitians recommend combining 1 cup water, 1 cup vinegar, 1 tablespoon baking soda and the juice of 1/2 lemon, and spraying that onto fruits and vegetables. Let stand for five minutes, then rinse in cold running water.

It's also important to scrub melons, squash, carrots, potatoes or any items that grow directly on the ground, even if they're organic and even if the peels aren't eaten. When a melon is sliced, for example, the knife can drag any contamination on the outside into the edible portion inside.

Even when food is organic, Bennett and other experts caution not to get fooled into thinking it's healthier. Studies that measure nutritional differences between organic and conventional produce belong to a relatively new genre of scientific research, and it's difficult to determine the relative nutritional benefits at the consumer level, says Alyson Mitchell, professor of food science at UC Davis. Variables such as post-harvest handling, environmental conditions and storage can all affect the nutritional profile.

But, Bennett adds, "When you grow plants organically, and you're not using pesticides or herbicides, plants use phytochemicals to fight the environment," which means they can contain higher levels of these beneficial chemicals.

Buying locally raised produce, whether organic or not, also can make the most of limited food dollars, as shown by the popularity of farmers' markets and the increasing number of supermarkets that identify the harvests of individual farms in their produce sections.

If you can talk to the person who raised it, "you can have more assurances that it's not grown with chemicals you don't want," Bennett says.

Regardless of their origin, however, eating more fruits and vegetables is a good thing. That's the first step, Bennett says. "Step two would mean, if you can, buy organic."

E-mail Lisa Wallace at food@sfchronicle.com.