Pico de gallo, which means "rooster's beak," has a bright, fresh flavor with just a bit of heat. Any hint of bitterness can ruin this easy-to-make salsa. One ingredient -- old cucumbers, preservatives or too much lime juice -- is usually the culprit. To prevent this problem in the future, start with fresh ingredients and add them slowly, tasting as you go.
The Vegetables
Since pico de gallo is an uncooked salsa, it's important to use the freshest produce you can find. Produce that's old can cause bitterness, especially if the salsa contains cucumbers. Choose Roma or plum tomatoes, which are meaty and flavorful, without being overly juicy. The tomatoes should be firm and fragrant, but not overly ripe. If you use cucumbers, avoid those that are shriveled or dry. Opt for red bell peppers, which are sweeter than green. Finally, taste each type of produce before you add it to the salsa. If it doesn't taste good on its own, it won't taste good in the salsa.
The Aromatics
Onions and garlic are essential to pico de gallo, but they can add bitterness if they're not fresh or if you use the wrong type. Use mild, sweet onions, such as red or white onions. You can also use scallions, if you prefer. Chop the onions and soak them in a little ice water to absorb some of their intensity. Drain the onions and blot them dry before you add them to the salsa. Use fresh garlic that's firm, but not hard or dried out. Avoid those with green centers, which may be bitter. Be careful when using bottled garlic: It's convenient, but it becomes bitter when it's past its prime.
The Seasonings
Sometimes a bitter flavor is caused by the liquids or ingredients you use to spice up pico de gallo. A tablespoon or two of lime juice adds freshness and punch, but any more than that can cause bitterness, especially as the pico de gallo is refrigerated. Ditto for vinegar. If you use vinegar, choose a mild-flavored variety, such as apple-cider vinegar, and add no more than a spoonful or two. Use fresh or canned chile peppers and fresh chopped cilantro. Some dried herbs, such as marjoram or oregano, can add a bitter flavor and don't really complement pico de gallo.
Commercial Pico de Gallo
Commercial pico de gallo contains preservatives to keep it fresh much longer than you'd store a homemade version. These preservatives may give the salsa a slightly bitter flavor, but a bitter flavor more likely is your cue that the salsa is no longer fresh. Buy only as much as you can use within a week or two and keep the salsa refrigerated. Take note of the "sell-by" date on the package.