Infused oils, vinegars, and syrups—created by steeping herbs, spices, fruit, or aromatics in liquid—are perfect for adding big bursts of flavor to food without a lot of work. A splash of raspberry vinegar on a sliced ripe peach or a drizzle of rosemary oil on roasted potatoes is all it takes to elevate a dish from simple to sublime. And when you understand why certain liquids absorb some flavors better than others, making your own infusions will be as easy as using them.
Start with good matchmaking. Infusing is the process of extracting an ingredient’s flavor into liquid. Any liquid can be infused—water, oil, vinegar, syrup, wine—but flavors don’t dissolve equally well in all liquids. The most flavorful infusions happen when an ingredient meets its ideal fluid.
All the tasty ingredients you use in the kitchen get their characteristic flavors from a variety of volatile aromatic chemicals. Some of these flavor compounds are chemically attracted to water; others prefer oil or alcohol. These preferences determine how well an ingredient’s flavor will infuse into a liquid.
Consider fresh basil. Put it in water and…yawn…there’s literally no chemistry. But when basil meets oil, the result is a heady, basil-scented infusion. The compounds that give basil its characteristic flavor are chemically attracted to fat molecules, so they rush out of the plant and dissolve in the oil. Mint’s flavor compounds, on the other hand, prefer water. It’s no coincidence that mint tea is one of the world’s most popular drinks, and basil tea is virtually unheard of.
Flavors love fat. The natural tendency of most flavors, whether they come from an herb, a spice, or an aromatic like garlic, is to dissolve in fat. So making an infused oil can be as simple as putting an ingredient in oil and letting the flavor compounds do what nature tells them. If it’s a good match, the infusion starts to happen in minutes at room temperature, although full flavor extraction can take a couple of hours, depending on the ingredient.
You can speed up the process by chopping the ingredient in a blender or food processor along with the oil. This liberates flavor compounds from their cell structures, allowing more of them to react with oil molecules at once. And if you add a little heat, it helps release the flavors of dried spices and chiles and sturdy herbs like lavender, rosemary, and oregano. But go easy: Too much heat can destroy flavor compounds.
Make infused oils in small batches and use them right away. If you make infused oils with fresh ingredients, it’s best to make only as much as you have immediate use for. Infused oils taste best when they’re fresh because flavor, by nature, is an unstable quality that changes over time. Also, harmful bacteria can grow in infused oils at room temperature.
The main food safety concern is botulism, a rare but dangerous illness caused by a bacterium found in soil. “Anything that grows in or near soil is likely to harbor it,” explains Dr. Linda Harris, a food microbiologist at the University of California at Davis. Exposed to air, the bacterium is dormant and harmless. But if it lingers for weeks in a moist, low-acid, oxygen-free environment, the organism can awaken and multiply. So if you submerge fresh herbs or aromatics in oil and store it at room temperature, you’re flirting with botulism. The organism can’t survive in acidic liquids (pH less than 4.6), so vinegar poses little risk. And as you’ll read in the section on infused syrup, sugar’s presence helps prevent bacterial growth.
All this doesn’t mean you can’t infuse oil with fresh ingredients; you just have to follow the safety tips below.
Simmering helps coax out flavors. Just because a flavor prefers oil doesn’t mean it won’t dissolve in water, or water-based liquids like sugar syrup. “Many compounds that prefer oil can also dissolve in water to some degree,” says flavor chemist Dr. Sara J. Risch.
Pastry chef Irit Isai uses simple sugar syrups flavored with vanilla, ginger, and rosemary to flavor fruit salads. Vanilla is extremely water soluble, so its flavor comes through loud and clear in syrup. But key flavor compounds in ginger and rosemary don’t really want to go into water. To coax them in, Ishai simmers the ingredients in sugar syrup—remember, heat helps ingredients release their volatile flavor compounds. And the sugar in the syrup acts as a flavor enhancer.
For best flavor, use infused sugar syrups soon after they’re made. “The flavor compounds in an infusion react to one another and essentially form new flavor compounds,” says Risch. This means the flavor will change over time, possibly for the worse.
If the syrups are refrigerated, bacterial growth isn’t a big concern because the sugar in the syrup acts as a preservative. Sugar molecules chemically engage the water molecules, keeping them too busy to interact with microbes. “Even though syrup seems watery, there’s actually not much water available for microbes to grow,” says Dr. Harris.
Infusion safety tips
Follow these guidelines for all infusions, whether oil, sugar syrup, or vinegar.
• Use sterilized jars, just as you would when canning.
• Thoroughly wash and dry all herbs and produce.
• As soon as the flavor has developed, strain out the ingredient until the liquid is clear. Coffee filters, rinsed and squeezed dry, are ideal; a cheesecloth-lined sieve will do.
• Use your infusions right away. Store any extra in the refrigerator. Use oils within 10 days, syrups within a few weeks, vinegars within 3 or 4 months.
• Discard any infusion that shows signs of spoilage, for example, bubbles, mold growth, or off odors.
The secret is time. Just as water isn’t the world’s greatest flavor extractor, neither is vinegar. When chemists talk about infusions, they actually consider water and vinegar as one and the same; most vinegars are about 95 percent water and 5 percent acetic acid. “Acid doesn’t have a big influence in terms of extraction,” explains Risch.
Berries infuse quickly into vinegar—their flavors are naturally attracted to water—but it can take weeks or longer to infuse many herbs and spices into vinegar because they don’t want to release their flavor compounds into that medium. Eventually, though, some of their flavor will go. “Steeping time is what matters most with vinegars,” says Kevin Vetter, corporate chef at McCormick, the spice company.
To hurry things along, crack spices or bruise herb leaves before dousing them in vinegar. This frees their flavors from the cell structures. You might also heat the vinegar to the temperature of a hot bath before pouring it over the fruit, herbs, or spices.
Although the infusion can take a while, flavored vinegars do keep well at room temperature because the acid inhibits the microbial growth that can quickly spoil flavored oils. But as with oils and syrups, if you love the flavor of an infused vinegar, use it sooner rather than later because its flavor will change over time.
The secret to making flavorful infusions is matching ingredients to their favorite liquids. You can always experiment, but here are some matches that work.
Oil (neutral flavored)
Lemon, orange, and other citrus peel
Raspberries, blackberries, blueberries