Let’s jump in the wayback machine to an early 1800s summer. Strawberries are hopping, raspberries are jumping, but both rot quickly once picked. There’s not a refrigerator or freezer in sight — at least, not for another few decades. So how do you preserve the harvest?

With jams and jellies, for one. Or in fruit wine. But you can also mash that fruit with some sugar, let it macerate, and then add vinegar to make an oft-imbibed Early American drink: shrub.

Years after it vanished from our glasses, shrub has made a comeback in the cocktail world. However, it can be hard to find behind a Long Island bar. At Treme in Islip, a jewel-toned raspberry shrub syrup is almost always on hand, ready for the Roffignac, the restaurant’s version of a long-standing New Orleans cocktail.

Shrubs are not easy to make, said Treme owner Josh Thompson — but worth the effort. “I like to expose people to new flavors,” said Thompson, and tart-sweet shrub definitely fits that bill. While some shrubmakers prefer cooking the fruit and the sugar, Thompson takes a more purist approach, cold macerating the fruit before straining out the seeds and blending with apple cider vinegar.

The Roffignac is a drink tied to Count Louis Philippe Joseph de Roffignac, New Orleans’ mayor in the 1820s. In Treme’s rendering, the raspberry shrub is combined with cognac, falernum, lime juice and club soda over ice. It’s zesty and quenching.

To make your own shrub with the berries of the moment, try this simple method:

1. Mash 2 cups of fruit with 1 1⁄2 cups of white sugar in a nonreactive bowl, cover and refrigerate for at least 24 hours and up to three days. (You can also try adding a sprig of rosemary or basil.)

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2. Remove and let the mixture strain over a sieve for an hour, making sure to press all juices from the pulp.

3. Combine the resulting syrup with 1 cup of apple cider or white-wine vinegar in a jar, and shake well to combine. Shrub keeps for up to a week in the refrigerator. A few teaspoons makes an excellent base for a sparkling-water mocktail — or splash in some booze to do some “adulting.”

Treme, 553 Main St., Islip; 631-277-2008, tremeislip.com