Why This Recipe Works: Sweet strawberries and tart rhubarb are a winning flavor combination. Often paired up to make a pie, this combo also makes a great spreadable jam, preserving the taste of summer. We started our recipe by finding the perfect balance of tart rhubarb to sweet strawberries, using twice as much rhubarb as strawberries to let the bright rhubarb flavor shine through. To reduce the natural stringiness of the rhubarb without having to peel it, we sliced it into smaller pieces, which had the added advantage of reducing the cooking time. We also found that a small amount of peeled, cored, and shredded Granny Smith apple enhanced the bright fruit flavor. The apple also helped create a more spreadable and cohesive jam. The final addition was two strips of lemon zest, which complemented the tart fruit. Fresh berries produce the best jam; choose small, fragrant berries that are just ripe and discard any fruit that is bruised and mushy. For safety reasons, it is imperative to use bottled lemon juice, not fresh-squeezed juice, in this recipe.
In step 1, use this guide when temping the jam at a higher elevation:
217-220 degrees for sea level
215-218 degrees for 1,000 feet
213-216 degrees for 2,000 feet
211-214 degrees for 3,000 feet
209-212 degrees for 4,000 feet
208-211 degrees for 5,000 feet
206-209 degrees for 6,000 feet
Cook: 20 minutes
Yield: two 1-cup jars
8 ounces strawberries, hulled and cut into 1/2-inch pieces (1 2/3 cups)
1 pound rhubarb, cut into 1/2-inch pieces (4 cups)
1 3/4 cups sugar
1/2 cup peeled and shredded Granny Smith apple (1/2 apple)
2 (2-inch) strips lemon zest
1 tablespoon bottled lemon juice
1. Place 2 small plates in freezer to chill. In large saucepan, crush strawberries with potato masher until fruit is mostly broken down. Stir in rhubarb, sugar, apple, lemon zest, and lemon juice and bring to boil, stirring often, over medium-high heat. When boiling, crush rhubarb gently with potato masher to help it break down, leaving some pieces intact. Once sugar is completely dissolved, boil mixture, stirring and adjusting heat as needed, until thickened and registers 217 to 220 degrees, 10 to 15 minutes. (Temperature will be lower at higher elevations; see above.) Remove pot from heat.
2. To test consistency, place 1 teaspoon jam on chilled plate and freeze for 2 minutes. Drag your finger through jam on plate; jam has correct consistency when your finger leaves distinct trail. If jam is runny, return pot to heat, simmer for 1 to 3 minutes longer before retesting. Remove lemon zest and skim any foam from surface of jam using spoon.
3. Meanwhile, place two 1-cup jars in bowl and place under hot running water until heated through, 1 to 2 minutes; shake dry.
4. Using funnel and ladle, portion hot jam into hot jars. Let cool to room temperature, cover, and refrigerate until jam is set, 12 to 24 hours. (Jam can be refrigerated for up to 2 months.)