The reason you put lemon juice in strawberry jam is because lemons contain citric acid, and this citric acid ingredient is good for gelling strawberry jam. Because citric acid can be fused with the pectin in the fruit, and play a role in the gel. So when we add lemon juice in making strawberry jam, it can make the jam thicken quickly.

Lime (Citrus aurantiifolia), also known in the literature as Lai Lemon, Green Lemon, and in China as Green Lemon and Sour Citrus, is a collective name for several of the species of the genus Citrus in the family Rutaceae, whose fruits are characterized by a pale yellowish-green globular, ellipsoidal, or obovate shape, about 4 to 5 centimeters in diameter.