Pineapple syrup is fresh pineapple juice — often simmered along with the trimmed core and skin, which carry real flavor most home cooks throw away — reduced with sugar into a concentrated, tropical-forward syrup. It does a different job than pineapple juice itself: where the juice adds both flavor and volume to a drink, the syrup adds concentrated flavor and sweetness without much extra liquid.
Why Not Just Use the Juice
Most tiki recipes already lean on fresh pineapple juice for body. A syrup is the move when a recipe wants that flavor concentrated into a small pour instead — layered alongside the juice rather than replacing it, or standing in where a drink wants pineapple's sweetness without adding another ounce of liquid to the shaker.
The Build
Fresh pineapple juice, ideally with the reserved trim (core and skin scraps) simmered in for extra flavor, reduces with sugar into a syrup thick enough to coat a spoon.
Don't Waste the Trim
The core and skin of a pineapple carry real flavor and pectin that plain juice misses — simmering them along with the juice before straining pulls more out of the fruit than juicing alone. Discard the solids after straining; they've given up what they have to give.
Storage and Shelf Life
About two weeks refrigerated — shorter than a cane or demerara syrup because fresh pineapple juice brings more water and less preservative sugar concentration than a straight sugar syrup. Watch for fermentation (a sharp, fizzy smell) and discard at the first sign.
Bottom Line
Worth making when a recipe specifically calls for pineapple syrup rather than juice — the concentration is the point, and juice alone won't substitute cleanly.