Flavored gelatin desserts have been made since the early 19th century, but required a great deal of work to clarify the gelatin and prepare it for use. The advent of Jell-o in the early 20th century made gelatin desserts quick and easy enough for any home cook to undertake. Jell-o makes approximately a 3 percent gelatin solution, enough to keep it firm in refrigeration or for a shorter time at room temperature. However, gelatin softens and melts at 95 degrees Fahrenheit, so you have to take care on hot days.
Keep Jell-o in the refrigerator until serving time. This is also advisable from a food-safety standpoint, since gelatin desserts are prone to spoilage, like other forms of protein.
Serve the Jell-o in frosted or refrigerated dishes or on a chilled platter to slow the warming process.
Cool the Jell-o to room temperature before placing it in the refrigerator. Slow cooling allows the gelatin molecules to form a stronger bond, which inhibits melting.
Add a teaspoon of unflavored gelatin to the recipe. Reinforcing the Jell-o with a small quantity of unflavored gelatin makes a stronger gel that resists melting.
Replace a portion of the water with alcohol when you make the Jell-o. Replacing 1 tablespoon of water in each cup with vodka or white rum helps the Jell-o form stronger molecular bonds, which help slow melting.
Store the Jell-o in a picnic cooler with several frozen gel packs if you don't have access to a refrigerator.