Pectin is a natural thickening agent commonly used in jelly-making. It helps the jelly set and achieve a firm texture. Perform a gel stage test to determine if the jelly has reached the desired consistency. Place a small amount of the jelly mixture on a chilled plate or spoon and let it cool for a few seconds. Push the mixture with your finger; if it wrinkles or holds its shape, it has reached the gel stage and should set properly. If it is still runny, continue boiling the mixture for a little longer and retest until the desired consistency is achieved.