A junket tablet in old recipes contains rennet, an enzyme that coagulates milk to create a soft, custard-like, milk-based dessert called junket (also known as curds and whey or rennet custard).
Rennet tablets and liquid are available to purchase. Rennet tablets however, are much ‘stronger’ than junket tablets so replacing a junket tablet with a fraction of a rennet tablet is difficult. It is easier to use the liquid rennet at a ratio of approximately 20 drops to one junket tablet.