Bromelain was a plant enzyme that appeared in several types of fruit and foliage, including – but not limited to – pineapple.
Lieutenant Malcolm Reed was allergic to this compound, but was able to consume it as a result of targeted treatments including regular injections, administered by Doctor Phlox. In 2151, Hoshi Sato used this information, given to her by Pholox, to determine what Reed's favorite food might be. He cautioned her that this fact did not necessarily mean pineapple was the food in question, but a frustrated Sato figured it was "close enough". (ENT: "Silent Enemy")
The idea of using bromelain as an allergen in "Silent Enemy" was devised by André Bormanis, shortly after Brannon Braga suggested Reed was somehow allergic to pineapple, which prompted Bormanis into doing some research into precisely what might cause Reed to be allergic to that fruit. He discovered bromelain was a potential allergen in pineapple and other food. Bormanis is allergic to the enzyme himself, due to regularly taking Lipitor, which doesn't mix well with bromelain. "I'm pretty sure Brannon had no idea about bromelain in pineapple," Bormanis stated, "but it worked perfectly." ("Silent Enemy" audio commentary, ENT Season 1 Blu-ray special features)