The best transition word or phrase to use in the context of the text is "Additionally." Here's the revised sentence:
"Inventing space food wasn’t an easy process, as meals had to meet several requirements. First, meals had to include a variety of nutritious ingredients that humans could digest easily. Additionally, the food had to be simple to prepare and consume; in a zero gravity environment, there’s no way to eat a bowl of soup without droplets dramatically drifting away!"