- Better Bread? Historically bakers had extremely short lives because without mixers they had to breathe in the flour dust all day. And the bread would have been extremely unevenly cooked (no temperature-regulated ovens), as well as being extremely salty (no preservatives).
- Pasta? Pasta is absolutely amazing. It is an extremely dense source of calories, which doesn't rot, is super-compact, and only requires water and heat to cook. Great for armies on the march.
- Coffee? Coffee's been around a long time, but high-pressure espresso is relatively new.
Other fairly basic staples are newer than you might think, like Potatoes (16th Cent.), Or Tomatoes (17th Cent.)
Actually that's a terrible idea. You want lemons, not limes. Really.
See http://idlewords.com/2010/03/scott_and_scurvy.htm for a discussion of where the myth to the contrary came from, and for the complex history of how the proper prevention of scurvy was discovered, forgotten, proved false, and only later rediscovered again. It also discusses why you really don't want to use limes.
Pasta is an amazing invention. It's functionally similar to hard tack except it can be prepared to an edible state very quickly with merely boiling water and also tastes much better.
Could you please clarify why whole grain flour pasta would not have nutrients? Unless you meant white flour pasta. And here is how to make it from scratch:
Some of these things would be helpful but you'd need to have them on you if you were dumped in the past. If you landed (now) in the Roman Empire how would you bring them potatoes? ("Just sail across the ocean, then go up the mountains on the other side of that continent and eat those root things there. Easy!")
Sorry, but you're wrong, and I think your racism is showing. Potatoes have been eaten for 7000 years, and tomatoes for 2500 years. They were introduced into Europe in the sixteenth century (and tomatoes weren't eaten there at first), but here in America, we've been eating them since well before medieval times.
Making sure to eat sources of vitamin C to prevent scurvy, having women who intend to get pregnant eat more folate to help their babies' brains...(Not to mention not drinking alcohol while pregnant!)
These might be practically difficult to realize just on brainpower alone.
For example: how do you avoid drinking alcohol if your only sources of drinking water are contaminated? You can boil the water — but you don't know a priori whether the pathogens or toxins in the water can be destroyed by boiling, or how much boiling is needed. If the toxins are things like arsenic, boiling the water will concentrate them, while various kinds of biological processing may filter them out.
And where do you get vitamin C? Tomatoes are a great source of vitamin C, but you can't just go around eating random local tomato-like plants; almost all of them are deadly. Plants like rhubarb have lots of vitamin C, but also lots of oxalic acid. Raw meat is also a good source of vitamin C, but raw meat can harbor many different pathogens. Are you going to build a microscope, synthesize some stains, and inspect a bunch of meat samples to see which ones bear pathogenic bacteria?
And once a couple of your human-subjects experiments go wrong, or just have coincidental unrelated deaths, you're a candidate for a witch-burning.
(Even in the best cases, modern hygiene is not without its drawbacks; the poliomyelitis epidemic is one example.)
