The USDA no longer offers a booklet on what you can bring into the United States. Instead, they have website, http://www.aphis.usda.gov/wps/portal/aphis/home, with guidance on what can and cannot be imported. You don't often get asked by your passengers what is okay and what isn't, but you as the pilot are still the one in jeopardy. You should have an idea of what is okay and where to look if you aren't sure.
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue. Note that I've updated this on 14 May 2015 but much of the data from the source websites is several years old. You should click on the websites given for possible updates. The USDA website does not easily link and they often change those links in an effort to make things harder than they should. I will try to point you in the right direction but they seem to change things every six months or so.
[http://www.aphis.usda.gov/wps/portal/aphis/home] — Click "Resources" / "Travelers International" and under "Related Links" select "Generally Allow Food and Agricultural Items by Category"
The following food items are generally allowed entry:
Remember, you must declare all food and agricultural products, including those listed above, to a CBP agriculture specialist or officer when you arrive in the United States.
The United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) maintains a website where you plug in the country or the food item and get a ruling on import. The web site is: epermits.aphis.usda.gov.
If you enter "Papaya," for example, you will find out you can never bring one to Hawaii. You are also restricted from bringing any papaya from Chile, Ecuador, or Malaysia to any port of the United States.
The web site works well but the list is rather large. If you are en route and don't have an Internet connection, the USDA publishes phone numbers for inquiries: 1-301-851-2046 or 1-877-770-5990.
[http://www.aphis.usda.gov/wps/portal/aphis/home] — Click "Resources" / "Travelers International" and then "Animal Products and By Products"
[USDA - APHIS] Individuals who fail to declare non-commercial agricultural items may be subject to penalties ranging from $1,100 to $60,000 per violation. These penalties are based on authorities granted to USDA through the Plant Protection Act and the Animal Health Protection Act.
If you discover a banned item on the aircraft it is best to declare it on inspection. I've done that and had the item confiscated, which was better than the times the item was discovered and the passenger was forced to write a check.
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 1.