There were already many food preservation techniques used by sailors and armies by the renaissance that allowed them to store food for months or years if need be. Flour can be kept and only cooked into bread/biscuits/etc as needed giving you a reliable source of carbohydrates. Dried and salted meats give you proteins. Certain cheeses can last a long time without refrigeration.
Fruit is your hardest major food group, but by the mid-1700s many sailing ships would carry oranges, lemons, and melons and by the the late 1700s, lemon juice was used to replace fresh fruit because it lasted longer. If you are going back with modern knowledge, your time traveler might know that adding fruit juice, dried peppers, or chilies to the menu would prevent scurvy and allow long term preservation of all your needed food groups.
As for bulk: the answer is plantations. Plantation style farming was common in many ancient civilizations such as Rome and Greece, and was later revived in the renaissance using Irish, Native American, and African slave labor. Basically, as long as you are willing to exploit a considerable % of your population to make all of your food while giving them less than their fair share, you can produce a large enough excess for a non-agricultural urban culture and permanent standing military without needing industrial farming. In fact, the revival of farmer exploitation in this manner is sometimes cited as a contributing factor to the the rise of urbanization that allowed the renaissance to happen.
There are also certain crops that offer notoriously good returns on your labor compared to others. Potatoes and Yams for example give a far better caloric return for your labor than wheat or berries. Focussing on high return crops like that would also allow you to better feed your people off of fewer farmers per capita.