Unfortunately, our sources for what the Ottoman army was eating during and around the conquest of Constantinople is rather scant. So, what I'm going to look at is what people were eating during this time period and especially what meals were being made in massive quantities for large groups of people.

In short, from primary sources around this time period, proteins from mutton, lentils, chickpeas, rice, and eggs would have been eaten regularly.

Perhaps our best source for this would be Özmucur and Pamuk's Real Wages and Standards of Living in the Ottoman Empire, 1489-1914, which looked at archival evidence of what foods were being purchased by charitable organizations, such as public kitchens. Overall, they considered "flour (mostly wheaten), rice, honey, cooking oil, mutton, chickpeas, lentils, onions, eggs, and fuel-grade olive oil." to be the most commonly consumed food items during this period, while, of those, "flour, rice, cooking oil, mutton, olive oil, and honey" would have been the most consistently and readily-available food items.

But, these are all just base ingredients. If we want to know what people actually ate and how they got their protein, Singer's Serving up Charity: The Ottoman Public Kitchen lays out what meals many people would have actually eaten. Singer first looks at Hurrem Sultan's mid-sixteenth-century public kitchen in Jerusalem, which served the following:

Each morning meal comprised rice soup, made with clarified butter; chick peas; onions; salt; and, according to the season, squash, yogurt, lemon, or pepper for additional flavor. In the evening, bulgar (crushed wheat) soup was made with clarified butter, chickpeas, onions, salt, and cumin. Meals always included bread.

As we can see, the meals are comprised of much of the same ingredients as determined by Özmucur and Pamuk's research. This includes: rice, chickpeas, onions, and flour. Protein, in this case, would mainly have come from the chickpeas and rice. In additional, these meals would have been able to be made cheaply in mass-quantities. This makes them perfect for feeding hundreds or even thousands of people reasonably.

However, while these soups don't include meats, that is not to say that meat was not served. According to Singer, "Friday nights (the night between Thursday and Friday); the nights of Ramadan; the nights of 'Agure, Mevlud, Regaib, and Berat; the great sacrifice festival (kurban bayramil/'Td al-adha) during the annual hajj (pilgrimage); and the celebrations marking the end of Ramadan (geker bayraml/'id al-fitr" would have all constituted special days in which different meals would be served. Dane, made with mutton and rice would be served on such days, alongside zerde, made with rice, honey, and saffron. Here we see more of our common ingredients, including: rice, mutton, and honey. So, in this case, people would be getting their protein from the rice and mutton.

And, these meals weren't just common in Jerusalem, but were common throughout the Ottoman public kitchens. Fatih Mehmed, following the conquest of Constantinople, established a public kitchen between 1463 and 1471. Here, even in the capital of the Ottoman Empire, Singer shows that the kitchen served a similar rice soup in the morning and wheat soup in the evening as the kitchen in Jerusalem nearly 100 years later. However, these common-day soups would also have featured more protein, such as mutton.

Alongside these public kitchens, we also have some accounts of foreigners traveling through the Ottoman Empire and remarking on the food. One such traveler was Nicholas de Nicola, French geographer and apparent part-time food critic, who wrote in his 1567 Discovrs et histoire veritable des navigations, peregrinations et voyages, faicts en la Tvrqvie:

the ordinary maner of eating of the Turks . . . farre differeth from ours, [which is] so superfluous, curious, and delicate . . . whereas to the contrary theirs is scant, bare, and grosse, without anye diversities of lardings, dressings, sawces, ioyces, and confections: their Cookes being very simple dressers of meat, as being neither dainty or delicate in ye dressing thereof. For the Turks do content themselves with slight meates & easily dressed . . . Bucks flesh, Goates fleshe, Mutton, Lambe, and Kidde, and certayne Hennes

This quote is presented in Dursteler's Bad Bread and the "Outrageous Drunkenness of the Turks": Food and Identity in the Accounts of Early Modern European Travelers to the Ottoman Empire.

(Note: These primary sources in the article provide a unique insight into everyday food in the Ottoman Empire, but some should be taken with a grain of salt. Many of the original authors are writing with an intent to make Turkish people seem barbaric compared to the West.)

Here, we actually see that meat was much more varied in the day-to-day life of people in the Ottoman Empire than previous sources suggested. Here we find, in addition to mutton: deer, goat, and chicken. However, their lesser popularity in public kitchens may imply that they were not as readily available or as cheap as mutton.

So, perhaps after all of this you're saying to yourself "This is all and good. I get what ingredients and meals would be common, but perhaps that doesn't accurately portray what a soldier's diet would be." So, let's look at sürsat. This was a system in which the Ottoman army would purchase food items at a discount in towns that it passed through. According to Inbaşi's, The Register of Expenditures of Murat IV's Bagdad Campaign, such food items as honey, rice, flour, bread, barley, mutton, and butter were sold to the army. Once again, we see the common protein of mutton being consumed. And with these ingredients, similar soups could be made as detailed earlier.

Overall, by looking at archival records of the purchases of public kitchens, the actual meals prepared, and sürsat imposed on towns, we can see the importance of mutton and other proteins, such as lentils, chickpeas, rice, and eggs in the common diet during this time period. Actual meats may have varied significantly, but the common use of mutton in public kitchens and in sürsat implies that it was a readily available and common meat.