The development of a workable system or science of nutrition had to await the development of modern chemistry with its significant advances at the end of the eighteenth century. Although the scientific evaluation of diets at the end of the nineteenth century focused on protein and energy, some realized that there were other requirements for a healthy diet. In particular, it was known that sailors on long voyages developed scurvy unless they periodically ate fresh green vegetables or fruit. As with ascorbic acid, the development of chemical analytic methods for determining the levels of each vitamin in foods and the inexpensive production of vitamins have enabled food manufacturers to fortify processed foods with them, without unduly raising their prices. Modern food industries have been able to process seeds and vegetables so as to extract the fat and sugar or, in the case of grains, to mill off the outer branny layers to yield white rice or white wheat flour.