1 oz. dry albumin yields 174 foot-tons of potential energy. 1 oz. fat „ 378 „ „ 1 oz. starch „ 135 „ „ 1 oz. cane sugar „ 129 „ „ 1 oz. glucose or lactose „ 122 „ „ In practical dietetics digestibility of food as well as chemical composition is an important factor. Furthermore metabolism in the body is not in every instance so complete as oxidation outside it. Hence estimates of potential energy can only be regarded as theoretically correct. Examination questions like the following are occasionally asked:— A man does work equal to 176.8 foot-tons in a day. Supposing that he eats only bread, how much will he require to give the amount of energy required, if bread contains 8 per cent. proteid, 1.5 per cent. fat, and 49.2 per cent. carbohydrate? On the above basis, from 100 ounces of bread the amount of potential energy obtainable is:— 8 × 174 = 1,392 foot-tons 1.5 × 378 = 567 „ 49.2 × 135 = 6,642 „ ——-—— Total energy = 8,601 „ obtained from 100 ozs. bread. Let _b_ = number of ounces of bread required to develop 176.8 foot-tons of energy. Then 8,601: 100:: 176.8: _b_. Therefore _b_ = +2.05 ounces+. CHAPTER VI. THE PREPARATION AND PRESERVATION OF FOOD. OBJECTS OF COOKING.—Food may be taken in its crude condition, as directly derived from the animal or vegetable world, or after it has undergone a preparatory process of cooking. Man is the only animal who cooks his food. Many foods, in the uncooked condition are almost entirely incapable of digestion by him—such as the proteid and farinaceous materials contained in the seeds of cereal and leguminous plants. But cooking, as a preparatory help to the digestion of food, is not equally required by all foods. Thus, fruit is commonly taken uncooked, and does not undergo any important alteration on cooking. Salads are taken uncooked, but not for their nutritive properties so much as for a relish to other foods, and for their quasi-medicinal properties. Milk, again, may be taken cooked or uncooked. The oyster is the only animal which is eaten habitually, and by preference, in the uncooked condition; and there is a physiological reason for this universal custom. The large fawn-coloured liver, which constitutes the delicacy of the oyster, is little else than glycogen, associated with its appropriate ferment diastase, so that the oyster is almost self-digestive. When cooked, the ferment is destroyed, and digestion of the oyster becomes more difficult. Cooking is intended—1. _To make the food softer_, and in part to mechanically disintegrate it, thus rendering it more easily masticated and digested. In fact, cooking, in the best sense, is an artificial help to digestion; and digestion may well be said to commence in the kitchen.