This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
Like all growing plants, the mushroom contains a very large proportion of water. The actual amount present varies greatly in different species. In the above table it will be seen that Polyporus sulphureus, with over 70 per cent. of water, has the least of any species mentioned, while the species of Coprinus and Agaricus have usually fully 90 per cent. water. The amount of water present, however, varies greatly in the same species at different seasons and in different localities, and with variations in the moisture content of soil and atmosphere, also with the age and rapidity of development of the individual plant.
The proportion of nitrogen in the dry matter of different species varies from 2 per cent. to 6 per cent. This comparatively high nitrogen content was formerly taken to indicate an unusual richness in proteid substances, which in turn led to very erroneous ideas regarding the nutritive value of these plants. The nitrogenous substances will be more fully discussed later, when we consider their nutritive value.
This consists of a variety of fatty substances soluble in ether. It varies greatly in quality and quantity in different species. The amount is usually from 4 per cent. to 8 per cent. of the total dry matter. It includes, besides various other substances, several free fatty acids and their glycerides, the acids of low melting point being most abundant. These fatty substances occur in the stem, but are much more abundant in the cap, especially in the fruiting portion. Just what nutritive value these fatty matters may have has never been determined.
The largest part of the dry matter of the mushrooms is made up of various carbohydrates, including cellulose or fungocellulose, glycogen, mycoinuline, trehalose, mannite, glucose, and other related substances. The cellulose is present in larger proportion in the stem than in the cap, and in the upper part of the cap than in the fruiting surface. This is doubtless related to the sustaining and protective functions of the stem and the upper part of the cap. Starch, so common as a reserve food in the higher plants, does not occur in the mushrooms. As is the case with the fats, no determination of the nutritive value of these substances has been made, but it may be assumed that the soluble cabohydrates of the mushrooms do not differ greatly from similar compounds in other plants.
The ash of mushrooms varies greatly. Polyporus officinalis gives but 1.08 per cent. of ash in dry matter, Pleurotus ulmarius gives 12.6 per cent., and Clitopilus prunulus gives 15 per cent. The average of twelve edible species gave 7 per cent. ash in the stem and 8.96 per cent. in the cap.
In regard to the constituents of the ash, potassium is by far the most abundant - the oxide averaging about 50 per cent. of the total ash. Phosphoric acid stands next to potassium in abundance and importance, constituting, on an average, about one-third of the entire ash. Oxides of manganese and iron are always present; the former averaging about 3 per cent. and the latter 5 per cent. to 2 per cent. of the ash. Sodium, calcium, and chlorine are usually present in small and varying quantities. Sulphuric acid occurs in the ash of all fungi, and is remarkable for the great variation in quantity present in different species; e. g., ash of Helvetia esculenta contains 1.58 per cent. H2 So4 while that of Agaricus campestris contains the relatively enormous amount of 24.29 per cent.
Any discussion of the bare composition of a food is necessarily incomplete without a consideration of the nutritive value of the various constituents. This is especially desirable in the case of the mushrooms, for while they are frequently overestimated and occasionally ridiculously overpraised by their friends, they are quite generally distrusted and sometimes held in veritable abhorrence by those who are ignorant of their many excellent qualities. On the one hand, we are told that "gastronomically and chemically considered the flesh of the mushroom has been proven to be almost identical with meat, and possesses the same nourishing properties." We frequently hear them referred to as "vegetable beefsteak," "manna of the poor," and other equally extravagant and misleading terms. On the other hand, we see vast quantities of the most delicious food rotting in the fields and woods because they are regarded by the vast majority of the people as "toad-stools" and as such particularly repulsive and poisonous.
Foods may be divided into three classes according to the functions they perform :
(a) To form the material of the body and repair its wastes.
(b) To supply energy for muscular exertion and for the maintenance of the body heat.
The formation of the body material and the repair of its wastes is the function of the proteids of foods. It has been found by careful experiment that a man at moderately hard muscular exertion requires .28 lb. of digestible proteids daily. The chief sources of our proteid foods are meats, fish, beans, etc. It has been as a proteid food that mushrooms have been most strongly recommended. Referring to Table I, it will be seen that nitrogen constituted 5.79 per cent. of the total dry substance of Coprinus comatus. This high nitrogen content, which is common to the mushrooms in general, was formerly taken to indicate a very unusual richness in proteid materials. It is now known, however, that there were several sources of error in this assumption.
Much of the nitrogen is present in the form of non-proteid substances of a very low food value. Another and very considerable portion enters into the composition of a substance closely related to cellulose. A third source of error was the assumption that all the proteid material was digestible. It is now known that a very considerable portion is not digestible and hence not available as food. Thus, notwithstanding the 5.79 per cent. of nitrogen in Coprinus comatus, we find but .82 per cent. in the form of actually available (i. e., digestible) proteids, or approximately one-seventh of what was formerly supposed to be present.
The digestibility of the proteids varies very greatly with the species. Mörner found the common field mushroom, Agaricus campes-tris, to have a larger amount of proteids available than any other species studied by him. Unfortunately, the digestibility of the American plant has not been tested. There is great need for further work along this line. Enough has been done, however, to demonstrate that mushrooms are no longer to be regarded as a food of the proteid class.
The energy for the muscular exertion and heat is most economically derived from the foods in which the carbohydrates and fats predominate.
The common way of comparing foods of the first two classes scientifically is to compare their heat-giving powers. The unit of measurement is termed a calorie. It represents the amount of heat required to raise a kilogram of water 1° Centigrade. (This is approximately the heat required to raise one pound of water 40 Fahrenheit.) A man at moderately hard muscular labor requires daily enough food to give about 3500 calories of heat-units. The major part of this food may be most economically derived from the foods of the second class, any deficiency in the .28 lb. of digestible protein being made up by the addition of some food rich in this substance.
In the following table the value of ten pounds of several food substances of the three classes has been worked out. Especial attention is called to the column headed "proteids" and to the last column where the number of heat-units which may be purchased for one cent at current market rates has been worked out.