METABOLIC PRODUCTS 267 



humus. Some of the carbon and other essential elements is converted 

 into bacterial and fungus protoplasm, which after death is subject to 

 decay. In the end, humus is converted into carbon dioxide, water, and 

 other simple compounds, which are used again. The importance of 

 humus as a soil constituent is ably discussed by Waksman (1938). In 

 addition to the carbon cycle, the fungi also play an important part in the 

 cycles involving the release and utilization of the other essential elements. 



FUNGI AS FOOD 



Many curious details about the early use of fungi as food have been 

 collected from classical and other sources by Buller (1914) and by Rolfe 

 and Rolfe (192G). The mushrooms were no doubt among the first fungi 

 used as food by man. Yeast became part of his diet when the arts of 

 brewing and baking were discovered. The widespread use of fermented 

 beverages, under certain dietetic circumstances, has an important bearing 

 on nutrition and health. J. S. Wallerstein (1939) has discussed primitive 

 brewing practices and the geographical distribution of the art. The beer 

 of the Middle Ages Avas turbid, owing to its content of suspended yeast 

 cells (Thaysen, 1943). 



The nutritive value of any food depends upon its composition and 

 digestibility and the assimilability of its hydrolytic products. The early 

 writers, in the absence of precise information, were of the opinion that 

 fungi had little value as food. The nutritive value of fungi, of yeast in 

 particular, will be discussed from the standpoint of protein content and 

 value, vitamins, fats, and minerals. 



Assuming good digestibility, the value of fungus protein is determined 

 by its amino-acid composition. Rose (1938), in a long series of careful 

 experiments, has determined which amino acids are essential for man and 

 animals. Some nine or ten amino acids were found to be essential (Table 

 20). If the protein part of a diet is deficient in a single essential amino 

 acid, nitrogen is lost from the body, or inefficient utilization of protein 

 results. More of a poor protein must be consumed in order to increase 

 the intake of essential amino acids to satisfactory levels. The amino- 

 acid composition of yeast and some other proteins is given in Table 46. 

 Yeast protein compares favorably w4th casein or meat with respect to 

 essential amino acids. 



Less complete data are available for the amino-acid composition of 

 fleshy fungi. According to Lintzel (1941), the proteins of Psalliota 

 campestris, Cantherella cibarius. Boletus edulis, and Morchella esculenta 

 are about equal to animal protein. From 100 to 200 gr. (dry weight) of 

 these mushrooms was required to maintain the nitrogen balance in a man 

 weighing 70 kg. Fitzpatrick et al. (1946) found the tryptophane content 

 of P. campestris to be 5 mg. per 100 g. 



