3. MUSHROOM MYCELIUM AS A SOURCE OF PROTEIN

The production of protein is an important aspect, which has received much attention from a great many workers studying the submerged culture of mushroom mycelium. As a rule protein in the mycelium was determined by the Kjeldahl method in which the total amount of nitrogen is ascertained and the crude protein content calculated by multiplication of the nitrogen content by a factor of 6.25, based on the assumption that the protein on an average contains 16% nitrogen and other nitrogenous compounds are present only in negligible amounts. The reported the crude protein content of mushroom mycelium varies considerably. For Morchella species values ranging from 10 to 55% were found [39,83, 93,126]. Tricholoma nudum contains 21 to 61% crude protein [39,63], Pleurotus ostreatus 24 to 35% [53] and Volvariella volvacea 34% [6]. Sugimori et al. [138] found that the protein content of Lentinus edodes mycelium varied with the carbon source. Grown on glucose, the mycelium contained 49% crude protein and on ethanol 58%. Lee et al. [89] found 48% crude protein in submerged grown mycelium of Agaricus bisporus.

The crude protein content of the mycelium of our strains of Agaricus bisporus andCoprinus comatus also depended on the composition of the medium. With lipids as carbon and energy source, a high dry weight of mycelium was observed, but the crude protein content was low (16% for Agaricus bisporus, see Table 18). In the CME medium the crude protein content of Agaricus bisporus was 38% [26] and in a medium based on skim milk it was 68%. Mycelium of Coprinus comatus grown in a medium based on skim milk contained 52% crude protein (Table 18).

The crude protein content, calculated from the results of the Kjeldahl method, is higher than the protein content determined by amino acid analysis after hydrolysis of the mycelial protein, because the total nitrogen content also includes non-protein nitrogen, especially from nucleic acids. From the amino acid analyses of LeDuy et al. [88] it can be calculated that in hydrolysates of Morchella mycelium 16 g of nitrogen represented 69 to 83 g of amino acids. This means, that only 62 to 75% of the crude protein (N x 6.25) was true protein.

We analysed amino acids in hydrolysates of the mycelium of Agaricus bisporus and Coprinus comatus, as well as free and bound amino acids in the medium before and after mycelial growth (Table 19); the samples were taken from the fermentor experiments, described in Chapter 5, in which the aeration and agitation conditions gave the highest yields of mycelium. The amino acids found in the mycelium of Agaricus bisporus represented 76% of the total nitrogen in the mycelium, which is similar to the result found by Leduy et al. [88] for Morchella mycelium. With Coprinus comatus this value was lower, namely 66%.

When the purpose of mushroom mycelium culture is to produce protein on an industrial scale, the nitrogen source that can be used is important. For efficient production of mycelial protein, the nitrogen source should either be a simple synthetic one or an inexpensive, complex substance not suitable for human or animal consumption, e.g. waste material. In this respect, the results with our strains, reported in Chapter 2, are not very promising, because they show the need for organic nitrogen sources and some essential amino acids. In order to study whether our strains could synthesise other amino acids in a complex medium, we made up the balance of each amino acid by subtracting the sum of the initial concentrations from the sum of the final concentrations. These balances are shown in Table 19. Although Coprinus comatus and Agaricus bisporus do not need all essential amino acids, the balances of all essential amino acids except threonine were negative, while the balances of only some non-essential amino acids (glycine, alanine and, with Coprinus comatus, also serine and aspartic acid) were positive. The balances of total amino acids were negative. We may conclude that the fermentation by mushroom mycelium of the media used in these experiments does not result in any nutritional enrichment, since essential amino acids are obviously used by the fungi to synthesise other nitrogenous cell material.

As stated in Chapter 2, the possibilities of finding complex, cheap media are not exhausted. Mushroom mycelium may be able to utilise complex organic material, not suitable for human consumption, thus converting it into edible and digestible mycelial protein.

Another disadvantage of our strains (and of mushroom mycelium in general) is the slow growth rate. We observed the most rapid growth of Agaricus bisporus and Coprinus comatus in the fermentors (Chapter 5), where both strains needed about one week to reach maximum yield. Even if this mushroom mycelium could improve the nutrient value or the digestibility of any organic material, it should generally be possible to discover a lower fungus or a yeast, that could do the same much faster. It would be useful to grow mushroom mycelium only when a substrate could not be used by other micro-organisms. In this respect we might think of materials that are used by mushroom mycelium in natural conditions, like wood (as was done by Yahagi [61]), humus or compost. However, comparatively little is known about the mechanisms by which mushroom mycelium attacks such substrates and about the resulting end products. Ecological relations with other microbes might be necessary for this breakdown.

A reason for growing mycelium of higher fungi instead of other fungi might be the fact that several mushrooms have been known to be edible for thousands of years, so that no toxicity problems are likely. There is, however, little experience with the use of mushrooms as the only or main source of protein in a human diet. Analyses of bound amino acids in mushrooms [7,8,81,100] and mushroom mycelium [61,88,100,138] showed the presence of all essential amino acids, but the concentrations of the sulphur containing amino acids were low. The same conclusion can be drawn from our amino acid analyses of Coprinus comatus and Agaricus bisporus mycelium (Table 19). For this reason, mushroom mycelium may be expected to be suitable for human and animal nutrition, if it is not the only source of amino acids, but is supplied with another kind of protein.

By pepsin digestion experiments Sugimori et al. [138] showed, that 80 to 87% of the nitrogen of Lentinus edodes mycelium could be digested and 90% of the nitrogen of Pleurotus ostreatus mycelium.

Fink et al. [40,41,42] examined the growth of rats, fed with fruit bodies of Agaricus bisporus and Boletus edulis as sole source of protein. The rats grew well on Boletus edulis over a period of 100 days, but died when fed only with Agaricus bisporus. Although addition of methionine and cystine to the diet based on Agaricus bisporus improved the nutritional quality, the difference between Agaricus bisporus and Boletus edulis was not explained fully by the difference in the amino acid compositions. Possibly a higher toxicity of Agaricus bisporus also plays a role.

Some toxicity tests have been made with mycelium of higher fungi, as reviewed by Worgan [160]. With 17 species of wood-rotting fungi, as well as with Tricholoma nudum and Morchella mycelium, no toxic symptoms were observed in animal feedings tests. We may conclude that if mushroom mycelium were to be considered in the future as an important part of the diet, further research on its toxicity would be necessary.

After these rather negative conclusions on the suitability of mushroom mycelium as a source of protein, we shall discuss in the next chapters a more promising aspect, namely the development of the characteristic mushroom flavour. When other fungi are used for the production of a blank flavoured protein, mushroom mycelium may provide an attractive flavour, making the resulting mixed product acceptable for human consumption.

REFERENCES TO CHAPTER 3