The technical advances made during recent decades have, along with myriad other implications, resulted in edible mushroom cultivation attaining global dimensions. Since cultivated mushrooms can be grown under different climatic conditions on cheap, readily available waste materials, they represent a solution to many of the world's current problems, including protein shortages, resource recovery and re-use, and environmental management.

The cultivation of edible mushrooms is a prime example of how low-value waste which is produced primarily through the activities of the agricultural, forest and food-processing industries can be converted to a higher value commodity useful to mankind.

Many varieties of mushrooms are valued greatly as nutritious food sources, as tonic foods, and as important sources of medicinal compounds anti-tumour/anti-viral agents and other pharmaceutically-active components. A number of proprietary products, including cosmetics, beverages and health foods, are marketed currently and the demand for such products is expected to increase.

Mushrooms are not chlorophyllous plants, i.e. they do not have the green pigment called chlorophyll that enables plants to utilize energy from sunlight to change chemicals into substances necessary for growth, a process commonly known as photosynthesis. Instead, mushrooms produce a wide range of extracellular enzymes, and it is these extracellular enzymes that enable them to degrade complex organic matter into soluble substances which can then be absorbed by the mushroom for purposes of nutrition.

The growth and fruiting of an individual mushroom species on a particular waste material will hence depend largely upon the ability of that mushroom to produce the enzymes essential to degrade the major components of the waste particular (or its growth `substrate'), and thereafter absorb it as food.

The materials that are most widely adopted for mushroom cultivation are `lignocellulosic' materials the major components of which are cellulose, hemicellulose and lignin. Together these three polymeric substances form the bulk of most plant cell walls. Dr. John Buswell and Prof. S.T. Chang of the Department of Biology have undertaken a study entitled `An investigation of extracellular enzyme production by selected edible mushroom species, and their ability to utilize different lignocellulosic wastes as growth substrate'. The main aim of this research is to investigate the production of three enzymes — cellulases, hemicellulases and ligninases — by five commercially important edible types of mushrooms, to decompose the cellulose, the hemicellulose, and the lignin respectively. This project won a competitive grant of HK$682,000 from the Research Grants Council in 1992.

The experimental results so far have revealed a wide range of ability among different edible mushrooms to produce the enzymes necessary to degrade individual components of lignocellulose. For instance, the researchers have found that the straw mushroom Volvariella volvacea would flourish on wastes that contain only small quantities of lignin, because this species is unable to produce any of the lignin-degrading enzymes, and hence unable to draw nutrition from wastes with a large amount of lignin. At the same time, it has been found that this species produces a family of cellulolytic enzymes, some of which possess novel catalytic properties.

Another commercially important mushroom, Pleurotus sajor-caju, can produce a broad spectrum of lignocellulolytic enzymes, and this is reflected in its ability to grow on waste residues of widely varying composition.

Various sophisticated techniques including fast-protein liquid chromatography and confocal microscopy have been adopted to purify indivdual lignocellulolytic enzymes and to investigate how the enzymes are secreted by the mushroom hyphae (the slender filaments which constitute the vegetative growth of the fungus). Genes encoding the production of cellulolytic enzymes in Volvariella volvacea are now being isolated and cloned to determine the effects of over-production on mushroom growth and fruit body yields. Also, since this mushroom does not appear to produce lignin-degrading enzymes, future research will be directed at inserting into Volvariella volvacea the genes from other fungi encoding the production of enzymes involved in lignin transformation. This is expected to extend the range of lignocellulosic wastes on which the mushroom is able to grow, and to improve yields.

Research findings are expected to facilitate the design of programmes for improving strain selection using modern molecular biological and genetic engineering techniques. The long-term value and significance of the research lies in the potential to improve the bioconversion of the organic substrate by different mushrooms, thereby increasing biological efficiency and improving mushroom yield.

In addition, the knowledge gained will be of relevance to the cultivation of other mushrooms, in particular those that are currently being used as sources of pharmaceutically useful metabolites and food additives.

The digestibility of lignocellulosic materials such as cereal straws is directly related to their lignin contents. Improved fungal growth on lignocellulosic waste in a manner that removes selectively the lignin content may also make it feasible to use spent mushroom compost as a superior animal feed. Furthermore, the spent compost will still contain the enzymes produced by the mushrooms for lignin degradation. Since some of these enzymes have been found to be effective in breaking down pollutants such as DDT and various chemical dyes, the spent compost may also have potential for use in bioremediation systems.

In overall practical terms, a better understanding of the processes involved in the bioconversion of organic wastes by edible fungi has far-reaching economic, social and environmental implications.