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Bioconversion of fruit and vegetable wastes

Robert Stanton

Botany Department, Faculty of Science, Malaya University, Kuala Lumpur, Malaysia

Evidence is accumulating that people have developed ingenious methods of fermentation to overcome inherent indigestibility, or toxicity, of the protective (fruit-coat constituents) and not-to-be-eaten components of plants by a combination of physical (to destroy the antimicrobial factors) and microbial methods, with or without leaching or throwing away the cooking wastes. These combined treatments act to dispose of alkaloids, glycosides (CN- and S-compound-containing), phenolics (bitterness principles) and their glycosides, saponins, sterols, toxic peptides, and anti-enzymatic factors, to name some of the commonly found compounds inhibiting consumption

Within limits, a useful method of ascertaining the causes of consumption inhibition is to bite into the normally uneaten portion, though it is advisable to be aware of the general content of secondary compounds in the plant family before doing so. The message will be clear to the human predator of acute astringency (e.g., capsule of the mangosteen, Garcinia mangostona) or acute bitterness (e.g., seed and rind of the papaya, Carica papaya). Other fruits are simply tough and full of aromatic compounds, though they are used by indigenous people. Examples include Mangifera spp. and Baccaurea spp. - the species B. griffithii is deliberately used as a flavouring and fermentation control agent. The pericarp of the nutmeg, Myristica fragans, is boiled and sweetened as a sweetmeat, but it is doubtful whether it is wise to consume it in large quantity.

Anaerobic fermentation is frequently employed concomitantly with purification by washing and decanting. The classical example of this is the treatment, before eating, of yams (Dioscorea spp.) and keladis, taros, cocoyams (aroid yams of the genera Colocasia, Xanthosoma, Amorphophallus), tapioca, cassava (Manihot esculenta Cranz), and the so-called cabbages of palms. Monkeys are fond of this last-named delicacy, but some species of cultivated palms are avoided because nature adds to the normal inhibitor - bundles of needlelike crystals of oxalic acid - a coating of highly toxic protein The needles serve to inject the protein into the mucous membrane of the mouth For many of the above examples, pectinolytic and mild cellulolytic bacterial action separates and weakens the starchcontaining parenchyma cells. Beta-glucosidases split the glucosides (the bacteria use the sugar), and the nitrites and sulphur compounds and degraded; other molecules may be eluted. The CN radical is capable of being used as both a nitrogen and carbon source by certain yeasts, but is normally disposed of. The main cause of a high incidence of CN toxicity is due to absorption of the acid through the skin by women preparing the food by grating the raw material for fermentation. Neuropathy symptoms (optical, peripheral) are linked with a low sulphur-amino acid diet where the body is incapable of detoxifying CN by the thiocyanate pathway.

Digestive disturbance due to sulphur compounds - thioglycosides, thioalkaloids, and thioethers - is widespread, and tolerance seems to vary greatly among individuals. The function of these compounds in plants is commonly bacteriostatic, though they also function as repellents and attackers. Addiction to durians (Durio zibethinus Murray) may be due to these compounds, and the so-called aphrodisiac effect is, I suspect, via a mild irritant action on vascular and mucous tissue. The sulphur compounds are dispersed in pickling, though their microbiostatic properties appear to have been unwittingly employed in pickle fermentations (e.g., the Korean kimchi). This is pickled Brassica sinensis cabbage with added chili peppers (Capsicum frutescens), pepper, ginger, and garlic, all containing bacterially active compounds.

In the fermentation of wastes from fruit and vegetables, fats, waxes, and longer-chain fatty acids may also play a role in inhibiting the fermentation. The situation is confused if the material is cooked prior to fermentation because without critical experimentation it may be difficult to distinguish between the effects of heat, or removal of the wax or fat, and the presence of microbial inhibitors in the original living plant cells.

State of the art of bioconversion

Regarding bioconversion, from the examples in the above review, it may be observed that:

a. by-product processing of fruit and vegetable wastes is biochemically strongly species-dependent;

b. the species may be ecosystem specific, though many have been subjected in recent years to inter-regional transfer;

c. substantial processing skill has been developed at the place of origin, but may not have been transferred with the original plant transfer. Thus, the apparent social reasons for nonuse may be lack of transfer of the appropriate technique.

Technical transfer

Inter-regional technical transfer has lagged behind crop transfer, and removal of biochemical contraints to use of residues may be effected by following the crop transfer with appropriate technical transfer.

The sophistication of the indigenous fermentation technology, although a process may appear simple, must not be under-rated, and technical transfer may be easiest among established communities having homologous crops. Because it is only a homology, the transfer may not be automatic and a block to full harvest utilization may occur. That is, the two plants of the agronomically homologous pair may occupy similar agro-ecological niches, but the plants' biochemistry, enabling them to occupy the respective homologous biological niches, may differ.

By contrast, the robustness of yeast fermentation has resulted in successful new developments in village technology in safe alcohol distillations in places as far apart as Nigeria and the Philippines. Even in Europe the itinerant technician (the distiller) takes his apparatus to the substrate, producing the schnapps for the individual farmer from his own brew.

At present this trend in domestic technology is most highly developed in Japan, and the Japanese experience is worth studying in this context. If you bake your own bread, it does not matter if the commercial bakers go on strike. Bringing the process to the raw material may be a good energy budgeting and waste elimination practice

In my view, the marriage of industrial technology in microbiology, enzymology, small-scale equipment, and the village or domestic processer has only just begun, and one may foresee a continued development of the concept of industrially produced starters and processing aids for the wide range of village-level processes.

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