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Perspectives on bioconversion of organic residues for rural communities

Introduction
Sources of available nutrients
The most suitable materials for bioconversion
Characteristics of residues
Bioconversion systems
Physical and chemical treatments
Microbial conversion
The animal conversion phase
Summary
References

P. van der Wal

Institute for Animal Nutrition Research (ILOB), Wageningen, Netherlands

Introduction

The shortage of food in the world recently prompted the Director General of FAO, Edouard Saouma, to reiterate the special need for food in densely populated rural areas of developing countries (1). We need new food sources, and we should not restrict ourselves to increasing supplies of existing ones to meet this demand (2). In our attempts to develop these potentials we should, however, avoid theoretical overkills (3).

In this paper, I shall try to take these points into account while studying the question of whether new sources can be tapped to a significant extent, and whether these new rural sources can provide food that is affordable, whole some, and acceptable organoleptically. In view of the latter point, I would like to emphasize that, especially in rural areas, consumers are extremely critical. This is by no means limited to developing countries only. In the Netherlands, too, children are taught, "What a farmer is not familiar with, he does not eat."

Sources of available nutrients

An inventory of nutrient sources is rather illuminating (see Figure 1). There are approximately 9,000 million ha of land in the world. Close to 50 per cent of this area consists of forests and shrub lands. Another 35 per cent is pasture and grassland, and 15 per cent is arable land. Of the produce grown on arable land, by far the major part is discarded as residue. This means that approximately 95 per cent of the land areas mentioned in Figure 1 could provide a source of nutrients yet untapped in the sense that their agricultural residues are not being utilized directly for human consumption.

Figure. 1. Land Production of Potential Nutrients

Food science and technology at present concentrate mainly on the 5 per cent of the annual production of potential nutrients that can be used after relatively simple forms of processing, such as cooking or baking. The 95 per cent residue needs considerable processing, be it physical, chemical, or via some form of bioconversion before it can be turned into suitable feed or food. This paper will concentrate on the products that we refer to as organic residues. They are what is left after agricultural production, sometimes left behind on the land (straw), on the farm (manure), or in agro-industries. They are relatively easily available for conversion into food.

Figure 2 shows some of the most important agricultural crops. There is a line along which we can divide these products. On the left side we find the percentage considered to be the main food component of the crops. They can be used without much processing by the human consumer (grain, oil, starch, vegetable protein). On the right side are the residues that comprise approximately two-thirds of the total production.

Figure. 2. Agricultural Crops

Two routes are available to convert these residues into useful products, as indicated in Figure 2: through feeding to animals, or by some industrial process.

Because these residues form the major part of agricultural production, their conversion into food via efficient and safe systems deserves far more attention than we have paid it so far.

Tremendous efforts are required to make such new systems operational. Therefore, we should concentrate on a limited number of the most promising ones and approach them in a multi-disciplinary fashion.

Those of us involved in developing new conversion systems will agree that the creation of acceptable food from novel sources usually requires animal conversion as a last step. Microbial conversion alone produces, in most cases, biomass, e.g., single-cell protein that is not accepted as a food by most consumers.

The most suitable materials for bioconversion

To identify the most suitable areas in which bioconversion of residues may be important and therefore worthwhile, the residues are divided into three categories, each susceptible to a common method of bioconversion. Cellulose-rich substrates form a total of more than 1,800 million tons annually of renewable resources (Table 1). They are to a great extent found in Asia, and it is therefore not surprising that they consist primarily of rice straw. The present use is often none; in some areas it is used for fuel. Straw could form an extensive base for reeding ruminants. There is no doubt that bioconversion would greatly improve the use of these materials, particularly in rural areas. To what extent in vitro SCP (single-cell protein) production can play a major role here depends greatly on local circumstances and on the results of research efforts in this field.

TABLE 1. Straw Production, 1974 (in millions of tons)

The second major residue category consists of starchy and sugary wastes (Table 2). Because their carbohydrates are more easily accessible, they require a somewhat less difficult form of SCP bioconversion. As shown in Table 2, cassava and sugar beets provide the greatest amount of residue. High productivity in relatively poor soil has made cassava a popular staple food, especially in countries most in need of food.

TABLE 2. Starchy, Sugary Residues (in millions of tons)

A third category of residue is manure, a by-product of all animal production systems. It is calculated that approximately 1,900 million tons of manure are produced per year.

Characteristics of residues

Residues are not used as foods because they are inedible without some form of bioconversion. Table 3 shows that the chemical composition of most residues is not well balanced. Straw contains 48 per cent crude fibre and 3 per cent crude protein.

TABLE 3. Chemical Composition (% dry matter) of Various Residues

* O. Kellner and M. Becker, Grundüge der Futterungslehre (1959).

** CVB Cattle Feed Table.

*** F. de Boer and A. Steg (Hoorn), report, "Megista mestdag," part II.

It is hardly a good product for human consumption. Grass has a better composition, but if it were to be used for monogastric consumers like man, there would still be severe problems because of the relatively high crude fibre content. Poor digestibility is another reason for rejecting unprocessed residues.

Table 4 gives the digestibility coefficients of some residues for ruminants. The organic matter of straw is only 38 per cent digestible even for ruminants. For monogastric organisms like man, poultry, and pigs, the coefficients are even lower. Grass is more digestible but less suitable for monogastrics. Digestibility of citrus and animal wastes is reasonable, but not particularly good.

TABLE 4. Digestibility Coefficients in Ruminants

* O. Kellner and M. Becker, Grundzüge der Futterungslehre (1959).

** CVB Cattle Feed Table.

*** F. de Boer and A. Steg (Hoorn), report, "Megista mestdag," part II.

A number of other reasons may make a residue undesirable. Logistic aspects and low drymatter content may be expensive to overcome. Seasonal variability often makes it difficult to manage the material by advanced technology. Chemical and microbial contamination and organoleptic or psychological unacceptability may preclude the use of some residues as food. The above characteristics all present problems that must be overcome if a residue is to be converted to food.

Bioconversion systems

Figure 3 shows the pathways for the bioconversion of residues into food. In the upper box, cellulose-rich, starchy and sugary residues, and animal manure are represented. The lower box shows the goal of bioconversion systems: food for man. In most cases this will be in the form of meat, milk, or eggs.

It is frequently said that there seems to be a certain competition for food between animals and man. One easily overlooks that this is the exception rather than the rule. Most animals are kept for the purpose of producing food for man. They are mainly converters (biological ones) of products inedible by man. As such, they do not compete significantly for human food supplies.

We have many options for making food from wastes. The ones by-passing the animals are represented by the dotted lines in Figure 3. Direct use as food is non-existent, otherwise the product would not be a waste. Chemical and physical treatments of waste seldom create food. Microbial conversion, either direct or after treatment, permits mushrooms to grow and favours the production of fermented oilseed cakes. Unfortunately, this method is not yet used for the conversion of millions of tons of residues to any significant degree.

The solid lines on the right side of the figure represent bioconversion systems making use of animals. Grass, straw, and quite an amount of poor-quality roughage follow the direct route to food. Animal feeds may also be wastes that are treated via chemical or physical means and/or by microbial conversion, which the animal also converts to food. The potential and efficiency of bioconversion should be exploited to a much greater degree. In general, the right side of Figure 3 shows the most realistic potential for bioconversion of the bulk of residues into food.

Figure. 3. Bioconversion of Residues into Food

Physical and chemical treatments

The alkali treatment of cellulose-rich materials like straw (1,800 million tons in rural areas) deserves special attention (Table 5). Digestibility for ruminants improves from 45 to 68 per cent when straw is treated. What does that imply? in the major rural areas of India, untreated rice straw provides hardly enough nutrients to maintain the live weight of cattle; in other words, it barely covers the animals' maintenance requirements Assuming that 90 per cent of the feed is used for maintenance, 10 per cent is available for increasing weight, for producing offspring, and for milk production. If digestibility were increased by 50 per cent, it would provide, as in the Table 5 example, a fivefold increase in nutrients available for meat and milk production.

TABLE 5. Effect of Treatment on Straw Digestion in Sheep

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