Leaf protein, a potential substitute for fishmeal and soybean meal, can be extracted from plant species hitherto rejected as unpalatable
by N.W. Pirie
Although the early papers advocating large-scale extraction of leaf protein (LP) usually contained some such phrase as “for use in feeding people and other non-ruminants,” their tone showed that more importance was attached to human than to animal use. It was obvious from work in the laboratory that LP was palatable when made with reasonable skill; it therefore seemed wasteful to feed animals, in any country where food was scarce, on something people could have eaten. The suggestion was sometimes made that work should start on animal-grade material and that better quality LP should later be made as human food. The objection to that suggestion is that difficulties were foreseen in winning acceptance as human food for something that had got the stigma of being animal feed. A curious reason for delayed recognition of the potentialities of LP as a feed for non-ruminant animals was the near unanimity of international organizations and governments in asserting that LP would be very expensive. No one with any experience of the grass-drying industry, and of the general character of the processes used in making LP, agreed with that conclusion. But it was reiterated for many years.
N.W. Pirie is at the Rothamsted Experimental Station, Harpenden, Herts. AL5 2JQ, United Kingdom.
When large-scale production of LP was advocated in 1942, one of the points emphasized was that the residue from which protein had been partially extracted would be a valuable fodder for ruminants, and that during protein extraction most of the water initially present in the crop is removed. In countries where the need for conserved fodder in winter limits the number of animals available to eat the spring flush, this removal of water is important because of the expense of evaporating water. When a crop is 10 percent dry matter (DM), 9 t of water have to be evaporated to get 1 t of DM; at 20 percent DM, 4 t; at 30 percent DM, 2.3 t; and at 40 percent DM, only 1.5 t. If drying equipment is to be operated continuously, it will sometimes be necessary to work with material at the one extreme because of rain or morning dew. Efficient pressing can produce an extracted residue at the other extreme. The recent increase in the cost of various forms of energy has focused attention on this method of removing water economically from fodder that is being conserved. The “grass” drying industry in industrialized countries usually operates on such a scale that, if a significant section of it adopted this economical method of working, the amount of LP produced would greatly exceed the amount that people in these countries could probably be persuaded to eat. As has often happened in the past when a process produces two or more products, with the passage of time their relative value inverts. Initially, LP was thought of as the main product with the residue as a useful by-product; now LP is often thought of as the by-product for which uses are sought.
When LP is the primary product, it is advantageous to extract as much of it as possible. When partly dewatered residue is thought of as the primary product, it is sometimes argued that there are advantages in less complete extraction. This is an illusion. Except when there is surface water on the crop, or when the crop has been heated before processing, or when the species used contains a large proportion of sappy stems, protein and water come out pari passu. Efficient extraction is therefore a usual concomitant of effective dewatering. To meet existing feeding stuffs quality standards it may be necessary to extract protein inefficiently, i.e. to make a residue containing> 2 percent N (on the dry matter) instead of 1.5 percent. Standards will however ultimately be brought into line with technical possibilities. Protein extracted for non-ruminant feed is more valuable than protein left unextracted as ruminant fodder.
The equipment used so far in attempts at commercial production was designed for handling sugarcane, fish, or cooked oilseeds. In these materials, the fluid is loosely confined, or is present as a free continuous phase, and has simply to be pressed out. The position is quite different with leaves; in them the juice is mainly contained in cells from which it has to be released by rubbing before it is worth while applying pressure. The economics of the process will be falsely assessed until more effort is put into designing equipment for this precise purpose.
Use of unfractionated leaf extracts
When LP is being made on a farm scale in conjunction with a small grass drying unit, it is convenient to feed the extracted juice directly to pigs or chickens without any fractionation. Recent annual reports from research institutes in Ireland, the United Kingdom and the U.S.S.R. comment briefly on satisfactory results with calves and pigs. Some difficulties may however be foreseen. Depending on the age of the crop and whether it is harvested with dew or rain on it, or at the end of a sunny afternoon, the protein content of the juice can vary by a factor of 5 and the DM content by a factor of 3. This complicates dietary compounding. All juices undergo coagulation and proteolysis after extraction — but at different rates. The products of proteolysis probably have the same nutritive value as the original protein, but the presence of curd could complicate juice distribution. Furthermore, some components of the juice from all species are of doubtful nutritional value in non-ruminants, and there are components in some species that are toxic or have unattractive flavours. For all these reasons, some fractionation of the juice seems advantageous.
Use of partly fractionated extracts
LP can be coagulated by acidification: it is usually coagulated by heating the extracted juice suddenly to > 70°. Sudden heating produces a compact, coherent curd that is easy to handle and it inactivates leaf enzymes before they have time to act. This minimizes proteolysis and, in leaves rich in chlorophyllase, the formation of pheophorbide. Pheophorbide in LP made from slowly heated lucerne (Medicago sativa) juice made pigs photosensitive. This is a very slight hazard with quick heating to 70°; it can be avoided completely by heating to 100°.
Part of the coagulum floats. It can all be made to float by bubbling air (or preferably N2) into the heated juice. Alternatively, it can all be made to sink by deaeration. The volume of compact sediment obviously depends on the protein content of the original extract. but its composition is relatively constant. This facilitates the accurate compounding of liquid feeds for calves or pigs. Furthermore, much of the material of questionable nutritional value for non-ruminants is removed from the sediment and, because of the diminution in volume, the wet sediment can be more economically preserved with acid or sulphite should short-term preservation be necessary.
Use of dried leaf protein
Heat-coagulated LP can be easily collected on a filter: acid-coagulated LP is troublesome on a filter but can be sedimented in a centrifuge. The filter cake can be preserved with the usual bacteriostatic agents. For prolonged conservation, dry LP is preferred. Unless care is taken, LP can be damaged by drying; in the rather unsystematic comparisons that have been made so far, the observed differences in nutritional value are probably more often the consequence of differences in drying conditions than in the use of different plant species. When dried in air, or in an oven, LP becomes hard and nearly black. The product is more attractive if it is partly dried and then ground finely before the drying is completed. However, when it is dried there is risk of damage through Maillard reactions if the filter cake is not washed to remove most of the sugars present in the original leaf extract. Carotene and xanthophyll are valuable components of LP whether it is used as a human or an animal food. As is well known, they are less rapidly destroyed during storage if the material is made slightly alkaline and is protected from access of air. Drying should be avoided whenever possible.
Unit used in many institutes for small-scale production of leaf protein. Two to three kg of crop per minute is fed from the tray into the pulper (middle of photograph). Pulp flies out to the right and is pressed between the tensioned belt and the perforated pulley at the extreme right. Extracted fibre is removed by a motor-driven auger to the left of the pulley. The belt is driven by the spring-loaded drum at its left-hand end.
General layout of the unit at the National Institute for Research in Dairying (Shinfield, Reading, United Kingdom) able to process 5 t (fresh weight) of crop per hour. The crop in the trailer on the left is carried by an elevator into the pulper in the centre; from this it goes into the belt-press. The extracted fibre is taken by the elevator on the right to a trailer for drying.
Woodham (in Pirie, 1971) assembled the results of experiments on laboratory animals and also on pigs and poultry. These showed, as would be expected from the amino acid analyses, that LP is a satisfactory substitute for fishmeal and is a little better than groundnut or soybean meal. More trials on the same species, and on fish, confirm the early results. The limiting amino acid appears to be methionine. There is some evidence that part of the methionine in LP is unavailable; more work on the extent to which this is the result of complex formation during separation and drying is urgently needed.
When made carefully, for human consumption, from suitable species, LP contains 9 to 11 percent N, i.e. it has 56 to 68 percent protein. When made for use as animal feed it seldom contains more than 50 percent because the fibre is usually less carefully removed by straining the initial extract, and surface dust is less completely removed from the crop before it is extracted. Most of the non-protein material is lipid (20 to 25 percent) and it is predominantly unsaturated. Because the lipid is a useful source of energy and essential unsaturated fatty acids, it would be unwise to remove chlorophyll and its breakdown products by solvent extraction. They appear to be harmless if pheophorbide formation is prevented. The amount of nucleic acid in LP depends on the interval between extraction and heat coagulation; if an extract from very young leaves is coagulated after a few minutes, the LP will contain 1 to 2 percent nucleic acid, but little will be present if leaf ribonuclease is allowed time to act.
Use of “whey”
Both for economy and to avoid local pollution it will be essential in largescale production to make use of the fluid separated from the coagulum. The simplest procedure is to take it back to the fields from which the crop came and so make use of the NPK in it. Depending on the weather, the age of the crop, and the conditions of coagulation, it contains 10 to 50 g dry matter per litre — about half of it carbohydrate. “Whey” at the more concentrated end of the range may be worth drying for use as animal feed. “Whey” has been used in several smallscale trials as a microbial substrate. This is probably the best ultimate use for it; detailed research will have to await regular supplies of “Whey” with predictable composition.
Sources of leaf
Several principles have to be borne in mind in selecting species for study. Protein extracts more readily from soft lush leaves than from those that are fibrous and dry; even when pulped with added alkali, acid leaves do not extract so well as those that give neutral extracts; leaves that give glutinous or slimy extracts are difficult to handle. It is obviously advisable to use leaves that can be harvested mechanically from a species that will regrow after cutting; this probably excludes tree leaves, though coppiced trees have potentialities. Equally obviously, mixed weeds from untended ground are useless—if the growth could be harvested mechanically and if the ground is being manured to ensure an adequate yield, it would be better to use the ground to grow a desirable species. Water weeds are an exception to that generalization; they often grow luxuriantly, but little is known about the extractability of the protein in them. Good yields of protein have come from leaves that are by-products, e.g. beans (Phaseolus sp. and Vicia faba), jute (Corchorus sp.), peas (Pisum sativum), ramie (Boehmeria nivea), potato (Solanum tuberosum) and sugar beet (Beta vulgaris).
In suitable places, large-scale production could depend on water weeds and by-products; it will more often depend on a crop grown specially for the purpose. Commercial thinking is dominated by the idea that lucerne is the best crop although there is successful semi-commercial production from grasses in Ireland and Scotland; they are suggested as sources in Southcast Asia. There is scope here for a great deal of research; the process of extraction enables species to be used which have hitherto been rejected as unpalatable because of texture or flavour, and novel types of cultivated plant may ultimately be used. In Countries with a long cold winter, one of the more valuable qualities in a crop destined for LP production is the ability to start growing early in spring Hudson, 1976).
There is work on LP in research institutes in at least sixteen countries. It is difficult to get reliable information on commercial interest. France-Lu-Zerne (Châlons-sur-Marne) appears to
Simple, experimental juice extractor seen from above. The auger feeds the crop at 1 to 2 kg per minute into a cylinder which is made of bars between which the juice can escape and where the leaves are disintegrated and pressed. When very soft leaves are used, the extra cover that is shown has to be attached to keep the leaf pulp from squeezing back to the left.
be the only company that is selling LP. As much importance is attached, in the advertising literature, to the carotene and xanthophyll in the product as to the protein. LP is about to come on to the market from two companies in the United Kingdom and possibly from companies in Japan, Sweden and the United States.
The main reason for this commercial interest is, as I have said, the importance attached to the partial dewatering of the residuce. Recent increases in the price of familiar protein concentrates, such as fish and soybean meal, have led to a search for possible alternatives. The annual yield of dry extracted protein from forage crops grown in the United Kingdom can be 2 t per hectare; in India it can be 3 t. These are experimental yields and it may not be economically realistic to strive for such yields in practice because they entail the use of large amounts of fertilizer. Nevertheless, the basic physiological point remains valid: in similar conditions, a forage crop that is harvested in such a manner that it remains photosynthetically active throughout most of the year is likely to outyield a seed crop that is relatively inactive photosynthetically while it is ripening. Techniques for separating forage into a fraction suited to ruminants and one suited to nonruminants seem therefore to have great potentialities.
Hudson, J.P. 1976. Food crops for the future. Proc. R. Soc. Arts, 124:572–585.
Pirie, N.W., ed. 1971. Leaf protein: its agronomy, preparation, quality and use. Oxford, Blackwell. 192 p.
Pirie, N.W. 1975. The effect of processing conditions on the quality of leaf protein. In Protein nutritional quality of foods and feeds, ed. by M. Friedman, p. 341–354. New York, Marcel Dekker.
Pirie, N.W. 1977. The extended use of fractionation processes. Phil. Trans. R. Soc. (B). (In press)