A laboratory-scale pulper for leafy plant material

BIOTECHNOLOGY AND BIOENGINEERING VOL. XI, PAGES 517-528 (1969)

A Laboratory-Scale Pulper for Leafy Plant Material

31. N. G. DAVYS and N. W. PIRIE, Rothamsted Experimental Station, Harpenden, Herts, United Kingdom

Summary

A machine is described that makes, from 2 to 3 kg samples of leaf, a pulp comparable to that made by the large-scale equipment used in leaf protein extraction. It is therefore suitable for use in agronomic experiments on leaf protein yield.

INTRODUCTION

The extraction of leaf protein involves two processes: the disinte- gration of the leaf and the expression of the protein-containing juice. For many years, in this and other laboratories, domestic meat mincers have been used for the first and squeezing the pulp by hand in a cloth for the second. Domestic meat mincers, both hand and power operated, are simple and widely distributed. On some crops they give repeatable results, but on others the scroll does not pull the charge smoothly into the barrel of the machine. The charge then has to be pushed in; this introduces a variable factor. The fiber in some types of leaf gets tangled round the cutter and stops the flow of charge completely. Stops for disentanglement introduce more variability. These faults are less obtrusive when a 2- rather than a 4-bladed cutter is used, and they can be minimized by giving the scroll and cutter independent drives and so breaking up the com- pacted mass that sometimes forms because of the constant relative positions of the cutter and scroll. These unavoidable elements of uncertainty in results depending on the domestic mincer convinced us that statements about the extractability of protein from crop plants, grown in a different institutes, would not be comparable until the

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518 M. DAVYS AND N. PIRIE

pulp was made in a manner that was less influenced by the skill of the operator. Furthermore, the percentage of the total protein of the leaf that is liberated into the juice, is systematically greater when pulp is made in the full-scale (about 1 ton of crop per hour) pulper' rather than in a domestic mincer. The difference is not consistent. Some comparisons are given in Table I.

TABLE I Comparison between a Domestic Mincer and the Large-Scale Pulpera

Protein nitrogen extracted X 100 Total nitrogen in crop , %

Crop Mincer Luge pulper

Wheat Wheat Wheat Rye Rye Oata GlWB Grass Red clover Barley and lucerne

50 29 34 46 28 26 30 34 18 27

56 63 43 59 46 32 53 3? 30 39

*Pulps from crops of varied age and protein content were preeaed by the method described in the accompanying paper' and the percentage of the total nitrogen of the leaf that appears as protein nitrogen in the juice was measured.

The machine described here works on the same principle as the large-scale pulper. Pulps made on the two machines have similar textures and similar proportions of the total leaf protein are liberated from a range of crops. It can be used with as little as 2 kg of fresh crop (though it is better to use 3 kg) and so is well adapted to agronomic work on plots that have been subjected to different types of hus- bandry. It, and the press described in the accompanying paper12 is also adapted for making quantitative measurements in the field, either on crops growing far from a laboratory, or on such relatively uniform wild growths as swamp and water weeds.

BIOTECHNOLOGY AND BIOENGINEERING, VOL. XI, ISSUE 3

PULPER FOR LEAFY PLANT MATERIAL 519

Description

The machine is a hammer mill ui th fixed beaters and drum, which is unperforated except for holes in the periphery a t either end to allow tangential feed and discharge (Fig. 1). The drum diameter is smaller a t the feed end to encourage unidirectional flow of wind and charge.

At the top, the feed tray rests on the vertical feed tube (55 mm bore) which is u-elded to the inlet end of the drum (273 mm 0.d. X 6 mm wall commercial steel tube, 143 mm long). The main part of the drum, which is also of commercial steel tube (324 mm-0.d. X 6 mm wall, 300 mm long), has a slot 430 X 38 mm for discharge. Drum ends and flanges are of 6 mm steel plate, welded in place. The flange of the small drum has six 8 mm studs which are used to locate and, by means of wing nuts, to secure the large drum. The large drum protrudes .i mm through its flange to hold an O-ring, so the studs are shouldered to prevent overtightening of nuts during assembly. The small drum also has welded to i t two pieces of 50 x 50 x 7 mm steel angle to which are bolted the two pillow blocks (double row, self-aligning ball bearings, 50 mm bore) in which the shaft turns. These pieces of angle extend beyond the bearings and are joined by a bar with two

The general arrangement is shown in Figure 2.

Fig. 1 . The drum assembled.


Fig. 2. General arrangement for laboratory use.

slots. This is used when mounting the machine on a Land Rover (Fig. 3).

The shaft (50 mm dia., steel E N 24T) is driven by a belt and extends through the length of the drum. It carries two end plates which support the hub (electrically resistance-welded steel tube, 140 mm 0.d. x 8 mm wall). Each of these end plates incorporates blades which sweep the inside of the drum ends. The outer plate and the nuts (1” UNF, left hand) clamping it on the shaft end are shown in Figure 4; this plate (7 mm steel) has a parallel bore and, when the nuts are fully tightened, meets a shoulder on the shaft after dishing 0.1 mm. The resultant clamping force is transmitted along the hub to the inner end plate (12 mm steel) (obscured in the photograph) and jams this plate’s conical bore on a mating portion



of the shaft, thus both locating the hub longitudinally and trans- mitting the drive. There are 58 beaters in six rows parallel to the axis of the hub. They are arranged as opposing pairs (9 pairs in the small drum, 20 in the large, are made from standard hexagon head setscrews (19 mm) with threads removed from the working end, and are long enough to clear the inside of the drums by 2 mm. They are pushed through clearance holes in the hub tube from the inside and secured by nuts and Dub0 nylon washers on the outside. The screw thread extends beyond the nuts to allow fractions of nuts to be added as balance weights. The holes in adjacent rows are staggered so that the beater tracks slightly overlap and the whole of the drum interior is swept. In developing our large machine we found unswept bands of up to 19 mm width tolerable, but the machine now described is designed to do the maximum amount of useful work in a small volume. A rotor with the pins butt-welded to the tube was tried and seemed equally effec- tive: whether it is preferable depends on local convenience.

This is not a necessary feature.

Fig. 3. The pulper removed from its laboratory stand and mounted on a Land Rover.


Fig. 4. The drum removed to show the beaters on the hub.

The beaters (16 mm diameter) are set on opposite ends of a diameter of the hub and the successive diameters are 12.5 mm and 120" apart. Because the beaters are on each end of a diameter, successive rows of beaters are 60" apart as can be seen in Figure 4. The arrangement is clearly not symmetrical. With one direction of rotation the following beater (i.e., the one lagging by SOo) lies on the next diameter, with the reverse direction it lies on the next-but-one. With pulp of the usual texture, and a t the speeds that we find suitable, the machine runs more smoothly and economically with the former arrangement. When rotation is to be clockwise, looking at the free end of the hub, each successive pair of beaters, starting from the inner pair, is therefore advanced 120" clockwise.

The clearance between the drum ends and the blades on the hub ends is only 2 mm; there is therefore a central hole in the discharge end of the drum to accommodate the shaft nuts, and a removeable cap is fitted to allow the use of a tachometer when desired.



Drive in the Laboratory The machine is mounted, as in Figure 2, on a stand so that its center

line is about 1.1 m from the floor and the drum can be cantilevered conveniently over a bench or table. The drive is contained within the stand and consists of a 5 h.p. motor (1415 rev/min) with a 48 tooth pulley coupled by a steel-cored timing belt (Fenner 900 H 100) to one of four pulleys which fit the pulper shaft, 36, 28, 22, and 20 teeth giving driven speeds of 1990, 2450, 3200, and 3600 rev/min., respec- tively (the prototype was fitted with an infinitely variable vee belt drive, but it was found that fine control was unnecessary). The motor is mounted on a hinged platform to enable quick adjustment of belt tension when changing the pulper pulleys. The direction of rotation is clockwise looking on the discharge end of the hub. This is dictated by the rotation of the power take-off of the Land Rover.

On the prototype we fitted two inlet, tubes, one on either side of the vertical center line of the machine, so that, whereas wit,h one the entry of crop was opposed by the rotating beaters, with the other it was assisted. The latter arrangement proved inferior because crop tended to be pulled into the machine faster than it could be dealt with and both the feed rate and uniformity of pulping suffered.

Drive in the Field The machine without its stand can be fastened to the drawbar of a

Land Rover and driven from the power take-off (Fig. 3). The machine is then located by the connection to the drawbar and the overhung weight is supported by tie rods from a bar across the back of the vehicle body. Two ‘A’ section vee belts are used with pulleys giving a 2:l increase in speed from the power take-off to the pulper shaft. Speed adjustment is by Land Rover gear and throttle, and measurement by hand tachometer. For good speed regulation when pulping it is best to select a gear that requires high engine revs to produce the required pulper speed (e.g., 3600 rev/min can be achieved in 3rd gear, and so this should be used rather than 4th).

RESULTS

Operation The dimensions of the long narrow feed tube were chosen so as to

make i t impossible for anyone’s fingers to get down to the beaters.


This is prudent in a machine designed for routine use but i t un- doubtedly impedes feeding and a wider entry tube would permit faster feeding. A short or tangled crop may have to be pushed in with a wooden plunger shaped so that it cannot reach the beaters; other types of crop may be drawn in so that they need restraint. With a little experience, the pulper can be fed uniformly a t rates up to 1.5 kg/min. The first 0.5 kg may not be properly pulped and should be discarded or passed through the pulper a second time.

In routine work, the pulper speed appropriate to the texture of the crop will already be known. With a novel plant species, a start should be made a t 3200 rev/min. There is no advantage in running a t such a speed that all the fibers are cropped short; but no undamaged fragments of leaf lamina should be visible. They stand out clearly because they are paler than the pulped material. Until experience has been gained, 2 kg samples of the same batch of crop should be pulped a t a speed on each side of the one thought suitable, and the amount of protein released should be measured by the method described in the accompanying paper.2 Table I1 gives the result of such a trial and shows how little difference is made by some variation in speed.

TABLE I1 Effect of Speed of Rotation on the Extraction of Protein

Protein nitrogen extracted X 100 Total nitrogen in crop , %

Hub speed, rev/min Crop 1990 2450 3200 3600

Thistle Nettles Grass ,Mustard Wheat Wheat Field beans

- 38.0 24.1 -.

31.0 - 43.7 53.6 - - 32.8 40.2

35. 3 - 39.2 41.4 - 11.6 45.7 49.2

52.4 - 57.6 56.6 43.8 12.1 45.4 44.9

The outer drum of the pulper is designed so that all parts in con- tact with the crop are accessible for cleaning with a brush and jet of water. For thorough cleaning, it is simplest to undo the nuts on the end of the shaft and remove the rotor completely.

RIOTECHNOLOGY AND BIOENGINEERING, VOL. XI. ISSUE 3


Fractionation The output of a grinding mill or pulper that is not empty a t the end

of a run may not have the same composition as the material retained. The error that this introduces into analytical work becomes smaller the larger the amount of material passed through the machine. This pulper is designed for use on the limited amount of material that is the product of plots in agronomic experiments. The compositions of the 200 to 500 g of material retained, and the material discharged, have therefore been compared when 2 to 3 kg of crop was being pulped. We had expected the trend, if any, to be towards the retention of drier and more fibrous material; in fact there is no consistent difference between the pulps retained and discharged. When 3 kg is pulped, less than one-sixth is retained and the greatest difference we have found would not make a difference of more than 3% between the compositions of the crop and the discharged pulp. This discrepancy is not serious and it could be diminished by mixing the retained and discharged pulps, by working with larger quantities of crop, or by making the clearances within the pulper smaller so that less pulp is retained. If the last course were adopted the pulper would lose much of its simplicity and cheapness.

TABLE I11 Comparison of Dry Matter and Nitrogen Content of Discharged and Retained

Pulp in the Laboratory Pulper

The results are set out in Table 111.

Discharged pulp Retained pulp

Crop 11M % N as '% DM DM % N a s % D M

Barley Bracken Buckwheat Cocksfoot Cow parsley Field beans Field beans Grass Mustard Pea haulm Rye Rye Sweet clover Thistle Wheat Wheat

29.2 14.9 10.4 11.9 11.5 16.2 15.6 21.0 18.2 14.0 9.8

13.4 12.0 8.3

12.0 12.2

1.7 3 . 1 2.7 2.0 3 . 5 2.0 2.6 1.8 2.0 2.8 2.8 4.3 2 .3 2.8 5.2 2.8

26.7 15.5 9 .8

11.8 11.6 14.6 14.4 19.8 15.0 14.1 10.7 13.6 11.4 10.0 13.0 12.7

2.2 3.3 3.1 2.0 3 . G 2.2 3.0 1.9 2.3 3.1 3.0 4 .3 2.3 3 . 0 5.0 3.0


Comparison with the Large-Scale Pulper : Extraction

A pulper of the general character of the one we have described' is not adapted for use with less than 100 kg of crop. On the other hand, measurements of protein extractability made in the laboratory by pulping leaves in a mortar, domestic mincer, or high-speed macera- tor, do not simulate the action of a large-scale machine and could give misleading ideas about extractability. Success in designing a pulper for agronomic work is assessed by the extent t o which the percentage of protein extracted by i t agrees with that extracted by the large machine. Some comparisons on crops harvested during May and June 1968 are assembled in Table IV. The agreement, though not perfect, is considerably better than in Table I.

TABLE IV

Comparison of Extraction of Protein N from Pulp Produced by Large-Scale and Laboratory Pulpers

Protein nitrogen extracted X 100 Total nitrogen in crop 9 %

Crop Large scale Laboratory

67.4 49.1 54 .4 63 .3 59 .9 51 .3 53 .4 63.7 46.6 44.9

67.6 52.2 57 .4 57 .6 56.1 56.2 51 .3 61.7 44.6 53.0

Comparison with the Large-Scale Pulper : Power Consumption

Although this machine is intended as an agronomic tool rather than a means of producing leaf protein, it seemed possible that com- parative measurements of the amount of work done on the crop could lead to improvements in the design of large-scale pulpers. The power consumed while 50 kg of three different crops were fed into the large machine, and 15 into the small one, was therefore measured. The



results, after subtracting the idling power, are shown in Table V. These results are admittedly crude, but the economy is so striking that i t suggests that a great deal of power is being expended uselessly in the large machine and that further experimentation on beater spacing and design is essential before any attempt is made to esti- mate the costs of leaf protein extraction.

TABLE V Comparison of Work Done on Crop by Large-Scale and Laboratory Pulpers

Power, kwh/ton of crop

Crop Large scale Laboratory

Rye 31 .5 12.7 Wheat 37 .5 18.0 Wheat 41.5 17.4 Rye grass 63.0 20.7

DISCUSSION

The nonuniformity of the products of field experiments defines a minimum amount of crop that should be pulped: the area occupied by a series of plots (and guard-strips) sown at different seed-rates, fertilized differently, and harvested at different ages imposes a practical limit on the size of each sample. The lower limit of 2 to 3 kg needed for the laboratory scale pulper described here was dictated partly by these considerations and partly by mechanics. Although designed to be as small as possible, the laboratory pulper could, in the course of an hour, pulp 50 to 100 kg of leaf. There is no obvious objection to making a somewhat larger machine with the same basic design. Whether this would retain the economy commented on above is a matter for experiment. If a continuous press, perhaps similar to the one we have already de~cr ibed ,~ were made of a size suited to this laboratory pulper, the unit would have the same capacity as our batch extractor or “village unit)’.4 It would not be as simple, and it would not be as adaptable to a diversity of prime movers, but we think i t would be more convenient and efficient than the “village unit” for making protein in 5 to 10 kg quantities in a research institute.


We thank Vauxhall Motors Ltd (Luton) for their cooperation in balancing the rotors, and Dr. J. Ahmad and Mr. G. Street for the analytical results. The comparisons between different aspects of design were made possible by grants from the Royal Society, through the International Biological Program, for extraction units that will be used at Aurangabad and Ibadan.

References 1. M. N. G. Davys and N. W. Pirie, Engineering, Lond., 190, 274 (1960). 2. M. N. G. Davys, N. W. Pirie, and G. Street, Biotechnol. Bioeng., 11,000 (1969). 3. M. N. G. Davys and N. W. Pirie, J. Agric. Engng Res., 10 (2), 142 (1965). 4. M. N. G. Davys and N. W. Pirie, J . Agric. Engng Res., 8 (l), 70 (1963).

Received February 3, 1969

BIOTECHNOLOGY AND BIOENGINEERING, VOL. XI , ISSUE 3

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A laboratory-scale pulper for leafy plant material