Figure 8.4 The steps in the initial processing of aloe gel. A: Harvest of leaves and transport to preliminary processing station or integrated processing plant. B: Leaf washing including initial wash in sodium hypochlorite solution (200 ppm), rinsing in 20 ppm hypochlorite solution, trimming off butts and tips, and culling of diseased or damaged leaves. This produces (C), washed and trimmed leaves. Next, the rind is removed, either by hand (D) or mechanically (E), producing gel fillets (F). These fillets are then lightly ground and cellulosic fibers removed by passage through stainless steel screen with 0.25 mm openings (G). This depulping separates pulp (H) from crude gel (I).
oriented toward removing organisms from the outside of the cuticular surface. The schematic diagram above shows the usual sequence of a wash tank with a 200 ppm hypochlorite sanitizing solution, or other suitable sanitizing agent. This feeds a rinsing line via a steel-link conveyor belt. The rinsing portion of the conveyor belt further washes the leaves with a 20 ppm hypochlorite spray. This spraying is followed by culling.
Conditions where mud is a problem can be addressed by increasing the size of the washing tank. A soft, brush scrubber can also be added to the rinsing line to aid in soil removal. However, often the best solution is to add a non-sanitizing pre-wash tank of about half the capacity of the main sanitizing wash tank. This primarily removes mud or adherent sand, sparing the wash tank for its sanitizing function. Although 200ppm hypochlorite might seem like a lot of oxidative power, it must be remembered that the humic matter in the soil washed from the leaves constitutes a significant input of reducing material. If there is significant mud adherent to the leaves, this hypochlorite oxidizer will quickly be consumed. Thus, under muddy conditions, a prewash tank avoids the necessity of frequent additions of hypochlorite. For monitoring the oxidizing power of the wash tank, a chlorine test kit, designed for use with swimming pools, offers a low cost method to monitor the hypochlorite adequacy of the wash tank.
Downstream from the rinsing assemblage (Figure 8.4, Process B) should be an area where damaged or diseased leaves can be culled out. Culling should be performed by a different crew than that responsible for filleting. This is because the production pressure on the filleting crew is to maximize throughput. If they are also responsible for a step crucial to maintaining quality, they will be pressured to discard fewer leaves in order to maintain speed and increase the amount of product. It is best to have separate groups of workers perform antithetical functions, culling with a goal of maximizing leaf quality versus filleting with a goal of maximizing production of gel fillets. Asking a single group to perform functions with conflicting goals usually results in a compromise wherein quality is sacrificed for throughput.
The product of this process is 'washed leaf (Figure 8.4, Product C). The overwhelming majority of washed leaves are further processed by either filleting into gel or grinding into whole leaf extract. However, within the last decade, a market has evolved for washed leaves. These are vended for approximately $2.00 per lb (US $4.00/kg) in the fresh, refrigerated state. At first these leaves were sold only in Hispanic specialty stores but at the time of writing they were being increasingly found in major chain grocery stores in the larger cities in Texas and the Northeast USA.
The next step in processing is the production of the gel fillet (Figure 8.4, Product F). The RGV variety of A. barbadensis is distinguished from the other cultivars of the species by its large leaves. These have been known to approach 1,300 grams in locations where rainfall is abundant (200cm per annum), nitrogen is sufficient (0.50% +) and the soil is well drained. These leaves are large enough to be efficiently stripped of their rind to produce gel fillets1. In most of the other species of Aloe, anthraquinone, chromones, etc. are harvested by:-
(i) incising the leaves and collecting the latex that exudes (as with the South African aloes);
(ii) harvesting the entire plant including roots, drying it, and either enclosing the ground, whole plant in a capsule or extracting the powder (in the Orient, especially A. arborescens);
(iii) grinding the freshly harvested leaves and processing them to extract the 'gel'1 (especially with A. barbadensis or A. arborescens), also called Whole Leaf Extract (WLE); and,
(iv) grinding and extracting the rind (a byproduct of fillet production in A. barbadensis).
However, it is only A. barbadensis leaves that are large enough to produce gel fillet economically. Anatomically, the leaf of A. barbadensis can be divided into three zones: the rind, the mesophyll, and the gel1 (Figure 8.5).
1 The use of the term 'gel' is a matter of contention between the aloe industry and the scientific community. Properly described, the scientific community reserves the use of the term gel to those materials with pseudoplasticity. Generally, these materials have significant viscosity in aqueous solution in the range of 0.05 to 0.5 g per 100 ml (0.05 to 0.5% total solids). The IASC defines 'aloe gel' as a water-white extract of A. barbadensis without regard to pseudoplasticity. The FDA recognizes the term 'gel' to the meaning usually employed by the scientific community rather than that promulgated by the IASC. We will use the term gel to connotate the usual scientific meaning and 'gel' to connotate the IASC meaning.
The rind contains multiple layers (see also Sheet 1, Figure 1 in McAnalley '935 patent, 1988). First is the outermost waxy cuticle, which acts as one of the barriers against moisture loss. Just beneath the waxy cuticle lies a region wherein reside the aloe-associated bacteria. Below this level is the chlorophyll-rich rind region where the bulk of photosynthesis occurs. The rind is rich in oxalic acid. Just below the rind and stripping away with it lies the mesophyll. This contains the xylem and phloem vascular bundles. The mesophyll contains the plant's highest concentration of anthraquinones and chromones. We presume that these secondary metabolites function as anti-feedant compounds. When the plant is well hydrated the mesophyll can be easily stripped away with the rind from the gel fillet if attention is paid to detail. Thus the 'rind' fraction (Figure 8.4, 'R' & 'M'), when meticulously prepared, contains both rind and mesophyl elements. This material is second only to the exudate as a source of biologically rich anthraquinones and chromones. However, unlike this exudate, rind is generally discarded as a by-product and is thus low in cost. Furthermore, if properly collected, without excessive exposure to oxygen, light and heat, isolated rind by-product, with mesophyll can be extracted without the tetrahydroxyanthraquinones being converted to red oxidation products or brown/black polyphenolic polymers.
The gel is in the inner parenchyma portion of the A. barbadensis leaf (Figure 8.5, G). When properly prepared it contains two elements. One is the liquid portion of the gel (Figure 8.4, Product I, defined by the IASC and ARF parameters) and the other is the cellulose-rich, fibrous, pulp (Figure 8.4, Product H). The ultimate test of a filleting system is its ability to prepare a gel fillet low in mesophyll anthraquinones.
Filleting is accomplished by one of two methods: manual removal of the rind with a knife (Figure 8.4, Process D) and filleting by machine (Figure 4, Process E). In either case, the tip of the leaf is first removed, the butt is trimmed off and the sides of the leaf are trimmed. This process is nicely illustrated by Sheet 1, Figure 2 in the McAnalley '935 patent (1988). Tip removal is usually accomplished using a knife at the culling table (Figure 8.4, Process B) although not usually to the extent that McAnnelley shows. The butt can either be trimmed with a knife at the culling table or by wire at the filleting table. Manual filleting generally takes place on a stainless steel table approximately 1 m wide with raised edges approximately 10 cm high. The length of the table varies depending on how many workstations are desired. Generally each worker, who stand on either side of the table, requires at least a meter of workspace. Table length tends to be in multiples of 2 m accommodating either four, six or eight workers. The flow of cleaned leaves is generally on the head of the table where the rind is removed and discarded to the side. Fillets with exuded pseudoplastic gel are removed from the foot of the table. Trimming and filleting tables are often equipped with stainless steel wire, set up on pegs about 1 cm above the surface. These provide a cutting edge for removal of the butt and trimming of the sides.
After trimming of the tip, butt and sides, the upper and lower surfaces of the leaf are removed. The rind on the flat side of the leaf is then often removed with a knife, discarding the rind. The gel is then scrapped or scooped with the knife away from the rind on the rounder side of the leaf. We have also seen the opposite sequence employed with equal success, discarding the rinds aside and passing the gel fillet down the table.
Mechanical filleting proceeds via a diametrical mechanism (Thompson, 1983). The leaf is placed tip first onto a conveyor belt, which rotates the leaf through 90°. Thus the leaf is carried into the frame of the machine with the broad axis of the leaf at a right angle to the horizontal. A vertical knife then bisects the leaf, splitting it along the broad axis. Almost simultaneously, a set of rollers presses the rind side of the leaf, firmly expressing the gel. The set and tension of the rollers determine how much of the mesophyll is expressed together with the gel. If roller pressure is too high, then the gel will be contaminated with the anthraquinones. If roller pressure is too low, then gel will be discarded with rind and mesophyll. If the reader has difficulty envisioning this system, the drawings of Thompson's '942 patent (1983) should be consulted.
The capacity and throughput of both manual and mechanical filleting is highly dependent on the size and quality of the leaves entering the system. In both systems the yield of gel increases as a power function of leaf size. There is virtually no lower limit to leaf sizes that can be manually filleted. Even the leaves below 100 g from an ornamental A. barbadensis houseplant can be hand filleted and are so by millions of households for the treatment of minor burns. However, when leaves less than 250g are manually filleted, the yield is usually only about 33% (w/w) gel and therefore the output per hour of gel is very low. The smallest leaves that can practically be filleted by machine are approximately 150g. At this size only about 25% of the wet weight of the leaf can be recovered as gel yielding c.37g/leaf. Since machine output is determined by leaf per minute throughput, the weight output of gel fillets per minute with small leaves is obviously very poor. For 500g leaves the recovery rate rises to about 50—55% of leaf weight, yielding 250—275 g of gel per leaf. For 1,000 g leaves the yield of gel approaches
75—80% or 750g per leaf. Since it takes essentially no more time to fillet a 200g leaf than it does to fillet a 1 kg leaf there is a disproportionate increase in gel yield with leaf weight. Therefore over the range of leaf weights of 300 to 1,000 g, a three-fold difference in weight, there is a six-fold increase in output (105g/leaf to 650g/leaf). Thus, a hidden benefit of proper agronomy is not only a higher gross weight yield per acre but a higher proportionate yield of gel and higher output on the filleting line.
Regardless of whether manual or mechanical filleting is performed, the equipment should be constructed of food-grade, high-quality stainless steel (Grade 316 preferable, 304 is acceptable with a polish finish) with smooth, polished welds. The best guide to filleting machines remains Thompson's 1983 U.S. Patent. Cottrell (1984) advises further modifications, the value of which are uncertain. Filleting machine plans are shown in Sheet 2 of the 1988 McAnalley '95 patent as exemplifying the prior art. As far as we know no one has ever manufactured these machines as their practicality is unclear. The Thompson Company is no longer in business but similar machines are currently being manufactured by Coastal Conveyor (Harlingen, Texas) and International Purchasing and Manufacturing (Harlingen, Texas). Needless to say, despite certain general principles described above, every filleting installation is slightly different depending on the desired throughput, available space and desired capital investment.
The advantages and disadvantages of manual versus mechanical filleting are endlessly debated and will not be resolved in this chapter. Suffice to say, carefully conducted hand filleting produces gel with lower levels of aloin, a quantitative measure of rind and mesophyll contamination, than the best machine fillets. On the other hand, we have seen machine filleted gel of extremely high quality. The most important factor is, 'what does the customer desire and what are they willing to pay?' Filleting technique becomes important only when the producer has mastered agronomy and sanitation, and few producers have mastered these basics.
The filleting process potentially yields two products, gel fillet and rind. Just as there is a small commercial market for whole, washed leaves there is a small commercial market for whole gel fillets (Product F in Figures 8.3 and 8.4). Generally, the fillets are packed into 55 gallon (200 liter) drums, preservatives are added to retard spoilage, and the drums are shipped refrigerated to cosmetic manufacturers who desire minimally modified aloe. Theoretically, the rind could be a rich source for extracting anthraquinones and chromones. However, to our knowledge, very few plantations systematically utilize rind as a feedstock; most rind ends up dumped on compost heaps.
The next and last step in preliminary processing involves removing the cellulosic fibers from the gel fillet. This is accomplished by very coarsely chopping the fillet as with an industrial grade garbage disposal. The coarsely chopped fillet is then passed through a depulper of the type employed in the citrus industry. The aloe industry almost exclusively uses depulpers manufactured by the FMC Corporation (San Jose, California), either the PF 200, MCF 200 or UCF-200A models. The 200 stands for the nominal pore size (the approximate fiber exclusion size in microns). Particles smaller than 200 ^m will readily pass through the final screen of the depulper. Furthermore, this sieve size allows the passage of the pseudoplastic gel without shearing the high molecular weight strands and reducing their viscosity. As pore size is decreased, it is more difficult to pass gels through the sieve because of their long-chain molecules. This effect is independent of viscosity but is highly dependent on flow rate and in fact defines the term 'gel'. These effects will be discussed in more detail as we discuss filtration. Removal of fiber yields the crude gel product (Figure 8.4, Product I).
With crude gel, as with the gel fillet (Figure 8.4, Product F), material that is essentially an intermediate can constitute a commercial product. The desirability of such a product is understandable since crude 'aloe gel' is close to the native gel. A small amount of crude gel is sold as such, without removal of bacteria by heat or filtration. Preservatives such as sulfite or benzoate are added in an attempt to prevent bacterial proliferation. Agents such as sorbate, citrate and ascorbate are added in an attempt to prevent oxidation. Unfortunately, these two problems are not easily solved without understanding and attacking the roots of the problems, which are bacteria and anthraquinones. Spoilage severely limits the usefulness of crude gel.
In our scientific studies we have extensively examined crude gel. We solved the problem of stabilization by freeze-drying directly from the crude gel — ARF Process A. This requires monopolizing a $650,000 machine for 60 hours to stabilize 100 liters of crude gel worth about $300 on the wholesale market — not a good return on investment. Thus, in our publications we continually emphasize that although ARF Process A materials are critical to our understanding of aloe gel in the native state, it is not an economically feasible product. Before proceeding on to Intermediary Processing, we will discuss a second processing pathway from leaf to extract — grinding the entire leaf rather than peeling it.
Slicing, grinding and whole leaf aloe extracts (WLE)
The tradition of preparing gel fillets of aloe leaves goes back approximately 60 years to the early days of the Hilltop Gardens where A. barbadensis (cv. RGV) appears to have first been commercially grown. Sometime within the last 30 years and certainly prior to 1989, a second method of processing was developed by grinding the whole leaf followed by extensive treatment with activated charcoal to remove the anthraquinones. This material, termed 'Whole Leaf Extract' or 'Whole Leaf Aloe,' is significantly cheaper than gel fillet for two reasons. First, the labor-intensive and time consuming filleting process is avoided. Second the limitations of gel recovery from small leaves is obviated. By grinding fresh leaves, 'gel' can be efficiently harvested from leaves as small as 100 to 200g. As the gel fillet seems to have arisen from the RGV Hispanic tradition of filleting out the gel to apply to the skin or put into a tea, so the WLE extract seems to be most closely related to the Oriental tradition of drying the whole aloe plant, especially A. arborescens, grinding it into a powder and ingesting the powder as a tonic to enhance vigor.
As alluded to above, although the whole leaf process was adapted to industrial practice long before Coats applied for the predecessor to his '811 patent in 1992, his description of the basic WLE process is apt. In 'Detailed Description of the Preferred Embodiment,' Coats lays out how the leaves are sanitized and then sliced and ground. His description of slicing and shearing 1000 lbs (about half a ton) of leaves in a 12 inch (c.30 cm) Fritz Mill within a minute or two is in accordance with what we have seen in the industry. We and others call this coarsely chopped material 'guacamole' by analogy with the peeled, deseeded, mashed pulp of avocado. Like real guacamole, the chopped whole aloe leafwill 'spoil' rapidly after preparation with exposure to air unless bacterial proliferation is retarded and oxidation prevented. A very limited amount of A. barbadensis gel is sold in this form, using various preservatives and anti-oxidants to retard spoilage. However, if the difficulties of stabilizing crude aloe gel from fillets are significant, the difficulties in preventing spoilage of WLE with preservatives alone are much more extreme. As a consequence the market for 'guacamole' is miniscule.
The next stage of preliminary processing of WLE converts the 'guacamole' to 'gel.' The primary process involved in this is termed 'depulping.' However, this is not as simple a process as described above because the 'guacamole' of fresh, raw WLE has a fiber content several hundred-fold greater than the fiber content of crude gel fillet. As a result it is customary to reduce viscosity and fiber by 'mixing the whole leaf with a cellulose-dissolving compound' (Coats, 1994). Coats recommends the use of Cellulase 4000, an excellent partially purified cellulase from Trichoderma reesei at a dose of 20g per 55 gallons (c.215 liters) of crude WLE. Although Coats does not specify the time and temperature for this process, the 'guacamole' is usually held with stirring at ambient (23—35 °C) until pseudoplasticity is broken (1—2 hours). This treatment certainly reduces the fiber content and viscosity of the WLE and allows it to easily pass through the depulper. However, there are other consequences of this cellulase treatment, as we shall later see. The product of depulping is a yellow liquid with a solids content of 1— 2g per dl and a viscosity little greater than water.
The above example illustrates the advantages of the WLE process — speed and economy. One or two minutes of grinding WLE can be compared with the hour or two it takes six workers to manually fillet a half ton of the same material. Machine filleting takes 15 to 25 minutes to process half a ton, depending on the size of the leaves and the design of the machine. WLE production requires neither the capital investment of the Thompson filleting machine nor the labor expense of hand filleting. The hour or two that the 'guacamole' sits and stirs with the cellulase is the only apparent penalty paid.
The limitations of the WLE process are subtler. The higher bacterial content of WLE means that the methods of removing bacteria must be a hundred to a thousand times harsher than those employed for high quality gel from fillets. WLE is rich in oxalic acid, which is more or less absent from the gel parenchyma and WLE is ten to one hundred times richer in anthraquinones than gel from fillet. This means that anthraquinone removal is mandatory. Lastly, the effects of cellulase are far more profound than Coats ever envisioned. By the time processing of WLE is completed, its polysaccharide has been destroyed and it can no longer rightfully be accorded the term gel.
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