We noticed an aloe plant struggling to maintain a foothold on the cold, rocky, windswept summit of the hill. Soon enough we noticed another, and yet another. Soon we realized that the entire hilltop was colonized by aloe plants. These must have been brought more than a century ago, directly from the Cape and planted on this desolate spot for laxative purposes by the staff of the semaphore station and if so were A. ferox Miller. However, the endurance of one plant does not equate with the optimal growth conditions for all species or for commercial cultivars.
One of the last things considered by most brokers of aloe feed stocks is agronomy. This consideration is partly because agronomy is probably the last item that consumer product manufacturers care about. However, in our collective experience, there is no item more important than agronomy in determining quantitative yields of gel per hectare, nor more essential to reproducible biological activity. Unfortunately, because we do not yet have chemical assays for everything in the broad spectrum of aloe biological activities, we cannot yet explain the exact agronomic parameters for the production of the best gel.
Water, nitrogen and soil pH
The three most critical factors for the production of aloe gel are the supply of water, the availability of nitrogen, and the acidity of the soil in which the plant grows. Out of the three factors, the most important is water, for although aloe is popularly conceived of as being a desert plant, in reality its growth rate is more highly tied to water availability than to any other factor. Approximately three decades ago it was realized that the anthraquinone content of Aloe sp. was dependent on the various inter-related factors (insolation, wind, moisture) which constitute water availability. As recently as 1992, Genet and van Schooten determined that water availability was the key determinant for growth of aloes in the sub-tropics. However, the only systematic study of the interaction of water, macronutrients, and micronutrients in the growth of A. barbadensis is the work of Dr Wang at Texas A&M University. Unfortunately, the results of this agronomic study were never published and therefore much of what can be said about the agronomy of aloes remains anecdotal rather than quantitative.
Four general principles can be stated with regard to the cultivation of A. barbadensis in the lower RGV. We focus on this locale because that is the location where A. barbadensis Miller (cv. RGV) has been cultivated for the longest time. One would therefore expect the agronomy to be well understood. Also, traditionally, this is where most of the aloe gel marketed in the United States is grown. First, the climate of this region is light and quite variable from year to year and severe frosts can happen. Second, adequate rainfall is irregular (average rainfall 26 inches, 66cm/annum). Droughts (25 cm/annum) lasting up to three years can destroy fledgling plantations. Third, the soil is often low in available nitrogen (<0.15%) and the soil pH is quite high (8.0—8.6). Fourth, due to the soil structure, correcting problems two and three by irrigation and fertilization can lead to salinization. The high calcium content of these soils makes it difficult to adjust the pH to the 6.8—7.6 range and correction of pH will be transient. These variables are interactive with each other and are further interactive with wind and insolation during periods were rain is suboptimal. This means that it is impossible to provide quantitative solutions for a given variable in the absence of controlled site studies. We can, however, provide the following approximate estimates.
Severe frosts occur every six to seven years in the lower RGV (three in the last 20 years; 1983, 1989 and 1991). This means that one risks their profitability if one aloe plants take four to five years (Coats, 1994) to come into full production. They can take this long because in many years the 100cm of rainfall and 0.25—0.40% nitrogen needed for minimally acceptable growth is not achieved. Higher growth rates (9—12 months to full production) could be achieved with optimal water (200cm per annum) and nitrogen (0.40—0.50%) inputs at a soil pH of ~7.4. But land and water are expensive in the lower RGV. The practical consequence of these factors is that production of A. barbadensis has shifted to areas where land is less expensive, frosts are less frequent, and rainfall is sufficient while water is cheaper. The RGV no longer dominates the world aloe gel market as it did ten years ago. Currently, production is distributed among plantations in Mexico, the Dominican Republic, Venezuela, Guatemala and North Africa. All of these locations feature, above all, an absence of killing frosts. This means that there is not a race to recover the investment before a frost wipes out the crop. Most of these locations are still less than optimal with regard to rainfall and have nitrogen deficient soils. We have no doubts that production of gel in these areas could be improved by such elementary measures as provision of adequate water and nitrogen.
Just as important as water availability is soil composition/porosity and pH. In soils where silt content is significant, drainage will be poor. In the lower RGV rainfall is intermittent and much of the total annual precipitation may occur in three or four deluges. Those locations with poorly draining soils will experience pooling of water on the surface. The roots of A. barbadensis are thick and shallow and possess relatively few root hairs. This may explain the predisposition of this plant to root rot. During the RGV droughts during the latter half of the 1990s, it was not uncommon to have episodes of aloe root rot following the occasional torrential rain that broke a year-long drought. Similarly, calcium-rich, high pH soils which drain poorly have a higher than expected problem with salinization. This association makes lowering the soil pH difficult since it exacerbates this salinization.
We tend to forget, when discussing agronomy, the spacing of plants and harvesting. However with the rising costs of land and water and the need to produce high quality leaves at minimal cost, these factors are becoming increasingly important. We have not uncommonly seen aloes planted at densities as low as 3,000 per acre. Where land is cheap, rainfall plentiful, intercropping is practiced and weed control is achieved by grazing, such a policy makes sense. We have also seen plantations using such practices with aloes planted at densities of 4,500 plants per acre yielding leaf size >800g. However, to maximize production per acre, densities of up to 6,000 plants per acre can be employed, provided optimal amounts of water are provided and soil nitrogen is maintained in the 0.40—0.50% range. A density of 6,000+plants/acre is most compatible with drip irrigation and the use of ground-cover poly-film for weed control. This sophisticated agronomy avoids the need for mechanical tillage which can easily disrupt the exceedingly shallow root system of A. barbadensis.
The procedure of aloe harvesting properly belongs in the next section since it is done only an hour or so before processing. Also, a major concern in harvesting is using techniques that discourage the proliferation of bacteria, again linking it with processing. However, harvesting is commonly performed by the same field hands that perform the other farming tasks. Harvest supervision is also performed by field managers. Therefore harvesting usually falls under the purview of farm operations.
Harvesting is one of several critical steps in the production of high quality aloe gel. Put another way, improper harvesting is the start of production of poor quality gel. We have taught that a quality control person should always be present during harvesting to make sure two essential steps are observed. First, leaves with tip necrosis should not be harvested for gel production. The necrotic areas of the leaf are where commensal organisms often proliferate. An attempt, during preliminary processing, to trim such areas out, merely contaminates the good gel and the entire processing area. Second, leaves should be treated gently and should be harvested in such a way as to keep the base of the leaf sealed. Puncture of the leaves facilitates entry of bacteria into the gel parenchyma. It is amazing how fast some of these subcuticular organisms can grow when introduced into gel. Enforcing these sanitary precautions will not make quality control popular but control of bacterial proliferation must start in the field.
The message is that proper processing of A. barbadensis gel requires a sophisticated farm manager. This person needs to understand soil chemistry. The farm manager must monitor soil nitrogen and pH and adjust soil treatment accordingly. Treatment of the soil can be with conventional sources of nitrogen, or manure in the case of 'organic' fields. We believe that the future of aloes in the lower RGV will utilize drip irrigation and perhaps poly-film weed control. A prudent manager should be prepared to handle these complexities. Finally, the farm manager should eagerly collaborate with Quality Control to enforce the best harvesting methods. Quality aloe gel requires quality agronomy.
Preliminary processing — cleaning, extracting and sanitization
The truck with baskets of aloe leaves or the tractor-drawn trailer with wire 'aloe cages' has just pulled up in front of the plant or farm processing station. The area is full of dust or mud and the leaves are coated with dirt mixed in with exuded aloe sap which is beginning to turn purple from the combination of exposure to air and sunlight. From the other end of the building must come a juice that is essentially sterile. That is the challenge. Failure means that the product will have problems with 'break-through' microbial growth, a cosmetic that turns black, or a biologically inactive product. Preliminary processing is where the battle over aloe quality begins.
Preliminary processing begins with sanitizing the outer surface of the freshly harvested leaf (Feedstock A in Figure 8.4) and proceeds through removal of the outer rind (Processes D or E) and expression of the gel fillet through a sieving device (Process G) to produce the gel (Product I). Washing begins with freshly harvested leaves. Ideally, leaves can be harvested and washed within two to four hours. However, this requires small batches (less than a ton) and quick transport from the field to either the field processing station or the integrated processing plant. In controlled studies (see Figure 11, H) we have found that if 24 hours elapse between harvesting and washing some biological activity is lost. Delaying washing after harvesting while preserving freshness requires meticulous planning and attention. Leaves must not be bruised and refrigeration should be immediate. Our recommendation is to keep the interval between harvesting leaves and washing and further processing to an hour or less.
Washing can be done in a number of ways (Figure 8.4, Process B). The thoroughness of washing will ideally be a function of how dirty the leaves are. If conditions are chronically muddy, more thorough washing will be required. Similarly, if downstream contamination of gel with soil-associated organisms is a problem, washing should be
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