Taxus and Taxol - A Compilation of Research Findings

Special Circular 150-99

Evaluation of Bin Drying of Taxus Biomass

Robert C. Hansen,
Department of Food, Agricultural, and Biological Engineering,
Ohio Agricultural Research and Development Center,
The Ohio State University,
Wooster, Ohio.

Ralph B. Shugert Jr.,
Staff Horticulturist,
Zelenka Nursery, Inc.,
Grand Haven, Mich.

Hala N. ElSohly,
Research Institute of Pharmaceutical Sciences,
School of Pharmacy,
The University of Mississippi,
University, Miss.

Edward M. Croom Jr.,
Research Institute of Pharmaceutical Sciences,
School of Pharmacy,
The University of Mississippi,
University, Miss.

Harold M. Keener,
Department of Food, Agricultural, and Biological Engineering,
Ohio Agricultural Research and Development Center,
The Ohio State University,
Wooster, Ohio.

Written for presentation at the 1992 International Winter Meeting sponsored by the American Society of Agricultural Engineers. Nashville Convention Center, Nashville, Tennessee, December 15-18, 1992. Reprinted with permission.

Summary

Standard grain-storage bins equipped with dryers were used to dry Taxus biomass. Drying temperatures were monitored and compared to specifications of 115 ± 15°F. Attempts to improve temperature control and to reduce variation were only marginally successful. The drying system was not capable of drying Taxus biomass to designated specifications.

Introduction

Clippings and needles from Taxus plants have been investigated as a source of Taxol, a promising drug for treatment of ovarian and breast cancers. Drying has been specified as a critical step in the preparation of Taxus biomass for storage. Drying is also generally recommended prior to the Taxol extraction process. The need for drying was established during preliminary laboratory research (Croom et al., 1991); however, attempts to dry Taxus clippings and/or plants on a commercial scale have not been reported in the literature.

Objectives

The objectives of this research were to measure drying temperature distributions while drying Taxus biomass in a standard grain-drying bin and to determine the effect of drying temperatures on Taxol yields.

Literature Review

Research identifying specifications for drying Taxus clippings and needles was reported by Croom et al. (1992). Optimum Taxol yields were obtained when drying temperatures were maintained between a lower limit of 100°F and an upper limit of 130°F. In addition, percent Taxol yields from needles were found to be significantly higher when the needles were dried while remaining attached to stems rather than being separated from stems before drying.

Hansen et al. (1993) used a thin-layer laboratory dryer to measure drying rates as a function of temperature for clippings, needles, and stems from Taxus x media 'Hicksii.' Based on their results, parameters were determined for a thin-layer drying equation for drying temperatures of 30°, 40°, 50°, and 60°C. The results showed that drying rates increased dramatically as drying temperatures increased. Also, stems were found to dry at a faster rate than needles, and needles dried at a faster rate than whole clippings.

Procedure

The drying study was conducted through the cooperation of Zelenka Nursery, Inc., on a farm about 10 miles south of Grand Haven, Michigan. A circular 27-ft.-diameter grain-storage bin, seven rings high, which was equipped with a perforated floor, was available. The floor provided space (18 inches high) for a plenum to which was attached a 10-hp, 3,450-rpm, 28-inch-diameter vane axial drying fan with a propane heating unit rated at 2 million Btu/hour. (See Figure 1.)

Results in this paper pertain to data collected while drying whole Taxus x media 'Hicksii' plants which averaged 36 inches in height. The plants had matured to the point where they were not marketable as ornamental plants and were therefore deemed to be culls. For most of the drying runs, roots were removed to reduce weight and contamination of the bin with soil.

Figure 1. Schematic drawing of the dying system showing the
location of temperature measurement points for Lot Nos. 105-109.

Plants identified by nine lot numbers were dried in various quantities and drying conditions as summarized in Table 1. As indicated in Note 3, Table 1, the fan and heater were replaced with new units during the time when Lot No. 103 was being dried. The temperature control system on the old dryer appeared to be defective. Since retention of Taxol content was apparently dependent on being able to hold drying temperatures at 115 ± 15°F, purchase of a new unit seemed to be prudent. Also, problems with the old heating unit had caused shutdowns at night while drying previous material. Specifications on the new fan and heater were 7.5- to 10-hp, 3,450-rpm, 24-inch-diameter vane axial fan with a 1,035,000 Btu/hr heater. In order to double drying capacity, a second 27-ft.-diameter bin was equipped identical to the first. Both bins were used to dry Lot Nos. 107, 108, and 109.

A Leeds and Northrup strip recorder was set up to monitor drying temperatures with thermocouples. Temperatures were recorded one channel at a time every 1.875 minutes. While moni- toring temperatures for one bin using 13 channels, a complete cycle of recordings required approximately 24 minutes and 20 seconds. When two bins were being monitored with 24 thermocouples, 45 minutes were required for a complete cycle. Chart speed was set at three inches per hour.

While drying Lot Nos. 101-104, 10 thermocouples were distributed to monitor temperatures in the bin on top of the perforated floor. In addition, one was placed in the transition channel between the burner and the bin, and one was placed in a shaded position outside to measure ambient temperatures. A schematic of the thermocouple layout is shown in Figure 2. After finishing the drying for Lot No. 104 and before starting the drying for Lot No. 105, the configuration for monitoring temperatures was changed so that measurements at the top of the plant material could be compared to measurements directly below at floor level. Also, temperatures a few inches away from the bin wall were recorded at one point in the bin. (See schematic in Figure 1.)

In an attempt to reduce drying time to avoid unnecessary heat loss at the bin wall and to improve energy use efficiency, foam insulation was applied on the outside of the wall of the bin and over the transition channel. The foam was applied two inches thick and about 20 inches high around the plenum and then narrowed to one inch of thickness to a total height of nine feet. The insulation was applied on Dec. 11 during the drying of Lot No. 104. Bin No. 2 was also insulated in the same manner before it was used for the first time on Dec. 26 for Lot No. 107.

Figure 2. Map of temperature measurement points
for Lot Nos. 101-104. The limited area of the floor
used for drying Lot Nos. 101-103 is also identified.

Various configurations for placement of the plant material on the bin floor were tried. Without static pressure build-up in the plenum that is typical for grain drying, wide distributions of airflow and drying temperatures occurred. Higher airflow rates and higher temperatures were measured on the side of the bin opposite the fan and heating unit. Therefore, Taxus material located on the hotter areas of the plenum would dry more quickly than those in cooler areas.

One strategy, used for Lot Nos. 101, 102, and 103, was to cover the cooler area of the floor with plywood and only dry using the hotter areas. (See layout in Figure 2.) While this procedure generally led to satisfactory results, drying capacity in terms of plant biomass dried per batch was severely restricted. Before drying Lot No. 101, a three-inch layer of pea gravel was distributed on the perforated floor in Bin No. 1 in an attempt to build up static pressure in the plenum and to reduce temperature variation from one part of the floor to the other. This strategy did not bring about measurable improvement, and therefore pea gravel was not applied to the floor in Bin No. 2.

Another strategy for distributing heat energy more uniformly throughout the plenum was the placement of a concrete block baffle, four-feet long and two blocks high, in a circular arc about four ft. away from the bin wall in line radially with the transition. (See Figure 1.) This was installed on Dec. 5 along with the new fan and burner for Bin No. 1. A similar baffle was installed in Bin No. 2.

Starting with Lot No. 104, plants were placed in upright positions (roots removed) and distributed across the entire floor. One or two layers of plants were placed horizontally across the top of the upright plants. With the new fan, burner, temperature controller, and baffle in place, temperature variation was expected to be significantly reduced. In addition, insulation was applied to Bin No. 1 during the drying of Lot No. 104 from which improvements in drying efficiency were expected.

Taxus biomass is considered to be dry when the needles become brittle. Evidence of brittleness is identified when needles break or snap as they are stressed or pinched longitudinally between the thumb and forefinger of an observer's hand. Biomass color also changes from green to light brown. The point at which brittleness occurs corresponds with a moisture content of 2.5 to 3.0 percent, wet basis.

Dried needles were successfully removed from the branches of shrubs by feeding them through an electrically powered, rotating drum to which rows of 0.5-inch-diameter by three-inch-long hard rubber fingers were attached. The machine is commonly known as a power-driven mechanical chicken plucker. Relatively small quantities of stems and bark were harvested with the needles. The harvested needles were successfully stored in an unheated outdoor shed after being wrapped in double layers of polypropylene, approximately 200 lbs. per pallet.

Discussion of Results

The drying schedule for Lot Nos. 101 to 109 (12 batches), showing starting and stopping times, is tabulated in Table 1. Increased drying times corresponded with efforts to increase batch size. As the number of plants per batch increased, the depth of the Taxus biomass in the bin increased from an average depth of 2.5 to 3.0 ft. to an average depth of 3.5 to 4.0 ft. Simultaneously, ambient temperatures were generally decreasing as starting dates progressed from late in November to early in January. Therefore, increasing drying times most likely resulted both from increasing biomass depth and colder days. Total drying time ranged from under four days to seven days. The last column in Table 1 shows that yields of dry needles as a percent of the weight of undried plants ranged from 17 to 30%. The moisture content of fresh, undried Taxus biomass studied to date ranged from 50 to 60 percent, wet basis.

Table 2 is a summary of drying statistics for the Lot Numbers identified in Table 1. Statistics for Lot Nos. 101 to 103 were based on thermocouple locations 3 to 8 since a significant portion of the drying floor was not used. (See Figure 2.) All locations were included in calculations for Lot No. 104 since the lot size was doubled and the entire floor was filled with plants. As indicated under the procedure section (see previously), thermocouple locations for Lot Nos. 105 to 109 were distributed as shown in Figure 1. Therefore, the results shown in Table 2 for Lot Nos. 105 to 109 were determined only from the five thermocouples located on the drying floor. (See Figure 1.)

As drying proceeded for each lot and as floor temperatures were monitored with the Leeds and Northrup recorder, the burner temperature was adjusted in an effort to meet drying specifications of 115 ± 15F°. Data from the strip charts were tabulated for each temperature reading approximately once every three hours as a basis for calculating mean and variation statistics summarized in Table 2.

Table 2 shows that mean temperatures for each lot generally were slightly below the target value of 115°F. Lot Nos. 108B1, 108B2, and 109B2 were nearly on target. Standard deviations ranged from a low for Lot No. 109B2 of 8°F to a high for Lot No. 103 of 14.5°F. As noted in Table 1, Lot No. 103 was subjected to numerous starts and stops caused by burner malfunctions. Additional measures of temperature variation at the plenum floor include minimum and maximum temperatures recorded and percentage of temperature recordings tabulated that were above, within, and below specification limits (see Table 2.)

Figure 3. Histogram of temperatures at the drying bin floor for Lot No. 103.	Figure 4. Histogram of temperatures at the drying bin floor for Lot No. 105.

Histograms of recorded temperatures graphically compare temperature distributions with the target temperature and specification limits (based on Celsius units) in Figure 3 for Lot No. 103 and in Figure 4 for Lot No. 105. A tolerance based on 45±Ê10°C is somewhat more relaxed than 115 ± 15°F. In both cases, drying temperatures averaged within 2.2°C of being right on the nominal temperature, and 80 percent or more of recorded temperatures at the floor were within upper and lower specification limits. The histogram for Lot No. 103 exemplifies temperature distributions before the new fan and heating unit were purchased and the baffle and insulation were in place. The histogram for Lot No. 105 is typical of results obtained after the improvements were installed. Although a significant reduction in temperature variation was not achieved as expected, the improvements did permit doubling and even tripling batch size without increasing temperature variation. There were some evidences of reductions in temperature variation in a few batches (see Table 2).

Figures 5 and 6 are plots of temperature histories for Lots No. 103 and 105, respectively, throughout the time periods that drying occurred for each batch. The problems that disrupted the drying of Lot No. 103 are noted in the graph in Figure 5. Even after the new drying unit was installed, note that maintenance of mean temperatures on a 45°C target was not attained. The dryer did not control temperatures to a prescribed set point. On the contrary, the temperatures seemed to parallel ambient temperatures.

Figure 5. Temperature histories of maximum, mean, and minimum floor temperatures compared to a history of ambient temperature for Lot No. 103. Interruptions in the drying process are noted.	Figure 6. Temperature histories of maximum, mean, and minimum floor temperatures compared to a history of ambient temperatures for Lot No. 105.

Figure 6 exemplifies the same concerns for Lot No. 105, although, in general, control seems to be improved.

Summary

Twelve batches of Taxus x media 'Hicksii' whole plants were dried in two 27-ft.-diameter grain-storage bins that were equipped with perforated floors, vane axial drying fans, and propane burners. Temperatures were monitored at the top surface of the plenum floor with thermocouples and a strip-chart recorder.

After temperature variations ranging from 30° to 40°F were recorded, efforts were made to reduce variation. The plenum and bin walls were insulated to reduce heat loss, a concrete block baffle was installed to diffuse heated air more uniformly throughout the plenum, and new drying units were added to improve temperature control. While these changes permitted drying more Taxus biomass per batch, reduction in temperature variation was small. Temperature control to a prescribed set point was poor. The drying system was not capable of drying Taxus biomass to specifications of 115 ± 15°F.

Acknowledgments

The authors gratefully acknowledge the dedicated assistance of Charles Wolters, Craig Schwander, and Dean Souden of Zelenka Nursery, Inc., Grand Haven, Michigan.

References

Croom, E. M. Jr., H. N. ElSohly, T. R. Sharpe, and J. D. McChesney. 1991. Research Institute of Pharmaceutical Sciences. School of Pharmacy. The University of Mississippi. Private communication.

Croom, E. M. Jr., E. S. El-Kashoury, and H. N. ElSohly. 1992. Effect of drying conditions on the Taxol content of the needles of ornamental Taxus. Second National Cancer Institute Workshop on Taxol and Taxus. Sept. 23-24, 1992. Alexandria, Va.

Hansen, R. C., H. M. Keener, and H. N. ElSohly. 1993. Thin-layer drying of cultivated Taxus clippings. Transactions of the ASAE. 36(6):1873-1877.