THIN-LAYER DRYING OF MARIGOLD FLOWERS AND
FLOWER COMPONENTS FOR PETAL REMOVAL
M. D. Buser, M. L. Stone, G. H. Brusewitz, N. O. Maness, D. P. Whitelock
ABSTRACT. Thin-layer drying characteristics of marigolds were determined as a function of air temperature and airflow
rate. Optimum drying conditions for petal separation were identified and xanthophyll reduction was associated with these
conditions. A thin-layer model was developed to describe the drying characteristics of marigolds. Optimum conditions for
petal detachment with thin-layer drying were found at an air temperature of 70°C and an airflow rate of 0.33 m3·s –1·m–2.
On the average, xanthophyll content decreased at higher drying temperatures.
Keywords. Marigold, Thin-layer drying, Xanthophyll, Petal removal.
T
he primary groups of natural coloring substances
in food are carotenoids, anthocyanins, porphyrins,
and chlorophylls (Walford, 1980). The carotenoids
are responsible for many of the brilliant red,
orange, and yellow colors of edible fruits and berries,
vegetables and mushrooms, flowers, and some animals
(Counsell, 1981; Minguez-Mosquera et al., 1993). The
carotenoids encompass both hydrocarbon carotenes and
oxygenated xanthophylls (Walford, 1984). Certain orange
Tagetes erecta L. (marigold) flower petals contain a large
amount of the xanthophyll pigment. Marigolds are fast
growing, erect brushy plants, with aromatic, feathery,
glossy, deep green leaves and large daisy like blossoms in
shades of yellow and orange. Marigolds typically grow to
heights of 1 m and have solitary flower heads ranging in
diameter from 7 to 13 cm. They are a genus of annuals that
flower continuously from early summer to autumn frost
(Still, 1994).
Tagetes extract is currently obtained by hexane
extraction of dried Tagetes erecta L. petals. (Davies and
Kost, 1988) The main coloring principle of the extract
consists of xanthophyll (oxyderivatives of carotenes) and
the primary pigments are palmitate esters of lutein,
particularly dipalmitate lutein (helenien). Tagetes extract
may also contain fats, oils, and other waxes naturally
occurring in the plant material. This orange oil-soluble
extract is currently used in dairy and fat-based products,
and as an additive to poultry feed (JECFA, 1992).
The current process for extracting xanthophyll from
marigolds consists of picking whole flowers, storage,
grinding, pelletization, and extraction (Stone and Kranzler,
1988). We propose a method of processing marigolds
which minimizes the storage of the harvested material,
optimizes the drying process, and reduces the quantity of
green material and seeds in the final product. This new
process consists of four steps: mechanically harvesting the
flowers, immediately drying the flowers, detaching the
petals from the remainder of the flower (the receptacle),
and extracting the xanthophyll pigments from the petal
components. Flower components are illustrated in figure 1.
Drying the wet material at air temperature and airflow
rates, which rapidly dry the petals and slowly dry the
Article was submitted for publication in October 1998; reviewed and
approved for publication by the Food & Process Engineering Institute of
ASAE in July 1999.
Approved for publication by the Director, Oklahoma Agricultural
Experiment Station. This research was supported by a grant from Oklahoma
Center for Advancement of Science and Technology and the Oklahoma
Agricultural Experiment Station. Mention of a trade name, proprietary
product, or specific equipment does not constitute a guarantee or warranty
by the United States Department of Agriculture and does not imply
approval of the product to the exclusion of others that may be available.
The authors are Michael D. Buser, Agricultural Engineer, and Derek
P. Whitelock, Agricultural Engineer, United States Cotton Ginning
Laboratory, Stoneville, Mississippi; Marvin L. Stone, Professor, and
Gerald H. Brusewitz, Regents Professor, Biosystems and Agricultural
Engineering, and Niels O. Maness, Professor, Horticulture and
Landscape Architecture, Stillwater, OK 74078. Corresponding author:
Michael D. Buser, USDA/ARS, U.S. Cotton Ginning Lab, PO Box 256,
Stoneville, MS 38776; voice: (601) 686-3096; fax: (601) 686-5483; email: [email protected].
Figure 1–Marigold schematic.
Transactions of the ASAE
VOL. 42(5): 1367-1373
1999 American Society of Agricultural Engineers
1367
receptacle, would reduce the mechanical work required to
separate the petals from the receptacle. Xanthophyll would
then be extracted only from the petal portion. Optimizing
the drying parameters to maximize petal detachment will
ultimately increase the processing efficiency and quality of
the end product.
Drying analysis is effectively accomplished through the
use of configuration-specific models, usually thin-layer
models. Researchers have developed numerous empirical
and theoretical thin-layer drying models for various
agricultural products (Tang and Sokhansanj, 1994; Troeger
and Butler, 1979; Hansen et al., 1993); marigolds are not
included in this group of products. Generally, these models
are based on thin-layer data characterizing the change in
moisture content and temperature of individual particles
under constant drying conditions. Thin-layer models
include the one-term exponential model (Henderson and
Pabis, 1961), Page’s model (Misra and Brooker, 1980), and
the two-term model used for products such as peanuts
(Sharaf-Eldeen et al., 1980). The equations for these
models are:
One-term exponential model:
M – Me = e – K×t
Mo – Me
(1)
Page’s model:
M – Me = e – K×t
Mo – Me
n
(2)
Two-term model:
M – M e = a × e – K1 × t + 1 – a × e – K 2 × t
Mo – Me
(3)
The information provided by the thin-layer models maybe
used to solve mass balance equations in dryer simulations
(Brooker et al., 1974; Troeger, 1982).
OBJECTIVES
The goal of this research was to determine the thin-layer
drying characteristics of marigolds. The specific objectives
were to:
1. Determine the drying coefficients for marigold
flowers, petals, and receptacles as a function of air
temperature and airflow rate.
2. Determine the drying temperature and airflow which
produces the optimum conditions for petal
detachment.
3. Determine the xanthophyll content in the marigold
petals after drying at various air temperatures and
airflow rates.
MATERIALS, EQUIPMENT, AND METHODS
Marigolds (Orange Lady variety) were transplanted to
experimental plots at the Oklahoma Botanical Garden and
Nursery complex in Stillwater, Oklahoma, in May of 1996.
1368
To reduce chemical and physical changes caused by
harvesting conditions, the thin-layer drying experiments
were conducted using fresh full-bloom flowers from late
June to mid July. Mature flowers were randomly harvested
by hand in early morning to avoid flower shrinkage due to
environmental drying. After harvesting, the samples were
immediately transported to the research laboratory.
Three dryers, designed for thin-layer drying
experiments, were used. Each dryer consisted of four
single-layer trays with wire mesh bottoms, 4.75-mm
openings. Each tray had a drying area of 552 cm2 for a
total bed area of 2208 cm2/dryer.
Air temperature and airflow rate through the bed were
the independent variables in this study. The measured
responses were the moisture content of the flower
components and a qualitative measure of petal detachment
ease. During a specific test, the air temperature and airflow
rate were held constant and moisture content data were
collected at preselected times. A range of different air
temperature and airflow rate combinations allowed
determination of the optimum drying conditions for petal
detachment ease.
Temperature and airflow rate combinations selected
were 55, 60, 65, and 70°C and 0.23, 0.28, and
0.33 m3·s –1·m–2. Preliminary tests were conducted to
determine the time delay before sampling and the sampling
period. These tests where based on the concept of
detaching petals with a low moisture content from
receptacles with a higher moisture content, the greater the
difference in moisture content the less work required to
detach the petals from the receptacles. A sampling period
of eight hours was selected with time delays before taking
the first sample of 120, 150, 180, and 210 min for
temperature settings of 70, 65, 60, and 55°C, respectively.
Three replications were performed for each temperature
and airflow rate combination requiring 36 tests. Utilizing
the three experimental drying beds available, three
replications of each temperature-airflow combination were
performed in a single day. Forty-eight specimens,
representing one replication, were randomly selected from
144 flowers harvested each day, individually weighed,
labeled, and placed in the trays of a dryer. This process was
repeated for the second and third replications.
The drying range for petal detachment ease (PDE) was
identified by selecting a flower from each drying period
and giving it a physical description representing the
detachment process. A numerical value (PDE code) was
assigned to the moisture status of the petals and receptacles
(table 1). PDE codes were based on the hypothesis that
petals detach from the receptacle best when the petals were
dry and the receptacles were wet. This condition is
described by PDE code three of table 1. Under these
conditions, the mechanical work required to detach the
petals from the receptacle was minimized. In addition, the
receptacle was more likely to remain intact therefore
decreasing the likelihood that petals would be
contaminated with receptacle components.
During thin-layer drying, sampling involved randomly
selecting three flowers every 30 min from a single test bed.
The three flowers refer to a sample, while a single flower
refers to a sub-sample. The first sub-sample, used in
determining the drying characteristics of the whole flower,
was weighed and placed in a tray. The petals of the next
TRANSACTIONS OF THE ASAE
Table 1. Numerical identifiers for qualitative measure
of marigold petal detachment ease (PDE)
PDE
Code
Receptacle
Petal
Description Description Drying Process Description
1
2
Wet
Wet
Wet
Damp
3
4
Wet
Damp
Dry
Wet
5
Damp
Damp
6
Damp
Dry
7
Dry
Wet
8
Dry
Damp
9
Dry
Dry
More drying needed.
Near optimum petal detachment
conditions.
Optimum petal detachment conditions.
Receptacles drying too fast and petals
drying too slow.
Receptacles drying too fast and petals
drying too slow.
Near the end of optimum petal
detachment conditions.
Receptacles over-dried for petal
detachment.
Receptacles over-dried for petal
detachment.
Petals and receptacles over-dried for
petal detachment.
sub-sample were detached from the receptacle. Both
components were weighed and placed in separate trays for
determination of the moisture content of the petals and
receptacles. The final sub-sample was separated the same
way as the second sub-sample and given a PDE code. The
petals were placed in a plastic bag and stored in a freezer
for xanthophyll tests and the receptacle was discarded.
After completion of the drying tests, the sub-samples for
determining moisture content were placed in a drying oven
at 120°C for 24 h to determine dry mass. This procedure
was repeated for each temperature and airflow rate
combination in the thin-layer drying experiment.
Mass losses recorded in the thin-layer drying
experiment were assumed to be entirely due to loss of
moisture. Initial and after drying moisture contents were
calculated and reported on dry basis for each whole flower,
petal, and receptacle sample collected during this study.
Temperature and relative humidity readings,
corresponding to the air conditions before each drying test,
were collected from the Stillwater mesonet site (Elliott et
al., 1994), which is 1.5 km away from the Biosystems and
Agricultural Engineering Laboratory. These readings were
determined to be adequate representations of the dryer’s
inlet air conditions, due to the low variation in humidity
ratio throughout the drying cycles. The relative humidity of
the air after it passed over the heaters was computed using
psychrometric relationships, these calculated values were
used to determine the potential moisture loss of the flowers
and flower components.
THIN-LAYER MODELING
A theoretical model, assuming a constant drying
coefficient, was developed to fit the experimental data. The
simple one term exponential drying relationship, as shown in
equation 1, has been found to be adequate in describing the
thin-layer drying of many biological materials (Brooker et
al., 1974). This equation implies that the change in moisture
content depends on the vapor pressure difference between
the air and material. Further, the equilibrium moisture
content accounts for the effects of relative humidity and
temperature of the inlet air on the drying process
(Sokhansanj et al., 1986). Since this model is based on
experimental data, information should not be extrapolated
VOL. 42(5): 1367-1373
beyond the experimental conditions for which the constant
drying coefficients were obtained.
The drying rate coefficients were determined by fitting
the data generated in the thin-layer drying experiment to
equation 1. These values were then represented in a linear
model as a function of air temperature and airflow rate.
This model is shown below.
K = C1 + C2 × T + C3 × V
(4)
RESULTS AND DISCUSSION
DRYING CHARACTERISTICS
The thin-layer drying experiment produced oven-dried
weights for the petals and receptacles. From these
measurements, it was determined that the dried petal to
dried receptacle mass ratio varied between 1 and 2.
Equilibrium moisture contents of Orange Lady
marigolds were determined in a separate test using saturated
salt solutions (Rahman, 1995). The equilibrium sorption
rate test used a variety of salts to generate equilibrium
moisture content data for the flowers, petals, and
receptacles corresponding to relative humidities of 11, 23,
33, 43, 52, 75, 86, and 97% at a temperature of 25°C. The
moisture content results were fitted to the GuggenheimAnderson deBoer (GAB) equation, an ASAE standard
equation for determining the equilibrium moisture content
(EMC) of plant-based agricultural materials and their
products (ASAE Standards, 1998). The GAB equation is:
MCD =
A × B × C × RH
1 – B × RH × 1 – B × RH + B × C × RH
(5)
This equation fit the EMC data for the flower, petals, and
receptacle with r 2 values of 0.96, 0.96, and 0.95,
respectively, over a range of 5 to 75% r.h. Table 2 lists the
Table 2. GAB coefficients for Orange Lady marigold
equilibrium moisture content data
Description
A
B
C
Flower
Petal
Receptacle
0.07
0.07
0.11
0.96
1.08
0.71
76.26
75.30
11.98
Figure 2–Equilibrium moisture contents for marigold flowers, petals,
and receptacles.
1369
coefficients for the GAB equation fit. Figure 2 shows the
fitted curves and EMC data of the flowers, petals, and
receptacles.
The drying rate coefficients for each replication of each
test were determined by fitting the computed moisture
ratios with the corresponding time interval to equation 1.
Tables 3-5 show the resulting drying coefficients (K) and
their r 2 values for the flowers, petal, and receptacles
respectively for all 12 combinations of air temperature and
airflow rate.
The drying rate coefficients determined from each test
replication were used to derive equations for calculating
drying rate coefficients based on a given air temperature
and airflow rate entering the drying bed. These equations
were determined by fitting the drying rate coefficients and
their corresponding air temperatures and airflow rates to
equation 4. The resulting equations are:
K = – 0.65 + 0.013 × T + 0.35 × V
(6)
for the whole flower with an r 2 of 0.98;
Table 3. Drying rate coefficients, K, and r2 values for
thin-layer whole flower data (3 replicates)
Airflow Rate
(m3·s–1·m–2)
0.23
Temp. (°C) K
55
(h–1)
0.15
0.23
0.14
0.25
0.28
0.22
0.28
0.36
0.25
0.34
0.40
0.32
60
65
70
0.28
r2
0.91
0.74
0.91
0.95
0.95
0.92
0.87
0.92
0.90
0.91
0.90
0.83
K
(h–1)
0.21
0.25
0.18
0.24
0.34
0.20
0.34
0.38
0.28
0.45
0.53
0.44
K = – 1.9 + 0.035 × T + 1.2 × V
0.33
r2
0.90
0.82
0.85
0.88
0.91
0.86
0.81
0.92
0.81
0.91
0.88
0.89
K
(h–1)
0.17
0.25
0.17
0.24
0.29
0.23
0.34
0.34
0.26
0.44
0.51
0.41
r2
0.88
0.89
0.79
0.84
0.91
0.79
0.93
0.88
0.84
0.88
0.93
0.85
Table 4. Drying rate coefficients, K, and r2 values
for thin-layer petal data (3 replicates)
Airflow Rate
(m3·s–1·m–2)
0.23
Temp. (°C) K (h–1)
55
60
65
70
0.16
0.37
0.14
0.36
0.44
0.29
0.43
0.59
0.61
0.55
0.80
0.47
0.28
0.33
r2
K (h–1)
r2
K (h–1)
r2
0.98
0.96
0.94
0.97
0.99
0.95
0.87
0.96
0.95
0.91
0.96
0.84
0.26
0.43
0.20
0.40
0.56
0.32
1.04
0.82
0.52
1.15
1.71
0.79
0.87
0.92
0.91
0.95
0.73
0.92
0.98
0.97
0.86
0.99
0.99
0.99
0.26
0.62
0.23
0.47
0.76
0.54
0.38
0.66
0.41
0.77
0.98
0.76
0.96
0.99
0.86
0.98
0.99
0.96
0.89
0.91
0.90
0.94
0.99
0.98
(7)
for the petals with and r 2 of 0.88; and
K = – 0.16 + 0.0053 × T + 0.095 × V
(8)
for the receptacle with an r 2 of 0.97. Based on the
coefficients of equations 6, 7, and 8, air temperature was
more important than airflow rate in determining the drying
coefficients. Figure 3 shows the measured and predicted
moisture contents using equations 6, 7, and 8 versus time
for an airflow rate of 0.28 m 3·s –1·m –2 and an air
temperature of 70°C.
PETAL DETACHMENT
The work associated with detaching the petals from the
receptacle increased as the physical characteristics of the
flower components deviated from PDE code three. When
both the petals and receptacle were wet, the work required
to detach the petals from the receptacle was greatly
increased. When both components were dry, the work
required to sort the petals from the receptacle was greatly
increased. The later was due to the shattering of the
receptacle caused by over drying.
To represent the effects of air temperature and airflow
rate on petal detachment, graphs with four regions were
generated: under-dried, over-dried, optimum drying, and
transition regions. The under-dried region refers to an area
on the graph where wet petals are expected and the overdried region refers to an area on the graph where dry petals
and dry receptacles are expected for a given air
Table 5. Drying rate coefficients, K, and r2 values for thin-layer
receptacle data. (3 replicates)
Airflow Rate
(m3·s–1·m–2)
0.23
Temp. (°C) K (h–1)
55
60
65
70
1370
0.16
0.17
0.15
0.19
0.19
0.16
0.19
0.22
0.15
0.20
0.28
0.18
0.28
0.33
r2
K (h–1)
r2
K (h–1)
r2
0.88
0.81
0.94
0.92
0.90
0.92
0.92
0.88
0.87
0.81
0.91
0.82
0.18
0.22
0.15
0.18
0.25
0.13
0.22
0.24
0.19
0.27
0.31
0.26
0.88
0.83
0.87
0.82
0.87
0.76
0.78
0.85
0.87
0.90
0.84
0.92
0.12
0.20
0.13
0.15
0.19
0.15
0.23
0.24
0.19
0.25
0.29
0.24
0.73
0.81
0.85
0.80
0.84
0.65
0.91
0.79
0.64
0.85
0.84
0.86
Figure 3–Moisture content profile based on thin-layer drying
coefficient equations at drying temperature of 70°C and airflow rate
of 0.28 m3·s–1·m–2.
TRANSACTIONS OF THE ASAE
temperature and drying time. The optimum drying region
refers to an area on the graph where dry petals and wet
receptacles are expected. The transition region is divided
into two sub-regions, wet-to-dry petals and wet-to-dry
receptacles. These two sub-regions combined refer to an
area on the graph where the petals and receptacle could be
wet, damp or dry. In addition to the drying regions, the
PDE codes from the petal detachment study were overlaid
on the graphs.
The profiles of the PDE codes (figs. 4-6) at airflow rates
of 0.23, 0.28, and 0.33 m3·s –1·m–2 show that the optimum
drying region increases with an increase in airflow rate. At
an airflow rate of 0.23 m3·s –1·m–2, the graphs indicate that
air temperatures in excess of 65°C are required for
optimum petal detachment. At an airflow rate of
0.33 m3·s –1·m–2, air temperatures in excess of 60°C are
required for optimum petal detachment. The graphs further
indicate that air temperatures less than 60°C are not
feasible for petal detachment. At air temperatures less than
60°C, petals and receptacles are drying at the same rate or
the receptacles are drying faster than the petals. Therefore,
petal detachment was found to be a function of both air
temperature and airflow rate.
XANTHOPHYLL CONTENT
The xanthophyll contents of interest corresponded to the
drying times associated with the transition and optimum
drying regions of the PDE profiles. The xanthophyll data in
the timeframes of these regions were selected from the
entire data set. Table 6 shows the means and standard
deviations of the xanthophyll contents. There was a large
variability in the xanthophyll content possibly due to large
variation among individual flowers. It was concluded that
single flower sampling during drying was not adequate for
comparing xanthophyll reduction with air temperature and
airflow rate. The large variability limited the conclusions
that could be made about the effects of temperature on
xanthophyll stability. A t-test, assuming unequal variances,
with an alpha of 0.05 resulted in a t-value of 2.4 and a
critical t-value of 1.7; indicating that high air temperature
during drying degrades xanthophyll content of the petals.
SUMMARY AND CONCLUSIONS
Drying rate coefficient equations were derived using a
thin-layer drying model from experimentally determined
moisture contents of Orange Lady marigolds. Statistical
differences in drying rate coefficients were detected
between all temperature and airflow rate combinations.
These differences could relate to handling, uniformity of
the airflow rate, or the variability of flower mass
composition. To compensate for these differences, the
drying rate coefficients of each replication were used to
generate a linear model for the drying rate coefficients as a
function of air temperature and airflow rate. These models
for predicting drying rate coefficients produced r 2 values
of 0.98, 0.88, and 0.97 for the flowers, petals, and
receptacles, respectively.
Based on the thin-layer drying experiment, it was
concluded that the drying rate coefficients for Orange Lady
marigold flowers and flower components are statistically
different. The petal components dried faster than the whole
flower, which dried faster than the receptacle components.
As the air temperature decreased, the difference in drying
rate coefficients for the whole flower and flower
components decreased. Further, drying air temperature was
more important than airflow rate in computing the drying
rate coefficients.
The identifying factors representing petal detachment
were characterized qualitatively during drying. By
Figure 4–Petal/receptacle drying regions based on petal detachment ease for drying at an airflow rate of 0.23 m3·s–1·m–2. Numbers (1, 2, . . . 9) on
the figure represent the actual petal detachment codes collected during the study.
VOL. 42(5): 1367-1373
1371
Figure 5–Petal/receptacle drying regions based on petal detachment ease for drying at an airflow rate of 0.28 m3·s–1·m–2. Numbers (1, 2, . . . 9) on
the figure represent the actual petal detachment codes collected during the study.
Figure 6–Petal/receptacle drying regions based on petal detachment ease for drying at an airflow rate of 0.33 m3·s–1·m–2. Numbers (1, 2, . . . 9) on
the figure represent the actual petal detachment codes collected during the study.
detaching the petals from the receptacle and describing
each component as wet, damp or dry, graphs were
1372
developed representing under-dried, over-dried, transition,
and optimum drying regions. Petal detachment was
TRANSACTIONS OF THE ASAE
Table 6. Xanthophyll contents (data associated
with the optimum PDE region)
Temp.
(°C)
No. of
Samples
60
65
70
23
25
32
Xantrophyll (mg of lutien/g of dry petal)
Mean
Standard Deviation
7.3
5.6
4.9
4.6
2.0
2.3
determined to be a function of air temperature and airflow
rate. As air temperature and airflow rate increase, the
optimum petal detachment region increased and the drying
time required to attain optimum petal detachment
decreased. At airflow rates of 0.23 and 0.33 m3·s –1·m–2, air
temperatures in excess of 65 and 60°C were required for
optimum petal detachment.
Xanthophyll content degraded at high drying air
temperatures; however statistically significant conclusions
could not be drawn because of high variability in the
xanthophyll content of the flowers.
It was concluded that the optimum conditions for petal
processing, in the temperature range of 55 to 70°C and
airflow range of 0.23 to 0.33 m3·s –1·m–2, were obtained at
an air temperature of 70°C and an airflow rate of
0.33 m3·s –1·m–2.
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NOMENCLATURE
a, n, A, B, C
C1, C2, C3
K, K1, K2
M
Me
Mi
MCD
RH
t
T
V
coefficients
linear coefficients obtained through regression
analysis
drying rate coefficient (h–1)
instantaneous moisture content (dimensionless)
equilibrium moisture content (dimensionless)
initial moisture content (dimensionless)
dry-basis moisture content (%)
relative humidity (%)
drying time (h)
air temperature (°C)
airflow rate (m3·s –1·m–2)
1373