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Chapter 3 _____________________________________________________________________________________________
166
3.1. INTRODUCTION
Drying, also known as dehydration is a process that removes moisture leading to a
dried product with reduced weight and volume, thereby provides better shelf life. Most of the
drying techniques apply heat on the product to remove moisture (Muller and Heindl, 2006). It
was observed that drying method had a significant effect on volatile oil content of aromatic
plants and its composition (Diaz-Maroto et al., 2003). Losses of selected components are
reported during the drying process; therefore appropriate drying method is vital for the quality
product. Processing methods may also improve the properties of naturally occurring
antioxidants or induce the formation of new compounds having antioxidant properties, so that
the overall antioxidant activity of plant (raw material) can remain unchanged or increased
despite the eventual loss of ingredients (Tomaino et al., 2005). Thermal drying technologies
attracted significant research and development for improved product with reduced cost, as
well as diminished environmental impact (Mujumdar and Law, 2010).
Drying of spices inhibits microorganism growth and forestalls certain biochemical
changes. However, it gives rise to other alterations such as changes in appearance and
alterations in aroma due to loss in volatiles or the formation of new volatiles as a result of
reactions e.g., oxidation, dehydration, rearrangement, esterification etc. In recent times,
applications of microwave drying were increased abundantly due to its advantages such as
higher drying rate, shorter drying time, rapid and volumetric heating, decreased energy
consumption, and better quality of the dried products (Zhang et al., 2006). Microwave drying
(MW) had gained popularity as an alternative drying method for a variety of food products
such as fruits, vegetables, snack foods, and dairy products (Wang and Sheng, 2006). One of
the main advantages in using the microwave heating as an alternate to convectional heating
was that the temperature and moisture gradients are in the same direction in the MW and aid
each other, whereas during convection heating, moisture move out of the material against
gradient of temperature (Murthy and Prasad, 2005). When biomaterials are subjected to a
microwave (MW) field, the wave penetrates directly into the material resulting in fast
volumetric heating (from the inside out). The quick energy absorption by water molecules
causes rapid evaporation of water, creating an outward flux of rapidly escaping vapor. In
Chapter 3 _____________________________________________________________________________________________
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addition to improving the rate of drying, this outward flux helps to prevent the shrinkage of the
tissue structure (Chua and Chou, 2005).
Dry ginger is a value added commodity of trade and utilized for manufacturing of
ginger powder, ginger oil, ginger essence, ginger oleoresins and soft drinks. All these value
added products are flavouring ingredients in various foods (Govindarajan, 1982).
Dehydration of ginger is a vital requirement to reduce the postharvest losses with minimum
changes in physical, chemical and organoleptic properties. Fresh ginger contains 80-90%
moisture and dried to final moisture content below 10% using sun-drying, which is the most
conventional method of drying and employed from earlier days. Since sun drying is weather
dependent and unhygienic due to microbial spoilage, insect attacks and dust contamination.
Therefore, mechanical dryers are employed which not only gives better quality product, but
also avoids the dependency on the weather and reduces the spoilage and contamination of
the product (Prasad et al. 2006).
Pungency is an important quality characteristic of ginger. The constituents
responsible for the pungent taste of ginger are a homologous series of phenolic ketones,
known as gingerols. In fresh ginger, the gingerols are identified as the major active
components. Shogaols are gingerol analogues with a 4, 5 double bond, resulting from
elimination of the 5-hydroxy group in alkyl side chain. The shogaol series of compounds,
even more pungent than the gingerols, are virtually absent in fresh ginger, and is derived from
the corresponding gingerols during thermal processing or long-term storage (Ravindran et al.,
2005). Generally, the dehydradation reaction of gingerol to shogaol takes place either
because of the acidic environment or as a result of the increase in temperature. It was
reported that gingerol was stable in the pH range 1 to 7 at 37°C; however, it starts degrading
at 60°C and above in aqueous solutions (Bhattarai et al., 2001).
During drying of ginger, the retention of the active components is essential for use in
medicinal applications. Hawlader et al., (2006) studied the drying of ginger rhizomes using
advanced drying methods and modified atmosphere. They reported that atmospheric heat
pump drying results in better retention of pungent components. Phoungchandang et al.,
(2009) compared the tray dryer, heat pump dryer, and mixed-mode solar dryer for drying of
ginger pieces. It was reported that mixed-mode solar drying showed high performance
Chapter 3 _____________________________________________________________________________________________
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potential compared to other drying methods. Ginger pieces were dried using mixed mode
solar drying, provided the shortest drying time and retained [6]-gingerol as high as that
obtained when using a heat pump dryer. However, information regarding antioxidant
properties of aqueous ethanolic extracts as affected by various drying methods was not
available.
Scope of the present study
The high moisture content leads to low shelf life of raw ginger make the transportation
and marketing an expensive proposition (Bartley and Jacobs, 2000). Also, the post harvest
losses due to mishandling, lack of wash / cleaning, immature harvesting etc were the
constraints for the farmers. Even sun drying of ginger is not practiced in many countries
because of the adverse climatic conditions. Therefore, it is imperative to study some
alternative cost-effective techniques for the production of value added products from fresh
ginger, which find market within the country as well as overseas. There are no reports in the
literature regarding drying of fresh ginger rhizomes using microwave energy. Hence, the
objective of the present study is optimization of drying of ginger using microwave energy and
its effect on yield, chemical composition of essential oil and non-volatile components ([6]-
gingerol content).
In this chapter, systematic study of chemical composition of volatiles from fresh,
microwave dried and conventional dried ginger are presented (fig. 3.1). Quality of ginger as
affected by drying methods was compared with that of convection-dried ginger. The effects of
microwave drying on the total polyphenol content (TPP) and antioxidant properties were also
evaluated using the Folin-Ciocalteau method, DPPH assay, reducing power, antioxidant
capacity assay and FRAP assay.
Chapter 3 _____________________________________________________________________________________________
169
3.2. METHODS
3.2.1. Convection drying (CD)
The slices of fresh ginger (as described in materials section) were uniformly spread in
aluminium trays to a thickness of 6-10 mm and dried in 48-tray convection dryer at 50 ± 4°C
for 12 h. During drying, the slices were mixed periodically at hourly intervals so as to facilitate
uniform drying. The dried flakes were packed in metalized polyester-polyethylene pouches.
3.2.2. Microwave drying (MW)
Drying treatment performed using digital microwave oven. The microwave oven was
set to different power levels (PLs 40, 60, 80 and 100 with microwave output power 385, 525,
660 and 800 W respectively) during drying experiments. The slices (250g / batch) were
evenly spread on the metal turntable and dried at each microwave power level. The contents
were mixed to avoid overheating and localized heating. Drying times were determined from
the average of triplicate experiments. Microwave-dried flakes were packed as mentioned
above.
Fig. 3.1. Flow diagram of the experimental work and scope of research
Fresh ginger
Convective drying at 50-54°C
Microwave drying at 800W, 660W, 525W, 385W
Moisture content
Colour analysis
Essential oil content
Non-volatile extraction
GC and GC-MS analysis
Antioxidant activity
Total polyphenolic content
HPLC analysis
Chapter 3 _____________________________________________________________________________________________
170
3.2.3. Moisture estimation
Moisture content of the fresh ginger slices and dried flakes obtained by different
drying conditions was determined as per ASTA (American Spice Trade Association)
procedure (ASTA, 1985) using Dean-stark apparatus and average of triplicate experiments
was provided.
3.2.4. Colour Analysis
Colour values of fresh ginger slices and dried ginger flakes were measured using
Hunter Labscan Spectro colorimeter. The L* coordinate ranged from 0 (black) to 100 (white)
indicates brightness, a* coordinate indicated red-green colour, and b* coordinate indicated
yellow-blue colour. Measurements were conducted in triplicates and mean values were
reported.
3.2.5. Isolation of essential oils and identification of compounds
Fresh ginger slices (500 g) were homogenised with water and resultant slurry was
subjected to hydro distillation in Clevenger type apparatus immediately. The Distillation was
carried out till the maximum quantity of oil was obtained (4-5 h). Dried ginger flakes (50 g
each) were ground and essential oil isolated as above. The pale yellow oils were dried over
anhydrous sodium sulphate and stored in dark glass bottles at 4°C for further analyses.
3.2.6. Chromatographic analysis
3.2.6.1 Gas chromatography (GC) analysis
GC analyses were carried out on a Fisons 8000 series gas-chromatograph equipped
with a flame ionization detector (FID) and a SE-30 capillary fused silica column [30 m length,
0.25mm I.D.; 0.25 µm polydimethyl siloxane (PDMS) film thickness]. The oven temperature
was held at 60°C for 8 min then increased at 4°C/min to 200°C, held for 5 min. Other
operating conditions were as follows: carrier gas, He (99.99%); inlet pressure, 75 kPa; with a
linear velocity of 20 cm/s; injector temperature, 220°C; detector temperature, 230°C; split
ratio, 1:25. Each sample was diluted in acetone (1:20 v/v) and one µl was injected. The
percentage composition of the oils was computed by the normalization method from the GC
peak areas. Individual peak areas calculated as average of three injections. Quantities
(µl/100g) of each component calculated using normalization method.
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3.2.6.2. Gas chromatography-mass spectrometry (GC-MS) analysis
The essential oils were analysed using Perkin Elmer GC equipped with quadrupole
mass spectrometer. A fused silica column SPB-1 (30 m length, 0.25 mm inner diameter, film
thickness 0.25 µm) coated with PDMS was used. Helium was used as carrier gas at a flow
rate of 1 ml/min. The oven temperature programme was as follows: 60°C (held 2 min), raised
to 150 °C at a rate of 2°C/min, then heated to 170°C at a rate of 1°C/min and finally increased
to 200°C at 4°C/min, and held for 2 min (76 min total run). Injector temperature was 220°C
and split ratio, 1:20. The mass spectrometry (MS) conditions were as follows: ionization
voltage, 70 eV; emission current, 40 mA; scan rate, 1 scan/s; mass range, 40-400 Da; ion
source temperature, 160°C. Each sample was diluted in acetone (1:20 v/v) and one µl was
injected. A mixture of aliphatic hydrocarbons (C8-C30) diluted with acetone was also injected
and analysed under similar conditions to calculate the Kovats’ retention indices using the
following equation (Jennings and Shibamoto, 1980).
I = 100 N + 100 n [log t' R (A) log t' R (N) ] / [log t' R (N + n) log t' R (N)]
t' R(N) and t' R ( N+ n) are the adjusted retention times of n-alkane hydrocarbons of
carbon number N and (N + n) that are respectively eluted before and after the sample t' (R).
The total ion chromatograms of the volatile oils were obtained using GC-MS.
3.2.7. Compounds identification
The constituents of the essential oils were identified by comparison of Kovats Indices
with those reported in the literature (Jennings and Shibamoto, 1980; Davies, 1990, NIST
Chemistry web book), and matching the fragmentation patterns in the mass spectra with
those stored in NIST mass spectral libraries and published in literature (Adams, 2001; NIST
Chemistry web book). The identification also confirmed by co-injection with an authentic
sample, wherever possible.
3.2.8. Solvent extraction
Dried flakes (both MW and CD dried) were ground to reduce the particle size to BS
25 mesh (<600 microns) using apex mill. Fifty grams of sample was loaded onto a glass
column and aqueous ethanol (50%) was added. It was allowed to percolate and stand
overnight at room temperature. The extracts (1000 ml) were collected and filtered through a
Chapter 3 _____________________________________________________________________________________________
172
Whatman filter paper No. 1. The filtrate was evaporated in a rotary evaporator (<50°C) until
dryness and the weight of each extract was recorded and percentage yield was calculated.
The extracts were analysed for total polyphenol content, [6]-gingerol content as well as
antioxidant properties.
3.2.9. Total polyphenol (TPP) content
The concentration of total polyphenol content in extracts was determined by the Folin-
Ciocalteau (FC) colorimetric oxidation/reduction method as described in chapter 1 (section
1.2.2.). TPP expressed as mg GAE /g.
3.2.10. High performance liquid chromatographic analysis of [6]-gingerol
Ginger extracts were analyzed on a Waters HPLC system consisting of a model 515
pump, a model 2487 dual wavelength absorbance detector was used. The analysis of the
extracts was conducted using a reversed phase C18 column SS Exsil ODS (SGE), particle
size 3 µm, i.d. 4.6, length 250 mm, pore size 80 Å. Acetonitrile-water (55:45, v/v) was used
as the mobile phase using isocratic elution. MW and conventional extracts (10 mg/ml) were
dissolved in methanol and injected. Operating parameters were as follows, injection volume,
10µl; flow rate, 1.0 ml/min. The eluting compounds were detected at λmax of 280 nm.
Triplicate injections were performed to ensure accuracy and reproducibility. [6]-gingerol was
accurately weighed and dissolved in methanol to produce stock standard solutions, which
was isolated from methanolic fraction of ginger extract and the structure elucidated employing
NMR spectral data (Rahath Kubra et al., 2011, chapter 1, Table 1.9). The stock solutions
were serially diluted to prepare working solutions for the calibration curves at five
concentration levels (12.5-200 µg). All the solutions were stored in amber glass bottles at
4°C. The calibration curve of the gingerol was prepared using the peak areas and
concentrations of working solutions. [6]-gingerol content of the samples was determined
using this calibration curve.
3.2.11. Antioxidant assays
The crude extracts from the MW and CD dried rhizomes were analyzed for the for
antioxidant activity by four methods, as follows.
Chapter 3 _____________________________________________________________________________________________
173
3.2.11.1. Radical scavenging activity
Antioxidant activity of the extracts was determined using the 1, 1’-diphenyl-2-picryl-
hydrazyl (DPPH) assay using the stable radical DPPH• according to the method of Blois
(1958) as mentioned in chapter 1 (section 1.2.3.1).
3.2.11.2. Reducing power
The reducing power of the extracts was evaluated (Oyaizu, 1986) as described in
chapter 1 (section 1.2.3.2).
3.2.11.3. Antioxidant capacity
The total antioxidant capacity of extracts along with the standard propyl gallate was
evaluated (Prieto et al. 1999) as reported in chapter 1 (section 1.2.3.3.).
3.2.11.4. Reducing ability (FRAP assay)
Total antioxidant activity (ferric reducing ability of plasma assay) of the extracts was
evaluated by modified method of Benzie and Strain (1996) mentioned in chapter 2 (section
2.2.7).
3.3. RESULTS AND DISCUSSION
3.3.1. Effects of drying methods on ginger
The present study aimed to optimise the drying conditions of ginger using microwave
energy. Drying times along with moisture, consumed energy, colour values and essential oil
contents of fresh and dried ginger samples are presented in table 3.1. The drying time
reduced significantly from 720 min for convective (CD) drying to 25-48 min in microwave
drying. Water molecules absorb microwave energy rapidly, resulting in rapid evaporation and
higher drying rates. Further, the total energy consumed was found to be almost same in MW
drying at different PLs and is comparable to optimally loaded convection dryer. It was
reported that the interior temperature of microwave-heated food was higher than the surface
temperature and moisture was being transferred to the surface more dynamically than during
CD (Diaz-Maroto et al., 2003).
Chapter 3 _____________________________________________________________________________________________
174
Table 3.1. Essential oil content and colour values of fresh and dried ginger rhizomes
Parameter FGy CGz Microwave dried
PL 100 PL 80 PL 60 PL 40
Moisture (%) 82 (±3)l 9 (±2) 7 (±2) 10 (±2) 9.5 (±2) 11 (±2)
Drying time (min) -- 720 25 32 40 48
Essential oil (% v/w) 3.2c 2.9bc 3.0bc 2.3a 2.0a 2.3a
Energy consumed
(KJ) -- -- 1200 1267 1260 1109
Colour (flakes)
L * (brightness) 53.9e 54.5f 49.8b 44.7a 50.1c 52.3d
a* (redness) 1.3a 4.9bcd 5.3cde 6.9e 5.5e 4.4bcd
b* (yellowness) 29.7d 17.9a 24.2c 23.5bc 23.1bc 18.9a
DE (total colour
difference) 47.3b 44.9a 50.1d 54.7e 49.8c 47.0b
y FG-Fresh ginger; z CG-Convection dried ginger; PL – microwave power level l Values expressed are mean of three experiments. Values followed by the same letter, within the same row, are not significantly different, p < 0.05 (DMRT test).
3.3.2. Effect of drying methods on essential oil content
In the present work, both the homogenised fresh ginger slices and ground dried
rhizome flakes were subjected to Clevenger’s hydrodistillation immediately to obtain good
quality essential oil. Macleod and Pieris (1984) reported that the quality of the essential oil
was found to be better, when the rhizomes were ground immediately prior to distillation. Yield
of the essential oil was generally dependent on a number of factors such as the ginger
variety, stage of maturity at harvest, method of preparation, drying and distillation
(Abeysekera et al., 2005). However, ginger variety, stage of maturity at harvest and
distillation method was same in the present study. Yields of essential oil obtained for fresh
and CD samples were 3.2 and 2.9 % (v/w) respectively (Table 3.1). The yields of the
essential oils were in the range of 2.0-2.3% (v/w), when MW PLs 40 (385W), 60(525W) and
80(660W) were used for drying. Losses were observed in the range of 30-40% when
compared to fresh and 20-30% compared to CD. This reduction in the essential oils at lower
microwave levels may be attributed to the temperature rise due to heat generation within the
Chapter 3 _____________________________________________________________________________________________
175
rhizome and due to the longer time exposure of sample to microwave radiation. These
conditions might have driven out the volatile components during drying. However, the yield of
essential oil was 3.0% for the sample dried at PL 100 (800 W) and was little higher than that
of CD sample. It was reported that the hydrophobic nature of volatiles also plays some role in
controlling the loss (viz., retention of sesquiterpene hydrocarbons at PL 100) of volatile
compounds during the drying process (Figiel et al., 2010). On the other hand, essential oil
obtained under microwave drying at PL 60 (525 W) showed substantial losses in most of
volatiles which may be due to expansion of the structure of the epidermis of plants. This effect
causes volatiles to be released into the air (Lerdau et al., 1997; Dίaz-Maroto et al., 2003).
3.3.3. Effect of drying on colour
The colour values (viz., L*, a* and b* values) of fresh ginger slices and dried ginger
flakes were presented in Table 3.1. Assuming the L*, a* and b* values determined for fresh
material were chosen as control for colour comparison. The flakes resulted from convection
drying are brighter (higher L* value) than the fresh slices. However, the brightness of the
microwave-dried flakes was decreased irrespective of the power level used. Less brightness
in microwave dried samples of garlic was reported earlier (Rao et al., 2007). Brightness of
the flakes resulting from microwave drying at PL 40 are closer to the fresh slices, whereas
drying at PL 80 was lowest (darker). The a* value of fresh slices was 1.3 indicating very low
redness. The a* values of the CD sample was 4.9, indicating redness was increased on
drying. The a* values of MW flakes were in the range of 4.4-6.9, indicating the increase in
redness. The PL 40 microwave dried sample had comparable a* values to that of the CD
flakes, while a* values of the other samples dried at PL 60 and PL 80 were largest, indicating
redness was higher. Colour shift in convection as well as microwave-dried samples to
redness reported earlier with garlic (Figiel, 2009). The b* value of fresh slices was largest
(viz., 29.7) indicating most yellow colour, while the b* value of CD was lowest (viz., 17.9)
indicating drastic decrease of yellow colour of the samples. The b* values were in the range
of 19.0-24.2 for MW flakes, indicating the decrease in yellowness and the samples were
moderate in yellow colour. The b* value of the sample dried at PL 40 showed a marked
decrease in the yellowness and was closer to the CD flakes, whereas the samples dried at PL
100, PL 80 and PL 60 showed comparable b* values and were moderate in yellow colour.
Chapter 3 _____________________________________________________________________________________________
176
3.3.4. Chemical characterization and quantification of essential oils
Chemical composition of essential oils from fresh as well as dried ginger samples
was determined and the identified components in the order of their elution on SPB-1 column
along with their molecular formula, molecular weight, Kovats indices and methods of
identification were presented [Table 3.2]. The identified compounds were classified as
monoterpene hydrocarbons, oxygenated monoterpenes, sesquiterpene hydrocarbons,
oxygenated sesquiterpenes, other aliphatic compounds and oxygenated aliphatic compounds
(Table 3.3). Majority of the components are derived from mevalonic acid pathway
biogenetically (Fig. 6). Terpenoids are derived from C5 isoprene units in the form of the
diphosphate (pyrophosphate) esters dimethylallyl diphosphate and isopentenyl diphosphate
and are the products of the mevalonate pathway (Dewick, 1997, 2002). Both the
monoterpenoids (such as neral, geranial, geraniol) and sesquiterpenenoids (such as β-
sesquiphellandrene, α-farnesene) are formed through a common biosynthetic route.
The relative concentrations (expressed in terms of microliters per 100g of dry weight)
of identified chemical components present in the essential oil extracted from the fresh as well
as the dried rhizomes using GC analysis presented in Table 3.2. Seventy-four compounds
constituting more than 99% of the fresh ginger oil (FGO) were characterized using GC (Fig
3.2) and GC-MS analysis (Fig. 3.3 to 3.8). The hydrocarbon content in FGO was 80.7% and
the oxygenated compounds were 18.5%. Zingiberene (23.5%) was the major sesquiterpene
hydrocarbon, followed by -farnesene (12.0%), β‐sesquiphellandrene (10.3%) and ar-
curcumene (5.5%). Main sesquiterpene alcohols present were (E)-nerolidol (0.5 %) and β -
eudesmol (0.4%). β‐phellandrene (9.3%) was the major monoterpene hydrocarbon followed
by camphene (6.2%), and α-pinene (2.1%). The major oxygenated monoterpene compound
was geranial (6.4%) followed by neral (3.7%), α-terpineol (0.5%) and linalool (0.3%).
Chapter 3 _____________________________________________________________________________________________
177
Fig. 3.2. GC chromatograms of ginger oil
(a) Fresh ginger oil (FGO); (b) Cabinet dried ginger oil (CGO); (c) Microwave dried at PL100;
(b) CGO
(a) FGO
(c ) PL100
......Continued
Chapter 3 _____________________________________________________________________________________________
178
Fig. 3.2. GC chromatograms of ginger oil
(d) Microwave dried at PL 80; (e) Microwave dried at PL 60 and (f) Microwave dried at PL 40
(d) PL 80
(e) PL 60
(f) PL 40
Chapter 3 _____________________________________________________________________________________________
179
8.50 18.50 28.50 38.50 48.50 58.50 68.50Time0
100
%
TIC3.31e9
36.856.81
6.37
3.63
9.84
8.27
35.6822.40
20.54
16.3513.07
35.41
29.59 30.44
37.54
38.30
39.62 42.82 46.84
8.50 18.50 28.50 38.50 48.50 58.50 68.50Time0
100
%
rahath1 Scan EI+ TIC
3.83e96.83
6.39
3.64
9.86
8.30
36.75
35.699.91
22.4020.57
16.3713.09
35.3929.59
37.4838.24
39.59 42.82 46.84
8.50 18.50 28.50 38.50 48.50 58.50 68.50Time0
100
%
rahath2 Scan EI+ TIC
2.82e936.80
6.77
6.35
9.79
8.26
35.66
9.8422.2520.46
16.32
35.3929.58
37.49
38.25
42.81 44.17
Fig. 3.3. Total ion chromatograms of ginger oil obtained using GC-MS
(A) Fresh ginger oil (FGO); (B) Cabinet dried ginger oil (CGO); (C) Microwave dried at PL 100
(a) FGO
(B) CGO
(C) PL 100
......Continued
Chapter 3 _____________________________________________________________________________________________
180
8.50 18.50 28.50 38.50 48.50 58.50 68.50Time0
100
%
TIC2.53e9
36.80
6.80
6.37
3.63
9.82
8.28
35.67
22.309.87 20.49
16.3435.40
29.58 30.44
37.49
38.26
42.84 44.21
8.50 18.50 28.50 38.50 48.50 58.50 68.50Time0
100
%
rahath3 Scan EI+ TIC
3.55e96.84
6.40
3.64
36.799.86
8.32
35.659.9122.3520.53
16.3635.40
29.59
37.50
38.25
42.84 44.21
8.50 18.50 28.50 38.50 48.50 58.50 68.50Time0
100
%
TIC2.95e9
6.83
6.39
3.64
36.78
9.85
8.30
35.69
9.9022.3420.52
16.3635.40
29.59 30.45
37.49
38.26
42.83 44.19
Fig. 3.3. Total ion chromatograms of ginger oil obtained using GC-MS
(D) Microwave dried at PL 80; (E) Microwave dried at PL 60 and (F) Microwave dried at PL 40.
(D) PL 80
(E) PL 60
(F) PL 40
Chapter 3 _________________________________________________________________________________________________
181
Fig. 3.4. Mass spectra of monoterpene hydrocarbons present in ginger oil
.....contd.
Chapter 3 _________________________________________________________________________________________________
182
Chapter 3 _________________________________________________________________________________________________
183
Fig. 3.5. Mass spectra of oxygenated monoterpene present in ginger oil
.....contd.
Chapter 3 _________________________________________________________________________________________________
184
Chapter 3 _________________________________________________________________________________________________
185
Fig. 3.6. Mass spectra of sesquiterpene hydrocarbons present in ginger oil
.....contd.
Chapter 3 _________________________________________________________________________________________________
186
.....contd.
Chapter 3 _________________________________________________________________________________________________
187
.....contd.
Chapter 3 _________________________________________________________________________________________________
188
.....contd.
Chapter 3 _________________________________________________________________________________________________
189
Chapter 3 _________________________________________________________________________________________________
190
Fig. 3.7. Mass spectra of oxygenated sesquiterpene present in ginger oil
.....contd.
Chapter 3 _________________________________________________________________________________________________
191
.....contd.
Chapter 3 _________________________________________________________________________________________________
192
.....contd.
Chapter 3 _________________________________________________________________________________________________
193
Chapter 3 _________________________________________________________________________________________________
194
Fig. 3.8. Mass spectra of other compounds present in ginger oil
.....contd.
Chapter 3 _________________________________________________________________________________________________
195
Chapter 3 _________________________________________________________________________________________________
196
Table 3.2. Concentrations of the volatile components (µl/100g of dry weight) of fresh and dried rhizomes of ginger
Peak no. Componentsx Molecular
formula M+ KI cal Identified
by o
Relative content (l/100g of dry weight)p
FGO CGO PL 100 PL 80 PL 60 PL 40
1 2-Pentanone-4-hydroxy, 4-methyl C6H12O2 116 802 KI, MS 14bc 15c 17d 14bc 12a 12a
2 2-Heptanol C7H16O 116 887 KI, MS 3a 6b - - - -
3 Tricyclene C10H16 136 921 KI, MS 4cd 5d 2ab 2ab 3abc 3bcd
4 α-Pinene C10H16 136 933 KI, MS, CI 68d 81e 43a 41a 61c 52b
5 Camphene C10H16 136 946 KI, MS, CI 197d 259e 133a 128a 189c 167b
6 6-Methyl-5-Hepten-2-one C8H14O 126 968 KI, MS 11d 12e 2b 1a 2b 3c
7 β-Pinene C10H16 136 971 KI, MS, CI 8c 10d 6a 6a 8c 7b
8 β-Myrcene C10H16 136 985 KI, MS, CI 28d 35e 24b 19a 26c 24b
9 α-Phellandrene C10H16 136 997 KI, MS, CI 9e 8d 7c 5a 9e 6b
10 p-Cymene C10H14 134 1013 KI, MS, CI 3b 6c 3b 2a 2a 2a
11 β-Phellandrene C10H16 136 1022 KI, MS, CI 296d 381e 183b 147a 230c 227c
12 Limonene C10H16 136 1022 KI, MS, CI - - 16 b 13a - -
Chapter 3 _________________________________________________________________________________________________
197
13 2-Nonanone C9H18O 142 1076 KI, MS 2b 2b tr 1a tr tr
14 Terpinolene C10H16 136 1079 KI, MS, CI 6d 6d 4bc 3a 4bc 4bc
15 Linalool C10H18O 154 1087 KI, MS, CI 10c 13d 4a 4a 4a 5b
16 2-Nonanol C9H20O 144 1091 KI, MS, CI 2b 3c tr 1a tr 1a
17 Cyclohexane-2-ethenyl-1,1-dimethyl-3-methylene C11H18 150 1107 MS 2b 2b 1a tr tr tr
18 Camphor C10H16O 152 1118 KI, MS 2 3 - tr tr tr
19 Camphene hydrate C10H18O 154 1130 KI, MS 2 3 - tr tr tr
20 3,7-dimethyl-1-octene C10H20 140 1135 MS 4d 5e 1a tr 3c 2b
21 Borneol C10H18O 154 1148 KI, MS, CI 31d 40e 12a 13ab 15b 18c
22 Cryptone C9H14O 138 1157 KI, MS - 3b - tr tr 1a
23 Terpinen-4-ol C10H18O 154 1161 KI, MS, CI 7d 9e 1a 2b 2b 3c
24 α-Terpineol C10H18O 154 1172 KI, MS, CI 16c 20d 6a 6a 7ab 9b
25 Myrtenol C10H16O 152 1179 KI, MS tr tr - - - -
26 Neral (Z-Citral) C10H16O 152 1217 KI, MS, CI 119d 140e 59a 61a 70b 82c
27 Geraniol C10H18O 154 1240 KI, MS, CI 10c 5b 4b 2a 5b 2a
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28 Geranial (E-Citral) C10H16O 152 1249 KI, MS, CI 206e 208e 84a 92b 106c 117d
29 Bornyl acetate C12H20O2 196 1268 KI, MS 1 tr 1 tr tr tr
30 2-undecanone C10H16O 170 1277 KI, MS 5a 8d 7c 6b 7c 7c
31 δ-Elemene C15H24 204 1329 KI, MS tr - tr tr tr -
32 Cyclosativene C15H24 204 1356 KI, MS 6bc 5ab 7c 6bc 4a 4a
33 Cycloisosativene C15H24 204 1358 KI, MS 2b 2b 2b 14c tr 1a
34 α-Cubenene C15H24 204 1366 KI, MS 18c 15b 22d 11a 10a 14b
35 β-Elemene C15H24 204 1380 KI, MS 10c 11d 18e 4a 9b 10c
36 cis--Bergamotene C15H24 204 1397 KI, MS 5d 4c 6e 1a 3b 4c
37 Caryophyllene C15H24 204 1402 KI, MS 2b 2b 2b 1a 1a 1a
38 Trans--Bergamotene C15H24 204 1426 KI, MS 3c 2b 3c 2b 1a 2b
39 -Guainene C15H24 204 1436 KI, MS 3b 2a 3b 13c 2a 2a
40 Aromadendrene C15H24 204 1444 KI, MS 17e 14d 19f 6a 10b 13c
41 β-Farnesene (E) C15H24 204 1448 KI, MS 7e 6d 8f 2a 4b 5c
42 Seychellene C15H24 204 1457 KI, MS 2b 2b 3c - 1a 2b
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43 Germacrene D C15H24 204 1464 KI, MS 36e 25b 37f 30d 22a 26c
44 ar- Curcumene C15H22 202 1468 KI, MS 176b 215d 239e 173b 110a 184c
45 Valencene C15H24 204 1474 KI, MS 35d 32cd 41e 30bc 23a 30b
46 γ- Gurjunene C15H24 204 1478 KI, MS 2b 2b - - 1a -
47 γ- Muurolene C15H24 204 1481 KI, MS 110d 86c 108d 77b 37a 74b
48 Zingiberene C15H24 204 1486 KI, MS 752d 429b 804e 560c 383a 436b
49 γ- Cadinene C15H24 204 1493 KI, MS 15 8 - - - -
50 α-Farnesene (E,E) C15H24 204 1497 KI, MS 383d 267b 425e 301c 212a 261b
51 Calamenene C15H24 204 1500 KI, MS 2 2 2 - - tr
52 Epi-bicyclosesquiphellandrene C15H24 204 1506 KI, MS 6b 7c 16e 8d 5a 7c
53 - Sesquiphellendrene C15H24 204 1511 KI, MS 331e 243c 344f 258d 174a 223b
54 cis- γ-Bisabolene C15H24 204 1517 KI, MS 8d 3a 9e 10f 5b 6c
55 -Elemol C15H26O 222 1525 KI, MS 8b 9c 12e 1a 11d 9c
56 Germacrene B C15H24 204 1536 KI, MS 30c 22a 30c 26b 22a 25b
57 E-Nerolidol C15H26O 222 1547 KI, MS 16b 15a 18d 16b 15a 17c
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58 Epiglobulol C15H26O 222 1569 KI, MS 10b 9a 15e 13c 13c 14d
59 Viridiflorol C15H26O 222 1580 KI, MS 1 - tr 1 tr 1
60 Caryophellene oxide C15H26O 222 1583 KI, MS 3 a - 4 b 3 a 3 a 4 a
61 10-epi -γ- Eudesmol C15H26O 222 1590 KI, MS 3 3 3 3 3 3
62 Zingiberenol C15H26O 222 1592 KI, MS 32a 32a 41d 37bcd 36abc 38cd
63 Dihydro cis α-copaene-8-ol C15H26O 222 1596 KI, MS 4a 5b 4a 4a 4a 4a
64 Trans-cadinol C15H26O 222 1608 KI, MS 8a 8a 10c 9b 9b 9b
65 - cadinol C15H26O 222 1614 KI, MS 2 2 2 2 2 2
66 -Eudesmol C15H26O 222 1618 KI, MS 12a 13b 14c 14c 14c 13b
67 Globulol C15H26O 222 1621 KI, MS 2a 3b 3b 3b 3b 3b
68 γ- Eudesmol C15H26O 222 1624 KI, MS 11a 14c 13b 13b 13b 13b
69 β-bisabolol C15H26O 222 1635 KI, MS 4c 4c 3b 1a 3b 3b
70 Cedr-8(15)-en-9--ol C15H24O 220 1646 KI, MS 2a 4c 2a 3b 2a 3b
71 Cubenol C15H26O 222 1652 KI, MS 5c 3b 7e 2a 7e 6d
72 - bisabolol C15H26O 222 1665 KI, MS 6b 5a 10e 7c 8d 8d
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73 Cedr-8(15)-en-10-ol C15H24O 220 1667 KI, MS 9a 16d 13e 21e 9a 11b
74 Cis--Copen-8-ol C15H24O 220 1687 KI, MS 5b 6c 7d 1a 6c 6c
75 Trans, trans-Farsenal C15H24O 220 1710 KI, MS 4b 4b 5c 7d 3a 4b
76 (Z)--Trans-Bergamotol C15H24O 220 1714 KI, MS 2a 2a 7e 6d 3b 4c
TOTAL CONTENTS
(l/100g of dry weight) 3174 2848 2965 2272 1978 2264
qtr –Traces (less than 0.5µl); xComponents are listed in order of elution in polar column (SPB-1) ; PRelative content (µl/100g of dry weight) obtained by GC peak area; oMS – Mass spectrum; CI – Co-injection; KI – Kovats indices. FGO- Fresh ginger oil; CGO- Cabinet dried ginger oil; PL 100- Microwave dried at 800W; PL 80- Microwave dried at 660W; PL 60- Microwave dried at 525W and PL 40- Microwave dried at 385W. rValues followed by the same letter, within the same row are not significant different , p < 0.05 (Duncan’s multiple-range test).
Chapter 3 _________________________________________________________________________________________________
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Table 3.3. Classification of the identified chemical components in essential oil of Zingiber officinale and their relative content
Components
Relative content (l/100g of dry weight)r
FGO CGO PL 100 PL 80 PL 60 PL 40
Chemical classes
Monoterpene hydrocarbons 619e 791f 421b 366a 533d 491c
Oxygenated monoterpenes 405e 445f 171a 182b 213c 241d
Sesquiterpene hydrocarbons 1963e 1402c 2149f 1534d 1040a 1330b
Oxygenated sesquiterpenes 146a 158b 190d 163b 165b 173c
Other aliphatic compounds 6d 7d 2a 2a 4c 3b
Other oxygenated aliphatic compounds 37c 45d 28b 22a 23a 25a
TOTAL CONTENTS 3174 2848 2965 2272 1978 2264
rValues followed by the same letter, within the same row are not significant different , p < 0.05 (Duncan’s multiple-range test). FGO- Fresh ginger oil; CGO- Cabinet dried ginger oil; PL 100- Microwave dried at 800W; PL 80- Microwave dried at 660W; PL 60- Microwave dried at 525W and PL 40- Microwave dried at 385W
Chapter 3 _________________________________________________________________________________________________
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3.3.5. Impact of drying on the essential oil components
The concentrations of the individual components were affected to different extents based
on the drying methodology (i.e., convection drying or microwave drying). Increases in the
quantities of certain compounds (Baritaux et al., 1992; Bartley and Jacobs, 2000) or the formation
of new compounds (e.g., Limonene and other related monoterpenoids) in some cases was
observed after drying, probably as a consequence of oxidation, dehydration reactions, hydrolysis
of glycosylated forms, or the release of substances following the rupture of cell walls (Huopalahti
et al., 1985).
All the essential oils obtained from dried rhizomes constituted sesquiterpene
hydrocarbons in the range of 48-72% of the total oil (table 3.3). This result was similar to the
earlier reports (Connell and Jordan, 1971; Onyenekwe and Hashimoto, 1999), who also reported
the sesquiterpenes, as the main constituents of dried ginger (Nigerian ginger variety).
3.3.5.1. Effect of convection drying on volatile components
Several reports revealed that drying in a convection oven leads losses in volatiles and
the losses were varying according to the drying temperature and drying time employed
(Raghavan et al., 1994; Venskutonis 1997). The GC analysis of the essential oil from convection
dried ginger (CGO) showed decrease (15%) in hydrocarbon content (2193 µl /100g dry weight),
whereas the oxygenated compounds (648 µl /100g dry weight) increased (10%) when compared
to FGO (Table 3.3). It was observed that in CGO, the relative percentage of monoterpenes and
related compounds increased (21%) significantly, while sesquiterpenes and related compounds
decreased (26%) compared to FGO. In particular, major sesquiterpene viz., zingiberene content
decreased by 43%, possibly owing to long exposure of ginger flakes to oxygen in convection
dryer, which might have resulted in the degradation of sesquiterpenes to monoterpenes. Due to
this, an increase in relative concentrations of major monoterpene hydrocarbons namely,
camphene (24%) and β-phellandrene (22%) was observed in CGO sample. The neral-to-geranial
(stereo isomers known as citral) ratios found in fresh and dried ginger oils in this study are in the
range of 0.57-0.70. In ginger essential oil, these are among the major oxygenated monoterpene
compounds. The loss of citral isomers during drying studies was reported earlier (Wohlmuth et
Chapter 3 _________________________________________________________________________________________________
204
al., 2006; Govindarajan, 1982). However, it was observed in the present study that the neral
content was increased (15%) and the geranial content was retained, when compared to that of
the fresh rhizomes.
3.3.5.2. Effect of microwave drying on volatile components
The GC analysis of the essential oils from microwave dried samples (PL 100, PL 80, PL
60 and PL 40) showed the presence of 69, 70, 71 and 70 compounds respectively. The
concentration of major sesquiterpenes such as zingiberene, α-farnesene, -sesquiphellandrene
and ar-curcumene were retained during MW drying, in fact, marginally increased by 7, 11, 4 and
36 % respectively in PL 100, when compared to FGO. The increase recorded may be attributable
to rupture of the plant cells in which the volatiles are stored. These compounds, along with
monoterpenes, are said to have a significant influence on the flavour of the dried product (Bartley
and Jacobs, 2000). At lower power levels, the concentrations of sesquiterpenes decreased and it
may be due to heat generation within the rhizome owing to the longer time exposure of sample to
microwave radiation. These might have degraded to monoterpenes (Diaz-Maroto et al., 2004).
Also, in MW dried samples, a significant loss of citral content (range 38-56%) was observed.
Quantity of ar-curcumene, which contributes to ginger aroma (MacLeod and Pieris,
1984), increased marginally in CGO and PL 100, when compared to FGO. Salzer and Furia
(1977) reported that the amount of ar-curcumene in the oil increases on heat treatment and
storage and concluded that ar-curcumene which might be low quantity in fresh oil was increased
due to the oxidative conversion of zingiberene and -sesquiphellandrene (Chen and Ho, 1989,
fig. 3.9). However, in case of oil obtained from PL 100, quantities of all the three sesquiterpenes
(viz., ar-curcumene, zingiberene and -sesquiphellandrene) increased along with others,
indicating that it may be due to greater release of these compounds owing to cell damage, as
observed in other drying studies (Bartley and Jacobs, 2000).
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+ [O]
+ [O]H
H
H
H
H
Zingiberene
-sesquiphellandrene
ar-curcumene
Fig. 3.9. Oxidative conversion of Zingiberene and β-sesquiphellandrene into ar-curcumene
3.3.6. Effect of drying on the extraction of non-volatile constituents
Ethanol and aqueous ethanol are the most suitable solvent for extracting phenolic
compounds as described earlier (chapter 1, section 1.3.1) and also due to their ability to inhibit
the action of polyphenol oxidases that cause the oxidation of phenolic compounds and their ease
of evaporation compared to water (Bravo and Mateos, 2008). Ethanol partially breaks down the
membranes of the plant cells thereby allowing the leaching of constituents. Therefore, aiming at
the maximum extractability of the polyphenol compounds, aqueous ethanol was chosen and also,
in preliminary studies it was observed that the aqueous ethanol (50%) resulted in higher yields
when compared to alcohol as solvent. The yield of the aqueous ethanol extracts obtained from
conventional dried as well as the microwave dried at different power levels (PL 40, PL 60, PL 80
and PL 100) are presented in table 3.4. The yields of the extracts were in the range of 11-15.2%.
Yield of the microwave-dried samples increased with increase in MW power levels. It was
observed that higher the processing MW power level, shorter the drying time. Higher yields could
be due to increased release of compounds from cell wall due to the breakdown of cellular
constituents from the matrices by the intense heat, thus components are more accessible during
the extraction (Mrkic et al., 2006).
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Table 3.4. Effects of drying methods on the yield, total polyphenols (TPP) and gingerol content of
the extracts
Parameters PL 100 PL 80 PL 60 PL 40 CD
Yield (%w/w) 14.5±0.2d 13.7±0.1c 11.8±0.2b 11.0±0.5a 15.2±0.2e
TPP (mg GAE/g) 80.81±1.4e
(1.17±0.02)*
69.60±1.0d
(0.95±0.02)
64.27±0.6c
(0.76±0.01)
59.03±0.3b
(0.65±0.03)
48.11±0.5a
(0.73±0.01)
[6]-gingerol content
(mg/g)
35.0±0.7e
(0.51±0.02)*
32.5±0.4d
(0.45±0.01)
31.2±0.2c
(0.37±0.01)
29.9±0.1b
(0.33±0.01)
22.8±0.5a
(0.35±0.02)
PL 100, PL 80, PL 60 and PL 40 are the MW power levels corresponding to microwave output power 385, 525, 660 and 800 W respectively and CD - convection dried ginger. Values expressed are mean of triplicate experiments. Values followed by the same letter, within the same row, are not significantly different, p < 0.05 (Duncan’s multiple-range test). * The values in parenthesis are the percentage of the compound released into the respective extract from raw material.
3.3.9. Effect of drying on the TPP and gingerol content
The content of extractable phenolic compounds in the ginger extracts was determined
through a linear gallic acid calibration curve (y = 0.0124x; R2 = 0.9971). The total phenolic content
(TPP) of the extracts varied from 48-80 mg GAE/g extract (Table 3.4). It was observed that TPP
content (59-80 mg GAE/g) of the MW dried samples increased with the increasing MW power
levels. Higher TPP contents at high MW power level could be due to increased release of
phenolics or bound phenolics owing to the breakdown of cellular constituents by the intense heat,
making polyphenols more accessible during the extraction (Toor and Savage, 2006). The
aqueous ethanol extract obtained from MW PL100 yielded higher quantity (14.5%) of extract as
well as the extract contained higher quantity of TPP content (80.8±1.4 mg GAE/g). Higher release
of TPP (1.17% GAE basing on raw material) was also observed. The aqueous ethanol extract
obtained by CD sample showed the least amount of TPP content when compare to the extracts
obtained by MW dried samples. Some of phenolic compounds present in ginger are lost /
degraded due to prolonged drying time in CD. Drying process would generally result in a
depletion of naturally occurring antioxidants in raw materials from plants (Tomaino et al., 2005).
Intense and/or prolonged thermal treatment may be responsible for a significant loss of natural
antioxidants, as most of these compounds are relatively unstable. Several studies have revealed
Chapter 3 _________________________________________________________________________________________________
207
that the phenolic content in the plants are associated with their antioxidant activities, probably due
to their redox properties, which allow them to act as reducing agents, hydrogen donors, and
singlet oxygen quenchers (Shahidi and Wanasundara, 1992; Pandey and Rizvi, 2009; Ismail, et
al., 2010).
The ginger extract was analysed by HPLC for the quantification of [6]-gingerol content.
[6]-gingerol was isolated by purification of the methanolic fraction of ginger extract (chapter 1;
section 1.2.9 and 1.3.4) and was used as the authentic standard for the preparation of calibration
curve (y =1575.7x where x was the concentration of standard gingerol (g/ml) and y was the total
peak area; R2= 0.9865). The PL 100 extract showed higher quantity (0.51%) of [6]-gingerol
compared to other power levels as well as the CD extract (Table 3.4). The aqueous ethanol
extract obtained by CD sample showed the lower quantity of [6]-gingerol content (0.35%; ~32%
less compared to PL 100; fig 3.10), which may be due to relatively unstable nature during CD and
undergoes oxidation in air (Denniff et al., 1981). Moreover, gingerol has acidic methylene protons
that are easily dehydrated to produce shogaols (Balladin et al., 1998; Denniff et al., 1981).
Gingerols and shogaols undergo the retro-aldol cleavages to form zingerone and corresponding
aldehydes (Mujumdar and Menon, 1995).
Fig 3.10. HPLC profiles of conventional dried and MW PL 100 dried ginger extract (* [6]-gingerol; CD-convectional dried ginger extract; MW PL 100- Microwave dried at power level 100 ginger extract)
A - MW PL 100 extract
B - CD extract
* A
B
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3.3.10. Effect of drying on the antioxidant activity of the extracts
Antioxidant activity was evaluated employing the free radical scavenging ability (DPPH
assay), reducing power, antioxidant capacity (phosphomolybdenum assay) and FRAP assay.
DPPH radical scavenging assay
It has been reported to be an easy and rapid method for determining antioxidant activities
of various plant materials (Chapter 1; section 1.3.3). The results of DPPH assay are presented in
fig. 3.11. It was observed that a dose response relationship was found in the DPPH radical
scavenging activity; the activity increased as the concentration increased. When compared to the
BHA, the extracts were found to be less efficient in the radical scavenging assay. The high
DPPH scavenging activity of MW drying can be explained owing to the presence of higher
quantity of TPP (as described in section 3.3.9), which might have been released due to the
disruption of the cell wall, from insoluble portion of the ginger or the formation of novel compound
having powerful donating ability (Choi et al., 2006). Among various extracts of ginger, MW PL
100 showed highest DPPH• scavenging activity (IC50 44.6 ± 0.15 µg/ml), followed by PL 80 (IC50
48.1 ± 1.4 µg/ml), PL 60 (IC50 50 ± 0.4 µg/ml) = CD (IC50 49.0 ± 0.5 µg/ml) and PL 40 (IC50 54.1 ±
0.8 µg/ml), while IC50 value of BHA was 8.1 ± 0.4 µg/ml .
Reducing power assay
Reducing power (RP) of the extracts was determined by measuring the change in yellow
color of test solution into various shades of green and blue colors at 700 nm (Chapter 1; section
1.3.3). Reducing powers of aqueous ethanolic extracts of ginger increased with increase in
concentration (Fig. 3.12.). Increased absorbance of the reaction mixture indicated increased
reducing power. At 100 ppm concentration, reducing power (absorbance at 700 nm) were in
order of PL 100 (0.165) > PL 80 (0.123) > PL 60 (0.120) > PL 40 (0.107) = CD (0.107). However,
the activity was less than the standard, ascorbic acid. Reducing powers of ascorbic acid and BHA
at 100 ppm were 0.85 and 0.61 respectively, which are significantly higher than that of extracts
tested in the present study. Reductones are also reported to react with certain precursors of
peroxide, thus preventing peroxide formation. The reducing power test, in which the capacity of
Chapter 3 _________________________________________________________________________________________________
209
breaking radical chain reactions was reflected, was considered to be a good indicator of
antioxidant capacity.
Fig. 3.11. Radical scavenging activity of the MW and conventional dried ginger extracts
Values are expressed as mean of triplicate determinations ± standard deviation; Different letters above the bars for the same concentration indicate statistically significant differences at P < 0.05.PL 100, PL 80, PL 60 and PL 40 are the MW power levels corresponding to microwave output power 385, 525, 660 and 800 W respectively and CD- convection dried ginger. BHA- Butylated hydroxyanisole.
Antioxidant capacity (Phosphomolybdenum method)
The phosphomolybdenum method (PM) being simple and independent of other
antioxidant measurements, its application was extended to plant polyphenols and related
compounds (Prieto et al, 1999). The antioxidant activity of different extracts depends on the
presence of polyphenols which may act as reductones (Chapter 1; section 1.3.3). The total
antioxidant capacity was quantitatively determined spectrophotometrically by measuring the
absorbance values because of phosphomolybdenum complex formed, and presented in fig. 3.13.
The order of antioxidant capacity as per absorbance values were: PG > PL 100 > PL 80 > PL 60
> PL 40 > CD. The antioxidant capacity of the propyl gallate showed 5458 ± 56 mol/g as
equivalents to ascorbic acid; whereas the MW dried extracts were in the range of 828-1197
Chapter 3 _________________________________________________________________________________________________
210
mol/g ascorbic acid equivalents. The antioxidant capacity of the CD extract was less (702 ± 24
mol/g ascorbic acid equivalents) than the MW dried extracts.
Fig. 3.12. Reducing ability of the MW and conventional dried ginger extracts
Values are expressed as mean of triplicate determinations ± standard deviation; the values in the same group are significantly different (p<0.05). PL 100, PL 80, PL 60 and PL 40 are the MW power levels corresponding to microwave output power 385, 525, 660 and 800 W respectively and CD-convection dried ginger
Fig. 3.13. Antioxidant capacity of the MW and conventional dried ginger extracts
Values are expressed as mean of triplicate determinations ± standard deviation; the values in the same group are significantly different (p<0.05). PL 100, PL 80, PL 60 and PL 40 are the MW power levels corresponding to microwave output power 385, 525, 660 and 800 W respectively and CD – convection dried ginger
Chapter 3 _________________________________________________________________________________________________
211
Ferric ion reducing ability of plasma (FRAP) method
The ability of the plants extracts to reduce ferric ions was determined using the FRAP
(Ferric ion reducing ability of plasma) assay developed by Benzie and Strain (1996). The
reducing capacity of a compound may serve as a significant indicator of its potential antioxidant
activity. The reducing ability of PL 100 was 20.27±1.2 µM of Fe (II)/g while that of the extracts of
other power levels were decreased with decrease in power and increase in the drying time i.e., in
the order PL 80 (19.28±1.88 µM) > PL 60 (18.96±1.27 µM) > PL 40 (18.13±1.91 µM) respectively.
The reducing ability of the CD was 16.10±1.26 µM of Fe (II)/g, less than the MW dried extracts.
Correlation studies between the total polyphenols content and antioxidant
activities showed a positive and significant correlation (p<0.05) between the TPP content and
DPPH. Antioxidant activity increased proportionally to the phenolic content and a linear
relationship between DPPH-radical scavenging activity and TPP was established. The results
also confirm that the polyphenolic compounds are responsible for other activities such as
reducing power and antioxidant capacity in ginger (Table 3.5.). Kikuzaki and Nakatani (1993)
reported that high antioxidant activity in ginger had been attributed to the active principal
component viz., [6]-gingerol. The correlation studies confirmed the relationship between
antioxidant potential and [6]-gingerol as well as TPP content.
Table 3.5. Correlation coefficient (R2) between total polyphenol content and antioxidant activity Correlation parameters
R2 value CD PL 100 PL 80 PL 60 PL 40
DPPH vs TPP
0.9562 (y=251.23x+3.85)
0.9329 (y=149.22x+10.04)
0.9414 (y=166.6x+8.39)
0.9621 (y=200.6x+3.24)
0.9972 (y=272.13x-0.52)
RP vs TPP 0.9992 (y=0.428x-0.02)
0.9995 (y=0.3751x-0.01)
0.9998 (y=0.2929x-0.001)
1 (y=0.369x-0.009)
0.9102 (y=0.4591x-0.022)
PM vs TPP 0.9982 (y=1.1131x+0.01)
0.9921 (y=0.8344x+0.044)
0.9945 (y=0.9809x+0.032)
0.9929 (y=0.9629x+0.029)
0.9887 (y=1.0055x+0.027)
[6]-gingerol vs TPP
1 (y=25.35x2.59)
1 (y=45.81x-10.81)
1 (y=37.1x-4.6)
1 (y=33.07x-1.87)
1 (y=29.13x+0.77)
[6]-gingerol vs DPPH
1 (y=26.24x-3.48)
1 (y=9.6x+25.4)
1 (y=15.6x+16.9)
1 (y=18.8x+12.4)
1 (y=24.2x+5.7)
TPP -Total polyphenols content (mg GAE/g); DPPH - Radical scavenging activity (%); RP - Reducing power (Abs at 700 nm); PM - Phosphomolybdenum method (Abs at 695 nm); [6]-gingerol content (mg/g) PL 100, PL 80, PL 60 and PL 40 are the MW power levels corresponding to microwave output power 385, 525, 660 and 800 W respectively and CD - convection dried ginger
Chapter 3 _________________________________________________________________________________________________
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3.4. CONCLUSIONS
In summary, the drying times were reduced significantly from 720 min in case of
convection drying to 25 min in case of microwave drying (PL 100). Moreover, the microwave
dried (PL 100) sample retained the maximum volatile fraction (3% v/w) followed by convection-
dried sample (2.9 %v/w) as compared to that of the fresh sample (3.2% v/w). Both fresh and
dried ginger oils contained zingiberene as the major constituent but in different quantities. The
concentration of zingiberene was marginally increased in PL 100 sample when compared to fresh
(FGO) sample, whereas it decreased by 43, 25, 49 and 42% in CGO, PL 80, PL 60 and PL 40
respectively. However, the colour criteria assessments showed that drying at PL 100 produced
the marginally higher redness and yellowness and lower brightness compared to convection-dried
sample.
The effects of MW drying on the total polyphenol content (TPP) and antioxidant
properties of aqueous ethanolic extract of ginger showed that extract yield, TPP content
increased with increase in MW power levels. This might be as a result of intense heat of MW
energy causing the release of cell wall phenolic compounds owing to breakdown of cellular
constituents. MW-dried (PL100, 800W) extract showed the highest retention of TPP, [6]-gingerol
contents as well as antioxidant activity, when compared to the CD ginger extract. The reduction
in the TPP, [6]-gingerol content as well as antioxidant activity at lower MW power levels may be
attributed to the temperature rise because of heat generation within the rhizome and also,
because of the longer time exposure of sample to microwave radiation. Hence, it could be
concluded that the optimum MW power level for drying of ginger slices was PL 100 for higher
retention of volatiles as well as for higher release of non-volatiles such as total polyphenols
including [6]-gingerol as per present studies.
