ORIGINAL RESEARCH
Effect of tamarind (Tamarindus indica L.) seed on
antioxidant activity, phytocompounds, physicochemical
characteristics, and sensory acceptability of enriched
cookies and mango juice
Sheilla Natukunda, John H. Muyonga & Ivan M. Mukisa
School of Food Technology, Nutrition & Bioengineering, Makerere University, P.O. Box 7062 Kampala, Uganda
Keywords
Antioxidant activity, condensed tannin,
flavonoid, phenolics, phytocompounds,
tamarind seed
Correspondence
Ivan M. Mukisa, School of Food Technology,
Nutrition and Bioengineering, Makerere
University, P.O. Box 7062 Kampala, Uganda.
Tel: +256 775414537;
Fax: +256 414533676;
E-mail: ivanmukisa@caes.mak.ac.ug
Funding Information
Swedish International Development
Co-operation Agency, Department for
Research Cooperation (Sida-Sarec).
Abstract
Tamarind seeds are not consumed despite their high antioxidative activity. In
this study, 0–10% tamarind seed powder (TSP) was incorporated into mango
juice and cookies. Total phenolics (Folin–Ciocalteu assay), antioxidant activity
(2,2-diphenyl-1 picrylhydrazyl (DPPH) radical scavenging assay), flavonoid (aluminum chloride assay), condensed tannins content (Vanillin-HCl assay), and
consumer acceptability (n = 50) of the products were determined. TSP increased
the pH and viscosity and reduced titratable acidity of juice. Incorporation of
TSP increased the: total phenolic content (6.84 ± 0.21 to 88.44 ± 0.8 mg GAE/100
mL); flavonoid (4.64 ± 0.03–21.7 ± 0.36 mg CE/100 mL); condensed tannins
(0.24 ± 0.01–21.81 ± 0.08 mg CE/100 mL) and total antioxidant activity
(4.65 ± 0.88–21.70 ± 0.03 mg VCE/100 mL) of juice. A similar trend was
observed for cookies. Maximum sensorially acceptable TSP levels were 1.5%
and 6%, respectively, for juice and cookies. TSP can thus be utilized as a source
of natural antioxidants in food products.
Received: 17 August 2015;
Accepted: 15 October 2015
Food Science & Nutrition 2016; 4(4):
494–507
doi: 10.1002/fsn3.311
Introduction
Epidemiological studies have consistently demonstrated
that consumption of plant-derived foods rich in bioactive
phytochemicals has a protective effect against oxidation
stress (Librandi et al. 2007; Ovaskainen et al. 2008; Galili
and Hovav 2014). Oxidative stress is strongly associated
with mutagenesis, carcinogenesis (Abnet et al. 2015), aging
(Everitt et al. 2006), and atherosclerosis in humans. The
bioactive phytochemical compounds thus decrease the risk
of chronic diseases, cardiovascular disease, cancer, and
degenerative diseases of the aging (Keservani et al. 2010).
494
Currently, industries are interested in developing valueadded products from the waste-by products generated
by both the food and agricultural processing industries
(Balasundram et al. 2006). The waste products including
seeds, peels, stalks, stems, and leaves of plants contain
substantial amount of phenolics and thus can be used
as cheap sources of natural antioxidants for pharmaceutical, cosmetic, and food application (Bucić-Kojić et al.
2009). Fruits and vegetable waste products including seeds
have been reported to have higher content of bioactive
phytochemicals than the edible portions (Soong and
Barlow 2004). Fruit seeds contain a variety of biologically
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc. This is an open access article under the terms of
the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
active phytochemical compounds, especially phenolic constituents, flavonoids, anthocyaninins, vitamin C, and
carotenoids. These phytochemicals positively influence
human health and indicate high antioxidant activity
(Pérez-Jiménez et al. 2008). Hence, it is considered crucial
to increase the antioxidant intake in the human diet and
one way of achieving this is can be through enriching
food products with seeds which are rich in
phytochemicals.
Tamarind (Tamarindus indica) seed is a by-product in
the tamarind pulp industry. Recently, a large amount of
the seed waste is discarded from the tamarind industry
(Oluseyi and Temitayo 2015). Tamarind seed is a rich
source of phytochemicals (Tsuda et al. 1994; Andabati
and Muyonga 2014) which consists of phenolic antioxidants, such as 2-hydroxy-3′,4′-dihydoxyacetophenone,
methyl 3, 4-dihydroxybenzoate, 3,4-dihydroxyphenyl
acetate and epicatechin (Sudjaroen et al. 2005; El-Siddig
et al. 2006). Tamarind seed extracts exhibit antioxidant
potential by reducing lipid peroxidation in vitro (Tsuda
et al. 1994) and anti-microbial activity. Tamarind seed
therefore has the potential of providing low cost nutritional and nutraceutical value.
In this study, tamarind seed powder (TSP was evaluated as a source of antioxidants for inclusion in two
commonly consumed products; cookies and mango juice.
Cookies and mango juice are regularly consumed by nearly
all age groups in developing countries. Cookies are popular
compared to other processed foods because of their low
cost, diverse taste, myriad shapes, and long shelf life
(Davidov-Pardo et al. 2012). Jesionkowska et al. (2009)
reported that cookies were selected by consumers as a
good vehicle for antioxidants. Cookies also contain fat
which possess’ sensory characteristics that are ideal to
mask the astringent flavor and taste associated with tamarind seed which is undesirable for most consumers.
Mango (Mangifera indica) is the most vital and widely
cultivated among the tropical and subtropical fruits (Akhter
et al. 2012). Thus, mango juice was selected because of
the abundance, seasonal availability, popularity, and distinctive flavor and taste. Mangoes also have a strong flavor
and substantial levels of pectin which can mask the astringency of the tamarind seed without affecting antioxidant
activity (Laaksonen 2011; Soultani et al. 2014).
While tamarind seeds are known to contain substantial
levels of bioactive phytocompounds, it was not clear if
they could be used to enhance nutraceutical properties
of widely consumed processed foods while retaining product acceptability. This study was therefore undertaken to
assess the effect of tamarind seed powder on the physicochemical properties, phytonutrient content, antioxidant,
and sensory properties of the enriched selected food
products.
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
Material and Methods
Materials
Fresh tamarind fruits were purchased from the local markets
in Mbale district, Uganda and transported to the Department
of Food Technology and Nutrition, Makerere University.
Fresh and fully ripened mangoes were obtained from the
local market in Kampala, Uganda and transported to the
Department of Food Technology and Nutrition, Makerere
University. The fruits were selected for uniformity in shape
and color, washed carefully with clean portable water and
stored in a refrigerator at 8°C prior to further use.
Commercial wheat flour (Bakhresa Grain Milling (U)
Ltd, Uganda), sugar (Kakira Sugar Works, Jinja, Uganda)
and sodium bicarbonate (Bidco oil Refineries, Nairobi,
Kenya) were purchased from the local supermarkets in
Kampala Uganda.
Methods
Preparation of tamarind seed powder
Tamarind fruit pods were manually cracked and from each
pod the pulp containing seeds was separated along with the
fibers. Tamarind seed with fibers were soaked overnight in
clean portable water (1: 3 w/v) to enable removal of the
pulp and fiber strands. The seeds were then washed using
clean distilled water. The cleaned seeds were sun dried in
a shaded for 14 days, milled into fine flour using a Wonder
mill (Grain of Truth Bread Company, Arlington, VA). The
flour was sieved using a 300 µm screen and stored in an
airtight container in a freezer at −18°C prior to further use.
Mango juice enrichment with tamarind seed powder
Preparation of mango juice
The mangoes were rinsed with portable water, peeled, and
sliced. Mango juice was then extracted using a blender (Royal
Philips, model HR 2167/40, Amsterdam, The Netherlands)
and filtered using a sterile cheese cloth. The filtered juice
was then blended with clean portable water (1: 3 of mango
juice: water) and sugar 3% (w/v) was added to make the
desired concentration of 9°Brix. The Mango juice was mixed
with TSP powder in different proportions 0%, 0.5%, 1.5%,
1%, 2% and 2.5% (w/v) using a portable hand mixer (sun
beam model SHM 100, Nu world industries Ltd, 2000).
Pasteurization treatment of tamarind seed mango
juice and storage conditions
The tamarind seed and mango juice blend of 600 mL in
a 1 L capacity container was pasteurized at 85°C with a
495
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
holding time of 25 sec (Moyer and Aitken 1980). The
beaker containing the juice was placed in a water bath
maintained at 95°C and the juice was heated for 20 min
while stirring until it reached 85°C. It was maintained at
this temperature for 25 sec before hot filling in amber
plastic bottles. The enriched juice was pasteurized to enable extraction of phytochemical compounds as well as
to destroy spoilage microorganisms(Andabati and Muyonga
2014). The juice was previously filtered with a clean sterile
cheese cloth before hot filling in amber plastic bottles.
The amber bottles containing juice were capped and cooled
to room temperature in a water bath. The bottles with
pasteurized juice were stored in the refrigerator at 8°C
subsequent to further analyses.
Cookie enrichment with tamarind seed powder
Preparation of flour mixtures and baking of
enriched cookies
method 943.02 (1995). Buffer solutions of pH 4.0 and
7.0 were used for periodical calibration of pH meter.
Three readings were performed for each replicate.
Color
The color of the juice was determined using a color tintometer (Lovibond Model E, AF-900; Tintometer Limited,
Salisbury. U.K.). The juice was filled in the one inch
optical glass cell which was placed into the cell channel.
Sample colors were matched by a suitable combination
of the three primary colors together with neutral filters
using a viewing tube and adjusting the color filter knobs.
The resulting set of Lovibond red, blue, yellow, and neutral
(RYBN) units were used to define the color of the mango
juice and TSP-enriched mango.
Viscosity measurements and analysis
Cookies were prepared according to the method number
10–50D AACC (2000) with some modifications in the recipe
(Table 1). Control samples were prepared without addition
of TSP. TSP-enriched cookies were prepared by substituting
wheat flour with of 2%, 4%, 6%, 8% and 10% TSP.
Tamarind seed powder was well blended with wheat flour,
sugar, and fat. The sugar and fat were creamed manually.
The baking flour, baking powder, and TSP were sieved
together and added to the cream before mixing it uniformly.
The cookie dough was rolled, sheeted to a thickness of
3.5 mm and cut using a circular mould (5 cm diameter).
Baking was done at 150°C for 40 min. After baking, cookies were cooled to room temperature and packed into
airtight polythene bags until further analysis.
Physicochemical analyses
pH
The pH of TSP-enriched mango juice samples and control
was measured with a pH-meter (pH meter, Hanna instrument H12210, Crison Instruments S.A., Barcelona, Spain)
at room temperature (~25°C) according to AOAC official
Viscosity of the tamarind seed-enriched mango juice and
control juice was measured using a Brookfield Viscometer
(Brookfield
LVDV-II+P,
Brookfield
Engineering
Laboratories, Inc., Middleboro, MA). TSP-enriched mango
juice of 500 mL was loaded into a glass beaker reservoir
(cylindrical shape) of 600 mL capacity and was allowed
to equilibrate at room temperature. Spindle LV- 1 was
used to measure viscosity of juice, using rotational speeds
ranging between 0.3 and 1.5 revolutions per minute (RPM).
The recording of the viscometer output commenced 3 min
after the onset of the experiment.
Total soluble solids
Total soluble solids content of the tamarind seed-enriched
mango juice and control was determined with a handheld
refractometer (Westover Model RHB-32; Southwest United
Industries, Tulsa, OK) using AOAC (1999) method 981.12.
The results were reported as oBrix at 20°C.
Titratable acidity
The total titratable acidity of the juices was determined
according to AOAC (1999) by titration, using 0.1N sodium
Table 1. Formulation for tamarind seed-enriched cookies.
Formulation description
Wheat flour (g)
Sugar (g)
Fat (g)
TSP (g)
Egg
Baking powder (g)
Control
2% TSP
4% TSP
6% TSP
8% TSP
10% TSP
250
245
240
235
230
225
80
80
80
80
80
80
100
100
100
100
100
100
0
5
10
15
20
25
1
1
1
1
1
1
3
3
3
3
3
3
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© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
hydroxide with phenolphthalein. The results were expressed
as % tartaric acid, using the weight of the molar mass
of tartaric acid as the equivalent weight of acid (Banigo
and Muller 1972).
pooled in an air tight container and stored in a freezer
at 4°C to be used in the determination of total phenolics content (TPC), total antioxidant activity (TAA),
total flavonoid contents (TFC) and total condensed
tannins (TCT).
Quantification of total carotenoids
Total carotenoids in tamarind seed powder-enriched mango
and control juice was determined according to the method
of Rodriguez-Amaya and Kimura (2004). Briefly, 2 mL
of TSP-enriched mango juice was extracted by mixing
with 50 mL cold acetone in the dark. The acetone was
removed through the slow addition of 250 mL double
distilled water to prevent the formation of emulsions. The
aqueous phase was discarded and this procedure was repeated four times until there was no residual no residual
acetone. The extract was then transferred through a funnel
into a 50 mL volumetric flask containing glass wool with
15 g of anhydrous sodium sulfate. The final volume was
adjusted with petroleum ether. Absorbance was measured
at 450 nm (Genesys 10-UV spectrophotometer, Thermo
Electron Corporation, Madison, WI) against petroleum
ether as a blank. The total carotenoid content was calculated using the following formula:
Total carotenoid content (𝜇g/mL) =
Determination of total phenolic content of
TSP-enriched products
The total phenolic content of the juices and cookies were
determined, using the Folin-Ciocalteu colorimetric method
(Makkar 2000) with some modifications. In brief, 100 µL
of the diluted juice (1: 10 of juice to water v/v) or
cookie extract was pipetted into a test tube covered with
aluminum foil and topped up to 0.5 mL with double
distilled water. Subsequently 0.25 mL of Folin–Ciocalteu
reagent (1 N) was added followed by 1.25 mL of sodium
carbonate (20% w/v) and the mixture homogenized using
a vortex. The mixture was then incubated at room temperature for 40 min to allow for color development.
Absorbance was measured at 725 nm (Genesys 10-UV
spectrophotometer, Thermo Electron Corporation) against
Absorbance × Total volume of extract × 104
Absorption coefficient of beta carotene (2592) × sample volume (mL)
Phytochemical analysis
Tamarind seed-enriched mango juices and cookies including controls (without TSP) were analyzed for total phenolic
content, total condensed tannins, total flavonoid content
and total antioxidant activity.
methanol as the blank. The total phenolic content was
determined using the standard gallic acid calibration
curve with varying concentrations (0.02– 0.125) mg/mL).
The total phenolic content was expressed as milligram
gallic acid equivalent (GAE)/100 mL of the enriched
mango juice and mg GAE/100 mg of the enriched
cookies.
Extraction of phenolics
Determination of total condensed tannins of
TSP-enriched products
The extraction method described by (Makkar 2000) was
used with slight modifications. Briefly, 100 mg of TSPenriched cookies and control was extracted, using 5 mL
of 50% methanol: 50% water solution (v/v). The falcon
tube containing the mixture was suspended in ultrasonic
water (Bransonic series, M 2800-E; Branson Ultrasonics
Co, Danbury, CT) and subjected to ultrasonic treatment
for 20 min at room temperature. The extract was
immediately cooled at 4°C in a freezer for 10 min and
then centrifuged at 3000 g for 10 min using a centrifuge
(Fischer scientific 225, Fisher Scientific Co. St. Louis,
MO). The supernatant was collected into a separate
tube and stored at 4°C. The pellet was then further
re-extracted under the conditions previously described
to ensure efficient extraction. The two supernatants were
Total condensed tannins were determined using the method
described by (Sun et al. 1998) with slight modifications.
Briefly, 1.5 mL of vanillin solution (4%) w/v was added
to 50 µL of diluted juice (1:10 of juice to water v/v) or
cookie extract in a test tube. Immediately, 0.75 mL of
concentrated HCl was added and the mixture vortexed.
The mixture was incubated at room temperature for
10 min to allow for color development. Absorbance was
read at 500 nm (Genesys 10-UV spectrophotometer,
Thermo Electron Corporation) with water as a blank. A
standard curve was developed using catechin standards
of varying concentrations (0.02 to 0.06 mg/mL). Total
condensed tannins values were expressed as mg catechin
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
497
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
equivalent/100 mL of the juice and mg catechin equivalent/100 g of cookies.
Determination of total flavonoids content of
TSP-enriched products
Total flavonoid content was determined using the method
of Muanda et al. (2011). In brief, 0.5 mL of catechin
standard solution or juice sample or cookie extract was
mixed with 2 mL of deionized water and 0.15 mL of
sodium nitrite (5% w/v). After 5 min, 0.15 mL of 10%
aluminum chloride was added followed by the addition
of 1 mL of molar sodium hydroxide after another 6 min.
Finally distilled water was used to adjust the total volume
to 5 mL and absorbance was read at 510 nm (Genesys
10-UV spectrophotometer, Thermo Electron Corporation).
A standard calibration curve was plotted using different
concentrations of catechin (0.002 to 0.125 mg/mL). Total
flavonoid content values were expressed in milligram catechin equivalents per 100 mL of juice (mg CE/100 mL)
and cookies in mg CE/100 g of enriched cookies.
Total antioxidant activity of TSP-enriched products
The total antioxidant capacity (TAA) of the tamarind seed
and products was determined, using the free-radical scavenging capacity by use of 1, 1-diphenyl-2-picrylhydrazyl
(DPPH) (Brand-Williams et al. 1995) with minimal modification (Stratil et al. 2006). Briefly, 50 µL of the tamarind
seed-enriched mango juice or cookie extract was added to
2.9 mL of freshly prepared 80% ethanol solution of 100 µM
DPPH. The mixture was vortexed and allowed to stand for
30 min in the dark at room temperature. Absorbance of
the resulting mixture was measured at 515 nm, using Genesys
10-UV spectrophotometer (Thermo Electron Corporation)
against a blank (80% ethanol). The free-radical scavenging
activity of the juice and cookies was calculated as follows
Scavenging activity (%)
)]
[ (
absorbance of sample
× 100
= 1−
absorbance of control
(1)
The antioxidant content was determined using a standard
curve of ascorbic acid (0.1–8 µg/mL). The results were
expressed as milligram vitamin C equivalents per 100 mL
of tamarind seed mango juice (mg VCE/100 mL) and
cookies as mg VCE/100 g of the cookie.
comprised of students from the Department of Food
Technology and Nutrition. The recommended minimum
number of panelists for assessing sensory acceptability of
a product is 50 since a big number best represents the
population (Hough et al. 2006). Each individual evaluated
eight sensory characteristics (appearance, taste, color, flavor,
consistency, mouth feel, sweetness and thickness) of the
enriched juices and control. Each sensory attribute was
rated on a 9-point Hedonic scale. The ratings on the 9-point
hedonic scale used were (9 = “like extremely”;8 = “like
very much”; 7 = “like moderately”; 6 = “like slightly”;
5 = “neither like nor dislike”; 4 = “dislike slightly”; 3 = “dislike moderately”; 2 = “dislike very much”; 1 = “dislike
extremely”) (Carr et al. 1999). Each subject received six
samples (unidentified, with randomly assigned three-digit
codes) of each juice (mango juice with tamarind seed and
a control). A control juice sample (without tamarind seed)
and five samples with different tamarind seed powder formulations (0.5, 1.0, 1.5, 2.0 and 2.5%). The panelists were
presented with 50 mL of each juice sample at room temperature under normal lighting conditions. The tamarind
seed-enriched juice and control samples were prepared the
day before and stored in a refrigerator. Bottled water was
provided to rinse the mouth between tasting samples.
Sensory analysis of tamarind seed-enriched cookies was
done as described above for the juices. However, cookies
were evaluated for color, appearance, texture, taste, flavor,
and overall quality. The samples comprised a control
cookie sample (without tamarind seed) and five samples
with different tamarind seed powder formulations (2%,
4%, 6%, 8% and 10%). Cookies were formulated according to the experimental design and prepared the day before
the evaluation day and stored at room temperature.
Data analysis
All experiments were conducted in triplicate. Statistical
analysis of the data was performed by analysis of variance
(ANOVA), using Student Edition of Statistix 9.0 software
(Analytical Software, Tallahassee, FL). A probability value
of difference P ≤ 0.05 was considered to denote statistical
significance. All data are presented as mean values ±
standard deviation (SD). Regression analysis was performed
to indicate the relationship between total phenolic and/or
flavonoid contents and antioxidant activity.
Results
Sensory evaluation of tamarind seed-enriched
products
Effect of tamarind seed powder on
physicochemical properties of mango juice
Sensory analysis of tamarind seed-enriched mango juice
involved the participation of 50 untrained panelists who
The addition of tamarind seed powder (TSP) to mango
juice significantly (P < 0.05) affected the total soluble
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© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
Table 2. Effect of tamarind seed powder (TSP) on total soluble solids, pH, titratable acidity and beta carotene of mango juice.
TSP concentration (%)
TSS (0Brix)
pH
%TTA
β-carotene
(µg/100 mL)
Control
0.5%
1.0%
1.5%
2.0%
2.5%
9.03 ± 0.04a
9.00 ± 0.00a
9.00 ± 00a
9.00 ± 00a
10.00 ± 00b
10 ± 00b
4.7 ± 0.01a
4.9 ± 0.01b
5.36 ± 0.01c
5.51 ± 0.20d
5.56 ± 0.01e
5.7 ± 0.00f
0.17 ± 0.01a
0.16 ± 0.00a
0.15 ± 0.00b
0.14 ± 0.01b
0.12 ± 0.00c
0.09 ± 0.00d
892.49 ± 11.79a
563.27 ± 26.73b
468.11 ± 8.90c
416.67 ± 20.45d
272.63 ± 17.81e
195.47 ± 4.45f
TTA, titratable acidity. Data are means of triplicate determination ± standard deviation. Mean values in the same column with different superscript
letters are significantly different (P < 0.05).
and 2.3 respectively. Tamarind seed-enriched mango juice
of 0.5% and 1% appeared paler than the rest due to a
higher yellow shade.
Effect of TSP on viscosity of mango juice
The viscosity of enriched mango juice increased with increase in tamarind seed powder concentration for all
viscometer speeds (Fig. 1). It is clear from Figure 1 that
addition of TSP to juices increases the viscosity of enriched
juices compared to the control.
Effect of tamarind seed powder on
phytochemical composition of mango juice
Figure 1. Effect of TSP on viscosity of mango juice.
solids, pH, titratable acidity, and beta carotene of mango
juice (Table 2).
The pH ranged from 4.7 to 5.7 and increased with
increase in tamarind seed concentration in the order of
control <0.5% <1% <1.5% <2.0% <2.5. Total acidity of
TSP-enriched mango juices decreased with increase in
TSP. The total carotenoids decreased in the order of control >0.5% >1% >2% >2.5%. Total soluble solids only
significantly increased at 2.0% tamarind seed powder
concentration.
Color
The color of mango juice varied at different concentration
of tamarind seed powder. The color of mango juices was
generally pale yellow to dull orange color on adding TSP.
The highest orange color (2.4 and 2.6 units) of the mango
juice was observed at 2% and 2.5% concentration, respectively. The juice appeared as yellow/orange with a
shade of yellow (2.3 and 1.9) at 2% and 2.5%, respectively.
The color of the juice at 0%, 0.5%, 1%, and 1.5% concentration was 1.5, 1.8, 1.8, and 0.9 orange values,
respectively, while the shade of yellow was 5.8, 6.0, 5.0,
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
Incorporation of tamarind seed powder in mango juice
formulation significantly increased the total phenolic, total
flavonoids, and total condensed tannin content as well
as the total antioxidant activity as compared to the control
(Table 3).
Addition of tamarind seed powder resulted in an increase
in the content of total phenolic from 6.54 ± 0.8 to
88.44 ± 0.21 mg GAE/100 mL of the juice. There was a
more than 13 fold increase in total phenolic content of
enriched juices that received higher TSP (2.5%) as compared to the control.
Mango juice enriched with TSP also showed a similar
pattern, with respect to total flavonoid content (TFC), with
the value for the 2.5% TSP – enriched juice increased by
20-fold compared to the control. The total antioxidant activity of the enriched juice increased from 4.64 ± 0.58 for
control to 21.70 ± 0.04 mg VCE/100 mL for mango juice
containing 3% TSP, while total condensed tannins increased
from 3.59 to 21.81 ± 0.08 mg CE/100 mL of the juice.
Effect of tamarind seed powder on
phytochemical composition of cookies
Cookies with tamarind seed powder had significantly
higher antioxidant activity and content of the different
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S. Natukunda et al.
Table 3. Effect of tamarind seed powder (TSP) on phytochemical composition of mango juice.
TSP concentration
TPC (mg GAE 100 mL−1)
TFC (Mg CE 100 mL−1)
TAA (Mg VCE 100 mL−1)
TCT (Mg CE 100 mL−1)
Control
0.5% TSP
1.0% TSP
1.5% TSP
2.0% TSP
2.5% TSP
6.54 ± 0.21a
19.50 ± 0.29b
29.60 ± 0.36c
43.90 ± 0.14d
56.06 ± 0.67e
88.44 ± 0.8f
1.04 ± 0.03a
8.36 ± 0.06b
11.87 ± 0.14c
13.06 ± 0.15d
17.75 ± 0.28e
22.48 ± 0.36f
4.64 ± 0.58a
8.84 ± 0.15b
13.96 ± 0.28c
17.91 ± 0.95d
20.33 ± 0.08e
21.70 ± 0.04f
0.24 ± 0.01a
3.59 ± 0.24b
8.62 ± 0.84c
11.33 ± 0.29d
15.99 ± 0.40e
21.81 ± 0.08f
TPC, total phenolic content; TFC, total flavonoid content; TAA, total antioxidant activity; TCT, total condensed tannins. Data are means ± standard
deviation from three independent experiments (n = 3). Mean values in the same column with different superscript letters are significantly different
(P < 0.05).
Table 4. Effect of tamarind seed powder (TSP) on phytochemical composition of cookies.
TSP concentration
TPC (mg GAE 100 mg−1)
TFC (mg CE 100 mg−1)
TAA (mg VCE 100 mg−1)
TTC (mg CE 100 mg−1)
Control
2.0% TSP
4.0% TSP
6% TSP
8% TSP
10% TSP
20.43 ± 0.29a
23.41 ± 0.31b
25.37 ± 0.20c
26.1 ± 0.05d
27.41 ± 0.09e
29.08 ± 0.23f
4.06 ± 0.06a
5.35 ± 0.07b
5.71 ± 0.05c
6.67 ± 0.29d
8.2 ± 0.08e
10.29 ± 0.07f
5.6 ± 0.01a
8.9 ± 0.07b
12.7 ± 0.08c
17.2 ± 0.06d
19.2 ± 0.03e
25.5 ± 0.04f
8.3 ± 0.73a
11.70 ± 1.06b
12.35 ± 0.5c
13.54 ± 0.27d
15.48 ± 0.43e
19.24 ± 0.40f
TPC, total phenolic content; TFC, total flavonoid content; TAA, total antioxidant activity; TCT, total condensed tannins. Data are means ± standard
deviation from three independent experiments (n = 3). Mean values in the same column with different superscript letters are significantly different
(P < 0.05).
bioactive compounds (Table 4). Total phenolics ranged
from 20.43 ± 0.29 for control to 29.08 ± 0.23 mg
GAE/100 g for cookies containing 10% TSP. Tannins
content ranged from 8.3 ± 0.73 to 19.24 ± 0.40 mg
CE/100 g (Table 4). Enriched cookies that received higher
TSP concentration (10%) showed an increase of fivefold
and 2.5 fold in both total antioxidant activity and flavonoid content, respectively, as compared to the control
juice (without TSP).
There was a significant positive correlation between total
phenolic content (TPC) and total antioxidant activity
(TAC) (R2 = 0.922, P < 0.05) as shown in Figure 2.
There was a significant positive correlation (R2 = 0.923,
P < 0.05) between TAC and TFC as shown in Figure 3.
Figure 2. Relationship between antioxidant activity and total phenolic
content mango juice enriched with TSP (R2 = 0.922).
The acceptability scores of all sensory attributes lowered
with increase in concentration of tamarind seed powder
in enriched mango juice (Table 5). Mango juice enriched
with 2.5% TSP received the lowest perceived scores in
all sensory attributes (Table 5). The general acceptability
of the enriched juice ranged from 4.9 to 8.1. Overall
Sensory attributes of 0.5%, 1%, and 1.5% mango–tamarind seed juices were well accepted with a score between
6.6 and 7 (like moderately). Addition of TSP up to
0.5% negatively affected all sensory attributes apart from
thickness. Majority the of panelists mentioned that 2.5%
TSP-enriched mango juices was more viscous/thicker
than 0%, 0.5%, 1.0%, 1.5% and 2%. Some panelists
also noted that 2.5% TSP-enriched mango juice was
more astringent in flavor compared to the other juices.
There was no significant difference (P < 0.05) between
0.5%, 1.0% and 1.5% TSP-enriched mango juice in terms
of flavor, taste, consistency, sweetness, and overall
acceptability.
Correlation of overall acceptability with taste
(r = 0.744), mouth feel (r = 0.738), sweetness (r = 0.783)
and flavor (r = 0.682) showed that each of the properties
500
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
Effect of tamarind seed powder on sensory
acceptability of enriched mango juice
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
Figure 3. Relationship between antioxidant activity and total flavonoid
content of mango juice enriched with TSP (R2 = 0.923).
is of high significance (P < 0.01) in determining the
acceptability of the TSP-enriched mango juice (Table 6).
Effect of tamarind seed powder on sensory
acceptability of enriched cookies
The scores of all sensory attributes lowered with increase
in concentration of tamarind seed powder in enriched
cookies (Table 7).
The results show that the addition TSP in cookie formulation resulted in lower scores in all sensory attributes
at high concentrations with an overall acceptability ranging
from 6.6 ± 1.47 to 7.8.1 ± 1.83. Addition of TSP up to
2.0% did not affect acceptability for appearance, color,
flavor, crunchiness and taste of the enriched cookies
(Table 7). Addition of tamarind seed powder above 8%
significantly (P < 0.05) reduces the acceptability for color
of the enriched cookies. Incorporation of tamarind seed
powder significantly affected texture in relation to the
control. In terms of overall acceptability tamarind seed
powder-enriched cookies were acceptable up to 10% tamarind seed powder (6.6 = like slightly). Although, 6%
tamarind seed powder-enriched cookie was not significantly
different from 8% tamarind seed-enriched cookie in terms
of overall acceptability, it differed in terms of flavor.
Discussion
Effect of tamarind seed on physicochemical
properties of mango juice and cookies
Addition of tamarind seed powder led to a significant increase
(P < 0.05) in pH of mango juice. Tamarind seed powder
has a pH 5.2 ± 0.01 (Oluseyi and Temitayo 2015) and therefore may be responsible for the increase of the pH of the
enriched mango juice that was observed. The reduction of
total titratable acidity is consistent with the increase in pH
of the enriched mango juices. The reduction in titratable
Table 5. Effect of tamarind seed powder (TSP) on sensory acceptability of enriched mango juice.
Inclusion level of TSP (%)
Attribute
0 (Control)
Appearance
Color
Flavor
Taste
Thickness
Consistency
Mouth feel
Sweetness
Overall acceptability
0.5
1.88a
1.0
1.37bc
7.9 ±
8.0 ± 1.71a
7.9 ± 1.09a
7.7 ± 1.41a
6.7 ± 2.03ab
7.2 ± 1.67a
7.4 ± 1.59a
7.7 ± 1.49a
8.1 ± 1.03a
6.9 ±
7.0 ± 1.07b
6.5 ± 1.50b
6.4 ± 1.41b
6.7 ± 1.76ab
6.4 ± 1.33b
6.3 ± 1.88bc
6.3 ± 1.69b
7.0 ± 1.46b
1.5
1.27b
2.0
1.24c
6.8 ±
6.7 ± 1.27b
6.0 ± 1.93b
6.2 ± 1.81b
6.9 ± 1.57a
6.4 ± 1.86b
6.3 ± 1.77c
6.1 ± 1.93b
6.7 ± 1.61b
2.5
1.48cd
6.5 ±
6.5 ± 1.29cb
5.9 ± 1.87b
5.8 ± 1.93b
6.0 ± 1.85b
6.4 ± 1.50b
6.0 ± 1.95b
5.9 ± 2.27bc
6.6 ± 1.99b
6.1 ±
6.0 ± 1.51c
5.2 ± 1.97c
4.9 ± 2.07c
6.3 ± 1.82ab
5.8 ± 1.85bc
5.3 ± 2.24db
5.2 ± 2.16c
5.3 ± 2.02c
5.9 ± 1.93d
6.1 ± 1.82c
4.9 ± 2.14c
4.8 ± 2.12c
5.9 ± 2.42b
5.4 ± 1.89c
5.2 ± 2.20d
4.9 ± 2.26c
4.9 ± 1.98c
Values are means ± standard deviation (n = 50). Mean values in the same row with different superscripts (a–d) are significantly different (P < 0.05).
Anchors for the hedonic scale used were: 9 = like extremely, 8 = like very much, 7 = like moderately, 6 = like slightly, 5 = neither like nor dislike,
4 = dislike slightly, 3 = dislike moderately, 2 = dislike very much, 1 = dislike extremely.
Table 6. Correlation coefficient of the acceptability selected sensory attributes and general acceptability.
Attribute
Flavor
Taste
Consistency
Mouthful
Sweetness
Overall acceptability
Flavor
Taste
Mouth feel
Sweetness
1
0.758**
0.552**
0.603**
0.758**
1
0.624**
0.707**
0.464**
0.528**
0.579**
0.485**
0.552**
0.624**
1
0.710**
0.603**
0.707**
0.710**
1
0.682**
0.744**
0.738**
0.783**
**P < 0.01.
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
501
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
Table 7. Effect of tamarind seed powder (TSP) on sensory acceptability of enriched cookies.
Inclusion level of TSP (%)
Attribute
0 (Control)
2.0
4.0
6.0
8.0
10
Appearance
Color
Flavor
Taste
Texture
Mouth feel
After taste
Crunchiness
Overall acceptability
7.7 ± 1.33a
7.4 ± 1.5a
7.6 ± 1.19a
7.5 ± 1.40a
8.4 ± 6.03a
7.3 ± 1.51a
7.3 ± 1.45a
7.7 ± 1.37a
7.8.1 ± 1.83a
7.6 ± 0.90a
7.4 ± 1.06a
7.5 ± 1.20a
7.4 ± 1.19a
7.2 ± 1.20b
7.2 ± 1.26ab
7.0 ± 1.59ab
7.6 ± 1.39a
7.7 ± 0.90ab
6.8 ± 1.61b
6.8 ± 1.53b
7.1 ± 1.52ab
7.0 ± 1.33ab
7.3 ± 1.30b
7.1 ± 1.16ab
6.8 ± 1.42abc
7.7 ± 0.92a
7.3 ± 1.15bc
6.6 ± 1.31bc
6.6 ± 1.45b
6.7 ± 1.29b
6.6 ± 1.40bc
6.7 ± 1.11b
6.8 ± 1.49ab
6.5 ± 1.50bc
7.5 ± 1.06ab
7.0 ± 1.07cd
6.8 ± 1.37b
6.8 ± 1.36b
6.4 ± 1.58c
6.6 ± 1.38bc
7.1 ± 1.30b
6.9 ± 1.35ab
6.8 ± 1.34abc
7.02 ± 1.41b
6.9 ± 1.03cd
6.2 ± 1.50c
6.0 ± 1.51c
6.1 ± 1.14c
6.3 ± 1.70c
6.6 ± 1.50b
6.6 ± 1.89b
6.4 ± 1.48c
7.1 ± 1.45b
6.6 ± 1.47d
Data are means ± standard deviation (n = 50). Mean values in the same row with different superscript letters are significantly different (P < 0.05).
Anchors for the hedonic scale used were: 9 = like extremely, 8 = like very much, 7 = like moderately, 6 = like slightly, 5 = neither like nor dislike,
4 = dislike slightly, 3 = dislike moderately, 2 = dislike very much, 1 = dislike extremely.
acidity may be attributed to tamarind seed powder since
there was increase in pH with increase in tamarind seed
powder concentration. The low titratable acidity and high
pH are associated with reduction in shelf life. Acidulants
like citric acid should be added to tamarind seed powderenriched mango juice to increase its shelf life (Pundhir
and Murtaza 2015).
Results of this study revealed that addition of tamarind
seed powder led to a concentration-dependent increase
in mango juice viscosity. These results are consistent with
observations by Kumar and Bhattacharya (2008) who
studied the rheological behavior of Tamarind Kernel
Powder (TKP) of varying concentration (2, 4, 6, 8, and
10%) and observed an increase in apparent viscosity with
increase in TKP concentration. The increment in viscosity
of the enriched mango juices may be attributed to large
proportions (65.1–72.2%) of galactoxyloglucan found in
tamarind seed (Bhattacharya et al. 1994). Galactoxyloglucan
is composed of a β- (1–4) linked D- glucan backbone
that is substituted with side-chains of α-d-xylopyranose
and β-d-galactopyranosyl (1-2)-α-d-xylopyranose linked
to (1–6) glucose residue (Yamanaka et al. 2000).
Galactoxyloglucan exhibits high water-holding capacity
with good stability to heat, acids, and shear and thus its
wide application in the food industry as a thickener, stabilizer, or starch modifier. It is used to improve rheological
and thermal properties of many products including, salad
dressing, mayonnaise, and stew (Nishinari et al. 2000).
This therefore implies that tamarind seed powder can
potentially be utilized as a food stabilizer.
The coloration of the enriched juices became more
orange and dull with increased concentration of tamarind
seed powder. A similar observation was made by Andabati
and Muyonga (2014) where addition of tamarind seed
powder to the tamarind pulp juice led to discoloration
of the enriched juices. This may be attributed to the
activity of polyphenol oxidase. TSP is rich in polyphenols
which are substrates for the polyphenol oxidases. In presence of oxygen, the enzymes catalyze the hydroxylation
of monophenols to diphenols and then subsequently to
corresponding quinine intermediates. It is the intermediates that are responsible for discoloration. More so, TSP
has a dark brownish color (Kumar and Bhattacharya 2008),
and therefore incorporation of TSP in mango juice decreases
its brightness. Color is very important when consumers
are making choices on food products (Tril et al. 2014).
It is therefore important to use low concentration of TSP
in juices to avoid discoloration/darkening of juices which
potentially has an effect on consumer preference. High
concentrations of TSP can be tried in other products
such as sausages whose color may not be affected (Tril
et al. 2014).
502
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
Effect of TSP on phytochemical composition
of enriched mango juices and cookies
The effect of TSP on phytochemical content of enriched
mango juice and cookies was concentration dependent.
In their studies Andabati and Muyonga (2014) found that
addition of tamarind seed powder in tamarind pulp juice
led to a significant enrichment in total phenolic compounds, total flavonoids and total antioxidant activity.
The observed increase in TFC, TPC and TAA in enriched
juices is attributed to the high phenolic content, flavonoid
content, and antioxidant activity in Tamarindus indica L.
seeds (Tsuda et al. 1994; Soong and Barlow 2004; Siddhuraju
2007). A similar observation was reported by Salgado et al.
(2012) where addition of antioxidant-rich pomegranate
peel (Punica granatum) extract to orange and tomato juice
led to increase in antioxidant activity. The total phenolic
content of TSP-enriched mango juices was in the range
6.54 ± 0.21 to 88.44 ± 0.8 mg GAE/100 mL. This is
S. Natukunda et al.
Antioxidant Potential of Tamarind Seed
higher than the phenolic content in tomato juice (5.97 mg
GAE/100 mL) (Owusu et al. 2015), passion fruit (27.1–
38.1 mg GAE/100 g) (Ramaiya et al. 2013), mango (6.25 mg
GAE/100 g) (Gorinstein et al. 1999), jackfruit 0.36 mg
GAE/100 g (Swami et al. 2012) and fruit juices enriched
with roselle fruit (53.7-10.8 GAE mg/100 g) (Mgaya et al.
2014). There was a 13-fold increase in the total phenolic
content at 2.5% of tamarind seed powder-enriched juice
compared to the control. This suggests that addition of
tamarind seed powder highly enhances the content of
bioactive compounds, thus increasing the nutraceutical
properties of the enriched mango juices. It is therefore
important to further utilize the seeds rather than just
discarding them as waste as it is a common practice currently. Flavonoid intake of about 14.33 mg/day has been
reported to reduce memory loss in elderly people
(Letenneur et al. 2007). Daily consumption of 110 mL
of 1.5% TSP-enriched mango juice is adequate to meet
flavonoid content of 14.33 mg/day. Therefore, the findings
from the current study show the potential of tamarind
seed-enriched mango juices in mitigating memory loss in
elderly people.
Antioxidant activity is very important in human health
mainly because of its free-radical scavenging activity and
protection against oxidative stress (Haghju and Almasi
2015; Valdes et al. 2015) and thus prevents development
of diseases such as heart disease and cancer (GonzálezVallinas et al. 2013; Farias et al. 2014). Numerous studies
have demonstrated that polyphenol and flavonoid compounds are the most effective antioxidative constituents
in fruits, vegetables and grains (Choi et al. 2007; Dykes
and Rooney 2007). This is consistent with significant positive correlation between total antioxidant activity and the
concentrations of total phenolics and total flavonoid in
tamarind seed-enriched juices observed in this study.
Similar relations have been reported for other foods such
as Chilean blackberries, barley, mushrooms, mulberries,
flaxseed, wheat, oats, ginseng rice, and bread enriched
with ginger (Choi et al. 2007; Céspedes et al. 2008;
Jayaprakasha et al. 2008; Shen et al. 2009; Balestra et al.
2011).
The addition of tamarind seed powder to mango juices
increased the content of total phenolics, flavonoid, antioxidant activity, and tannin in the enriched cookies in a
concentration-dependent manner. In a related study Ajila
et al. (2008) observed that incorporation of mango peel
powder (0, 5, 10, 15, 20%) increased total phenolics and
antioxidant activity in a concentration-dependent manner.
The assay of DPPH-scavenging activity showed that TSP
was a good source of active compounds, and adding it
significantly enhanced the antioxidant properties of the
cookies. A similar study Mildner-Szkudlarz et al. (2013)
reported that incorporation of white grape pomace
(residue) significantly increased the content of phenolics
and antioxidants in biscuits.
The beta carotene levels decreased with increase in
tamarind seed powder concentration in enriched juices.
Addition of TSP may have contributed to dilution of
beta carotene content. However, it was added in quite
small quantities up to 2.5% to account for all the change.
Another reason is probably formation of the complexes
between the proteins in the seed powder with carotenoids
in the enriched mango juices. This reduces carotenoid
concentration in juice with increase in tamarind seed
(Sweeney and Marsh 1971).
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
503
Effect of tamarind seed powder on sensory
properties of enriched products
Addition of tamarind seed powder in mango juice resulted
in reduced scores in all the sensory attributes including
color, flavor, consistency, thickness, and general acceptability. In a related study, Salgado et al. (2012) observed
low scores when pomegranate peel powder was added to
orange and tomato juice. Both powders have high phenolics and tannins which elicit undesired astringent taste
(Kumar and Bhattacharya 2008; McRae and Kennedy
2011). Phenolic compounds highly correlate inherently
with sensory characteristics of food, such as color, astringency, bitterness, and flavor (Mousavinejad et al. 2009).
High levels of phenolics and tannins can elicit negative
consumer reactions (Drewnowski and Gomez-Carneros
2000). Astringency is the drying, roughing, and puckering
of the epithelium of the oral cavity. The perception of
astringency results from binding and subsequent precipitation of tannins with salivary proteins and glycoproteins.
This interaction acts as a water barrier resulting in a
tactile sensational loss of lubrication in the oral cavity
(Kielhorn and Thorngate III 1999). This therefore explains
the reduction in scores in sensory acceptability with increase
in concentration of TSP in the current study. The lowest
score on the sensory attributes was recorded at the highest
concentration of 2.5% of the TSP in mango juice. According
to Lawless et al. (2012), addition of high level of polyphenol compounds in food formulations negatively affects
the sensory attributes and acceptability of finished foods,
resulting in changes such as increased bitterness and
astringency. The sensory acceptability results of this study
confirm the association of astringency to poor acceptability.
The low consumer acceptance ratings for the thickness
and flavor of the enriched juices scores were substantiated
by the findings on viscosity and phytochemical analysis
findings in the current study. There was no significant
difference in acceptance for flavor, taste, thickness, and
overall acceptability between 0.5%, 1.0%, 1.5%, respectively,
while at 2.0% and 2.5% there was a significant difference
Antioxidant Potential of Tamarind Seed
S. Natukunda et al.
of the enriched juices. The results of this study show
that juice with 2% and 2.5% TSP had significantly lower
acceptability compared to the control and those with lower
TSP concentrations. This therefore suggests that 1.5% is
the maximum concentration possible that can be added
to enrich mango juice that can have commercial appeal
because it was the acceptance limit of sensory evaluation
by the tasters.
While consumers may be interested in products with
high levels of bioactive products, palatability and taste
are key determinants of acceptability. This, therefore, suggests that a concentration of 1.5% of TSP would be an
appropriate level to cater for consumer acceptability as
well as providing health benefits of phenolics.
Incorporation of tamarind seed powder into cookies
resulted in reduced scores in all the sensory attributes.
Sensory evaluation studies showed flavor, color, taste,
crunchiness, and overall general acceptability of cookies
containing tamarind seed were as acceptable as those of
control cookies up to 2% level of tamarind seed incorporation and any further increase led to lower scores.
The enriched cookies also became relatively harder compared to the control. Tamarind seed powder contains
galactoxylose which has high water-binding capacity
(Bhattacharya et al. 1994) and this may explain the hardness of the enriched cookies. The addition of TSP influenced the color of each of the cookies. In the cookies
made with 10% TSP panelists commented on the unappealing dark color. This may be due to the brown of the
tamarind seed (Kumar and Bhattacharya 2008). These
comments were reflected in the significantly lower acceptance scores for enriched cookies color compared with
the control.
At higher TSP concentration, the acceptability for mouth
feel and after taste scores was also low. It is known that
the polyphenolic compounds contribute to the astringency
of enriched cookies because of the interaction between
phenolics, mainly procyanidins and the glycoproteins in
saliva (McRae and Kennedy 2011). The tamarind seed
has high phenolics and tannins which elicit undesired
astringent taste (Kumar and Bhattacharya 2008) which
makes products with tamarind seed powder have lower
consumer appeal. Based on all these sensory attributes,
participants preferred a control cookie, rather than enriched cookies. This is consistent with the study by Bakke
and Vickers (2011), in which, the bitterness from added
wheat germ extract decreased bread liking. In terms of
overall acceptability, cookies enriched with (6%, 8%, 10%)
TSP were not significantly different. However, 8% and
10% TSP-enriched cookies had significantly lower scores
for flavor (6.4) and color (6.0), respectively. On that basis,
the 6% TSP-enriched cookie was selected as the maximum
acceptable TSP concentration for TSP-enriched cookies.
Other efforts have been made to add bioactive-rich
components to processed foods. (Ajila et al. (2008); Hooda
and Jood (2005); Mildner-Szkudlarz et al. (2013)) added
mango peel powder, fenugreek, and white grape pomace
powder to biscuits and reported no negative effect with
levels of up to 10% (Sudha et al. 2007) added apple
pomace to biscuits and found that up to 20% could be
incorporated without compromising acceptability.
504
© 2015 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc.
Conclusion
Incorporation of the tamarind seed powder into mango
juice and cookies significantly increases their content of
bioactive phytochemicals with an associated increase in
the antioxidant activity. Based on sensory analysis, it may
be concluded that the amount of TSP that can be added
to mango juice and cookies need to be limited to 1.5%
and 6%, respectively, to ensure consumer acceptability.
The findings confirm the potential to utilize tamarind
seed powder as a source of natural antioxidants and stabilizer in our search for good human health.
Acknowledgments
This study was funded through a grant from the Swedish
International
Development
Co-operation
Agency,
Department for Research Cooperation (Sida-Sarec)
Program of Makerere University.
Conflict of Interest
The authors did not declare any conflict of interest.
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