Physical and nutritional properties of edamame seeds as influenced by stage of development

Abstract

Changes in physical, chemical and anti-nutritional characteristics of two vegetable soybean varieties (‘Asmara’ and ‘Mooncake’) during seed development were investigated. Pods were sampled weekly for 6 weeks starting from developmental stage 5 (R5) to 8 (R8). Changes over time in measured attributes were similar but the rate of change differed among the two varieties. In both varieties seed moisture content and intensity of green color decreased as seed developed with the most significant decline observed in the last 2 weeks of sampling. Seed weight peaked at R6 then gradually decreased thereafter, while seed hardness increased throughout the sampling period with ‘Asmara’ recording significantly higher seed hardness at R8. For both varieties, protein accumulation occurred mostly in the later stages, while significant lipid accumulation was observed in the early stages of development. Among the sugars, fructose content decreased with seed development, sucrose content increased to R6 before decreasing, and there was rapid accumulation of raffinose and stachyose in the last 2 weeks of sampling. Total phenolic content decreased between R5 and R6 but increased with further seed development. Tannin and phytate content in seed increased throughout the sampling period. Changes in trypsin inhibitory activity varied with variety reaching a maximum at R6 for ‘Asmara, and closer to maturation for “Mooncake’. Our data provide physical, chemical and anti-nutritional basis for harvesting vegetable soybean at R6 namely: peak seed weight and sucrose content, lower oligosaccharide and anti-nutrients values and intense green color.

Full text

ORIGINAL PAPER
Physical and nutritional properties of edamame seeds as
influenced by stage of development
Yixiang Xu
1
•Arrieyana Cartier
1
•Daniel Kibet
1
•Krystal Jordan
1
•
Ivy Hakala
1
•Stephanie Davis
1
•Edward Sismour
1
•Maru Kering
1
•
Laban Rutto
1
Received: 8 June 2015 / Accepted: 1 December 2015 / Published online: 12 December 2015
ÓSpringer Science+Business Media New York 2015
Abstract Changes in physical, chemical and anti-nutri-
tional characteristics of two vegetable soybean varieties
(‘Asmara’ and ‘Mooncake’) during seed development were
investigated. Pods were sampled weekly for 6 weeks start-
ing from developmental stage 5 (R5) to 8 (R8). Changes over
time in measured attributes were similar but the rate of
change differed among the two varieties. In both varieties
seed moisture content and intensity of green color decreased
as seed developed with the most significant decline observed
in the last 2 weeks of sampling. Seed weight peaked at R6
then gradually decreased thereafter, while seed hardness
increased throughout the sampling period with ‘Asmara’
recording significantly higher seed hardness at R8. For both
varieties, protein accumulation occurred mostly in the later
stages, while significant lipid accumulation was observed in
the early stages of development. Among the sugars, fructose
content decreased with seed development, sucrose content
increased to R6 before decreasing, and there was rapid
accumulation of raffinose and stachyose in the last 2 weeks
of sampling. Total phenolic content decreased between R5
and R6 but increased with further seed development. Tannin
and phytate content in seed increased throughout the sam-
pling period. Changes in trypsin inhibitory activity varied
with variety reaching a maximum at R6 for ‘Asmara, and
closer to maturation for ‘‘Mooncake’. Our data provide
physical, chemical and anti-nutritional basis for harvesting
vegetable soybean at R6 namely: peak seed weight and
sucrose content, lower oligosaccharide and anti-nutrients
values and intense green color.
Keywords Vegetable soybean Seed Development
stage Physical Chemical Anti-nutritional quality 
Harvest
Introduction
Vegetable soybean [Glycine max (L.) Merr.], also known as
edamame, is a class of food-grade soybean varieties that are
generally harvested when the pods are fully filled and still
green [1]. Compared to a long history of edamame production
and consumption in East Asia (China, Taiwan, Japan and
Korea), the edamame industry is still at its infancy in the
United States [2]. However, the popularity of edamame is
increasing as US consumers become aware of its high nutri-
tional value and related health benefits [1]. Each year 25,000
to 30,000 tons of edamame are consumed in the US with a
majority of them being imported from China and Taiwan [3].
In the past few decades, a lot of effort has been made to
promote edamame as a vegetable crop in the US [4–10].
Unlike grain-type soybean which is used primarily for
vegetable oil and soy protein products, edamame is con-
sumed fresh as a snack after boiling or as a vegetable. The
attributes that differentiate edamame from regular soybean
include dark green color at maturity, soft texture, large seed
size, and a sweet and less beany flavor [11]. In addition,
edamame contain high levels of protein, vitamins, minerals,
dietary fiber and isoflavones [12,13]. The health benefits
associated with consumption of edamame include increased
bone density, reduction of cholesterol levels, prevention of
cardiovascular disease, and reduction in mammary and
prostate cancers [14]. However, similar to most plant protein
Ivy Hakala and Stephanie Davis have contributed equally to this
work.
&Yixiang Xu
yixu@vsu.edu
1
Agricultural Research Station, Virginia State University,
Petersburg, VA 23806, USA
123
Food Measure (2016) 10:193–200
DOI 10.1007/s11694-015-9293-9

sources, soybean contains some anti-nutrients such as phe-
nolics, tannins, phytates, and trypsin inhibitors. Although
these compounds are reported to be lower in edamame than
in grain-type soybean, their presence, even in small quan-
tities, will reduce nutrient availability through interference
with either protein digestion or mineral bioavailability [15].
Development and maturation in soybean can generally be
divided into two phases: vegetative (six stages) and repro-
ductive (eight stages) as presented in Table 1[11,16]. Of the
eight reproductive stages, R5 and R6 describe seed develop-
ment, while R7 and R8 describe seed maturation [16]. Nor-
mally, edamame is harvested at the R6 stage when the
immature seeds are fully developed but the pods are still
green. It is well known that seed physical parameters,
chemical composition and nutritional quality are a function of
developmental stage. Changes in seed chemical composition
of grain-type soybeans, including starch, oil, protein and
sugar content during seed development and maturation have
been investigated [17–19]. Saldivar et al. [11] also evaluated
change in seed chemical composition of five food-grade
soybean genotypes during development and maturation.
Currently, there is limited information on the changes
that occur in edamame physical and chemical properties
during seed development and maturation. In this work, we
evaluated dynamic changes in physical, chemical and anti-
nutritional properties of two edamame varieties from seed
formation to maturity (approximately 6 weeks). The intent
is to present a full picture of edamame physical and
chemical attributes at different developmental stages and
thereby identify a harvest window that assures the greatest
nutritional benefit for human consumption.
Materials and methods
Materials
Two vegetable soybean varieties (var ‘Asmara’ and
‘Mooncake’) grown at Randolph Farm, Virginia State
University were used to conduct this study in the fall of
2014. Pods were sampled weekly for 6 weeks starting from
developmental stage R5. Immediately after harvest, pods
were separated from the vines, hand sorted to remove
debris, and shelled. Shelled beans from each sample were
divided into two portions. The first portion was used
directly to determine physiochemical properties, while the
second portion was immediately freeze dried then ground
using a micro-mill grinder (Bell-ArtMini Mill) to pass a
2 mm sieve for use to measure nutritional, anti-nutritional,
and antioxidant properties.
Physical properties of seeds
Water content and color
Seed water content was measured using a Sartorius MA35
moisture analyzer (Goettingen, Germany). A Minolta CR-
400/410 chromameter (Minolta Camera Co., Ltd, Osaka,
Japan) was used to determine ‘L* ‘ (lightness) from black
(0) to white (100), ‘a*’ (red (?a) to green (-a) color), and
‘b*’ (yellow (?b) to blue (-b) color) values. Intensity of
green color was calculated as -a*/b*.
Seed weight and hardness
Seed weight was determined by weighing three randomly
selected samples of 100 seeds per variety per sampling
period. The hardness of 20 individual seeds was measured
according to Gu
¨zel and Sayar [20] using a TA-XT2 texture
analyzer (Texture Technologies Corp., Scarsdale, NY/
Stable Micro Systems, Godalming, Surrey, UK) equipped
with a 5 cm diameter aluminum cylinder probe. Each seed
was placed in a stable resting position with the interface of
the cotyledons approximately parallel to the surface of the
probe and then compressed to 50% of its original height at
a test speed of 0.2 mm/s. Seed hardness was quantified as
the peak of compression force.
Table 1 Soybean developmental stages [11,16]
Vegetative stages Reproductive stages
Stages Description Stage Description
VE Emergence R1 Beginning bloom (At least one flower)
VC Cotyledon stage R2 Full bloom (An open flower at one of two upper nodes)
V1 First trifoliate leaf R3 Beginning pod (3/1600 pod on the upper four nodes)
V2 Second node R4 Full pod (3/400 long pod on one of four upper nodes)
V3–V5 Third to fifth nodes R5 Beginning seed (1/800 long seed in four upper nodes)
V6 Sixth node (flowering will soon start) R6 Full green size seed on at least one of the four top nodes
R7 Beginning maturity (one normal pod on the main stem)
R8 Full maturity (95 % of pods reach mature color)
194 Y. Xu et al.
123

Chemical composition of seeds
Proximate composition
Analyses of crude protein, crude lipid, ash, and carbohy-
drate were performed using methods described by the
Association of Official Analytical Chemists (AOAC) [21].
Crude protein was analyzed using the dry combustion
method with a Vario MAX CN analyzer (Elementar
Americas, Inc., Mt. Laurel, NJ, USA). Oil was measured
using Soxhlet apparatus. Ash was determined using a
preheated muffle furnace (600 °C), and carbohydrate con-
tent (%) was determined by subtracting the total percentage
of other components from 100.
Sugar composition
Sugars were extracted from ground seed (1 g) using water
as the solvent and analyzed by high pressure liquid chro-
matography (HPLC) using a Waters HPLC-2695 and a
Waters 410 differential refractometer (Milford, MA) fol-
lowing methods optimized by Johansen et al. [22]. Dif-
ferent sugars in extracts were identified by comparing
retention times with known standards. For quantification,
trehalose was used as an internal standard, and the con-
centration of different sugars expressed as g/100 g of beans
(dry weight basis).
Anti-nutritional composition of seeds
Total phenolics and tannins
Freeze-dried soybean meal (200 mg) was mixed with
10 ml of aqueous acetone (70 %) and subjected to ultra-
sonic treatment for 20 min at room temperature, followed
by centrifugation for 10 min at approximately 4700 9gat
4°C. The supernatant was collected and kept on ice and
the pellet re-extracted with two 5-mL aliquots each of
70 % aqueous acetone as described above. The super-
natants were pooled prior to determination of total phenolic
(TPC) and tannin content following the Folin-Ciocalteu
method [23] with some modifications. Extracts were mixed
with Folin-Ciocalteu reagent (10 %) and 7.5 % sodium
carbonate solution and placed in the dark for 1 h before
measuring absorbance at 725 nm using a spectropho-
tometer. TPC was expressed as gallic acid equivalent
(GAE) on a dry weight basis (mg/g sample). Tannin con-
tent was presented as the difference between total and
simple phenolics. To determine simple phenolics, the
extract was first mixed with insoluble polyvinylpyrrolidone
(PVPP) powder (100 mg) to adsorb tannins then the
supernatants were reacted with Folin reagent as described
above. Similarly, tannin content was expressed as gallic
acid equivalent (GAE) on a dry matter basis (mg/g
sample).
Phytate and trypsin inhibitor activity
Phytate was analyzed using a rapid colorimetric procedure
[24]. Freeze dried samples were extracted with
hydrochloric acid (2.4 %) for 1 h then centrifuged at
4700 9gfor 10 min. The supernatant was diluted with
distilled water before passing through an AG1-X8 anion-
exchange resin to remove inorganic phosphorus and other
contaminants. Phytate was eluted with 0.7 M sodium
chloride, and content was measured based on the reaction
between the ferric ion and sulfosalicylic acid in Wade
reagent using a spectrophotometer at 500 nm.
Trypsin inhibitory activity (TIA) was determined fol-
lowing methods described by Kakade et al. [25]using
benzoyl-DL-arginine-p-nitroanilide hydrochloride (BAPNA)
as a substrate for trypsin. BAPNA is subjected to hydrolysis
by trypsin to produce yellow-colored p-nitroanilide. Trypsin
inhibitors in soybean extracts inhibit the production of yel-
low color, and the degree of inhibition by extracts (TIA) was
measured at 410 nm using a spectrophotometer. The results
are expressed in terms of trypsin units inhibited (TUI) with a
TU being an arbitrary measure representing an increase of
0.01 in absorbance under prevailing assay conditions.
Statistical analyses
Three replications were used to obtain average values and
standard deviations for all tests. All results were analyzed
with SAS version 9.4 statistical software (SAS Institute
Inc., Cary, NC). Tests of significance were evaluated using
a significance level of a=0.05. Two-way ANOVA was
used to determine the percentage total variation by variety,
harvest-week, and their interaction. One-way ANOVA and
Tukey’s multiple comparison test were used to evaluate the
significance of differences within and variety between
harvest weeks.
Results and discussion
Changes in physical properties of seeds
Changes in seed water content, intensity of green color,
weight, and hardness of two edamame varieties during
development and maturation are presented in Table 2.
Physical properties play a critical role in the acceptability
and marketability of edamame [26,27]. Both varieties had
high initial water content (72.3 % for ‘Asmara’ and 71.0 %
for ‘Mooncake) at R5. Water content decreased signifi-
cantly (P\0.05) during seed development dropping to
Physical and nutritional properties of edamame seeds as influenced by stage of development 195
123

13.0 % for ‘Asmara’ at maturity (R8). Both varieties had
an initial green color intensity of 0.61 that gradually
decreased with seed development. The seeds remained
green to R6 but rapidly turned yellow thereafter with green
color intensity declining to 0.07 for ‘Asmara’ and 0.13 for
‘Mooncake’ at the end of 6-week sampling period.
According to Borrmann et al. [28], the loss of green color
in seed during maturation is due to chlorophyll degradation
and formation of colorless chlorophyll derivatives. Mean
seed weight in both varieties increased significantly in the
first 3 weeks, peaking at R6 followed by a gradual decrease
for the remaining sampling period. The initial increase in
weight during seed development is a result of dry mass
accumulation while the decline in weight during matura-
tion is due to a reduction in seed moisture [29,30]. Seed
hardness in both varieties increased during seed develop-
ment and maturation reaching a maximum at the end of the
sampling period with the highest hardness of 52,882 gf
observed in ‘Asmara’. Seed hardness at maturity is attrib-
uted to reduced water content and the development of a
thick seed coat. ‘Asmara’ is an early maturing genotype
that progresses through the different stages at a faster rate
reaching R6 within 3 weeks and R8 in six. On the other
hand, ‘Mooncake’ matured at a slower rate, reaching R6 in
4 weeks and late R7 at the end of the 6-week study period.
Changes in chemical composition of seeds
Changes in proximate composition (protein, lipid, ash, total
carbohydrate, and fiber) of the two edamame varieties are
presented in Table 3. There was no significant difference in
protein content between the two varieties up to R6 fol-
lowed by a gradual increase thereafter to maturity, a
finding that is consistent with that of Saldivar et al. [2].
Protein content is an important nutritional index for soy-
bean. Although both varieties had similar protein levels at
maturity, ‘Mooncake’ had significantly higher protein than
‘Asmara’ at the R6 stage (when edamame is harvested), a
desired trait in edamame [2]. Lipid accumulation in seed
occurred mostly between R5 and R6 and thereafter
remained constant except in ‘Asmara’ that recorded a
significant increase at R8. This is consistent with a previous
report on oil change during seed development and matu-
ration in soybean [2]. In the first 4 weeks of sampling, ash
content fell to a low of 3.89 % for ‘Asmara’ and 3.58 % for
‘Mooncake’ before increasing with further seed develop-
ment and maturation. Both varieties showed reduced total
carbohydrate with increased protein and lipid accumulation
during seed development.
Soluble sugar content including monosaccharides
(fructose, glucose), disaccharides (sucrose), and oligosac-
charides (raffinose and stachyose) in edamame seed are
presented in Table 4. Seed from both two varieties had
high initial fructose contents of 2.59 g and 2.57 g/100 g for
‘Asmara’ and ‘Mooncake’, respectively, but these levels
significantly decreased with seed development to final
values of 0.40 g/100 g for ‘Asmara’ and 0.56 g/100 g for
‘Mooncake’. Glucose content was low throughout seed
development for both varieties, slightly increasing to R6
before gradually decreasing. Sucrose was the most abun-
dant soluble sugar in both varieties with initial levels of
6.51 g/100 g for ‘Asmara’ and 6.86 g/100 g for ‘Moon-
cake’. Sucrose content increased with seed development
and reached maximum levels of 9.16 g/100 g for ‘Asmara’
and 9.34 g/100 g for ‘Mooncake’ at R6. High sucrose
content at harvest is a desirable trait for edamame to
Table 2 Changes in physical properties in edamame seeds during development and maturation
Variety Weeks Water Content (%) Intensity of green color (-a/b) Seed weight (g/100 seeds) Hardness (g
f
)
Asmara 1 (R5) 72.3 ±1.00
a
0.61 ±0.03
a
7.6 ±0.44
c
3253 ±420
e
2 64.7 ±1.52
b
0.59 ±0.04
a
40.6 ±1.37
b
4090 ±580
de
3(R6) 59.5 ±2.46
bc
0.55 ±0.07
ab
50.5 ±3.07
a
6734 ±794
d
4 54.6 ±3.41
c
0.46 ±0.05
b
49.3 ±0.66
a
8699 ±1381
c
5 33.2 ±2.18
d
0.22 ±0.08
c
37.9 ±0.61
b
12,243 ±1227
b
6 (R8) 13.0 ±4.10
e
0.07 ±0.01
d
28.5 ±0.65
c
52,882 ±3381
a
Mooncake 1 (R5) 71.0 ±0.87
a
0.61 ±0.02
a
20.9 ±1.56
e
2827 ±534
e
2 63.6 ±1.07
b
0.56 ±0.02
ab
39.1 ±0.37
d
4107 ±608
d
3 62.3 ±1.20
bc
0.54 ±0.02
ab
48.2 ±1.34
b
6102 ±977
c
4(R6) 56.9 ±3.41
c
0.51 ±0.05
b
52.1 ±1.91
a
7007 ±1621
c
5 46.2 ±4.59
d
0.14 ±0.05
c
44.1 ±1.03
c
8330 ±1382
b
6(late R7) 39.3 ±1.88
e
0.13 ±0.04
c
40.2 ±0.56
d
10,312 ±1028
a
Data are expressed as mean ±standard deviation (n =3, n =20 for hardness)
Means followed by the same letter within a variety indicate no significant (P[0.05) difference between harvest weeks
196 Y. Xu et al.
123

provide the sweet flavor. Sucrose concentration decreased
slowly from R6 to late R7 or R8 stages. Raffinose content
remained low up to R6 for ‘Asmara’, and thereafter
increased rapidly, while rapid accumulation of raffinose
occurred in the last 2 weeks for ‘Mooncake’. Initial sta-
chyose levels were 0.36 g/100 g for ‘Asmara’ and 0.71 g/
100 g for ‘Mooncake’. Stachyose content decreased to the
lowest level at R6 and then dramatically increased in the
last 2 weeks, reaching 5.13 g/100 g for ‘Asmara’ and
3.51 g/100 g for ‘Mooncake. Raffinose and stachyose are
indigestible sugars associated with flatulence and other
stomach discomfort [31], and low oligosaccharide content
at harvest is a desirable trait in edamame. The formation of
oligosaccharides requires sucrose, and the accumulation of
raffinose and stachyose during the later stages of seed
development explains the decrease in sucrose content [32].
Change in anti-nutritional compositions of seeds
Changes in anti-nutritional properties (total phenolic con-
tent, tannins, phytate, and trypsin inhibition activity) in the
two edamame varieties during development and maturation
are presented in Table 5. Total phenolic content slowly
decreased during early seed development reaching a low
point (3.20 mg GAE/g for ‘Asmara’ and 2.82 mg GAE/g
for ‘Mooncake’) at R6 before increasing with further seed
development. For tannins, ‘Asmara’ had an initial content
of 1.66 mg GAE/g in the first week that significantly
increased to 1.89 mg GAE/g in the second week. There
was no significant change in tannins between the second
and fifth week followed by a sharp increase in the last week
of sampling (R8 stage). For ‘Mooncake’, there was no
significant difference in tannin content in the first 4 weeks
Table 3 Changes in proximate
composition (g 9100 g
-1
dry
weight) in edamame seeds
during development and
maturation
Variety Weeks Protein Lipid Ash Total carbohydrate
Asmara 1 (R5) 32.1 ±0.35
d
15.8 ±0.41
c
4.53 ±0.09
a
47.6 ±0.80
a
2 33.3 ±0.22
d
17.1 ±0.18
b
4.24 ±0.14
ab
45.4 ±0.18
b
3 (R6) 34.8 ±0.49
c
17.2 ±0.28
b
4.14 ±0.16
b
43.9 ±0.57
c
4 35.8 ±0.09
bc
17.6 ±0.17
b
3.89 ±0.17
c
42.7 ±0.30
c
5 36.0 ±0.03
b
17.4 ±0.06
b
4.12 ±0.03
b
42.5 ±0.13
c
6 (R8) 37.6 ±0.40
a
18.2 ±0.12
a
4.20 ±0.10
ab
39.9 ±0.50
d
Mooncake 1 (R5) 34.9 ±0.42
b
15.1 ±0.09
c
5.49 ±0.38
a
44.7 ±0.47
a
2––––
3 34.7 ±0.57
b
17.2 ±0.10
b
4.69 ±0.10
b
42.4 ±0.19
b
4(R6) 36.1 ±0.05
a
18.2 ±0.19
a
3.58 ±0.06
d
42.1 ±0.57
b
5 36.9 ±0.53
a
18.9 ±0.32
a
4.14 ±0.09
c
39.1 ±0.12
c
6(late R7) 37.7 ±0.31
a
18.5 ±0.27
a
4.12 ±0.01
c
39.7 ±0.11
c
Data are expressed as mean ±standard deviation (n =3)
Means followed by the same letter within a variety indicate no significant (P[0.05) difference between
harvest weeks; –Not applicable
Table 4 Changes in sugar
content (g 9100 g
-1
dry
weight) in edamame seeds
during development and
maturation
Variety Weeks Fructose Glucose Sucrose Raffinose Stachyose
Asmara 1 (R5) 2.59 ±0.55
a
0.13 ±0.01
bc
6.51 ±0.09
d
0.08 ±0.01
c
0.36 ±0.07
c
2 1.22 ±0.02
b
0.19 ±0.02
ab
8.93 ±0.30
a
0.06 ±0.00
c
0.14 ±0.04
d
3 (R6) 0.85 ±0.03
bc
0.21 ±0.07
ab
9.16 ±0.08
a
0.46 ±0.01
b
0.11 ±0.02
d
4 0.72 ±0.02
bc
0.28 ±0.01
a
7.66 ±0.26
b
0.51 ±0.08
b
0.08 ±0.01
d
5 0.53 ±0.02
c
0.16 ±0.02
b
7.28 ±0.10
bc
1.05 ±0.04
a
2.58 ±0.08
b
6 (R8) 0.40 ±0.07
c
0.09 ±0.03
c
6.96 ±0.05
cd
1.09 ±0.02
a
5.13 ±0.72
a
Mooncake 1 (R5) 2.57 ±0.06
a
0.14 ±0.01
c
6.86 ±0.21
c
0.01 ±0.00
c
0.71 ±0.07
b
2–––––
3 0.86 ±0.02
b
0.24 ±0.02
b
8.16 ±0.14
b
0.19 ±0.04
b
0.24 ±0.04
c
4 (R6) 0.79 ±0.01
b
0.21 ±0.01
b
9.34 ±0.22
a
0.16 ±0.05
b
0.07 ±0.01
d
5 0.65 ±0.03
c
0.35 ±0.03
a
6.75 ±0.14
c
0.94 ±0.05
a
3.06 ±0.11
a
6 (late R7) 0.56 ±0.02
d
0.17 ±0.01
c
6.46 ±0.06
c
1.03 ±0.08
a
3.51 ±0.10
a
Data are expressed as mean ±standard deviation (n =3)
Means followed by the same letter within a variety indicate no significant (P[0.05) difference between
harvest weeks; –Not applicable
Physical and nutritional properties of edamame seeds as influenced by stage of development 197
123

(1.52–1.58 mg GAE/g) but there was a significant increase
to 1.95 mg GAE/g in the fifth week, a level that did not
change in subsequent harvests. Tannins are astringent
polyphenols that precipitate proteins and chelate metal ions
thereby reducing their bioavailability [33]. They accumu-
late during seed development and protect mature seed
against pathogens, herbivores, and hostile environmental
conditions [34].
Phytate levels were 1.27 and 2.13 mg/g for ‘Asmara’
and ‘Mooncake’, respectively at the start of the experi-
ment. During seed development and maturation, phytate
content exhibited a continuous and approximately linear
increase for both varieties with both having approxi-
mately the same final content (11.0 mg/g). These phytate
levels are similar to those previously reported by Raboy
and Dickinson [35] for developing grain-type soybean
seed. Phytate, formed during seed maturation, is the
principal storage form of phosphorus in plant tissues [36]
and plays an important role in mineral availability [37].
At R5 (the first week of sampling), TIA was 34.4 TUI/mg
for ‘Asmara’ and 27.2 TUI/mg for ‘Mooncake’. There
were varietal differences in TIA during seed development.
In ‘Asmara’, TIA initially increased with seed develop-
ment to 43.7 TUI/mg at R6 before decreasing, while there
was no change in ‘Mooncake’ during the first 2 weeks.
After that, ‘Mooncake’ TIA significantly increased to 45.8
TUI/mg at week 5. Our results are consistent with pre-
vious reports on changes in TIA with seed development in
grain-type soybean [38–40].
Table 5 Changes in anti-nutritional properties in edamame seeds during development and maturation
Variety Weeks Total phenolics (mg TAE/g
dry sample)
Tannins (mg TAE/g dry
sample)
Phytate (mg/g dry
sample)
Trypsin inhibitor activity (TUI/mg
dry sample)
Asmara 1 (R5) 3.58 ±0.04
c
1.66 ±0.09
c
1.27 ±0.14
f
34.4 ±1.31
bc
2 3.25 ±0.08
cd
1.89 ±0.11
b
4.45 ±0.35
e
38.8 ±2.69
ab
3 (R6) 3.20 ±0.07
d
1.85 ±0.10
b
6.34 ±0.24
d
43.7 ±2.93
a
4 3.78 ±0.15
c
1.94 ±0.12
b
7.54 ±0.51
c
37.0 ±0.67
bc
5 4.13 ±0.20
b
2.05 ±0.19
b
9.38 ±0.27
b
37.4 ±1.09
bc
6 (R8) 4.94 ±0.14
a
2.43 ±0.34
a
11.1 ±0.33
a
32.6 ±1.15
c
Mooncake 1 (R5) 3.30 ±0.13
b
1.52 ±0.16
b
2.13 ±0.24
d
27.2 ±1.49
c
2– – – –
3 3.07 ±0.05
bc
1.56 ±0.08
b
4.05 ±0.74
cd
27.4 ±0.38
c
4(R6) 2.82 ±0.07
c
1.58 ±0.10
b
5.26 ±0.17
c
41.0 ±0.44
b
5 4.05 ±0.09
a
1.95 ±0.10
a
8.35 ±0.40
b
45.8 ±1.31
a
6 (late
R7)
4.00 ±0.05
a
1.94 ±0.03
a
11.6 ±0.51
a
48.4 ±1.50
a
Data are expressed as mean ±standard deviation (n =3)
Means followed by the same letter within a variety indicate no significant (P[0.05) difference between harvest weeks
TAE stands for tannic acid equivalent; –Not applicable
Table 6 Effects of variety, week and their interaction on edamame
seed attributes
Attributes Variety Week Variety 9week
Pr [FPr[FPr[F
Physical properties
Water content 0.0002 \0.0001 \0.0001
Intensity of green color 0.0166 \0.0001 \0.0001
Seed weight 0.9515 \0.0001 \0.0001
Hardness \0.0001 \0.0001 \0.0001
Proximate composition
Protein 0.0755 \0.0001 \0.0001
Lipid \0.0001 \0.0001 \0.0001
Ash 0.0024 \0.0001 \0.0001
Total carbohydrate \0.0001 \0.0001 \0.0001
Sugar compositions
Fructose 0.6464 \0.0001 0.3059
Glucose 0.0161 0.0002 0.0083
Sucrose 0.4729 \0.0001 \0.0001
Raffinose 0.7356 \0.0001 0.0313
Stachyose 0.2871 \0.0001 \0.0001
Anti-nutritional compositions
Total phenolics \0.0001 \0.0001 \0.0001
Tannins \0.0001 \0.0001 0.1541
Phytate \0.0001 \0.0001 \0.0001
Trypsin inhibitor activity 0.087 \0.0001 \0.0001
Significance level P\0.05
Pr probability, FFvalue
198 Y. Xu et al.
123

Effects of variety, week and their interaction
on edamame seed attributes
Harvest week significantly affected all edamame seed
attributes measured. Significant variety and harvest week
interactions were also observed for most attributes except
fructose and tannins indicating that change over time of
measured attributes differed among the two varieties
(Table 6). These results are generally in agreement with
those reported by Saldivar et al. [2] for changes in chemical
composition during soybean seed development.
Conclusion
A clear picture of change in physical, chemical, and anti-
nutritional attributes of edamame at different develop-
mental stages is presented by this study. Results show that
even with slight variation by genotype, vegetable soybean
varieties are quite similar to grain-type soybean. Our
results also confirm the benefits of harvesting edamame at
R6 namely: peak seed weight and sucrose content, lower
values of oligosaccharide and anti-nutrients, and intense
green color. This study provides useful information
regarding physical and nutritional quality of edamame to
breeders, food processors and consumers.
Acknowledgments The authors thank Dr. Anwar Hamama and
Mrs. Naomi Pearson for technical support. This work was conducted
at Virginia State University Agricultural Research Station (Journal
Series number 325).
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