17 isoflavones, but significantly decreased the content of the malonylglucoside forms
malonyldaidzin, malonylglycitin, and malonylgenistin Xu and Chang 2008. Therefore, this research indicated that isoflavones from phenolic group obtained
from 80 acetone extract were the responsible components affecting the changes of antioxidant capacity of tempe during heating.
On the other hand, the decline of antioxidant compounds in tempe was due to the increase of water soluble antioxidant subtances to 2 of salt solution.
Thermal process might decrease firmness and adhesion of cell walls Van Buren 1979, thus it induced releasing bound phenolic compounds, such as flavonoids,
accumulated in the vacuoles Brecht et al. 2008. Briefly, as longer heating time and higher temperature, the bound antioxidant components were more liberated
from cell and leached into salt solution.
Figure 7 Effects of heating on DPPH scavenging capacity of tempe at 75, 85, and 95
o
C.
0,00 1,00
2,00 3,00
4,00
30 60
90 120
DP P
H sca
v en
g in
g ca
p a
ci ty
m M
AA Eg
Time min
Tempe
75ºC 85ºC
95ºC 0,00
1,00 2,00
3,00 4,00
30 60
90 120
DPPH sc a
v en
g in
g c
a p
a city
m M
o f
AA Eg
Time min
Salt Solution
75ºC 85ºC
95ºC
0,00 1,00
2,00 3,00
4,00
30 60
90 120
DPPH sc a
v en
g in
g c
a p
a city
m M
AA Eg
Time min
Total of Tempe and Salt Solution
75ºC 85ºC
95ºC
18
4.2.2 Total Phenolic Content
The total phenolic contents TPC were measured using the Folin- Ciocalteu assay, which was based on the transfer of electrons from phenolic
compounds to the Folin-Ciocalteu reagent in alkaline medium forming blue complexes that can be detected spectrophotometrically at 750
–765 nm Singleton et al. 1999, Cai et al. 2004. As can be seen from Table 3, the fresh tempe
contained phenolic compounds of 1.45 mg of GAEg. When it was subjected to the thermal processing, the TPC changed with the same trend as antioxidant
capacity Figure 8.
Figure 8 Effects of heating on total phenolic content of tempe at 75, 85, and 95
o
C. The increase of TPC occured in salt solution, whereas TPC of tempe
decreased during heating period of 120 min indicating that heat treatments caused water-soluble phenolic compounds leaching into heating medium. Totally heating
0,00 1,00
2,00 3,00
4,00
30 60
90 120
To ta
l p
h en
o li
c co
n te
n t
m g
of G
AEg
Time min
Tempe
75ºC 85ºC
95ºC 0,00
1,00 2,00
3,00 4,00
30 60
90 120
To ta
l p
h en
o li
c co
n te
n t
m g
o f
G AEg
Time min
Salt Solution
75ºC 85ºC
95ºC
0,00 1,00
2,00 3,00
4,00
30 60
90 120
To ta
l p
h en
o li
c co
n te
n t
m g
of G
AEg
Time min
Total of Tempe and Salt Solution
75ºC 85ºC
95ºC
19 of samples increased the total amount of TPC in tempe and salt solution. Heating
of tempe might cause the degradation of polyphenols and release of bound phenolic compositions from the vacuoles Brecht et al. 2008. According to
correlation analysis see Table 4, there were significant correlations between TPC and DPPH scavenging capacity p 0.05 at 75, 85, and 95
o
C in all components of samples. It can be assumed that phenolic compounds strongly influenced the
antioxidant capacity of tempe. The changes of phenolic content in tempe was affected by composition of
phenolic acids. Raw and boiled yellow soybean contained free phenolic acids both benzoic type, such as gallic acid, 2,3,4-trihydroxybenzoic acid, vanillic acid and
protocatechualdehyde, and cinnamic type, such as chlorogenic, sinapic, and trans- cinnamic acid. Moreover, conjugated phenolic acids were also detected in both
raw and boiled yellow soybean, such as benzoic type gallic, protocatechuic, 2,3,4-rihydroxybenzoic, p-hydroxybenzoic, gentistic, syringic, vanillic acid,
protocatechualdehyde and vanillin and cinnamic type caffeic, p-coumaric, m- coumaric, o-coumaric, sinapic and trans-cinnamic acid Xu and Chang 2008.
Thermal processing caused complex variations in phenolic acid profiles of soy products. For instance, boiling soymilk caused significant increases in free
gallic, protocatechuic, 2,3,4-trihydroxybenzoic, sinapic acid, and subtotal benzoic acids Xu and Chang 2009, whereas boiling treatment of yellow soybean
significantly increased 2,3,4-trihydroxybenzoic acid Xu and Chang 2008. In addition, pressure steaming of yellow soybeans caused the increase of benzoic
acid about 23 and also the TPC value about 35. The elevated total phenolic values were correlated to the release of phenolic substances with reactivity toward
Folin-Ciocalteu reagent from polymerized structural substances in cell walls upon thermal processing Xu and Chang 2008.
Table 4 Correlations between antioxidant capacity and total phenolic flavonoid content.
Correlation Coefficient
a
r TPC
TFC 75
o
C 85
o
C 95
o
C 75
o
C 85
o
C 95
o
C DPPH
scavenging capacity
Tempe 0.77 0.89 0.68 0.09 0.83 0.51
Salt solution 0.81 0.69 0.89 0.16
0.32 0.04
Total of tempe and salt solution
0.81 0.49 0.79 0.10 0.21
0.05
a
correlation is significant at the 0.05 level.
4.2.3 Total Flavonoid Content
The total flavonoid contents TFC in tempe was estimated by colorimetric assay involving AlIII-flavonoid complexes formed in solution Deng and Van
Berkel 1998. The results of spectrophotometric analysis showed that before heating the flavonoid compounds of tempe was 0.77 mg of CAEg see Table 3.
According to Figure 9, the TFC was relatively constant during thermal treatments at three different temperatures 75, 85, and 95
o
C. Similar heat effects were found
20 in heating of tomatoes at 88
o
C for 2, 15, and 30 min, which resulted no significant changes in the total flavonoid content Dewanto et al. 2002. The results of
correlation analysis see Table 4 indicated that no significant correlations existed between total flavonoid content and DPPH scavenging capacity p 0.05 except
tempe at 85 and 95
o
C.
Figure 9 Effects of heating on total flavonoid content of tempe at 75, 85 and 95
o
C. In the other hand, Xu and Chang 2009 have studied that traditional stove
cooking increased by 20-23 TFC, steam injection cooking increased by 28-65 TFC, direct Ultra High Temperature UHT processing increased by 48-90 TFC,
and indirect UHT processing increased by 74-113 TFC. The effects of heating at different temperature on changes of TFC in this research can not be observed
distinctly. The spectrophotometric method used were less sensitive to analyze the changes of TFC during thermal treatments. The flavonoid compounds were poorly
occuring in yellow soybean Xu and Chang 2007, thus the more sensitive
0,00 0,30
0,60 0,90
1,20 1,50
30 60
90 120
To ta
l fla
v o
n o
id c
o n
te n
t
m g
o f
C A
Eg
Time min
Tempe
75ºC 85ºC
95ºC 0,00
0,30 0,60
0,90 1,20
1,50
30 60
90 120
To ta
l fla
v o
n o
id c
o n
te n
t
m g
o f
C A
Eg
Time min
Salt Solution
75ºC 85ºC
95ºC
0,00 0,30
0,60 0,90
1,20 1,50
30 60
90 120
To ta
l fla
v o
n o
id c
o n
te n
t
m g
of C
A Eg
Time min
Total of Tempe and Salt Solution
75ºC 85ºC
95ºC
