21 methods were required to study the trend of changes in total flavonoid
components, such as High Performance Liquid Chromatoraphy HPLC. The predominant flavonoid compounds in soybean are isoflavones which
its retention and distribution were significantly affected by thermal processing Xu and Chang 2008, Xu and Chang 2009. Although the TFC in this study was
relatively constant, the antioxidant capacity from 80 acetone extract elevated during heating at 75, 85, and 95
o
C see Figure 7. These findings indicated that thermal treatments transformed some compounds which had lower antioxidant
status to higher antioxidant power Xu and Chang 2009. The aglycone forms of isoflavones had stronger antioxidant capacity than the glucosides Hayes et al.
1977, Pratt and Stafforini 1979.
75
o
C 85
o
C
95
o
C Figure 10 Force versus time curves of texture profile analysis during thermal
treatments of tempe.
22
Figure 11 Effects of
heating on
hardness, springiness,
stickiness, cohesiveness, and chewiness of tempe at 75, 85, and 95
o
C.
0,00 0,20
0,40 0,60
0,80 1,00
1,20
10 20
30
H a
rd n
ess k
g
Time min
Hardness
75ºC 85ºC
95ºC 0,00
0,20 0,40
0,60 0,80
1,00 1,20
10 20
30
S p
rin g
in ess
Time min
Springiness
75ºC 85ºC
95ºC
0,00 0,20
0,40 0,60
0,80 1,00
1,20
10 20
30
S tick
in ess
Time min
Stickiness
75ºC 85ºC
95ºC 0,00
0,20 0,40
0,60 0,80
1,00 1,20
10 20
30
Co h
esi v
en ess
Time min
Cohesiveness
75ºC 85ºC
95ºC
0,00 0,20
0,40 0,60
0,80 1,00
1,20
10 20
30
Ch ew
in ess
Time min
Chewiness
75ºC 85ºC
95ºC
23
4.3 Effects of Heating on Physical Quality
4.3.1 Textural Profiles
Texture profile analysis TPA basically simulates the mastication process of humans to give force versus time curves as a function of deformation. Before
heating the fresh tempe had textural characteristics as followed: 1.19 kg of hardness, 0.38 of springiness, 0.21 of stickiness, 0.22 of cohesiveness, and 0.14 of
chewiness. Thermal processing effectively impacted on hardness, whereas other parameters, such as springiness, stickiness, cohesiveness, and chewiness, were
remainly constant when subjected to heating for 30 min Figure 11.
The main texture attribute of vegetables is hardness or firmness Bourne 1989 because it has the best correlation to the consumer acceptance Bourne
1982. According to force-time curves see Figure 10, hardness is the maximum force needed to compress the sample. Most published studies have described
thermal degradation of texture based on hardness characteristic and it is possible to model changes based on other properties, but their relation to consumer
perception has not been well established Rizvi and Tong 1997.
The firmness of various bean was strongly correlated with soluble pectin content increasing due to thermal treatment Huang and Bourne 1983.
Intercellular adhesive material, such as pectin, held the firm texture of cell wall structure. When broken by heat, the pectin was depolymerized, so the resistance
of plant tissue to fracturability loss decreased during heating Loh and Breene 1982. In addition, thermal treatments resulted in plasmolysis which reduces the
turgor pressure of cell wall and caused softening texture Rao and Lund 1986.
The presence of salt NaCl in the medium accelerated the solubility of pectin Van Buren 1986. Hence, the more salt dissolved in solution, the texture of tempe
became softer.
4.3.2 Visual Appearance
Appearance of the food product is the first thing influencing on buying ability of consumers. This is characterized by the color which Hunter L a b model
is often used to analyze the color changes. Another color system recommended by CIE is the L C h system Wrolstad and Smith 2010. Figure 12 shows typical
results on the changes of color parameters of tempe surface for 12 min at different temperatures. Visual inspection could not differ color changes of samples, but the
instrumental analysis using Minolta Chromameter was able to detect the color differences from the surface of samples. The color parameters of fresh tempe were
L = 64.00 lightness, a = -0.30 greenness, b = 3.69 yellowness, c = 3.70 chroma, and H = -88.93 hue see Table 3. During thermal processing, the
lightness, yellowness, chroma, and hue value significantly increased, but the greenness tended to decline slightly.
24
Figure 12 Effects of heating on lightness, greenness, yellowness, chroma, and hue of tempe at 75, 85, and 95
o
C.
64,00 64,50
65,00 65,50
66,00
2 4
6 8
10 12
Lig h
tn ess
L
Time min
Lightness
75ºC 85ºC
95ºC -2,00
-1,50 -1,00
-0,50 0,00
2 4
6 8
10 12
G re
en n
ess a
Time min
Greenness
75ºC 85ºC
95ºC
3,00 3,50
4,00 4,50
5,00
2 4
6 8
10 12
Ye ll
o w
n ess
b
Time min
75ºC 85ºC
95ºC 3,00
3,50 4,00
4,50 5,00
2 4
6 8
10 12
Ch ro
m a
C
Time min
75ºC 85ºC
95ºC
-83,00 -82,50
-82,00 -81,50
-81,00
2 4
6 8
10 12
H u
e h
Time min
75ºC 85ºC
95ºC -83,00
-82,50 -82,00
-81,50 -81,00
2 4
6 8
10 12
H u
e h
Time min
75ºC 85ºC
95ºC
25 According to Clydesdale and Ahmed 1978, object-light interactions may
affect on color measurement of samples, such as reflection from the surface, refraction into the object, transmission through the object, diffusion, and
absorption within the object. Heating tempe in salt solution caused the increase of water content entering into the tissue. When the source light from chromameter
came on the tempe surface, it would be resulted increasing intensity of object-light interactions. In this way, increasing water content of tempe produced the more
reflected light from the tempe surface. This might increase the lightness of tempe and change the other color propeties.
The change of chroma and hue parameters corresponded to the intensity of yellowness and greenness. The elevation of hue angle was influenced by the
increase of yellowness and the decrease of greenness intensity. The chroma had almost similar value to yellowness. Hence, the reduction of greenness value did
not significantly affected on the chroma of tempe.
A food system which contain a carbonyl group of reducing sugar and an amine group of free amino acids subjected to high temperature treatment should
experience the Maillard reaction involving the formation of brown pigmen Kim and Lee 2009. It is well known that tempe contains high protein 23-55 and
also some sugars Kwon et al. 2010. When tempe was heated for certain time, the carbonyl group and amine group might interact to form brown color of Maillard
reaction. But, the presence of salt in heating medium could decrease the rate of Maillard Reaction due to the decrease of water activity value BeMiller and Huber
2008. Therefore, it can be assumed that the use of 2 salt solution as heating medium might be effectively retain the visual appearance of tempe.
4.4 Kinetic Modeling of Quality Changes
Kinetic modeling is one of the quantitative modeling used as indicators of food quality degradation. A systematic way is done to stimulate an appropriate
design process, thus the quality retention of product can be managed well Van Boekel 2008. According to Hindra and Baik 2006, the rate of quality change
strongly relates with to two main factors, product composition moisture content, water activity, pH, etc. and environmental factors time, temperature, etc..
Furthermore, kinetic order reaction is determined by the characteristics of chemical reaction in the product. Most heat induced reactions are assumed to obey
first order kinetic which the reaction rate is proportional to the reactant concentration or quality properties Holdsworth and Simpson 2007.
Determination of quality indicators induced by heating was based on a logaritmic relationship describing the quality attributes as a function of variable
process time and temperature obeying first order kinetic reaction. However, in this study not all tested properties could be modeled well as thermal kinetic
parameters because some of them were not affected significantly by process conditions, such as the total flavonoid content, hue, springiness, stickiness,
cohesiveness, and chewiness. The heat sensitive quality parameters of tempe, such as antioxidant capacity, TPC, hardness, lightness, yellowness, greenness, and
chroma, could be modeled well as first-order reaction with different tendency.
