6 2006. Quantification of antioxidant capacity in the foods can be determined by
DPPH 2,2-diphenyl-1-picrylhydrazyl scavenging capacity assay. This method is based on the reduction of DPPH in alcoholic solution in the presence of a
hydrogen-donating antioxidant due to the formation of the non-radical form DPPH-H in the reaction. The antioxidants were able to reduce the stable free
radical DPPH to the yellow coloured diphenyl-picrylhydrazine showing a maximum absorbance at 517 nm Gülçin 2012. When DPPH radicals encounter a
proton-donor substrate such as an antioxidant, the radicals would be scavenged and the absorbance is reduced Blois 1958, Gülçin et al. 2009.
Plant phenolics have been one of the major groups of primary antioxidant compounds acting as reducing agents, hydrogen atom donators, singlet oxygen
scavengers, and transition metal ion chelators Rice-Evans et al. 1996, Karaman et al. 2009, Gülçin 2012. The FCR Folin-Ciocalteu Reducing assay has been used
to measure a total phenolics in the products Prior et al. 2005, relying on the transfer of electrons from phenolic compounds to the FCR in alkaline medium
forming blue complexes that can be detected spectrophotometrically at 750
–765 nm Singleton and Rossi 1965, Singleton et al. 1999, Cai et al. 2004, Song et al.
2010. FCR or Folin’s phenol reagent is a mixture of phosphomolybdate and phosphotungstate used for the colorimetric assay of phenolic and polyphenolic
antioxidants Singleton et al. 1999.
Flavonoids are naturally phenolic antioxidants occuring in living cells as glycosides and may break down to their respective aglycone and sugar by
enzymes or acid-heat treatments Shahidi et al.1992, Yao et al. 2004. Antioxidant mechanisms of flavonoids were divided into free radical chain breaking, metal
chelating, and singlet oxygen quenching, with the inhibition of enzymatic activity Clifford and Cuppett 2000. The total flavonoid contents TFC was estimated by
colorimetric assay involving AlIII-flavonoid complexes formed in solution Deng and Van Berkel 1998.
2.5 Effects of Heat on Quality Attributes
Term quality is refered to the composite of the characteristics that differentiate individual units of a product and have significance in determining the
consumer acceptability Kramer and Twigg 1966, such as color, texture, nutritional
value, and sensory attributes. Quality loss involves both subjective factors like taste that cannot be readily quantified, and quantifiable factors such as nutrient
degradation Awuah et al. 2007. Despite destroying microorganisms effectively, thermal processing can induce permanent changes to the nutritional and sensory
attributes of foods depending on the process severity.
External apperances, both color and texture, are the most important factors that have to be considered before accepting or rejecting a food product. Some
researchers studied the effect of thermal processing on the color and texture of food products: peas Smout et al. 2003, Garrote et al. 2008, faba beans Revilla
and Vivar-Quintana 2008, carrot Rastogi et al. 2008, Peng et al. 2014, potatoes Alvarez and Canet 2002, Moyano et al. 2007, Rattan and Ramaswamy 2014,
green asparagus Lau et al. 2000, red chili puree and paste Ahmed et al. 2002,
7 soybean Gandhi and Bourne 1991, Xu and Chang 2008
b
, tempe Handoyo and Morita 2006.
Thermal softening can be due to changes in cell wall matrix polysaccharides celluloses, hemicelluloses, pectins etc. which depend on many
factors such as pH, types and amounts of various salts present in the plant cell wall. Water uptake by polysaccharides results in reduction of cohesiveness of cell
wall matrix thereby resulting in lower adhesion. Other reason for softening is due to loss of turgor pressure which is pressure of cell components against the cell
wall and it is due to the water content inside the cell Lund 1982. The thermal treatments can result in plasmolysis which reduces the turgor pressure and it is
responsible for softening of food Rao and Lund 1986.
Thermal processing influences the nutritional value of some food products by changing the total of antioxidant capacity. Boiled and steamed eclipse black
beans exhibited significantly lower antioxidant activities than raw beans in total phenolic content TPC, DPPH free radical scavenging activity DPPH, and
oxygen radical absorbing capacity ORAC Xu and Chang 2008
a
. As compared to the raw soybeans, all processing methods caused significant decreases in TPC,
DPPH, ORAC, total flavonoid content TFC, condensed tannin content CTC, monomeric anthocyanin content MAC, and ferric reducing antioxidant power
FRAP, and in black soybeans Xu and Chang 2008
b
. In the other hand, thermal processing elevated total antioxidant activity
and bioaccessible lycopene content in tomatoes and produced no significant changes in TPC and TFC. The increase in total antioxidant activity of the heat-
processed tomatoes was due to the increased amount of lycopene as a major phytochemical in tomatoes and other bound phytochemicals released from the cell
matrix Dewanto et al. 2002. However, high pressure processed tomato and carrot purées had significantly higher antioxidant capacities when compared to
thermally treated samples Patras et al. 2009.
2.6 Kinetics of Quality Degradation
Some of quality degradation reaction in food product can be described well by first order kinetic reaction. This model was characterized by a straight line
when the logarithm of the texture property was plotted against heating time Rizvi and Tong 1997.
Generally, thermal softening in fruits and vegetables was suited to the first order rate with the firmness as the primary texture attribute Rao and Lund
1986, Bourne 1987,
Rizvi and Tong 1997
.
The heat-induced degradation of natural pigments and browning reaction also followed the first order reaction
Villota and Hawkes 2007. The equations for a degradation reaction for first order are Van Boekel
2008: -
dC dt
=kC
-ln C
C =kt
