5 In addition to being a source of proteins, tempe has several interesting
functional properties from its bioactive compounds. Soy isoflavones are recognized for their benefits to human health, reducing menopause symptoms and
lowering the risk of chronic diseases, such as cardiovascular ones, osteoporosis, and breast, prostate and colon cancer
Matsuo et al. 1997, Alekel et al. 2000, Badger et al. 2005, Lee et al. 2005, Ma et al. 2008, Dong and Qin 2011. Tempe also
contains g
amma amino butyric acid GABA which has antihipertensive effects Aoki et al. 2003,
Tsai et al. 2006
.
2.3 Antioxidant in Tempe
An antioxidant is any substance that significantly delays or inhibits the oxidation of an oxidizable substrate Halliwell and Gutteridge 1989, Sies 1993,
Halliwell 1995. Antioxidant compounds can scavenge free radicals and increase shelf life by retarding the process of lipid peroxidation, which is one of the major
reasons for deterioration of food products during processing and storage Halliwell 1997. Antioxidants protect the human body from free radicals and
ROS effects that can retard the progress of many chronic diseases as well as lipid peroxidation Gülçin 2012.
Natural antioxidants are found in almost all plants, microorganisms, fungi, and animal tissues Pokorny 1999. Phenolic compounds are secondary plant
metabolites that are thought to be an integral part of both human and animal diets Gülçin 2006. The predominant of natural antioxidants are phenolic compounds
such as tocopherols, flavonoids, phenolic acids, and lignans, as well as carotenoids and ascorbic acid Ames et al. 1993, Chen et al. 1996, Gülçin 2012.
Nitrogeneous compounds such as oligopeptides, proteins, and melanoidins also have antioxidative activity Chen et al. 1996.
According to Hoppe et al. 1997, tempe is reported to be antioxygenic. Fermentation of tempe causes increased antioxidative capacity Chang et al. 2009.
The antioxidant components of soybean flour have been shown to be isoflavones, phospholipids, tocopherols, amino acids, and peptides
Gyӧrgy et al. 1964, Naim et al. 1974, Naim et al. 1976, Hayes et al. 1977, Murakami et al. 1984. The
predominant isoflavones in tempe were genistein 5,7,4 ’-trihydroxyisoflavone
and daidzein 7,4 ’-dihydroxyisoflavone, produced by digesting genistin and
daidzin with β-glucosidase during fermentation Murakami et al. 1984.
Isoflavones contributed only one-thirds of the antioxidative capacity in tempe and the other two-thirds were contributed by the peptides derived from proteolyzing
soy proteins Sheih et al. 2000. Esaki et al. 1996 found 3-hydroxyanthranilic acid HAA as another antioxidative substance formed in tempe.
2.4 Antioxidant, Phenolics, and Flavonoids in Food
The terms of antioxidant activity and antioxidant capacity have different meanings. Activity refers to the rate constant of a reaction between a specific
antioxidant and a specific oxidant. Capacity is a measure of the amount as a mole of a given free radical scavenged by a sample MacDonald-Wicks et al.
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,
