2.6.2. Hydroperoxide index
Lipid hydroperoxides in soymilk were determined by using the method described by Ostdal, Andersen, and Nielsen (2000) with some modifications. The reaction consists of the oxidation of ferrous to ferric ion by hydroperoxides in the presence of ammonium thiocyanate to produce ferric thiocyanate whose absorbance can be measured at 500 nm. Soymilk (0.4 mL) was mixed with water (1.6 mL). Then 2 mL of methanol were added and stirred. Chloroform (4 mL) was added and vortexed for 30 s approximately. After centrifugation for 10 min at 12,000g, 1 mL of the chloroform phase was transferred to a test tube and mixed with 1 mL of Fe (II)/ thiocyanate in methanol/chloroform. The mixture was allowed to react for 5 min at room temperature before the absorbance at
500 nm was read. Soymilk samples were analyzed in triplicate, and
data were expressed as meq peroxide/L of sample as described by
Hornero-Méndez, Pérez-Gálvez, and Mínguez-Mosquera (2001).
The calibration curve was made according with methodology
described by FIL- IDF (1991). Measure is based on spectrophotometric reading at 500 nm of a series of dilutions that contain Fe3þ in
chloroform/methanol (70:30). Analysis was carried out 10 h after
treatment and after 15 days of storage at 4 C.