4C under nitrogen atmosphere. Acid

4C under nitrogen atmosphere. Acid-activated clay, Tonsil 278,
was utilized in the bleaching procedure. Acid-activated bleaching
clays are preferably employed than natural bleaching earths due
to their demonstrated better efficiency during the adsorption process (Rossi et al., 2003; Sathivel, 2010). The bleaching clay was
kindly provided by Süd-Chemie España, S.L. (Toledo, Spain).
2.2. Bleaching procedure
DNSO was subjected to bleaching under partial vacuum
(70 mmHg vacuum pressure). Sixty grams of DNSO were introduced with a given clay amount into a three-neck round-bottom
flask. The samples were stirred at 300 rpm and kept at the desired
temperature using an oil bath controlled by a thermo-regulator.
The three experimental factors considered for this study were temperature, clay amount and contact time. A factorial experimental
design comprising 27 runs was executed, in which each input
variable was set at three levels: 90, 110 and 130C for temperature; 1, 3 and 5 wt% for clay amount; and 20, 40 and 60 min for
contact time. After the contact time was completed for each run,
the adsorbent was removed from the oil by centrifugation. The
oil samples were stored under nitrogen at 4C in 50 mL Falcon
tubes covered with aluminum foil until analysis.
2.3. Determination of oxidation parameters
2.3.1. Free fatty acids
The free fatty acids (FFA) content of the oil samples was determined according to the standard ISO 660:2009 (ISO, 2009). This
method is based on the titration of the oil, suitably diluted with
an ethanol-ethyl ether mixture, with a potassium hydroxide solution employing phenolphthalein as indicator. Results are expressed
as percentage of oleic acid.
2.3.2. Peroxide, p-anisidine and totox values
The peroxide value (PV) of the oil samples was determined
according to the standard ISO 3960:2007 (ISO, 2007). The PV method is based on the titration with a sodium thiosulfate solution of
the oil diluted with an acetic acid-isooctane mixture and then treated with potassium iodide. Results are expressed as miliequivalents per kg of oil.
The anisidine value (AV) of the oil samples was determined
according to the standard ISO 6885:2006 (ISO, 2006). The AV method is based on the reaction ofp-anisidine diluted in acetic acid with
thea- andb-unsaturated aldehydes (primary 2-alkenals) present
in the oil. Results are expressed as 100 times the increment of
absorbance produced by this reaction, measured at 350 nm.
Totox is a comprehensive oxidation index calculated from a
weighted sum of peroxide value (PV) andp-anisidine value (AV)
by applying the following equation:
Totox¼2PVþAV ð1Þ
2.3.3. Oxidative stability
The Rancimat test was employed to determine the oxidative
stability of the oil samples. A Metrohm Rancimat model 743 (Methrom Instruments, Herisau, Switzerland) was utilized. A stream of
filtered, cleaned and dried air at a rate of 20 L/h was bubbled into
3 g of oil samples contained in reaction vessels. These vessels were
placed in an electric heating block which was set at a temperature
of 100C. Effluent air containing volatile organic acids from the oil
samples were collected in a measuring vessel with 60 mL of distilled water. The conductivity of the water was continuously recorded and the induction period (IP) was automatically
determined by the apparatus. Rancimat induction period was expressed as resistance time (in hours) of the oil to oxidation