InTables 2 and 3, it is also observ

InTables 2 and 3, it is also observed that the proposed quadratic
models explain the variability of the data to a large extent, with
coefficients of determination,R
2
, being around 0.90 for most of
the output variables except for FFA.
For the optimization of the bleaching process, totox value was
chosen as measured variable to indicate the total oxidation of the
oils, while hue-angle and chroma were selected as output variables
indicating the final color of oil samples.
Firstly, the goodness of the fit for these three variables was
proved.Fig. 1plots the measured values against the predicted ones
for totox (Fig. 1a), hue-angle (Fig. 1b) and chroma (Fig. 1c). The
data are correlated by means of a regression line whose equation
is inserted in each figure. Also shown in each figure are the dotted
lines representing a deviation of ±10% between experimental and
model values.
Secondly, by means of the second-order models obtained above
and employing response surface methodology, contour maps were
generated, where the totox, hue-angle and chroma were plotted
against temperature (C) and time (min), as shown inFig. 2a–c.
Adsorbent concentration was set at the maximum level assayed,
5 wt%. It was due to the fact that, within the experimental range,
higher clay amount resulted in bleached oils of superior quality.
In Fig. 2, it was observed that the hue-angle and chroma followed
a pronounced curved surface. It is due to the significance of clay
percentage quadratic effect in the case of hue-angle and to the significance of temperature and clay amount quadratic effects in the
case of chroma.
By optimizing the quadratic models, optimum values for totox,
hue-angle and chroma were found, marked as circles in the contour plots (Fig. 2). In order to optimize the quality of the bleached
oils in terms of oxidation products, a minimum totox value is required. InFig. 2a, the optimum value for the totox, 21.38, was located at the maximum levels assayed for the three input
parameters, 130C, 60 min and 5 wt%. According to Sathivel
(2010), during the bleaching process, peroxides are broken down
to aldehydes and ketones as a consequence of the high temperatures employed. Subsequently, these secondary oxidation products
are adsorbed onto the activated clay surface. It is therefore reasonable to conclude that the optimum bleaching conditions for totox
reduction were: (a) the maximum temperature evaluated, 130C,
which allowed the highest rate for peroxides decomposition maintaining a proper activation of the adsorbent; (b) the highest clay
amount, 5 wt%, and (c) the highest contact time, 60 min, which
maximized the adsorption of secondary oxidation products.
Additionally, optimizing the color quality in oil samples implies
maximizing hue-angle, obtaining yellowish oils, and to minimize
chroma, reducing the color intensity (Huang and Sathivel, 2010).
InFig. 2b, the maximum value predicted for hue-angle is observed
to be 89.19. This optimum value was found at the highest adsorbent
concentration, 5 wt%, and in the vicinity of the maximum level assayed for the time of 56.6 min. Nevertheless, a temperature of
99.2C was obtained as the optimum temperature for the reduction
in red color.Fig. 2c shows the optimum value predicted for chroma,
81.76, which was within the experimental region: 49.4 min and
109.4C, employing the highest level of clay (5 wt%). Moderate
temperatures, in the range of 99–110C, were required to obtain
an optimum color for the bleached oil. This can be explained by
the fact that working in this range of temperature the following
phenomena may occur: (i) a better activation, in terms of pigments
removal, of the acid-activated earth employed (Tonsil 278), (ii) a
reduction in the oxidation of colorless components which cause
alteration in the oil color, and (iii) a less pronounced fixing of the
existing color pigments (Antoniassi et al., 1998; Crexi et al., 2010).
The results stated above indicate that in order to obtain optimum values for totox, hue-angle and chroma, different process
conditions such as temperature and time of contact are required.
It is observed that temperature increase had a positive effect on
the minimization of totox, obtaining its minimal value at 130C.
On the other hand, moderate temperatures, in the range of 99–
110C, were required to obtain hue-angle and chroma optimal values. Considering time conditions, it is noticed that although each
evaluated optimal point gave a different value, in general, the three
output variables were optimized at long contact time conditions,
close to the experimental upper bound, ranging from 49 to 60 min