Experimental Procedures
Materials
Degummed, neutralized and bleached soybean oils, rapeseed oils, maize
oils and sunflower seed oils were used for this study. Crude oils were
obtained from solvent extracted or screw pressed oil from Wilmar Group,
and the crude oils conformed the internal standards of TFA≤0.10% were
selected. All chemicals used in this study for analyses were of
analytical grade and purchased from Sigma-Aldrich (St. Louis, MO, USA).
Conventional deodorization
Neutralized and bleached oils were deodorized in an industrial
deodorizer soft column, which was designed by Alfa Laval and has been
widely used in industrial vegetable oil refining in recent years. The
incoming bleached oils were firstly deaerated and then heated by heat
exchanger and high-pressure steam to the final operating temperature of
248-250°C
under reduced pressure (<2 mbar). Steam was applied as the
stripping gas (approximately 1% of the oil weight). From the final
heater the oils flowed by gravity into the deodorizer, through the
deodorizer and out to be cooled by heat exchange with incoming bleaching
oil. During flow through the deodorizer, the oil firstly underwent a
continuous stage for fast FFA removal in a structured packing unit for a
short time (5-10 min), and then a long time (90-120 min) holding stage
for heat bleaching and deodorization.
Optimization of deodorization temperature
Deodorization temperatures were preliminarily optimized between
220-240°C in the soft column or tray column, in order to investigate the
relationship between temperatures and TFA generation. Meanwhile
deodorization temperatures were optimized between 225-235°C in a packed
column, to investigate the relationship between temperatures and TFA
generation. Other parameters (deodorization time, vacuum, stripping
steam etc.) were not simultaneously adjusted as variable and remained at
their best according to the actual conditions in each plant, to obtain
deodorized oils with low FFA, light color and few odors.
Dual columns with dual temperatures deodorization
A new DCDT deodorizer was designed, upgraded and implemented in a large
number of plants of Wilmar Group. This new deodorizer contained dual
independent columns (tray column and packed column). Some plants
combined independent tray column and packed column together, and some
others firstly separated the soft column into a packed column and tray
column, and then strung them in order (Fig. 1). During vacuum
deodorization, oil from bleaching was heated by heat exchange with oil
out from packed column to a moderate temperature (205°C for soybean oil
and rapeseed oil, 210°C for maize oil and sunflower seed oil), after
which it first passed through the tray column to undergo heat bleaching
and deodorization for a long time (60-90 min). When the oil came out
from the tray column, it was heated again by high-pressure steam to a
higher temperature (225°C for soybean oil and rapeseed oil, 230°C for
maize oil and sunflower seed oil) and then passed through the packed
column for a short time (5-10 min) to undergo final deacidification and
deodorization. The deodorized oil was then cooled by heat exchange with
bleached oil to below 60°C.
Trans fatty acids analysis
TFA content was quantitatively analyzed by gas chromatography (GC)
according to the national standard of China (GB 5009.257-2016).
Free fatty acids and acid value determination
Free fatty acids contents of oils were calculated as acid value divided
by 1.99 according to the AOCS Official Method 940.28, by titration. Acid
values were determined according to the AOCS Official Methods Cd 3d-63.
Peroxide
values determination
Peroxide values were determined according to the AOCS Official Methods
Cd 8-53.
COLOR analysis
Lovibond color was measured using Lovibond Tintometer (Model F,
Lovibond, England) according to the ISO method 15305:1998. The R value
was calculated as follows: R= (R+Y/10)/2, where R and Y represent the
red unit and yellow unit, respectively.
The
type of cuvette used to determine the color has a path length of 133.4
mm.
Sensory evaluation
Ten trained panelists sniffed deodorized oil samples (150 g), which were
filled in opaque bottles (220 ml) to avoid the influence of the color in
the decision. These samples included the deodorized oils obtained at
different deodorization conditions. Bottles were capped for more than 15
min prior to being tested. After smelling each sample, panelists were
asked to score the samples according to the following four basic options
of scale: “it does not smell”, “it smells, or seems to smell of
refined oil”, “it slightly smells of undesirable odors”, and “it
smells of undesirable odors” (coded with numbers from 1 to 4,
respectively).