Question
A company producing olive oil has occasionally a lot that is unfit for normal use
owing to off-flavors. Purification processes remove the off-flavors but also the highly
desirable natural flavor of the oil. The company wants to find another outlet for
these lots and considers that the very special composition of olive oil may lead to a
useful solid fat by complete hydrogenation. The company’s laboratory is asked to do
some trials and they make blend of 20%fully hydrogenated and 80%unmodified oil,
to study crystallization behavior. Suppose that you have to do experiments in which
the aim is to obtain (a) very small crystals, (b) rapid crystallization, or (c) fully stable
and largish crystals. What temperature regime would you apply in each of these
cases? The fatty acid composition of the oil is 70%O, 15%Li, 10%P, and 5%others,
of which about half is C18.
Answer
The triglycerides obtained after hydrogenation will be for the most part SSS, the rest
being largely made up of S and P. This means that the composition of the fat is quite
similar to that of the solid fraction in the system discussed with reference to Figure
15.22. Consequently, we can directly read off the clear points for thea- andb^0 -forms
from that figure atc¼0.2 (mass and molar fractions will be virtually identical for the
system studied); the values are 30 and 50 8 C, respectively. For theb-form, we may
assume that the clear point is given by the highest melting component, i.e., SSS.
Assuming that the distribution of fatty acid residues over the triglycerides is random,
SSS will make up about 0.87^3 ¼0.66 of the hydrogenated fat; hence, its molar
fraction in the total mixture is about 0.13. Applying the Hildebrand equation (15.8)
and the data for SSS in Table 15.3, we obtain for thebclear point 63 8 C. The
questions asked can now roughly be answered. (a) To obtain small crystals,
nucleation has to be fast, which needs undercooling to well below theaclear point.
For instance, mixing one part of the melted hydrogenated fat, temperature, e.g.,
758 C, with 4 parts oil of 10 8 C yields a temperature of 23 8 C, i.e., 7 degrees below thea
clear point. When the mixture then is continuously stirred, many small crystals will
result. (b) To assure fast crystallization, the specific surface area of the crystals has to
be large (hence many small crystals), and the supersaturation has to be high. Initial
cooling thus can be as mentioned under (a). However, owing to crystallization, the
supersaturation will decrease; this is aggravated by the temperature rise caused by
the release of the heat of fusion. Assuming the latter to be about 170 kg?kJ^1 , and
taking into account that the specific heat of oil equals about 2.1 kJ?kg^1 ?K^1 (Table
9.2), the heat release will lead to a temperature increase by 0.2 6 170/2.1¼16 K. It
will thus be desirable to cool the crystallizing mixture. (c) To ensure stable crystals,
the crystallizing temperature should be above theb^0 and below thebclear point, i.e.,
between 50 and 63 8 C. At such a temperature, nucleation will be extremely slow, and
it would thus be necessary briefly to precool the system to a lower temperature (to be
experimentally estimated).