59
Palm Oil
Fractionation of palm oil:
Current status, future possibilities
Ralph E. Timms
The following article is based on an award
address given by the author at the World
Conference and Exhibition on Oilseeds and
Vegetable Oil Utilization: Processing, ByProducts, Biodiesel, Specialty and Functional
Oils, and New Applications & Technologies,
held in Istanbul, Turkey during August
14–16, 2006.The author was the 2006 recipient of the Timothy L. Mounts Award,
which recognizes research accomplishments relating to the science, technology, or
applications of edible oils or derivatives in
food products.
Palm oil is the world’s most produced oil. About half is fractionated to give various oleins (liquid fractions), stearins (highermelting fractions), and mid-fractions (intermediate- or middle-melting fractions) with a wide variety
of melting properties. I shall briefly
consider current fractionation
technology, its limitations, and
how it might be improved in the
future.
Fractionation means fractional crystallization. The process takes place in
stages:
Melt oil completely (with or without an organic solvent).
Cool under controlled conditions
until crystal nuclei form.
Allow the crystals to grow, mature
and agglomerate.
Separate the crystals (stearin) from
the liquid (olein). The proportions of
stearin and olein are determined by the
phase behavior of the system, the temperature of crystallization being the most important factor. The whole process takes
What is the future of the fractionation of palm oil (derived from palm fruit “drupes” and
the world’s most produced oil)?
several hours—up to 24 hours in some
cases.
Although for economic reasons most
palm oil is fractionated without a solvent,
it is worth considering this more fundamentally. Two questions asked at Unilever
in the 1970s were:
Do organic solvents offer better
triglyceride separation efficiency than dry
fractionation, i.e., fractionation without an
added solvent?
Which is the best solvent?
At the time, Loders Croklaan in the
United Kingdom was using acetone,
Aarhus in Denmark was using hexane, and
Glidden Durkee in the United States was
using 2-nitropropane. We developed a theoretical approach to answer these questions, which I shall explain briefly.
Triglycerides do not form Ideal Solutions with organic solvents, so we applied
Regular Solution theory and the concept of
information
2
Energy of mixing = ∆Hm = Vm (δ1 – δ2) ϕ1ϕ2
where:
Vm = mean molar volume of solution
δ1, δ2 = solubility parameters of each
component
ϕ1, ϕ2 = volume fractions of the two
components
The energy of mixing is thus proportional
to the square of the difference between the
solubility parameters of triglyceride and solvent. Solubility Parameters are widely used in
the chemical industry and are available from
tables. We also added an entropy of mixing
term (∆Sm > 0), because triglycerides and common organic solvents are very different in size.
60
January 2007, Vol. 18 (1)
TABLE 1.
Application of Regular Solution Theory
δ
Tripalmitin
Acetone
Hexane
Acetone & Hexane (50:50)
a
8.3
9.9
7.3
8.9
(δ1 – δ2)2
—
2.6
1.0
0.4
a δ, solubility parameter.
FIG. 1. Solubility of PPP (tripalmitin) in acetone and hexane.
Solubility Parameters. Unlike an Ideal Solution, a Regular Solution is formed with absorption of heat (∆Hm > 0), but with the
same entropy of mixing as an Ideal Solution. (See information box
for relationship).
An application of the theory is shown in Table 1. The observed solubility of tripalmitin in the solvents is in the order predicted mathematically. We have the saying “like dissolves like,”
and if we say that like means having the same solubility parame-
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ter, we can see that this is true. The effect of mixing solvents can
be seen in more detail in Figure 1. The observed and predicted results agree well. There is a clear increase in solubility as acetone
is added to hexane, even though the solubility in acetone is much
lower than in hexane.
In Table 2 are shown solubilities of triglycerides in acetone
and hexane. For ease of comparison, solubilities in acetone are set
equal to 1. The relative solubilities of tripalmitin (PPP) and oleodipalmitin (POP) are almost the same in acetone and in hexane.
Thus for fractionating palm oil, where we separate triglycerides
such as PPP, POP, and POO, the solvent makes almost no difference; but for tripalmitin and trilaurin (LLL), where the molecular
sizes are very different, there is a clear difference in solubilities
between acetone and hexane.
In Table 3 are shown solubilities of a triglyceride and diglycerides in acetone and hexane. For ease of comparison, solubilities
of POP are set equal to 1. The relative solubilities of the diglycerides are much greater in acetone than in hexane. Furthermore,
in hexane the diglycerides are less soluble than POP, so they
would be crystallized with the less soluble triglycerides such as
PPP in the stearin.
For the final decision on which solvent is the best to use, we
need to consider other things, such as absolute solubilities and the
mutual solubility of triglycerides. Nevertheless, we can draw
some useful conclusions:
For palm oil triglycerides, all solvents show similar selectivity.
Hexane is the preferred solvent when the aim is to produce a good olein.
Acetone is the preferred solvent when the aim is to produce a POP-rich mid-fraction.
Where no organic solvent is used, diglyceride separation efficiency is intermediate between that for acetone and hexane, but
tending to be more like acetone, i.e., concentrating the majority of
the diglycerides in the olein. Thus, overall we can conclude that
for palm oil, dry fractionation is almost as effective in separating
the desired glycerides as acetone or hexane.
To complete the fractionation process, the desired solid
triglycerides in the crystals need to be separated from the liquid
triglycerides. These liquid triglycerides are distributed in three locations:
in solid solution with the solid triglycerides;
in the uncrystallized bulk oil;
in the uncrystallized oil that is physically trapped or entrained in the crystals.
The extent and type of solid solutions formed depend primarily on the fundamental phase behavior of the fat being crystal-
Palm Oil
61
TABLE 2.
Relative Solubilities of Triglycerides
TABLE 3.
Relative Solubilities of Tri- and Diglycerides
Glyceride a
Glyceride a
Acetone
Hexane
1
1
1
35
32
5
PPP
POP
LLL
POP
P(OH)O
P(OH)P
a
a
Acetone
1
14
0.2
Hexane
1
0.6
0.007
For abbreviations, see Table 2.
PPP, tripalmitin; POP, palmitoyl-oleoyl-palmitoyl trigylceride; LLL, trilaurin.
lized, although high degrees of supercooling tend to increase the
extent of formation of solid solutions. However achieved, once
the solid solution has formed, the separation step can do nothing
to change the composition of the solid phase. The uncrystallized
bulk oil is relatively easily removed from the crystals, although
some liquid oil always remains as a surface layer. The entrained
oil is more difficult to remove. The separation step must address
the problem of reducing the level of entrained oil in the final solid
fraction. Three methods have been developed to achieve this:
Centrifugation, either using the LipofracTM or Lanza
process with a detergent solution to wet the crystals in an aqueous
phase or using Nozzle Centrifuges;
Vacuum Filtration using drum or belt filters; and
Pressing using vertical hydraulic presses and filter cloths
or automatic membrane presses.
Automatic membrane filter presses are now the preferred
choice for new dry fractionation plants.
In Table 4, I compare acetone fractionation, as an example
of the best possible separation, with the best achievable with dry
fractionation using membrane presses with high pressure and narrow chamber width. For dry fractionation, the latest membrane
presses have achieved a significant lowering in the level of entrainment, from over 60% in earlier plants to 30%, but there is still
a long way to go to match the almost zero entrainment of solvent
fractionation. With the present type of membrane press it is difficult to see how the entrainment level could be lowered much further.
So what might be possible in the future to improve the fractionation process further? I believe there are three possibilities
worth considering:
Countercurrent crystallization;
Improved separation using new technologies;
Reconsider solvents.
Counter-current crystallization is commonly applied in the
chemical industry, but has not been used for oils and fats because
of their slow crystallization and poor solid-liquid phase separation. However, with the improved separation efficiencies now
available the process was reconsidered by Unilever researchers.
Using a two-stage counter-current process, for the same stearin
yield they were able to improve the ratio of trisaturated (SSS) to
disaturated (S2O) triglycerides in the stearin from 1.35 to 1.80
compared with a conventional two-stage series process, a significant improvement in separation efficiency. SSS is particularly desirable for use in interesterified hardstocks for zero-trans blend
formulations.
Future developments to improve
the process still further should consider
countercurrent crystallization and
fundamentally new separation
techniques, and should reconsider
the use of solvents.
The benefits of reducing the chamber width and increasing
the pressure in presses are clear, but conventional presses probably cannot achieve much better results. One way to achieve higher pressures and smaller chamber widths could be to press
TABLE 4.
Comparison of Solvent Crystallization with the Best Dry Fractionation Technology
Year
Company
Process
mid 1960s
Unilever
2000
Desmet
Continuous Acetone Tube Crystallizer
+ Belt Filter
+ washing with pure solvent
Batch Dry Crystallizer
+ Membrane Filter Press (25 mm
chamber width & 30 bar
squeezing pressure)
Yield
(%)
IV of
Stearin
Entrained oil
(% of stearin)
10–11
~8
~5
~17
~32
~30
62
information
Further reading
Fractionation—Current Status and Future Prospects in a
Low-Trans World, SCI Oils & Fats Group meeting, Ghent, Belgium 22–23 November 2005. Netlink: http://tinyurl.com/
yxpp76.
van den Kommer, M., and C.N.M., Keulemans, Developments in Dry Fractionation of Fats, symposium Fractional Crystallization, London, March 9, 1994, and other papers from
this symposium (Netlink: http://tinyurl.com/y56lkb).
Timms, R.E., Fractional Crystallization—The Fat
Modification Process for the 21st Century, European Journal
of Lipid Science and Technology 107, 48–57, 2005.
between rollers in an ever-reducing thickness, as in milling operations. Pressures greater than 100 bar with “chamber” widths of
1 mm could then be achieved. A pulsing pressure might allow the
crystals to relax and more liquid to exude, just as, when one
squeezes a sponge, it is always best to squeeze and then relax
the pressure before squeezing again in order to get out the most
liquid.
January 2007, Vol. 18 (1)
Finally, should we reconsider solvents? Costs are high because of their flammability and the need to redistill for re-use. The
great advantage of solvent is the ability to wash the crystals. To
limit costs, perhaps solvent could be used just to wash the crystals
after dry fractionation has done its best? Supercritical CO2 has
been investigated in the laboratory, but does it have commercial
potential? Methyl esters are now readily available as biodiesel
and could easily be removed in the deodorizer. Perhaps there are
still some flammable solvents that have not been fully evaluated?
We have seen that dry fractionation is capable of similar
triglyceride separation efficiency as solvent fractionation, but
that the practical process of dry fractionation is limited by the
problem of separating the solid and liquid phases. Future developments to improve the process still further should consider countercurrent crystallization and fundamentally new separation techniques, and should reconsider the use of solvents. Fractionation
has come a long way in the last 50 years, but the process is still
capable of further improvement.
Along with three colleagues, Ralph Timms founded Britannia Food
Ingredients, which is now a major European supplier of cocoa butter, cocoa butter equivalents, and other speciality fats. He can be
reached via e-mail at [email protected].
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