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6.7
Soybean Oil Composition for Biodiesel
Neal A. Bringe
Soybean Oil Composition
The composition of soybean oil can be modified to improve the usefulness of soy-
beans for food and fuel applications. Molecular marker, traditional breeding, and
transgenic technologies enable seed companies to incorporate modified oil traits
into high-yielding germplasm. It takes several years to deliver a modified oil com-
position to the marketplace; thus, it is prudent to select the right targets in the early
stages of development.
Benefits sought by the biodiesel industry are improved oxidative stability and
improved cold flow properties. These two properties are linked. In some situations,
neat biodiesel has to be heated to ensure flow. The warm temperatures increase the
rate of fatty acid oxidation. Thus, improvements in cold flow can reduce the stabil-
ity target required to meet commercial needs.
The key fatty acids limiting the cold flow quality of biodiesel are palmitic
(16:0) and stearic acids (18:0) as illustrated by the melting point of the fatty acid
methyl esters (FAME) (Table A-1 in Appendix A). Polyunsaturated fatty acids
(PUFA) improve cold flow properties but are most susceptible to oxidation. Thus
one has to identify an optimum level of PUFA. Food processors’ demand for
PUFA must be considered. If a large segment of the food processing industry
rejects the oil, the costs of segregating the grain will prohibit the practical use of
the extracted oil for fuel purposes. Linoleic acid is a primary source of fried food
flavor compounds such as 2,4 decadienal (1), and oleic acid is a source of fruity,
waxy, and plastic tasting odors such as 2-decenal (2,3). The proportions of these
fatty acids have to be selected to balance flavor and shelf-life objectives. Good
potato chip flavor was obtained with an oil having 68% oleic and 20% linoleic
acid (4). PUFA are essential in the diet and play a role in cardiovascular health
particularly when they replace saturated fat (5,6). Thus, we hypothesized an opti-
mized soybean oil composition for food and fuel use that retains ~24% PUFA
(Table 1).
A synthesized biodiesel with the targeted composition was prepared from
mixtures of pure FAME (>99% purity; modification from Table 1: 18:1 was
73.3% and the “other” category was 0%). The cold flow properties of the oil were
compared with controls (Table 2). It was apparent from these data that the cold
flow properties of the target biodiesel composition can be comparable to or better
than those of petroleum diesel. Additional data from other biodiesel compositions
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TABLE 1
Compositions of Typical Soybean Oil (Control), a Modified Composition, and a Target
Soybean Oil Composition
Control (%) USDAa line (%) Target (%)
18:1 21.8 31.5 71.3
18:2 53.1 52.7 21.4
18:3 8.0 4.5 2.2
16:0 11.8 5.2 2.1
18:0 4.6 4.1 1.0
Other 0.7 2.0 2.0
aUnited States Department of Agriculture.
were added to the data from this study, and an exponential relation was found
between the saturated fat content of biodiesel and cloud point (Fig. 1). The effect
of lowering the saturated fat level of biodiesel from 15 to 10% on cloud point was
relatively minor compared with a change to a 3.5% saturated fat content.
The ignition quality of the synthetic biodiesel was tested using an Ignition
Quality Tester (IQT) (see Chapter 6.1). The derived cetane number (CN) was
55.43 ± 0.4, the same as pure oleate methyl esters (10). The good CN of the syn-
thetic biodiesel was attributed to the high-oleic fatty acid content (73%) of the
biodiesel. When soybeans (United States Department of Agriculture line) were
used to make biodiesel that had reduced saturated fat without large increases in
oleic acid, the average derived CN of the biodiesel was 46.5 using the IQT. PUFA
have low CN and do not compensate well for a reduction in saturated fatty acids in
a new fuel composition. Palmitate and stearate had CN of ~75, whereas that for
methyl linolenate was 33 (10). A CN of 46.5, if repeatable in engine tests, could be
problematic, given the minimum CN of 47 in biodiesel specification D6751.
Biodiesel with low polyunsaturated fat levels, especially lower C18:3 fatty
acids, should also emit lower levels of nitrogen oxides. This expectation follows
from linear correlations found between the level of biodiesel unsaturation (mea-
sured by the iodine value), the density of biodiesel, and nitrogen oxides emissions
TABLE 2
Cold Flow Properties of Fuelsa
Cloud point Pour point Cold filter plugging
Sample (°C) (°C) point (°C)
Soy methyl estersb 20 –10 –20
Biodiesel, USDA line –40 –60 –10
#2 Diesel –11 –18 –17
Synthesized biodiesel –18 –21 –21
aUsing ASTM Methods (D2500, D97, IP3991D6371); USDA, United States Department of Agriculture.
bSource: Reference 7.
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Fig. 1. Soy biodiesel
cloud point and
saturated fat content.
Data for samples with
8, 15, and 23%
(hydrogenated soy)
saturated fat are from
References 7, 8, and
Saturated fatty acids (% of total) 9, respectively.
(11). Thus, reduced levels of fatty acid unsaturation should decrease the density of
biodiesel and decrease nitrogen oxide emissions. Nitrogen oxide emissions from a
1991 DDC Series 60 engine may be predicted from the density of the biodiesel: y =
46.959(density) – 36.388, R2 = 0.9126. The density of the synthetic biodiesel (0.8825
g/mL) gave a calculated nitrogen oxides emission of 5.05 g/(bhp⋅h), an improvement
over soy biodiesel tested with the 1991 DDC Series 60 engine [5.25 g/(bhp⋅h)] (10).
Full exhaust emissions testing is required to confirm any effect.
Conclusion
The oil of the target composition (Table 1) is suitable for use as a feedstock to produce
biodiesel because biodiesel from the oil has improved cold flow, improved ignition
quality (CN), improved oxidative stability, and presumably reduced nitrogen oxide
emissions. The challenge is to create soybean oil near the target composition without
sacrificing soybean yield. The composition also must be marketed successfully and
tested in foods so that the soybeans are sought as an improved source of vegetable oil.
These accomplishments will enable soybeans to be grown on a large percentage of the
total acres and create value across the food and fuel chain.
References
1. Polorny, J., Flavor Chemistry of Deep Fat Frying in Oil, in Flavor Chemistry of Lipid
Foods, edited by D.B. Min and T.H. Smouse, AOCS Press, Champaign, IL, 1989, pp.
113–155.
2. Neff, W.E., Odor Significance of Undesirable Degradation Compounds in Heated
Triolein and Trilinolein, J. Am. Oil Chem. Soc. 77: 1303–1313 (2000).
3. Warner, K., W.E Neff, C. Byrdwell, and H.W. Gardner, Effect of Oleic and Linoleic
Acids on the Production of Deep-Fried Odor in Heated Triolein and Trilinolein, J.
Agric. Food Chem. 49: 899–905 (2001).
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4. Warner, K., P. Orr, L. Parrott, and M. Glynn, Effects of Frying Oil Composition on Potato
Chip Stability, J. Am. Oil Chem. Soc. 71: 1117–1121 (1994).
5. Hu, F.B., J.E. Manson, and W.C. Willet, Types of Dietary Fat and Risk of Coronary Heart
Disease: A Critical Review, J. Am. Coll. Nutr. 20: 5–19 (2001).
6. Kris-Etherton, P., K. Hecker, D.S. Taylor, G. Zhao, S. Coval, and A. Binkoski, Dietary
Macronutrients and Cardiovascular Risk, in Nutrition in the Prevention and Treatment of
Disease, edited by A.M. Coulston, C.L. Rock, and E.R. Monsen, Academic Press, San
Diego, 2001, pp. 279–302.
7. Lee, I., L.A. Johnson, and E.G. Hammond, Use of Branched-Chain Esters to Reduce the
Crystallization Temperature of Biodiesel, J. Am. Oil Chem. Soc. 72: 1155–1160 (1995).
8. Peterson, C.L., J.S. Taberski, J.C. Thompson, and C.L. Chase, The Effect of Biodiesel
Feedstock on Regulated Emissions in Chassis Dynamometer Tests of a Pickup Truck,
Trans. ASAE 43: 1371–1381 (2000).
9. Kinast, J.A., Production of Biodiesels from Multiple Feedstocks and Properties of
Biodiesels and Biodiesel/Diesel Blends. Report 1 in a Series of 6. NREL/SR-510–31460,
2003.
10. Bagby, M.O., B. Freedman, and A.W. Schwab, Seed Oils for Diesel Fuels: Sources and
Properties, ASAE Paper No. 87-1583, American Society of Agricultural Engineers, St.
Joseph, MI, 1987.
11. McCormick, R.L., M.S. Graboski, T.L. Alleman, and A.M. Herring, Impact of Biodiesel
Source Material and Chemical Structure on Emissions of Criteria Pollutants from a Heavy-
Duty Engine, Environ. Sci. Technol. 35: 1742–1747 (2001).
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