Food Lipids: Chemistry, Nutrition, and Biotechnology

ecome the preferred processing methods because the oil is more completely recovered.
For a long time, the rule of thumb has been that materials containing more
than 30% oil require pressing, either hard pressing or prepressing prior to solvent
extraction. Hard pressing involves squeezing as much oil as possible; prepress solvent
extraction involves squeezing out only part of the oil before subjecting the
partially deoiled material to more complete extraction with solvent. The recent adoption
of the expander has largely done away with this rule, and even high-oil content
materials can be solvent extracted today with no or little prior oil extraction [5,6].
Direct solvent extraction, without any prior pressing or expanding, has long been the
most widely practiced method, the oil is more completely recovered, and the oil and
meal are economically recovered undamaged by heat. Both prepress solvent extraction
and direct solvent extraction are depicted in Figure 2.
1. Seed Storage
Oilseeds are often harvested at moisture contents higher than levels that allow for
long-term storage and must be dried for safe storage. Increasingly, farmers are storing
oilseeds, particularly soybeans, on the farm to take advantage of higher prices that
are paid later in the crop year.
Storage for extended periods at moisture contents exceeding critical moisture
levels will damage oilseeds, reducing the yields of oil and protein, and diminishing
the quality of the oil (notably darker color and higher refining loss). Seeds at harvest
are alive and respire, converting seed mass to CO2 and other metabolites, albeit at
low rates when the moisture content is below the critical moisture level. The critical
moisture level for safe storage varies with the seed specie: usually, the higher the
oil content, the lower the critical moisture value (Table 1). At moisture contents
exceeding the critical moisture level, respiration rate increases, and the seed can even
germinate and become subject to fungi attack. Respiration and germination liberate
heat and, when there is insufficient aeration, the heat further accelerates these reactions.
Under extreme conditions, the seed may become scorched or even catch on
fire (especially cottonseed). Modern seed storage facilities employ temperature-monitoring
systems to alert elevator and storage operators when seed temperatures exceed
critical set points. Then the seed is moved to another bin to disperse hot spots and/
or aerated (blowing air through the seed). The percentage of seed that is heat damaged
is often a factor in the U.S. grades and standards (e.g., soybeans).
Overdrying can increase seed fragility, leading to excessive breakage during
handling, storing, and processing. In the case of soybeans (a dicot), the cotyledon is
prone to splitting into halves when the hull becomes separated from the cotyledon
(meat) during conveying and transporting, a problem that becomes worse when the
beans are overdried. Splits are undesirable because they are difficult to separate from
foreign matter, and the oil deteriorates at a faster rate. Oil from soybean splits is
higher in free fatty acids, phosphatides, iron, and peroxides due to activation of
catabolic enzymes [7]. Oils from field- and storage-damaged seed usually are poor
in flavor [8]. For these reasons, damaged kernels and splits are also factors in the
U.S. grades and standards for soybeans.
When oilseeds are received at the crushing plant (Fig. 3), samples are often
taken for analysis of moisture, foreign matter, damaged seed, oil, protein, and free
fatty acid contents. Shipments arriving with similar values for these analyses may
be segregated based on actions that are required to minimize further degradation.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.