The oil we use in cooking every day typically comes from plant seeds. Olive, sunflower, rape and palm are common examples, with sesame, coconut, peanut, walnut, cotton and soya also available, though less well known. (Triffid oil is, of course, fictitious.). This array of plants that can store oil easily and in large enough quantities to be commercially viable raises the question of how this happens.

The question becomes more pertinent when we remember that oil and water do not mix. We can shake olive oil and water together in a closed, colourless container and, for a short time, observe what looks like a mush, but the oil and water separate after only a few minutes. We know plant material contains water (up to 96%); we also know that the oil is there too. The seeds in which the oil is stored are small and we cannot see the form the oil takes, suggesting the oil is in microscopic portions.

Work using light microscopes reveals this to be the case. Using staining techniques, the bodies in which the oil is stored are visible under a light microscope. This tells us about the compartmentalisation of oil storage in plant seeds, but not how it seems to be energetically favourable*: it looks for all the world that the oil is in blobs in a watery system (a cell). It looks like what food technologists call an oil-in-water emulsion: small blobs of oil suspended in water.

This on its own is impossible for any length of time, as we observe. There must be a third component that satisfies the energetic requirements of the apparent observation of oil in water. That third component is a mono-layer of amphiphilic molecules. Like simple lipids, this mono-layer comprises a surface that is hydrophilic (likes water) and one that is lipophilic (likes fat).

This is an elegant compromise between the requirements of storing oil in a seed. However, it raises as many questions as it answers: how does the plant control this important organelle? What is the mono-layer made of exactly? Do the components of this mono-layer have other uses?

The answers to these questions share a few connections. We know from a recent study detailing the molecular composition of this organelle [1], that it is dominated by a group of protein isoforms known collectively as oleosins. These proteins are amphiphilic, but also have other properties.

For example, many seeds that use oil as an energy storage desiccate on maturing, i.e., lose some of their water. We know that lipid systems are sensitive to water, and so when the amount changes, it can have a profound effect on lipid behaviour. However the oil bodies, the organelles that store the oil, remain in tact. This is clearly an important and transferrable feature: all oil bodies yet discovered comprise large amounts of oleosin. It turns out that the oleosin component is not only an amphiphile, but one that can confer a measure of flexibility on the mono-layer it comprises. The oleosin fraction of the surface is typically the dominant one; about 60% of the mass of the oil body mono-layer in Helianthus annuus (common sunflower) was found to be oleosins [1].

This suggests that this protein may have a powerful part to play in other applications – and there are a surprising number. In fact, anything with an oil-in-water emulsion that is in any way sensitive, may be improved by the use of oleosins. Current speculation and applications under investigation include a molecular vaccine delivery mechanism (increasing the stability of the vaccinating molecular species) [2], a way of immobilising enzymes in a way that is recyclable [3-5] and a way of improving the shelf-life of edible oil-in-water emulsions [6-9].

In fact, the list goes further than these examples, and is fast approaching the magnitude of plants that make oil bodies in quantity. The scope for a flexible mixture of lipidic and proteinaceous amphiphiles is therefore something keenly anticipated, and not just by lipid chemists.

References and Notes

[1] Furse et al., The Lipidome and Proteome of Oil Bodies from Helianthus annuus (common sunflower), Journal of Chemical Biology, 2013, 6, 63-76. DOI: 10.1007/s12154-012-0090-1 .
[2] H. Deckers et al., Immunogenic formulations comprising oil bodies, 2004, United States Patent.
[3] C. J. Chiang, C. Y. P. Chen, J. T. C. Tzen, Efficient system of artificial oil bodies for functional expression and purification of recombinant nattokinase in Escherichia coli, Journal of Agricultural Food Chemistry, 2005, 53, 4799–4804. doi:10.1021/jf050264a
[4] J. H. Liu, L. B. Selinger, K. J. Cheng, K. A. Beauchemin, M. M. Moloney. Plant seed oil-bodies as an immobilization matrix for a recombinant xylanase from the rumen fungus Neocallimastix patriciarum, Mol Breed, 1997, 3, 463–470. doi:10.1023/a:1009604119618
[5] J. R. Liu, et al., Cloning of a rumen fungal xylanase gene and purification of the recombinant enzyme via artificial oil bodies, Applied Microbiology Biotechnology, 2008, 79, 225–233. doi:10.1007/s00253-008-1418-1 .
[6] I. Fisk, R. Linforth, A. Taylor, D. A. Gray, Aroma encapsulation and aroma delivery by oil body suspensions derived from sunflower seeds (Helianthus annuus). European Food Research Technology, 2011, 232, 905–910. doi:10.1007/s00217-011-1459-z
[7] I. Fisk, D. A. White, M. Lad, D. A. Gray, Oxidative stability of sunflower oil bodies. , European Journal of Lipid Science and Technology, 2008, 110, 962–968. doi:10.1002/ejlt.200800051
[8] D. R. McCaskill, F. Zhang, Use of rice bran oil in foods, Food Technology, 1999, 53, 50–52 .
[9] C. V. Nikiforidis, V. Kiosseoglou. Physicochemical stability of maize germ oil body emulsions as influenced by oil body surface−xanthan gum interactions. Journal of Agricultural Food Chemistry, 2009, 58, 527–532. doi:10.1021/jf902544j

*This typically means the conformation of the system that requires the least energy. It is also used to describe a conformation of the system that does not require more energy than the system possesses.