Extraction of walnut oil body and its demulsification based on thin film drying-vacuum filtration technology

The oil body is one type of oil-storing organelle with a triglycerides matrix core that is coated by a protein-phospholipid membrane. Oil bodies can be isolated from oilseeds by aqueous extraction processing, and then directly processed into cream-like food ingredients. Nevertheless, the demulsification of the oil body is another meaningful direction for the utilization of the oil body. Much effort has been made into the demulsification of the oil body, such as the physical (e.g., freeze-thaw), chemical (e.g., surfactant), and enzymatic methods (mainly proteases). However, these methods have either a low demulsification rate or a high cost (adding chemicals or enzymes). In this study, a simple method without enzymes and chemicals was designed to demulsify the walnut oil body. Firstly, the water extract was prepared from the peeled walnut kernels, and the water extract was then separated into the light phase (oil body cream), intermediate phase (skim), and heavy phase (precipitate) by low-speed centrifugation (2 862 g, 15 min). Secondly, the oil body cream was demulsified by combined thin film drying and vacuum filtration technology, in order to obtain the walnut oil and the by-product rich in phospholipids and membrane proteins. The distribution of lipids and proteins in the three centrifuged fractions was systematically investigated to examine the mechanism of demulsification of the oil body during film drying. The results showed that the lipids in peeled walnut kernels were mainly distributed in the oil body cream (85.69% of total lipids in peeled walnut kernels), while the proteins were mainly distributed in the skim (23.58% of total proteins in peeled walnut kernels) and precipitate (65.04% of total proteins in peeled walnut kernels). The Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (Tricine-SDS-PAGE) protein profiles showed that the proteins in the skim were mainly composed of albumin and globulin, and those in precipitate were mainly composed of glutenin. The moisture content (20.68%) of the oil body cream gradually decreased during the thin film drying process. In the beginning, the oil body cream was gradually thickening, and then gradually was soft until it became a liquid. The liquid material could be separated into walnut oil (81.78% of the total lipids in walnut kernels) and filter cake (phospholipid-membrane protein concentrate) by vacuum filtration. The composition analysis showed that the phospholipid-membrane protein concentrate were composed of 67.23% of neutral lipids, 19.41% of proteins, 6.61% of phospholipids and 6.75% of other components (such as sphingosine). Tricine-SDS-PAGE results showed that more than 50% of the proteins in the phospholipid-membrane protein were oil body membrane proteins, indicating better emulsifying activity. The demulsification mechanism of the oil body was examined by confocal laser scanning microscope and conductivity analysis. The results showed that the distance among oil bodies was shortened with the evaporation of water, and gradually coalesced during the thin film drying, resulting in the reduction of the specific surface area of the coalesced oil body. As a result, the protein-phospholipid membrane was squeezed out from the coalesced oil body. With continuous coalescence, the coalesced oil droplets became larger and larger until release of free oil, and the free oil caused a sharp decrease in conductivity during the thin film drying and almost achieved to 0. This finding can provide a novel and simple strategy for the oil body demulsification, particularly with the guiding significance for the comprehensive value-added utilization of the walnut oil body.