Study On The Natural Settling Process Of Sesame Oil And Its Influencing Factors

Sesame oil is a brown oil with unique flavor obtained by pressing sesame seeds roasted at a temperature of 180～200℃.The presence of colloidal impurities such as phospholipids,proteins,and sugars in freshly produced sesame oil can affect the stability of the oil and cause flavor abnormalities if not removed in time.Sesame oil is usually refined using natural settling methods to remove impurities,but the natural settling process takes a considerable amount of time and does not guarantee the complete precipitation of undesirable colloidal flocculated particles,which affects the quality of sesame oil,especially its flavor.The natural sedimentation process involves the flocculation of colloidal impurities into agglomerates and then settling by gravity,so this study first describes the natural sedimentation process and then investigates the flocculation of colloidal impurities and the physical sedimentation process after flocculation.In this study,the natural settling process of sesame oil and its effect on the process is investigated by dehulling and changing the acid value of sesame seeds,followed by a stepwise study of the flocculation process and the settling of flocculent particles;a non-destructive in situ flocculation monitoring device,gathering beam reflectometry,is used to investigate the flocculation process and the flocculent characteristics in sesame seed oil with different moisture contents;the effect of temperature on the settling process at room temperature(10-30°C)is studied using a simulated vessel,and the settling process and particle distribution are calculated based on a computational fluid dynamics technical approach to modeling the settling of flocs.The main research results are as follows:(1)The natural sedimentation process was investigated by determining the composition of sesame gross oil and precipitate based on the sesame oil obtained before and after dehulling,in combination with the precipitate content,phospholipid content and the pattern of change of the precipitate layer during the sedimentation process.The results show that compared to unpeeled sesame seed oil,peeled sesame seed oil is approximately 5%more efficient in the linear settling phase.The results for samples of different depths showed that the phospholipid content and acid value in the upper and lower layers of the oil did not differ significantly after the linear settling stage,so the plant did not need to separate the oil into layers.More than 90%of the sesame oil precipitate was fat,with a small amount of water,phospholipids,protein and sugar forming gum mingles,with significant aggregation of water and phospholipids occurring.The initial height of the sesame oil precipitate layer is low and the linear settling stage is faster in compression,resulting in a higher yield of approximately3%of the final oil.Therefore,the peeling of sesame seeds facilitates the natural settling process of sesame oil and results in higher yields.(2)Since free fatty acids affect the solubility and charge of colloidal impurities,the effect of adding oleic acid on the natural sedimentation process was investigated by altering the free fatty acid content of sesame oil.The results showed that the addition of oleic acid increased the potential of colloidal particles from 2.64 m V to 12.9 m V compared to the sesame oil without oleic acid,which increased the electrostatic repulsion between the particles and was detrimental to the flocculation and sedimentation process.The linear settling efficiency of the two samples without oleic acid was 55.69%and 65.83%,respectively,while the linear settling efficiency was significantly reduced by the addition of5%and 10%oleic acid to 32.92%,28.21%and 27.55%,27.64%respectively.After 20 days of natural settling,a large number of colloidal impurities were still present in the samples with the addition of oleic acid and the settling process was slow,the increase in free fatty acids would increase the solubility of the colloidal impurities in the oil.The oleic acid added samples did not form a precipitate layer until after day 3 and the structure was loose and difficult to compress.Therefore,the increase in free fatty acids causes an increase in the acid value which is detrimental to the natural settling process of sesame oil.(3)The natural settling process of sesame oil is mainly a colloidal impurity flocculation process.In this study,the flocculation process of colloidal impurities was accelerated by adjusting the water content based on the FBRM technique to investigate the flocculation process and the characteristics of the flocs.The results of the particle number,mean chord length and cumulative distribution curves measured by the FBRM showed that 1%water was most favorable for the flocculation of colloidal impurities in sesame oil and that there was a linear relationship between the number of flocculated particles and the amount of water added.The liquid crystal phase of the flocculent after the addition of water was observed to be larger by polarized light microscopy and was a colloid formed by phospholipids and water,whereas the composition and chord length of the flocculent formed without the addition of water was similar to the particle size of the sesame oil body,so the structure of the flocculent particles was probably a spherical structure of the phospholipid-protein-oleosin membrane encapsulated triacylglycerol.The flocculation process is reversible,and the efficiency of the resolubilization increases with increasing temperature,with the optimum resolubilization temperature being 60°C.(4)Colloidal impurities flocculate to form particles that settle under the influence of gravity.A computational fluid dynamics(CFD)-based flocculation model was developed and the effect of room temperature(10-30°C)on the sesame oil settling process was investigated.The results showed that the experimental settling efficiencies were 22.89%,38.24%,43.54%,48.30%and 65.43%at 2 h,5 h,8.8 h,10.8 h,22.8 h and 48 h,while the settling efficiencies of the simulated values with a density of 993 kg/m~3were 22%,38.36%,44.32%,45.49%and66.87%.The small differences indicate good agreement between the CFD simulations and experiments.The simulated value with a density of 1093 kg/m~3is closer to the experimental value,while the 925 kg/m~3simulation settles more slowly in the early stage and settles more quickly in the late stage at 1193 kg/m~3,indicating that the floc particle density is in the range of 993 kg/m~3～1093 kg/m~3.The large particles settled almost completely after 1 day of the simulation,but the small particles were still suspended in large amounts in the oil,which may be one of the reasons for the reappearance of the’black oil foot’in the sesame oil during the shelf life.The slope of the linear settling phase was the highest and the intercept the lowest at 30°C during the experimental phase,indicating that the settling was the fastest and that the temperature had less influence on the subsequent settling process.