Concern for the environment, safety and costs has promoted the development of the method for extracting soybean oil by an aqueous process. An advanced aqueous extraction of soybean oil assisted by adding free oil was established in this study, which recovered 81% of the oil from soybeans with 20.73% crude oil content and produced a de-oiled residue with 4.7% residual oil. The acid or peroxide value of the recovered oil met the Chinese national standard for first class refined oil, which was lower than that produced by solvent extraction or high temperature pressing. No wastewater was produced during the aqueous extraction of oil. The removal of the oil by the addition of oil and a small amount of water generated a residue (solids) containing all the protein, which represents 2/3 of the revenue in the soybean process. The protein-rich residue can be further processed to produce a protein isolate with high purity (e.g. > 90%) by using a higher amount of water. It can also be used as a nutritious ingredient or raw material for the production of many food products, among other applications.
The nutritional value of soybeans has been applauded by scientists in the literature because they provide high quality oil and proteins (Weingartner,
The disadvantages of solvent extraction and high temperature pressing which are currently used for the commercial production of soybean oil throughout the world have been extensively reported (Tu
The aim of this study was to develop a new aqueous procedure for extracting soybean oil by adding free oil since the current recovery yield achieved by the aqueous extraction process reported in the literature appears to be < 80%. This new aqueous method for extracting soybean oil is based on the aggregation of hydrophilic groups of compounds with and without hydrophobic groups via hydrogen bonds with the prevention of their solubilization or dispersion by adding the proper (small) amount of water as a prerequisite. This method is different from the aqueous method reported in the literature which relies on the complete dispersion of the compounds without hydrophobic and hydrophilic groups by using large quantities of water.
Soybeans were obtained from the Chengdu People Nutritious Food Factory, China. Their moisture content was 10.35 ± 0.11%. The oil content on the dry basis of baked and peeled soybeans was 20.73 ± 0.20%. Soybean oil (first-class refined oil according to Chinese national standard) was obtained from the Arowana Business, China.
The soybeans were baked at 120 °C for 5 min. Then the baked soybeans were cooled and peeled. The peeled soybeans were rolled into slices and then extruded via a cold spiral press (made by Ai Bang Agricultural and Horticultural Machinery Plant, China) at 20 °C. Soybean oil (3 parts) was mixed with the rolled and extruded soybeans (5 parts). After mixing well, the mixture was ground by hand and passed through a 100 mesh sieve. These procedures prepared the slurry (mixture of soybeans and refined soybean oil added (SSO)). Every 10.00 g SSO was weighed into a container and mixed with sodium chloride (0.1000 g). Then, 0.00, 1.00, 1.10, 1.20, 1.25, 1.30, 1.35, 1.40, or 1.50 mL of water were added into a container. The mixtures were agitated at 65 °C for 10 min. After agitation, the mixture of free oil and aggregated hydrophilic group was transferred into a 20 mL centrifuge tube and centrifuged at 4000 r/min for 30 min. This step was repeated three times. Finally, the residue at the bottom of the centrifuge tube was pressed by cold pressing at room temperature via a spiral cold press (made by Ai Bang Agricultural and Horticultural Machinery Plant, China). Three replicates of each sample were performed. The oil content in the de-oiled and dried residue from the spiral cold press was determined. The oil yield (OY) was calculated using the following formula:
where Soc, Roc, WBPSS and WTDDR represent the oil fraction of the original baked and peeled soybean sample (dry basis), the oil fraction of the de-oiled residue which was dried after pressing by the spiral cold press, the weight (g) of the baked and peeled soybean sample and the weight (g) of the total dry de-oiled residue, respectively.
The soybeans were baked at 100 °C, 110 °C, 120 °C, 130 °C, or 140 °C for 0 or 5 min. The procedure for preparing SSO, the separation of oil and the method for determining OY were the same as that described in 2.2.1
The soybeans were baked at 120 °C for 5 min. The rest procedure of preparing SSO, the separation of oil and the method of determining OY were the same as that described in 2.2.1
The soybeans were baked at 120 °C for 5 min. The procedure for preparing SSO, the separation of the oil and the method for determining OY were the same as that described in 2.2.1
The soybeans were baked at 120 °C for 5 min. The procedure of preparing SSO, the separation of the oil and the method for determining OY were the same as that described in 2.2.1
The procedure was the same as that described in “2.4”. The only difference was that the refined oil was not added in this experiment. The OY was measured by the same method as that described in 2.2.1
The procedure was the same as that described in “2.4”. The only difference was that after agitation the mixture of free oil and aggregated hydrophilic group (a large rigid particle formed because of hydrogen bonds) was separated by the following methods: A (Centrifuging three times), B (Centrifuging + cold spiral pressing) and C (Cold spiral pressing three times). The OY was measured by the same method as that described in 2.2.1
The soybeans were baked at 120 °C for 5 min. The procedure for preparing SSO was the same as that described in 2.2.1
Crude fat was measured according to Chinese national standards GB 5009.6-2016. Moisture content was measured according to Chinese national standards GB 5009.3-2016. Acid value and peroxide value were determined according to Chinese national standards GB 5009.229-2016 and GB 5009.227-2016, respectively. All experiments were carried out in triplicate, and the mean and standard errors were calculated and reported for each determination.
The means of the data were analyzed by One-way ANOVA using IBM SPSS Statistics. The level of significance was set at P < 0.05.
The results indicated that no oil from the soybean sample was extracted and 52.43% of the refined oil added was absorbed instead when no water was added. The effect of the amount of water added on the OY is indicated in
Effect of amount of water added on the final yield of soybean oil.
The results mentioned above mean that the addition of water is essential for the extraction of soybean oil. The addition of water should be necessary for the formation of hydrogen bonds among the hydrophilic compounds such as proteins, saccharides, phospholipids, and free fatty acids for them to aggregate together into a large rigid and sticky particle. This may also enhance the hydrophobic interaction between oil molecules. These reactions caused the release of oil from the aggregated hydrophilic particle. However, the addition of an excessive amount of water was harmful to the extraction of soybean oil. The reason for this may be that some hydrophilic compounds such as free fatty acids, phospholipids and phenolic compounds dissolved in the water so that emulsification was enhanced and OY was reduced.
The results from this experiment showed that the OY (56.18 ± 0.74%) obtained without baking was not significantly different from that (56.79 ± 0.87%) obtained at 100 °C baking temperature (P > 0.05). The effect of baking temperatures from 100 to 130 °C is shown in
Effect of baking temperature on the final yield of soybean oil.
The results mentioned above should indicate that baking at an appropriate temperature improved the efficiency of extracting soybean oil by using an aqueous solution though increased by only 2% in the OY. The reason for obtaining this effect may be that baking can deactivate lipase in soybeans and a baking temperature which is too high may denature storage proteins, which increases their oil-binding capacity and therefore decreases the OY. The deactivation of lipase can reduce the hydrolysis of neutral oil which facilitates emulsification and reduces OY during aqueous extraction. In fact, whole oil seeds can lose neutral oil during storage if their storage temperature and moisture content are high enough. Therefore, it is reasonable to believe that the loss of neutral oil in the soybean seed slurry with the addition of water and agitation is very likely if lipase is not deactivated. Another advantage of baking is that the flavor of the oil recovered can be improved.
The results from this experiment are shown in
Effect of amount of sodium chloride added on the final yield of soybean oil.
Therefore, the addition of sodium chloride greatly increased the efficiency of extracting soybean oil using an aqueous solution. The increment in OY resulting from the addition of sodium chloride was > 8%. This means that the addition of sodium chloride is critical for achieving high OY. The addition of salt to the water can result in changes in the surface tension and surface charge of the protein or carbohydrate, which increases the polarity of the hydrophilic group. This effect can increase the non-covalent interaction of the polymer with water by the action of ionic bonds, which then extrude the oil from the aggregated polymer. The addition of salt can also increase the water density and therefore increase the density difference between the aqueous sediment and the supernatant oil. However, the soybean OY dramatically decreased when the amount of salt further increased to higher than 0.08 g, which may be caused by a decrease in the availability of water.
Effect of agitation temperature and time on the final yield of soybean oil.
Therefore, the agitation temperature greatly increased the efficiency of extracting soybean oil using an aqueous solution. The increment in the OY resulting from the appropriate increase in agitation temperature was > 17%. This means that the appropriate agitation temperature is critical for achieving high OY. Furthermore, the increment in OY resulting from the increase in agitation time was ca. 2% when other operating conditions were fixed. For achieving high OY, this factor should not be ignored.
From
Effect of milling time on the final yield of soybean oil.
The results from this experiment indicated that the OY was only 4.76 ± 0.31% (
Comparison of the final yield of soybean oil by different separating methods.
According to the experiments described in 3.1, the critical operating conditions or parameters from the step of agitation upstream were established and optimized. Then the efficiency of different methods for separating free oil and aggregated hydrophilic group (solid) were compared when the established and optimized operating conditions or parameters were employed.
This study found that A (centrifuging three times), B (centrifuging + cold spiral pressing) and C (cold spiral pressing three times) produced a de-oiled residue containing 8.94 ± 0.15%, 4.78 ± 0.09% and 4.88 ± 0.09% residual oil (dry basis), respectively. The OY corresponding to this residual oil is shown in
According to the experimental results described in 3.1 and 3.2, a feasible procedure for efficiently extracting oil from soybeans was established, and described in 2.4. The process flow diagram of this procedure is shown in
Process flow diagram of producing soybean oil by aqueous extraction assisted by the addition of free oil.
The result of our final test on this procedure for producing soybean oil indicated that the OY was 81.00% (4.70% residual oil content in the de-oiled residue on dry basis). The OY obtained by using the method developed in this study was much higher than that (73%) obtained by ohmic heating and the enzyme assisted aqueous extraction method (Pare
The oil produced by the method developed in this study had an acid value of 0.19 (mg KOH/g) and 4.81 peroxide value (mmol/kg) which met the Chinese standard for first-class refined oil. This method produced oils with higher quality as compared to solvent extraction and high temperature pressing.
In this study, the removal of oil by the addition of oil and a small amount of water generated a residue (solids) containing all the protein, which represents 2/3 of the revenue in the soybean process. The protein-rich residue can be further processed to produce protein isolate with high purity (e.g. > 90%) by using a higher amount of water. It can also be directly used as a nutritious ingredient or raw material for the production of many food products, among other applications.
The aqueous method of extracting oil from soybeans assisted by adding free oil which was developed in this study produced high OY (81.00%). The reason for the efficiency of this method using a small amount of water rather than using the large quantities of water reported in the literature should be that these two methods have different mechanisms. The method developed in this study added the proper amount of water with its quantity (liquid:material ratio < 1:5) enough to result in interactions (mainly via hydrogen bonds) between hydrophilic groups of compounds which may also contain hydrophobic groups (e.g. phospholipids, proteins, amino acids, saponins), but not enough to solubilize or disperse any of them as a prerequisite. The aggregation of hydrophilic compounds into a large and rigid particle repels oil so that a free oil phase is obtained. The addition of free oil enhanced this kind of reaction. On the other hand, the traditional method using large quantities of water (liquid:material ratio > 2:1) primarily relies on the full solubilization or complete dispersion of compounds having both hydrophilic and hydrophobic groups (e.g. phospholipids, proteins, amino acids, saponins) which facilitate the formation of stable emulsion with oil (Campbell and Glatz,
The reasonably low water content in the defatted meal with high quality, no wastewater generated during the extraction of oil, the high OY and good quality of oil recovered should show that the aqueous method developed in this study is superior to traditional or enzyme-assisted aqueous method which uses large quantities of water, solvent extraction and traditional high temperature expressing.
With regard to the comprehensive utilization of soybeans, the new aqueous method developed in this study is comparable to solvent extraction and superior to the enzyme-assisted aqueous method and pressing at high temperature. The aqueous method developed in this study is comparable to pressing at high temperatures and superior to solvent extraction and enzyme-assisted aqueous method in terms of reducing equipment investment. The method developed in this study is superior to solvent extraction, enzyme-assisted aqueous method and pressing at high temperatures in terms of reducing production cost. Furthermore, the method developed in this study is superior to solvent extraction and enzyme-assisted aqueous method for the purpose of decreasing adverse effects on the environment.
The experimental method described in this paper established operational variables to obtain 81% oil recovery rate which was better than other methods such as hot-pressing alone or cold-pressing alone, or traditional aqueous method using large quantities of water including those enzyme-assisted, except for solvent extraction. Therefore, the methods used in this study were adequate, and resulted in that the aim of the work was clearly established. Further optimization such as surface response experimentation may be a valuable attempt to further improve the oil recovery rate. However, it may be better left for future study since the operational variables were good to obtain a meaningful oil recovery rate from the point view of commercial processing of soybeans.
In 2.2.1., we clearly indicated that the mixture of soybeans and refined soybean oil added (SSO) was prepared by grinding 3 parts refined soybean oil with 5 parts rolled and extruded soybeans to pass through a 100 mesh sieve by hand. All experiments were performed by using SSO as a study material. In 3.1.1., it has been clearly described that no oil from SSO was extracted and 52.43% of the refined oil added was absorbed instead when no water was added (by centrifuging three times followed by pressing or by hot-pressing alone or by cold-pressing alone). When the proper amount of 6.1% aqueous salt solution was added to SSO followed by proper agitation, free oils were released and the solids aggregated into a sticky and large particle. The majority of the oil released can be directly poured out of the agitator container. Only minor oil needs to be collected by pressing or centrifuging. If the oil is collected by pressing alone, it may contain a tiny amount of solids which must be removed by centrifugation. If the soybean was directly treated by hot pressing alone, the recovery rate of oil was less than 70% while cold pressing alone had a much lower recovery rate of oil. In 3.1.6., it has been clearly indicated that only 4.76% oil was recovered when no refined oil was added. Therefore, the addition of water and refined oil is essential for the extraction of oil from soybeans. Others are auxiliary measures.
Because the purpose of this study was to establish a new aqueous method for the extraction of oil from soybeans, de-oiled meal was just a by-product and its properties were not analyzed in details. However, since soybean is a kind of edible seed, the de-oiled meal should be applicable in the food industry.
Baking was an efficient method for deactivating lipase and therefore preventing the loss of neutral oil during extraction and decreasing the acid value of the oil produced, which should be applied to any kind of aqueous method. It should be noted that dry-heating is not likely to reduce the solubility of protein and may cause an adverse effect on oil quality.
The extraction of oil by this method at such a low temperature should produce oil with a quality similar to or even better than that obtained by cold-pressing. Such surfactants as phospholipids, free fatty acids, and amino acids should remain in the aggregated hydrophilic particle, which was the reason that the oil produced by this new aqueous method was clean and had high quality.
Aqueous extraction assisted by adding free oil was very efficient and had the potential of being a classic procedure for processing soybeans in the future and forever. The addition of an appropriate amount of both free oil and water was found to be essential and critical for extracting oil from soybeans. The amount of water and sodium chloride added must be appropriate since an excess of them was found to be harmful to the recovery of free soybean oil. The temperature for baking soybeans and agitating the soybean slurry containing free oil added was also found to be critical for achieving high OY, but it must be appropriate since a temperature which was too high was harmful to the recovery of free soybean oil. The extension of milling time of the soybean slurry containing added free oil positively correlated with OY. No wastewater was produced by using the aqueous extraction method developed in this study. The method developed in this study also produced high quality de-oiled residue which can be directly used in the food industry or further processed to produce protein isolate with high purity (e.g. > 90%) by using a higher amount of water. This method may be applied to the processing of any oil seed similar to soybeans which have a low oil content.