Microencapsulation efficiency of linseed oil
The ME of linseed oil for the points of the experimental design is shown in Table 1. The ME values of linseed oil were found to be between 54.6% (value for design point 2) and 90.7% (the value for design point 3). The results showed that higher ME values were obtained with a high concentration of encapsulating wall (30% wall material concentration, 14% oil concentration, and MD/GA wall type). According to Young et al. 25, ME can rise by increasing the concentration of solids in the wall solution. According to Jafari et al. 26, the most important factor determining the retention of food oils during spray drying is the dissolved solids content in the feed. A high solid content of the prepared emulsion increases retention principally by reducing the time required to form a semi-permeable membrane at the surface of the drying particle. Moreover, higher total solids lead to the increase of emulsion viscosity, preventing circulation inside the droplets, and, thereby, resulting in a rapid skin formation. With respect to the oil content, several reports have observed that higher oil loads generally result in poorer retention or lower encapsulation efficiency and higher surface oil content of the powder 15, 27–30.
Different biomolecules have been microencapsulated by spray drying using GA as a microencapsulation agent due to the efficient protection offered by this wall material 31. Researchers have tried to use a blend of GA with other wall materials and/or to replace GA completely 32. They have shown that MD can successfully replace a part of GA as the wall material, and they have determined the best ratios between them. Turchiuli et al. 16 studied the microencapsulation of vegetable oil in a solution of MD/GA (2:3) by spray drying and observed that the amount of free or non-encapsulated oil on the surface of solid microcapsules was low (<2% of total oil). Krishnan et al. 33 reported the microencapsulation of cardamom oleoresin by dryer spraying using GA, MD, and a modified starch as wall materials. The microcapsules were evaluated by organic solvent extraction of the non-volatile for 6 wk and the stability of volatile and non-volatile compounds was determined. The results showed that GA presented higher protection for the oleoresin than MD and modified starch. Krishnan et al. 34 reported the microencapsulation of cardamom oleoresin by spray drying using binary and ternary mixtures of GA, MD, and modified starch as the wall materials. The results obtained indicated that the mixture 4:1:1 of GA/MD/modified starch showed a higher protection from oxidation than the GA used alone.
The selection of the proper carrier is one of the most important factors in the microencapsulation of oils, since it can affect the viscosity of in-feed emulsion 35, 36. Although, MD shows low viscosity, is a poor emulsifier, and does not promote good retention of compounds during the spray drying process, it does protect encapsulated ingredients from oxidation 31, 37. On the other hand, GA is an excellent carrier for microencapsulation providing good retention of oils upon drying 38. The use of GA and MD mixtures for the microencapsulation of oils is a good compromise between cost and effectiveness as wall materials.
Table 2 shows results concerning the magnitude of average difference by control factors and working levels. The greater the difference, the higher the influence of the control factors on the ME, and it is possible to identify the working level that produces the best result. The concentration of encapsulating wall (Factor A) showed a higher influence on the ME of linseed oil with a difference of 19.6 units between the response of working level 2 and working level 1, followed by the oil concentration (Factor B) with a difference of 16.49 units between the two working levels. Factor C (type of wall material) did not generate an important impact on the response, with a difference of 5.09 units.
Table 2. Levels selected as greater is better in the results obtained for the microencapsulation efficiency response of linseed oil by spray drying
|Difference ΔN(2 − 1)||19.6||16.49||5.09|
Figure 1 shows the degree of incline of the slope as response. The greater the difference between level 1 and level 2 for a variable, the higher the change magnitude of the response. Therefore, the concentration of wall material (variable A) presented the highest slope compared to the oil concentration (variable B). Consequently, the higher the wall material concentration and the lower the oil concentration, the higher the ME.
It should be taken into account that increasing the solids concentration in the emulsion is favorable for an optimal viscosity, which leads to a suppression of the internal circulations and oscillations of droplets, thereby, improving retention 26. However, should the optimal limit of viscosity be increased, the retention will decrease due to a greater exposure during atomization, the slow formation of discrete droplets during atomization, and difficulties in droplet formation 26. As mentioned before, lower oil loads result in higher retention 15, 27–30, as shown in this study.
Regarding the type of wall material, this variable did not produce any significant change in the response since the values were close to the average, restricting the effect of control factors on the response. Furthermore, the combination (30% Factor A, 14% Factor B, and MD/GA Factor C) ensures optimal conditions for obtaining a higher ME of linseed oil. In addition, the Taguchi method made it possible to determine which factors of the process optimization most affect product quality with the minimum amount of assays.
The results of the ME were subject to an ANOVA with the aim of determining the degree of influence of the control factors on the response. The Fexp values were 19.83, 14.03, and 1.34 for Factors A, B, and C, respectively. The ANOVA demonstrated that the determination coefficients (R2) for the independent variables concentration of wall material (A), oil concentration (B), and type of wall material (C) were 51.0, 36.0, and 3.4% respectively, to which A and B contributed with 87% of the response. The analysis showed that the most significant factor (p ≤ 0.05) was the concentration of wall material with a contribution (R2) of 51% to the ME. However, the type of wall material did not significantly affect (p ≥ 0.05) the ME.
The determination coefficient for these three variables was significant (p ≤ 0.05), with R2 = 90.4%, indicating a high affinity or association of independent variables with the ME of linseed oil.
The OTE for ME was determined by means of Eq. (4), considering factors A and B together, which contributed 87% of the response.
where T = 70.1 is the total average of responses of experimental runs, A and B are the control factors, and sub-indices 1 and 2 corresponding to working levels 1 and 2.
The OTE obtained (88.2%) represents the contribution per working level of the two control factors.
As reported by Imagi et al. 17, a lower percentage of free or non-encapsulated oil would indicate an efficient microencapsulation process, thereby, improving the quality of the microencapsulated product.
The samples of design points 1 and 3 obtained from spray drying were analyzed by SEM.
Figure 2a shows the microcapsules obtained by design point 3, which reached the highest ME. The particle surface is smooth and free of pores, which is essential for the stability of the microcapsule as the pores facilitate the entry of oxygen and the exit of the encapsulated material and, therefore, a decrease in the oxidation of compounds such as fatty acids. In recent studies, Fuchs et al. 41 encapsulated a vegetable oil by spray drying using MD and GA as the carriers, leading to the formation of small particles (<50 µm). The results showed that the encapsulated oil was protected against oxidation, contrary to the case of non-encapsulated oil.
Figure 2. Morphology of micropasules by SEM obtained for (a) design point 3 and (b) design point 1. The particle surface of the microcapsules obtained by design point 3 is smooth and free of pores. The microcapsules of design point 1 produced with GA showed collapsed or shrunken particles with rough surfaces.
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Microcapsules with lower ME (design point 1) produced with GA showed morphological differences. Figure 2b shows collapsed or shrunken particles with rough surfaces.
According to Sheu and Rosenberg 42, microcapsules obtained by spray drying using polysaccharides as the wall material show remarkable surface cracks and the formation of dents or collapsed particles, which have been attributed to the effects of the composition of encapsulating material, atomization, and the drying parameters, among others. It has also been reported that microcapsules with GA mixed with MD and modified starch as wall materials showed improved morphology, obtaining a more spherical shape 34.
Figure 3 clearly shows the presence of oil globules in the microcapsules detected by the emitted fluorescence using confocal microscopy.
Figure 3. Confocal micrographies of MLO staining with nile red. The presence of oil globules was detected in the microcapsules by the fluorescence emitted using confocal microscopy.
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Particle size distributions for microcapsules obtained from design points 1 and 3 were measured by laser diffraction. For design point 1, the 10% of volume of particles were distributed less to 5.8 µm, whereas, 90% of volume of particles were distributed less to 46.6 µm. The distribution mean was 17.6 µm. For design point 3, the 10% of volume of particles were distributed less to 7.3 µm, whereas, 90% of volume of particles were distributed less to 67.4 µm. The distribution mean was 23.1 µm showing a homogeneous distribution of microcapsules.
Fatty acid composition
To determine the fatty acid profile in this study (Table 3), the sample with the highest ME yield of linseed oil by spray drying (point design 3) was analyzed. Pure linseed oil was used as control in order to evaluate the effect of the microencapsulation treatment by spray drying on the fatty acid profile of linseed oil.
Table 3. Fatty acid profile of linseed oil and microencapsulated linseed oil
|Fatty acid||Linseed oil||Microencapsulated linseed oil|
|Fatty acid (%)|
Fifteen fatty acids in extracted linseed oil were identified and quantified (Table 3). ALA was found in non-encapsulated linseed oil (40.35%) and in MLO (34.97%). Linoleic and oleic acids were also identified in non-encapsulated oil (14.95 and 10.55%, respectively), and MLO (8.45 and 5.46%, respectively).
The results in Table 3 showed that the homogenization of the emulsion and/or the microencapsulation process by spray drying affected the fatty acid composition of linseed oil. When comparing the percentage of microencapsulated oil with the untreated oil (control), a decrease in the percentage of fatty acids was observed, probably due to the physical process of encapsulation and chemical reactions that might change the nature of the original linseed oil. The amount of linolenic acid decreased slightly; however, the amounts of oleic and linoleic acid decreased by almost 50%.