Evaluation of the properties of microcapsules containing Ziziphora clinopodiodes extract stabilized by gum Arabic, whey protein isolate, guar gum and their combinations

Introduction: Due to the increasing of awareness and consumer demand for healthy and natural foods without synthetic preservatives, in the recent years the food industry has paid increasing attention to using natural preservatives. Plant extracts and essential oils (EO) are a broad class of natural food preservatives that are generally recognized as safe (GRAS). Microencapsulation is the most common technique that has been utilized to increase the stability of bioactive compounds. The benefits of microencapsulation of herbal extracts and essential oils includes: protection of flavor components from evaporation and destructive changes, promotion of easier handling, better mixing with food matrix, release controlling and increasing bioavailability (Donsi et al., 2011). Ziziphora clinopodiodes (known as Kakuti) is one of the most commonly consumed herbal plants belonging to the family of Lamiaceae and it is native to Turkey and Iran. The major phenolic compounds of Z. clinopodiodes essential oil (ZEO) or its extract (ZE) are include pulegone, 1,8-cineole, thymol, carvacrol, p-cymene and limonene (Sonboli et al., 2010). The functional properties of Z.clinopodiodes extract (ZE) and ZEO including strong antioxidant, antibacterial, antifungal and antiviral activities have been fully recognized in previous researches. The selection of a suitable wall material is critical to have a successful microencapsulation. Whey protein isolate (WPI), gum Arabic (GA) and guar gum (GG) are some of most frequently used biopolymers for encapsulation of bioactive compounds in food industry applications (Kuck et al., 2017). There is no report on the encapsulation of ZEO or ZE. The aim of this research was to evaluate and compare the potential of GA, WPI and GG either alone or in combination with each other in the microencapsulation of Ziziphora clinopodiodes extract (ZE) by ultrasonication method.
Material and methods: The GG, GA and WPI were used for encapsulation of ZE in the single form or in combination with each other. After encapsulation, the suspensions were freeze dried and the powder of microcapsules was used for further analyses. The particle size, zeta potential, and encapsulation efficiency were determined. Also, the total phenol content, antioxidant activity and color properties of microcapsules were analyzed. Structural and morphological properties of them were also characterized by FT-IR, XRD and SEM analyses. The one-way ANOVA method by using SPSS software was used for statistical analysis of obtained data.
Results and discussion: Particle size analysis showed that the WPI stabilized sample had the lowest particle size (292.7 nm). The performance of GA and GG was improved after combination with WPI. By considering encapsulation efficiency (EE%), particle size, PDI and Zeta potential values, the compatibility of WPI with GA was more than GG. The highest EE% (86.91%) belonged to GA/WPI sample. Total phenol content and antioxidant activity were in their maximum level for GA/WPI mixture. Among the used wall materials, GA had the highest Zeta potential (-20.5 mV). Since the isoelectric point of WPI is around pH 5 and pH of dispersions was about 7, the net charge of these microcapsules was also negative (-15.5 mV). GG is a weak polyelectrolyte and had the lowest Zeta potential (-6.6 mV). All of the powders had a whitish color. There was no significant difference between L* and a* values of samples (p˃0.05). But b* was increased significantly when WPI and GA were used instead of GG. Yellowish color of WPI and GA caused to increase b*. This yellow appearance was more distinct in GA in comparison to WPI. The initial TPC of ZE before enclosing in microcapsules was equal to 346.91 mg GAE.100g^-1 DW. The TPC of all samples was lower than initial amount in fresh ZE. Between microcapsules, those that had the higher EE showed a higher TPC too. TPC of GA was higher than WPI followed by GG. When the mixture of wall materials was used, TPC was increased and the GA/WPI sample had the highest TPC (212.81 mg GAE.100g^-1 DW). GG and GA/WPI with 21.67% and 32.11% respectively, had the lowest and highest antioxidant activity. Spherical shape of microcapsules was observed by SEM for all samples expect to GG/GA. WPI stabilized microcapsules exhibited the lowest particle size with uniform distribution. Polydispersity and mean diameter was higher for GA microcapsules and these parameters increased for GG microcapsules. FT-IR analysis confirmed the formation of new interactions between wall materials (expect to GG) and ZE ingredients. XRD test revealed that the crystallinity of wall materials was decreased after ZE incorporation but WPI contained samples were resistant against structural alterations. Crystallinity index of GG, GA and WPI coated microcapsules was equal to 6.54%, 23.13% and 9.76%, respectively. Crystallinity index of 11.5%, 25.21% and 19.66% was calculated for GG/GA, GG/WPI and GA/WPI microcapsules, respectively. By considering of applying similar sonication and drying conditions on all samples, these results indicate that the ZE incorporation had no distinct effect on crystallinity of WPI but it disrupt the structure of polysaccharide based encapsulating agents.
Conclusion: In this work the performance of different wall materials and their combinations was evaluated in microencapsulation of ZE. WPI and GA/WPI combination showed the lowest particle size and the best encapsulation efficiency, respectively. In the total phenolic content study, the mixture of GA/WPI performed better in protecting the ZE phenolic compounds leading to increase the antioxidant activity of microcapsules. SEM images indicated that the successful fabrication of microcapsules with GA, WPI and GG is possible but it is better to use their combination. The best morphological characteristics with spherical microparticles were obtained when WPI was mixed with GA. Formation of new interactions between GA, WPI and ZE ingredients was approved by FT-IR analysis. But XRD test indicated that these interactions lead to change in structural characteristics of biopolymers. However, in spite of new bond formation, WPI preserved its semi-crystalline structure when used alone or in combination with polysaccharides. Generally, by in comparison of individual wall materials, the WPI was the best encapsulating agent but its performance was increased via blending with other biopolymers. The compatibility of WPI with GA was more than GG and by considering the set of results; the combination of GA/WPI presented the best behavior. Further studies are required for exploring the performance of these microcapsules for their application in food formulation.