A low viscosity alginic acid sodium salt from brown algae (Sigma Aldrich, Spain) was used as shell material. The core material was an essential oil, specifically, Rosemary oil (Ensencias Lozano, Spain). In order to observe the oil presence as a core material it was used a natural dye, Verde Cornasol C (Prochimac, Switzerland).

Calcium chloride (Sigma Aldrich, Spain) was used as cross-linking material [16] .

Experimental parameters were optimized in a previous research [17] [18] . They are showed in Table 1.

The alginate solution was prepared with 4% (w/w). The alginic acid sodium salt was mixed with distilled water and kept in the fridge during 24 h, in order to obtain the right viscosity and stabilize the emulsion.

Cross-linking agent, calcium chloride, was prepared at 0.5 M.

1% of dye solution in the rosemary oil was used in order to observe and control the encapsulated oil, by spectrophotometry.

Microcapsules were obtained by BUCHI B-390at room temperature. The internal nozzle diameter (core) was 0.2 mm and the external nozzle diameter (shell) was 0.4 mm. Potential and frequency values were 250 V and 350 Hz. Oil flow and alginate flow were controlled by injection pump (900 mL/h and 300 mL/h respectively), this difference is due to the solution viscosity.

Microcapsules were filtered and washed three times with distilled water in order to eliminate all the calcium chloride solution in the microcapsules wall (Figure 1 and Figure 2).

Microcapsules were dried at 60˚C during 2 hours. This temperature was selected due to the alginate gels formed at temperatures between 0˚C to 100˚C, and because they do not melt when heated [19] .

Microcapsules morphology was studied in an optic microscope AM4115ZT Dino-Lite Edge (Dino-Lite Digital Microscope, Taiwan) with a Dino-Capture Software.

The experiments were done with dry and non-dry microcapsules. They were repeated four times to compare the results between different microcapsules.

The microcapsules particle size distribution was measured by a Coulter^® Counter apparatus (Multisizer 3, Coulter Electronics, Northwell, UK). The particle size was expressed as the equivalent volume diameter and three replicates were performed for each batch of microcapsule. To reduce error, an average curve was calculated and analyzed.

Samples were diluted 1:20 in an Isoton II Solution (Akralab, Spain) up to 10 mL. Measures were performed at room temperature.

Differential scanning calorimetry Mettler-Toledo 821 (Mettler-Toledo Inc., Switzerland) was used to determinate the glass transition temperatures (Tg) of the dried microcapsules. Approximately 10 mg of sample (dry microcapsules) was prepared in aluminum pans in air atmosphere. The curves were obtained according to the following heating program: to 0˚C and 220˚C at 5˚C/min.

Termogravimetry (TGA/SDTA 851, Mettler-Toledo Inc., Switzerland) was used to determine the stability of the microcapsules in high temperatures. Approximately 70 μl of sample (dry microcapsules) was prepared in aluminum pans. The thermal program was located between 0 and 220˚C at 10˚C/min under nitrogen gas with a flow rate of 60 ml∙min⁻¹.

Control release was made with a spectrophotometric technic UV (Evolution 60S, Thermo Scientific, USA). It was necessary to use a dye in this technique due to the fact that rosemary oil is colorless.

The experiments were made with 0.5 gr of non-dry and dry microcapsules. They were repeated three times. To release the oil, it was put the same quantity of microcapsules and ethanol, and then sonicated for 10 minutes.

Previously, a calibration curve at 640 nm was made and the oil content was calculated by Lambert-beer method.

Rosemary essential oil and the microcapsules were tested to determinate antifungal/antibacterial activity, based on Standard ASTM E 2149-13. The contact time of the material with fungi was 24 h and the culture medium used was Sabouraud Dextrosa Agar (Sharlab, Spain). The contact time of the material with bacteria was 24 h and the Culture medium used was plate Count Agar (Sharlab, Spain). The Incubation temperature was 35˚C ± 2˚C.

Experiments were performed with dry microcapsules.

A Trace 1300 GC gas chromatograph equipped with a programmed split/splitless injector, an AI 1310 automatic sampler, and coupled to a single quadrupole mass detector (ISQ LT) was used to perform the GC-MS analysis (all from Thermo Scientific, San José, CA, USA). A Zebron ZB-5MS (30 m × 0.25 mm i.d., 0.25 μm thickness) column was used (Phenomenex Torrance, CA, USA). Helium was used as carrier gas, at a flow rate of 1 mL/min.

The injector temperature was 250˚C. Samples were injected in the split mode (1:30). The injection volume was 1 μl. The oven temperature was programmed as follows: 40˚C for 2 min, then 310˚C at 7˚C/min, maintained for 5 min.

The results obtained show that non-dry microcapsules were not completely spherical. The core material has a spherical shape and the oil with dye is inside the wall, Figure 3.

The results of the morphology study are shown in Table 2.

Due to the core morphology, dry microcapsules presented more spherical shape than non-dry microcapsules.

Non-dry microcapsules area was bigger than the of the dry microcapsules. This is due to water loss in the drying process.

Certain differences can be observed between size microcapsules obtained (Table 2) and the external nozzle diameter (0.4 mm). This is due to equipment parameters like vibration frequency, flow rate, electrode tension,

separation distance of the nozzle to the bath, the effect of gravity, the surface tension of the inducing gelation solution, the gel time, alginate viscosity, etc., all of them have influence in the microcapsules sizes [20] .

Figure 4 shows the particle size distribution of non-dry and dry microcapsules, certain differences can be observed, and also that these results meet with the morphology results.

Non-dry microcapsules contain water, so their size is bigger than the one of dry microcapsules. Otherwise, dry microcapsules, having a smaller size, are present in a higher number.

The particle size is smaller than expected (external nozzle diameter was used 0.4 mm), this is due to the test used an isotonic solution with ions. It is possible that these ions replace the sodium/calcium ions in the alginate polymer.

Alginate microcapsules were obtained because sodium alginate needs to come into contact with divalent ions such as calcium (Ca²⁺) to form the gel. As soon as sodium alginate is added to a solution of calcium chloride, a gel forms as the sodium ions (Na⁺) are exchanged with calcium ions. Depending on the concentration of calciumions, the gels are either thermoreversible (low concentrations) or not (high concentrations). In this case, the gel (shell of microcapsule) is reversible and the ions present in an isotonic solution replace of calcium ions and the gel structure is affected [21] .

Figure 5 shows a differential scanning calorimetric diagram.

Rosemary oil curve shows that at 130˚C, it decomposes. This is an important value to consider in the drying process.

Alginate, wall material and microcapsules present a step at 80˚C, this corroborates that the drying process used is the correct.

Figure 6 shows three curves corresponding to the rosemary oil, wall material and microcapsules.

Rosemary oil curve shows that at temperatures above 130˚C it decomposes.

Wall material and alginatecurve presents two steps. The first one, until 100˚C is caused by the external part moisture and the second one, 100˚C - 180˚C, is due to the water present within the microcapsules.

As a result of the delivery assays, it was observed that oil quantity in non-dry and dry microcapsules is the same. Thus corroborating that, the drying process used is correct and therefore no loss of oil in it (Table 3 and Table 4).

Rosemary essential oil was tested as an antimicrobial and antifungal. It presented an excellent reduction at the bacteria and fungi test (Table 5).

Figure 7 shows the contents of the five major components that have been identified in the samples of rosemary essential oil. α-pinene, β-pinene, cineole, camphor and cariophyllene. All of them areterpenes. Scientific literature presents the same major components (cineole, camphor, and α-pinene), which are present also in this study [22] . Some terpenes like pinene and camphor have antimicrobial properties [23] . Also, some of the main compounds found (cineole and camphor) in the rosemary essential oil have antifungal properties [24] .