## 3 Results and Discussion
### Effect of Crosslinker Concentration  
In order for complex coaervate microcapsules to withstand the thermal and mechanical stress of spray drying, the capsule walls are commonly hardened using crosslinkers like transglutaminase [8, 27, 28], glutaraldehydehyde [14, 27, 29], tannic acid [10] or tripolynphosphate [29], or via simple addition of, for example, Maltodextrin [23], corn syrup [5] or even poly(ethylene glycol) [25] or non-ionic surfactants such as lutensol [9] prior to spray drying. Here, we report the effect of a crosslinker based on an oxidatively modified Zulkowsky type food starch. The Zulkowsky starch was oxidized according to a standard procedure using sodium meteperiodate [21]. This method results in the ring-opening of the glucose units within the starch between the C2 and C3 carbons, with concurrent formation of a dialdehyde, making it an ideal candidate for the crosslinking of protein domains. In order to assess the effect on complex coaervate capsule size, stability and microencapsulation yield, three different crosslinker concentrations were used; 2:5, 5 and 10 wt% with respect to BSA. As mentioned above, control samples without crosslinkers did not withstand spray drying, and so were excluded from further analysis. Optical microscopy images of complex coaervate microcapsules before (upper row) and after (bottom row) spray drying are shown in Figure 6, with corresponding size distributions as measured by laser diffraction shown in Figure 7. As discussed above, the general trend is that spray dry  
Figure 5: Microencapsulation yield of peppermint essential oil after spray drying for different Polymer:Oil ratios. Note that crosslinker concentration was kept constant at 5 wt% with respect to BSA. ing results in a narrowing of the microcapsule size distribution. Interestingly, while the size distributions for 2.5 and 5 wt% crosslinkers are almost identical, the microcapsules obtained with a 10 wt% crosslinker are significantly larger. Moreover, for the 10 wt% crosslinker, spray-dried microcapsules were found to aggregate significantly within 24 h after redesign in DI water, whereas no such dispersion instability was observed for the lower crosslinker concentrations. This indicates that above a threshold concentration, the crosslinker increases microcapsule stickiness. Since the crosslinker only acts on the protein component of the wall materials (BSA), one explanation could be that increasing crosslinker concentration could induce surface defects or even porosity, by selective contraction of the polypeptide domains. The effect of crosslinker concentration on microencapsulation yield is shown in Table 1 and Figure 8. Spray drying is often challenging for coacervates due to their typically soft-shell nature and the high shear forces experienced in the peristaltic feed pumps and the narrow nozzle, to minimize such problems we used a wider-than-standard tip for all the samples contained in our study (1.4 mm in place of 0.7 mm). To minimize the evaporation of the peppermint oil, inlet and outlet temperatures were kept as low as possible whilst still obtaining dry particles, as such, inlet temperature was held between 135-145 \({}^{\circ}\)C, resulting in outlet temperatures of 54-60 \({}^{\circ}\)C. The lowest crosslinker concentration used here--2.5 wt% with respect to BSA--resulted in the highest microencapsulation yield observed after spray drying, at 54%. [PERSON] et al. [23] have reported complex coacervation and subsequent spray drying of orange essential oil using a wall combination of whey protein isolate (WPI) and GA at WPI:GA = 1:2 and a Polymer:Oil ratio of 2:1, using Maltodextrin DE 10 (Maltodextrin:WPI = 2:1 _w:w_) as a hardener, giving a microencapsulation efficiency of 53%. Using soybean protein isolate (SPI) and GA as wall materials (SPI:GA = 1:1), a Protein:Oil ratio of 2:1 and Maltodextrin and PEG as additives, [PERSON] et al.