3.3.3. Temperature Stability

Bioactive-component-loaded carrier systems for food application might be exposed to different temperature treatments during the production cycles. Figure 7 shows the effect of temperature on the size distribution and ζ-potential of quercetin-loaded HZ, HZ-chi and HZ-chi/pec particles. Quercetin-loaded HZ particles had ζ-potential values between +41–+48 mV (Figure 7B). Size of quercetin-loaded HZ particles increased as temperature increased and showed two peaks above 40 ◦C (Figure 7A), possibly attributed to rearrangement of zein molecules and exposure of nonpolar groups followed by a collapse of hollow structure [55]. This is different from solid zein particles, those were colloidally stable when heated at pH 4 and 80 ◦C for 120 min [43]. Likewise, Figure 7A shows that size of quercetin-loaded HZ-chi particles increased with increasing temperature, but two peaks were observed above 70 ◦C, suggesting that the chitosan coating improves the particle stability against heat treatment. Their ζ-potential was +32 mV at 30 ◦C and increased to a range of +45 and +48 mV at 40–80 ◦C and to +56 mV at 90 ◦C. The changes might be due to that the realignment of zein and chitosan at higher temperatures possibly leads to inter/intramolecular interactions [5,56]. The quercetin-loaded HZ-chi/pec particles were stable when the temperature was increased to 40 ◦C (Figure 7A). Then, their size gradually increased upon further increase in temperature and showed two peaks at 90 ◦C. Their ζ-potential was kept between −25 and −32 mV at all the investigated temperatures. These results indicate that the pectin coating further improves the colloidal stability of quercetin-loaded HZ-chi particles. It is presumed that steric stabilization of pectin coating inhibits the collision of zein particles with more exposed reactive functional groups at the higher temperature, thus preventing increment in size and particle aggregation [57].

**Figure 7.** Effect of temperature on size distribution qHZ (blue), qHZ−chi (red) and qHZ−chi/pec (black) (**A**) and ζ−potential (**B**) of quercetin−loaded composite hollow zein particles.
