3.2.2. Microwave assisted extraction
TPC increases with the use of microwave extraction in com-parison with CSE. Indeed, the MAE at a power of 100 W leads to a TPC of 23.40 ± 0.12 g GAE/kg whereas the control gives a TPC of 22.13 ± 0.36 g GAE/kg. These two extractions
occur at a temperature of 67◦C so we can conclude that there is an improvement of extraction yield by the use of microwaves. Indeed, microwave irradiation accelerates the rupture of cells by causing a sudden temperature rise and internal pressure increase in the plant cell walls (Inoue et al.,2010; Jawad and Langrish, 2012) which alters the physical properties of biological tissues and enhances its porosity.This allows better penetration of solvent through the matrix which facilitates the extraction of flavonoids (Kratchanovaet al., 2004; Mandal et al., 2007). Moreover, it appears that the increase of microwave power from 100 to 200 W leads to an increase of phenol content from 23.40 ± 0.12 to 26.88 ± 0.78 gGAE/kg of orange peel powder. However, above 200 W, the TPC decreases to reach 23.81 ± 0.41 g GAE/kg of orange peel pow-der at 400 W. This phenomenon could be explained by the fact that higher microwave power induces a higher temperature which improves the phenol extraction until a temperature level for which phenols were degraded. So, the microwave power to obtain the highest content of total phenol and total flavonoid was found to be 200 W (200 W correspond to 76◦C).This idea is confirmed by Hayat et al. (2010) who indicate that the increase of the microwave power increases the extraction yield of phenolic acids from mandarin peel up to 160 W, and then extraction yield decreases.