The results showed that the yield of pectin increased with extraction time. The pectin yield was 9.21 g/100 g of peel d.m. at 30 min boiling and increased to 10.90 g per 100 g of peel d.m. at 60 min boiling. The same trend occurred in the study of Kulkarni and Vijayanand (2010), where the pectin yield was found to be 5.59 g/100 g of peel (d.m.) at 30 min boiling and 9.06 g/100 g of peel (d.m.) at 60 min boiling (the other conditions of extraction were temperature of 98.7 °C, pH 2.0 ± 0.02 and solid/liquid ratio of 1:30). Guo et al. (2012) found 15.45 g of pectin per 100 g of orange peel (d.m.) with an extraction time of 60 min, temperature of 80 ± 2 °C and solid/liquid ratio of 1:50.
The galacturonic acid content was higher than 65% d.b. for 30 min and 60 min of extraction, but pectin extracted for 30 min had higher galacturonic acid content (82.40 g per 100 g of pectin d.m.) compared to pectin extracted for 60 min (70.35 g per 100 g of pectin d.m.), which may be due to the partial degradation of pectin. In this case, it is possible to observe the depolymerization of pectin when the extraction time was 60 min, indicating that much time in high temperature can affect the structure of polysaccharide.
To investigate the change in the microstructure caused by the extraction process, Fig. 3 shows the scanning electron micrographs of the surface of pectin. Fig. 3A shows the micrographs for the extraction conditions of 85 °C, 30 min, pH 2.0 and 1:30 solid/liquid ratio, which were the conditions that promoted the highest galacturonic acid content. The micrographs showed parallel fibers with some damage in the fiber structure, which may have resulted from the extraction process (Fig. 3B will be discussed later).