Pectin extraction from citrus waste: structural quality and yield with mineral and organic acids

Pectin is a renewable polysaccharide valued for its gelling, stabilising, and encapsulating properties, with broad applications in food, pharmaceutical, and industrial sectors. However, extraction conditions critically affect its yield, structural integrity, and functional performance. Despite citrus peel being a major source of pectin, large amounts remain underutilised as waste. This study systematically investigates how different acid types influence the extraction efficiency and structural quality of pectin derived from citrus peel. Dried citrus peel powder was extracted using four acids—sulphuric, hydrochloric, acetic, and citric—under controlled conditions at 80 °C. Extractions were performed at a fixed time of 90 min for all acids, with additional time trials for sulphuric acid. Extracted pectins were evaluated for gravimetric yield, colour, solubility, degree of esterification (DE) by titration and FTIR, and structural features using FTIR and 1H NMR spectroscopy. Results showed that sulphuric and hydrochloric acids yielded the highest pectin recoveries (30–35% and 20–25%, respectively) but caused significant degradation, evident from dark colour, broad FTIR peaks, low DE (<10%), and poor solubility. In contrast, acetic and citric acid extractions resulted in moderate yields (8–15%) but preserved the pectin backbone and maintained higher DE (>30%) compared to the mineral acid-extracted samples and the commercial low methoxyl (LM) standard, as confirmed by clear FTIR and NMR profiles. These findings demonstrate the trade-off between extraction yield and structural integrity, underscoring the potential of mild organic acids to produce high-quality pectin suitable for value-added applications. Optimising acid type and extraction conditions can support sustainable waste valorisation and expand the industrial use of citrus-derived pectin.