Sweet lime (Citrus limetta) peel waste drying approaches and effect on quality attributes, phytochemical and functional properties

Sweet lime (Citrus limetta) peels are a rich and valuable bioactive residue obtained from the citrus processing industry. The high moisture content of the peels can be reduced by drying and dried peels could be used as an ingredient to enhance the nutritional value of foods. In this study, different drying operations viz; infrared drying (IRD), vacuum drying (VD), tray drying (TD), hot-air multimode drying (HAD), and evacuated tube solar drying system without heat pipe (SD1) and with heat pipe (SD2) were explored for their effect on drying behavior and quality characters of sweet lime peels. Proximate, total phenol, total flavonoid, DPPH radical scavenging activity, color profile, and Fourier Transform Infrared (FTIR) spectroscopy of fresh and dried sweet lime peel were analyzed. The results showed that the time taken for IRD was the lowest followed by VD and HAD. The IRD was a suitable technique for retention of crude fibre, ash, and crude protein content. However, VD exhibited a higher L* value and a lower b* value which indicated good retention of color. The fresh peels exhibited total phenol, total flavonoid, and DPPH radical scavenging activity of 25.60 mg GAE/g, 18.85 mg QE/g, and 32.79%, respectively. Besides, peels dried by IRD, TD, & HAD exhibited a significant increase in antioxidants activity compared to fresh peels and those subjected to other drying methods. FTIR spectroscopy confirmed the presence of phenols in the dried peels. IRD of sweet lime peels could be an effective way to preserve the nutrients, phenols, flavonoids, and antioxidants.