Production of functional food ingredients from coconut meal by stepwise subcritical fluid treatment

Abstract

Green process to obtain functional components from agricultural and food industry by-products is one of the current global trends. This study aimed to investigate a stepwise approach in subcritical fluid treatment of coconut meal, a by-product from coconut milk process, for producing potential functional ingredients. Firstly, an established supercritical CO2 extraction of coconut meal was investigated. The operational parameters, i.e., pressure, temperature, CO2 flow rate, and particle size affected the oil yields, fatty acid composition, medium-chain fatty acids, total phenolic content, and antioxidant activity. The highest oil yield (24.04% wb, 99.30% recovery) was obtained under moderate conditions. The oil from supercritical CO2 extraction with large particles exhibited the highest medium-chain fatty acids (79.11% of total fatty acid content). Total phenolic content and antioxidant activity increased with extraction pressure. Secondly, subcritical ethanol extraction of coconut meal was investigated as a novel method for oil extraction. The highest oil recovery (81.49%) occurred with 100% ethanol and an 8:1 solvent-solid ratio at 100 °C for 45 min, comparable to results at 120 °C. The extracted oil met standard values for crude coconut oil in terms of chemical properties. The subcritical ethanol-extracted oils exhibited higher total phenolic content and antioxidant activity compared to the Soxhlet hexane-extracted oil. Finally, the subcritical ethanol-defatted coconut meal was used as a raw material for subcritical water hydrolysis, followed by ethanol precipitation. The obtained ethanol-insoluble precipitates contained mainly carbohydrates (89.09–98.88%) with small amounts of protein and 5-hydroxymethyl-2-furaldehyde. The molecular weights of the final product ranged from 0.83 to 34.70 kDa. The highest yield of the final product was from the treatment at 220 °C (10.76 g/100 g defatted coconut meal), while the treatment at 230 °C possessed the highest content of mannooligosaccharides with a degree of polymerization of 2–6 (46.12%). The obtained carbohydrates showed antioxidant capacities and enhanced in vitro growth of lactobacilli and bifidobacteria. Acetic and propionic acids were the major short-chain fatty acids formed by fermentation. This study demonstrated that these eco-friendly supercritical and subcritical fluid treatment can be applied individually or as a stepwise method to obtain functional ingredients from coconut meal including coconut oil with high medium-chain fatty acids and mannooligosaccharides with prebiotic properties, which can be used in food products.

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