Ultrasound-Assisted Hot Water Extraction of Red Amaranth Leaves Extract and Application As a Natural Colorant in Glutinous Rice Balls

Lok, Agnes Ying Zhen (2026) Ultrasound-Assisted Hot Water Extraction of Red Amaranth Leaves Extract and Application As a Natural Colorant in Glutinous Rice Balls. Final Year Project (Bachelor), Tunku Abdul Rahman University of Management and Technology.

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Abstract

The rising demand for natural food colorants has highlighted red amaranth leaves as a promising source of betacyanins, natural pigments known for their vibrant colour and antioxidant properties. However, conventional hot water extraction is often limited by prolonged processing time and pigment degradation. This study aims to investigate the potential of ultrasound-assisted hot water extraction of red amaranth leaves extract on betacyanins recovery and to evaluate the application of the optimized extract as a natural colorant in glutinous rice balls. Red amaranth leaves powder (2.5%, w/w solution) was subjected to ultrasound pretreatment (5, 10, and 15 min) followed by hot water extraction at 50 ℃ (30, 60, and 90 min). Extracts were analyzed for color, ascorbic acid content, total betacyanins content (TBC), total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activities (ABTS, DPPH, and FRAP). The optimum condition of ultrasound-assisted hot water extraction was selected its application in glutinous rice balls. The red amaranth leaves extract was incorporated into glutinous rice balls and compared with a commercial red colorant during storage at -18 °C and 4 °C for three storage days (2, 5, and 10). Results showed that both ultrasound pretreatment and hot water extraction times significantly affected the properties of red amaranth leaves extract (p < 0.05). The optimal condition was achieved at 15 min ultrasound in combination with 60 min hot water extraction, producing red amaranth leaves extract with significantly highest TBC of 10.24 ± 0.24 mg/100 g and TFC of 963.99 ± 5.82 mg QE/100 g of TFC as well as 19.03 ± 0.12 of color L*, 37.93 ± 0.12 of color a*, 23.00 ± 0.10 of color b*, 26.47 ± 0.15 of color change, 199.53 ± 12.80 mg/100 g of ascorbic acid content, 663.68 ± 6.10 mg GAE/100 g of TPC, 1428.31 ± 6.15 mg TE/100 g of ABTS, 1507.55 ± 10.59 mg AAE/100 g of DPPH, and 1144.55 ± 3.25 mg FeSO2E/100 g of FRAP. The storage time and temperature significantly affected (p < 0.05) the stability of color and betacyanins of red amaranth leaves extract in glutinous rice balls. Glutinous rice balls colored with red amaranth leaves extract exhibited significantly higher (p < 0.05) retention of redness (71.14–92.03%) and TBC (39.07–81.72%) than those prepared with commercial red colorant, particular over 10 days at storage temperature of -18 °C. This study demonstrates that ultrasound-assisted hot water extraction is an effective method to improve the efficiency of betacyanin recovery from red amaranth leaves, and the optimized extract shows strong potential as a natural alternative to synthetic food dyes in food applications.