Mohd Rozlan, N. A., and N. F. Che Pa. “A Systematic Review: Optimization of Extracts Psidium Guajava L. By Using Ultrasound-Assisted With Soxhlet Extraction in Application of Food Preservation”. Progress in Engineering Application and Technology, vol. 2, no. 1, June 2021, pp. 12-26, https://publisher.uthm.edu.my/periodicals/index.php/peat/article/view/864.