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J. Cosmet. Sci., 71, 411–424 (November/December 2020) 411 Determination of Required HLB Values for Citrus unshiu Fruit Oil, Citrus unshiu Peel Oil, Horse Fat and Camellia japonica Seed Oil YU-YEON LEE and KYUNG-SUP YOON , Department of Chemistry and Cosmetics, Jeju National University, Jeju-do 63243 , Korea (Y.-Y.L., K.-S.Y.) Accepted for publication August 5, 2020 . Synopsis In the present study, the required hydrophilic lipophilic balance (HLB) values of Citrus unshiu fruit oil (CUFO), Citrus unshiu peel oil (CUPO), horse fat (HF), and Camellia japonica seed oil were determined empirically by preparing oil-in-water (O/W) emulsions. Lipophilic and hydrophilic surfactants were prepared in various ratios in O /W emulsion. The droplet size of the emulsion was measured using a particle size analyzer, and the turbidity was measured using a turbidity meter and a ultraviolet (UV)-vis spectrophotometer. According to the Orafi diya– Oladimeji method, the HLB value of the emulsion having the minimum dispersion ratio, the minimum droplet size, and the maximum turbidity degree was determined as the required HLB value for each essential oil. Based on these methods, the required HLB values of CUFO, CUPO, HF, and Camellia japonica seed oil were determined as 14.75–14.90, 15.35–15.40, 6.30–7.06, and 5.94–6.30, respectively. INTRODUCTION Essential oils are commonly used as preservatives and fragrances in cosmetics. However, in recent years, the medical values of essential oils such as antimicrobial properties, and their other various applications, for example, as food additives, organic pesticides, and natural insecticides are being actively explored. The oil-in-water (O/W) emulsion is one of the preferred techniques to minimize evaporation of the volatile essential oils, and thereby increase their shelf life (1–4). However, the O /W emulsion is thermodynamically unstable and can lead to various types of phase separations. These phase separation phe- nomenon can pose a problem to the functionality and stability of the active ingredient in the product, so the emulsifi cation technology in cosmetics is very important (5–7). To produce a stable emulsion, emulsifi ers or combinations of emulsifi ers are used with the required hydrophilic lipophilic balance (HLB) values depending on the oil phase. Optimal stability is achieved when the HLB value of the surfactant mixture is close to that of the required oil (8–13). Required HLB means that when two or more emulsifi ers Address all correspondence to Kyung-Sup Yoon at ksyoonjh@jejunu.ac.kr.