According to the Code of Federal Regulations, the maximum residual ozone level is 0.4 mg/l when water is bottled (FDA, 2012). However, there are no official regulations regarding a measurement method for residual ozone in liquid foods such as apple juice. This may be due to
the rapid decomposition property of ozone. Therefore, we analyzed residual ozone by the indigo method which is commonly used to determine the concentration of ozone in water. After ozone treatment, residual ozone concentration in all samples decreased to below 0.4 mg/l when maintained at 4 °C for 6 h. We also confirmed that residual ozone increased as the treatment temperature decreased, correlating with the result of a previous study (Achen and Yousef, 2001). They reported that the concentration of residual ozone was greatest at 4 °C,
not at 22 or 45 °C.