3. Results and discussion
3.1. Onion addition increases total soluble solid of apple juice
Generally, apple juice is expected to have proper total soluble solid. To examine the effect of onion addition on total soluble solid of apple juice, we measured Brix values using a hand refractometer. Heated apple juice with different concentrations (0.5–2.5%, v/v) of onion solution showed a dose-dependent increase in Brix value (Fig. 1). The Brix value also increased in unheated apple juice supplemented with onion at higher concentrations (1.25–2.5%, v/v) (Fig. 1). However, heated apple juice without onion did not show an increased Brix value compared to unheated juice, suggesting
that onion addition was responsible for the increase in Brix value instead of heating. Therefore, onion supplementation can increase total soluble solid of apple juice, and heated onion may be more effective.