2.3. Measurement of total soluble solid
The brix of the apple juice supplemented with onion solution was measured by a hand refractometer (N-1E, Atago, Tokyo, Japan). A degree Brix is a unit of measurement for the total soluble solid of liquids. A liquid has one degree Brix (=1% Brix) if it has the same refractive index as a solution of 1 g of sucrose in 100 g of solution.