What is the method used for testing maturity of some citrus fruits?

The method used for testing maturity of some citrus fruits is often a refractometer. This instrument measures the sugar content in the fruit juice, which correlates to the fruit's maturity and flavor. As citrus fruits mature, their sugar content increases, and using a refractometer allows growers and producers to determine if the fruit has reached the optimal sweetness for harvesting.

A hydrometer primarily measures the specific gravity of liquids, which isn't as directly related to fruit maturity. A pycnometer is used for measuring the density of liquids but is not typically applied for assessing fruit maturity. A pH meter measures acidity or alkalinity, which can provide some insights into fruit quality but does not directly indicate maturity in the way that sugar levels do.