A process for stabilizing a liquid that contains a first type of polyphenol includes measuring the liquid's spectrum, comparing it to a reference spectrum, based on the comparison, estimating an amount of the first type of polyphenol, based on the amount, adding a specified quantity of an insoluble stabilizing agent to the liquid, feeding the stabilizing agent and the liquid to a filter unit, at the filter unit, separating the stabilizer from the liquid, and discharging the liquid. The specified quantity is based on the amount.
