Copied with permission from https://www.arishtam.com/product-page/aristam-homebrewing-guide and https://www.amazon.in/Arishtam-HomeBrew-Probiotics-Indias-homebrew-ebook/dp/B07WSXSCQY
Human tongue is very susceptible to sourness and hence standardization of acidity is the first step towards standardization of process. A simple advice would be to use acid blend (mixture of citric acid, mallic acid and tartaric acid) to reduce the pH and add Calcium Carbonate (CaCO3) to increase it. Avoid sodium carbonate (baking/cooking soda), as it can give a slight salty taste.
However, few additional aspects need to be taken into account:
The fermented beverages are a blend of organic acids and long chain molecules. As a result, an ordinary diode based hand held pH meter will give erroneous readings. It is always good to measure Total Acidity (using titration method) along with a simple pH reading.
Carbonic Acid formed due to dissolved CO2 can also impact the reading. Hence, it is recommended to de-gas them before testing. Wines are usually filled using a vacuum bottle filler so that they are de-gassed and tastes less sour.
Litmus paper: Although it is easiest, I would not recommend. The pigments in the drink interferes with the readings. Also, color chart reading is often subjective and prone to a lot of errors. So, a calibrated digital pH meter is recommended.
Acidic medium is preferred by yeast and lacto bacillus as it retards the growth of molds and other harmful microbes. This is the reason why most artisans will measure the starting pH (acidity in wort and in the fresh must) as well.
In wines, the pigment color stability is linked with optimal pH.
Please calibrate the equipment and reagents before testing.