Why does tooth decay start when the pH of mouth is lower than 5.5?
Actually the pH of mouth comes down due to presence of bacteria which secrete acids which causes that yellow plaque(tartar) initially.
Due to these acids being secreted the enamel of your tooth will slowly start decomposing and eventually your tooth decay symptoms will start.
This is why the dentist-supervised home-bleaching products had a mean pH of 6.48,while the over-the-counter whitening products had a mean pH of 8.22.(The high pH is to neutralize the damage caused to your mouth by those acids and also to kill the bacteria).
When the ph is less than 5.5 then the bacteria which is formed due to the dipositon of sugar in our mouth releases acids wich causes the enamel to corrode. This is how the tooth decays.