The reason Costco's $4.99 rotisserie chicken tastes soapy is because it contains phosphate along with other ingredients which can cause a soapy taste in the chicken.
Although most people should not be able to taste the phosphate but some people are more sensitive to the phosphate than others and will taste the phosphate in the Costco's $4.99 rotisserie chicken.
The Costco's $4.99 rotisserie chicken does not taste soapy to everyone but when you're more sensitive to the phosphate you will taste a soapy taste to it which can also occur in other foods with phosphate as well.
For some palates, phosphates can taste strange and chemical-like, the same way cilantro tastes like soap to some people.
Although now, a new phenomenon is surrounding the rotisserie chickens that's even more alarming than a foul flavor.
Customers are reporting illness after eating the Costco chicken.
Chickens are injected with phosphate.
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid.
It most commonly means orthophosphate, a derivative of orthophosphoric acid, a.k.a. phosphoric acid H₃PO₄.