A method of producing a frothed liquid, such as a milk shake or whipped cream is disclosed. The method involves the steps of filling a container (which may be of PET) to approximately one third capacity, then filling the remaining headspace with pressurised gas, for example at 120 psi, and sealing the container. Once the contents have reached equilibrium, the seal is breached to produce the frothed liquid. The method, and containers, disclosed have many significant advantages over known arrangements, in particular, there are significant cost savings associated with the method and containers of the present invention.