Today in 1994, US Patent 5334400 A was issued, an invention of Robert Purdham, assigned to Guinness Brewing Worldwide Limited, for his “Beverage Package.” Here’s the Abstract:

A sealed beverage package has a bottle 1, a neck 4 of which provides a narrow mouth 5. A hollow insert 7 is submerged in beverage 13 in the bottle. A headspace 14 in the bottle is at a pressure greater than atmospheric and when the package is opened to de-pressurise the headspace 14, gas and/or liquid under pressure in the insert 7 is ejected through an aperture 8 into the beverage to cause a head of froth to develop on the beverage 13. The insert 7 is retained in position by an open framework 9 of struts 10 which extend between the insert and a retaining ring 11 that frictionally engages in a bore 6 adjacent to the mouth 5 of the bottle. The insert 7 can be attached to the framework 9 or formed integral therewith.

It’s also described thusly:

The present invention relates to a beverage package and a method of forming such a package. More particularly it concerns a sealed package of the kind containing beverage having gas in solution and within which beverage is located a hollow insert containing gas under pressure which, upon opening of the package, causes a gas and/or liquid to be ejected from the insert by way of a restricted orifice to cause, or assist in, the formation of a head of froth on the beverage by the evolution of gas dissolved therein. The ejection of the gas and/or liquid from the hollow insert results from the opening of the package when a headspace of the beverage which, in the sealed package, is at a pressure greater than atmospheric rapidly reduces to atmospheric pressure so that a pressure differential is created which permits the gas and/or liquid under pressure greater than atmospheric in the insert to be ejected into the beverage in the container.