The pisco sour's initial recipe was that of a simple cocktail. According to Peruvian researcher Guillermo Toro-Lira, "it is assumed that it was a crude mix of pisco with lime juice and sugar, as was the whiskey sour of those days." As the cocktail's recipe continued to evolve, the bar's registry shows that customers commented on the continuously improving taste of the drink. The modern Peruvian version of the recipe was developed by Mario Bruiget, a Peruvian from Chincha Alta who worked under the apprenticeship of Morris starting on July 16, 1924. Bruiget's recipe added the Angostura bitters and egg whites to the mix. Journalist Erica Duecy writes that Bruiget's innovation added "a silky texture and frothy head" to the cocktail.