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The ingredient that primarily contributes to the sour aspect of sour cocktails is fruit juice. Sour cocktails typically balance sweet and sour flavors, with the sourness often derived from citrus juices like lemon or lime. These juices contain organic acids, such as citric acid, which impart a fresh, tangy flavor that is essential to the character of sour cocktails. This sour component is crucial in creating harmony with the sweet elements, such as syrups or liqueurs, and it plays a vital role in enhancing the overall flavor profile of the drink.

While other ingredients may influence the cocktail's taste, they do not primarily provide the sourness that defines this style of drink. Syrups are used to add sweetness, herbs can contribute aromatic and flavor complexity, and carbonated water typically adds a refreshing effervescence but does not contribute to the sourness of the cocktail. Thus, fruit juice is the key element that delivers the critical sour characteristic in these beverages.