There's A Good Reason Why McDonald's Can't Call Its Shakes 'Milkshakes' - Tasting Table
Briefly

Milkshakes, a beloved treat on diner menus, are given a creative twist by McDonald’s, which omits 'milk' from 'milkshake' to comply with differing state dairy regulations. This decision stems from the brand's use of a reduced-fat soft serve that may not meet traditional milkshake standards across all states. For example, in Massachusetts, a milkshake cannot include cream and is instead called a 'frappe,' highlighting the varying criteria. Therefore, McDonald's shakes utilize alternative blends and ingredients, distancing themselves from customary definitions of milkshakes.
McDonald's deliberately avoids the term 'milkshake' to sidestep varying state regulations, leading to inconsistencies in what qualifies as a milkshake across states.
In Massachusetts, traditional milkshakes are classified as 'frappes' since they can't contain any cream, illustrating the complexity of dairy classifications in different states.
Read at Tasting Table
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