Mayonnaise, to the average shopper, is a creamy, white spread for sandwiches, salads, and dips. But in the eyes of the arbiters at the U.S. Food and Drug Administration, mayonnaise is a food that must contain eggs yolks, among other specific ingredients. That’s where Unilever (ULVR:LN), parent company of Hellmann’s and Best Foods, finds issue with Hampton Creek’s fast-growing eggless, mayonnaise-like product dubbed Just Mayo. The food giant is accusing the San Francisco startup of misrepresenting its spread as mayonnaise.

Unilever has cause for concern as the second-biggest day of U.S. mayonnaise consumption—the day after Thanksgiving—is fast approaching. (The Fourth of July holds the top mayo spot.)

The mayonnaise business faces real challenges. Data from Euromonitor International show Hellmann’s and Best Foods’ dollar sales in the U.S. both dropped in 2013, and the researcher expects them to fall again this year. The company’s legal complaint blames Just Mayo for stealing market share. “As Hampton Creek’s distribution and advertising increases,” the filing warns, ”the irreparable harm to Unilever will continue and worsen.”

(Blommberg Business)