6:25 AM PT -- A rep for Nestle USA tells TMZ, "The allegations are baseless and we will vigorously defend ourselves. All Nestle products and labels comply with FDA and USDA regulations."

Lean Cuisine has always touted itself as the healthy alternative ... which is why some incredulous consumers have sued the diet giant, claiming its food fare is filled with preservatives.

Courtney Ross filed a class action suit against Nestle, which manufactures Lean Cuisine frozen dinners, claiming she went to a CVS in NYC and bought the Four Cheese Pizza, paying what she claims is a premium for a preservative-free meal.

Ross says to her chagrin, she checked the label and saw the pie contained citric acid, designed to preserve flavor and freshness. She says other companies fess up when they use citric acid, acknowledging it's a preservative. She mentions Hungry Man, Jimmy Dean and DiGiorno.

And Ross then unleashes a litany of Lean Cuisine meals which contains citric acid, including Asian-Style Pot Stickers, Shrimp Alfredo, Mushroom Mezzaluna Ravioli and Ranchero Braised Beef.