The U.S. Food and Drug Administration has launched a criminal investigation into a Pennsylvania cheese factory it says has put fillers such as wood pulp and cellulose into its grated parmesan cheese products that are distributed to some of the nation's largest grocery store chains.

The FDA said that it received a tip and then discovered evidence that Castle Cheese has provided cheese labeled as “100% grated Parmesan” that contained no Parmesan cheese for almost 30 years. The cheese was marketed under Target's Market Pantry brand and two other Associate Wholesale brands, Always Save and Best Choice. Testing showed that no Parmesan was used to manufacture these products; instead, the ingredients were a mixture of Swiss, mozzarella, white cheddar and cellulose.

Michelle Myrter, president of Castle Cheese, is expected to plead guilty to criminal fraud charges and faces up to a year in prison and a $100,000 fine.

Bloomberg News conducted its own investigation, purchasing grated Parmesan products and having then tested for adulterated content by an independent laboratory. In a report on its findings, Bloomberg said that Walmart's Great Value 100% Grated Parmesan Cheese tested at 7.8% cellulose; Jewel-Osco's Essential Everyday 100% Grated Parmesan Cheese registered 8.8% cellulose and Whole Foods' 365 brand tested at 0.3% Kraft tested at 3.8% cellulose.

According to the Center for Dairy Research, an acceptable level of cellulose in grated Parmesan cheese is between 2-4%. Bloomberg reported that hard cheeses like Parmesan are more susceptible to having fillers added since they are more expensive to produce.

Bloomberg says the FDA was tipped off by a former Castle Cheese plant manager who was fired by the company in 2012. Castle Cheese filed for bankruptcy in 2014.

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