The FDA is warning pasta and pizza lovers that cheese labeled “100 percent Parmesan” are often filled with cheese substitutes—like wood pulp.

Yes, you’ve been eating wood, thanks to companies like Castle Cheese, which produced Parmesan cheese containing no actual Parmesan for almost 30 years. The president of the company, which supplied megastores like Target, is scheduled to plead guilty this month to charges that carry a sentence of up to a year in prison and a $100,o00 fine, according to Bloomberg.

Neil Schuman—who runs Arthur Shcuman Inc. the largest seller of hard Italian cheese in the U.S.—estimates a whopping 20 percent of such cheese are mislabeled.

Bloomberg ran experiments on store-bought grated cheese to test for wood pulp content in a lab and found that many cheeses have cellulose, which is made from wood pulp. Essential Everyday 100% Grated Parmesan Cheese, from Jewel-Osco, contained 8.8 percent cellulose. Wal-Mart Stores Inc.’s Great Value 100% Grated Parmesan Cheese had 7.8 percent cellulose. Whole Foods 365 brand didn’t list cellulose as an ingredient, and tested at 0.3 percent. Kraft had 3.8 percent cellulose.

Representatives at these stores said they were “investigating” these findings.

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Write to Eliana Dockterman at eliana.dockterman@time.com.