Up to 800 staff at 2 Sisters Food Group are staging the first of three 24-hour strikes.
Unite said workers at three sites in the West Midlands walked out in a dispute over pay and conditions.
Staff mainly deal with packing chicken, but also supply fish and baked and frozen products to shops including Tesco and Marks & Spencer.
The strike affects the firm’s plants in Smethwick, West Bromwich and Lincoln Street in Wolverhampton.
Unite’s regional officer Des Quinn said staff had voted overwhelmingly to take a stand against a “paltry” pay offer.
More strikes are planned for 18th and 21st December.