Serratia marcescens Causes the Brown Discoloration of Frozen Steamed Stuffed Buns during Resteaming

Brown discoloration was observed in the crust of commercial frozen steamed stuffed buns (FSSBs) during resteaming. Culture-dependent and culture-independent analyses demonstrated that Serratia marcescens, a prodigiosin-producing species, was more abundant in spoiled samples than in unspoiled samples. Inoculation of experimental FSSBs with S. marcescens isolated from spoiled FSSBs confirmed that this species causes brown discoloration of FSSBs during resteaming. S. marcescens formed prodigiosin only between 15 and 28 degrees C but brown discoloration appeared only upon resteaming after storage at 4 degrees C. High-performance liquid chromatography analyses revealed that prodigiosin was absent from yellow-brown FSSBs. The pigmentation observed during resteaming is thus likely attributable to the intermediate 2-methyl-3-amylpyrrole. These findings provide valuable insights into the microbial contamination of FSSBs and will facilitate the prevention of spoilage of FSSBs.