Inhibition of Morganella morganii Growth and Histamine Production by GRAS Products

Introduction: Histamine-producing bacteria (HPB; e.g., Morganella morganii [Mm]) are in saltwater/estuarine environments and on gills and within intestines of scombroid fish (e.g., tuna) where they can proliferate and produce histamine (Hst) over a wide temperature range.  Hst production can occur under moderate (e.g., 7.2°C) abuse conditions and in raw unfrozen fish.

Purpose: This study evaluated use of GRAS (generally recognized as safe) products to inhibit growth and Hst production by Mm on raw tuna at various storage temperatures.

Methods: Two GRAS products (2% and 4% DV™ [vinegar-based] and1% ProTek™ [NaCl:acetate:diacetate]) were evaluated using Bioscreen C (TSB + 0.1% yeast extract [YE], 30°C, 18 h) for effectiveness against pure cultures of Mm (3 strains; 5 log CFU/ml).  Subsequently, 25-g portions of raw tuna (9 replicates) were surface-inoculated with Mm strain 11 at 4 log CFU/g and treated with sterile deionized water (DI, control) and 15% DV™ solution (≤ 2% pick-up) using a prototype sprayer.  Mm numbers were determined by MPN real-time PCR immediately following treatment, weekly during cold storage (4°C, 3 weeks), and daily during temperature abuse (18°C, 3 days; 25°C, 2 days).  Hst (ppm) was determined by the modified AOAC 977.13 fluorometric method. 

Results: With Bioscreen C, the greatest reduction in Mm growth (turbidity), 90 - 107%, was produced by 4% DV™; growth was reduced 47 - 64% by 2% DV™ and 31 - 43% by 1% ProTek™.  Mm numbers on DV™- and DI-treated tuna did not increase during 3 weeks at 4°C.  Mm numbers on DV-treated fish were 1.6 - 3.6 and 1.3 - 3.5 log CFU/g lower than on DI-treated controls during the incubation periods at 18°C and 25°C, respectively (P < 0.001).  DV™ reduced Hst production by 300 ppm after 2 days at 25°C.

Significance: DV™ could be used as a control measure to restrict growth and Hst production by Mm during processing and storage of raw tuna.