Mesophilic and psychrotrophic bacteria from meat and their spoilage potential in vitro and in beef

Abstract

Mesophilic and psychrotrophic populations from refrigerated meat were identified in this study, and the spoilage potential of microbial isolates in packaged beef was evaluated by analyzing the release of volatile organic compounds (VOC) by gas chromatography-mass spectrometry (GC/MS). Fifty mesophilic and twenty-nine psychrotrophic isolates were analyzed by random amplified polymorphic DNA-PCR, and representative strains were identified by 16S rRNA gene sequencing. Carnobacterium maltaromaticum and C. divergens were the species most frequently found in both mesophilic and psychrotrophic populations. Acinetobacter baumannii, Buttiauxella spp. and Serratia spp. were identified among the mesophilic isolates, while Pseudomonas spp. were commonly identified among the psychrotrophs. The isolates were further characterized for their growth at different temperatures and their proteolytic activity in vitro on meat proteins extracts at 7 degrees C. Selected proteolytic strains of Serratia proteamaculans, Pseudomonas fragi, and C. maltaromaticum were used to examine their spoilage potential in situ. Single strains of these species and mixtures of these strains were used to contaminate beef chops that were packed and stored at 7 degrees C. At time intervals up to 1 month, viable counts were determined, and VOC were identified by GC/MS. Generally, the VOC concentrations went to increase during the storage of the contaminated meats, and the profiles of the analyzed meat changed dramatically depending on the contaminating microbial species. About 100 volatiles were identified in the different contaminated samples. Among the detected volatiles, some specific molecules were identified only when the meat was contaminated by a specific microbial species. Compounds such as 2-ethyl-1-hexanol, 2-buten-1-ol, 2-hexyl-1-octanol, 2-nonanone, and 2-ethylhexanal were detectable only for C. maltaromaticum, which also produced the highest number of aldehydes, lactones, and sulfur compounds. The highest number of alcohols and ketons were detected in the headspace of meat samples contaminated by P. fragi, whereas the highest concentrations of some alcohols, such as 1-octen-3-ol, and some esters, such as isoamyl acetate, were produced by S. proteamaculans. In conclusion, different microbial species can contribute to meat spoilage with release of different volatile compounds that concur to the overall quality decrease of spoiling meat.

FIG. 1.

Total ion current chromatograms obtained for meat samples contaminated by S. proteamaculans M42 by HS-SPME-GC/MS analysis. (A) 42M, after 30 days of storage (t30); (B) 42M, t15; (C) 42M, t0.

FIG. 2.

Total ion current chromatograms obtained for Mix t30 (a), 42M after 30 days of storage (t30) (b), 9P t30 (c), and 25P t30 (d) samples by HS-SPME-GC/MS analysis. * and **, quantified volatile metabolites detected at 30 days for only one and two species, respectively, among the three considered.

FIG. 3.

Quantitative indications (mean values, μg/5 g of contaminated meat samples) of some representative volatile metabolites, including alcohols (1-octen-3-ol; 2-ethyl-1-hexanol, and 3-methyl-1-butanol) (A), aldehydes (hexanal, nonanal, and decanal) (B), and carbonilic compounds (isoamyl acetate, ethyl hexanoate, ethyl octanoate, ethyl decanoate, and 2-nonanone) (C).