The Use of MALDI-TOF MS for Microbial Identification of Discolored Vacuum-Packaged Beef

Vacuum-packaging technologies have been developed over recent decades to extend the shelf-life of beef and to provide consumers with standardized, high-quality products. However, muscles stored under vacuum can still undergo discoloration, likely when animals are exposed to adverse pre-mortem weather conditions (high air humidity and/or large temperature fluctuations). Even if microbial activity is assumed to play a role in discoloration processes, relationships between bacterial populations and beef discoloration remain largely unexplored. This manuscript aims to identify potential microbiological germs related to discoloration in vacuum-packaged beef. To assess the microbiological basis of beef discoloration, total viable and anaerobic counts were quantified and bacterial identification using MALDI-TOF MS was performed in both discolored (n = 12) and control (n = 12) meat groups. The samples were collected at approximately 5-8 days post-mortem from the storage facility of a German slaughterhouse. Unexpectedly, the pH values in the control samples were consistently higher than those in the discolored samples, however a significant color difference was found (p < 0.05). Discolored muscles appeared to have higher lightness (L*) values, but had lower redness (a*) and overall color intensity (h_a/b) compared to the control samples, which can also be associated to higher superficial metmyoglobin formation. Principal component analysis (PCA) and network plot were performed using eight different variables of the data matrix. Both statistical tools confirmed that there were differences between discolored and control groups when measured at both test points. It seemed that the microbial activity in both groups of samples could not entirely explain the discoloration processes happening during beef aging in vacuum-packages.