Protease-Producing Pseudomonas in Cold-Stored Milk: Mechanisms of Quality Deterioration: A Review

Abstract

This review article addresses a major problem in the dairy industry, namely the role that protease-producing bacteria, Pseudomonas, play in the deterioration of the quality of cold-stored milk. Although cooling slows the growth of most bacteria, it creates an ideal environment for the proliferation of psychrotrophic bacteria, such as Pseudomonas. These bacteria are widespread in dairy environments and can contaminate milk after pasteurization. The main problem is the ability of Pseudomonas to produce heat-resistant protease (proteolytic) and lipase (lipid-lyzing) enzymes, which remain active even after pasteurization. These enzymes target and break down essential milk proteins (such as casein), resulting in: Changes in texture: gelation, increased viscosity and undesirable texture, changes in flavor: release of bitter peptides due to casein degradation, free fatty acids that cause rancid or soapy flavors due to lipolysis, synergistic effects: Proteolysis and lipoprotein work together to accelerate the degradation process and produce unwanted flavor compounds at a faster pace.

The article reviews in detail the protease systems in Pseudomonas, which include mineral enzymes (e.g. AprX) and serine enzymes, and how their production is regulated at the genetic and environmental level (e.g., temperature, pH, quorum sensing). It also discusses the impact of storage conditions, especially temperature fluctuations and oxygen availability, on bacterial growth and enzymatic activity.

In terms of detection and monitoring, the article points out that traditional methods (microbial transplantation) are slow and impractical for routine monitoring. Therefore, biochemical methods (e.g., enzymatic activity assays) and molecular methods (e.g., PCR and metagenomics techniques) are being used for rapid and accurate diagnosis of degrading potential.