Potential harmful of extracellular proteases secreted by Pseudomonas fluorescens W3 on milk quality

Abstract

Pseudomonas fluorescens is one of the main psychrotrophic bacteria that endangers milk quality because they can secret heat-resistant extracellular enzymes.

The effect of extracellular protease secreted by Pseudomonas fluorescens W3 on the quality of UHT milk

“…The molecular weight of protease AprX was usually between 40 and 50 kDa (Zarei et al, 2020). In our previous study, the protease AprX was identified from the extracellular secretions of P. fluorescens W3 by proteomics (Wang et al, 2021), and this also proved that the heat-resistant protease secreted by P. fluorescens W3 was AprX. In addition to the protease AprX, we have also identified a variety of proteases, but these enzymes did not show activity on the zymography after heating, indicating that they have no heat resistance or poor heat resistance.…”

“…2.1 | Bacterial and culture conditions P. fluorescens W3 was a predominant AprX secretion psychrophilic bacteria isolated from raw milk in Heilongjiang Province of China (Wang et al, 2021), and stored in the Food Science and Engineering Laboratory of Harbin Institute of Technology at −80°C. Before use, the strain needed to be inoculated into skim milk medium at 2% (v/v) inoculation volume, and cultured at 30°C, 150 rpm for 24 h for activation.…”

“…Heat sterilization can effectively kill microorganisms including psychrotrophic bacteria, but some heat-resistant enzymes including protease and lipase secreted by psychrotrophic bacteria remain partly active after sterilization (Salgado et al, 2020;Stoeckel et al, 2016;Xin et al, 2017). Although pasteurization cannot completely inactivate psychrotrophic bacteria, pasteurized dairy products have a shorter shelf-life and require low-temperature storage, so the residual psychrotrophic bacteria and protease are less harmful to pasteurized milk (Wang et al, 2021). However, these thermostable protease and lipase secreted by psychrotrophic bacteria caused adverse effects such as gelation, sediment, peculiar smell, and fat floating in other dairy products (D'Incecco et al, 2019).Psychrophilic bacteria is a general term for a class of bacteria that can grow under low temperature (−15~20°C).…”

Proteolytic Traits of Psychrotrophic Bacteria Potentially Causative of Sterilized Milk Instability: Genotypic, Phenotypic and Peptidomic Insight

“…Moreover, the temperature-dependent production of heat-stable proteases was demonstrated for Pseudomonas spp. [32]; recently, Wang et al reported that the AprX-producing P. fluorescens W3 had a lower ability to secrete extracellular proteases at 4 • C than at 30 • C [33]. The same authors observed that P. fluorescens W3 grew slower and showed lower proteolytic activity under low temperature conditions.…”

“…Different studies highlighted that UHT milk gelation is strictly related to psychrotrophic bacteria content in starting milk, since maximum thermostable protease expression is achieved when the psychrotrophic strains reach the late exponential phase of growth (10 7 -10 8 CFU/mL) [3,19,37]. In this regard, Alves et al and Wang et al reported that P. fluorescens W3 and P. fluorescens 07A expressed higher level of (thermostable) proteases at refrigerated conditions to compensate for the decreased enzymes' activity [33,38]. This could explain why the growth of psychrotrophic bacteria in milk kept at low temperature for a long time can be detrimental for the physical stability of the derived sterilized milk, but not for the starting refrigerated raw milk.…”