Evolution under different storage conditions of anomalous blue coloration of Mozzarella cheese intentionally contaminated with a pigment-producing strain of Pseudomonas fluorescens

Abstract

Several widespread occurrences of anomalous blue coloration of Mozzarella cheese have been recorded in the United States and some European countries. Official laboratory analysis and health authorities have linked the occurrences to contamination of the processing water with strains of Pseudomonas fluorescens, although several experts questioned how to unequivocally link the blue color to the presence of the microorganism. To establish a method to determine whether a given Pseudomonas spp. strain is responsible for the defect and study the evolution of the coloration under different storage conditions, we developed an in vitro system for the evaluation of blue coloration of Mozzarella cheese intentionally contaminated with strains of P. fluorescens. The purpose of the system was to determine whether P.fluorescens strains, isolated from Mozzarella cheese with anomalous blue coloration, were able to reproduce the blue coloration under controlled experimental conditions. Thirty-six trials of experimental inoculation of Mozzarella cheese in different preservation liquids were conducted using various suspensions of P.fluorescens (P. fluorescens ATCC 13525, P.fluorescens CFBP 3150, and P. fluorescens 349 field strain isolated from blue-colored Mozzarella cheese) at different concentrations and incubated at different temperatures. Growth curves of all tested P.fluorescens strains demonstrated that after 3 d of incubation the concentration was generally >10 6 cfu/g of Mozzarella cheese incubated in either tryptic soy broth (control) or conditioning brine. Prolonged incubation for 5 d at either 20°C or 8°C led to concentrations up to 10 9 cfu/g of Mozzarella cheese incubated in tryptic soy broth and up to 10 8 cfu/g of Mozzarella cheese incubated in preservation liquid. All Mozzarella cheeses inoculated with the field strain of P. fluorescens, except those opened 1 h after packaging and stored at 8°C, showed the characteristic anomalous blue coloration, which appeared from 1 to 72 h after opening the packaging, and was proportional to colony count, duration of storage, and storage temperature. With the proposed system, which enabled a larger number of samples to be analyzed under controlled experimental conditions and a large amount of data to be generated in a short time, we described precisely how and under which conditions the presence of P. fluorescens in Mozzarella cheese is responsible for the anomalous blue coloration. The system will help producers intercept contaminated batches and help consumers avoid the conditions under which the defect can appear.

Setup of a rapid method to distinguish among dead, alive, and viable but not cultivable cells of Pseudomonas spp. in mozzarella cheese

“…population is close to 8 log cfu/g of cheese. This result is in accordance with the previous studies (Sechi et al, 2013;Cenci-Goga et al, 2014) where the authors correlated the appearance of the anomalous blue coloration of mozzarella cheese to a specific P. fluorescens cell number, thus confirming the reliability of the method here described.…”

Pseudomonas azotoformans Belonging to Pseudomonas fluorescens Group as Causative Agent of Blue Coloration in Carcasses of Slaughterhouse Rabbits

“…In agreement with other studies [30], the isolation of P. fluorescens-related microorganisms from the water sample and from the labelling gun suggests the importance of good sanitization procedures. The abnormal coloration observed on the carcasses started from the label inoculation point, indicating that the carcass contamination was probably due to the labelling gun.…”

“…Accordingly, the labelling gun should be disinfected frequently during slaughtering operations, to improve gun cleaning and decontamination when labelling the carcasses. As previously suggested [30], the alteration induced by P. fluorescens was directly correlated to storage time in addition to environmental temperature. In the two butcher shops under investigation, there was a longer storage time in the cold rooms (of about five days) before sale than in the other butcher shops, where the carcasses were stored for less than 24/36 h. Moreover, bacterial load seems to play a role in the appearance time of the alteration.…”

Metabolomics and microbiological profile of Italian mozzarella cheese produced with buffalo and cow milk

“…In 2010, the Italian agricultural authorities found that some BM from an area south of Rome (Italy) had fallen below standards after traces of cow's milk were found; moreover, anomalous blue colouration of Mozzarella cheese contaminated with a pigment-producing strain of Pseudomonas fluorescens has been reported (Cenci-Goga et al, 2014 Many investigations have been carried out to characterise BM. They dealt with establishing methods to detect the addition of cow's milk to buffalo milk in mozzarella preparation (Addeo et al, 1995), and with characterising its microbiological profile (Coppola et al, 1988;Romano, Ricciardi, Salzano, & Suzzi, 2001;Ercolini et al, 2012).…”