The researchers said the trigger for the study published in Food Control was the fact there has been minimal research conducted previously on the utilization of antimicrobial films for the inhibition of undesirable microorganisms in cheese during storage.
Cheese is s ready-to-eat products considered potentially risky in terms of contamination by pathogens.
Indeed, Listeria contamination of Prolactal cheese was linked to the death of seven people in Austria and Germany earlier this year, with the cheese manufacturer subsequently concluding that human error and failure to adhere to the company control system were most likely the causes of the contamination.
The French researchers said that that as Listeria can survive under the conditions of cheese making and is able to grow at refrigeration temperatures evaluation of methods to hinder bacterial growth during storage is critical.
Nisin, they added, is a peptide bacteriocin produced by certain strains of Lactococcus lactis, and is the only bacteriocin recognized as safe by the World Health Organization (WHO) for use within food production, and the authors said that it exhibits antimicrobial activity toward a wide range of Gram-positive bacteria,
The authors said that nisin was incorporated into sorbitol-plasticized sodium caseinate films at 1000 IU/cm2 and the films were prepared by casting methods.
Mini red Babybel cheese was chosen as a semi-soft cheese model, said the researchers.
The antimicrobial activity evaluation was based on the contact between antimicrobial films and surface-contaminated as well as in-depth contaminated cheese samples during a seven day storage period at temperatures of 4°C, they added.
The active films, said the researchers, resulted in a reduction of 1.1 log CFU/g reduction in L. innocua counts in surface-inoculated cheese samples after 1 week of storage as compared to control samples.
Regarding in-depth inoculated cheese samples, antimicrobial effectiveness was found to be dependent on the distance from the contact surface with the films containing nisin to the cheese matrix, they found.
The researchers explained that inactivation rates obtained were 1.1, 0.9 and 0.25 log CFU/g with slice 1 (distance from contact surface d = 1 mm), slice 2 (d = 2 mm) and slice 3 (d = 3 mm), respectively.
They claim that these results show the potential application of these antimicrobial films as a promising method to overcome problems associated with post-process contamination, thus enhancing the safety and extending the shelf life of food products.
Further research, said the authors, might focus on producing active films with higher nisin concentration and also evaluate the effectiveness of the antimicrobial films for a longer storage period to determine the maximum food shelf life extension as well as the efficiency against other undesirable microorganisms such as mould and yeast.
Source: Food Control
Published online ahead of print: doi: 10.1016/j.foodcont.2010.02.004
Title: Importance and efficiency of in-depth antimicrobial activity for the control of listeria development with nisin-incorporated sodium caseinate films
Authors: L Cao-Hoang, L Grégoire, A Chaine, Y Waché