Effect of Modified Atmospheres on the Growth and Extracellular Enzyme Activities of Psychrotrophs in Raw Milk

Abstract

The conservation of food products within a controlled atmosphere is efficient in packaging. To extend the cold storage of raw milk, the effects of five gas atmospheres enriched with carbon dioxide and nitrogen were investigated. Treated and control milk were stored at 7 °C for 10 days and analyzed for microbial counts, pH, proteolysis and lipolysis. The addition of CO2, N2, or their mixture had a significant inhibitory effect on psychrotrophic growth. The generation times of these microorganisms were significantly longer in treated milk, particularly for yeasts where they amounted to 16.63 h. The maximum inhibition was observed when a gas mixture of 50 % CO2 and 50 % N2 was used. As a result, psychrotrophic growth was affected to 98 % whereas this inhibition did not exceed 78 % when CO2 and 41 % N2 were applied. Milk treatment under the conditions of 50 % CO2 and 50 % N2 gave significantly lower counts for all groups of psychrotrophs being more efficient against Enterobacteriaceae with 99.5 % of inhibition. Storage of raw milk under the tested atmospheres had a different effect on extracellular enzyme productions. Significant decreases in protease and lipase activities were observed during the storage at 7 °C. These enzyme activities were not detectable with pure CO2 and a 50 % CO2 and 50 % N2 mixture. N2 has shown to be the less efficient treatment against lipases (65 %) and proteases (95 %). With regard to growth, the course of the pH and the protease and lipase activities, the tested gas mixture of 50 % CO2 and 50 % N2 was more suitable for extending the shelf life of raw milk.