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.

Potential of Nitrogen Gas (N2) Flushing to Extend the Shelf Life of Cold Stored Pasteurised Milk

“…The pH of the pasteurised milk was not affected by the treatments; the initial pH values ranged from 6.69 to 6.87. Following N 2 -flushing, the pH values were between 6.82 and 6.87, compared to 6.60 and 6.87 for the corresponding controls, which indicates that the pH of pasteurised milk is not altered following the continuous application of N 2 , as already found for raw milk [24,27]. …”

“…Numerous studies have reported the inhibition of bacterial growth following the addition of carbon dioxide to raw milk [20–24]. However, the treatments also promote the acidification of milk or the modification of its sensory properties.…”

“…However, the treatments also promote the acidification of milk or the modification of its sensory properties. Two studies investigated the use of nitrogen (N 2 ), considered an inert gas, to inhibit bacterial growth and the spoilage potential present in raw milk [24,25]. Both studies investigated a “closed system” with no possibility of gas exchange between the milk flasks and the environment.…”

Effect of nitrogen gas flushing treatments on total antioxidant capacity and ascorbic acid content in raw bovine milk during cold storage

“…Thus, controlling psychrotrophic bacterial growth during cold storage of raw milk is important for both -technological and human health aspects. Several studies have indicated that the combination of modified atmosphere treatments (based on the use of CO 2 , N 2 or mixtures of both) combined with cold storage may have a significant potential to extend shelf life of raw milk (Martin et al, 2003;Dechemi et al, 2005;Rajagopal et al, 2005). Previously, our research group showed that continuous N 2 gas flushing when applied in a so-called "open system" improved the microbiological quality and shelf life of raw and pasteurized milk samples during cold storage (Munsch-Alatossava et al, 2010a,b; Munsch-Alatossava et al, 2011; Munsch-Alatossava et al, 2013).…”

N2 Gas Flushing Limits the Rise of Antibiotic-Resistant Bacteria in Bovine Raw Milk during Cold Storage

“…Two studies examined the use of N 2 gas to prevent bacterial growth in raw milk that was kept in a “closed system” (Murray et al, 1983; Dechemi et al, 2005). By considering an “open system,” both culture-dependent investigations and DNA barcoding studies revealed that no pathogen, no spoilage bacteria or any anaerobe was clearly advantaged by the N 2 gas flushing treatment when it was applied to raw milk at the laboratory scale, despite the fact that 10 4 - fold lower bacterial counts differentiated the N 2 -flushed from non-flushed cold-stored raw milk samples (Munsch-Alatossava et al, 2010a,b; Gschwendtner et al, 2016).…”