**5. Spoilage features of psychrotrophs**

 The reputation of psychrotrophs as key spoiling bacteria in cold-stored raw milk is extensively documented, due to their production of various enzymes, which can degrade the major milk constituents. The heat treatments, such as pasteurisation or UHT, which target bacterial populations do not affect much the hydrolytic enzymes, characterised by remarkable heat stability. Recently, the heat stability of proteases, lipases and phospholipases from selected raw milk isolates was determined in one study: after 142°C for 4 s, *Acinetobacter* frequently showed remaining lipase and phospholipase activities, whereas *Pseudomonas* exhibited highest protease activities [35].

Consequently, the spoilage is not limited to raw milk but occurs also at the level of milk-derived products: various defects or technological failures were linked to enzymatic activities [36]. Psychrotrophs and their enzymes significantly impact the quality of dairy products, which implies economic consequences.

If proteases and lipases received major attention, less studies describe other enzymatic types; however, the production of phospholipase C (PLC) was evidenced for key spoiling genera such as *Pseudomonas* and *Bacillus*. PLC causes the disruption of the milk fat globule membrane (MFGM) and is also described as a heat stable enzyme [35–37]. In our recent investigations concerning phospholipids, by a lipidomics-based approach, we also observed that PLC production was a common feature of psychrotrophic bacteria; but, consequent to bacterial growth in raw milk during its cold storage, we also evidenced the presence of various types of bacterial phospholipases that promoted hydrolysis of phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine species, together with an increase of phosphatidic acid; the changes imply the implication of phospholipases C, A, D and sphingomyelinase C activities, and show that phospholipolysis in raw milk involves many enzymatic types [38]. The analyses also revealed the presence of various lysophospholipids (LPLs) (resulting from PLA activity) in cold-stored raw milk: the fact that numerous reports described multiple physiological or pathophysiological roles of LPLs [39, 40] calls for further investigations on the significance of LPLs in cold-stored raw milk.

Like for the producing bacteria, only considered as problematic when associated to technological failures, the enzymes synthesised by these bacteria are also mainly considered under a technological point of view and to a lesser extent on their eventual impact on human health.

## **6. Antibiotic resistance of psychrotrophs**

Earlier observations showed that bacterial isolates, retrieved from raw milk samples that apparently spent a longer time in cold storage, also exhibited higher antibiotic resistance (AR) or multi AR features [41]. We also observed that psychrotrophic bacterial populations are more risky in terms of AR compared to their corresponding mesophiles [30].

Recently, a study that evaluated the efficiency of the activated lactoperoxidase system (LPS) and N2 gas flushing to hinder bacterial growth in raw milk showed that N2 seemed to favour a more diverse bacterial community at 6°C, less heavily loaded with antibiotic multi-resistance features, compared to LPS [42].

*Quality and Safety of Bovine Raw Milk: Present Challenges and Technological Solutions DOI: http://dx.doi.org/10.5772/intechopen.83507* 

Numerous reports pointed *Pseudomonas* and *Acinetobacter* as key genera associated with raw milk and spoilage of dairy products [11, 24, 35, 43–45]. The WHO, which considers that AR is nowadays one of the highest threats to global health, to food security and development, has ranked the bacterial species *Pseudomonas aeruginosa* and *Acinetobacter baumannii* as critical priorities regarding AR [46].
