Changes in phosphoglyceride composition during storage of ultrahigh-temperature milk, as assessed by 31P-nuclear magnetic resonance: possible involvement of thermoresistant microbial enzymes

Soluble phosphoglycerides were studied in ultrahigh-temperature (UHT) milk by 31P-nuclear magnetic resonance. It was shown that, during storage of UHT milk, manufactured from raw milk with poor microbial quality, glycerophosphocholine and glycerophosphoethanolamine disappeared in parallel with an increase in alpha-glycerophosphate (GP). Storage at 10, 20, and 30 degrees C showed a faster transformation as the temperature increased. UHT milk samples manufactured from raw milks with better microbial quality and submitted to severe heat processes did not display changes in phosphoglycerides during storage. Screening of commercial UHT milks showed variations regarding the presence of GP, while in pasteurized milk samples, the appearance of GP occurred when the commercial life had been exceeded. Inoculation of sterile milk with Pseudomonas fluorescens NCIB9046 and incubation at 10 degrees C supported that changes in phosphoglycerides could be the consequence of a phosphodiesterase activity of bacterial origin, able to survive UHT processing. A similar behavior was observed between this activity and proteolytic activity. The potential application of the detection of these compounds as spoilage predictor indices is discussed.