Flavor, enzymatic and microbiological profiles of pressure-assisted thermal processed (PATP) milk

The main objective of this study was to evaluate the application of high pressure processing (HPP) and pressure-assisted thermal processing (PATP) as an alternative technology to process high quality fluid milk. The specific objectives of this work were to compare the microbial load, chemical stability and flavor profiles of HPP and PATP milk to that of HTST pasteurized and ultra high temperature (UHT) processed milk. Milk (2% milkfat) was subjected to combined pressure-heat treatment using a factorial 3x1x3 model at temperature (32, 72 and 105°C), pressure (650 MPa), and time (0, 1, and 5 min). Milk samples were processed within 72 hr, packed in light -- protected polyethylene teraphtalane (PET) bottles without head space, and stored at either room temperature (25+/-1°C) or refrigeration (4+/-1°C) conditions depending on the treatment applied. The shelf life of pressure treated milk samples was examined over a period of 20, 45 and 90 d. Additionally, pasteurized (78+/-0.8°C for 18 sec) and UHT processed (138+/-1°C for 2 sec) milk samples obtained from commercial source were analyzed along with pressure treated milk. The quality of milk samples was analyzed by the following tests: (1) Microbiological tests: Total plate count (TPC) and spore-forming bacteria survival analyses (Bacillus stearothermophilus and B. amyloliquefaciens). (2) Residual Plasmin activity in milk samples was measured using a BODIPY FL-Casein spectrofluorometric assay. (3) The chemical stability of milk samples was assessed by measuring the extent of proteolysis and lipolysis. Proteolysis was evaluated by SDS-PAGE analysis and lipolysis was measured by a modified copper soap method. (4) The flavor profile of milk samples was evaluated using selected ion flow tube mass spectrometry (SIFT-MS) to identify and quantify volatile organics compounds in milk on real time basis. Preliminary data showed that the application of pressure treatment is capable of rendering milk with longer shelf life than HTST pasteurized and UHT milk. Up to 6-log reduction was obtained when a suspension (N0 = 7x106 spore/mL) of B. stearothermophilus was inoculated in UHT milk treated at 700 MPa for 3 min at 105°C. Similarly, a 7-log reduction was achieved when a suspension (N0 = 1x107 spore/mL) of B. amyloliquefaciens was inoculated in UHT milk treated at the conditions described above. Pressure treatments at room temperature (32+/-1°C) delivered milk with similar microbial load than pasteurized milk. The microbial population of milk further decreased with increasing temperature (72 and 105°C). Up to 4 log reduction (N0 = 2x104 CFU/mL) was obtained in PATP milk samples processed at 650 MPa and 105°C for 0, 1 and 5 min, respectively. Storage temperature had an effect on the microbial growth rate in PATP milk. Microorganisms recovered in pressure treated milk processed at 650 MPa and 72°C for 0, 1 and 5 min stored at refrigeration conditions showed slower growth rates than pressure-treated samples stored at room temperature conditions. No additional efforts were made to characterize the microorganisms recovered during storage. No significant proteolysis was observed in pressure-treated samples at the end of their shelf life. Plasmin inactivation rates increased with increasing temperature. Although the enzyme was not completely inactivated, combinations of mild temperature with ultra high pressure were sufficient to inactivate the enzyme to levels similar to UHT processes. However, lipolysis was enhanced by pressure, holding time during pressurization and storage temperature. The formation of volatile aroma compounds in HPP and PATP milk was different than in milk processed with traditional heat treatments. Pressure treatments enhanced only the formation of straight-chain aldehydes and sulfur compounds; whereas higher concentrations of methyl ketones and straight-chain aldehydes were observed in pasteurized and UHT milk samples. These results indicate that pressure treatment is capable of rendering milk with quality characteristics close to pasteurized and UHT milk. However, processing temperature and storage conditions have a significant role on the quality of milk.