Browsing by Subject "UHT milk"

Plasmin is an enzyme which is an important factor affecting both flavor and shelf life of UHT milk. Plasmin hydrolyzes mainly β, αs1- and αs2-caseins into γ-caseins and proteose peptones. During storage, proteolysis by plasmin causes bitterness and gelation. The objective of present study was to investigate time-temperature combinations for preheat treatment of UHT milk to decrease plasmin activity thus improving the shelf life and quality of existing product. In experimental research, 14 different UHT milk samples were manufactured with 7 different preheat processes and two different types of milk: prehydrolyzed (A) and posthydrolyzed (B). Samples were analyzed for their extent of proteolysis by two different methods: SDS-PAGE and RP-HPLC. SDS-PAGE gave qualitative information on the casein degradation and RP-HPLC gave quantitative information. Also, sensory analyses were made by eight trained employees at Arla Ltd (Sipoo, Finland). There were significant differences in casein degradation of samples at 2 and 4 months. All samples had some proteolysis after 4 months of storage. The largest amount of casein degradation was found in samples 5, 7 and 11, whereas the least proteolysis found in samples 1, 2, 13 and 14. Same results came from both SDS-PAGE and RP-HPLC. The sensory evaluation showed differences between the samples regarding bitterness at 2 and 3 months. The samples which were evaluated most bitter in sensory analysis, showed also most proteolysis in other analyses (samples 5, 7 and 11). The rest of the samples were quite equal in all evaluated features. The impact of hydrolysis point of lactose was unclear in this study. The aim of this study was accomplished since optimal processing conditions were found: 1, 2, 13 and 14 and the optimization of the process was possible based on the results from this study.