Isıl İşlem ve Homojenizasyonun UHT Sütlerde Protein Yapısı Üzerine Etkilerinin İncelenmesi

Abstract

Depending on the severity of the heat treatment applied to milk, physicochemical changes such as denaturation of serum proteins, κ-casein/β-lactoglobulin complex formation, calcium phosphate precipitation, Maillard reaction may occur. The aim of this study is to examine the possible effects of UHT method and homogenization sequence (before or after heat treatment) on proteins in UHT milk produced using different production technologies (direct injection and indirect tubular heat treatment). As a result of the research, it is thought that the technologies to be used in the production of milk-based liquid products to be developed for different target groups will be decisive in the decision-making process. UHT milk samples were produced in a local dairy company according to the stated purposes. Raw milk and boiled (100 oC, 10 min) milk samples were used as a control group.

Isoelectric casein was obtained by bringing the pH of raw milk and heat-treated milk to 4.6. At the same time, colloidal casein was obtained from all milk samples using ultracentrifugation (135000xg, 60 min, 20 oC). Serum and pellet separated by centrifugation were preserved by freezing and then freeze-dried. The possible effects of the heat treatment method and homogenization order on milk protein conformation were investigated by analysis the samples with vibrational methods. For this purpose, Attenuated Total Reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and Raman spectroscopy (1064 nm laser source) were used. In this way, important information was obtained about the α-helix, β-sheet structures, and changes in proteins depending on amide (A, B, I-VII) bonds and other conformational bonds. Generally, compared with raw milk, it was demonstrated by vibrational analysis methods that the combination of UHT application and homogenization differentiated the distribution of milk proteins in the serum phase and caused changes in protein conformation.

According to ATR-FTIR or Raman spectra of milk samples, it was determined that the technologies (direct or indirect) used in UHT milk production and homogenization applications were effective on protein conformations. Although there are no significant differences in peak profiles between the Raman spectra of the samples in general, it has been determined that the peak intensities vary according to the UHT application model. Principal component analysis (PCA), one of the chemometric methods, was used in the evaluation of the measurements. As a result of the studies, as the homogenization order and heat treatment method applied to milk changed, differences were observed especially in the Amid-I 1700-1600 cm-1 region, which is used to determine the secondary structure of proteins, and in the Amid-II 1600-1500 cm-1 region, which gives general information about proteins. As a result, the changes in the secondary structure of milk proteins with the effect of temperature can be determined easily and quickly by using the geometric orientation in the structures of dipole-moment changes, hydrogen bonds, α-helix, β-sheets and β-turns, and UHT milk production modifications can be designed.