UHT milk contains multiple forms of αS1-casein that undergo degradative changes during storage

Influence of Ultra-Heat Treatment on Properties of Milk Proteins

Milk can be considered one of the primary sources of nutrients for the mammalian neonate. Therefore, milk and milk-based products, such as infant formula, whey protein isolate, different varieties of cheese, and others are prepared to meet the nutritional requirements of the consumer. Due to its significant nutritional components and perishable nature, a variety of pathogenic microorganisms can grow and multiply quickly in milk. Therefore, various heat treatments can be employed for the improvement of the shelf life of milk. In comparison to pasteurized milk, due to excessive and severe heating, UHT milk has a more cooked flavor. During storage, changes in the physicochemical properties of milk can lead to off-flavors, undesirable browning, separation of fat, sediment formation, or gelation during the subsequent storage. Several important factors such as processing parameters, time-temperature abuse (storage condition), and packaging type also influence the quality characteristics and consumer acceptance of the milk; however, the influence of heat treatments on milk protein is inconstant. The major protein modifications that occur during UHT treatment are denaturation and aggregation of the protein, and chemical modifications of its amino acids. These UHT-induced protein alterations can change digestibility and the overall biological influence of the intake of these proteins. Therefore, this review is focused on the influence of UHT on the physicochemical and structural attributes of milk proteins during storage. There are many indications of milk proteins present in the UHT milk, and milk products are altered during processing and storage.

Changes in Milk Protein Interactions and Associated Molecular Modification Resulting from Thermal Treatments and Storage

We investigated protein modifications that occur during short- and long-term storage of raw, pasteurized, and ultra-high-temperature processed (UHT) milks using RE-HPLC and redox proteomics. The RE-HPLC results show that casein dissociation and whey protein/κ-casein association occurred in both pasteurized and UHT milk. The extent of protein interactions was more pronounced in UHT milk after storage. The redox proteomics analyses show that primary structural level protein modifications were not correlated to processing type on the of day processing but did occur and increase during storage. Methionine oxidation was the most significant type of oxidative modification in all samples, particularly in the caseins. Methionine oxidation increased in the UHT-treated milk samples with longer storage times, especially in the micelle-phase proteins, likely due to the increasing exposure of these proteins as they migrated to the serum phase. Glycated and lactosylated early-stage Maillard reaction products were also found after heat treatment, particularly in UHT-treated milk, with the levels of these products maintained and generally increased with increasing storage time. PRACTICAL APPLICATION: Understanding changes in protein modification during heat processing and storage of liquid milk products may help develop a model to predict the quality and shelf-life stability of heat treated milk products.

Influence of Kluyveromyces marxianus on proteins, peptides, and amino acids in Lactobacillus-fermented milk

With increasing application of yeast in fermented milk, in order to study the effect of yeast on milk protein during the fermentation process, the effects of the presence of Kluyveromyces marxianus in milk fermented by Streptococcus thermophilus and Lactobacillus bulgaricus were investigated. After fermentation, the amino acid, protein, and peptide contents were analyzed by ultra-performance liquid chromatography, two-dimensional gel electrophoresis, and liquid chromatography-mass spectrometry, respectively. After the addition of K. marxianus for fermentation, 25 protein spots changed significantly. These were mostly caseins and bovine serum proteins, and the content of total free amino acids increased by 16.30%; ten types of bioactive peptides were identified. Furthermore, the number of peptide types in milk fermented by K. marxianus increased significantly compared with milk fermented by Lactobacillus. K. marxianus is considered to promote proteometabolism in milk when added with Lactobacillus, generate flavor compounds, and improve the digestion and absorption character of milk.