Abstract
Milk samples with twelve combinations of κ- and β-casein (CN) and β-lactoglobulin (β-Lg) variants were obtained to investigate the effect of protein variant on the mechanism/s of age gelation in ultra-high temperature (UHT) skim milk. Only milk groups with κ-CN/β-CN/β-Lg combinations AB/A1A2/AB and AB/A2A2/AB suffered from the expected age gelation over nine months storage, although this could not be attributed to the milk protein genetic variants. Top-down proteomics revealed three general trends across the twelve milk groups: (1) the abundance of intact native proteins decreases over storage time; (2) lactosylated proteoforms appear immediately post-UHT treatment; and (3) protein degradation products accumulate over storage time. Of the 151 identified degradation products, 106 (70.2%) arose from β-CN, 33 (21.9%) from αs1-CN, 4 (2.7%) from β-Lg, 4 (2.7%) from α-La, 3 (2%) from κ-CN and 1 (0.7%) from αs2-CN. There was a positive correlation between milk viscosity and 47 short peptides and four intact proteoforms, while 20 longer polypeptides and 21 intact proteoforms were negatively correlated. Age gelation was associated with specific patterns of proteolytic degradation and also with the absence of the families Bacillaceae, Aerococcaceae, Planococcaceae, Staphylococcaceae and Enterobacteriaceae, present in all the non-gelling milk groups pre-UHT.
Highlights
Ultra-high temperature (UHT) treatment (135–150 ◦ C for 2–6 s) is widely used to sterilize milk and extend its shelf-life up to 4–6 months at room temperature [1]
These parameters show that all the milk groups generally had similar initial parameters apart from the casein micelle diameter, which have previously shown to be related to the genetic variants of κ- and β-casein [36]
We have shown that during the storage of genetically selected ultra-high temperature (UHT) milk there is a progressive increase in the proteolytic degradation of the major milk proteins, whether milk displayed age gelation or not
Summary
Ultra-high temperature (UHT) treatment (135–150 ◦ C for 2–6 s) is widely used to sterilize milk and extend its shelf-life up to 4–6 months at room temperature [1]. UHT milk processing, whilst producing sterile products, induces major physicochemical changes to the milk components, the heat-sensitive whey proteins These physicochemical changes can lead to a suite of negative sensory attributes such as browning, bitterness, particle formation and gelation that consumers reject, dictating the shelf life of UHT milk rather than shelf life restriction from microbial infection [3]. The mechanisms of these defects have been previously reviewed by several authors [2] with age gelation being the most prominent defect affecting UHT milk. It complexes with κ-casein (κ-CN) forming a β-Lg-κ-CN
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