Abstract
Acidification of milk destabilizes casein micelles (CMs) and results in network formation. This process is fundamental during the manufacturing of dairy gels such as cheese and yoghurts. Understanding the structural alterations of CMs during this process aids to predict the physicochemical and sensory properties of dairy gels. Herein we utilize direct Stochastic Optical Reconstruction Microscopy (dSTORM) to visualize individual hydrated CMs and characterize pH-dependent changes in CM size distributions. CMs were immobilized, fixed, labelled with caseins-specific primary antibodies, and imaged by dSTORM using Alexa Fluor 647 conjugated secondary antibodies. While antibodies specific to κ- and β-casein were used to stain CMs, only β-casein antibodies enabled reproducible imaging of micelles across the entire chosen pH window. Furthermore, CM’s structural evolution was studied at acidic pH values representing the conditions during acid milk gel formation and at three elevated pH values. dSTORM imaging of casein aggregates at pH 4.5, below the isoelectric point of caseins, showed that β-casein distribution throughout the protein network and resolved nano-sized pores within the structure. Moreover, automated and quantitative image analysis revealed that the average size of CMs increased upon alkalization to pH 7.5 and 8.3, whilst narrow size distributions were found upon acidification to pH 5.5.
Highlights
Milk is composed of casein micelles (CMs) that are dispersed in a solution containing additional whey proteins, lactose and salt (Ann Augustin et al, 2011)
While CMs can retain their stability during many harsh food processing conditions such as drying, heating, freezing and concentrating, their destabilization leads to coagula formation into a gel which is essential in the production of dairy foods such as cheese and yoghurt (Ann Augustin et al, 2011). pH is one of the influencing parameters that can destabilize CMs and affect their microstructural features such as their size and overall composition (Dalgleish & Corre dig, 2012a)
A control was prepared in the absence of CMs, by introducing primary and secondary antibodies into the imaging chamber assembled from a coverslip coated first with 0.1% (v/v) PLL and 4% (w/v) formaldehyde
Summary
Milk is composed of casein micelles (CMs) that are dispersed in a solution containing additional whey proteins, lactose and salt (Ann Augustin et al, 2011). These use stimulated emission depletion microscopy (STED) to distinguish the dependence of dairy gel properties on induction method or milk type (Glover, Ersch, et al, 2019), to characterize the distribution of fat droplets within the network (Glover, Bisgaard, Andersen, Povey, Brewer & Simonsen, 2019), and to measure the dynamic moisture loss (Glover et al, 2020) These handful of pioneering SRM studies in food science demonstrate the potential of SRM techniques to image complex food matrices with a high spatial resolution in their hydrated state with chemical specificity through labeling with suitable markers of the different substituents. The size distributions of individual CMs at pH 5.5, 7, 7.5 and 8.3 were quantified from high-resolution dSTORM images using MATLAB enabled auto mated image analysis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
