The aggregation of proteins is of fundamental relevance in a number of daily phenomena, as important and diverse as blood coagulation, medical diseases, or cooking an egg in the kitchen. Colloidal food systems, in particular, are examples that have great significance for protein aggregation, not only for their importance and implications, which touches on everyday life, but also because they allow the limits of the colloidal science analogy to be tested in a much broader window of conditions, such as pH, ionic strength, concentration and temperature. Thus, studying the aggregation and self-assembly of proteins in foods challenges our understanding of these complex systems from both the molecular and statistical physics perspectives. Last but not least, food offers a unique playground to study the aggregation of proteins in three, two and one dimensions, that is to say, in the bulk, at air/water and oil/water interfaces and in protein fibrillation phenomena. In this review we will tackle this very ambitious task in order to discuss the current understanding of protein aggregation in the framework of foods, which is possibly one of the broadest contexts, yet is of tremendous daily relevance.

http://iopscience.iop.org/article/10.1088/0034-4885/76/4/046601/pdf

The self-assembly, aggregation and phase transitions of food protein systems in one, two and three dimensions

When skim milk at pH 6.55 was heated (75 to 100 degrees C for up to 60 min), the casein micelle size, as monitored by photon correlation spectroscopy, was found to increase during the initial stages of heating and tended to plateau on prolonged heating. At any particular temperature, the casein micelle size increased with longer holding times, and, at any particular holding time, the casein micelle size increased with increasing temperature. The maximum increase in casein micelle size was about 30-35 nm. The changes in casein micelle size were poorly correlated with the level of whey protein denaturation. However, the changes in casein micelle size were highly correlated with the levels of denatured whey proteins that were associated with the casein micelles. The rate of association of the denatured whey proteins with the casein micelles was considerably slower than the rate of denaturation of the whey proteins. Removal of the whey proteins from the skim milk resulted in only small changes in casein micelle size during heating. Re-addition of beta-lactoglobulin to the whey-protein-depleted milk caused the casein micelle size to increase markedly on heat treatment. The changes in casein micelle size induced by the heat treatment of skim milk may be a consequence of the whey proteins associating with the casein micelles. However, these associated whey proteins would need to occlude a large amount of serum to account for the particle size changes. Separate experiments showed that the viscosity changes of heated milk and the estimated volume fraction changes were consistent with the particle size changes observed. Further studies are needed to determine whether the changes in size are due to the specific association of whey proteins with the micelles or whether a low level of aggregation of the casein micelles accompanies this association behaviour. Preliminary studies indicated lower levels of denatured whey proteins associated with the casein micelles and smaller changes in casein micelle size occurred as the pH of the milk was increased from pH 6.5 to pH 6.7.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022029902005903

Association of denatured whey proteins with casein micelles in heated reconstituted skim milk and its effect on casein micelle size

Pressure treatment of β-lactoglobulin (β-LG), whey protein concentrate (WPC), whey protein isolate and skim milk has been explored by many groups using a wide range of techniques. In general terms, heat treatment and pressure treatment have similar effects: denaturing and aggregating the whey proteins and diminishing the number of viable microorganisms. However, there are significant differences between the effects of the two treatments on protein unfolding and the subsequent thiol-catalysed disulfide-bond interchanges that lead to different structures and product characteristics. Application of a range of techniques has given insight into the subtle differences between the pathways from native proteins to the final product mix. This review covers some of the techniques used and their strengths, and the probable pathways from native protein to the final products. β-LG is one of the most pressure-sensitive proteins and α-lactalbumin (α-LA) is one of the most pressure resistant. In a heated WPC system, bovine serum albumin is very sensitive and β-LG is more resistant. In a heated milk system, β-LG reacts with κ-casein (κ-CN) and not with αS2-CN, but, in pressure-treated milk, β-LG forms adducts with either κ-CN or αS2-CN. In both treatments, the role of β-LG is central to the ongoing reactions, involving α-LA and κ-CN in heated systems but involving κ-CN, αS2-CN and α-LA in pressurized systems. Industrial relevance High hydrostatic pressure (HHP) processing, as opposed to heat treatment, has received much attention recently as a means of processing milk proteins. This review examines the differences in the denaturation pathways that give rise to different final products.

Interactions of milk proteins during heat and high hydrostatic pressure treatments — A Review

Whey proteins have many benefits due to their high nutritional value and their various applications in food products. A drawback of whey proteins is their instability to thermal processing, which leads to their denaturation, aggregation, and, under some conditions, gelation. As thermal processing is a major treatment in the processing of milk and milk products, its influence on whey proteins has been extensively studied. Understanding the mechanisms involved during each stage of denaturation and aggregation of whey proteins is critical to devising ways of improving their stability. These aspects are reviewed in this paper. Also covered are approaches to preventing or reducing heat-induced aggregation of whey proteins. Inhibition of aggregate formation has considerable potential for alleviating the problems that arise from the instability of whey proteins.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/1541-4337.12105

Stability of Whey Proteins during Thermal Processing: A Review

Fouling of heat exchangers is a problem in the dairy industry and costs billions of dollars every year. It has been studied extensively by researchers around the world, and a large number of studies are reported in the literature. This review focuses on the mechanisms of milk fouling, investigating the role of protein denaturation and aggregation as well as mass transfer. We also endeavor to review the effect of a number of factors which have been classified into 5 categories: (1) milk quality, (2) operating conditions, (3) type and characteristics of heat exchangers, (4) presence of microorganisms, and (5) transfer of location where fouling takes place. Different aspects have been discussed with the view of possible industrial applications and future direction for research. It may not be possible to alter the properties of milk since they are dependent on the source, collection schedule, season, and many other factors. Lowering the surface temperature and increasing the flow velocity tend to reduce fouling. Reducing the heat transfer surface roughness and wettability is likely to lower the tendency of the proteins to adsorb onto the surface. The use of newer technologies like microwave heating and ohmic heating is gaining momentum because these result in lower fouling; however, further research is required to realize their full potential. The presence of microorganisms creates problem. The situation gets worse when the microorganisms get released into the process stream. The location where fouling takes place is of paramount importance because controlling fouling within the heat exchanger may yield little benefit in case fouling starts taking place elsewhere in the plant.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1541-4337.2006.tb00080.x

A Critical Review of Milk Fouling in Heat Exchangers

Reconstituted whole milk was heated using pilot-scale heating equipment. Kinetic and thermodynamic parameters for the irreversible denaturation of β-lactoglobulins A and B and α-lactalbumin were determined. α-Lactalbumin denaturation was first order, whereas both β-lactoglobulin variants had a reaction order of 1.5. Arrhenius plots for all three proteins showed an abrupt change in temperature dependence. In the low-temperature range, the thermodynamic parameters were ascribed to typical denaturation processes in which the unfolding of the protein tertiary structure is the rate-determining step. At higher temperatures, these parameters were in the range expected for typical condensation reactions, suggesting that aggregation processes may be rate-determining in this temperature range. The rate constants for β-lactoglobulin denaturation were independent of the initial protein concentration at all temperatures. For α-lactalbumin at temperatures below 85 °C the rate constants may have been dependent on the in...

Anthony B. McKenna

Papers

Reaction kinetics of thermal denaturation of whey proteins in heated reconstituted whole milk