![]() The availability of thiol groups tends to decrease mainly in the samples pre-heated at a high temperature, maybe due to thiol exchange and intermolecular disulphide bonds, forming a polymeric structure observed by DLS in the samples pre-heated at 73 and 90 ☌. Indeed, more accessible DTNB (Ellman’s assay) to thiol groups may be indirectly linked to the new conformation assumed by α-helix at 65 ☌ pre-heating. Reducing the dimensional scale, a view of thiol groups may offer a study of thermal impact on another part of β-Lg structure. A slight increase in λmax was observed for the 65 ☌ pre-heating temperature that is linked to the surrounding changing of Trp residues, meaning a conformational protein change. More in detail, fluorescence spectroscopy allowed to highlight the local β-Lg structure and observe the surrounding conditions of Trp residues. The observation of denaturation ΔH by DSC did not highlight the conformational changing of β-Lg structure at the low temperatures, but it did in the samples pre-heated at 73 ☌. Biophysical tools and Ellman’s method were used to characterise β-Lg structure on the dimensional scale: from the complete structure, to thiol group accessibility and availability. To better understand these events, the second part of the work started and it led to the characterisation of β-Lg structure at the molecular level: indeed, changes in protein conformation were observed after the pre-heating treatments at several temperatures (51, 56, 65, 73, 90 ☌). These events may be connected with the different physico-chemical surroundings: minerals and pH influence the protein ability to interact with each other and with other solution components lactose may act as a protein protection from denaturation. An increased pre-heating time at T(onset) was observed too, and the inability of ST2 samples to obtain the gel, not even after a long time. Gelation assay highlighted the ability of ST1 samples to obtain the gel structure, the decreasing of gel strength and the increasing of delay in gelation time. Preliminary characterisation of WPI by DSC showed a higher T(onset) for ST1 sample than for ST2 and also an asymmetrically shaped endothermic peak for ST2. Then, they were thermally treated at 80 ☌ to obtain a gelled structure. Samples were pre-thermally treated several times at their T(onset) (55 ☌ for ST1 52 ☌ for ST2) identified by DSC. To study macroscopically the gel, the first part involved two kinds of samples, which differ in their composition: Sample Type 1 (ST1) was the whole WPI re-suspended in water Sample Type 2 (ST2), had low molecular weight molecules removed and replaced by a phosphate buffer. The experimental plan was divided in two main parts. For this in-depth study β-Lactoglobulin (β-Lg) was chosen since it is the most abundant protein in milk whey and it is assumed that this protein with its thiol group is partly responsible for aggregation and gelation. Hence, our objective is to study more in detail the pre-heating step impact on protein structure, especially at low temperatures (that is, before the denaturation peak of the protein, close to T(onset)). As a gelling ingredient, in particular, pre-heating processes, (even at low temperatures, below 65 ☌) it may change the gel structure, maybe due to protein pre-activation, probably linked to conformational changes of the whey proteins. WPI powder was chosen for its characteristics: it is easily reconstituted and it is obtained by low-temperature spray drying process in terms of its formulation, it represents a classical ingredient for food factories: it is extensively used to get gels. Indeed, it is necessary to set up the drying process and the physico-chemical conditions of whey protein suspensions in such a way to have less impact on their structure. Texturing and emulsion stabilising abilities of a whey powder are well known and are linked to production processes. ) or also as protein supplements in foods. Our work deals with whey milk powders as by-products from cheese manufacture, which becomes a very important ingredient in food processing, due to its technological properties (gelling, emulsifying, foaming.
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