An arXiv preprint reports that an intermediate-scale scattering feature in casein micelles began to fade before dynamic light scattering (DLS) detected aggregation as reconstituted milk moved toward a gel. The changing X-ray pattern was more compatible with a porous “snowball” model than with a diffuse core-shell model. The authors interpret the result as consistent with κ-casein having a structural role inside the micelle as well as at its surface.
Following the transition
The experiment used low-heat skim-milk powder reconstituted at 100 g.L−1 in ultrapure water. The mixture had an approximately 10% casein-micelle volume fraction and a casein concentration of 25.8 g.L−1. After warming it to 38 °C, researchers added camel chymosin at 1 IMCU per gram of casein. RP-HPLC, DLS and small-oscillatory rheometry monitored κ-casein hydrolysis, micelle aggregation and gel formation.
Small-angle X-ray scattering (SAXS), a way of following structure through an X-ray scattering pattern, supplied a new frame every 20 seconds. The data were read across low-, intermediate- and high-q regimes, which correspond to different structural scales.
Hydrolysis came first in the time course. Its extent followed a first-order curve with a characteristic time of 540 seconds. DLS-measured particle size began to increase at about 1000 seconds, when χenz was approximately 0.85. Rheology estimated the gel time at about 1750 seconds, when G′ exceeded G′′.
A disappearing signal
In the intermediate-q range, a bump near q≈0.15 nm−1 progressively vanished and was nearly indistinguishable at the gel point. After gelation, the region showed a q−4 Porod-like regime. At low q, the fractal exponent—a measure of the pattern across scale—was about 2–2.1 at gel time and rose slowly to about 2.3 afterward.
The changing feature did not amount to a comparable loss of total scattering. QI increased while QII decreased from 200 seconds, before DLS-detected aggregation at about 1000 seconds. The total invariant Q and high-q partial integral QIII remained approximately constant, consistent with scattering intensity being redistributed between regimes rather than total scattering power changing substantially.
What the models suggest
The researchers compared the measured spectra with two structural simulations. The snowball model reproduced the loss of the intermediate bump without reducing the intensity associated with the micelle-scale pattern. The diffuse core-shell model predicted a reduction in that intensity that the measurements did not show.
On the authors’ reading, the match is consistent with κ-casein contributing to the micelle’s internal organization rather than being confined to surface stabilization. That is a structural interpretation of the scattering trajectory, not a direct molecular map.
At higher q, the existing features stayed essentially unchanged during gelation, but a second peak at q=0.4 nm−1 became visible as intermediate-regime scattering declined. The authors interpret the peak as a pre-existing correlation among calcium-phosphate nanoclusters that became visible when Porod/interface scattering weakened, not necessarily as a new structural motif formed during gelation.
The result has clear boundaries
The number of independent preparations and replicates was not reported, and fit uncertainty for the hydrolysis time course or the rheological gel-time estimate was not provided. The evidence comes from the reported reconstituted skim-milk laboratory system and its experimental conditions.
Because hydrolysis, aggregation and gelation were followed together in one time course, the study shows temporal association and model compatibility rather than a unique molecular mechanism. The simulations do not uniquely determine micelle architecture or a kinetic pathway, and the 0.4 nm−1 peak remains an interpretive assignment. Whether the proposed organization holds under other milk compositions and physical conditions remains open.
Paper data and sources
Original title: Unmasking the internal structure of casein micelles through enzymatic hydrolysis: A SAXS study
Authors: Julien Bauland, Ghazi Ben Messaoud, François Boué et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
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