Two-step aging dynamics in enzymatic milk gels

Abstract: Colloidal gels undergo a phenomenon known as physical aging, i.e., a continuous change of their physical properties with time after the gel point. To date, most of the research effort on aging in gels has been focused on suspensions of hard colloidal particles. In this letter, we tackle the case of soft colloidal "micelles" comprised of proteins, where gelation is induced by the addition of an enzyme. Using time-resolved mechanical spectroscopy, we monitor the viscoelastic properties of a suspension of colloidal micelles through the sol-gel transition and its subsequent aging. We show that the microscopic scenario underpinning the macroscopic aging dynamics comprises two sequential steps. First, the gel microstructure undergoes rapid coarsening, as observed by optical microscopy, followed by arrest. Second, aging occurs solely through a contact-driven mechanism, as evidenced by the square-root dependence of the yield stress with the elastic modulus measured at different ages of the gel. These results provide a comprehensive understanding of aging in enzymatic milk gels, which is crucial not only for a broad range of dairy products, but also for soft colloids in general.

• The paper demonstrates a two-phase aging process in enzymatic milk gels, with rapid microstructural coarsening followed by contact-driven stiffening.
• It utilizes time-resolved mechanical spectroscopy, USAXS, and confocal microscopy to monitor the sol-gel transition and track evolving viscoelastic properties.
• The study introduces a time-connectivity superposition model linking yield stress and elastic modulus, providing key insights for dairy processing and soft material design.

Two-step Aging Dynamics in Enzymatic Milk Gels

The paper "Two-step Aging Dynamics in Enzymatic Milk Gels" explores the aging processes of milk gels induced by enzymatic treatment. The authors use time-resolved mechanical spectroscopy and structural methods to explore the viscoelastic properties of these gels throughout their sol-gel transition and subsequent aging.

Methodology and Experimental Setup

The research employs time-resolved mechanical spectroscopy to monitor the viscoelastic properties during the sol-gel transition. A suspension of colloidal micelles, initiated by adding an enzymatic coagulant, allows the examination of the gel's evolving microstructure and mechanical behavior over time. Complementary techniques such as ultra-small-angle X-ray scattering (USAXS) and confocal microscopy provide insights into the underlying structural changes.

Findings and Analysis

Sol-Gel Transition and Viscoelastic Properties

During the sol-gel transition, the gel's viscoelastic properties show significant evolution. The elastic modulus ($G'$) and viscous modulus ($G''$) increase rapidly after an initial latency, marking the enzymatic destabilization of casein micelles. The loss tangent $\tan \delta$, a measure of the balance between dissipative and elastic responses, decreases exponentially (Figure 1). This behavior highlights the formation and evolution of the gel network.

Figure 1: Scaling factors vs. time at various volume fractions. (a)-(b) Scaling factors $G_0$ and $\omega_0$ as a function of the elapsed time since the gel point $t-t_{g}$.

Two-step Aging Mechanism

The aging dynamics unfold in two distinct phases:

1. Coarsening Phase: Initially, the gel microstructure undergoes rapid coarsening characterized by large-scale structural rearrangements. This phase significantly increases gel elasticity until the microstructure arrests at a critical time $\tilde{t}_c$.
2. Contact-driven Aging: Beyond $\tilde{t}_c$, viscoelastic properties continue to evolve through contact-driven interactions. Despite the absence of long-range structural changes, the gel's mechanical strength increases as evidenced by the square root relationship between the yield stress and elastic modulus (Figure 2).

   Figure 3: Evolution of the microstructure through the sol-gel transition and subsequent aging. (a) USAXS intensity $I$ vs. wave vector $q$ at selected aging times: $\tilde{t}$.

Time-Connectivity Superposition Principle

The viscoelastic spectra reveal a time-connectivity superposition principle, akin to time-cure principles in polymer gels. By rescaling spectra based on the aging time, a master curve emerges, spanning multiple orders of magnitude and indicating self-similar evolution of the viscoelastic properties (Figure 3).

Figure 2: Yield stress $\sigma_y$ vs. elastic modulus $G^{\prime}$ measured at different ages.

Practical Implications and Theoretical Insights

This study provides critical insights into the rheological behavior of enzymatic milk gels, which have extensive applications in dairy processing and other soft colloidal systems. The understanding of the aging processes, particularly the implications for gel strength and stability, can inform developments in food technology and materials science.

Conclusion

The research article presents a comprehensive examination of the aging dynamics in enzymatic milk gels, elucidating a complex two-step aging process. These findings advance the understanding of soft colloidal gels, offering valuable guidance for industrial applications and setting the stage for future research in this domain.