Sorbitol Changes How Milk Protein Builds Tiny Fibers

The study looks at how a common sugar alcohol, sorbitol, affects the way κ‑casein, a protein found in milk, forms long fiber‑like structures called amyloids. In ordinary milk, κ‑casein keeps fat droplets together, but when the protein is stressed it can aggregate into ordered fibrils. Researchers added different amounts of sorbitol to κ‑casein solutions and watched the fibers form. Fluorescent dye tests showed that sorbitol removes the slow start phase of fiber growth, so the fibers appear more quickly while their final growth rate stays similar. When more than 500 millimoles of sorbitol were present, the fibers became noticeably shorter and much stiffer. Atomic force microscopy revealed that this stiffness comes from tighter packing of the protein chains inside each fiber, even though the overall cross‑section shape did not change.Small‑angle X‑ray scattering confirmed that the basic internal structure of the fibers remains the same despite the size and rigidity changes. Computer simulations at the atomic level indicated that sorbitol shifts how flexible the protein parts are: it makes middle sections pack tighter and allows the ends to move more freely, resulting in fibers that are globally longer yet mechanically stronger. These observations show that sorbitol acts like a molecular switch, linking the environment of the protein solution to how its fibers look and feel. The findings give new insight into how sugar alcohols can control protein self‑assembly in foods and suggest ways to tailor the texture of food products that contain amyloid fibers.