Shelf-life stability of CBS-based compound chocolate: a kinetic approach to physico-chemical, rheological, and structural changes

This study aimed to determine the shelf-life limiting factors of Cocoa Butter Substitutes (CBS) based milk chocolate stored at 20 °C and 40.0% RH for 330 days by monitoring physico-chemical, rheological, textural, visual, and molecular (FT-IR) changes. Kinetic modeling (zero-order) was applied to quantify the degradation rates (k) of critical quality parameters. Results indicated that the samples exhibited exceptional oxidative and microbiological stability; the Peroxide Value (PV) remained at 0.00 meq O₂ /kg, and no microbial growth was detected throughout the storage period, attributed to the low water activity (aw < 0.400). However, physical and visual degradation emerged as the primary factors limiting shelf life. Moisture content increased from 1.05 to 1.65 g/100 g, which, alongside a rise in free fatty acids, acted as an effective plasticizer, driving a distinct textural softening and drastically reducing the structural product hardness over time. While zero-order kinetic models successfully predicted the rate of deterioration with high regression coefficients (R² > 0.89) for hardness, color, and FFA content, the rheological properties of the molten matrix (Casson and Herschel-Bulkley parameters, thixotropy) exhibited highly fluctuating, nonlinear dynamic behavior. Visual analysis revealed a linear darkening trend; the samples lost luminance and whiteness index (WI), with the total color difference (ΔE) exceeding the sensory perception threshold (ΔE > 5.0) after 240 days. These macroscopic changes were substantiated by molecular analysis; FT-IR spectra indicated shifts in lipid-protein interactions. Consequently, while the product remained chemically safe and oxidatively stable for 11 months, its shelf life based on instrumental quality indicators was effectively limited to 8 months due to visual matting surpassing the human perception threshold and pronounced textural softening.