Effect of coffee lyophilizate addition on the physicochemical properties and bioactive profile of yogurt during storage.

This study investigated the effect of caffeinated Arabica and decaffeinated coffee lyophilizate (0.6 g/100 g), incorporated either before or after fermentation, on the phenolic compound profile, caffeine and diterpene content, antioxidant activity, and physicochemical properties of yogurt during 20 d of refrigerated storage. Yogurts were produced from commercially available pasteurized and homogenized cow's milk (2.0% fat) using a thermophilic starter culture comprising Streptococcus thermophilus, Lactobacillus acidophilus, and Bifidobacterium animalis ssp. lactis (Lyofast SAB 440B). Total phenolic losses during lyophilization amounted to 14.0% and 19.0% for caffeinated and decaffeinated preparations, respectively, with feruloylquinic acid isomers showing the highest susceptibility to degradation (losses of 55-72%). Coffee enrichment resulted in an approximately 9-10-fold increase in total phenolic content relative to control yogurt, with overall phenolic retention reaching 73-85% of theoretical values. After-fermentation addition yielded higher extractable phenolic content, while before-fermentation addition was associated with lower extractable phenolics but superior hydroxyl radical scavenging capacity (OHSC), potentially reflecting the formation of polyphenol-protein complexes during fermentation. Oxygen radical absorbance capacity (ORAC) increased significantly during storage in before-fermentation variants, suggesting progressive release of bound phenolics through starter culture-mediated proteolysis. Decaffeinated lyophilizate added before fermentation produced the firmest yogurt gel, an effect potentially attributable to polyphenol-casein interactions and possible caffeine-mediated modification of starter culture activity. Addition timing rather than coffee type was the primary determinant of surface optical properties. The majority of bioactive compound profiles and physicochemical parameters showed no systematic changes during the 20-d storage period, although significant storage-related changes were observed for selected antioxidant activity indices, textural parameters, surface gloss, and diterpene content in specific treatment groups. These findings indicate that coffee lyophilizate is a promising functional ingredient for yogurt enrichment, with the timing of addition being a critical process parameter that influences bioactive compound availability, antioxidant functionality, and textural quality.