Metabolic and microbial stability of protein-fortified fermented dairy drinks: Effect of base and storage

Abstract

Aging, particularly in the elderly, is marked by physiological changes that alter protein requirements and utilization, while higher protein intake helps maintain nutritional status and physical function. Fermented dairy products improve protein digestibility and calcium bioavailability, offering benefits for the elderly. Whey protein is a suitable fortifier due to its solubility, digestibility, and acceptance. For these reasons, this study aimed to formulate and analyse whey protein–fortified fermented dairy drinks suitable for home preparation and acceptable to elderly consumers in terms of nutritional, functional, and sensory properties. To facilitate self-care or caregiver-assisted intake, the drinks were designed to be prepared in advance and stored for consumption throughout the day. Changes in sensory attributes, microbial load, physicochemical properties (pH and color), organic acids, sugars, and free amino acids (FAA) were evaluated. Regarding sensory quality, all formulations were free of off-flavors, and notable differences in their profiles allowed for adaptation to different preferences. Microbial counts remained high (>8 log CFU g⁻¹) throughout storage, with quark and kefir-drinks showing the highest stability. The addition of whey protein decreased lightness and increased a* and b*, whereas storage at 24h20C maximized kefir acidification. Lactic acid was the predominant organic acid, and lactose and glucose levels decreased over time due to microbial activity. Finally, storage at 24h20C led to enhanced proteolytic activity; notably, kefir significantly doubled their essential FAA content. Overall, all formulations were shelf- stable under the tested conditions.