The effect of apple pomace on the texture, rheology and microstructure of set type yogurt

Valorization of apple pomace (AP) as a novel food ingredient is a promising way to efficiently utilize a rich source of phytochemicals and dietary fibre, while also contributing to reducing food waste. In the present study the physico-chemical and functional properties of freeze-dried AP powder were investigated along with the potential of AP powder as a dairy ingredient with focus on interaction of such a complex matrix with milk proteins under acidification and its consequences on the structure of the dairy gel matrix. The addition of 1% AP (w/w) before fermentation favored aggregation of casein micelles at an early stage of fermentation causing onset of gelation at higher pH (5.9) compared to the control (pH 5.3). However, no such effect was noticed at concentrations of AP up to 0.5%. A significant increase of gel firmness and cohesiveness was observed with increasing AP concentration from 0% to 1% during cold storage (28 days), indicating a reinforcement of the structure of the undisturbed casein gels. Such an effect might be attributed to the gelling capability of pectins and other soluble fibres released from AP into milk as well as the gel structure reinforcement by the insoluble AP particles. Confocal images of the intact casein gels indicate a uniform cellular structure with thick casein strands well interconnected at 0.5% AP concentration, which may be an optimal level of AP for a well-structured set-type yogurt gel under the conditions employed in this study.