Shaping the structure of blends from sunflower press cake and whey proteins through heat treatment and fermentation

The current food system suffers from the inefficient use of resources, including the generation of side streams of low economic value, but still containing nutritional components. One potential approach to reach a more sustainable food system is to reintroduce such side streams into a circular value chain, and valorise them in novel food products, preferably in an unrefined or minimally refined manner. Moreover, blending side streams from different industries might be a suitable way to exploit functional synergies of structuring components such as proteins and polysaccharides, and to improve the nutritional value of the final food matrix. In this study, we combined the side streams from sunflower oil production (i.e., sunflower seed press cakes) and cheese manufacture (i.e., whey and whey proteins) to obtain novel food matrices containing valuable proteins, structuring polysaccharides, as well as lactose and minerals facilitating the fermentation. Press cake, whey powder, and whey protein concentrate were dispersed in milk ultrafiltrate to prepare different blends with varying sunflower protein to whey protein ratio (100:0–0:100) but equal dry matter (~26%) and protein content (~10%). Structure formation as a function of heating temperature was studied by heating the blends to 80, 120, or 140 °C in a Rapid Visco Analyser (RVA), either undisturbed or under continuous, moderate shear applied by a paddle rotating at 160 rpm. The bulk viscosity of the unheated blends increased with increasing press cake concentration (0–22.5%) due to the higher polysaccharide to protein ratio. As observed from the torque profiles measured by the RVA, heat treatment at 120 and 140 °C increased the viscosity of the blends. A lower heating temperature, 80 °C, barely affected the sunflower components, but resulted in some denaturation of whey proteins, thereby increasing the viscosity of blends low in press cake and high in whey proteins. Confocal microscopy revealed that undisturbed heating at 120 and 140 °C resulted in the formation of a homogeneous gel network, whereas heating under moderate shear fostered the formation of a more heterogeneous structure, with protein aggregates dispersed in a continuous matrix. Fermentation trials using three different co-cultures, each comprising one strain of lactic acid bacteria and one yeast strain, were conducted on the blend with the highest press cake content (22.5%) for maximum valorisation of the side stream. The samples were heated in an autoclave at 120 °C for 5 min without agitation prior to inoculation, as a homogeneous structure was assumed favourable for the fermentation. The pH development during fermentation at 26 °C was recorded, and samples were withdrawn for analysis after 12, 24, and 48 h. Small amplitude oscillatory shear rheology showed no significant changes in the storage modulus with fermentation, and confocal microscopy revealed a homogeneous microstructure for the unfermented and all fermented blends. This research provides important insights in the structure formation during processing of biomacromolecule blends and shows the potential of fermentation as a mean to stabilise side stream blends and modulate their sensory properties while only minimally affecting their physical appearance.