Flow process and heating conditions modulate the size and properties of whey protein aggregates

Whey protein (WP) have been extensively studied in the past and are commonly used for their functional and nutritional properties in food products. Recently, a particular interest was drawn in designing whey protein aggregates by adjusting physicochemical (pH, salt and protein concentration) and processing conditions in order to obtain new functional properties, depending on aggregate structure and size obtained (compact sphere, fibril or strand like). Namely, WP aggregates obtained at pH 7 and 80°C present a fractal structure that could be interesting for increasing dairy products texture. In this view, the aim of this study was to assess the influence of pilot scale continuous process conditions (temperature rise kinetics, outlet heating temperature, Reynolds number value) on the structure, size and properties of experimentally elaborated WP aggregates. Aggregates were obtained from the heat treatment of a WP solution (5% w/w), with a pH adjusted at 7 and a salt concentration of 0,01 M. The heat treatment pilot included a feed tank, a preheating exchanger to increase the temperature up to 50°C or 55°C, a tubular heat exchanger to achieve the target outlet heating temperature (80°C or 85°C) and finally a water-bath for a static holding. Moreover, it was possible to vary in an independent way the temperature rise kinetics and the flow regime depending on the mass flow rate and the tubular heat exchanger inner diameter chosen (6 to 22,5 mm). WP aggregates were separated and analysed by Asymetrical Flow Field-Flow Fractionnation coupled with Multi-Angle Light Scattering and Differential Refractometer (AF4-MALS-DRi) and by Transmission Electron Microscopy (TEM). After 80°C heat treatment, three populations of aggregates were observed in the solution : non-aggregated proteins, and 2 types of WP aggregates with an average radius of gyration (Rg) of 10 nm for the first, and 60 nm to 100nm for the second one. It was shown that both temperature rise kinetics and flow regime affected mean size of this latter : namely decreasing Reynolds number from 10000 to 2000 induced an increase of the aggregates size. Moreover, increasing temperature led to the formation of a fourth population of larger aggregates (Rg ≥200nm). These results show that process parameters are efficient levers to modulate the size and structural properties of fractal aggregates in a semi-industrial scale and should be carefully assessed in order to monitor their functional properties