*3.3. Non-Thermal Processing*

Non-heat processing (such as ultrasonication and high pressures) has long been researched to modify the functional property of soy proteins. Ultrasound is well-known energy and time-saving technique. The use of ultrasound in various industries is becoming more common. Ultrasound destroys noncovalent interactions and disulfide bonds through thermal, mechanical, and chemical effects [18], thus causing protein subunits to dissociate and aggregate, resulting in the modification of solubility, emulsifying, foaming, and gelation properties [19]. Jambrak et al. [20] found that ultrasound treatment (20 kHz) increased the emulsifying and foaming ability of soy proteins, which might be ascribed to the higher adsorption at the emulsion droplet interface of the denatured soy protein [10]. It has been revealed that ultrasound treatment increased the surface hydrophobicity and zeta potential value of soy-protein isolate–citrus-pectin electrostatic complexes, and significantly decreased emulsion droplet size was observed. The results showed that ultrasonic cavitation effects changed the structure of both biomacromolecules and increased the electrostatic interactions between soy-protein isolate and citrus pectin, both of which led to the complex's improved emulsifying properties [21]. Albano et al. [22] have also observed reduced particle size in the soy protein and pectin complex stabilized emulsion after ultrasonication. Ren et al. [23] compared the effects of ultrasonic cavitation treatment and hydrodynamic cavitation on the functional properties of soy protein isolate. The particle size and viscosity of SPI were reduced and the surface hydrophobicity was increased after ultrasonic cavitation or hydrodynamic cavitation treatment, resulting in improved solubility, emulsifying activity index, emulsion stability index, and foaming capacity. However, significantly decreased foam stability was observed after ultrasonic cavitation and hydrodynamic cavitation treatment, which might be ascribed to the weaker protein-protein interaction as reflected by the decreased viscosity.

When proteins are subjected to high pressures (HP), it is known that protein molecules would undergo conformational changes, which may lead to modification of the emulsifying properties. Several studies have been conducted to see whether high-pressure treatment can be applied to change the emulsifying properties of soy proteins. Molina, Papadopoulou [9] reported that the emulsifying activity was increased by high-pressure treatment at neutral, while no improvement was observed for the emulsifying stabilities and solubilities. They observed the highest emulsifying activity indexes were 400 MPa and 200 MPa for βconglycinin and glycinin, respectively. Puppo et al. [24] compared the effect of high pressure on emulsifying properties of soy protein isolate at acidic (pH 3.0) and alkaline (pH 8.0) conditions. High-pressure treatment (200 MPa) of SPI at alkaline condition induced a reduction of droplet size and an increase of depletion flocculation. High-pressure treatment induced a significant increase of adsorbed proteins at the oil–water interface at both pH conditions. Torrezan et al. [25] found in both the low pH range (2.66–4.34) and the nearneutral range (5.16–6.84), increasing the soy protein concentration (0.32–3.68%) caused a reduction in emulsifying activity index. In acidic conditions, the emulsifying activity index was higher at low-pressure treatments whereas, in the near-neutral pH range, the best emulsifying activity was at the middle range of pressure treatment. The critical effect of the initial protein concentration was confirmed by Wang et al. [26], who observed higher EAI value when lower soy protein concentration (1%) was applied for high-pressure treatment.

Apart from some common non-heating processes that have been applied for modification of the emulsifying properties of soy protein, pulsed electric fields (PEF) [27], extrusion process [28], and radiation [29] have also been studied.
