Abstract
The ultrasonic effect on the physicochemical and emulsifying properties of three animal proteins, bovine gelatin (BG), fish gelatin (FG) and egg white protein (EWP), and three vegetable proteins, pea protein isolate (PPI), soy protein isolate (SPI) and rice protein isolate (RPI), was investigated. Protein solutions (0.1–10 wt.%) were sonicated at an acoustic intensity of ∼34 W cm−2 for 2 min. The structural and physical properties of the proteins were probed in terms of changes in size, hydrodynamic volume and molecular structure using DLS and SLS, intrinsic viscosity and SDS-PAGE, respectively. The emulsifying performance of ultrasound treated animal and vegetable proteins were compared to their untreated counterparts and Brij 97.Ultrasound treatment reduced the size of all proteins, with the exception of RPI, and no reduction in the primary structure molecular weight profile of proteins was observed in all cases. Emulsions prepared with all untreated proteins yielded submicron droplets at concentrations ≤1 wt.%, whilst at concentrations >5 wt.% emulsions prepared with EWP, SPI and RPI yielded micron sized droplets (>10 μm) due to pressure denaturation of protein from homogenisation. Emulsions produced with sonicated FG, SPI and RPI had the similar droplet sizes as untreated proteins at the same concentrations, whilst sonicated BG, EWP and PPI emulsions at concentrations ≤1 wt.% had a smaller droplet size compared to emulsions prepared with their untreated counterparts. This effect was consistent with the observed reduction in the interfacial tension between these untreated and ultrasound treated proteins.
Highlights
Proteins perform a vast array of functions in both the food and pharmaceutical industries, such as emulsification, foaming, encapsulation, viscosity enhancement and gelation
Bovine gelatin (BG), fish gelatin (FG), Egg white protein (EWP), Pea protein isolate (PPI), soy protein isolate (SPI) and rice protein isolate (RPI) as a function of time are shown in Fig. 1 and Table 2
This study showed that ultrasound treatment (20 kHz, ~34 W cm-2 for 2 min) of animal and vegetable proteins significantly (P < 0.05) reduced aggregate size and hydrodynamic volume, with the exception of RPI
Summary
Proteins perform a vast array of functions in both the food and pharmaceutical industries, such as emulsification, foaming, encapsulation, viscosity enhancement and gelation. This functionality arises from the complex chemical make-up of these molecules (O’Connell & Flynn, 2007; Walstra & van Vliet, 2003). Once at the interface proteins undergo surface denaturation and rearrange themselves in order to position their hydrophobic and hydrophilic amino groups in the oil and aqueous phase respectively, reducing the interfacial tension and overall free energy of the system (Caetano da Silva Lannes & Natali Miquelim, 2013; McClements, 2004). Proteins provide several advantages for emulsion droplet stabilisation, such as protein-protein interactions at interfaces, and electrostatic and steric stabilisation due to the charged and bulky nature of these biopolymers (Lam & Nickerson, 2013; McClements, 2004; O’Connell & Flynn, 2007)
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