Effect of low frequency ultrasound on the solubility and physical characteristics of reconstituted milk protein powders

This study has adopted a systematic approach to identify the extent to which 20 kHz ultrasound affects the particle size and solubility of different protein solutions containing 4%, 7% and 10% whey proteins, caseins or a mixture at a range of energy densities (15-400 J/mL) and various pH (4.0-9.0). Milk proteins hold a prominent place in the food industry as they are used in many manufactured products for their functionality and most importantly nutritional benefits. Various dairy ingredients are currently used in food processing, with notable constituents including whey protein concentrate (WPC), whey protein isolate (WPI), milk protein concentrate (MPC), and sodium caseinate (Na-CN). To produce these fundamental ingredients, the starting material of bovine milk is fractionated and spray dried to render it into a powder form. Subsequent use in manufacturing of these ingredients requires them to have a high solubility as the most crucial aspect to ensure the overall quality of the product.

Over the years, various technological innovations have been developed for the food manufacturing industry; one such novel innovation with commercialisation potential is low frequency (20 kHz) ultrasound, which has been extensively researched to determine its effect on milk ingredients and commercial viability. Research to date has revealed that low frequency ultrasound generates acoustic cavitation, which is a process that releases vast amounts of energy, in the form of heat, pressure and shear, which can be exploited for various food processing purposes.

In research for this thesis, the physical shear generated by sonication at a frequency of 20 kHz was used to treat reconstituted protein solutions. WPC, WPI, MPC and Na-CN were reconstituted in water to 4%, 7% and 10% (w/w) protein concentrations at various pH (4.0, 4.6, 6.7 and 9.0) levels. The reconstituted suspensions were then sonicated at a range of energy densities (15, 30, 150, 300 and 400 J/mL), which correspond to a contact time of 26 seconds, 51 seconds, 4 minutes and 27 seconds, 8 minutes and 54 seconds and 11 minutes 37 seconds, respectively. The particle size, surface charge and solubility were measured before and after sonication. In the case of reconstituted sodium caseinate, the solution viscosity was also determined.

Previous literature has shown that ultrasound reduces the particle size of suspensions and it was hypothesised that low frequency ultrasound will improve the solubility of reconstituted protein powders by reducing the size of the large insoluble aggregates. Past studies, however, only explored the effects of sonication at random and often excessive and unjustified energy densities and a systematic study was required to identify optimal treatment conditions that serve a practical purpose. Furthermore, the pH of sonicated solutions was neglected and a pragmatic and comprehensive study exploring a range of alkali, pH neutral and acidic environments was lacking. Since the solubility of the powders is mainly affect by the composition, a systematic design was incorporated into this study to identify the effects of ultrasonic treatment on protein suspensions containing only whey proteins (WPC and WPI), containing only casein proteins (Na-CN) and containing a mixture of casein and whey proteins (MPC).

The results demonstrate that low frequency ultrasound reduces the particle size of the aforementioned reconstituted dairy ingredients when larger aggregates are present in the first 26 seconds of treatment. For example, the particle size prior to sonication for 10% protein WPC solutions reconstituted to pH 6.7 was 31 µm and after 26 seconds sonication or the equivalent to an applied energy density (ED) of 15J/mL, the particle size was reduced to approximately 17µm. The particle size continues to reduce as the ED increases to 30 J/mL and up to 150 J/mL for all pH and protein concentrations trialled. Beyond this ED, however, there were minor changes (p > 0.05) in the particle size. Furthermore, the data exhibit that particle size reduction corresponds to improved solubility. The data indicated that protein solutions at pH 4.0-4.6 were close to the isoelectric point (IEP) for all dairy ingredients studied. This indicates that at the IEP, the solubility was at its lowest due to protein aggregation. Under extreme alkali and acidic environments, the protein solutions sometimes gelled and the solubility and particle size could not be determined as was the case for sodium caseinate at pH 4.0 and 4.6.

From the data, it is indicated that the solubility was improved and particle size was initially reduced after 15J/mL sonication. However, the solubility and particle size become consistent at 150 J/mL sonication and there is neither further reduction in particle size nor improvement in solubility. The highest solubility for all dairy ingredients was achieved by adjusting the pH to 9.0.

Findings suggested that there is little to no effect on the surface charge of the protein solutions, which is consistent with prior research findings, namely, that low frequency ultrasound generates a physical effect rather than a chemical effect.