The effects of selected non-thermal interventions on various aspects of the hydrolysis of sodium caseinate by Protamex®

In this thesis the potential applications of non-thermal technologies, including pulsed electric fields, low frequency ultrasound and high pressure, at various stages in the production process of protein hydrolysates were investigated. These technologies have the potential to induce structural changes at a molecular level to protein substrates.The first area investigated was the use of selected non-thermal technologies as pre-treatments for a caseinate substrate prior to enzyme hydrolysis. It has been shown by other authors that pulsed electric field, low frequency ultrasound and high pressure treatments can induce protein denaturation under specific settings and this may increase their susceptibility to subsequent hydrolysis. The pre-treatments of 10% (w/v) sodium caseinate employed were: pulsed electric field treatment of 34 kV cm-1 with a total specific energy of 276.81 kJ kg-1, low frequency ultrasound at a frequency of 20 kHz (amplitudes of 170 µm and 136 µm) and a high pressure treatment of 400 MPa, each applied for 30 minutes. The results from this investigation indicated that low frequency ultrasound applied for 30 minutes at an amplitude of 136 µm is the most promising pre-treatment as a ~33% reduction in the hydrolysis time was achieved, while all other pre-treatments investigated had, at best, a marginal effect on the hydrolysis rate.Low frequency ultrasound (24 kHz) was also applied during enzymatic hydrolysis of sodium caseinate to investigate if the cavitational effect could lead to increased mixing in the system, thus leading to an enhanced rate of hydrolysis. Amplitudes of 20, 40 and 60 µm were investigated with stirring speeds ranging from 100-600 rpm and protein concentrations of 5-15%. It was found that the rate of the hydrolysis could be enhanced by applying ultrasound but only under very specific conditions. For a stirring speed of 400 rpm with a protein concentration of 10% (w/v), application of ultrasound at amplitudes of 20 µm and 40 µm decreased the hydrolysis time by ~25% and ~45% respectively. All other ultrasound conditions investigated lead to either an increased hydrolysis time or had a marginal effect over the control indicating that there is a narrow optimum range of conditions in which this phenomenon occurs.Finally high pressure was compared to thermal treatments as a means to inactivate enzyme activity (e.g. for the termination of a hydrolysis reaction). The enzyme, Protamex®, was subjected to thermal and high pressure treatments when dispersed in water or in a 10% (w/v) sodium caseinate substrate solution. The presence of the caseinate substantially increased the stability of the enzyme to heat treatment. Similarly the enzyme was more pressure susceptible in water than in the presence of sodium caseinate, again indicating a protective effect of the substrate. Over 90% inactivation was achieved at pressures ≥400 MPa with holding times of 30 minutes in water, whereas this level of inactivation required 600 MPa in the sodium caseinate solution. No significant inactivation occurred at pressures <400 MPa.During the course of this work it became apparent that these non-thermal technologies only have a positive effect on hydrolysis under a very narrow range of conditions and careful process optimisation is required. All of the investigations conducted were also at laboratory scale and up-scaling of these experiments may result in different outcomes.

A hard copy of this thesis is available in UCD Library, thesis 12619