Potential of ultrasound technology towards improvement of protein gel systems as affected by thermal and high-pressure processing as pre-treatment.
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The potential of ultrasound (US) to modify structural and functional properties in complex protein food systems has gained more interest in recent years.US-enhanced gelling properties have been demonstrated in acid milk-protein gel, which is a cornerstone step in the manufacture of dairy products such as yoghurt. Mechanistic studies have associated US-assisted gel-forming and -stabilising properties with improved homogenisation, increased surface hydrophobicity, and temperature effects on promoting protein denaturation. On the other hand, beneficial effects of high-pressure processing (HPP) on the protein gels strength have been reported. Furthermore, synergistic effects of US and heat have been demonstrated on acid gelation, implying that US- assisted gelation may be influenced by the prior properties of the milk proteins in the sample. The objective of this study was to investigate how different milk protein properties induced by heat treatment and high-pressure processing (HPP) will influence the effect of subsequent US treatment on the gelation and gel properties of the resulting acid milk gel. Reconstituted skim milk was thermally processed at 85oC for 20 min or HPP treated at 400 or 600 MPa for 15 min, with or without subsequent US treatment at 68 kHz, 500 W for 15 min at 30oC. In addition, untreated milk with and without sonication were considered. All samples were acidified with glucono-δ-lactone at 30°C until pH ≤ 4.6 to form the milk gel.Gelling and gel properties were characterised by the change in pH, gelation time, and the viscoelastic and textural properties and microstructure of the final gel. The untreated and treated milk samples were assessed by particle size distribution, zeta potential, surface hydrophobicity, concentration of reactive and total sulfhydryl groups, transmission electron microscopy, texture and rheological analysis, gel electrophoretic analysis and Fourier-transform infrared spectroscopy. The untreated milk retained most of the milk protein properties which were mostly in their native state, that is, undenatured. The resulting gel structure was characterised by low resistance to deformation which can be due to lack of denaturation in the milk protein, hence reduced interaction between the protein during gel formation. The introduction of US treatment to the untreated slightly increased the milk acidity and denatured the milk casein resulting in weak gel characterised by lower resistance to large deformation and reduced elasticity.The impact of heat treatment on milk protein structure mainly concerned the denaturation of the whey proteins and caseins where the whey protein, alpha La seemed to have a protective effect against denaturation or dissociation. Heat treatment of milk prior to US treatment allowed for increased interaction among the denatured proteins. The introduction of US further modified the heat-induced changes by increasing the level of native β-Lg and less absence or low intensity of β-sheet peak in the protein structure resulting in more brittle gel. This can be attributed to US cavitation effect and the increased level of native whey protein in the serum phase after US. The high-pressure treatment at 400 and 600 MPa (especially at 400 MPa) increased the milk pH but shortened the gelation time attributed to an HPP induced increase in calcium ion activity. The firmer gel formed were characterised by increased resistance to deformation, with more interconnectivity between the mechanically disrupted milk proteins. The introduction of US homogenised the HPP treated milk, dissolving the aggregates, and reducing the particle size. US modified the HPP effect on the already denatured protein, with high level of denatured α-La observed, native α-casein, as well as reduced level in serum phase κ-casein resulting in strong but brittle gel. This study has demonstrated that effect of US on the gelation and gel properties of the acid milk gel is affected by the prior properties of milk proteins as induced by heat treatment and HPP prior to US treatment. The degree of protein denaturation in the milk induced by pre-treatment, was concluded to be the key factor which determines the subsequent US effects on the gel properties.