High Pressure Processing of High Concentration Milk Protein Systems

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Milk proteins represent the structural basis for traditional dairy food products such as cheese and yogurt. The gelation processes used for manufacturing these products, including fermentation by lactic cultures and enzymatic coagulation, require extended reaction time, and are not very easy to control. Recently, high pressure processing (HPP) has caught the interest of the food industry as an alternative nonthermal processing method to induce in-package gelation of milk proteins. The literature review in Chapter One summarizes the current knowledge on the pressure-induced milk protein transformations and gelation. To date, little is known about how pH, calcium, and the addition of milk coagulating enzymes influence the structure and properties of HPP-created milk protein gels. Such details are essential for designing new functional milk protein gels, as these factors are critical for the conformation and functionality of milk proteins, particularly caseins. To cover this knowledge gap, this dissertation demonstrates how HPP can create different milk protein structures at different pH, calcium and chymosin level, and explores the mechanism responsible for the formation of these structures. The effect of pH and calcium on the gel structural properties of milk protein gels HPP-created by pressure treatment of milk protein concentrate (MPC) was investigated in Chapter Two. The rheological properties and microstructure were both pH- and calcium-dependent. Reconstituted MPC suspensions with 12.5% protein were able to form gels after pressurization to 600 MPa for 3 min, at pH above 5.3. An increase in pH (5.3 – 6.6) resulted in stronger gels with higher values of elastic moduli (G’). At a pH close to neutral (6.6), adding calcium to milk protein samples further increased gel strength. SEM micrographs showed more porous, aggregated microstructures with pH reduction or calcium addition. These findings suggest that pH and calcium affected the structure of HPP-created milk protein gels by influencing electrostatic interactions among proteins and shifting the calcium phosphate balance in the system. They also indicate the potential of HPP for creating a whole new palette of textures and structures, as well as new pathways from native dairy proteins to interesting and nutritious high protein dairy foods. One limitation of this process is that pressure-induced gelation leads to casein gels that are mainly noncovalently cross-linked by hydrogen bonds and hydrophobic interactions, which have a finite energy and lifetime. In order to further improve the gel strength and stability, the concurrent use of milk coagulating enzymes and HPP was introduced in Chapter Three. The combined HPP-chymosin treatment led to extensive protein aggregation and network formation at 5 °C, in a much shorter time (3 min) compared to conventional enzymatic coagulation (around 30 min). Gel strength (G’) doubled by adding chymosin, reaching as high as 104 Pa, which is characteristic of a strong gel. The markedly improved gel strength indicated that rennet did maintain its activity at 600 MPa, and substantially enhanced the gel structure by pressurization. Moreover, during the 4 weeks of refrigerated storage, the HPP chymosin gels showed higher gel hardness and better structural stability compared to HPP gels without chymosin. These results clearly show that enzymatic coagulation under pressure can create gel structures that are stable during 28 days of refrigeration. Overall, the findings of this work demonstrate that controlled and fast structural modification of high concentration protein systems can be achieved by HPP. This has significance for the development of milk-protein based products with novel structures and textures, built-in safety, and extended shelf life.

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106 pages


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Gelation; High Pressure Processing; Milk protein; Nonthermal; Stability; Structure


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Union Local


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Committee Chair

Moraru, Carmen I.

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Padilla-Zakour, Olga I.
Davis, Andrew M.

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Food Science and Technology

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Ph. D., Food Science and Technology

Degree Level

Doctor of Philosophy

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Government Document




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dissertation or thesis

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