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Title: Physicochemical characterization of liver paste and its main constituents
Other Titles: Fysicochemische karakterisering van leverpastei en constituenten
Authors: Steen, Liselot
Issue Date: 9-Jan-2015
Abstract: Macroscopic properties such as mouth feel, texture and stability are important quality attributes of liver paste, determining consumer acceptance. Liver paste typically contains a significant amount of liver (proteins) and adipose tissue or lard and their functional behaviour is expected to be important for the macroscopic properties of this product. This research focused on understanding the mechanisms determining the macroscopic properties of liver paste. Not only the physicochemical characteristics of the constituents but also the microstructure of liver paste were studied. Knowledge of their respective relation to the macroscopic properties is important in maintaining or optimizing the quality of this product and in the development of new products (e.g. clean label products). In a first chapter, the macroscopic characteristics of liver paste (emulsion stability, texture and mouth feel) were studied, whereby the effect of liver/fat ratio (35/35 and 20/50) and salt (0 and 1.8%) was investigated. As the liver/fat ratio decreased, the fat binding properties decreased while the creaminess perception increased. The hardness, however, was not influenced by the liver/fat ratio. Salt increased the fat binding properties and hardness of liver paste and resulted in a less spreadable liver paste.The second chapter studied the microstructure of liver paste by means of rheology and light microscopy and the relationship with the macroscopic properties. The rheological properties of the intermediates (liver batter and liver paste batter) were also studied in order to understand how the structure of liver paste is formed during liver paste processing. For the conventional liver paste (35/35, 1.8% salt), it was shown that liver proteins were present in the continuous phase and that the fat globules were surrounded by a protein layer. Both intermediates and liver paste were characterized as weak gel-like emulsions with the elastic modulus (G′) higher than the viscous modulus (G″). G′ and G″ of liver paste were higher in magnitude compared with both intermediates which shows that structure building occurred during pasteurization and cooling: the more pourable liquid-like liver paste batter is converted to a more solid-like spreadable liver paste. This structure building is caused by a combination of fat crystallization and protein denaturation and aggregation. Generally, the values of the viscoelastic parameters of liver paste batter and liver paste increased with the addition of salt. With salt, a stronger and more stable liver paste was obtained which may be attributed to solubilisation of salt soluble liver proteins, making them more available to act as emulsifier. Salt however affected the viscoelastic properties of liver batter in the opposite way: a weaker structure was formed with salt. A microstructure with smaller fat globules was obtained with the addition of salt, explaining the increased hardness and fat binding properties. A higher liver/fat ratio (35/35 versus 20/50) only increased the viscoelasticproperties of liver paste batter while liver paste was not affected. This might be attributed to the crystallization of the fat in the liver paste with a high fat/liver ratio, which besides the structural potential of liver proteins, also aids to structure building of liver paste. However, a higher liver/fat ratio did increase the linear viscoelastic region in both liver paste batter and liver paste, indicating that the structural contribution of liver proteins is more stress resistant than the structure generated by fat crystallization. The micrographs showed a heterogeneous structure with bigger fat globules and fat channels with a decrease in the liver/fat ratio, causing decreased fat binding properties.Next, the functional properties of liver protein fractions were investigated and compared to commercial protein ingredients (sodium caseinate, porcine globin and porcine albumin). Their emulsifying, gelling and foaming properties as well as their molecular weight distribution and surface hydrophobicity were studied. Two protein fractions were characterized: water soluble (WSLP) and water+salt soluble liver proteins (W+SSLP) (= salt soluble liver proteins (SSLP) in the presence of WSLP). The effect of different salt concentrations was also investigated (0, 1.8 and 3.4% NaCl). Both WSLP and W+SSLP displayed good emulsifying properties while their gelling properties were rather weak. An increase in salt concentration decreased the emulsifying properties of WSLP while the effect on W+SSLP was less pronounced. The gel forming ability of W+SSLP containing 0% was higher compared to this at high salt concentrations, probably due to a stronger intermolecular network of non-solubilized myofilaments compared to the network formed of more solubilized myofibrillar proteins at high salt concentrations. The same conclusions could be drawn for the WSLP, indicating that with phosphate solution also myofibrillar fragments were extracted as demonstrated by SDS-PAGE. Higher salt concentrations shifted the gel temperature of both WSLP and W+SSLP to lower temperatures. No relation could be found for the effect of salt between the functional properties of liver proteins in model systems and the microstructure and macroscopic properties in liver paste. In the last chapter of this research, differential scanning calorimetry and real-time X-ray diffraction using synchrotron radiation were used to study the isothermal crystallization formation mechanism of lard at 18, 20, 22 and 24°C. At 18°C, lard crystallized in three steps. A potential mechanism for these three steps was proposed. In the first step, part of the melt (the trisaturated triacylglycerols (TAGs)) crystallized in alfa crystals adopting a double length structure (2L), while the second step consisted of a polymorphic transition of these 2Lalfa crystals to ߒ crystals with a triple length structure (3L). Extra 3Lߒ crystals consisting of monounsaturated TAGs were also formed directly from the melt. In the third and last step, ß crystals were formed due to a second polymorphic transition of trisaturated 3Lߒ crystals to ß crystals adopting a 2L structure. Above a cut-off temperature of 20°C lard crystallized in two steps: no formation of alfa crystals could be observed and 3Lߒ crystals (trisaturated and monounsaturated TAGs) were formed directly from the melt. This proposed mechanism implies that lard crystallization is characterized by an overlap of fractionated crystallization and polymorphic transitions.The results obtained in this work showed more insight in the microstructure and macroscopic properties of liver paste. In addition, it was demonstrated that liver paste is mainly stabilized by liver proteins surrounding fat globules. Correlation of the functional behaviour of the constituents studied in model systems compared to liver paste was however not evident.
ISBN: 978-90-8826-392-7
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Bioengineering Technology TC, Technology Campuses Ghent and Aalst
Microbial and Molecular Systems, Campus Kulak Kortrijk
Technologiecluster Bioengineering Technologie
Microbial and Molecular Systems - miscellaneous

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