Meat has gained great popularity all over the world for its abundant nutrients and delicious flavor. Along with the production of meat, large amounts of animal by-products are generated but underutilized. Protein hydrolysis can recover protein from meat by-products, however, the resultant protein hydrolysates are often associated with off-flavors such as bitterness. This thesis aimed to improve the taste of protein hydrolysates derived from porcine hemoglobin and meat through enzymatic treatments, and to characterize the changes in sensory and physicochemical properties of the modified hydrolysates.
Plastein reaction refers to the peptide aggregation reaction induced by protease, and has been reported to reduce the bitterness of protein hydrolysates. Alcalase and papain were used to catalyze the protein hydrolysis and plastein reaction of porcine hemoglobin and meat. In terms of plastein formation efficiency shown by turbidity and particle size, Alcalase was more effective than papain, and hemoglobin hydrolysates acted as a better substrate than meat hydrolysates. It was hydrophobic interactions rather than covalent bonds that played a key role in the generation of plastein. The location of aggregative amino acid sequences in precursor proteins involved in the formation of plastein were investigated using LC-MS/MS and visualized by Peptigram.
Another strategy is to use γ-glutamyl transfer reaction, which involves production of γ-glutamyl peptides, thereby enhancing the kokumi sensation of protein hydrolysates. Kokumi sensation describes the savory taste of the food with complex mouthfeel and continuous aftertaste. γ-Glutamyltranspeptidase (GGT) was added to the protein hydrolysates from both raw materials for γ-glutamyl transfer reaction. After the reaction, most sensory attributes of protein hydrolysates (particularly from hemoglobin) were greatly enhanced. Without adding extra glutamine, the glutamine content in meat hydrolysates was higher than that in hemoglobin hydrolysates, which led to more kinds and a higher total concentration of γ-glutamyl dipeptides in γ-glutamylated meat hydrolysates. Addition of extra glutamine at 10 and 20 mM promoted the synthesis of γ-glutamyl dipeptides in both hydrolysates, but had little effect on kokumi enhancement.
Furthermore, hydrolysates and γ-glutamylated hydrolysates from both raw materials were incorporated into emulsion-type pork sausages (2% and 4%). Addition of all hydrolysates and γ-glutamylated hydrolysates (especially from hemoglobin) could enhance most of the sensory properties of sausages. However, the sensory differences among sausages were mainly caused by the concentrations of hydrolysates and to a less extent due to whether hydrolysates contained γ-glutamyl peptides. Besides, kokumi enhancement by γ-glutamyl transfer reaction was only observed with hemoglobin hydrolysates.
Overall, the results of this thesis increase the knowledge of improving the taste of protein hydrolysates through enzymatic treatments, and provide ideas for the potential use of protein hydrolysates from meat by-products as natural food ingredients.