Nutritional and techno-functional characterization of protein isolates obtained from porcine heart

Given the growing demand for protein sources and the premise of meeting global requirements, there is a need for more efficient use of by-products generated in the slaughter process, like pig heart, which in turn are highly nutritious and also have the capacity for techno-functional application. The pH-shifting technique using aqueous solutions allows the efficient recovery of proteins of high nutritional quality and with improved functional properties, but the muscle protein extraction could be improved using saline solutions that directly influence the yield and final quality of the protein isolate. However, the muscle heme pigments oxidize during extraction, generating a blackish color of the isolates, making their use in foods less attractive. An alternative would be to use nitrite salts in the extraction solution since they interact with heme pigments forming a reddish heat-stable nitrous pigment. Thus, this study aimed at obtaining and characterizing proteins isolated from the porcine heart (HPI), extracted with aqueous (HPI_aq) or saline (HPI_sal) solutions, stabilized or not with sodium nitrite. Treatments did not influence the isolates' protein content (89.0%), and HPI_sal had a higher salt-soluble (53.8 vs. 47.0%) and lower insoluble (19.2 vs. 24.1%) proteins content than HPI_aq. The total heme pigments and collagen contents were unaffected by the extraction solution, but they were higher in samples treated with nitrite (921.6 vs. 707.6 μg hematin per g). Saline extraction only reduces the color intensity (C* = 9.1 vs. 12.7), but HPIs extracted with nitrite had a lighter (L* = 62.4 vs. 59.2), reddish (h = 65.7 vs. 69.5°) and less intense color (C* = 26.5 vs. 30.7). The oil retention, water absorption, and emulsifying capacity (EC) of isolates were not affected by the extraction solutions, but the saline extraction reduced the emulsion stability (ES) and the nitrite-based-treatment reduced EC and ES. The isolates contained high levels of iron (165.8 mg kg^-1) and calcium (53.7 mg kg^-1), with higher sodium (85.6 vs. 8.7 mg g^-1) and heme iron (62.5 vs. 58.7 mg kg^-1) content in HPI_sal than HPI_aq. All the essential amino acids (EAAs) were found in the isolates, with a higher EAA index (160 vs. 128) and estimated protein efficiency (51 vs. 41) in HPI_aq than HPI_sal. The results indicate that the HPI_sal has a high potential to be applied as a techno-functional protein ingredient in meat product formulations, while the HPI_aq has the most interesting profile to be used as a food supplement. Treatment with nitrite should be chosen according to the application and interest of the food manufacturer.