Changes of alanyl aminopeptidase activity and free amino acid contents in biceps femoris during processing of Jinhua ham
Introduction
Jinhua ham, one of the famous dry-cured hams in the world, is appreciated for its characteristic flavor in most Asian countries. The development of the desired aging flavor requires a long processing time. Intense proteolysis has been observed during dry-curing process (Martín et al., 1998, Molina and Toldrá, 1992), giving rise to an important collection of free amino acids (Buscailhon et al., 1994, Cordoba et al., 1994, Sforza et al., 2001) that can directly contribute to flavor such as sweet, sour or bitter tastes, or indirectly contribute as precursors of volatile flavor compounds. So the presence of free tyrosine and lysine, for example, has been related to the development of aged flavor in Parma ham (Careri et al., 1993).
The free amino acids in dry-cured hams are mainly generated from muscle proteins and peptides by the actions of aminopeptidases (1-aminoacylpeptide hydrolases) (Toldrá, Flores, & Sanz, 1997). Muscle aminopeptidase activities, including alanyl, arginyl, leucyl, tyrosyl and pyroglutamyl aminopeptidase activities, were reported in both raw and dry-cured ham and had good stability even after 8 months of curing (Toldrá et al., 1992). Alanyl aminopeptidase (AAP) accounts for 83% of the total porcine skeletal muscle aminopeptidase activities and demonstrates broader substrate specificity toward aromatic, aliphatic, and basic aminoacyl bonds (Flores, Aristoy, & Toldrá, 1996). Therefore, it may be the aminopeptidase that takes the most important role in the generation of free amino acids during dry-cured ham processing.
Aminopeptidase activity and free amino acid generation are very important in dry-cured ham production for their involving in the development of characteristic ham flavor. However, relevant information from Jinhua ham processing has not been well documented up till now. Thus, the main objective of the present work was to trace the changes of AAP activity and free animo acid contents in biceps femoris during Jinhua ham processing and evaluate the possible contributions of AAP and amino acids to Jinhua ham flavor development.
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Materials
Sixty trimmed green hams weighting 6.2–6.9 kg from hind legs of local cross swine (5–6 months, 90–100 kg) from Lanxi (Zhejiang province of P.R. China) were used to produce Jinhua hams. 7-Amido-4-methyl-coumarin (AMC), Ala-AMC was purchased from Bachem Feinchemikalien (Bubendorf, Switzerland) and other chemical agents, purchased from Sigma (St. Louis, MO).
Jinhua ham processing
Jinhua ham processing was carried out under natural condition as prescribed by traditional processing technology in Zhejing Provincial Food
Changes of potential AAP activity during processing
The results of potential AAP activity after each stage of Jinhua ham processing were shown in Table 2. Porcine muscle possessed very strong potential AAP activity, but it decreased gradually during processing from 201,635.43 U g−1 at prior-salting to 6147.11 U g−1 at the end of post-aging. Salting and sun-drying process greatly reduced AAP activity (P < 0.05) and about 27.11% and 35.88% of AAP activity losses were observed during salting and sun-drying stages, respectively. No statistically
Discussion
Past research work has revealed that the activities of all the aminopeptidases detected decreased gradually along the processing of Spain dry-cured ham and AAP showed the highest exopeptidase activity along the full process (Toldrá, Aristoy, & Flores, 2000). About 20–27% of AAP activity discovered in fresh ham was remained after 8-month processing (Toldrá et al., 1992, Toldrá et al., 1997). Our work proved similar decline tendency of AAP activity during Jinhua ham processing, but the activity
Conclusion
Muscle AAP possessed very strong potential activity that decreased gradually during Jinhua ham processing and about 3.05% of prior-salting activity was left in the products. Temperature, salt content and pH value significantly affected AAP hydrolyzing activity, which made AAP actual activity accounting for less than 5% of its corresponding potential activity at each stage of Jinhua ham processing. Even so, AAP could always show considerable actual activity. The concentrations of all the free
Acknowledgements
This work was supported by Grant 2002AA248031 from the 863 Program of China governments and project 03092 from China Ministry of Education. The authors acknowledge the staff of University Center Lab, Henan Agricultural University, PR China, for their help when some research work was done there. We thank Howard Swatland from University of Guelph, Ontario, Canada, for his work in improving the article.
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