Changes of alanyl aminopeptidase activity and free amino acid contents in biceps femoris during processing of Jinhua ham

Abstract

Sixty experimental Jinhua hams were processed by a traditional method. The potential alanyl aminopeptidase (AAP) activity in biceps femoris was determined. The effects of temperature, salt content, sodium nitrate content and pH on muscle AAP were evaluated using response surface methodology. Porcine muscle was found to possess very strong potential AAP activity that decreased gradually during processing from 201,635 U g⁻¹ before salting to 6147 U g⁻¹ after aging. Temperature, pH and salt content had significant exponential effects on AAP activity (P < 0.001). Both temperature and salt content interacted with pH in their effects on AAP activity (P < 0.01). However, 0–50 mg L⁻¹ sodium nitrate had no detectable effect on AAP activity (P > 0.05). The regression model showed muscle AAP maintaining its activity all through Jinhua ham processing, indicating that muscle AAP may generate free amino acids during the processing and storage of Jinhua ham. The concentrations of free amino acids increased significantly (P < 0.05) during Jinhua ham processing, except for arginine and cystine. The concentrations of most free amino acids were 5–20 times higher in the final product than in hams before salting. Final concentrations exceeded thresholds for sensory detection, thus implicating an important role of free amino acids in the determination of Jinhua ham flavor.

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.