BACKGROUND: Ginger rhizome (Zingiber officinale Roscoe) contains ginger proteases and has proteolytic activity. Ginger proteases have been used for tenderizing meat but rarely for milk clotting. The purpose of this study was to purify ginger proteases and to research their biochemical characteristics.
RESULTS: The milk clotting activity (MCA) and proteolytic activity (PA) of the proteases was stable after storage at 4 degrees C for 24 h. The MCA and PA of fresh ginger juice with 0.2% L-ascorbic acid remained stable for 6 days at 4 degrees C. When under storage at -80 degrees C for 2 months, the MCA and PA of the fresh ginger juice and acetone precipitate were still high. Two peaks with protease activity were purified from a DEAE FF ion-exchange column; the specific activity (units mg(-1) protein) of the MCA (MCSA) and PA (PSA) for the first peak was significantly higher than the second peak (P < 0.05). The protease activity of the ginger proteases was significantly inhibited by E-64, leupeptin, and iodoacetic acid. Zymography results showed that two protease fractions purified from ginger juice with 62 and 82 kDa had a higher PA against alpha- and beta-casein than against kappa-casein.
CONCLUSION: The ascorbic acid addition significantly stabilized the MCA and PA of ginger proteases. The protease inhibition test suggested that ginger proteases belonged to the cysteine type. The biochemical characteristics of ginger protease described in this paper can provide useful information for making new milk curd products. (C) 2009 Society of Chemical Industry
