Inhibition of specific degradation of 57-kDa protein in royal jelly during storage by citrate buffer

We have previously shown that ethylenediaminetetraacetic acid (EDTA) suppresses the storage-dependent degradation of 57-kDa protein, which is a possible marker for freshness and quality of royal jelly (RJ) through the inhibition of a proteinase in RJ. We suggested that EDTA could be useful as a preservative agent to maintain the quality of RJ. Here, we report the effects of other metal chelators. di- or tricarboxylic acids such as citric acid and malic acid, on proteinase activity in RJ and the specific degradation of 57-kDa protein during storage. Various carboxylic acids inhibited the proteinase activity in RJ, but did not suppress storage-dependent degradation of 57-kDa protein. However, when RJ was stored with various carboxylate buffers (pH 4.0) at 40 deg C, the degradation of 57-kDa protein during storage was suppressed through the inhibition of proteinase in RJ. Among the buffers, citrate buffer (pH 4.0) effectively inhibited the decrease of 57-kDa protein concentration in RJ during storage. These results suggest that citrate buffer (pH 4.0) could be available as a new preservative agent to maintain the quality of RJ.