Causes and prevention of rutin (quercetin-3-O-rhamnoglucoside) crystal deposition in pickled green asparagus (Asparagus officinalis).

摘要

Crystals that form on surfaces of pickled asparagus stems and factors that affect their formation were studied. The studies were designated to observe crystals in fresh and pickled asparagus, to determine the crystal composition, to determine the conditions promoting crystal formation, and to prevent or retard rutin crystallization in asparagus pickles. Fresh asparagus tissue, retorted low acid and acid asparagus, and crystals from pickled asparagus were observed under light and polarizing light microscopy (PLM). After methanolic extraction, rutin content was determined in different areas of fresh asparagus. Crystal composition was determined by HPLC. Rutin oxidation, thermolability, solubility and rutin desupersaturation in model systems with and without thickener agents were assessed. Acidified asparagus was retorted or pasteurized, and the effect of processing on rutin crystallization was established. Pretreatments were applied to destroy rutin crystals on asparagus surface. Processed asparagus was treated with different salts, thickeners, and beta-glucosidase in the brine. Samples with preformed crystals were treated with beta-glucosidase. Rutin crystals were observed in fresh and low acid and acid retorted asparagus. The maximum rutin content was in asparagus tips and chlorenchyma tissues. Crystals on pickled asparagus were identified as rutin by PLM and HPLC. Retorted low acid and acid asparagus contained more soluble pectin, and tissue disruption than pasteurized pickled asparagus, which was why retorted asparagus contained only microscopic crystals. Rutin increase in brine and crystallization on asparagus spears occurred simultaneously, indicating that crystallization did not occur due to brine desupersaturation. Crystallization occurred at sites where rutin crystals were already present in fresh spears. Pre-treating asparagus with 0.2 M NaHCO3 and adding 25 mg/L ferrous gluconate or 50 mg/L iodate and adding pectin in the cover brines delayed crystallization. beta-Glucosidase did not prevented crystallization but destroyed the crystals after they were formed in pickled asparagus producing quercetin, which formed crystals that did not adhere to the stems. In conclusion, rutin crystallization was not prevented in pickled asparagus because: crystals existed in fresh asparagus, rutin solubility was low at pickles pH, and the sites for the crystallization were not destroyed by the pasteurization.