39B-6

L. C. WU and K. J. Siebert. Food Sci. & Tech. Dept., Cornell University, Geneva, NY 14456

JUSTIFICATION: After-bottling hazes may form in clear apple juices, most frequently due to protein-polyphenol complexation. Very little is known of the nature of the apple juice haze-active proteins.

OBJECTIVE: The objective was to isolate and characterize haze-active proteins from apples.

METHODS: Haze-active proteins were removed from juice by polyvinylpolypyrrolidone (PVPP) treatment. Hydrophobic proteins were extracted from apples with acetone and purified by hydrophobic interaction chromatography followed by solid phase extraction with C18 or SAX cartridges. Salt-extractable, hydroxyproline-rich apple proteins were also investigated. The molecular weights, isoelectric points and haze-forming activities of the isolates were determined.

RESULTS: PVPP treatment (5 g/L) of Jonagold apple juice reduced total polyphenols by 64.3%, and total protein by 18%. The protein reduction presumably occurred because haze-active protein that was complexed with polyphenol attached to the PVPP. Electrophoresis of juice treated with several PVPP levels showed that protein bands with molecular weights of 15 and 28 kDa were significantly reduced by PVPP treatment. Hydrophobic and hydroxyproline-rich proteins in Jonagold apples were isolated and characterized. Electrophoresis showed the molecular weights of three isolates were 12, 15, and 28 kDa, respectively. Isoelectric focusing revealed that the 12 and 15 kDa proteins were basic (pI > 9.6 and 9.0, respectively), and the 28 kDa protein was acidic (pI 5.2 - 5.8). Amino acid analysis indicated that the isolated proteins were low in aromatic amino acids, had moderate proline content, and were high in serine. The haze-forming activity of the isolated proteins increased with increasing proline content. These results suggest that apple haze-active proteins bind to polyphenols via their proline-rich areas and then form aggregates. The high content of serine may be indicative of oligosaccharide linkage points in these proteins.

SIGNIFICANCE: These results suggest that the proline content of proteins is related to their haze-forming activity.