Influence of processing and storage on the phenolic composition of apple, pear and white grape juice

Graduation date: 1989Methodology for characterization of the phenolic profile of\ud apple, pear and grape juice was developed. It utilized High\ud Performance Liquid Chromatography (HPLC) for separation of\ud individual phenolic compounds and Diode Array Detection for\ud recording the Ultra-violet (UV) spectrum of chromatographic\ud peaks. The high resolution achieved and the ability of recording\ud more than one wavelength simultaneously, allowed for\ud quantitation of phenolic acids (cinnamics and benzoics), flavonol\ud and dihydrochalcone glycosides, arbutin and hydroxymethylfurfural\ud (HMF) with minimum sample preparation. Quantitation of\ud procyanidins, however, required preliminary isolation and\ud analysis in a separate run. Procyanidin isolation based on\ud Sephadex IH-20 resulted in high recoveries and degree of\ud reproducibility.\ud The methodology was applied to study the influence of\ud variety, post-harvest storage, SO₂, initial high temperature short time (HTST) heat treatment, enzymatic clarification,\ud fining, bottling, concentration and storage on the phenolic\ud composition of apple, pear and grape juice. Processing with SO₂\ud inhibited extensive phenolic oxidation during pulping and\ud resulted in increased phenolic yields. Initial HIST treatment\ud protected phenolics extracted in the juioe from degradation\ud during processing. Cinnamic hydrolysis occurred during enzymatic\ud clarification. The hydrolytic activity was removed with fining\ud or bottling. There was no appcirent reduction of phenolics with\ud fining. Procyanidins were sensitive to the heat applied during\ud bottling and concentration. Storage of juice concentrates for\ud nine months at 25°C resulted in formation of HMF, considerable\ud degradation of cinnamics and dihydrochaloones and total loss of\ud procyanidins and flavonols.\ud The effect of diffusion extraction at different teirperatures\ud on the phenolic composition of apple juice from different\ud varieties was also studied. Up to a three-fold increase in\ud cinnamics and a five-fold increase in dihydrochalcone glycosides\ud were measured in diffusion extracted juice relative to juice\ud extracted with conventional pressing. The increase in\ud procyanidins and flavonols was even higher