Graduate Thesis Or Dissertation
 

Color of strawberry juice and concentrate as influenced by heating and storage temperatures

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  • This investigation was conducted to study the effects of blanching and storage temperatures on the color of strawberry juice and concentrate. Strawberry juice and concentrate (42°βrix) were prepared from blanched crushed Tioga strawberries. Crushed strawberries were blanched, in a steam-jacketed kettle, at 70°, 80°, and 90°C for 1 min. Ascorbic Acid (AA), at a level of 0.01%, was added to one batch and hoped to counteract polyphenol oxidase (PPO) activity in the early stages of processing. Samples were then stored at 1°, 21° and 38°C and the chemical composition as well as the color parameters were monitored throughout the study. Blanching at 70°C was found to heat-inactivate PPO in strawberries. Generally, blanching protected the compositional constituents, such as, anthocyanins (ACN), AA, leucoanthocyanins, flavanols and total phenolics. Initially, blanched samples contained higher concentrations of these chemicals than the unblanched (control) samples. Samples blanched at 80°C contained highest concentrations followed by those blanched at 90° and 70°C. This protective effect was markedly reduced in samples blanched with added AA. AA decomposition products seem to initiate other degradation reactions. There was a considerable decrease in the juice chemical constituents upon concentration. A particular striking decrease was observed with AA where samples lost more than 75% of their initial content. All samples experienced gradual losses of their chemical constituents accompanied by loss of the bright red color associated with freshly prepared juice and concentrate. The losses increased with increased storage temperatures. Storage at 38°C, as a result, was found to be a major factor in the loss of the chemical constituents and the rapid deterioration of the color. Concentrate samples developed burnt flavor by the third week and lost all ACN and AA contents by the end of the storage period. The loss was much less on storage at 21°C and least at 1°C. At any storage temperature, samples blanched at 80° and 90°C retained more of their constituents as well as better color quality parameters when compared to the control or those blanched at 70°C. Samples blanched with added AA experienced greater losses of their color values and chemical constituents, particularly ACN and AA, when compared to samples blanched without AA. ACN and AA degradation in samples stored at 1°C followed first order reaction kinetics, while browning formation in these samples and in juice samples stored at 21°C followed zero order reaction kinetics. Rate constants suggest a major role for AA in the deterioration of color. Moreover, they also suggest a protective effect on blanching on the overall quality of the juice and concentrate. Blanching at 80°C for 1 min was the optimum processing temperature investigated, while low storage temperatures are, naturally valuable in retaining better color quality strawberry juice and concentrate.
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