- 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
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.