Fruit juices color changes

Citrus fruits contain citric acid, so their juices are acidic. This is shown here by the color changes on the indicator paper. Acids taste sour. 

Most of these reports have shown yeasts to be the predominant fungi involved in juice spoilage (Parish and Higgins, 1989). Yeast spoilage of fruit juice can result in formation of haze, production of CO2 and off-odors, and changes in color. 

Sometimes the proposed method is an indirect procedure to monitor the excess of oxidant or the color change of an auxiliary reagent such as the method described for the assay of azathioprine, either in pure form or in pharmaceutical formulations the methods are based on the oxidation of the drug with excess N-bromosuccinimide or chloramine-T and determining the consumed reagent by the decrease in color intensity of celestine blue or gall-ocyanine, respectively. Another example is the determination of ascorbic acid in fruit juice and pharmaceuticals on the basis of its inhibition effect on the reaction between hydrochloric acid and bro-mate. The decolorization of methyl orange due to the reaction products was used to monitor the reaction at 510 nm. 

Fruit juices generally develop undesirable color changes and a stale flavor during storage [90]. While off-flavors due to the Maillard reaction are quite conunon, few studies other than those noted have been successful in identifying the components responsible for staleness. 

A new concept for microtitration using a LOV system with sequential injection of monosegmented flow has been proposed (Jakmunee et al., 2005). The performance of the system was demonstrated by the assay of acidity in fruit juices based on acid-base neutralization. Standard/sample solutions containing citric acid, indicator, and sodium hydroxide were sandwiched between air segments and then aspirated upon flow reversal and removal of the air segments, the change in the absorbance of the indicator color was monitored. 

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