Organic Acid Analysis
The tartness of pure fruit juices is largely due to a group of organic acids. The particular acids present, and their relative abundances, can be diagnostic features of the juices.
For example, the acidity of apple juice is largely due to malic acid. Figure 6 summarizes the malic acid content of commercial apple juice. The mean value was 0.45 g/100g, with most samples between 0.3 and 0.6 g/100g. Dilution of apple juice solids with added sugar will result in declines in the malic acid content.
Natural malic acid is chiral, and consists entirely of the L- isomer. Added malic acid can be detected by determination of the unnatural D- isomer. This can be done either by enzyme assay with D-malate dehydrogenase, or by chiral HPLC techniques. Figure 7 shows a chiral HPLC chromatogram of two apple juices, one containing added malic acid. The D-malic acid peak is clearly visible in the latter chromatogram.
Isocitric acid is a minor organic acid found in most fruit juices. It can be easily measured by enzyme assay. Sugar addition to orange juice will result in reduced isocitric acid levels. Since isocitric acid is an expensive substance commercially, it is difficult to mask sugar substitution by back addition of isocitric acid. Since the quantities of citric and isocitric acids are correlated, high citric/isocitric ratios can also be used as an indicator of citric acid addition in some juices.
Organic acid analysis can also be ueful in detecting additions of less expensive fruit juices, such as white grape juice or pear juice. For example, additions of grape juice to other juices can be detected by the presence of tartaric acid in the juice.
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