By means of the SPE-cartridge module Syncore® Analyst parallel evaporator the number of samples analyzed per day can be increased by three times. The method was assessed successfully with 17 samples, the analytical results of which are detailed and evaluated.
As cider pears have a lower market value than cider apples, it is frequently suspected that apple juice is diluted with the cheaper pear juice. According to Swiss law, apple juice can contain a maximum 10% of pear juice and pear juice a maximum 10% of apple juice. For any other ratios a different designation, e.g. sweet must or fruit juice has to be used, so that the composition is clear for the consumer.
The question posed is how to check that such a regulation has been followed. To ensure the purity of a fruit juice, the SLMB (Swiss Handbook for Food) suggests the analysis of proline.
According to these guidelines, a pure apple juice should contain <15 mg/l of proline, whereas pear juice has a proline content of 30 – 250 mg/l. Additionally, according to the SLMB, pear juice contains higher amounts of sorbitol (averaged 15g/l vs. 4g/l) and higher amounts of citric acid (0.1 – 4 vs. 0.05 – 0.2 g/l) compared to apple juice.
Our own analyses have shown increased concentrations of proline and sorbitol in apple juice, indicating the presence of excess pear juice, but the parameters are unreliable for a quantitative assessment of the actual contents.
The reason for this is that although these compounds are present in both apples and, in higher concentrations, in pears, the natural deviations are relatively high. Thus, these parameters only allow for a more or less qualitative conclusion concerning the actual pear content.
Another group of compounds used for the determination of fruit juice concentrations are the phenolic glycosides. Schieber et. al. describe a method to detect any addition of pear juice by determining the presence of isorhamnetin-3-glucoside. Furthermore Spanos and Wrolstad show that the phenolic profiles of apple and pear juices differ most obviously when comparing the content of 4-hydroxyphenyl-ß-D-glucopyranoside, trivially named arbutin. During subsequent years, other research groups drew the same conclusion. As to the analytic proof, the advantage of arbutin compared to other phenolic glycosides is the fact that the molecule is acid-resistant.