Lemon juice is deceptively simple in composition, comprising water, sugars, organic acids (principally citric acid) and small amounts of amino acids, vitamins and phenolic compounds. As with most fruit juice analysis, the total titratable acidity and soluble solids content are frequently used as defining parameters for lemon juice, too. Total titratable acidity includes all the acidic substances in juice that react with sodium hydroxide, with citric acid being the predominating organic acid. Of various approaches to measuring acidity the Thermo Scientific™ Gallery™ Automated Photometric Analyzer provides a reliable method for routine analysis of citric acid in juices. The major soluble solids in lemon juice are sugars and the total soluble solids content is determined using a refractometer such as the Pro-Juice refractometer.
Economic Adulteration Is Profitable
Unfortunately there is a huge natural variability in the composition of lemon juice, meaning that acidity and solids content alone are not sufficient to detect economic adulteration, the most common being by dilution with water, especially if citric acid and sugars are also added.
Poor lemon harvests, for example due to adverse weather conditions, can be significant drivers for adulteration of lemon juice and even small additions of water can increase profit margins. In response to this threat, there have been a large number of studies focusing on developing and refining techniques for the detection of adulteration of fruit juices in particular employing isotope ratio mass spectrometry (IRMS) to identify whether additions have been made of substances derived from photosynthetic pathways other than from a citrus tree. However, as natural variation of the isotopic distribution of fruit juice components is relatively large, a high level of adulteration would be required to allow its detection by traditional IRMS.
New Approaches to Detecting Adulteration
A recent publication in Food Additives & Contaminants Part A by Wang & Jablonski from the US Food & Drug Administration describes alternative targeted and non-targeted approaches to detect adulteration of lemon juice. Twenty-two batches of lemons were obtained from USA, Chile and Mexico and were freshly squeezed to prepare juices which, in each case, were adulterated by adding an aqueous solution containing 5% (w/w) citric acid and 6% (w/w) sucrose. Zero, 40% and 70% additions were made to each juice to provide 66 different samples which were divided into five replicates for analysis.
Lemon juice samples were subjected to a minimal pre-treatment by extracting with dimethyl formamide (DMF) and collecting the upper layer after centrifugation. Both targeted and untargeted analysis of these samples was investigated employed LC-high resolution mass spectrometry using an Exactive™ Orbitrap MS operating at 100,000 resolution. For the targeted approach, seven components separated by LC including hesperidin and eriocitrin, which were the two major phenolic compounds were measured. Unfortunately, the natural variation in levels of these components between juices from different sources made it impossible to detect adulteration. Dilution with as much as 50% water would only bring those juices with the higher levels of these marker compounds down to average levels. The alternative untargeted approach relies not on identification, but on fingerprinting based on high resolution mass spectrometric profiles and then using the power of chemometric techniques for discrimination between juices. This approach was successful at detecting high levels of adulteration (70%), but could not discriminate when there was a 30% dilution with water, acid and sugar. The detection of lemon juice adulteration therefore remains a challenging problem with the authors of this paper concluding that the answer is probably to employ a combination of multiple analytical techniques including LC-MS and IRMS.