Richard W. Dapson writes, on 15th October 2021:
Detection of formaldehyde in milk.
Get your cell phones ready: there is a new use for Schiff’s reagent! But ﬁrst, let’s review what Schiff’s reagent is and does.
The classic Schiff reagent is a decolorized version of pararosaniline (CI 42500) or basic fuchsine. The red dye is rendered colorless by treating it with sodium metabisulﬁte or other agents that generate sulfurous acid. H2SO3 attaches to the central carbon atom of the conjugated (aromatic) system responsible for the dye’s color, thereby disrupting the arrangement of alternating single and double bonds and destroying the color. Because the reagent contains primary amine groups (–NH2), it can combine with aldehydes, upon which the color is restored. In histochemistry, this reaction is commonly used to detect certain mucins and other carbohydrates in a staining sequence called the PAS (periodic acid-Schiff) test. Carbohydrates that contain a pair of adjacent hydroxyl (–OH) groups (called glycols or vic-hydroxyls) can be oxidized to aldehydes by periodic acid. The aldehydes react with colorless Schiff’s reagent and positive tissue sites become magenta.
Hugo Schiff (1834-1915) was a chemist in Europe and published a series of papers in French and German during the 1860s on the reaction between aldehydes and amines. He was not a histologist and his reactions did not ﬁnd use in histology until Feulgen used Schiff’s reagent to demonstrate nuclear DNA in 1924, and Hotchkiss (1948) and McManus (1946) applied it to tissue carbohydrates. Since then the Feulgen reaction and the PAS test have been staples of the histochemist’s tool box.
So what’s new with Schiff’s reagent? Earlier this year, three researchers from the University of Sao Paulo, Brazil published a study whose title was a real eye-opener: Silva AFS, Goncalves IC, Rocha FRP (2021) Smartphone-based digital images as a novel approach to detect formaldehyde as a milk adulterant. Food Control 125: 1-7.
Formaldehyde in milk?! Yes! In developing and poor countries, formaldehyde is used to extend the shelf-life of milk and to mask the poor hygiene practices in milk production and storage. While formaldehyde in milk is banned in certain countries, it does occur there and elsewhere, including Brazil, as I have conﬁrmed through a Brazilian colleague.
The research used milk samples spiked with formaldehyde. The formaldehyde was then quantitatively and qualitatively measured with a variety of chemical and physical laboratory tests, all of which were expensive and time consuming. Results from these tests were used to validate the new procedure, which involved putting a few drops of Schiff reagent into adulterated samples of milk, then simply taking a photograph of the samples with a smartphone equipped with a free app that measures the intensity of the color. The technique can be used in the ﬁeld or in the lab by food inspectors. It is fast, economical and involves only one reagent, Schiff’s.
Those phones just keep getting smarter.
For a detailed history of Hugo Schiff and various Schiff reagents, see Dapson RW (2016) Schiff and pseudo-Schiff reagents: the reactions and reagents of Hugo Schiff, including a classiﬁcation of various kinds of histochemical reagents used to detect aldehydes. Biotechnic Histochem. 91: 522-531.