Formaldehyde in alcoholic beverages: large chemical survey using purpald screening followed by chromotropic Acid spectrophotometry with multivariate curve resolution

Abstract

A strategy for analyzing formaldehyde in beer, wine, spirits, and unrecorded alcohol was developed, and 508 samples from worldwide origin were analyzed. In the first step, samples are qualitatively screened using a simple colorimetric test with the purpald reagent, which is extremely sensitive for formaldehyde (detection limit 0.1 mg/L). 210 samples (41%) gave a positive purpald reaction. In the second step, formaldehyde in positive samples is confirmed by quantitative spectrophotometry of the chromotropic acid-formaldehyde derivative combined with Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS). Calculation of UV-VIS and (13)C NMR spectra confirmed the monocationic dibenzoxanthylium structure as the product of the reaction and disproved the widely cited para,para-quinoidal structure. Method validation for the spectrophotometric procedure showed a detection limit of 0.09 mg/L and a precision of 4.2-8.2% CV. In total, 132 samples (26%) contained formaldehyde with an average of 0.27 mg/L (range 0-14.4 mg/L). The highest incidence occurred in tequila (83%), Asian spirits (59%), grape marc (54%), and brandy (50%). Our survey showed that only 9 samples (1.8%) had formaldehyde levels above the WHO IPCS tolerable concentration of 2.6 mg/L.

Figure 1

Hypothetical reaction products of the chromotropic acid-formaldehyde reaction [7]. The monocationic dibenzoxanthylium structure (1) is the more likely product (see text).

Figure 2

Reaction mechanism for the determination of aldehydes using purpald according to Hopps [8].

Figure 3

Automated steam distillation device for the determination of formaldehyde in alcoholic beverages: (1) Kjeldatherm digestion tube with acidified sample, (2) steam inlet tubing, (3) Viton-cone, (4) distillation condenser cooled at 1°C, (5) distribution head, (6) keyboard and display, (7) distillate outlet tubing, and (8) cooled graduated flask as receiver containing ice and sulfuric acid (25%), in which the outlet tubing must be submerged to avoid losses.

Figure 4

VIS-Spectra of formaldehyde (FA) and acetaldehyde (AA) reaction products with purpald (a) and chromotropic acid (b).

Figure 5

Results of the screening using purpald reagent.

Figure 6

Calculated UV-VIS spectra of the possible reaction products of the chromotropic acid-formaldehyde reaction. Structure (1): monocationic dibenzoxanthylium structure; structure (2): para,para-quinoidal structure (for structural formulae see Figure 1).

Figure 7

Experimental pure formaldehyde spectra (a) and resolved MCR-ALS (b), MILCA (c), and SIMPLISMA (d) spectra of formaldehyde and two interferences.