Effects of hypochlorite as a disinfectant for hydroponic systems on accumulations of chlorate and phytochemical compounds in tomatoes

Abstract

Recently, official food surveillance discovered high residues of chlorate (ClO₃ ⁻) and perchlorate (ClO₄ ⁻) in different plant-based foods, which was the start of a big discussion in the EU Commission. There is currently no knowledge about possible ClO₃ ⁻ uptake quantities in vegetable, when hypochlorite (ClO⁻) is used as a disinfectant in plant production facilities. Therefore, the present study is focused on the identification of interactions between ClO⁻ applications and ClO₃ ⁻ accumulations in fruit, as well as findings in terms of suitable concentrations of ClO⁻ to ensure food safety. Primary and secondary metabolites were analyzed as well. As such, on-site produced potassium hypochlorite (1 % KClO) solution was supplemented using proportional injection control once a week for 90 min, as a disinfectant, into a recirculating tomato production system (NFT) until a free chlorine concentration of 1 mg L⁻¹ (D I) and 2 mg L⁻¹ (D II) was reached, respectively. The chlorate (ClO₃ ⁻) content in fruit increased from 0.01 mg (Control) to 0.22 mg (D I) and 0.25 mg ClO₃ ⁻ kg⁻¹ FW (D II). A critical assessment of these values is given in the discussion section. Contrary to the expectations, a co-occurrence of ClO₃ ⁻ and ClO₄ ⁻ in fruit was not found. Compared to the control, the fruit contents of lycopene were increased by 21.3 % (D I) and 33.5 % (D II) and those of ß-carotene by 9.2 % (D I) and 23.9 % (D II), when calculated on a fresh weight basis. These results changed slightly when these substances were calculated on a dry weight basis. Furthermore, ClO₃ ⁻ induced stress in fruit. In this context, a significant correlation (r) and a significantly increased slope (m) compared to zero were found between ClO₃ ⁻ and lycopene (r = 0.74; m = 0.10), as well as ß-carotene (r = 0.70; m = 0.01). The content of soluble solids and that of titratable acids remained unaffected.