Redistribution and introduction of heavy metals during wheat milling: A process-based study

Abstract

Heavy metal contamination in food products poses a significant threat to global food safety and public health. In this study, the impact of wheat milling processes on the concentrations of ten heavy metals, including aluminium (Al), arsenic (As), cadmium (Cd), chrome (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), nickel (Ni), and zinc (Zn) was determined across 43 milling streams. The assessment was conducted across key processing stages, including intake, pre-cleaning, cleaning, and tempering, as well as the subsequent milling phases of breaking, reduction, and crushing. Wheat samples were analyzed for nine metals (Al, Cd, Cr, Co, Cu, Fe, Pb, Ni, and Zn) using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), while As was determined by Atomic Absorption Spectrometry (AAS). No As was detected in both raw wheat samples or any stage of milling stages, indicating no associated contamination risk. Coarse fractions, such as bran and germ, contained higher levels of heavy metals, particularly Al and Zn, while refined flour fractions exhibited significantly lower concentrations due to the removal of outer grain layers. Al levels decreased from 13.63 mg/kg in raw wheat to 0.36 mg/kg in refined flour. Cd, which was not detected in earlier stages, appeared at 0.58 mg/kg during crushing, indicating potential metal transfer caused by mechanical abrasion. These findings highlight the role of milling in reducing heavy metal content and underscore the need for strict control of external sources and regular equipment maintenance to ensure the safety of wheat-based products.