Wheat is a versatile crop. A range of products can be produced from wheat, such as flour, cereals, pasta, beer, starches and glucose syrups. The range of products place it in high demand and as such also put it in the eyes of regulators. In the case of wheat, trace metals have been identified as a risk and levels have been regulated.
The soil that crops grow in can have trace metals present. As the crop grows and absorbs nutrients from the soil metals can also be absorbed. Trace metals can be toxic if consumers ingest enough from their foods to reach high enough levels.
Many governments have defined maximum permitted levels of trace metals in many foods, in particular arsenic, mercury, lead, cadmium, tin, chromium, antimony. The US Food and Drug Administration (FDA) does constantly monitor potentially toxic materials. A lot of strict regulations are targeted at food that is frequently consumed by children. For example, infant rice cereals now have a proposed maximum of 100 ppb of inorganic arsenic, the level that corresponds to a limit proposed by the European Commission for the production of food for infants and young children.
Other countries have legislated maximums for trace metals in wheat. Australia’s food standards code has Schedule 19 ‘Maximum levels of contaminants and natural toxicants’. For example, the limit for total arsenic in cereals is 1 mg/kg, cadmium in wheat 0.1 mg/kg, lead in cereals is set at 0.2 mg/kg. The European Union has The European Food Safety Authority (EFSA) Regulation (EC) No 629/2008 3.2.13 setting maximum levels for certain contaminants in foodstuffs. China has GB 2762-2012: National Food Safety Standard Maximum Levels of Contaminants.
In Australia, Europe and China, the various regulations have set maximum levels for cadmium for wheat (under grains and grain products). In Australia and China that is 0.1 mg/kg while in Europe it is 0.2 mg/kg (cereals though have lower level 0.1 mg/kg as mentioned above). China further specifies a maximum level for total arsenic at 0.5 mg/kg (0.2 mg/kg for cereal-based food for infants), total mercury at 0.02 mg/kg, lead at 0.2 mg/kg* (0.5 mg/kg** for cereals) and chromium at 1.0 mg/kg. Chinese old standard GB 2762—2005 also specified the maximum levels of total rare earths oxides in food, where maximum level for grains is 2.0 mg/kg, which is valid in the new version of the standard.
The concentration of trace metals in wheat can be measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Wheat samples must be prepared before using ICP-MS. This requires a microwave assisted digestion step using acid digestion (nitric or aqua regia). The sample can then be injected into the plasma source as an aerosol. The heat of the plasma ionises elements that can then be measured, based on their mass, using Mass Spectrometry (ICP-MS), giving concentrations of trace metals if they are present. Collison mode in ICP-MS helps to achieve very low detection limits for arsenic by removing interferences.
Due to the versatility of wheat as a food material, the potential for accumulation of heavy metals to toxic levels need to be monitored. Recognising this governing bodies, to reduce this risk, have put regulations in place to control maximum levels of various trace metals. Using ICP-MS, levels of trace metals can be monitored by producers.
*Grains and grain products [with the exception of cereal, gluten, assorted cereal porridge, wheat and rice products with fillings]
** Cereal, gluten, assorted cereal porridge, wheat and rice products with fillings.