Factors That Influence Food Contamination

Contaminants can range in severity:

• from simply causing food to be unappealing or un-wholesome, such as finding a hair or insect fragment in one’s soup
• to dangerous causing acute, chronic and/or debilitating illness, such as contamination with certain toxins
• and life threatening, such as can be the case with food allergens. In food chemistry and medicinal chemistry, the term "contamination" is used to describe harmful intrusions, such as the presence of toxins or pathogens in food or pharmaceutical drugs.

Unfortunately, there is no single ‘one stop’ test for contaminants. Contaminant testing methods are specific to the target contaminant and draw on virtually every testing technology in the food chemist’s tool box - from simple physical observation, separation and microscopic observation, through solvent extractions, and Enzyme Linked Immunosorbent Assays (ELISA) to the most advanced Mass Spectrometer Chromatography methods. Because contaminant testing is so diverse and potentially expensive, testing for all contaminants in all food would be highly impractical if not impossible. Yet, as mentioned, contaminants can pose serious even life-threatening risks in specific instances. For this reason, food safety concerns dictate that foodstuffs and ingredients be analyzed from a comprehensive risk based hazard analysis perspective which constructs a matrix considering the likelihood of a specific contaminant, as well as its potential severity, for each item.

Factors for consideration should include:

• Contaminants historically associated with a particular commodity such as mycotoxin infection, or insect infestation in cereal grains. These can be further impacted by specific seasonal or regional factors such as incidence of crop stress from drought or flooding etc.
• Whether there may be risks of cross-contamination from shared harvesting, transporting, or processing equipment, as with allergens
• Geographic origin of commodities concerns, such as the likelihood of heavy metals or radiation contamination from the soil or water of a particular region
• Pesticides commonly associated with the production of food in a region and/or a given commodity.
• Economic motivation for food fraud such as dilution or substitution of high cost commodities with less expensive ingredients, for example adding corn syrup to honey or substituting lower grades of olive oil for ‘Extra Virgin’ grade
• Length and complexity of the supply chain for a given commodity impacting the ease of access to commodities, and the probability of economic fraud or culturally motivated poisoning
• History and audit frequency of a given supplierThe most commonly considered classes of contaminants include, but are not necessarily limited to, toxins, residues, allergens, and genetically modified organisms (GMO).

Toxins

Toxins are poisonous substances that are a specific product of the metabolic activities of a living organism and is usually very unstable and notably toxic (Merriam-Webster, 2018). Heavy metals and mycotoxins are two types of food related toxins.

Heavy Metals

There is no good definition of heavy metals. A metallurgist would consider heavy metals based on their density while a physicist would be concerned with the atomic number. From the perspective of food chemistry, heavy metals are considered to be toxic elements. For this reason mercury, lead, cadmium, and arsenic are the most commonly tested for heavy metals in food. Analyses of heavy metals are typically analyzed by one of the following procedures: wet chemistry or instrumental methods.Wet ChemistryThe “legacy” method of wet chemistry is a colorimetric procedure described in USP 231. In this procedure, an ashed sample is reacted in a sulfur containing test solution which causes the formation of insoluble sulfides of any heavy metals present. These in turn darken the solution which is visually compared to a standard solution of lead prepared at the upper specification limit (typically 10 ppm). This method, while currently cited quite commonly in specifications for food and pharmaceutical preparations, is a screening procedure as opposed to a specific quantitative test because it is not specific to any one toxic element and does not determine a specific quantity. Rather, it detects toxic elements as a group being below or above a specific threshold limit.

Instrumental Methods

Instrumental methods can quantify heavy metals, as well as other elements of nutritional significance, in food. The most common of these, listed from oldest to the most modern, are:

• Atomic Absorption Spectroscopy (AAS)
• Atomic Emission Spectroscopy (AES)
• Inductively Coupled Plasma – Mass Spectrometer (ICP-MS)

Atomic Absorption Spectroscopy (AAS) uses a high heat source (either an acetylene flame, or graphite furnace) to atomize tiny droplets of the sample which has been sprayed as a mist (nebulized). The atoms in turn absorb radiation, which is passed through the reaction chamber at a specific narrow wave length. The reduction of radiant energy is measured by a detector and compared to that of standard solutions to determine the concentration of the sample. This procedure can only analyze for one element at a time and requires changing the lamp (radiation source), along with re-calibration, for each element. A graphite furnace AA uses an electrically heated graphite tube (furnace) as the heat source for atomization and offers increased sensitivity (lower detection). Cold Vapor AA is another specific variation of this procedure and used exclusively for the analysis of mercury. It remains as one of the most common methods currently used for mercury. However, AAS is an older procedure that was commonly used for many years and has been largely replaced over the last two decades by other methods.

Atomic Emission Spectroscopy (AES) is similar to AAS in that a heat source is used to excite atoms in a nebulized mist. It differs in principle from AAS in that the spectral energy emitted from this excitation is the source of radiation measured, as opposed to measuring the absorption of energy from the external lamp. By using the elemental atoms themselves as the source of spectral energy, multiple elements can be analyzed simultaneously, or in sequence, from a single sample aliquot without changing lamps or re-calibrating. The most popular versions of these instruments in current use employ a heat source known as inductively coupled argon plasma (ICAP or ICP) in which radio frequency (RF) energy is used to heat a stream of the inert gas argon to temperatures as high as 10,000⁰ K.

Inductively Coupled Plasma – Mass Spectrometer (ICP-MS) is the most recent adaptation of the ICP technique, in which the detection of elements, after ionizing them via an ICP torch, is accomplished by means of a mass spectrometer (MS). The MS directly measures the ionized element by determining their atomic mass by measuring their degree of deflection as they are passed through a strong magnetic field. ICP-MS allows for greater sensitivity, compared to AES detection with detection limits in the part per trillion range (for the diluted sample preparation injected into the instrument) (Nielsen, 2017).

Challenges with Heavy Metals

Toxic elements are present in almost all samples by modern methods, with detection limits in the parts per billion (ppb) range. For this reason, tolerance thresholds need to be based on epidemiologic evidence, not a simple binary detect / non-detect standard. Heavy metals are often naturally occurring. For example:

• Toxicity varies greatly based on form or “species”
• Arsenic in rice & seafood – much less toxic than inorganic arsenic
• Inorganic arsenic – the highly toxic form of arsenic found most often in soil and ground water
• Methyl mercury – formed from inorganic mercury by microbes, volcanoes, forest fires
• Chromium +3 (III) and Chromium +6 (VI)- While chromium III is not toxic, and in fact is considered an essential trace nutrient, chromium VI (or hexavalent chromium) is toxic and a carcinogen

Mycotoxins

Mycotoxins are produced by molds of the Aspergillus, Fusarium and Penicillium genera. Their production is determined by environmental factors such as temperature, humidity, pest attack, and plant stress during the growing season and subsequent storage of grains.