Mineral Oil Hydrocarbons
Mineral oils are widespread food contaminants, and edible oils, like many other foodstuffs, are often contaminated. The lack of robust analytical methods and proper toxicological evaluation make it difficult to set a tolerance level. The best way to avoid mineral oil contamination is to prevent it by complying with good manufacturing practices, and the best solution to reducing mineral oil contamination in edible oils is through refining, especially during deodorization. This paper gives an overview of contamination sources, levels in some edible oils, regulatory aspects, analytical methods, and strategies for mitigation during refining
Refined oil quality is primarily evaluated by traditional parameters, such as free fatty acid content, oxidative stability, color, odor, and taste. In addition, high-quality food oils will contain low amounts of trans fatty acids, high amounts of natural antioxidants (tocopherols), and very low or no “process” contaminants, such as polycyclic aromatic hydrocarbons (PAH), mycotoxins, pesticides, dioxins, polychlorinated biphenyls (PCB), phthalates, 3-monochloropropane-1,2-diol (3-MCPD), glycidyl esters (GE), trans fatty acids, di-alkyl ketones, and mineral oil hydrocarbons (Fig. 1). Contamination with mineral oil hydrocarbons (MOHs) is not new but has attracted more attention recently.
They are categorized into two main groups: mineral oil saturated hydrocarbons, or MOSH (paraffins and naphthenes), and mineral oil aromatic hydrocarbons, or MOAH (aromatics). MOSH correspond to straight and branched openchain alkanes and largely alkylated cyclo-alkanes; MOAH are alkylated and non-alkylated polyaromatics (Fig. 2). MOHs exclude hydrocarbons naturally occurring in food and hydrocarbons from synthetic origin. They are usually classified according to their carbon number: total MOSH refer to the sum of C10-C16, C16-C20, C20-C25, C25-C35, C35-C40 and C40-C50 fractions, and total MOAH to the sum of C10-C16, C16-C25, C25-C35 and C35-C50 fractions.
MOHs are contaminants that can be present in food due to environmental pollution (air, soil), use of lubricants for machinery during harvesting and food production, food contact materials, processing aids, food additives, and other factors. They often come from malpractices in process operations. A contamination of walnut oil with a foodgrade lubricant during refining was highlighted in 2009, followed in 2011 by a contamination of milk fat during production (2). In most cases, proper monitoring of good manufacturing practices is the best solution to avoiding such problems. The application of benchmark levels to identify unusually high loads of MOHs is informative (3). These levels are oriented to the market and allow manufacturers to respond appropriately; they are an effective tool for minimization. It is also important to monitor MOHs contents to better understand their relative presence in food commodities; variable levels of MOSH (up to 50 mg/kg) can still be detected in large food categories. Oil seeds, fats, and oils can accumulate MOHs from all kinds of sources along the production chain. A general overview is given in (4) for vegetable oils like sunflower oil, olive oil, palm oil, sesame oil, walnut oil, rapeseed oil, argan oil, poppy seed oil, olive oil, olive pomace oil, cottonseed oil, and grapeseed oil; contamination is also reported in oils for feed production. A specific survey is described in (5, 6) for olive oils. Contents depend on the variety: 8 to 90 mg/kg for MOSH and 1 to 13 mg/kg for MOAH, mostly in the C25-C35 fractions. Highly contaminated oils contain heavier MOSH (C35-C50). The average MOSH and MOAH contents (C10-C50) in a series of oils are presented in (7). The highest amounts are found in coconut oils (MOSH up to 40 mg/kg and MOAH above 15 mg/kg); the contents largely depend on suppliers, and some special grades do not have MOAH. In MOSH, the major fractions are C16-C40; in MOAH the major ones are C16-C35. MOSH in soybean are also high (up to 35 mg/kg), although MOAH are usually low (< 2 mg/kg). Rapeseed, palm kernel, and sunflower oils, are also reported to have low MOAH and MOSH levels below 5–10 mg/kg. Higher contents in MOSH (> 25 mg/kg) and in MOAH (> 2.5 mg/kg) are detected in palm oil; however, palm oil from sustainable sources with low MOH contents is preferred for infant formula applications. A pilot study on MOSH status in crude palm oil was conducted by Sime Darby Plantation Palm Oil Mills and published in 2019 (8). Results revealed that MOSH in Hazard Analysis Critical Control Point (HACCP)-certified palm oil mill was at the range of 10–17 mg/kg, while non-HACCP certified-mill had an average of 44.8 mg/kg. Premium CPO showed lower MOSH, with 10.7 mg/kg compared to standard CPO at 21.9 mg/kg. The study did not establish any correlation between the free fatty acid and the MOSH contents. A survey of MOSH and MOAH in different vegetable oils is shown in a 2020 article (9). Cocoa butter had the highest contents: up to 162 mg/kg for MOSH and 55 mg/kg for MOAH. MOSH were mostly observed in the ≤C35 faction, and MOAH in the ≤C24 fraction; presence of benzothiophene, dibenzothiophene, and alkylated naphthalenes was detected. Palm oil contained up to 124 mg/kg of MOSH (in ≤C35 and in > C35 fractions, in lower extent in ≤C16 and ≤C24 fractions) and 39 mg/kg of MOAH (mostly in ≤C35 and > C35 fractions). Sunflower oil showed lower contents: less than 17 mg/kg of MOSH (in ≤C35 fraction) and no MOAH. In 2009, other authors (10) had already described similar levels for sunflower oil and concluded that environmental contamination and harvesting were critical factors for accumulation in the seed oil. All new data will help fill the missing gaps needed for proper risk assessment and make it possible to establish a tolerable daily intake (TDI) for MOSH in the next EFSA (European Food Safety Authority) opinion due December, 2022
MOHs pose potential health risks in animal studies; Sprague Dawley rat seems to be a good model for humans, but it is not yet confirmed. Scientific understanding of MOHs has advanced significantly, but knowledge gaps remain, particularly regarding the risk to humans.
The risks associated with MOSH are different from those of MOAH. MOSH are neither carcinogenic nor mutagenic;there is no direct indication of harmful effects during accumulation - even though formation of granulomas in organs of Fischer rats is reported. Critical effect was nevertheless identified by EFSA, with NOAEL (No Observed Adverse Effect Level) value of 19 mg/kg body weight/day.
MOAH are carcinogenic, mutagenic, and hormone-disruptive. Carcinogenic potential seems to correlate with increasing number of aromatic ring systems; MOAH are suspected to contain bio-accumulative genotoxic compounds, mainly related to 3- to 7-ring aromatic substances. Today, no safe thresholds are clearly defined, and presence in foods is a concern.