What are ochratoxins?
Ochratoxins are a small group of chemically related toxic fungal metabolites (mycotoxins) produced by certain moulds of the genera Aspergillus and Penicillium growing on a wide range of raw food commodities. Some ochratoxins are potent toxins and their presence in food is undesirable.
The ochratoxins are pentaketides made up of dihydro-isocoumarin linked to β-phenylalanine. The most important and most toxic ochratoxin found naturally in food is ochratoxin A (OTA). The only other ochratoxin found in food is ochratoxin B, which is rare and much less toxic. Other structurally related ochratoxins include ochratoxin C, α and β. These have been isolated from fungal cultures, but are not normally found in foods. The remainder of this section therefore refers specifically to OTA.
What foods can be contaminated?
In surveys, OTA has been found in a very wide range of raw and processed food commodities all over the world. It was first reported in cereals, but has since been found in other products, including coffee, dried fruits, wine, beer, cocoa, nuts, beans, peas, bread and rice. It has also been detected in meat, especially pork and poultry, following transfer from contaminated feed.
OTA levels in different food products vary, but are generally low in properly stored commodities (mean value less than 1 μg/kg for cereals from temperate regions). However, much higher concentrations can develop under inadequate storage conditions. Levels of up to 6,000 μg/kg and 5,000 μg/kg have been reported in Canadian wheat and UK barley respectively, but the concentrations found are usually below 50 μg/kg. The major contributors to OTA in the diet in Europe are cereals and wine. Coffee was thought to be important in this respect, but is now considered less significant. Pork products have also been suggested as a significant dietary source.
How do they affect human health?
OTA is a potent nephrotoxin and causes both acute and chronic effects in the kidneys of all mammalian species tested. The sensitivity of different species varies, but a level of 200 μg/kg in feed over three months is sufficient to cause acute damage to the kidneys of pigs and rats. There are no documented cases of acute OTA toxicity in humans.
OTA is also genotoxic (damages DNA) and teratogenic (damages the foetus) and is considered a probable carcinogen, causing renal carcinoma and other cancers in a number of animal species, although the mechanism for this is uncertain. It is also reported to have adverse effects on the immune system in some species. The evidence for carcinogenicity in humans is not conclusive, but in view of the evidence for other mammalian species, the presence of OTA in food and feed must be considered undesirable. Some toxicologists suspect that OTA may be a very significant food contaminant from a public health point of view.
OTA has been detected in human blood and breast milk, demonstrating dietary exposure. Daily intakes have been estimated at between 0.2 and 4.7 ng/kg bodyweight. In 2006, the European Food Safety Authority (EFSA) derived a tolerable weekly intake (TWI) of 120 ng/kg bodyweight for OTA in the diet, based on the latest scientific evidence.
Where do they come from?
In tropical and sub-tropical regions, OTA is produced mainly by Aspergillus species, particularly the widespread A. ochraceus. But in temperate climates (Canada, Northern Europe and parts of South America), the main producer is Penicillium verrucosum.
OTA production by A. ochraceus is favoured by relatively high temperatures (13oC to 37oC), but P. verrucosum grows and produces the toxin at temperatures as low as 0oC. A. ochraceus is able to produce OTA at water activities down to 0.80, while the lower limit for significant toxin production by P. verrucosum is thought to be about 0.86. Both are considered to be storage fungi, rather than field contaminants or plant pathogens, and toxin production occurs mainly when susceptible commodities are stored under inappropriate conditions, particularly at high moisture levels.
Are they stable in food?
OTA is a relatively heat-stable molecule and survives most cooking processes to some extent, although the reduction in concentration during heating depends on factors such as temperature, pH and other components in the product. For example, heating wet wheat at 100oC for 2.3 hours gave a 50% reduction in OTA concentration, but in dry wheat, the same reduction took 12 hours.
Processes such as coffee roasting and baking of cereal products and biscuits can produce significant losses in OTA levels, but processes like pasta manufacture produce little reduction. OTA also survives brewing and winemaking and can be found in a variety of processed consumer food products.
OTA is destroyed by acid and alkaline hydrolysis and by the action of some oxidising agents.
How can they be controlled?
The ability of OTA-producing fungi to grow on a wide range of food commodities and the persistence and ubiquity of OTA in the food chain mean that control is best achieved by measures designed to prevent the contamination of foods using HACCP-type techniques. Detection and removal of OTA-contaminated material from the food supply chain is also important for imported products.
For primary producers
Both A. ochraceus and P. verrucosum are considered to be storage fungi rather than field fungi. Pre-harvest controls are therefore limited to harvesting susceptible crops at the correct moisture level and stage of maturity.
For cereals, the most important and effective control measure in post-harvest handling and storage is the control of moisture content and hence, the water activity of the crop. Ensuring that susceptible crops are harvested at a safe moisture level, or are dried to a safe level immediately after harvest is vital to prevent mould growth and OTA production during storage. In tropical and sub-tropical climates stored grains must be dried rapidly to an Aw value of below 0.8 and this level must be maintained throughout storage to prevent A. ochraceus growth. In temperate regions a target moisture content of 18% for grain drying is recommended, together with rapid cooling of grain if hot-air drying is used. This should be followed by further drying down to a moisture level of 15% (UK Code of Good Storage Practice).
Other important cereal storage factors are effective cleaning of grain stores and handling equipment between crops, and fumigation to prevent insect infestation. In tropical regions, the use of controlled atmosphere storage to control insects may also help to inhibit mould growth.
Rapid and effective drying is also important in the control of OTA production in other commodities, especially coffee. For dried fruits, minimising mechanical and insect damage during handling and storage helps to prevent the entry of moulds into the fruit before drying.
For food processors
Monitoring raw material quality is the most effective control for processed foods. Any ingredient that displays visible mould growth should not be used. Testing for the presence of OTA in susceptible materials, such as barley for brewing, may be necessary in some cases.
Physical separation of contaminated material can be an effective means of reducing OTA levels in contaminated commodities. Mouldy grain should not be used for food, or for animal feed.
There has been little practical evaluation of chemical decontamination methods for OTA to-date, but an ammoniation process has been shown to be effective for cereals.
For enforcement agencies
Some countries monitor imported commodities that are susceptible to OTA contamination, such as grains and coffee beans, by sampling and analysis. A number of analytical methods have been developed based on TLC, HPLC and ELISA and there are also rapid screening kits available. However, moulds and mycotoxins in bulk food shipments tend to be highly heterogeneous in their distribution and it is essential to ensure that an adequate sampling plan is used to monitor imported materials.
Are there rules and regulations?
A number of countries, particularly in Europe, have regulations governing OTA in food and feed and most include maximum permitted, or recommended levels for specific commodities.
The EU has set limits for OTA in cereals, dried vine fruits, roasted coffee beans and ground coffee, soluble coffee, wine and grape juice. Limits vary according to the commodity, but range from 2-10 μg/kg. The limit for unprocessed cereals is 5.0 μg/kg, but for processed cereal products intended for direct human consumption it is 3.0 μg/kg. The limit for dried vine fruits is 10 μg/kg. There is also a limit of 0.50 μg/kg for OTA in processed cereal-based foods for infants and young children.
In 2010, additional limits were set for OTA in spices and liquorice products. The maximum permitted level for spices, including chilli powder, paprika, pepper, nutmeg, and turmeric, is set at 30 μg/kg until mid 2012, when it will be reduced to 15 μg/kg. The limit for liquorice root is 20 μg/kg and for liquorice extract it is 80 μg/kg.
Switzerland applies a limit of 5.0 μg/kg for all foods except cereal based infant foods, where the limit is 0.5 μg/kg, and Turkey has set limits of between 3.0 and 10 μg/kg for various food commodities.
Few other countries outside Europe have imposed limits for OTA, but a number have proposals to do so. Uruguay sets a limit of 50 μg/kg for rice, cereals and dried fruits and Canada sets a limit of 2,000 μg/kg for OTA in pig and poultry feed