What are glycoalkaloids?
Many plants in the potato family (Solanaceae) contain glycoalkaloids, and they are considered to be natural toxins. They are active as pesticides and fungicides and are produced by the plants as a natural defence against animals, insects and fungi that might attack them.
The plant glycoalkaloids are toxic steroidal glycosides and the commonest types found in food plants are α-solanine and α-chaconine, with α-solanine (C45H73NO15) being the more toxic of the two.
What foods can be contaminated?
Amongst the most widely cultivated food crops, aubergines, tomatoes and potatoes are in the Solanaceae family, but the levels of glycoalkaloids in tomatoes and aubergines are generally quite low and are not a concern. The glycoalkaloids of most relevance to food safety are those occurring in the potato, since even in commercially available tubers destined for human consumption a residual level of these compounds is always present.
The predominant toxic steroidal glycosides in potato are α-solanine and α-chaconine. They occur in potato tubers, peel, sprouts and blossoms and their concentration in tubers depends on a number of factors, such as cultivar, maturity and environmental factors.
In the UK, the total glycoalkaloid level in tubers destined for human consumption is generally in the range 25 – 150 mg/kg fresh weight, but considerably higher levels have been recorded for certain commercial varieties. For example, the Lenape potato variety was withdrawn from commercial growing in Canada and the USA as it contained unacceptably high levels of glycoalkaloids. In Sweden, a conditional sales ban had to be imposed on potato tubers of the commercially established variety Magnum Bonum harvested in 1986, as they contained potentially toxic levels of glycoalkaloids.
How do they affect human health?
Most cases of suspected potato poisoning involve only mild gastrointestinal effects, which generally begin within 8-12 hours of ingestion and are resolved within one or two days. However, reported symptoms have included nausea and vomiting, diarrhoea, stomach cramps and headache. More serious cases have experienced neurological problems, including hallucinations and paralysis, and fatalities have also been recorded.
Although glycoalkaloids are suspected to be the cause of these symptoms, there is little data to confirm this. One study examined case reports of poisoning incidents and estimated that glycoalkaloid doses of 2-5mg/kg bodyweight would be enough to cause symptoms in humans and that 3-6mg/kg bodyweight could be fatal. However, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) considered that the evidence implicating glycoalkaloids in potato poisoning cases was not convincing. The Committee concluded that levels of α-solanine and α-chaconine normally found in potatoes (20-100 mg/kg) were not of toxicological concern.
Nevertheless, JECFA and others have expressed concern about glycoalkaloids in skin-on potato products, such as crisps, that became widely available in the mid 1990s. Glycoalkaloid concentrations of up to 720 mg/kg were found in ‘green-skinned’ crisps, compared with a maximum of 150 mg/kg in normal crisps.
Apart from their toxicity, glycoalkaloids are also associated with a bitter taste and burning sensation in the throat.
How common is illness?
Although suspected potato poisoning is rare, a number of incidents have been documented. For example in 1979, a group of 78 schoolboys in London consumed a meal containing potatoes stored for a number of weeks before use. They suffered from Diarrhoea, vomiting, circulatory, neurological and dermatological problems, with 17 of the boys being hospitalised. A second incident occurred in Sweden in 1986, when 11 people became ill after consuming Magnum Bonum variety potatoes.
Where do they come from?
Although glycoalkaloids in potatoes are produced naturally by the plant, certain factors can have a significant effect on the levels present.
The highest concentrations of glycoalkaloids are usually associated with areas that are undergoing high metabolic activity, such as potato flowers, young leaves, sprouts, peels and the area around the potato ‘eyes’. Small immature tubers are normally high in glycoalkaloids since they are still metabolically active.
Exposure to light
Exposure to light has a significant effect on the concentration of both total and individual glycoalkaloids. Potatoes that become sunburned during growth and start to ‘green’, owing to lack of soil cover, tend to taste very bitter as a result of their high glycoalkaloid content.
In retail outlets, tubers may be displayed under fluorescent lighting and this can increase glycoalkaloid concentration. Studies have indicated that replacing fluorescent lights with mercury lighting for potatoes on display would significantly reduce glycoalkaloid content and improve food safety.
Storage at very low temperatures (0 – 5°C) results in more bitter-tasting potatoes and thus more glycoalkaloids than storage at higher temperatures (up to 20°C). On the whole, storage at lower temperatures will prolong potato quality, but at very low temperatures (0 – 5°C), stress becomes a factor and glycoalkaloid accumulation starts to occur.
Any type of injury or damage to the tuber will result in the accumulation of glycoalkaloids. Disease, insect attack or rough handling, during or after harvest, will all initiate glycoalkaloid synthesis (as it is a defence response). Damaged potatoes from retail generally contain elevated levels of glycoalkaloids.
Are they stable in food?
Glycoalkaloids are relatively stable in potatoes and levels are not affected by boiling, freeze-drying, or dehydration. Microwave cooking has only a limited effect, but cooking at temperatures at or above 170oC is more effective at reducing.
How can they be controlled?
For primary producers
The amounts of total and individual potato glycoalkaloids are genetically controlled. The most effective way of obtaining low levels is to select varieties that are initially very low in glycoalkaloids.
For food processors
In normal tubers, potato glycoalkaloids appear to be concentrated in a small 1.5 mm layer immediately under the skin, therefore, with normal tubers, peeling will remove between 60% – 95% of the glycoalkaloids present. However, if the tubers are very high in glycoalkaloids, peeling will remove only up to 35%, as diffusion into the deeper tissues occurs at higher concentrations. Unfortunately, peeling or slicing also elicits a stress response in the tubers and causes a slow rise in glycoalkaloid levels. If long delays occur before subsequent processing, glycoalkaloids can accumulate.
The heat stability of glycoalkaloids means that only high-temperature processing, such as deep-frying, has any significant effect on levels in potatoes.
Are there rules and regulations?
Although there is no specific legislation governing glycoalkaloid levels in potatoes, the generally accepted safe upper limit is considered to be 200mg glycoalkaloids per kg of fresh potato. Plants of the Solanum family are included in the EFSA Compendium of Botanicals that have been reported to contain toxic, addictive, psychotropic or other substances of concern, published in 2009.