What are glycoalkaloids?
Many plants in the Solanaceae family 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 a-solanine and a-chaconine, with a-solanine (C45H73NO15) being the more toxic of the two.
Occurrence in foods
Amongst the most widely cultivated food crops, aubergines, tomatoes and potatoes are in the Solanaceae family; however, the levels of glycoalkaloids in tomatoes and aubergines are generally quite low and are therefore 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 a-solanine and a-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, environmental factors and stress conditions.
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. As an example, the Lenape potato variety was withdrawn from commercial growing in Canada and the USA as it contained unacceptably high levels of glycoalkaloids.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.
Effects on health
Most cases of suspected potato poisoning involve only mild gastrointestinal effects, which generally begin within 8-12 hours after ingestion and resolve 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 suspected potato poisoning is rare, a number of incidents have been documented, and a few of the more recent ones are tabulated below (taken partly from a review by McMillan and Thompson, 1979; Quart. J. Med. 48, 227-243):
|Potato poisoning incidents|
||7 family members ate greened potatoes
||Extreme exhaustion, restlessness, rapid breathing, loss of consciousness. Death of 2 family members|
||In Cyprus, 60 people consumed young potato shoots and leaves as a vegetable
||Headache, nausea, vomiting, diarrhoea, fever, throat irritations. One death|
||382 North Koreans affected following consumption of rotten potatoes
||Pain, nausea, vomiting, facial oedema, respiratory failure, cardiac arrest. 52 hospitalised and 22 deaths|
||78 London schoolboys consumed potatoes left over from a previous term
||Diarrhoea, vomiting, circulatory, neurological, dermatological problems. 17 hospitalised|
||11 people in Sweden consuming Magnum Bonum variety potatoes
||Nausea, vomiting, pain, headache|
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, a toxicological monograph produced by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 1992 states that "Glycoalkaloids are not acutely toxic by the oral route in laboratory animals even at very high doses (up to 1g/kg bodyweight) in some species." The Committee considered that the evidence implicating glycoalkaloids in potato poisoning cases was not convincing. JECFA concluded that levels of a-solanine and a-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.
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.
Stability in foods
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 170 oC is more effective at lowering levels.
The amounts of total and individual potato glycoalkaloids are genetically controlled. The most effective way of obtaining low levels is to select breed varieties that are initially very low in glycoalkaloids.
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, in potatoes with a high level, diffusion into the deeper tissues occurs. 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.
Other processes give little or no reduction in the concentration of these compounds.
Gamma irradiation has been shown to control glycoalkaloid levels, particularly in damaged tubers. Treatment with certain chemicals, most of which function as sprout inhibitors, has also been shown to control glycoalkaloid accumulation.
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.
Sources of Further Information
Friedman M., McDonald G.M.,
Potato glycoalkaloids: chemistry, analysis, safety and plant physiology.
Critical reviews in Plant Sciences, 1997, 16 (1), 55-132
Glycoalkaloids in Solanaceae.
Food Reviews International, 1994, 10 (4), 385-418.
JECFA review 1992
Potato glycoalkaloid toxicity article (Cornell University)
US National Toxicology Program literature review (a-solanine and a-chaconine) 1998