Cadmium and Glycoalkaloids in Raw Potatoes

July 1996 March 1997

Report prepared by Andrew Rigg

1. OBJECTIVE

1.1 To determine the compliance of potatoes for cadmium to the Australia and New Zealand Food Authorities (ANZFA) Food Standards Code (the Code).

1.2 To determine the levels of potato glycoalkaloids, a -Solanine and a -Chaconine in potatoes.

2. Background

2.1 Cadmium

Cadmium is a highly toxic element which has been described as one of the most dangerous elements in the food and environment of man (Vons et al, 1987). Cadmium showed its danger by the presence of itai-itai disease in Japan in the late 1960s which caused a great deal of human suffering and death. According to Reilly (1991, at page 146),

  • [t]he ingestion of [high concentrations of] cadmium in food and drink can rapidly result in feelings of nausea, vomiting, abdominal cramp and headaches . . . Long-term ingestion of cadmium causes serious renal damage, as well as bone disease leading to brittleness and even collapse of the skeleton. These latter were among the most obvious and alarming of the symptoms seen in the notorious itai-itai disease outbreak in Japan.

Dietary modelling indicates that potatoes contribute the major proportion of cadmium in the total dietary intake (approximately 4.3 mg/day or 46.7%) (ANZFA, 1997). It is likely that the true dietary intake for Australians is somewhere within the range of 915 m g/day. Sources of cadmium in the diet are shown in the following table (ANZFA, 1997):

 

Commodity

Mean Cadmium Intake (m g/person/day)

% Total Cadmium Intake
for whole population

Potatoes

4.26

46.7

Wheat

1.48

16.3

Cocoa and cocoa paste

1.14

12.5

Meat (mammalian)

0.67

7.3

Crustaceans

0.32

3.4

Peanuts

0.28

3.1

Liver (mammalian)

0.23

2.6

Root & Tuber Vegetables (except potatoes)

0.23

2.5

Leafy Vegetables

0.17

1.9

Molluscs

0.13

1.4

Fish

0.10

1.1

Kidney (mammalian)

0.06

0.6

Rice

0.05

0.6

Total

9.12

100

 

2.2 Glycoalkaloids

Despite being a staple food in the Australian diet, the potato tuber contains toxic glycoalkaloids that cause sporadic outbreaks of poisoning in humans. Probably because glycoalkaloids are natural compounds present in a staple food that has been used for millennia, their effects on humans has not been investigated as for other chemicals regulated in the Code. As a result there is no prescribed maximum permitted concentration (MPC) in the Code, however, the literature indicates that levels of glycoalkaloids in commercial tubers are normally less than the widely accepted safety limit of 200 mg/kg fresh weight.

Synthesis of glycoalkaloids can be markedly stimulated in tubers subjected to physiological stresses such as physical damage (eg. bruising or cutting) which can arise during harvest or transit and by microbial and/or insect attack. In addition, exposure to light stimulates chlorophyll synthesis which causes greening of tubers and in turn stimulates glycoalkaloid synthesis. A number of studies have demonstrated that the glycoalkaloids are concentrated in the outer 1.5 mm layer of potato tubers. Skin-on and peel based products could therefore pose a higher potential risk than whole potatoes. It is significant to note that heat processing does not inactivate potato glycoalkaloids.

D.B Smith et al (1996) states that

  • [g]lycoalkaloid poisoning elicits a wide variety of symptoms ranging from gastrointestinal disorders, through confusion, hallucination and partial paralysis to convulsions, coma and death but it is thought to stem from one or both of two distinct modes of action. The first is inhibition of the enzyme acetylcholinesterase . . . Neurological symptoms such as weakness, confusion and depression . . . are likely manifestations of this antiacetylcholinesterase activity. Chaconine and solanine are equally potent inhibitors of acetylcholinesterase.

  • The other major biological action of [glycoalkaloids] is their ability to disrupt sterol-containing membranes. This action is thought to be responsible for damaging cells in the gastrointestinal tract and also in other tissues or organs into which [glycoalkaloids] are transported following absorption.

3. Standards

Standard A12 of the Code states potatoes having a MPC of 0.05 mg/kg for cadmium. This standard is likely to be changed in the near future to 0.1 mg/kg for cadmium (ANZFA, 1997).

There is no standard for glycoalkaloid levels in potatoes, although a safety limit of 200 mg/kg fresh weight is widely accepted.

4. Results

4.1 Samples Received

There were 42 samples of potatoes collected over the period of July 1996 to March 1997 from various supermarkets and grocers in the ACT.

Figure 1 shows the varieties of potatoes sampled. Pontiac and Desiree were most commonly sampled followed by a couple of Chats, Bintje, Idaho and Toolangi. Other varieties included Sebago, King Edward, Spunta and Manuels. Unknown samples were commonly labelled brushed or washed potatoes.

 

graph
graph


Every sample of potatoes was also sub-sampled into potatoes which constituted "Good", "Bad" and "Green". This categorisation was very subjective and often impossible because in some instances, for example, all the potatoes were of "good" quality only. This categorisation was done in order to determine if any significant difference in levels of glycoalkaloids could be detected. Figure 2 shows the relative proportions these subsamples.

The categories were defined as follows:

(a) Good those potatoes with no obvious damage, blemishes or greening.

(b) Bad those potatoes with obvious damage and blemishes like cuts and bruises.

(c) Green those potatoes with some greening on its surface.

Potatoes that were both green and damaged were was not chosen for analysis and discarded.

4.2 Sample Results

4.2.1 Cadmium

In total, there were 89 subsamples formed from the samples and of these, 53 were tested for Cadmium levels. The average concentration was 0.03 mg/kg with all the results below 0.05 mg/kg except for one sample of bad brushed potato with a level of 0.20 mg/kg. This was very unusual because the good and green subsamples both had acceptable results.

Due to time constraints and acceptable Cadmium levels in those samples tested (on the whole), it was decided not to test the remaining few.

4.2.2 Glycoalkaloids

The average concentration of chaconine detected was 28.7 mg/kg (range 064 mg/kg) and for solanine, the average was 14.5 mg/kg (range 051 mg/kg). The total concentration of both chemicals in the samples averaged 43.2 mg/kg (range 0103 mg/kg).

The comparison between the three groups is shown in the following table:

 

 

Total Glycoalkaloids (mg/kg)

 

Good

Bad

Green

Average

42.1

43.2

44.1

Min

0

0

0

Max

83

103

91

 

5. Discussion

Cadmium levels in the potatoes where within the acceptable MPC and pose little public safety concern.

From the testing of glycoalkaloids in raw potato, there were no samples exceeding the "accepted" level of 200 mg/kg. One sample had half this level at 103 mg/kg, however the mean concentration of 43.2 mg/kg shows that there is minimal concern for glycoalkaloids.

There appears to be no significant difference between the three categories (good, bad, green) of samples. An explanation for this is that the categorisation was at times very arbitrary and those samples considered bad and green were sometimes only slightly so. The results seem to indicate that potatoes even with some greening or visible bruising or marks are not likely to have significantly higher levels of glycoalkaloids. The fact that staff had difficulty to find such subsamples, indicates that the majority of potatoes in the market are of good quantity. Green and bad potatoes were more often found in prepacked bags of potatoes.

6. Conclusion

Raw potatoes sold in the ACT had detected minimal levels of Cadmium and glycoalkaloids and are unlikely to pose any public health and safety concern. They were found to be mostly of good visual appearance having minimal greening or bruising occurring.

7. Recommendations

Anecdotal evidence suggests that it is already well known in the community that green potatoes should be avoided, but it is less well known that damaged tubers should also be avoided. As glycoalkaloids are concentrated in the outer layer of potato tubers (Smith et al) it would be appropriate to recommend that consumers peel potatoes before consumption, especially if the potatoes show signs of greening or damage.

It is recommended that a follow up survey be conducted on potato products such as skin-on crisps and baked potato peels which are available in a number of take away shops. Because such products contain more of the glycoalkaloid containing parts of the tuber they pose a potentially higher health risk to consumers.

8. References

Australia New Zealand Food Authority, Food Standards Code, incorporating amendments up to and including amendment 32, December 1996.

ANZFA, Proposal P144 Review of the Maximum Permitted Concentrations of Cadmium in Food: Executive Summary, Full Assessment Report, February 1997.

Food Act 1992 (ACT), reprint as at 28 February 1994.

Smith, D.B et al, Potato glycoalkaloids: Some unanswered questions, in Trends in Food Science and Technology, Vol. 7, p. 126, 1996.

Reilly C, Metal Contamination of Food, 2nd Edition, Elsevier Science Publishers Ltd, London, 1991.

Vons G, Hovens JPC, & Delft WV, Food Additives and Contamination, vol. 4, p. 73, in: Reilly C, Metal Contamination of Food, 2nd Edition, Elsevier Science Publishers Ltd, London, pp 140151, 1991

9. Acknowledgments

This report could not have been possible without the work of Simon Christen, Helen Kivela, Margaret Woolcock, Christine Wixson and Fiona Wojtas.