Thursday March 6 2008
Pre-soaked potatoes have less colour when cooked
“Soaking potatoes cuts cancer risk,” according to The Daily Telegraph’s headline, suggesting that soaking potatoes in water before frying them can cut the levels of a potentially cancer-causing chemical by half.
The Daily Mirror also reports that scientists have found that soaking potatoes reduces amounts of acrylamide, a chemical that is formed “when starch-rich foods are cooked at high temperatures”. Apparently, two hours of soaking reduces levels by 48%, half an hour by 38% and just washing them lowers levels by 23%.
There is evidence of a link between acrylamide and cancer in rats, though evidence for the link in humans is limited and it is classified as a “potential human carcinogen”. Until further research better establishes whether or not acrylamide does cause cancer in humans, it seems sensible to consider limiting consumption of it where possible.
The results of this study suggest that soaking raw potato for two hours before cooking to make chips leads to reduced acrylamide in the cooked product. People attempting this should be aware that pre-soaked potatoes also have reduced colour when cooked, so they should not be left to brown longer as this may reverse the effects that the pre-soaking has.
Where did the story come from?
Rachel Burch and colleagues from Leatherhead Food International, the University of Reading, the British Potato Council and the Food Standards Agency carried out the research. The study was funded by the UK Food Standards Agency. The study was published in the peer-reviewed: Journal of the Science of Food and Agriculture.
What kind of scientific study was this?
Acrylamide is a naturally occurring chemical that forms in carbohydrate rich food when it is baked or fried. Animal experiments have shown that acrylamide causes cancer in rats, and as a precautionary measure, has been classified as a potential carcinogen in humans. Steps have subsequently been taken to reduce the amount of acrylamide in processed foods.
The aim of this research was to research acrylamide levels in home cooked food - specifically chips made from fresh potatoes - and ways to help consumers minimise the formation of the chemical. The researchers said that this involved looking at factors over which the consumer had little or no control (the variety of potato and how it was stored pre-purchase) and those that they could (how they stored and cooked their potatoes). A summation of the research can therefore be divided into results that are relevant to the food industry and those that are relevant to the consumer.
The study was a controlled experimental study in which researchers compared potatoes stored and treated under different conditions. In the part of the experiment that is relevant to industry, three different potato varieties – Desiree, Maris Piper and Cabaret – were purchased and stored under usual conditions (12°C for one week followed by a reduction in temperature to 3.5° at a rate of 0.5° per day at 95% relative humidity). During storage the varieties were sampled at six weeks, 16 weeks and 34 weeks. The potatoes that were to remain in storage for 16 and 34 weeks were sprayed with a chemical to prevent sprouting. At each sampling point, raw french fries were produced using a mechanical chipper.
In part of the experiment some of the french fries were pre-treated in a variety of ways before they were cooked. Pre-treatment involved; washing under tap water for 30 seconds, soaking in water for 30 minutes, and soaking in water for two hours. The different properties of the fries– both raw and cooked - were then compared.
The acrylamide content of cooked fries that hadn’t been pre-treated and those that had were compared, as were the sugar and asparagine content (a precursor to acyrlamide) in raw chips.
What were the results of the study?
The researchers found that the longer that raw potatoes were stored, the higher the levels of aspargine. This was most pronounced with Desiree potatoes and was confirmed further by the greater acylamide levels in the cooked potatoes that had spent longer in storage.
Washing raw fries before cooking them reduced eventual acrylamide content by 23%, while 30 minutes of soaking reduced this by 38% and two hours of soaking by 48%.
The researchers make the point that the pre-treated fries had less colour than untreated ones, and that if pretreated fries were cooked until they were the same colour as the untreated ones, that the acrylamide content may indeed be just as high.
What interpretations did the researchers draw from these results?
The researchers conclude that pre-treating fries through soaking or washing can reduce acrylamide formation in the final product. However, this is “when cooking is stopped at a texture-determined endpoint”.
This final detail is an important point as colour formation during cooking is also reduced by soaking, meaning that people watching their fries might let them continue to cook until they are darker brown. This in turn might increase the acrylamide levels to those seen in untreated potatoes.
What does the NHS Knowledge Service make of this study?
Acrylamide is present in large amounts in high starch foods that have been cooked at high temperatures. Studies in rodents have shown a link between acrylamide exposure and cancer, though the evidence for such a link in humans is limited. There is currently no guidance on what is considered a safe amount to eat.
This study has several facets. The researchers were sampling different varieties of potatoes, that were stored for different lengths of time and that underwent different pre-treatments before being made into fries. These findings may be relevant to the potato industry:
- Storage at low temperatures increases levels of sugars (which are associated with increased acrylamide levels after cooking).
- Longer storage increases the level of asparagine (associated with acrylamide levels) in Desiree potatoes.
- For all varieties, the concentration of acrylamide in cooked fries (unwashed) increased according to storage period, i.e. potatoes stored longer resulted in fries with higher levels of acrylamide, though this was very pronounced with the Desiree potatoes.
For the consumer, the finding that pre-treatment before cooking has an effect on acrylamide levels in the fries are relevant to everyone who fries potatoes. However, the effects of pre-treatment may depend on how long the potatoes were stored in the first place. The effects of soaking for 30 minutes were compared in potatoes stored for six, 16 and 34 weeks, while the effects of soaking for two hours was only assessed in potatoes that were stored for six weeks. Potatoes that were stored for longer had potentially higher levels of acrylamide.
From this study it would seem that potatoes which result in the lowest level of acylamide after cooking are Maris Pipers that have been stored for only six weeks and have been soaked in water for two hours before cooking.
Until further research clarifies whether there is a direct link between acrylamide and cancer in humans, it would seem sensible to limit the amount taken in through diet where possible. There are other reasons to avoid fried foods – such as the link between saturated fats and cardiovascular health – but if they are eaten, the suggestions for pre-treatment offered by this study could be considered.
Sir Muir Gray adds...
Not only soaking them in water, but boiling them in water rather than in fat would be even better.