Curious Cook in the New York Times: Double-dipping

In today's Curious Cook column in the New York Times, I write about a new study of the microbiological consequences of double-dipping, or re-using a chip to scoop up a second helping of dip.

Acrylamide again: Gingerbread and hartshorn

One more surprising discovery about acrylamide that I neglected to include in my recent roundup. Among the foods with the highest known acrylamide levels is gingerbread as it's traditionally made in northern Europe--so high that in Holland, gingerbread consumption alone accounts for something like a sixth of the total year-round acrylamide intake. This is despite the fact that wheat flours don't contain nearly as much asparagine as potatoes.

Over the last couple of years, Thomas Amrein and colleagues at the Swiss Federal Institute of Technology in Zurich have shown that the culprit is the traditional leavening agent: not familiar baking soda or powders, but ammonium bicarbonate, which is sometimes called hartshorn because it was originally obtained by heating deer antlers. Ammonium carbonate is especially suited to thin, dry cookies, because when heated it releases ammonia and carbon dioxide gases, but no water. So the cookies cook and dry out faster, and the pungent ammonia gets completely baked out, rather than lingering in a thicker mass. (The European gingerbreads are made from dough sheeted to about 7 mm thick, or a shade over 1/4 inch, baked for 10 minutes at 360-380 degrees F, 180-190 degrees C.)

The problem with ammonium carbonate is the ammonia that it releases during baking. It reacts with glucose and fructose in the dough to form unusual molecules that in turn react very efficiently with asparagine to form acrylamide. The Swiss gingerbread dough is made with glucose, fructose, and honey; molasses also contains a lot of glucose and fructose. Ordinary table sugar, sucrose, is not vulnerable to attack by ammonia.

So it's easy to make low-acrylamide gingerbread: either use a standard sodium bicarbonate leavener, or use table sugar for the sweetener, or both.

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Amrein, T.M. et al. Acrylamide in gingerbread: critical factors for formation and
possible ways for reduction. J. Agric. Food Chem. 2004, 52, 4282-88.

Amrein, T.M. et al. Investigations on the promoting effect of ammonium hydrogencarbonate on the formation of acrylamide in model systems. J. Agric. Food Chem. 2006, 54, 10253-61.

Acrylamide: an update

Acrylamide is the industrially useful but toxic chemical that reared its ugly head among our potato chips and fries in 2002, when Swedish food chemists first thought to look for it in foods. Acrylamide is known to cause cancer and other illnesses in animals, and nerve damage in humans. It's suspected to be a human carcinogen as well. Acrylamide concentrations in drinking water are regulated by the Environmental Protection Agency, and the Swedish chemists found food levels hundreds of times higher than the EPA limit.

Since 2002, we've learned a lot about acrylamide in food. It's formed in both food manufacturing and in our kitchens during the heating of carbohydrate-rich foods (potatoes, grains and their products) at temperatures high enough to cause browning and the development of the desirable flavors typical of browned foods (above about 250 degrees F, 120 degrees C). It arises from the reaction of a particular amino acid, asparagine (the major amino acid in potatoes), with glucose and other sugars. Among the foods that contribute most to our daily intake of acrylamide are french fries and potato chips, breakfast cereals, cookies, and coffee. Breads, toast, pies and cakes, and corn snacks are also significant sources. Recently acrylamide was even detected in Japanese green teas, which contain asparagine and are finished by roasting at 250-280 degrees F, 120-140 degrees C.

Food chemists are coming up with a lot of new information about acrylamide levels in foods and ways to minimize them. Here are some recent findings.

A study from Donald Mottram and colleagues at the University of Reading shows that the addition of a little citric acid and the amino acid glycine reduces the acrylamide levels on cooked potatoes without reducing flavor.

Mottram and others have also found that the asparagine content of wheat flours, and their tendency to form acrylamide, depend strongly on the wheat variety, growing conditions, and farming methods. Inadequate sulfur nutrition can increase acrylamide production by a factor of 5. Less refined flours with more of the outer protein-rich aleurone layer produce more acrylamide, as do flours made from sprouted wheat.

Felix Escher and colleagues at the ETH in Zurich have found that during the last stage of potato frying, when the moisture content of the potato falls below 20%, the energy required for acrylamide production increases, while flavor production continues unaffected. This means that acrylamide levels can be minimized by lowering the oil temperature toward the end of the frying process.

What we don't yet know about acrylamide is the actual risk that it poses in our current diet. That bottom-line question is under active investigation by scientists in several countries. The fact that coffee consumption isn't associated with a higher cancer risk suggests to me that other protective factors in food compensate for its presence. But until we know for sure, it's prudent to go easy on fries and packaged snacks.

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Mizukami, Y., et al. Analysis of acrylamide in green tea . . . . J. Agric. Food Chem. 2006, 54, 7370-77.

Low, M.Y. et al. Effect of citric acid and glycine addition on acrylamide and flavor in a potato model system. J. Agric. Food Chem. 2006, 54, 5976-83.

Muttucumaru, N. et al. Formation of high levels of acrylamide during the processing of flour derived from sulfate-deprived wheat. J. Agric. Food Chem. 2006, 54, 8951-55.

Claus, A. et al. Influence of agronomic factors and extraction rate on the acrylamide contents in yeast-leavened breads. J. Agric. Food Chem. 2006, 54, 8968-76.

Amrein, T.M. et al. Influence of thermal processing conditions on acrylamide generation and browning in a potato model system. J. Agric. Food Chem. 2006, 54, 5910-16.

Green potatoes may not be as toxic as we thought

Beware of green potatoes, and peel every trace of green away: that's been standard advice for decades, and for good reason. When potatoes are exposed to light, these underground tubers interpret it as a sign that they're no longer completely buried in the soil. So they produce chlorophyll pigments to help them make use of the light's energy, and they produce bitter toxins to discourage animals from eating them. The toxins, alkaloids called solanine and chaconine, are about as powerful as their better-known cousin strychnine. They apparently interfere with the structure of all our cell membranes and also with the processing of a nerve transmitter (they inhibit acetylcholinesterase), which can cause hallucinations and convulsions. Because the color change in a potato parallels its accumulation of alkaloids, greenness is used as an indicator of toxicity and therefore irreversible spoilage. It's estimated that around 15% of the US potato crop is discarded on account of greening. But until recently, there has been little careful study of the toxin levels found in typical American potato varieties exposed to the light levels in typical markets.

N. Richard Knowles and colleagues at Washington State University exposed four different potato varieties (White Rose, Yukon Gold, Dark Red Norland, Russet Norkotah) to simulated store lighting for 10 days. They found that the correlation between color and alkaloid content was variable, with greening sometimes outpacing alkaloid accumulation. The highest alkaloid contents were found in the skins, where they sometimes exceeded the recommended maximum, an important fact for producers of potato-skin products. However, none of the varieties developed dangerous levels in the flesh.

These findings suggest that most greened potatoes need not be discarded. But the authors make two cautionary points. Potato alkaloids at low levels may have subtle toxic effects of which we're not yet aware. And because these alkaloids remain in the bloodstream for longer than 24 hours, people who eat potatoes every day may gradually accumulate toxic levels.

So it looks as though the occasional green potato is fine, but it's still not a good idea to buy them by the bagful.



Grunenfelder, L.A. et al., J. Agric. Food Chem. 2006, 54 (16), 5847.