Nitrates and Nitrites in Food and Vegetables

Capture33Why not have a salad for breakfast, especially when it contains Mom and Dad’s home-grown lettuce? Because they either use no fertilizer or in the odd year composted manure, it’s lower in nitrates ( I measured about 250 ppm, relatively low for lettuce). Fresh manure can contaminate it with both nitrates and bacteria, while most synthetic fertilizer will also increase the concentration of the ion and more significantly.

If the recommended amount of inorganic fertilizer (200 kg·N·ha−1) is used as a standard of comparison, lettuce augmented with organic fertilizers (200 kg·N·ha−1) have significantly longer and wider leaves, higher shoot, and lower concentrations of nitrate.  ncbi.nlm.nih.gov/pmc/articles/PMC4025000/

The amount of nitrate from synthetic Other vegetables and fruits known to be high in nitrates include spinach and strawberries. Here’s more on why nitrates can be harmful and what foods contain them.

  1. Why worry about nitrates (NO3) and nitrites NO2?

Here’s the latest health effect summary(italicized) from the Environmental Protection Agency

A)                 Excessive levels of nitrate in drinking water have caused serious illness and sometimes death. The serious illness in infants is due to the conversion of nitrate to nitrite by the body, which can interfere with the oxygen-carrying capacity of the child’s blood. This can be an acute condition in which health deteriorates rapidly over a period of days. Symptoms include shortness of breath and blueness of the skin.

One author (Martijn Katan ) who is a nutritionist argued here (http://ajcn.nutrition.org/content/90/1/11.full#ref-3) that what’s being described above (methemoglobinemia ) is actually caused by bacterial contamination and subsequent production of nitrogen monoxide. But that was one conclusion from an experiment in the 1950s. Meanwhile, reputable medical sources (such as http://www.nlm.nih.gov/medlineplus/ency/article/000562.htm) point out that nitrites and nitrates can cause methemoglobinemia , and it can also be brought about by a host of other compounds according to several sources that show up in Google Scholar under “causes of  methemoglobinemia”.

B)                 Drinking water levels which are considered “safe” for short-term exposures: For a 10-kg (22 lb.) child consuming 1 liter of water per day, a ten-day exposure to 10 mg/L total nitrate/nitrite.

C)                  Chronic: Effects of chronic exposure to high levels of nitrate/nitrite include diuresis, increased starchy deposits and hemorrhaging of the spleen.

D)        Cancer: There is inadequate evidence to state whether or not nitrates or nitrites have the potential to cause cancer from lifetime exposures in drinking water.

So it is still a possibility given that their presence and impact can combine with that of known carcinogens in the environment. Nitrates and nitrites are not themselves carcinogenic but they can form cancer-causing nitrosamines in the gut.

  1. What foods are sources of nitrates and nitrites?

Bacon particularly along with cold cuts and hot dogs have nitrites added to them either in the form of sodium nitrite or celery salt. But erythorbic and ascorbic acids are also added to prevent nitrosamine formation. Meanwhile spinach, especially, and lettuce can also have concentrations of nitrate that can be as high as those of bacon. But it depends on how they’re grown! If they are grown with composted manure, legume-enriched soil or with no fertilizer at all, these vegetables will have safe levels of nitrates. What elevates their concentration is liquid synthetic nitrates, some other nitrogen fertilizers and fresh manure, especially that of chickens. It should also be noted that outer leaves of lettuce have more nitrates than the inner leaves, since they have more time to concentrate them.

3. What happens to nitrites in processed meats?

Nitrites from cold cuts that have been rendered red by the addition of nitrites (hot dogs that  are made from byproducts are pale in color without nitrite) can form nitrosamines . Certain antioxidants added along with nitrites are supposed to prevent this. It works in most cases except in fried bacon, cured meats and salami, which contain between 2 and 11 ppb of nitrosamines(specifically, N-nitrosodimethylamine). Interestingly the nitrosamines in raw fish are volatile and do not show up in the analyses of cooked fish.

4. Can Excess Nitrates from Fruits and vegetables End Up As Nitrosamines?

Salivary glands and microbes in saliva are capable of converting some nitrates to nitrites, which subsequently, under the stomach’s acidic conditions can end up as nitrosamines. Some nitrites end up as nitrogen monoxide, which can have beneficial effects.

Another factor to consider is the serving size of cured meats and salami relative to that of a salad. Four slices of a cold cut has twice the mass of a serving of lettuce. So even if the compared foods contain an equal concentration of nitrites/nitrates, eating treated meat would expose you to twice the amount. Add lettuce to the cold cuts and that amount rises further.

Sources:

http://www.agroecology.org/documents/Joji/leafnitrate.pdf

J Sci Food Agric. 2014 Mar 15; 94(4):773-8.

http://ajcn.nutrition.org/content/90/1/11.full#ref-3

http://www.nlm.nih.gov/medlineplus/ency/article/000562.htm

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Becoming A Culinary Chemist

Created Feb 20 2011 – 9:00pm Updated 2014- August

My first brush with molecular gastronomy came decades ago with attempts to convince my Italian mother that the shape of pasta noodles actually affects their taste. Although all forms are made with semolina(from durum wheat), their shapes affect texture which is part of the taste experience. Fusilli, bucatini and farfalle also have a range of surface to volume ratios. It is plausible that this leads to noticeable differences in salt absorption and causes varying amounts of sauce to cling to them. Anyone who has cooked has definitely noticed that the time of cooking varies inversely with the surface area of noodles. As the time decreases, it creates a narrower window to capture that optimal al dente texture.

The al dente texture itself is caused by a network of coagulated protein in which starch is embedded. If overcooked, the greater absorption of water causes more starch to coagulate. The continuous network of protein then breaks down into discrete masses, the starch reaches the surface and the pasta becomes soft and sticky.

The next set of common reactions that could catch the fancy of a chemist are of the Maillard type. With the addition of heat, the amino group (NH2) of amino acids attacks the carbonyl group (C=O) of a reducing sugar eventually leading to a range of brownish and appetizing compounds. Here are some examples of baking and cooking products that include compounds from Maillard reactions:

1) Bread crust

2) Boiling of maple syrup

3) Roasting of almonds, coffee or cocoa nuts

4) Beer-making (where’s the heat you may wonder? It’s the heat of fermentation.)

5) Baking of cookies

6) Browning of sauce on meat.

The first compound formed from the amino attack is an N-substituted glycosylamine. But the hexagonal ring of this molecule then breaks up, undergoes another rearrangement with the help of a pH-change to produce an Amadori compound. What happens to this type of compound depends on pH, but in either case the NH2 group is lost forming ketones.(A compound with C=O group sandwiched between carbons.) In the next stage, these compounds are split into some of the brown compounds that we taste and smell. While Maillard reactions are taking place, amino acids can decompose into Schiff bases that eventually produce cereal like flavours and those of roasted nuts, bread and meat. Unless a sweet sauce is added to meat, the browning seen upon cooking is not a Maillard reaction. Rather it results from the oxidation of the Fe2+in myoglobin to the Fe3+ state. This is part of the reaction where the myoglobin protein is denatured to hemichrome. And when a cooked leaf loses its green colour, it is because chlorophyll has lost its Mg2+ ion. Like most reactions it depends on enzymes. In this case if you want to maintain the green colour, a little baking soda can be added. (not too much or you’ll gain both colour and bitterness) The higher pH from HCO3 prevents the enzyme from converting chlorophyll into pheophytin.

If you are a non-meat eater, you won’t mind me switching the topic from meat to seafood. Astaxanthin is a compound related to the carotene in carrots. It is pink and found in shrimp. Normally while the shrimp is alive, the pink colour of astaxanthin is not evident because it is bound to a protein, which changes its colour. But the heat of cooking uncoils the protein, unsheathing the same pigment that keeps flamingos feathers pink.A similar explanation applies to the blue/green to red colour change for cooked lobsters. Astaxanthin’s structure is similar to that of carotene. But the extra C=O group in astaxanthin increases the alternating single-double bond network, which makes it easier for electrons to get excited to higher energy levels. Compared to carotene, astaxanthin needs less energy or that of a longer wavelength for electronic excitation. This is consistent with the fact that astaxanthin reflects color of a longer wavelength: pink instead of orange. By the way, if you find a flamingo feather, keep it. Astaxanthin sells for $ 7000/kg! Of course a feather will have a negligible fraction of astaxanthin, so look for a lost flamingo instead.We often distort reality by trying to operate by simple rules of thumb. This certainly applies to vitamins in fruits and vegetables. Cooking certainly reduces the concentration of vitamins in food, but it does not destroy them, as shown in the table below:Effect of Heat on Vitamins(Source: USDA)

Food(100g) Vit A in Raw(IU) Vit A in Cooked(IU)
retention
Vit C in Raw(mg) Vit C in cooked(mg)
retention
carrots
28129
17202 61% 5.9 3.6 61%
red peppers 5700 2760 48% 190 163 86%
broccoli
3000
1967 66% 93 65 70%

And where in the fruit and vegetable are the vitamins located? Does the peel contain a lot of nutrients? The answer varies. In the case of the potato, the peel does have fiber and minerals, and baking with the peel prevents the escape of some vitamin C during the cooking process. I have not been able to verify the claim that most of the vitamin C is in the flesh just underneath the peel. In the case of mangos, vitamin A is distributed evenly throughout the orange pulp, which is coloured by similar beta carotene molecules. But the peel, especially when ripe, has antioxidants, carotenes and vitamin C. Apple peels are not devoid of nutrients either: they contain minerals (K+, Mg2+), antioxidants and fiber.

Here’s a great example of another endothermic reaction in the kitchen: cooking avocados. Some compounds in the avocado will be converted into bitter alkaloids with heat. But if the avocados are added towards the end of a recipe to minimize the amount of heat absorbed, then the amount of bitter-tasting products will be kept to a minimum.

Finally an update following some experiments I did in the kitchen after reading Herbert This’ book, Molecular Gastoronomy.

Yes it’s OK to scream at kids for leaving the fridge door open! If the room temperature is at 20 C, leaving the fridge open for just 20 seconds raises the temperature of the air in the fridge from 8 to 18 C because the cold dense air easily slips out, while warmer room air diffuses in. The Institute of Food and Agricultural Sciences at the University of Florida says that being careless with opening and closing your fridge door wastes 50 to 120kWh a year. I did the math, and if every one of the 12 million+ households in Canada was sloppy, we’d be wasting $84 million worth of electricity (at Quebec prices). Since not all of Canada uses hydro, that’s tied in to a lot of needless pollution.
vitC
This is not the lemon-juice trick. At breakfast I treated the left side with ascorbic acid(vitamin C) instead, which also inhibits the enzymes that otherwise speed up the oxidation of polyphenols. Apples are low in vitamin C(about 5 mg per 100 g), but they won’t be any more after this treatment.
In theory, freshly brewed coffee should be left to cool to on its own to about 75 C, and then add cold milk if you’re looking for the quickest way to reach optimum temperature(55 C). This is because the rate of cooling is at its maximum when the difference between room and coffee temperature is highest. But when I checked my brewer pot’s temperature, and more importantly that of the quickly poured coffee, it was nowhere near the 100 C that Herve This had mentioned. It was already down to 68.8 C. So when I added milk it dropped to 54 C immediately! But I had no clue that such a temperature is comfortable to the tongue and upper palate.(but just 4 more degrees than that stings the tongue!)
Photo: Science In the Kitchen---Notice that the egg on top of the picture has an off-centered yolk. The bottom one does not. I partially followed molecular gastronomist Herve This' advice, While it was cooking, I rotated one of the eggs to prevent the less-dense yolk from rising, but he recommended rotation for 10 minutes. Instead I hypothesized that there is enough coagulation of proteins after 5 minutes to prevent the yolk from rising. So I only rotated one egg, once a minute for 5 minutes, turned off the heat, and left the eggs in the pot with the lid on for 5 more minutes. When I took them out, the water was still at 90 oC, still hot enough to create hard boiled eggs. (Only 68 oC is sufficient to coagulate all proteins in the egg.)
Photo: Science In the Kitchen—Notice that the egg on top of the picture has an off-centered yolk. The bottom one does not. I partially followed molecular gastronomist Herve This’ advice, While it was cooking, I rotated one of the eggs to prevent the less-dense yolk from rising, but he recommended rotation for 10 minutes. Instead I hypothesized that there is enough coagulation of proteins after 5 minutes to prevent the yolk from rising. So I only rotated one egg, once a minute for 5 minutes, turned off the heat, and left the eggs in the pot with the lid on for 5 more minutes. When I took them out, the water was still at 90 oC, still hot enough to create hard boiled eggs. (Only 68 oC is sufficient to coagulate all proteins in the egg.)
Photo: We may be like cats and dogs because we eat, but animals can't cook, devise recipes or learn about the science of cooking. So what happens when you make spaghetti? Without absorbing water, the protein and embedded starch granules are too hard. As both nutrients absorb water, the starch swells up and the network of protein keeps the starch contained. But if you overcook the pasta, the granules will break out of the network, cling to the surface of the noodles and make them sticky. Can this be prevented?  The protein network is strongest at a slightly acidic pH,  so Herve This' recommended adding 15 ml of lemon juice to the pot of cold water. It worked like a charm. I normally cook it 2 minutes below the suggested time; instead even with 3 more minutes, the spaghetti was firm, not sticky and did not get any complaints from my anti- al dente family. Oil does not help and mineral water makes it worse, apparently. With that in mind I only added the salt 30 seconds before turning off the heat and stirred well.
Photo: We may be like cats and dogs because we eat, but animals can’t cook, devise recipes or learn about the science of cooking. So what happens when you make spaghetti? Without absorbing water, the protein and embedded starch granules are too hard. As both nutrients absorb water, the starch swells up and the network of protein keeps the starch contained. But if you overcook the pasta, the granules will break out of the network, cling to the surface of the noodles and make them sticky. Can this be prevented? The protein network is strongest at a slightly acidic pH, so Herve This’ recommended adding 15 ml of lemon juice to the pot of cold water. It worked like a charm. I normally cook it 2 minutes below the suggested time; instead even with 3 more minutes, the spaghetti was firm, not sticky and did not get any complaints from my anti- al dente family. Oil does not help and mineral water makes it worse, apparently. With that in mind I only added the salt 30 seconds before turning off the heat and stirred well.

Sources:

  • Barham, Peter. Molecular Gastronomy. Chem Rev. 2010 April 14; 110(4): 2313–2365 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2855180/
  • Atkins, P.W. Molecules. Scientific American Library. 1987
  • McGee, Harold. On food and cooking: the science and lore of the kitchen. Simon&Schuster. 2004
  • C.M. Ajilaa, S.G. Bhata and U.J.S. Prasada Rao. Valuable components of raw and ripe peels from two Indian mango varieties. Central Food Technological Research Institute
  • United States Department of Agriculture usda.org
  • http://www.plantphysiol.org/cgi/reprint/39/6/1056.pdf

 

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