Citric acid

Properties

General
Name	Citric acid
Chemical formula	C6H8O7
Formula weight	192.13 amu
Synonyms	2-hydroxy-1,2,3-propanetricarboxylic acid
CAS number	77-92-9
Phase behavior
Melting point	426 K (153 °C)
Thermal decomposition temperature	448 K (175°C)
Acid-base properties
pKa1	3.15
pKa2	4.77
pKa3	5.19
Solid properties
ΔfH0	-1543.8 kJ/mol
S0	252.1 J/mol·K
Cp	226.5 J/mol·K
Density	1.665 ×103 kg/m3
Safety
Acute effects	Skin and eye irritant.
Chronic effects	None.
More info
Properties	NIST WebBook
MSDS	Hazardous Chemical Database
SI units were used where possible. Unless otherwise stated, standard conditions were used. Disclaimer and references

Citric acid is a weak organic acid found in citrus fruits. It is a good, naturalpreservative and is also used to add an acidic (sour) taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of almost all living things. It also serves as an environmentally friendly cleaning agent and acts as an antioxidant.

Citric acid exists in a variety of fruits and vegetables, but it is most concentrated inlemons and limes, where it can comprise as much as 8% of the dry weight of the fruit.

Citric acid's chemical formula is C₆H₈O₇ and its structure is shown at right. This structure is reflected in its IUPAC name 2-Hydroxy-1,2,3-propanetricarboxylic acid.

Contents [hide]

1 Properties 2 History 3 Production 4 Uses 5 Safety

Properties

The physical properties of citric acid are summarized in the table at right. The acidity of citric acid results from the three carboxy groups COOH which can lose aproton in solution. If this happens, the resulting ion is the citrate ion. Citrates make excellent buffers for controlling the pH of acidic solutions.

Citrate ions form salts called citrates with many metal ions. An important one iscalcium citrate or "sour salt", which is commonly used in the preservation and flavoring of food. Additionally, citrates can chelate metal ions, which gives them use as preservatives and water softeners.

At room temperature, citric acid is a white crystalline powder. It can exist either in an anhydrous (water-free) form, or as a monohydrate that contains one water molecule for every molecule of citric acid. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating it above 74°C.

Chemically, citric acid shares the properties of other carboxylic acids. When heated above 175°C, it decomposes through the loss of carbon dioxide and water.

History

The discovery of citric acid has been credited to the 8th century Islamic alchemistJabir Ibn Hayyan (Geber). Medieval scholars in Europe were aware of the acidic nature of lemon and lime juices; such knowledge is recorded in the 13th centuryencyclopedia Speculum Majus (The Great Mirror), compiled by Vincent of Beauvais. Citric acid was first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, who crystallized it from lemon juice. Industrial-scale citric acid production began in 1860, based on the Italian citrus fruit industry.

In 1893, C. Wehmer discovered that Penicillium mold could produce citric acid fromsugar. However, microbial production of citric acid did not become industrially important until World War I disrupted Italian citrus exports. In 1917, the Americanfood chemist James Currie discovered that certain strains of the mold Aspergillus niger could be efficient citric acid producers, and Pfizer began industrial-level production using this technique two years later.

Production

In this production technique, which is still the major industrial route to citric acid used today, cultures of Aspergillus niger are fed on sucrose to produce citric acid. After the mold is filtered out of the resulting solution, citric acid is isolated by precipitating it with lime (calcium hydroxide) to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid.

Alternatively, citric acid is sometimes isolated from the fermentation broth byextraction with a hydrocarbon solution of the organic base trilaurylamine , followed by re-extraction from the organic solution by water.

Uses

Most citric acid is used as a flavoring and preservative in food and beverages, especially soft drinks; it is denoted by E Number E330. Citrate salts of various metals are used to deliver those minerals in a biologically available form in many dietary supplements. The buffering properties of citrates are used to control pH in household cleaners and pharmaceuticals.

Citric acid's ability to chelate metals gives it use in soaps and laundry detergents. By chelating the metals in hard water, it lets these cleaners produce foam and work better without need for water softening. Similarly, citric acid is used to regenerate the ion exchangematerials used in water softeners by stripping off the accumulated metal ions as citrate complexes.

It is used in the biotechnology and pharmaceutical industry to passivate high purity process piping in lieu of using nitric acid, since nitric is a hazardous disposal issue once it is used for this purpose, while citric is not.

In the United Kingdom, pharmacies control the sale of Citric acid. Citric acid is a popular buffer used to increase the solubility of streetheroin in Scotland. Single-use citric acid sachets have been used as an inducement to get heroin users to exchange their dirty needles for clean needles in an attempt to decrease the spread of AIDS and hepatitis. See the .pdf article here. Other acidifiers used for brown heroin are ascorbic acid, acetic acid, and lactic acid: in their absence, the drug abuser will often substitute lemon juice or vinegar.

Safety

Citric acid is recognized as safe for use in food by all major national and international food regulatory agencies. It is naturally present in almost all forms of life, and excess citric acid is readily metabolized and eliminated from the body.

Contact with dry citric acid or with concentrated solutions can result in skin and eye irritation, so protective clothing should be worn when handling these materials

Why Are Babies Born with Blue Eyes?

Question: Why Are Babies Born with Blue Eyes?

Answer: You inherit your eye color from your parents, but no matter what the color is now, it may have been blue when you were born. Why? Melanin, the brown pigment molecule that colors your skin, hair, and eyes, hadn't been fully deposited in the irises of your eyes or darkened by exposure to ultraviolet light. The iris is the colored part of the eye that controls the amount of light that is allowed to enter. Some other animals are born with blue eyes, too, such as kittens.

Melanin is a protein. Like other proteins, the amount and type you get is coded in your genes. Irises containing a large amount of melanin appear black or brown. Less melanin produces green, gray, or light brown eyes. If your eyes contain very small amounts of melanin, they will appear blue or light gray. People with albinism have no melanin in their irises and their eyes may appear pink because the blood vessels in the back of their eyes reflect light.

Melanin production generally increases during the first year of a baby's life, leading to a deepening of eye color. The color is often stable by about 6 months of age. However, several factors can affect eye color, including use of certain medications and environmental factors. Some people experience changes in eye color over the course of their lives. People can have eyes of two colors. Even the genetics of eye color inheritance isn't as cut-and-dried as was once thought, as blue-eyed parents have been known (rarely) to have a brown-eyed child!

How To Make a Rainbow in a Glass Density Demonstration

You don't have to use lots of different chemicals to make a colorful density column. This project uses colored sugar solutionsmade at different concentrations. The solutions will form layers, from least dense, on top, to most dense (concentrated) at the bottom of the glass.

 

Difficulty: Easy

Time Required: minutes

Here's How:

1. Line up five glasses. Add 1 tablespoon (15 g) of sugar to the first glass, 2 tablespoons (30 g) of sugar to the second glass, 3 tablespoons of sugar (45 g) to the third glass, and 4 tablespoons of sugar (60 g) to the fourth glass. The fifth glass remains empty.
2. Add 3 tablespoons (45 ml) of water to each of the first 4 glasses. Stir each solution. If the sugar does not dissolve in any of the four glasses, then add one more tablespoon (15 ml) of water to each of the four glasses.
3. Add 2-3 drops of red food coloring to the first glass, yellow food coloring to the second glass, green food coloring to the third glass, and blue food coloring to the fourth glass. Stir each solution.
4. Now let's make a rainbow using the different density solutions. Fill the last glass about one-fourth full of the blue sugar solution.
5. Carefully layer some green sugar solution above the blue liquid. Do this by putting a spoon in the glass, just above the blue layer, and pouring the green solution slowly over the back of the spoon. If you do this right, you won't disturb the blue solution much at all. Add green solution until the glass is about half full.
6. Now layer the yellow solution above the green liquid, using the back of the spoon. Fill the glass to three-quarters full.
7. Finally, layer the red solution above the yellow liquid. Fill the glass the rest of the way.

Tips:

1. The sugar solutions are miscible, or mixable, so the colors will bleed into each other and eventually mix.
2. If you stir the rainbow, what will happen? Because this density column is made with different concentrations of the same chemical (sugar or sucrose), stirring would mix the solution. It would not un-mix, like you would see with oil and water.
3. Try to avoid using gel food colorings. As you can see in my photo, it is difficult for young children to mix them into the solution.
4. If your sugar won't dissolve, an alternative to adding more water is to nuke the solutions for about 30 seconds in the microwave or to use warm water in the first place. If you heat the water, use care to avoid burns.
5. If you want to make layers you can drink, try substituting unsweetened soft drink mix for the food coloring, or four flavors of sweetened mix for the sugar plus coloring.

What You Need:

• sugar
• water
• food coloring
• tablespoon
• 5 glasses or clear plastic cups

blue sky red sunset

Whenever it's not completely filled with clouds, we can see that the sky is blue. As the sun rises and as it sets, it looks red. These two observations are related, as this experiment will show.
You will need the following materials:

• a flashlight
• a transparent container with flat parallel sides (a 10-liter [2½-gallon] aquarium is ideal)
• 250 milliliters (1 cup) of milk

Set the container on a table where you can view it from all sides. Fill it ¾ full with water. Light the flashlight and hold it against the side of the container so its beam shines through the water. Try to see the beam as it shines through the water. You may be able to see some particles of dust floating in the water; they appear white. However, it is rather difficult to see exactly where the beam passes through the water.
Add about 60 milliliters (¼ cup) of milk to the water and stir it. Hold the flashlight to the side of the container, as before. Notice that the beam of light is now easily visible as it passes through the water. Look at the beam both from the side and from the end, where the beam shines out of the container. From the side, the beam appears slightly blue, and on the end, it appears somewhat yellow.
Add another ¼ cup of milk to the water and stir it. Now the beam of light looks even more blue from the side and more yellow, perhaps even orange, from the end.
Add the rest of the milk to the water and stir the mixture. Now the beam looks even more blue, and from the end, it looks quite orange. Furthermore, the beam seems to spread more now than it did before; it is not quite as narrow.
What causes the beam of light from the flashlight to look blue from the side and orange when viewed head on? Light usually travels in straight lines, unless it encounters the edges of some material. When the beam of a flashlight travels through air, we cannot see the beam from the side because the air is uniform, and the light from the flashlight travels in a straight line. The same is true when the beam travels through water, as in this experiment. The water is uniform, and the beam travels in a straight line. However, if there should be some dust in the air or water, then we can catch a glimpse of the beam where the light is scattered by the edges of the dust particles.
When you added milk to the water, you added many tiny particles to the water. Milk contains many tiny particles of protein and fat suspended in water. These particles scatter the light and make the beam of the flashlight visible from the side. Different colors of light are scattered by different amounts. Blue light is scattered much more than orange or red light. Because we see the scattered light from the side of the beam, and blue light is scattered more, the beam appears blue from the side. Because the orange and red light is scattered less, more orange and red light travels in a straight line from the flashlight. When you look directly into the beam of the flashlight, it looks orange or red.
What does this experiment have to do with blue sky and orange sunsets? The light you see when you look at the sky is sunlight that is scattered by particles of dust in the atmosphere. If there were no scattering, and all of the light travelled straight from the sun to the earth, the sky would look dark as it does at night. The sunlight is scattered by the dust particles in the same way as the light from the flashlight is scattered by particles in milk in this experiment. Looking at the sky is like looking at the flashlight beam from the side: you're looking at scattered light that is blue. When you look at the setting sun, it's like looking directly into the beam from the flashlight: you're seeing the light that isn't scattered, namely orange and red.
What causes the sun to appear deep orange or even red at sunset or sunrise? At sunset or sunrise, the sunlight we observe has traveled a longer path through the atmosphere than the sunlight we see at noon. Therefore, there is more scattering, and nearly all of the light direct from the sun is red.

Red sky at night

Meaning

This is the first part of the weather-lore rhyme:

Red sky at night; shepherds delight,
Red sky in the morning; shepherds warning

Sometimes the phrase involves sailors rather than shepherds - both have a more than usual interest in the weather.

Origin

The saying is very old and quite likely to have been passed on by word of mouth for some time before it was ever written down. There is a written version in Matthew XVI in the Wyclif Bible, from as early as 1395:

"The eeuenynge maad, ye seien, It shal be cleer, for the heuene is lijk to reed; and the morwe, To day tempest, for heuen shyneth heuy, or sorwful."

The Authorised Version gives that in a more familiar form:

"When it is evening, ye say, It will be fair weather: for the sky is red. And in the morning, It will be foul weather to day: for the sky is red and louring."

There are many later citations of the saying in literature, including this from Shakespeare, in Venus & Adonis, 1593:

"Like a red morn, that ever yet betoken'd wreck to the seaman - sorrow to shepherds."

So, that's where it originated but why?

There are many proverbs and stories concerning the weather from mediaeval England. For example, the notion that the weather on St. Swithin's Day (15th July) predicts the weather in England for the next 40 days:

St Swithin's Day, if it does rain
Full forty days, it will remain
St Swithin's Day, if it be fair
For forty days, t'will rain no more

This prediction is nonsense and the weather on that day has no more significance than any other.

When rhymes like that were established England had a primarily rural and maritime economy and weather was consequently of life and death importance. There was no accurate means of forecasting the weather, so the tendency to make the most of what little information they had to go on, and occasionally to put two and two together and make five, is hardly surprising.

The 'red sky at night' rhyme is more than an old wives' tale though and has some meteorological foundation - in England at least.

To explain why we'll need to know why clouds sometimes appear red and how that may be used to predict the weather. Firstly, why do clouds often appear red in the morning and evening?

- Sunlight is broken into the familiar rainbow spectrum of varying-wavelength colours as it passes through the atmosphere.

- The blue/violet end of the spectrum is diverted more than the red/orange.

(This is the same mechanism that causes us to see the sky as blue incidentally, but that's getting rather off our point)

- When the sun is low in the sky, at dawn and dusk, sunlight travels through more atmosphere than at other times of day. The red wavelength is better able to go on a direct course and be reflected back off clouds, whereas the blue light is more scattered before reaching the cloud and is therefore less visible. So, we see the clouds as red as the light that is reaching them is primarily red.

...and how does that help predict the weather?

- The weather in the UK comes from the west, i.e. the wind is primarily westerly.

- The sun rises in the east and sets in the west.

- If there is broken cloud in the morning we may look to the west and see red light reflecting back from the cloud, i.e. 'red sky in the morning'. As the clouds are coming towards us there must be a chance of rain, at least an increased chance compared with the cloudless period we had just enjoyed.

- Likewise for 'red sky at night'. If we see red clouds in the evening they will be in the east and have already passed us by, giving a good chance of clear skies and fine weather 

Why is Milk white?

Why is Milk white?Those chalky-white mustaches that color our lips after chugging down a refreshing ice-cold glass of creamy milk is caused by the protein called Casein. Rich in calcium, Casein helps contribute to milk's white color. In addition, the cream that is found in milk contains white colored fat. The more cream in milk the more white it is. Low and non-fat milk appear more grayish rather than white because they contain less cream. Another reason milk looks white to our naked eyes is because some objects do not absorb very much light. Rather than absorb light, these objects reflect light. For instance, red colored objects reflect only red light and absorb the other colors of light in the rainbow spectrum. The molecules that make up Casein and cream reflect light. That's why milk is white. Did you know? Calcium and Vitamin D are the two ingredients of milk that make it such a healthy beverage. While calcium is a major nutrient for us that gives us strong bones and healthy teeth, Vitamin D is a nutrient that is needed by humans to produce healthy bones. While milk is being squeezed from the cow's udders, it leaves the cow's body at a high temperature of 101 degrees Fahrenheit. Then the milk is quickly cooled and stored at 40 degrees Fahrenheit. In order to make 9 gallons of milk a day, a cow must drink 18 gallons of fresh, clean water (2 gallons of water for every gallon of milk). Doctors and nutritionists recommend that everyone should drink about 2 glasses of milk a day to stay healthy and prevent osteoporosis (the disease that causes the deterioration of our bones) The average cow produces 90 glasses of milk per day. Milk usually arrives at grocery stores about 2 days after is it pumped from the cow. Chocolate milk contains the same essential ingredients as white milk--calcium, vitamin D, vitamin B12, high quality protein, and magnesium. The only difference between white and chocolate milk is that chocolate milk contains about 60 more calories because of the sucrose and other nutrient sweeteners that are added to it.

Milk

 A glass      of milk

Milk most often means the nutrient fluid produced by the mammary glands of female mammals. It provides the primary source of nutrition for newborns before they are able to digest more diverse foods. It is also processed into dairy products such as cream, butter, yoghurt, ice-cream, gelato,cheese, casein, whey protein, lactose, dried milk, and many other food-additive and industrial products.

It can also be used to mean

• the white juice and the processed meat of the coconut in, more or less, liquid form, used especially in Thai, Indian (Kerala), and Polynesian cuisine.
• a non-animal substitute such as soya milk, rice milk, and almond milk.

Human milk is often fed to infants through breastfeeding, either directly or by the female expressing her milk to be saved and fed later. As colostrum, it carries the mother's antibodies and intestinal bacteria to the baby.

Contents

1 Composition and nutrition 2 Cow's milk 2.1 Varieties and brands 3 Other milk animals 4 Curdling 5 Distribution 6 Ethical issues 7 See also 8 External links 9 Other

Composition and nutrition

The composition of milk varies greatly among different mammals.

• Human breast milk is thin and high in lactose, its primary sugar.
• Cow's milk, in contrast, is lower in sugar and higher in protein, and is composed of about 3.5% to 6.5% milkfat, 4% to 8.5% milk solids and about 88% water. Its main protein (80%) is casein, while whey proteins make up most of the rest.

Lactose in milk is digested with the help of the enzyme lactase produced by the bodies of infants. In humans, production of lactase falls off towards adulthood (depending on the person's ethnic origin), in many cases to the point where lactose becomes indigestible, leading to lactose intolerance a gastrointestinal condition that afflicts many.

There is some controversy over whether consumption of cow's milk is good for adult humans. While milk is often touted as healthy for its significant amount of calcium, required for healthybone growth and nerve function, there is some disputed research to suggest that proteins in milk interfere with the use of its calcium to form bones by increasing the acidity level of the blood and triggering a response which balances that acidity level by leeching calcium that is presently in bones. However breeds of cattle produce milk that is significantly different from that of others as do different mammals' from others. Such factors as the lactose content, the proportion of and size of the butterfat globule and the strength of the curd, formed by the human enzymes digesting the milk, can differ from breed to breed and mammal to mammal.

Milk has also been linked in a small number of studies to osteoporosis, cancer, heart disease,obesity and high blood pressure. Because of milk's high protein content, and inability of the body to digest it, it is unable to fully absorb the calcium. In some countries where dairy is rarely used, such as China, these diseases are rare, although it is unclear whether dairy consumption is a cause.

Cow's milk

Cow's milk is produced on an industrial scale for human consumption.

Varieties and brands

Cow's milk is generally available in several varieties. In some countries these are:

• full cream (or "whole" in North America)
• semi-skimmed ("reduced fat" or "low fat", about 1.5-1.8% fat)
• skimmed (about 0.1% fat)

Milk in the U.S. and Canada is sold as

• "whole" varieties
• "2 percent" (reduced fat)
• "1 percent" (low fat)
• "1/2 percent" (low fat)
• "skim" (very low fat)

Full cream, or whole milk, has the full milk fat content (about 3-4% if Friesian- or Holstein-breed are the source). For skimmed or semi-skimmed milk, all of the fat content is removed and then some (in the case of semi-skimmed milk) is returned.

The best-selling variety of milk is semi-skimmed; in some countries full-cream (whole) milk is generally seen as less healthy and skimmed milk is often thought to lack taste.

Whole milk is recommended to provide sufficient fat for developing toddlers who have graduated from breast milk or infant formula.

There are many brands of milk currently, most milk brands vary little from each other. These brands include:

• Alpenrose
• Anchor (a brand from Fonterra)
• Dairy Farmers
• H.P. Hood
• Leche Caparra
• Leche Suiza
• Lucerne
• Parmalat
• Pauls (a subsidiary of Parmalat in Australia)
• Pura
• Shamrock Milks
• Tenuvah
• Vinamilk

Milk is the state drink of Minnesota, Pennsylvania,Maryland and South Carolina.

Other milk animals

In addition to cows, the following animals provide milk for dairy products:

• Sheep
• Goats
• Horses
• Asses (donkeys)
• Camels (including the South American camelids)
• Yaks
• Water buffalo
• Reindeer

In Russia and Sweden, small moose dairies exist [1]. Donkey and horse milk have the lowest fat content, while the milk of seals contains more than 50% fat. [2]

Curdling

When raw milk is left standing for a while, it turns sour. This is the result of fermentation: lactic acid bacteria turning the milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products such as cheese and yogurt.

Pasteurized cow's milk, on the other hand, spoils in a way that makes it unsuitable for consumption, causing it to assume a disgusting odor, which alone may induce vomiting in sensitive persons, and pose a high danger of food poisoning if ingested. The naturally-occurringlactic acid bacteria in raw milk, under suitable conditions, quickly produce large amounts of lactic acid. The ensuing acidity in turn prevents other germs from growing, or slows their growth significantly. Through pasteurization, however, these lactic acid bacteria are mostly destroyed, which means that other germs can grow unfettered and thus cause decomposition.

In order to prevent spoilage, milk can be kept refrigerated and stored between 1 and 4 degrees Celsius. Most milk is Pasteurized by heating briefly and then refrigerated to allow transport from Factory Farms to local markets. The spoilage of milk can be forestalled by using ultra-high temperature (UHT) treatment; milk so treated can be stored unrefrigerated for several months until opened. Sterilized milk, which is heated for a much longer period of time, will last even longer, but also lose more nutrients and assume a still different taste. Condensed Milk, made by removing most of the water, can be stored for many months, unrefrigerated. The most durable form of milk is milk powder which is produced from milk by removing almost all water.

UHT milk is very popular in Europe, whereas in North America, most of the milk sold is pasteurized.

Distribution

Prior to the widespread use of plastics, milk was usually commercially distributed to consumers inglass bottles. In the UK, milk can be delivered daily by a milk man who travels his local milk round (route) on an electric milk float, although this is becoming less popular as a result of supermarkets selling milk at cheaper prices. In New Zealand in some urban areas milk is still delivered to customers' homes.

Glass containers are rare these days. Most people purchase milk in plastic jugs or bags or in waxed-paper cartons. Ultraviolet light from fluorescent lighting can destroy some of the proteins in milk, so many companies that once distributed milk in transparent or highly translucent vessels are starting to use thicker materials that block the harmful rays. Many people feel that such "UV protected" milk tastes better. (However, few people have ever tasted fresh, unprocessed, milk straight from the cow.)

In the United States, milk is commonly sold in gallon, half-gallon and quart containers (U.S. customary units) of rigid plastic or waxed cardboard. The U.S. single serving size is usually the half-pint. In much of Canada, a 1 1/3 litre plastic bag (sold as 4 litres in 3 bags) is the most common, while 2 litre, 1 litre and 500 millilitre cartons are also available. In Europe, metric sizes of 500 millilitres, 1 litre (the most common), 2 litres and 3 litres are commonplace (in the UK, some stores instead still stock the equivalents of old Imperial sizes: 568 ml (1 pint), 1.136 l (2 pints), 2.273 l (4 pints)).

Condensed milk is distributed in metal cans and powdered milk is distributed in boxes or bags.

Ethical issues

Many people concerned about animal welfare (especially vegans) do not drink milk. An increasing number of dairy cows are being raised on factory farms, which some people consider cruel. On many farms, the calves are separated from their mothers within days of birth to prevent the calf from drinking the milk so that humans can drink it instead. Some of the calves born by dairy cows are raised in crates for veal and are slaughtered three to eighteen weeks later. On many farms, once a dairy cow's milk production decreases, she is also slaughtered at an age that is a fraction of her natural lifespan. Some people also believe that the use of bovine growth hormone to increase milk production in cows is unethical. For these reasons, either in an attempt to reduce animal suffering or to prevent animals from being killed, some people choose to not consume milk. Some also object to drinking milk for environmental reasons.