Alcohol

Alcohols are compounds which has got OH group as their functional group. Alcohols are present naturally in many product like ethanol and methanol.

Alcohols are classified with number of OH groups attached. For example when the alcohol contains only one OH it is called as mono hydric alcohol for example ethanol whose formula is CH₃CH₂OH, when the compound contains two OH group it is called as di hydric alcohol for example as in ethylene glycol CH₂OH-CH₂OH. The alcohol is called as tri hydric alcohol when there is three OH group attached in the compound for example as in glycerol CH₂OH-CHOH-CH₂OH.
Another important way of classifying the alcohol is by the carbon in which the alcohol is attached.

What is Alcohol?

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Alcohol is an organic compound containing hydroxyl functional group. Ethanol a main constituent of beverages and medicines and glycol common antifreeze are some examples of alcohols.

The general formula of alcohol is R-OH, where R can be alkyl group. Depending on the nature of alkyl group (R) the alcohol are classified as primary, secondary, tertiary, vinyl, allyl and benzyl alcohol.

Synthesis of Alcohols

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Alcohols can be synthesized by any of the following ways.

1. By the hydration of alkenes

Alkenes on hydration in the presence of dilute acids give alcohols. For example ethylene on hydration gives ethyl alcohol.

CH₂=CH₂ + H₂O → CH₃-CH₂-OH

Hydration of propene gives 2-propanol in according to Markovnikov's rule. The reaction involves protonation of alkene followed by addition of water.

2. By reduction of aldehydes, ketones, acyl chlorides

Alcohols are synthesized by reduction of aldehyde with lithium aluminum hydride. Reduction of aldehyde generally gives primary alcohols. For example acetaldehyde on reduction gives ethyl alcohol.

CH₃-CHO → CH₃-CH₂-OH

Similarly ketones on reduction give secondary alcohols. For example ketone on reduction gives 2-propanol.

Acyl chlorides like acetyl chloride on strong reduction give alcohols. Acetyl chloride on strong reduction gives ethyl alcohol.

CH₃-COCl → CH₃CHO → CH₃-CH₂-OH

3. By hydrolysis of Grignard reagent

Grignard reagent like methyl magnesium chloride reacts with formaldehyde to give a primary alcohol. Here methyl magnesium chloride reacts with formaldehyde which on further hydrolysis gives ethanol. If we want to change the carbon chain length in primary alcohol, the same should be changed in alkyl part of Grignard reagent.

CH₃-Mg-Cl + HCHO → CH₃-CH₂-OMgCl
CH₃-CH₂-OMgCl + H₂O → CH₃-CH₂-OH + MgCl(OH)

Similarly Grignard reagent on reacting with acetaldehyde followed by hydrolysis gives secondary alcohol. For example methyl magnesium chloride reacts with acetaldehyde followed by hydrolysis gives 2-propanol. Grignard reagent on reacting with acetone followed by hydrolysis gives tertiary alcohol.

Nomenclature of Alcohols

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Alcohols are named after the parent alkane chain by replacing ‘ane’ in the parent alkane chain with ‘ol’. For example in CH₃ –OH there is one carbon atom and so the parent alkane name is methane. If we replace ‘ane’ with ‘ol’, we will get methanol. Hence, the compound name is methanol.

If the compound contains branches then the longest chain is selected as parent chain and other alkyl groups are treated as substituents. For example in the following compound the longest chain contains five carbon atoms and hence the compound name is 2 methyl Butanol.

If the compound contains more than two carbon atoms then the position of attachment of OH group should be indicated. For example the following compound is named as 2-Butanol.

In case of compounds containing more than one functional group, the order of priority should be followed. In this halogens will have least priority than alcohols and other functional groups like acids will have more priority than alcohol.

List of Alcohols

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1. Primary alcohol

Here the functional group is attached to a primary carbon atom ( a carbon atom which is connected to exactly one carbon atom). Ethanol is an example for primary alcohol.

2. Secondary alcohol

Here the functional group is attached to a secondary carbon atom which is exactly connected to two carbon atoms. 2-propanol is an secondary alcohol.

3. Tertiary alcohol

Here the functional group is attached to a tertiary carbon atom which is exactly connected to three carbon atoms. 2-methyl-2-propanol is an example for tertiary alcohol.

4. Vinyl alcohol

Iit is an alcohol where the functional group is directly attached to a carbon containing double bond. Prop-1-ene-1-ol is an example for vinyl alcohol.

5. Allyl alcohol

It is an alcohol where the functional group is directly attached to a carbon which is connected to unsaturated carbon. Prop-2-ene-1-ol is an example for allyl alcohol.

6. Benzyl alcohol

Here the functional group is attached to a carbon chain containing the benzene ring. The structure of benzyl alcohol is given below.

7. Dihydric alcohol

The compounds containing two hydroxy group are called as dihydric alcohol.
Ethylene glycol is an example for dihydric alcohol.

8. Trihydric alcohol

The compounds containing three hydroxy group are called as trihydric alcohol.
Glycerol is an example for trihydric alcohol.

Physical Properties of Alcohols

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• Alcohols are colorless liquids with characteristic smell.
• They are high boiling liquids due to presence of hydrogen bonding.
• They are soluble in water.
• Their boiling point increases with increase in the length of carbon chain and increase in the number of hydroxyl groups.
• So, ethylene glycol will have more boiling point that ethanol.

Chemical Properties of Alcohols

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Alcohol are polar molecules due to the presence of OH functional group. The OH functional group can release proton in solution and hence alcohol are slightly acidic.

In the second case the OH functional group can altogether be replaced. So the reaction of alcohols are classified as

1. Reactions involving acidic hydrogen
2. Reactions involving hydroxyl functional group

Reactions of Alcohols

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1. Reaction with sodium

Alcohols react with sodium to give sodium alkoxide and hydrogen. This is the characteristic reaction of alcohol and is often used in organic analysis to identify the alcohol.

2CH₃-CH₂-OH + 2Na → 2CH₃-CH₂-ONa + H₂

2. Reaction with acids (Esterification reaction)

Alcohols condense with acids in the presence of concentrated sulfuric acid to give ester. For example ethyl alcohol condense with acetic acid to give ethyl acetate.

CH₃-CH₂-OH + CH₃-COOH → CH₃-CH₂-O-CO-CH₃ + H₂O

3. Reaction with acidified potassium permanganate (oxidation reaction)

Primary alcohols on oxidation give aldehyde which easily undergoes oxidation to give carboxylic acid with same number of carbon atoms. This is characteristic reaction of primary alcohol as secondary alcohols on oxidation gives ketone which is difficult to oxidize to further.

CH₃-CH₂-OH → CH₃-CHO → CH₃-COOH

4. Reaction with PCl₅, PCl₃, SOCl₂

Alcohols react with chlorinating agents like phosphorus pentachloride, phosphorus trichloride and thionyl chloride to give chloro alkanes.

CH₃-CH₂-OH + PCl₅ → CH₃-CH₂-Cl + POCl₃ + HCl

CH₃-OH + SOCl₂ → CH₃-Cl + SO₂ + HCl

5. Reaction with ammonia

Alcohols react with ammonia to give a mixture of amines. The reaction yields depends on the concentration of ammonia and alcohol.

CH₃-OH + NH₃ → CH₃-NH₂ + H₂O

CH₃-NH₂ + CH₃-OH → CH₃-NH-CH₃ + H₂O

CH₃-NH-CH₃ + CH₃-OH → CH₃-N-(CH₃)₂ + H₂O

6. Dehydration reaction

Alcohols undergo intra-molecular dehydration with concentrated sulfuric acid to give alkene. For example, ethanol on dehydration gives ethene and propanol on dehydration give propene.

CH₃-CH₂-OH → CH₂=CH₂ + H₂O
CH₃-CH₂-CH₂-OH → CH₃-CH=CH₂ + H₂O

At elevated temperature alcohols undergo intermolecular dehydration to give ethers.

CH₃OH + CH₃OH → CH₃-O-CH₃ + H₂O

Alcohol

Alcohols are compounds which has got OH group as their functional group. Alcohols are present naturally in many product like ethanol and methanol.

Alcohols are classified with number of OH groups attached. For example when the alcohol contains only one OH it is called as mono hydric alcohol for example ethanol whose formula is CH₃CH₂OH, when the compound contains two OH group it is called as di hydric alcohol for example as in ethylene glycol CH₂OH-CH₂OH. The alcohol is called as tri hydric alcohol when there is three OH group attached in the compound for example as in glycerol CH₂OH-CHOH-CH₂OH.
Another important way of classifying the alcohol is by the carbon in which the alcohol is attached.

What is Alcohol?

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Alcohol is an organic compound containing hydroxyl functional group. Ethanol a main constituent of beverages and medicines and glycol common antifreeze are some examples of alcohols.

The general formula of alcohol is R-OH, where R can be alkyl group. Depending on the nature of alkyl group (R) the alcohol are classified as primary, secondary, tertiary, vinyl, allyl and benzyl alcohol.

Synthesis of Alcohols

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Alcohols can be synthesized by any of the following ways.

1. By the hydration of alkenes

Alkenes on hydration in the presence of dilute acids give alcohols. For example ethylene on hydration gives ethyl alcohol.

CH₂=CH₂ + H₂O → CH₃-CH₂-OH

Hydration of propene gives 2-propanol in according to Markovnikov's rule. The reaction involves protonation of alkene followed by addition of water.

2. By reduction of aldehydes, ketones, acyl chlorides

Alcohols are synthesized by reduction of aldehyde with lithium aluminum hydride. Reduction of aldehyde generally gives primary alcohols. For example acetaldehyde on reduction gives ethyl alcohol.

CH₃-CHO → CH₃-CH₂-OH

Similarly ketones on reduction give secondary alcohols. For example ketone on reduction gives 2-propanol.

Acyl chlorides like acetyl chloride on strong reduction give alcohols. Acetyl chloride on strong reduction gives ethyl alcohol.

CH₃-COCl → CH₃CHO → CH₃-CH₂-OH

3. By hydrolysis of Grignard reagent

Grignard reagent like methyl magnesium chloride reacts with formaldehyde to give a primary alcohol. Here methyl magnesium chloride reacts with formaldehyde which on further hydrolysis gives ethanol. If we want to change the carbon chain length in primary alcohol, the same should be changed in alkyl part of Grignard reagent.

CH₃-Mg-Cl + HCHO → CH₃-CH₂-OMgCl
CH₃-CH₂-OMgCl + H₂O → CH₃-CH₂-OH + MgCl(OH)

Similarly Grignard reagent on reacting with acetaldehyde followed by hydrolysis gives secondary alcohol. For example methyl magnesium chloride reacts with acetaldehyde followed by hydrolysis gives 2-propanol. Grignard reagent on reacting with acetone followed by hydrolysis gives tertiary alcohol.

Nomenclature of Alcohols

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Alcohols are named after the parent alkane chain by replacing ‘ane’ in the parent alkane chain with ‘ol’. For example in CH₃ –OH there is one carbon atom and so the parent alkane name is methane. If we replace ‘ane’ with ‘ol’, we will get methanol. Hence, the compound name is methanol.

If the compound contains branches then the longest chain is selected as parent chain and other alkyl groups are treated as substituents. For example in the following compound the longest chain contains five carbon atoms and hence the compound name is 2 methyl Butanol.

If the compound contains more than two carbon atoms then the position of attachment of OH group should be indicated. For example the following compound is named as 2-Butanol.

In case of compounds containing more than one functional group, the order of priority should be followed. In this halogens will have least priority than alcohols and other functional groups like acids will have more priority than alcohol.

List of Alcohols

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1. Primary alcohol

Here the functional group is attached to a primary carbon atom ( a carbon atom which is connected to exactly one carbon atom). Ethanol is an example for primary alcohol.

2. Secondary alcohol

Here the functional group is attached to a secondary carbon atom which is exactly connected to two carbon atoms. 2-propanol is an secondary alcohol.

3. Tertiary alcohol

Here the functional group is attached to a tertiary carbon atom which is exactly connected to three carbon atoms. 2-methyl-2-propanol is an example for tertiary alcohol.

4. Vinyl alcohol

Iit is an alcohol where the functional group is directly attached to a carbon containing double bond. Prop-1-ene-1-ol is an example for vinyl alcohol.

5. Allyl alcohol

It is an alcohol where the functional group is directly attached to a carbon which is connected to unsaturated carbon. Prop-2-ene-1-ol is an example for allyl alcohol.

6. Benzyl alcohol

Here the functional group is attached to a carbon chain containing the benzene ring. The structure of benzyl alcohol is given below.

7. Dihydric alcohol

The compounds containing two hydroxy group are called as dihydric alcohol.
Ethylene glycol is an example for dihydric alcohol.

8. Trihydric alcohol

The compounds containing three hydroxy group are called as trihydric alcohol.
Glycerol is an example for trihydric alcohol.

Physical Properties of Alcohols

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• Alcohols are colorless liquids with characteristic smell.
• They are high boiling liquids due to presence of hydrogen bonding.
• They are soluble in water.
• Their boiling point increases with increase in the length of carbon chain and increase in the number of hydroxyl groups.
• So, ethylene glycol will have more boiling point that ethanol.

Chemical Properties of Alcohols

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Alcohol are polar molecules due to the presence of OH functional group. The OH functional group can release proton in solution and hence alcohol are slightly acidic.

In the second case the OH functional group can altogether be replaced. So the reaction of alcohols are classified as

1. Reactions involving acidic hydrogen
2. Reactions involving hydroxyl functional group

Reactions of Alcohols

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1. Reaction with sodium

Alcohols react with sodium to give sodium alkoxide and hydrogen. This is the characteristic reaction of alcohol and is often used in organic analysis to identify the alcohol.

2CH₃-CH₂-OH + 2Na → 2CH₃-CH₂-ONa + H₂

2. Reaction with acids (Esterification reaction)

Alcohols condense with acids in the presence of concentrated sulfuric acid to give ester. For example ethyl alcohol condense with acetic acid to give ethyl acetate.

CH₃-CH₂-OH + CH₃-COOH → CH₃-CH₂-O-CO-CH₃ + H₂O

3. Reaction with acidified potassium permanganate (oxidation reaction)

Primary alcohols on oxidation give aldehyde which easily undergoes oxidation to give carboxylic acid with same number of carbon atoms. This is characteristic reaction of primary alcohol as secondary alcohols on oxidation gives ketone which is difficult to oxidize to further.

CH₃-CH₂-OH → CH₃-CHO → CH₃-COOH

4. Reaction with PCl₅, PCl₃, SOCl₂

Alcohols react with chlorinating agents like phosphorus pentachloride, phosphorus trichloride and thionyl chloride to give chloro alkanes.

CH₃-CH₂-OH + PCl₅ → CH₃-CH₂-Cl + POCl₃ + HCl

CH₃-OH + SOCl₂ → CH₃-Cl + SO₂ + HCl

5. Reaction with ammonia

Alcohols react with ammonia to give a mixture of amines. The reaction yields depends on the concentration of ammonia and alcohol.

CH₃-OH + NH₃ → CH₃-NH₂ + H₂O

CH₃-NH₂ + CH₃-OH → CH₃-NH-CH₃ + H₂O

CH₃-NH-CH₃ + CH₃-OH → CH₃-N-(CH₃)₂ + H₂O

6. Dehydration reaction

Alcohols undergo intra-molecular dehydration with concentrated sulfuric acid to give alkene. For example, ethanol on dehydration gives ethene and propanol on dehydration give propene.

CH₃-CH₂-OH → CH₂=CH₂ + H₂O
CH₃-CH₂-CH₂-OH → CH₃-CH=CH₂ + H₂O

At elevated temperature alcohols undergo intermolecular dehydration to give ethers.

CH₃OH + CH₃OH → CH₃-O-CH₃ + H₂O

Organic Chemistry

The root word of organic chemistry, 'Organic' means that the compounds were synthesized from living organisms in the past. Still now organic chemistry reactions are involving the synthesis of organic compounds from living organism like starch, cellulose etc. Introduction to organic chemistry Organic Chemistry is a sub division of Chemistry study and it deals with the scientific study of structure, properties and the compositions of compounds. This is also considered as the chemistry of carbon containing compounds. Every living organisms, irrespective of plants and animals are composed of organic compounds and anyone with an interest in life would definitely like to know more about the molecules involved in these life processes and also need to have the basic understanding of organic chemistry. What is Organic Chemistry? Organic chemistry is the branch of chemistry dealing with compounds containing carbon-carbon bonds. These carbon compounds are special in nature because most of them covalent in nature and they are highly volatile. As organic compounds have some distinguishing characters they are differentiated from rest of the chemistry and studies separately. The next interesting thing is the number of organic compounds. As carbon can from a long chain due to its catenation ability (an ability to form long chains with itself and with other atoms) it can form many number of compounds. Hence we have to study the properties of organic compounds separately. Organic Chemistry Definition Hence organic chemistry is the branch of chemistry dealing with organic compounds made up of covalent carbon chain. This branch is unique in studying the properties of organic compounds as all of them are covalent and they undergo different set of reactions from Inorganic compounds. Functional Groups in Organic Chemistry One of the special feature of organic chemistry which differentiates it from Inorganic chemistry is the compounds will form a pattern called homologous series. Every organic compound will have a specific part or group where the reactivity is more. This part is called as functional group in the organic compound. All the organic compounds with same functional group will fall under the same homologous series. They will have same chemical properties which make the study of organic chemistry much more easier. For example in the following compounds the hydroxy (-OH) is the functional group and all the compounds are called as alcohols with same physical and chemical properties. The special nature of functional group is all the compounds with same functional group will have same chemical properties. But the physical properties may differ with the number and nature of carbon chain. For example both methanol and butanol will fall under alcohol series. Both will react with sodium to liberate hydrogen gas. 2CH3OH + 2Na → 2CH3ONa + H2 2CH3CH2CH2OH + 2Na → 2CH3CH2CH2ONa + H2 Organic Chemistry Reactions Back to Top Organic chemistry reactions are different from inorganic chemistry reactions. As most of the organic compounds are covalent in nature, the organic chemistry reactions involves the cleavage of the covalent bonds and forming of new bonds. A covalent bond is made up of two electrons and cleavage of such bond may happen in such a way that both the electrons are taken away by one atom resulting in formation of ions. On the other hand the bonding electrons may be equally divided between atom giving rise to free radicals. Hence organic chemistry reactions proceed by the formation of ions or free radicals. 1. Hence the organic chemistry reaction may be classified as Free radical reaction where radicals are formed and initiate the reaction, For example bromination of methane involves formation of bromine free radical to proceed the reaction. Hence the reaction is free radical reaction. CH4 + Br. → CH3Br Nucleophilic reaction where negative ions are produced and attack on positive sides. For example the carbonyl carbon is partially positive charged and negative ions will attack on the carbon easily. Hence all the reactions in aldehyde and ketone are nucleophilic reactions involving negative ions. Electrophilic reaction where positive ions are produced and attack on negative sides. For example benzene ring is a rich source of Π electrons. Hence all the reactions will proceed with the attack of this electron by positively charged ions. Hence all the reactions in the benzene ring are electrophilic reactions. 2. The organic chemistry reactions may also further divided as Addition reaction: where an atom or group is added across an unsaturated bond. For example addition of bromine with ethylene gives di-bromo ethane. CH2=CH2 + Br2 → CH2Br-CH2Br Elimination reaction: where a molecule is eliminated from an organic compound to give unsaturated compound. For example ethyl bromide on elimination in the presence of alcoholic KOH gives ethene. CH3-CH2-Br → CH2=CH2 + HBr Oxidation reaction: It is the type of reaction where oxygen is added or hydrogen is removed from an organic compound. For example ethyl alcohol on strong oxidation in the presence of acidified potassium permanganate gives acetic acid. Reduction reaction: It is the type of reaction where oxygen is removed or hydrogen is added to an organic compound. For example acetone on reduction with lithium aluminium hydride gives 2-propanol. Condensation reaction: It is the type of reaction where two organic compounds combine together to give one compound by elimination of simple molecules like water, HCl etc. For example condensation of acid and alcohol gives ester. CH3-CH2-OH + CH3COOH → CH3CH2OCOCH3 + H2O Polymerization reaction: It is the reaction in which small organic molecules called as monomers combined together to give a large chain of macro molecule called as polymer. Polymerization of vinyl chloride gives polyvinyl chloride, shortly called as PVC. Substitution reaction: It is the reaction in which an atom or group is replaced by another atom or group in organic compound. For example chloromethane reacts with potassium hydroxide to give methanol and potassium chloride. CH3-Cl + KOH → CH3OH + KCl

Wine and chemistry

I’ve been reading The Billionaire’s Vinegar recently, the story about wine unearthed allegedly owned by our third President, Thomas Jefferson. While I’m only half way through, this “mystery” revolves around the discovery and authenticity of wine bottles with the inscription “Th J” and the intense bidding wars for these prizes unleashed at auction. You can read my review on the book by clicking here. What caught my eye was a discussion about the roll that oxygen plays in the maturation or spoiling of wine, in this case, a wine allegedly two hundred year old. It caused me to pause, reminding me that basic chemistry is so vital to the quality of the end product that we enjoy.

Oxygen was discovered by Joseph Priestly in 1774 when upon burning Mercury Oxide, noted that an odorless gas allowed a candle flame to burn far longer than anticipated. In 1775, Priestly placed a mouse in a closed jar with oxygen and to his astonishment, it survived 30 minutes and was revived without incident. Oxygen is the third most abundant element in the universe after hydrogen and helium and constitutes just short of 21% of our atmosphere. There are two forms of the element that make life possible for all of us. One is diatomic oxygen (two atoms combined to form O2) and the other O3 or Ozone, a layer high up in the atmosphere that protects us down here from the hazards of ultra violet radiation. Ironically, Ozone is a pollutant at the surface and can be a component of smog.

Ok, enough basic chemistry. Let’s get down to the Dr. Jekyll/Mr. Hyde personality of oxygen during and after wine production. As a primer, let’s establish the stages of wine production in very simple terms and then parse out how oxygen does its work: there’s harvest, crushing, pressing (the process of forcing juices from the grapes) and fermentation, storage, bottling and at some delightful point, popping the cork or unscrewing the cap.

In the big picture, oxygen can be a friend, an enemy or both to any given wine. Big, bold tannic red wines with lots of astringency benefit from some oxidation, oxygen facilitating the break down of these tannins, allowing them to reform into polymers. These polymers have a soft and smoother mouthfeel, making the wine more balanced. Alternatively, give oxygen the freedom to react with alcohol for a prolonged period of time and you risk transforming the alcohol into acetic acid (the acid in vinegar).

Oxygen begins to react with grapes post harvest as soon as they’re crushed, making it advantageous to transport harvested grapes in shallow containers where “gravity” pressing is less likely to occur. Many vintners prefer to have the crushing and destemming apparatus close to the pressing vats in order to reduce oxygen contact. Unless strict precautions are taken during pressing, it’s a given that oxygen will react with the juices causing some browning or oxidation of the liquids. Think about how cut apples, avocados or newspapers turn yellowish-brown upon exposure to oxygen. To counter this natural process, many vintners add small quantities of sulfur dioxide to the mixture, inactivating the enzymes responsible for oxidation.

Depending upon the category and style of wine desired, the vintner has a choice during the pressing and fermentation phase of either exposing or limiting the must (skin, stems, seeds and juices) to oxygen. If the wine is destined to be a bold and tannic red, the must is likely to be an astringent “soup” that could benefit from oxygen exposure. This can be accomplished by performing this phase in an open wood or cement vessel, allowing some oxygen seepage and leaving the top open. Stirring and punching down the cap will serve to expose all of the must to air, as well. This results in the slow breakdown of some of the more astringent elements such as tannins which then recombine into compounds (polymers) that yield a softer feel. (If this process occurs in the environment of a closed bottle, you’ll find a carpet of these polymeric sediments layered in the most dependent part of the bottle after a period of time).

Alternatively, if the red is to be light weight in style and targeted for current consumption (vs. aging), minimal oxygen contact is desired, the focus being on retaining the light and lively freshness of the wine. This can be accomplished by timely crushing, fermentation and transferring the must to a stainless steel fermentation tank (or a wood vessel but for just a short time in order to limit oxygen exposure).

The delicate flavors and aromas of white wines are far more susceptible to oxidation than reds requiring vintners to be extra diligent in the process. Over oxidation in whites can spoil the fresh, delicate flavors and produce very perceptible and unpleasant acetaldehyde and vinegary aromas. Thus it’s very important that the crushing process be accomplished in the most efficient manner. The issue is not quite as sensitive with reds as their complexity often “hides” potential defects.

Fermentation of the red must takes place prior to pressing in a vessel that can range from wood to cement to stainless steel. The vessel can be closed or open allowing the must to have less or added contact with oxygen. Again, this decision often lies with the anticipated weight of the wine.

Alternatively, fermentation in whites occurs after the juices are pressed off. Many whites are fermented in closed, stainless steel vats in order to limit oxygen contact. CO2 produced from the fermentation process in concert with adding an inert gas such as Argon can be very efficient in excluding oxygen. Fresh, acid driven wines such as Sauvignon Blanc and German Rieslings would be good candidates for this style of fermentation. Some whites, such as Chardonnay, do well with barrel fermentation in order to give the wine that full and buttery personality so often associated with the California style. Here, the wood allows for some very slow diffusion of oxygen into the wine, which some vintners feel adds additional flavor nuances. Unoaked or “naked” Chardonnays have become more common in recent years, these wines stored in stainless steel containers to eliminated oxygen and retain the pure freshness of the Chardonnay grape.

Once the fermentation phase is complete, the juices are them removed and stored. Depending upon the wine, oxygen contact will be either allowed or restricted. As you might gather, the meaty and tannic red wine will likely welcome a little oxygen contact in order to slowly soften the elements. These wines are usually stored in wood vessels (barrels) allowing for very slow oxygen diffusion during the storage process. Lighter reds and the lighter, acid driven whites such as Sauvignon Blanc are likely to be stored in stainless steel tanks in which the ullage space will be filled with inert gas to limit oxidation.

Once the storage process is complete and bottling is begins, the producer has the option of spraying carbon dioxide and an inert gas such as Argon into the bottle to reduce exposure to oxygen. This can be very crucial, especially if the wine is a lively white or red to be consumed in the near term. On the other hand, a deep, bold and tannic driven wine meant for long term aging, not only would not suffer from a little remaining oxygen, but may well benefit in the months and years ahead, the magic of chemistry softening the tannins and bringing the wine’s elements into ideal balance. This is how the truly collectible reds from regions such as Bordeaux and Burgundy find their way to the top of the quality charts.

Meanwhile, I’ll be fascinated to continue on through The Billionaire’s Vinegar to read the tasting notes of those who have been offered such a unique opportunity to taste history. If the bottles of wine containing the inscription “Th J” really do represent an era of wine bottling 200 years ago, how do you think the wine will taste?

Measuring Moisture Contents In the Snack Food and Baking Industries

Measuring Moisture Contents In the Snack Food and Baking Industries

Article

With the steady rise and success of chips, corn curls, cookies, breads etc., the snack food and baking industries have now become more competitive then ever. The most powerful and destructive natural force on earth has taken a stand in the industry as one of the most commonly measured properties in food materials &em; WATER.
Almost daily, we are asked two very important questions: Why is moisture content important? and How can we test for moisture? Knowing the moisture content of the materials used throughout the snack food and baking process has become one of the most important concepts in the industry. The process begins with the arrival of raw ingredients. At this time, a Certificate of Analysis is presented to the purchasing plant to verify the validity of the raw materials, which includes the moisture content. Caveat Emptor &em; Buyer Beware. The cost of many raw materials is based upon weight. Water is an inexpensive ingredient, and manufacturers often try to incorporate as much water as possible, without exceeding the legal maximum requirement. A simple moisture test on incoming raw ingredients could save the purchasing plant thousands of dollars. Are you paying for free water?

During the food manufacturing process, an understanding and knowledge of the moisture content is necessary to fully comprehend the behavior of foods during processing, mixing, and drying. The mixing, and the certainty of the correct addition of the initial ingredients optimizes high quality, consistent finished products. Carpe Diem &em; Seize the Day. It is at this critical stage of the manufacturing process that moisture content can make or break the product. Excess moisture in mixes can cause clumping and the moisture content will continue to increase during storage, causing the product to deteriorate. A simple moisture test in this area will allow easy adjustments to be made to conditioners and throughout the blending process to keep moisture low and under control.

Ad Extremum &em; At Last. The final product is the apple of the eye for the manufacturer; the pride and joy of the job; a successful finished product. The texture, taste, appearance, stability and also the shelf life of your final product depend on the amount of water it contains. Don’t waste the taste and have finished products returned because of molding or customer complaints. Knowing the appropriate moisture content will allow your product to be successfully manufactured and sold in one of the worlds leading industries. Checking the moisture content on the finished product will allow verification that your product is the cream of the crop.

Because moisture is such a critical part of all snack food and baking products, the critical question is brought to our attention... So now what? The moisture content of all samples are determined by measuring the mass of the material before and after the water is removed by evaporation:

% Moisture = Initial Sample Weight - Final Sample Weight X 100Initial Sample Weight

Temperature and time are standardized in the loss-on-drying methods to obtain both accurate and repeatable results. However, a standard drying condition or time for all materials does not exist. The optimum drying conditions must be individually determined for each product. Convection, Vacuum and Infrared Drying Moisture Analyzers are the most common methods for moisture analysis.

In a convection oven, the original sample is placed in the oven at a specified temperature and time. Once the product has finished drying, the above equation is used to determine the moisture content. ASTM recognizes the oven method as the standard for testing moisture. However, samples that contain large amounts of carbohydrates have a risk of undergoing chemical changes, and therefore should not be dried in a convection oven. This method can also take up to 24 hours for accurate and reliable results.

Similarly, with the vacuum oven a random sample is taken, pre-weighed, and placed in the oven. However, vacuum ovens use reduced pressure, typically 25-100 mm Hg, which allows for faster drying times when compared to the convection ovens. An air inlet and outlet carries the moisture lost from the material out of the oven, which prohibits the accumulation of moisture in the oven. Because the boiling point of water in a vacuum oven is reduced, drying can be completed faster than a convection oven and problems with degradation of heat sensitive substances can be reduced. This method does not satisfy the urgency of rapid moisture results and will take between 3-6 hours in length and can be quite costly.

With the heightened interest in improving quality control, an oven test would not ideally satisfy the necessity of the quick results needed to check incoming raw materials, baking mixes, and final product previous to be being packaged. If the end product was taken from the final production line and tested for 24 hours in the oven, and an incorrect moisture content was read, thousands of pounds of bad product would have been produced and distributed. Is the risk of one bad test worth the risk of loosing thousands of dollars and more importantly the risk of losing loyal customers daily?

Determined as a well suited instrument for quick weight loss by the American Institute of Baking (see below chart), the Infrared Moisture Balance offers a high quality and a fast paced method of drying using inexpensive equipment. The chart below clearly indicates that the Infrared Moisture Balance produces repeatable and accurate results when compared with the standard oven method. Infrared Moisture Balances not only test samples significantly faster, they save floor and counter space, reduce heat in the work area and significantly boost production rates. Also, this saves energy and reduces costs in product manufacturing. Infrared Moisture Balances are easily integrated into existing production lines and QC testing areas.

Table IX
Taken from AIB Technical Bulletin
Volume XV, Issue 6

Moisture Content of Yellow Layer Cake

Method	Time	Temp.	Lamp	PS	1-Day Pre-Drying	2-Day Pre-Drying
Oven Method
AACC 44-40	5 hour	100 C	-	-	33.4 +/- 0.1	29.2 +/- 0.00
AACC 44-15A	1 hour	130 C	-	-	33.2 +/- 0.1	29.1 +/- 0.0
Moisture Balance
Digital	6 min	-	125 C	80	33	28.9
Digital	5 (6) min	-	125 C	90	33	29.1
Digital	5 (6) min	-	125 C	100	33.2	29.2
Digital	7 min	-	250 R	60	33.5	29.1
Digital	7 (6) min	-	250 R	70	33.5	29.1
Digital	7 (6) min	-	250 R	80	33.7	29.2

- Values within ( ) refer to 2-day pre-drying
-125 Watt clear lamp or 250 Watt red lamp
-(PS) Power setting

With an Infrared Moisture Balance, a small random 5-10 gram sample of material is taken and placed in a heating chamber of the instrument. The beginning weight of the sample is recorded. The water molecules in the material evaporate because the heat from the infrared heat source is absorbed by the sample. With results in as fast as 3 to 20 minutes, percent moisture or percent solids can easily be determined once the sample is dried. Results are easily reproducible and accurate with the infrared moisture balances because the distance between the heat source and sample remain the same between tests.

With easy access to Infrared Moisture Analyzers throughout production and laboratories, moisture results can be performed numerous times in a single hour! When compared with the 24-hour oven method, the quick results can help to create leading products in this competitive industry. With infrared testing, the mystery can be taken out of moisture testing.

Determination of Moisture in powder product

Determination of Moisture and Total Solids

Introduction

Moisture content is one of the most commonly measured properties of food materials. It is important to food scientists for a number of different reasons:

• Legal and Labeling Requirements. There are legal limits to the maximum or minimum amount of water that must be present in certain types of food.
• Economic. The cost of many foods depends on the amount of water they contain - water is an inexpensive ingredient, and manufacturers often try to incorporate as much as possible in a food, without exceeding some maximum legal requirement.
• Microbial Stability. The propensity of microorganisms to grow in foods depends on their water content. For this reason many foods are dried below some critical moisture content.
• Food Quality. The texture, taste, appearance and stability of foods depends on the amount of water they contain.
• Food Processing Operations. A knowledge of the moisture content is often necessary to predict the behavior of foods during processing, e.g. mixing, drying, flow through a pipe or packaging.

It is therefore important for food scientists to be able to reliably measure moisture contents. A number of analytical techniques have been developed for this purpose, which vary in their accuracy, cost, speed, sensitivity, specificity, ease of operation, etc. The choice of an analytical procedure for a particular application depends on the nature of the food being analyzed and the reason the information is needed.

3.2 Properties of Water in Foods

The moisture content of a food material is defined through the following equation:

%Moisture = (m_w/m_sample)´ 100

Where m_w is the mass of the water and m_sample is the mass of the sample. The mass of water is related to the number of water molecules (n_W) by the following expression: m_w = n_wM_w/N_A, where M_w is the molecular weight of water (18.0 g per mole) and N_A is Avadagro's number (6.02 ´ 10²³ molecules per mole). In principle, the moisture content of a food can therefore be determined accurately by measuring the number or mass of water molecules present in a known mass of sample. It is not possible to directly measure the number of water molecules present in a sample because of the huge number of molecules involved. A number of analytical techniques commonly used to determine the moisture content of foods are based on determinations of the mass of water present in a known mass of sample. Nevertheless, as we will see later, there are a number of practical problems associated with these techniques that make highly accurate determinations of moisture content difficult or that limit their use for certain applications. For these reasons, a number of other analytical methods have been developed to measure the moisture content of foods that do not rely on direct measurement of the mass of water in a food. Instead, these techniques are based on the fact that the water in a food can be distinguished from the other components in some measurable way.

An appreciation of the principles, advantages and limitations of the various analytical techniques developed to determine the moisture content of foods depends on an understanding of the molecular characteristics of water. A water molecule consists of an oxygen atom covalently bound to two hydrogen atoms (H₂O). Each of the hydrogen atoms has a small positive charge (d+), while the oxygen atom has two lone pairs of electrons that each has a small negative charge (d-). Consequently, water molecules are capable of forming relatively strong hydrogen bonds (O-H^d+ « ^d-O) with four neighboring water molecules. The strength and directionality of these hydrogen bonds are the origin of many of the unique physicochemical properties of water. The development of analytical techniques to determine the moisture content of foods depends on being able to distinguish water (the "analyte") from the other components in the food (the "matrix"). The characteristics of water that are most commonly used to achieve this are: its relatively low boiling point; its high polarity; its ability to undergo unique chemical reactions with certain reagents; its unique electromagnetic absorption spectra; and, its characteristic physical properties (density, compressibility, electrical conductivity and refractive index).

Despite having the same chemical formula (H₂O) the water molecules in a food may be present in a variety of different molecular environments depending on their interaction with the surrounding molecules. The water molecules in these different environments normally have different physiochemical properties:

• Bulk water. Bulk water is free from any other constituents, so that each water molecule is surrounded only by other water molecules. It therefore has physicochemical properties that are the same as those of pure water, e.g., melting point, boiling point, density, compressibility, heat of vaporization, electromagnetic absorption spectra.
• Capillary or trapped water. Capillary water is held in narrow channels between certain food components because of capillary forces. Trapped water is held within spaces within a food that are surrounded by a physical barrier that prevents the water molecules from easily escaping, e.g., an emulsion droplet or a biological cell. The majority of this type of water is involved in normal water-water bonding and so it has physicochemical properties similar to that of bulk water.
• Physically bound water. A significant fraction of the water molecules in many foods are not completely surrounded by other water molecules, but are in molecular contact with other food constituents, e.g. proteins, carbohydrates or minerals. The bonds between water molecules and these constituents are often significantly different from normal water-water bonds and so this type of water has different physicochemical properties than bulk water e.g., melting point, boiling point, density, compressibility, heat of vaporization, electromagnetic absorption spectra.
• Chemically bound water. Some of the water molecules present in a food may be chemically bonded to other molecules as water of crystallization or as hydrates, e.g. NaSO₄.10H₂0. These bonds are much stronger than the normal water-water bond and therefore chemically bound water has very different physicochemical properties to bulk water, e.g., lower melting point, higher boiling point, higher density, lower compressibility, higher heat of vaporization, different electromagnetic absorption spectra.

Foods are heterogeneous materials that contain different proportions of chemically bound, physically bound, capillary, trapped or bulk water. In addition, foods may contain water that is present in different physical states: gas, liquid or solid. The fact that water molecules can exist in a number of different molecular environments, with different physicochemical properties, can be problematic for the food analyst trying to accurately determine the moisture content of foods. Many analytical procedures developed to measure moisture content are more sensitive to water in certain types of molecular environment than to water in other types of molecular environment. This means that the measured value of the moisture content of a particular food may depend on the experimental technique used to carry out the measurement. Sometimes food analysts are interested in determining the amounts of water in specific molecular environments (e.g., physically bound water), rather than the total water content. For example, the rate of microbial growth in a food depends on the amount of bulk water present in a food, and not necessarily on the total amount of water present. There are analytical techniques available that can provide some information about the relative fractions of water in different molecular environments (e.g., DSC, NMR, vapor pressure).

3.3. Sample preparation

Selection of a representative sample, and prevention of changes in the properties of the sample prior to analysis, are two major potential sources of error in any food analysis procedure. When determining the moisture content of a food it is important to prevent any loss or gain of water. For this reason, exposure of a sample to the atmosphere, and excessive temperature fluctuations, should be minimized. When samples are stored in containers it is common practice to fill the container to the top to prevent a large headspace, because this reduces changes in the sample due to equilibration with its environment. The most important techniques developed to measure the moisture content of foods are discussed below.

3.4. Evaporation methods

3.4.1. Principles

These methods rely on measuring the mass of water in a known mass of sample. The moisture content is determined by measuring the mass of a food before and after the water is removed by evaporation:

Here, M_INITIAL and M_DRIED are the mass of the sample before and after drying, respectively. The basic principle of this technique is that water has a lower boiling point than the other major components within foods, e.g., lipids, proteins, carbohydrates and minerals. Sometimes a related parameter, known as the total solids, is reported as a measure of the moisture content. The total solids content is a measure of the amount of material remaining after all the water has been evaporated:

Thus, %Total solids = (100 - %Moisture). To obtain an accurate measurement of the moisture content or total solids of a food using evaporation methods it is necessary to remove all of the water molecules that were originally present in the food, without changing the mass of the food matrix. This is often extremely difficult to achieve in practice because the high temperatures or long times required to remove all of the water molecules would lead to changes in the mass of the food matrix, e.g., due to volatilization or chemical changes of some components. For this reason, the drying conditions used in evaporation methods are usually standardized in terms of temperature and time so as to obtain results that are as accurate and reproducible as possible given the practical constraints. Using a standard method of sample preparation and analysis helps to minimize sample-to-sample variations within and between laboratories.

3.4.2. Evaporation Devices

The thermal energy used to evaporate the water from a food sample can be provided directly (e.g., transfer of heat from an oven to a food) or indirectly (e.g., conversion of electromagnetic radiation incident upon a food into heat due to absorption of energy by the water molecules).

Convection and forced draft ovens. Weighed samples are placed in an oven for a specified time and temperature (e.g. 3 hours at 100 ^oC) and their dried mass is determined, or they are dried until they reach constant mass. The thermal energy used to evaporate the water is applied directly to the sample via the shelf and air that surround it. There are often considerable temperature variations within convection ovens, and so precise measurements are carried out using forced draft ovens that circulate the air so as to achieve a more uniform temperature distribution within the oven. Samples that contain significant quantities of carbohydrates that might undergo chemical changes or volatile materials other than water should not be dried in a convection or forced draft oven. Many official methods of analysis are based on forced draft ovens.

Vacuum oven. Weighed samples are placed under reduced pressure (typically 25-100 mm Hg) in a vacuum oven for a specified time and temperature and their dried mass is determined. The thermal energy used to evaporate the water is applied directly to the sample via the metallic shelf that it sits upon. There is an air inlet and outlet to carry the moisture lost from the sample out of the vacuum oven, which prevents the accumulation of moisture within the oven. The boiling point of water is reduced when it is placed under vacuum. Drying foods in a vacuum oven therefore has a number of advantages over conventional oven drying techniques. If the sample is heated at the same temperature, drying can be carried out much quicker. Alternatively, lower temperatures can be used to remove the moisture (e.g. 70^oC instead of 100 ^oC), and so problems associated with degradation of heat labile substances can be reduced. A number of vacuum oven methods are officially recognized.

Microwave oven. Weighed samples are placed in a microwave oven for a specified time and power-level and their dried mass is weighed. Alternatively, weighed samples may be dried until they reach a constant final mass - analytical microwave ovens containing balances to continuously monitor the weight of a food during drying are commercially available. The water molecules in the food evaporate because they absorb microwave energy, which causes them to become thermally excited. The major advantage of microwave methods over other drying methods is that they are simple to use and rapid to carry out. Nevertheless, care must be taken to standardize the drying procedure and ensure that the microwave energy is applied evenly across the sample. A number of microwave oven drying methods are officially recognized.

Infrared lamp drying. The sample to be analyzed is placed under an infrared lamp and its mass is recorded as a function of time. The water molecules in the food evaporate because they absorb infrared energy, which causes them to become thermally excited. One of the major advantages of infrared drying methods is that moisture contents can be determined rapidly using inexpensive equipment, e.g., 10-25 minutes. This is because the IR energy penetrates into the sample, rather than having to be conducted and convected inwards from the surface of the sample. To obtain reproducible measurements it is important to control the distance between the sample and the IR lamp and the dimensions of the sample. IR drying methods are not officially recognized for moisture content determinations because it is difficult to standardize the procedure. Even so, it is widely used in industry because of its speed and ease of use.

3.4.3. Practical Considerations

1. Sample dimensions. The rate and extent of moisture removal depends on the size and shape of the sample, and how finely it is ground. The greater the surface area of material exposed to the environment, the faster the rate of moisture removal.
2. Clumping and surface crust formation. Some samples tend to clump together or form a semi-permeable surface crust during the drying procedure. This can lead to erroneous and irreproducible results because the loss of moisture is restricted by the clumps or crust. For this reason samples are often mixed with dried sand to prevent clumping and surface crust formation.
3. Elevation of boiling point. Under normal laboratory conditions pure water boils at 100 ^oC. Nevertheless, if solutes are present in a sample the boiling point of water is elevated. This is because the partial vapor pressure of water is decreased and therefore a higher temperature has to be reached before the vapor pressure of the system equals the atmospheric pressure. Consequently, the rate of moisture loss from the sample is slower than expected. The boiling point of water containing solutes (T_b) is given by the expression, T_b = T₀ + 0.51m, where T₀ is the boiling point of pure water and m is the molality of solute in solution (mol/kg of solvent).
4. Water type. The ease at which water is removed from a food by evaporation depends on its interaction with the other components present. Free water is most easily removed from foods by evaporation, whereas more severe conditions are needed to remove chemically or physically bound water. Nevertheless, these more extreme conditions can cause problems due to degradation of other ingredients which interfere with the analysis (see below).
5. Decomposition of other food components. If the temperature of drying is too high, or the drying is carried out for too long, there may be decomposition of some of the heat-sensitive components in the food. This will cause a change in the mass of the food matrix and lead to errors in the moisture content determination. It is therefore normally necessary to use a compromise time and temperature, which are sufficient to remove most of the moisture, but not too long to cause significant thermal decomposition of the food matrix. One example of decomposition that interferes with moisture content determinations is that of carbohydrates.

C₆H₁₂O₆ 6C + 6 H₂O

The water that is released by this reaction is not the water we are trying to measure and would lead to an overestimation of the true moisture content. On the other hand, a number of chemical reactions that occur at elevated temperatures lead to water absorption, e.g., sucrose hydrolysis (sucrose + H₂O fructose + glucose), and therefore lead to an underestimation of the true moisture content. Foods that are particularly susceptible to thermal decomposition should be analyzed using alternative methods, e.g. chemical or physical.

6. Volatilization of other food components. It is often assumed that the weight loss of a food upon heating is entirely due to evaporation of the water. In practice, foods often contain other volatile constituents that can also be lost during heating, e.g., flavors or odors. For most foods, these volatiles only make up a very small proportion and can therefore be ignored. For foods that do contain significant amounts of volatile components (e.g. spices and herbs) it is necessary to use alternative methods to determine their moisture content, e.g., distillation, chemical or physical methods.
7. High moisture samples. Food samples that have high moisture contents are usually dried in two stages to prevent "spattering" of the sample, and accumulation of moisture in the oven. Spattering is the process whereby some of the water jumps out of the food sample during drying, carrying other food constituents with it. For example, most of the moisture in milk is removed by heating on a steam bath prior to completing the drying in an oven.
8. Temperature and power level variations. Most evaporation methods stipulate a definite temperature or power level to dry the sample so as to standardize the procedure and obtain reproducible results. In practice, there are often significant variations in temperatures or power levels within an evaporation instrument, and so the efficiency of the drying procedure depends on the precise location of the sample within the instrument. It is therefore important to carefully design and operate analytical instruments so as to minimize these temperature or power level variations.
9. Sample pans. It is important to use appropriate pans to contain samples, and to handle them correctly, when carrying out a moisture content analysis. Typically aluminum pans are used because they are relatively cheap and have a high thermal conductivity. These pans usually have lids to prevent spattering of the sample, which would lead to weight loss and therefore erroneous results. Pans should be handled with tongs because fingerprints can contribute to the mass of a sample. Pans should be dried in an oven and stored in a descicator prior to use to ensure that no residual moisture is attached to them.

3.4.4. Advantages and Disadvantages

· · Advantages: Precise; Relatively cheap; Easy to use; Officially sanctioned for many applications; Many samples can be analyzed simultaneously

· · Disadvantages: Destructive; Unsuitable for some types of food; Time consuming

3.5. Distillation Methods

3.5.1. Principles

Distillation methods are based on direct measurement of the amount of water removed from a food sample by evaporation: %Moisture = 100 (M_WATER/M_INITIAL). In contrast, evaporation methods are based on indirect measurement of the amount of water removed from a food sample by evaporation: %Moisture = 100 (M_INITIAL - M_DRIED)/M_INITIAL. Basically, distillation methods involve heating a weighed food sample (M_INITIAL) in the presence of an organic solvent that is immiscible with water. The water in the sample evaporates and is collected in a graduated glass tube where its mass is determined (M_WATER).

3.5.2. Dean and Stark Method

Distillation methods are best illustrated by examining a specific example: the Dean and Stark method. A known weight of food is placed in a flask with an organic solvent such as xylene or toluene. The organic solvent must be insoluble with water; have a higher boiling point than water; be less dense than water; and be safe to use. The flask containing the sample and the organic solvent is attached to a condenser by a side arm and the mixture is heated. The water in the sample evaporates and moves up into the condenser where it is cooled and converted back into liquid water, which then trickles into the graduated tube. When no more water is collected in the graduated tube, distillation is stopped and the volume of water is read from the tube.

3.5.3. Practical Considerations

There are a number of practical factors that can lead to erroneous results: (i) emulsions can sometimes form between the water and the solvent which are difficult to separate; (ii) water droplets can adhere to the inside of the glassware, (iii) decomposition of thermally labile samples can occur at the elevated temperatures used.

3.5.4. Advantages and Disadvantages

• · Advantages: Suitable for application to foods with low moisture contents; Suitable for application to foods containing volatile oils, such as herbs or spices, since the oils remain dissolved in the organic solvent, and therefore do not interfere with the measurement of the water; Equipment is relatively cheap, easy to setup and operate; Distillation methods have been officially sanctioned for a number of food applications.
• · Disadvantages: Destructive; Relatively time-consuming; Involves the use of flammable solvents; Not applicable to some types of foods.

3.6. Chemical Reaction Methods

Reactions between water and certain chemical reagents can be used as a basis for determining the concentration of moisture in foods. In these methods a chemical reagent is added to the food that reacts specifically with water to produce a measurable change in the properties of the system, e.g., mass, volume, pressure, pH, color, conductivity. Measurable changes in the system are correlated to the moisture content using calibration curves. To make accurate measurements it is important that the chemical reagent reacts with all of the water molecules present, but not with any of the other components in the food matrix. Two methods that are commonly used in the food industry are the Karl-Fisher titration and gas production methods. Chemical reaction methods do not usually involve the application of heat and so they are suitable for foods that contain thermally labile substances that would change the mass of the food matrix on heating (e.g., food containing high sugar concentrations) or foods that contain volatile components that might be lost by heating (e.g. spices and herbs).

3.6.1. Karl-Fisher method

The Karl-Fisher titration is often used for determining the moisture content of foods that have low water contents (e.g. dried fruits and vegetables, confectionary, coffee, oils and fats). It is based on the following reaction:

2H₂O + SO₂ + I₂ ® H₂SO₄ + 2HI

This reaction was originally used because HI is colorless, whereas I₂ is a dark reddish brown color, hence there is a measurable change in color when water reacts with the added chemical reagents. Sulfur dioxide and iodine are gaseous and would normally be lost from solution. For this reason, the above reaction has been modified by adding solvents (e.g., C₅H₅N) that keep the S₂O and I₂ in solution, although the basic principles of the method are the same. The food to be analyzed is placed in a beaker containing solvent and is then titrated with Karl Fisher reagent (a solution that contains iodine). While any water remains in the sample the iodine reacts with it and the solution remains colorless (HI), but once all the water has been used up any additional iodine is observed as a dark red brown color (I₂). The volume of iodine solution required to titrate the water is measured and can be related to the moisture content using a pre-prepared calibration curve. The precision of the technique can be improved by using electrical methods to follow the end-point of the reaction, rather than observing a color change. Relatively inexpensive commercial instruments have been developed which are based on the Karl-Fisher titration, and some of these are fully automated to make them less labor intensive.

3.6.2. Gas production methods

Commercial instruments are also available that utilize specific reactions between chemical reagents and water that lead to the production of a gas. For example, when a food sample is mixed with powdered calcium carbide the amount of acetylene gas produced is related to the moisture content.

CaC₂ + 2H₂O C₂H₂(gas) + Ca(OH)₂

The amount of gas produced can be measured in a number of different ways, including (i) the volume of gas produced, (ii) the decrease in the mass of the sample after the gas is released, and (iii) the increase in pressure of a closed vessel containing the reactants.

3.7 Physical Methods

A number of analytical methods have been developed to determine the moisture content of foods that are based on the fact that water has appreciably different bulk physical characteristics than the food matrix, e.g. density, electrical conductivity or refractive index. These methods are usually only suitable for analysis of foods in which the composition of the food matrix does not change significantly, but the ratio of water-to-food matrix changes. For example, the water content of oil-in-water emulsions can be determined by measuring their density or electrical conductivity because the density and electrical conductivity of water are significantly higher than those of oil. If the composition of the food matrix changes as well as the water content, then it may not be possible to accurately determine the moisture content of the food because more than one food composition may give the same value for the physical property being measured. In these cases, it may be possible to use a combination of two or more physical methods to determine the composition of the food, e.g., density measurements in combination with electrical conductivity measurements.

3.8 Spectroscopic Methods

Spectroscopic methods utilize the interaction of electromagnetic radiation with materials to obtain information about their composition, e.g., X-rays, UV-visible, NMR, microwaves and IR. The spectroscopic methods developed to measure the moisture content of foods are based on the fact that water absorbs electromagnetic radiation at characteristic wavelengths that are different from the other components in the food matrix. The most widely used physical methods are based on measurements of the absorption of microwave or infrared energy by foods. Microwave and infrared radiation are absorbed by materials due to their ability to promote the vibration and/or rotation of molecules. The analysis is carried out at a wavelength where the water molecules absorb radiation, but none of the other components in the food matrix do. A measurement of the absorption of radiation at this wavelength can then be used to determine the moisture content: the higher the moisture content, the greater the absorption. Instruments based on this principle are commercially available and can be used to determine the moisture content in a few minutes or less. It is important not to confuse infrared and microwave absorption methods with infrared lamp and microwave evaporation methods. The former use low energy waves that cause no physical or chemical changes in the food, whereas the latter use high-energy waves to evaporate the water. The major advantage of these methods is that they are capable of rapidly determining the moisture content of a food with little or no sample preparation and are therefore particularly useful for quality control purposes or rapid measurements of many samples.

3.9 Methods to Determine Water in Different Molecular Environments

The overall water content of a food is sometimes not a very reliable indication of the quality of a food because the water molecules may exist in different environments within foods, e.g., "bound" or "free". Here "bound water" refers to water that is physically or chemically bound to other food components, whereas "free water" refers to bulk, capillary or entrapped water. For example, the microbial stability or physicochemical properties of a food are often determined by the amount of free water present, rather than by the total amount of water present. For this reason, it is often useful for food scientists to be able to determine the amount of water in different molecular environments within a food. A variety of analytical methods are available that can provide this type of information.

3.9.1. Vapor pressure methods

A physical parameter that is closely related to the amount of free water present in a food is the water activity:

where, P is the partial pressure of the water above the food and P₀ is the vapor pressure of pure water at the same temperature. Bound water is much less volatile than free water, and therefore the water activity gives a good indication of the amount of free water present. A variety of methods are available for measuring the water activity of a sample based on its vapor pressure. Usually, the sample to be analyzed is placed in a closed container and allowed to come into equilibrium with its environment. The water content in the headspace above the sample is then measured and compared to that of pure water under the same conditions.

3.9.2. Thermogravimetric methods

Thermogravimetric techniques can be used to continuously measure the mass of a sample as it is heated at a controlled rate. The temperature at which water evaporates depends on its molecular environment: free water normally evaporates at a lower temperature than bound water. Thus by measuring the change in the mass of a sample as it loses water during heating it is often possible to obtain an indication of the amounts of water present in different molecular environments.

3.9.3. Calorimetric methods

Calorimetric techniques such as differential scanning calorimetry (DSC) and differential thermal analysis (DTA) can be used to measure changes in the heat absorbed or released by a material as its temperature is varied at a controlled rate. The melting point of water depends on its molecular environment: free water normally melts at a higher temperature than bound water. Thus by measuring the enthalpy change of a sample with temperature it is possible to obtain an indication of the amounts of water present in different molecular environments.

Spectroscopic methods

The electromagnetic spectrum of water molecules often depends on their molecular environment, and so some spectroscopy techniques can be used to measure the amounts of water in different environments. One of the most widely used of these techniques is nuclear magnetic resonance (NMR). NMR can distinguish molecules within materials based on their molecular mobility, i.e., the distance they move in a given time. The molecular mobility of free water is appreciably higher than that of bound water and so NMR can be used to provide an indication of the concentrations of water in "free" and "bound" states.