Food Chemistry (intro to fats and rancidity; antioxidants)

The amount of each food group depends on factors such as weight, gender and occupation.

Know the terms saturated and unsaturated

Know the terms cis and trans (fatty acids)– The terms cis and trans are from Latin, in which

cis means "on the same side" and trans means "on the other side" or "across".

Food –any substance (processed or other wise) intended for human consumption that excludes tobacco, cosmetics and drugs.

Nutrients-any substance obtained from food and used by the body to maintain & repair body tissue.

cis-but-2-ene

trans-but-2-ene

Guidelines for healthy eating

Fats and oils

Are triesters (triglycerides)

Are made according to the following reaction

Long chain + propane- Fats &

carboxylic acid 1,2,3-triol oils

(fatty acid) (glycerol)

Melting Points of Fats*The melting points of f.a. increase with the increase of molar mass.Due to the stacking of a tetrahedral arrangementVan der Waals Forces increase

It also depends on the amount of unsaturationStearic(C17H35COOH)(69.6oC)

and Linoleic

C17H31COOH (-5.0oC) contain

same number of Cs but L has 2 d.b.

As the bond angle changes from 109.5o (tet) to 120o(d.b), the molecules are not able to stack making a kink

As unsaturation increases, the lower the melting point (or the easier it is to break bonds)

Geometric isomerism in Fats: Cis and Trans

Unsaturation can either be Cis or transTrans fatty acids can pack more closely, have higher melting points than cis-fatty acidsThe packing arrangements above explain why unsaturated fats (oils) have lower melting points than saturated fatsExamples of fats and oils: palm, coconut, butter, shortening, tallowEx. of unsaturated oils: olive, canola and peanutEx. of polyunsaturated oils: safflower, sunflower, linoleic, linolenic.

cis-but-2-ene

trans-but-2-ene

ASSIGNMENT: 1

1.Draw the structural formulas and give the IUPAC names for: stearic acid, palmitic acid, oleic acid and linoleic acid. Why are these acids referred to using their common names?

2.Draw the structural formula formed from one molecule of propane-1,2,3-triol, one molecule of stearic acid, one of palmitic and one of oleic. How would you classify this molecule?

3.What is meant by the term omega-three fatty acid?

Chemical Stability of Fats and Oils

Unsaturated fats are less stable than saturated fats because the double bond in unsaturated fats can react with water and undergo hydrolysis in the presence of heat or enzymes.They can also react with

oxygen (auto-oxidation)light (photo-oxidation)

hydrogen (hydrogenation)

Addition of hydrogen(Hydrogenation)

Shelf Life

Packaged food will display on the labelbest if used by, sell by, best before, use byor display until.

Shelf life - when the food no longer maintains the expected quality desired by the consumer because of changes in smell, texture and appearance(color) and microbial spoilage. A food passed its shelf life may be safe to consume, but optimal quality is no longer guaranteed.

Shelf Life (best before, use by)

Rancidity

Occurs in lipids, and is perceived by our senses as when things smell bad, sour or not quite right (IB gone off), owing to a disagreeable smell taste, texture or appearance.

Is usually always caused by either by:

hydrolysis of triesters,

or oxidation of fatty acid chains

The Hydrolysis of Fat

Fat

H2COOCR1

IH2COOCR2

IH2COOCR3

+ water

+ 3H2O

>acid1

R1COOH

+acid2

+R2COOH

Acid3

+R3COOH

GlycerolPropane-1,2,3-triol

H2C-OH

IH2C-OH

IH2C-OH

Hydrolytic Rancidity

Is the breaking down of lipid into its fatty acids and glycerol (last slide)

This is the reverse of esterification

It occurs more rapidly in the presence of lipase, and with heat and moisture.

In deep frying, the water present in the food and high temperatures increases the rate of hydrolysis to fatty acids.

Oxidative Rancidity

Is due to the oxidation of the fatty acid chains, typically by the addition of oxygen across the carbon-to-carbon double bond in unsaturated fatty acids. Mackerel contains a high proportion of unsaturated fatty acids. It is prone to oxidative rancidity in the presence of metal ions or enzymes via a free radical mechanism catalyzed by light.This photo-oxidation leads to the formation of hydroperoxides. They have the formula R-O-O-H

and break down to form free radicals due to weak O-O bond.

Examples of Fatty Acids and the foods that they are found in

CarbohydratesEmperical formula of Cm(H2O)n

Our bodies use these as an energy source

They are found in plant foods such as cereals, fruits, grains, and vegetables.

Plants produce these through photosysthesis– Energy is provided by light– Many different carbohydrates are produced in

plants– Sugar are low mass carbohydrates

MonosaccharidesThe simplest carbohydrates

Empirical formula of CH2O

They are either aldehydes or ketones

All have one carbonyl group (C=O)

All have at least two hydroxyl (-OH) groups.

Examples– Glucose, fructose, ribose

All soluble in water (due to the –OH)

MonosaccharidesClasses of monosccharides– Aldose – contains one aldehyde group per

molecule– Ketose – contains one ketone group per

molecule– Triose*– Tetrose– Pentose*– Hexose*

* Most common sugars

MonosaccharidesHexose sugars have the formula C6H12O6

– As with most organic compounds, there are more than one isomer

D-glucose (an aldose sugar)– The most important – Used for cellular respiration

D-fructose (a ketose sugar)– Found in fruits

glucose

Straight chain in solid form

Ring in aqueous solutions– Due to internal reaction– -OH on C5 attacks the carbonyl group

2 isomers in ring form (anomers)– α and β– -OH on C1 appear on different sides

Fructose

When in aqueous solution, fructose as with most monosaccharaides, undergo an internal reaction to form a ring structure.

Draw the ring structure of fructose

DisaccharidesFormed from a condensation reaction (formation of 1 water) of two monosaccharides

There are many disaccharides– Most important are maltose, lactose, and sucrose– Maltose

α-D-glucose + α-D-glucose = maltose

β-D-glucose + β-D-galactose = lactose

α-D-glucose + β-D-fructose = sucrose

PolysaccharidesPolysaccharides are condensation polymers– A condensation reaction is a chemical reaction in which two

molecules (or functional groups) combine to form one single molecule, together with the loss of a small molecule

Polysaccharides store energy– Glucose is stored in plants as starch (a polysaccharides)– Glucose is stored in animal cells as glycogen (a polysaccharides)

Starch is a polymer of α-D-glucose– Branched polymer called amylopectin – 70%– Unbranched polymer called amylase – 30%

StarchStarch is a polymer of α-D-glucose

– Branched polymer called amylopectin – 70%

– Unbranched polymer called amylose – 30%

CelluloseCellulose is a polymer of α-D-glucose also

– Branched polymer containing about 10000 glucose molecules and is found in the cell walls of plants

– Our bodies do not have the necessary enzymes to break down this compound

– Does not provide nutritional value, but is valuable as fiber

ProteinsProteins have diverse roles– They are largely responsible for the structure of the body. We are

mostly built of proteins.– They act as the tools that operate on the molecular level.

They act as catalysts (enzymes) which speed up metabolic reactions

They act as carrier molecules for transporting oxygen in the blood

They act as structures in blood cells able to help fight disease

They act as hormones

The structure of proteins– Proteins are polymers– Their monomer unit are amino acids

Each amino acid contains an amino group (-NH2) and a carboxylic acid group (-COOH) bonded to the same carbon atom

They are called 2-amino acids because the amino group is bonded to C2

The central carbon (C2) is also bonded to a hydrogen and an R

The identity of the R defines the amino acid.

ProteinsAmino Acids cont.– There are about 20 different amino acids– Each is given a three letter abbreviation

The smallest amino acid is glycine (Gly)

– A complete list can be found in Table 19 of the IB Data booklet

Amino acids are classified by chemical nature– This is determined by the R group– Usually based on differing polarities of the Rs

Protein synthesis– The H from the amino group and the OH from the carboxylic acid

group condensate to form H2O

– The bond formed between the C and N is called a peptide bond.– Dipeptide – Tripeptide– Polypeptide

ProteinsPractice– Draw a tripeptide with the following sequence: cys-val-asn

ProteinsProtein Structure – Primary structure – the sequence of the amino acids– Second structure – orientation of links

Hydrogen bonding causes there to be an attraction between various groups within the chain. This attraction causes folding or bent alignments of the protein.

– Tertiary structure – overall three dimensional shapeHydrogen bonding between polar groups on the side chains

Salt bridges (ionic bonds) formed between –NH2 and –COOH groups

Dipole-dipole interactions

Van der waals forces between non-polar groups

Disulfide bridges formed between two cystine molecules from difference positions along the polymer chain.

Essential amino acids – these cannot be produced by our bodies and must be supplied in the diet– Typically found more in animal protein than vegetable.

Most lack lysine

Minimizing the rate of Rancidity(traditional ways)

FermentationPreservingPicklingSaltingDrying Smoking

Newer methods include processing, packaging and the use of additives

Prolonging Shelf life of foods:Processing Packaging AdditivesRefrigeration-slows lipase

Using an inert gas to minimize the contact with oxygen

Sodium sulfite, sodium hydrogen sulfite, and citric acid delay onset of enzymatic browning

Reducing light Using hermetic sealing or low-gas permeability packing film

Sodium and potassium nitrite and nitrate for curing meat, fixing color & stop bac

Keep out moisture by smoking, or adding salt or sugar

Keeping jars full to minimize the amount of air in space above oil

Sodium benzoate and benzoic as antimicrobial in juices, carbonated drinks, pickles sauerkraut

Sealing in tincans

Sorbic acid, propanoic aic, calcium propanoate for delaying mold and bac from breads and cheeses

Ethanoic acid and benzoic acid delay mold and bac in pickled meats and fish with added flavor

Synthetic antioxidants

Butylated hydroxyanisole (BHA)*

Butylated hydroxytoluene (BHT)*

Propyl galate (PG)*

Trihydroxybutyrophenone (THBP)*

Tert-butyl hydroquinone (TBHQ)*

All contain a phenolic group, many have a tertiary Carbon bonded to 3 methyl groups (tert-butyl group), Both these groups are free radical scavengers. They remove free radicals and prolong shelf life.

Natural Antioxidants (delay oxidation)

Vitamin C*(biochem) (ascorbic acid): found in citrus fruit, green leafy veggies, strawberries, red currant, potatoes

B-carotene*: found in carrots, squash, broc,sweet potatoes, cantalope, melon, peaches, apricots

Vitamin E (tocopherols):Found in wheat germ, nuts, seeds, green leafyVeggies, veg oils (canola) and soybeans

Selenium: found in fish, shellfish, red meat, eggs, grains, chicken and garlic

Antioxoidants in food: Advantages and disadvantages

Naturally occurring Vit. C, E and carotenoids reduce risk of heart disease and cancer by inhibiting free radical formation

Consumers perceive synthetic antioxidants to be less safe, as they do not occur naturally

Vitamin C is vital for the production of hormones and collagen

Natural antioxidants are more expensive and less effective than synthetic antioxidants, and can also add unwanted color and leave aftertaste

Beta-carotene can be used as an additive in margarine to provide it with a yellow color and act as a precursor for vitamin A synthesis

Synthetic antiox are classfied as food additives, and need to be regulated by policies and legislation to ensure their safe used in food.

They are believed to enhance the health effects of other foods, and boost overall health and resilience

Policies regarding the sate use and labeling of food additives can be difficult to implement and monitor, esp in developing countries and across borders

Antioxidants in Traditional FoodPreviously mentioned Vitamin C* and Carotenoids*Flavonoids are another class of antioxidants– Are found in citrus fruits, green tea, red wine,

oregano and dark chocolate (having > 70%)– Have been linked to lower levels of LDL

cholesterol and blood sugar, will reduce high blood pressure and prevent the development of cancer cell growth

– Tumeric is a yellow substance used in Indian cooking. Active ingredient is curcumin having 2 phenolic groups on the structure*

ColorDerived naturally or artificially

Pigment: naturally occurring color found in cells of plants and animals.

Dye: is a food grade, synthetic, water soluble color.

Both of the above are colored because of their ability to absorb light in the visible region of the em spectrum and transmit the remaining light of the visible spectrum that has not been absorbed. The light transmitted is known as complementary color. Thus a dye or pigment that absorbs red light will transmit a blue-green color.

Practice

In your notebook describe on the atomic level the interaction between wavelengths of light and electrons as a specific color is observed.

Why Poppies are red.Why do you think poppies are red?

Reason it out from the last statement….

They contain an anthocyanin called cyanidin

The sap of poppies is acidic and under these conditions the cyanidin absorbs light at 375 nm and 530 nm. 375 nm is in the invisible UV range and 530 is in the blue-green region. So… the poppies transmits the complementary color of blue-green which is red.

Naturally occurring pigments:

PorphyrinsComplex ring-shaped molecules with a metal atom in the center

A coordination complex is a metal surrounded by an array of atoms or molecules, usually ligands

This means that the bond with the metal are dative bonds

HaemPigments in red meat

Commonly found in hemoglobin

Fe2+ or Fe3+

Naturally occurring pigments:

Porphyrins

Chlorophyll Green pigment in plants

Mg2+

Chlorophyll a – CH3 absorbs violet-blue and red-orange

Chlorophyll b – CHO absorbs blue – more water soluable

Chlorophyll*Contain a group called porphin which has 4 nitrogen atoms in the center and forms a stable structure with Mg ion. The stability of chlorophyll depends on the pH. Basic pH of 9 is stable acidic pH of 3 is not.When plants are heated the cell membrane breaks down releasing acids and lowering the pH. At lower pH, Mg ion is replaced by H+ ions creating an olive brown pheophytin complex.The breakdown of the cell makes it light sensitive as well.Was first synthesized in 1962 by Bob Woodward. Nobel prize in 1965.

HaemAlso contains a porphin ring, but it is complexed to an Fe ion. During oxidation, O binds to purple-red myoglobin* (Mb) and red oxymyoglobin (MbO2) forms. In both, the form of Fe is 2+ or iron (II). Auto-oxidation can take place where the Fe (II) turns into Fe(III).

When Fe (III) forms called metmyoglobin (MMb), it has a undesirable red-brown color.

So packaging attempts to keeps O away so this doesn’t happen. CO2 can be injected into packing as well to stop the above.

MbO2 / Mb / MMb

(red) (purple-red) (brown)

Fe 2+ Fe 2+ Fe 3+

Naturally occurring pigments:

CarotenoidsFat-soluble due to the long chain of C-H bonds being non-polar

Long-chain hydrogcarbon molecules

yellow to red in color

A large majority are produced by algae

Most common natural pigmentsSalmon and flamingos – pink

Lobsters and crabs – blue-green

Low levels are found in grasses and dairy productsButter is yellow

In the body they are converted into vitamin A

Also important as an additive to processed foods

β-carotene and lutein are examples

CarotenoidsContain alternate Carbon to Carbons single and double bonds (d.b.) when bonded together account for the fact that they are colored. The structure and visible spectrum of beta-carotene* absorbs light in the visible range of violet-blue and so it transmits yellow light. Because of this unsaturation, they are sensitive to oxidation which can also be catalyzed by light, metals and hydroperoxides. This changes the bonding causing bleaching of color, loss of Vit A*(Biochem) activity and it smells.Are stable under 50oC pH from 2-7 and aren’t degraded by most most food processing. When heated the trans isomer changes to cis.

Naturally occurring pigments:

AnthocyaninsMost widely occurring pigments in plants

Responsible for the pink, red, purple, and blue colors in fruits and vegetables.

Very water soluble

Many are red in acidic conditions and blue at higher pH

All have the same basic structureThese vary by the number of hydroxyl groups

The colors vary as glucose is coupled as R

AnthocyaninsIn aqueous solution anthocyanins exist in different forms, which are in equilibrium with each otherChanging the pH and the temperature affects the position of equilibrium and thus the predominant species responsible for the color.They are most highly colored at low pH (acidic) and at low temperatures.When exposed to heat the equilib moves to the right and compds are less thermodynamically stable. Hence loss of color and browning.

Quinoid *(A) / flavylium* (AH+) / carbinol* / chalcone* (blue) (red) (colorless) (colorless)

Anthocyanins form complexes with Al3+ anc Fe 3+ , These are present in tin cans causing fruit to become discolored

Factors affecting the color and stability of pigments

Any factors that change the structure of a molecule, because that will affect the precise wavelength of the visible light that the pigment absorbs and thus the complementary colors transmitted.These factors include:OxidationTemperature changespH changesPresence of metal ions

Synthetic Colorants (dyes)Are added to make the food look more appealing.Many countries who list the ingredients, many times will list them by a numbering system either by E numbers or the INS system.Unfortunately many dyes are carcinogenic (can cause cancer) ex. 4-dimethylaminoazobenzene. Anyhow it was added to butter until 1938.Many different countries have very different regulations. Some are permitted in some and banned in others. There is a need for an international legislation on colorants. Practical!

Non-enzymatic Browning of FoodNatural browning such as a cut apple is un-desirable vs browning by cooking is desirable.Foods high in carbohydrate content, especially sucrose and reducing sugars such as glucose, and lacking nitrogen-containing compounds can be caramelized. Even tho this can be accomplished by heating, the chemistry of it is far from simple. Both sucrose and glucose when caramelized form many different products among them acids, sweet and bitter derivatives, volatile molecules with a caramel aroma and brown-colored polymers.Factors that increase the rate of C include acid catalysis at pH values < 3 or >9. Temp > 120oC required. Ex browning of baked egg dishes. Boiling has no effect (<120oC)

Non-enzymatic Browning of Food

Enzymatic Browning of Food

Fruit and veg containing the enzyme polyphenoloxidase

Up to 50% of all fruit and veg losses are caused by this browning

Produces brown pigments and is detrimental to quality

Mailliard reactionFor browning N containing foods this reaction involves the reaction of a carbohydrate, either a free sugar or one bound up in starch, with the amine group attached to an amino acid, which may be free or part of a protein chain.It involves a condesation reaction between a reducing sugar and the amine group.A.A. lysine*(biochem) is best at producing browning where cysteine*(biochem) is the least. Milk has lysine and easily browns…Because moisture lowers the temp, in order to make good stew, the meat is browned first in hot oil to bring out the flavors before adding any liquid.Other browners, milk choc, heating sugar to make fudge…

Non-enzymatic browning of food

Factors affecting the Maillard reactionpH

Type of amino acid

Type of sugar

Temp

Time

Presence of oxygen

Water activity

Other food components

These factors determine the final color and aroma

Genetically Modified (GM) FoodsGenetic engineering involves the process of selecting a single gene for a single characteristic and transferring the sequence of DNA from one organism to another.GM foods are produced or derived from a genetically modified organism.GM foods can be substantially different or not very different as the conventional food in composition, nutrition, taste, smell, texture and functional characteristics.Ex of GM food is FlavrSavr Tomato: it’s ripening gene has been extended so the tomato can stay on the vine longer to achieve maximum flavor before ripening. This also prolongs its shelf-life.

Benefits of GM foodsImprove the taste, flavor, texture, nutritional value and shelf-life of food.

Anti-cancer substances could be incorporated and exposure to less healthy fats reduced.

Plants can be made more resistant to disease, herbicides and insect attack. Similarly GM foods can increase resistance to disease in animals, increase productivity and feed efficiency to give higher yields of milk and eggs.

Environmentally friendly bio-herbicides and bio-insecticides can be formed. GM foods can lead to soil, water and energy conservation and improve natural waste management.

Potential Concerns for GM FoodsThe outcomes to alterations is uncertain, as not enough is known about how genes operate.

They may cause disease, as antibiotic-resistant genes could be passed to harmful mico-organisms making them resistant.

Genetically engineered genes may escape to contaminate normal crops, with unknown effects

They may alter the balance of delicate ecosystems as food chains become damaged.

There are possible links to an increase in allergic reactions (particularly for those involved in food processing).

TextureFood often appears homogeneous. However, many ingredients in food are completely immiscible, and so will form separate phases within the food. The reason why the food often appears to be homo is that these phases are very small.

A dispersed system is a kinetically stable mixture of one phase within another largely immiscible phase.

There are several types of d.s. and usually the continuous phase is a liquid:

Suspension: solid dispersed in a liquid (molten choc)

Emulsion: liquid dispersed in a liquid (cream)

Foam: gas dispersed in a liquidwer (beer)

EmulsifiersAre substances that aid the dispersal of the droplets below to stabilize them preventing them from forming larger globules and separating outTwo types of food emulsions (water-in-oil; oil-in -water)Ex WOL is a dispersion of water droplets in oilEx LOW is like milk or salad dressing and manufactured foods.Good emulsifiers will tend to be soluble in both fat and water. Ex lecithin (pure or in egg yolk), milk protein and the salts of fatty acids*.To physically make an emulsion mechanical energy is required to mix it in to cause dispersion. (why we beat, mix and whisk in cooking).Stabilizers can also be added to prevent emulsions and foams from separating out into distinct phases.