Unit 3, Chapter 5 notes: by Alison ReavellRoy
Food Chemistry 2: Carbohydrates, Lipids, and Proteins
5.1 Food Carbohydrates
The Structures of Sugars
Monosaccharides are single sugar molecules that are not chemically bonded to other
sugar molecules. A simple sugar can also be termed as an organic alcohol since it is a
molecule that contains carbon atoms attached to the –OH (alcohol) groups.
Monosaccharides that contain 3 carbon atoms in their structure are called trioses; 5
carbon atoms are called pentoses; 6 carbon atoms are called hexoses.
Important monosaccharides found in foods are the hexoses: glucose, fructose and
Fructose, is sweeter and more soluble than glucose. Glucose is the most common
monosaccarides in food.
Glucose is considered an aldose because its carbonyl group (C=O) at carbon atom 1 is
in the form of an aldehyde. ; Fructose is considered a ketose because its carbonyl group at
carbon atom 2 is in the form of a ketone.
Monosaccarides exist in more than one structural form; straight chain or fisher
projection molecules, and ring or cyclic Hathworth projection form.
Disaccharides are two monosaccarides join together by a glycosidic bond.
Three important food disaccarides are sucrose, lactose and maltose Sucrose is a common table sugar and is widely used in food preparation in its crystalline
form. Lactose is found only in milk and dairy product.
The Functional Properties of Sugars
Sugar molecules contain two important, reactive functional groups: the carbonyl group (
C=O) and the alcohol group(OH). The alchol group is important for solubility and
sweetness, while the carbonyl group is important for reducing activity and the millard
browning reaction, which can cause color and flavor changes.
Reducing sugars are sugars that contain the aldehyde or ketone carbonyl group. Reducing
sugars react with other substances through oxidationreduction chemistry to produce a
reduced substance plus the oxidized sugar molecule. All monosaccharides and certain
disaccharides are reducing sugars.
Dextrose equivalent relates to solubility, reduction action, viscosity, and many other
properties. Dextrose equivalent is a measure of the percentage of glycosidic bonds
hydrolyzed in disaccharides and polysaccharides, which indicates the level of reducing
sugar present. The higher the Dextrose Equivalent, the more soluble, and the greater the
reducing ability of a sugar.
Two important browning reactions can occur in sugars: Maillard browning and
Reducing sugars react with the amino acids through the Millard reaction, to produce
brown color pigments in foods. Millard browning is the browning of foods as a result of the millard reaction, and the brown pigments that form are called melanoidins. (baked
bread and cooked meats is due to this reaction)
The Millard reaction known as a nonenzymatic browning because enzymes are not part
of the reaction, is a complex sequence of chemical reactions.
The Millard reaction can be viewed as a sequence of three chemical reactions:
condensation, rearrangement, and polymerization.
Caramelization: Is the formation of brown caramel pigments as a result of applying heat
energy to sugars. The temperature required approximately 200C. Protein material,
including enzymes, is not required for carmelization. The color used in cola beverages is
created by caramelizing sucrose in the presence of ammonia bisulfite.
The formation of a crystalline structure, or crystallization, implies threedimensional
arrays of unit cells into a solid form. It will depend on factors such as the moisture,
temperature and concentration of the sugar in a food system. A crystal is a solid made up
of units in repeating pattern. Sugars crystallize in two stages, the first stage is transfer of the sugar molecule to the
surface of a crystal and the second is the incorporation of the sugar into the crystalline
Humectancy refers to a substance that has an affinity for moisture. Humectants are able
to hydrogen bond with water molecules, making water less available for microbial
The hydrolysis of sucrose to its component monosaccharides is carried out if sweeter
product is desired than when sucrose is present alone in a product. The mixture of the two
monosaccharide end products, called invert sugar, is typically created in food products
through the deliberate application f the enzyme invertase.
Oxidation and reduction
Oxidation of the RCOH(aldehyde) group in sugars cause a loss of sweetness and coverts
the aldehyde to an acid(HOC=O) group.
Sweetness and texturizing
See relative sweetness of sugars on a weight basis table 5.3
Polysaccharides and Their Functional Properties
These large carbohydrate structures are composed of joined sugar units and come in only
two sizes: oligosaccharides and polysaccardies. Complex carbohydrates of 10 or
fewer(typically nonreducing) sugar units are called oligosaccharides. Polysaccharide
molecules are usually at least 40 or more sugar units in size, potentially comprising
hundreds or more.
Sugars can exist in two structural forms, called alpha and beta. The forms differ on
whether the OH group attached to the first carbon atom projects downward (alpha) or
Betaglucans are polysaccahrides of glucose similar to cellulose, but less linear,
occurring in oats, barely and yeast.
A watersoluble form of enzymetreated oat flour containing betaglucan soluble fiber,
called oatrim, can be used as a fat replacer and texturizing ingredient.
Cellulose The most abundant of all carbohydrate polymers, cellulose composes of plant
cell wall material. Although similar to amylose, the nearly insignificant structural
differences accounts for the strength of cellulose fibers, making it impossible for human
digestive enzymes to hydrolyze them.
Dextrins and Maltodextrins Dextrins are polysaccaharides derived from starch, linear arrays of glucose units bound
by a alpha1,4 glycosidic linkages. They are produced commercially by hydrolyzing
especially the amylose portion of starch. This process is termed pyroconversion or
dextrinization. since heat is used to carry out the reaction, the products are often referred
to as pyrodextrins.
Dextrins can be used as a 4kcal/gram fat replacer that can replace all or some of the fat in
a variety of products.
Maltodextrins are polysaccharide fragments derived from starch hydrolysis. In the food
industry, corn starch is the typical source of material used to generate maltodextrins.
Maltodextrins can be used as a fat replacer, texture modifier, or bulking agent.
Fructooligosaccharides are naturally occurring sugars consisting of multiple units of
sucrose joined to one, two, or three fructose molecules via glycosidic bond to the fructose
portion of the sucrose molecule.
Fructooligosaccharides are known as prebiotics. These substances promote the growth of
probiotics, bacterial organisms believed to be beneficial to health.
Inulin: dietary Fiber
The substance called inulin is a fructooligosaccharide that functions as a soluble dietary
fiber. It occurs naturally in plants such as chicory root, onions, asperagus and Jerusalem
In reducedfat or nonfat systems, inulin provides a creamy mouthfeel through texture
Pectin substances are high molecular weight polysaccharides found in plant cell wall
The term used for the proportion of methyl esters in a pectic substance is degree of
Although pectic substances are commonly referred to as pectin or methyl pectate , three
chemically distinct substances have been identified:
Protopectin: refers to nonmethylated galacturonic acid polymers found in immature fruit.
Pectinic acid is a methylated galacturonic acid polymer produced during ripening.
Pectic acid is a shortchain demethylated derivative of pectinic acid associated with over
LM pectin is defined as having less than 50 percent of the carboxylic acids esterfied with
methanol, while HM pectin is defined as having more than 50 percent of the carboxylic acids esterified with methanol. HM pectin occurs naturally in fruits while LM pectin is
chemically modified pectin.
Pectin/Gelation because pectins are able to form colloidal dispersions, sols, and gels, they
are widely used in the food industry. The inclusion of pectins in pie fillings to stiffen the
texture is but one example. Pectin gels are systems containing a large volume of water
within a three dimensional solid network.
Starch is a polysaccaride derived from plant sources such as corn, potatoe, rice and
wheat. A starch molecule is a polymer of 200 or more glucose units, having both
branched (amylopectin) and unbranched (amylose) regions to its molecular structure.
These starch polymers do not exist in free state in plants. Instead they occur as discrete,
spherical aggregates called starch granules. An example of a chemically modified starch
is pregelatinized starch, a type that increases product thickness with minimal thermal
Starch gelatinization: Starches do not form true solutions with water because starch
molecules are too large. But when starch is heated in water, the bonds joining the starch
fractions amylose and amylopectin are weakened or loosened, which allows water
molecules to move in and form molecules to move in and form hydrogen bonds. This
results in the gelatinization phenomenon. During gelatinization, heated starch granules
absorb water and swell in size; it is an irreversible reaction.
Starch pasting and gel formation: A starch paste is a viscoelastic starch and water system
that possesses both thick liquidlike (viscous) and solidlike (elastic) properties. Pasting
encompasses three changes in starch: swelling, exudation, and disruption.
Following gelatinization and pasting is termed gelation. With respect to starch, gelation
refers to the formation of a gel from a cooled paste. A starch gel is a rigid, thickened
starch and water mixture that has properties of a solid.
Starch retrogradation after heating a cooling the starch polymers amylose and
amylopectin and the intergranule matrix starch fragments can reassociate into an ordered
structure, which represents a loss of entropy to the system. This process is defined as
retrogradation and is due to intramolecular hydrogen bond formation, between linear
amylose molecules especially.
An increased tendency to release water from the gel, called syneresis, also occurs.
Vegetable gums are plant hydrocolloids substances derived from plants that distribute in
water as colloidal dispersions. They are composed of longchain polymers of various
hexoses and pentoses. Sources of food hydrocolloids include exudate gums such as gum
asabic, carageenan(a seaweed extract), guar gum(a plant seed gum), methyl cellulose
(chemically modified cellulose), and xanthan gum, a substance produced by
microorganisms. They are important food processing as ingedients in reducedfat foods, such as salad dressings, and processes meats, to increase viscosity, and they also behave
5.2 Food Lipids
Structure and Types of Lipids
Lipids are organic substances that are relatively nonpolar. Lipids, as fats, oils, waxes, are
only slightly soluble in water but very soluble in organic solvents, owing to the presence
of hydrocarbon chain structures. They are considered lipophilic or hydrophobic
Fats and oils
Fats and oils are chemically known as tricylglycerols. Triacylglycerols, or triglycerides,
as they are often called, are triesters of glycerol and fatty acids.
Saturated and unsaturated fats, Fats and oils are mixtures of fatty acids differing in chain
length and degree of unsaturation. A fatty acid chain is termed saturated if it does not
contain any carbon tocarbon double bonds. In saturated fatty acids, each carbon atom
(except the ones at each end) has two hydrogens attached. A fatty acid chain is termed
unsaturated if it does contain carbontocarbon double bonds.(eg. Monounsaturated or
One special case of polyunsaturated fatty acids is the omega fatty acid. An omega fatty
acid has a certain number of carbon atoms between the terminal CH (meth3l) group and
the last double bond(the one furthest from the COOH end) in the fatty acid.
The higher the degree of unsaturation, the greater the reactivity provided that the double
bonds occur in a series with a single bond in between, categorizing them as conjugated
Cis and trans fats
Unsaturated fatty acids come in two configurations, defined by their structure at the
double bonds. The cis configuration, the hydrogen atoms bonding in the C=C are located on the same side of the double bonds. In the trans configuration, the hydrogen atoms
attached to the carbon atoms of the double bond are opposite to each other. For the most
part, unsaturated fatty acid double bonds exist in foods in the cis rather than the t