Carbohydrates FDST205 Handout

7/30/2019 Carbohydrates FDST205 Handout

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Carbohydrates in Food Systems

Randy Wehling

Dept. of Food Science & Technology

University of Nebraska-Lincoln

rwehling1@unl.edu

Importance of Carbohydrates

Carbohydrates make up over ___% of the biological

matter in the world

__________ is the most abundant organic chemicalon earth

_______ (table sugar) is produced commercially in

larger quantities that any other pure organicchemical

Importance of Carbohydrates in

the Diet

In the US, carbohydrates account for about ________

of the calories consumed by the average person

In some parts of the world, carbohydrates may

account for as much as ___% of an average

individual’s caloric intake

Classes of Carbohydrates

Monosaccharides: The smallest molecules thatmaintain the properties associated withcarbohydrates

Oligosaccharides: __ → ___ monosaccharide units ___________ linked together

Polysaccharides: Long chains of monosaccharides

__________ linked together. Occur as a means of ______ storage, or as __________ components.Chains can be ______ or ________.

Monosaccharides

Are ____________ aldehydes or ketones.Monosaccharides containing an aldehyde are knownas _______; ketone containing monosaccharides are

_______.

Contain __ → __ carbon atoms

Have the general formula (CH2O)n

The suffix “- ___” is used to signify a sugar (glucose,etc.)

Isomers

Isomers – Compounds that have the same molecular

formula, but differ in their arrangement of

atoms in space

__________ isomers – Compounds with the same

molecular formulae, but different

structures. They can have different

functional groups or be ________ isomers

Stereoisomers – Atoms of two molecules areattached in the same order, but have a

different orientation in space

Effect of Orientation of Hydroxyl Groups on

Chemical and Physical Properties

glucose galactose 1) Solubility 50% 32%

2) Melting point 146° C 170° C

3) Sweetness more less

4) Enzyme action fast slow

Aldoses

Reactions of Monosaccharides

Monosaccharides do

not give positive

tests for aldehydes

or ketones

The carbonyl reacts

with an hydroxyl on

the molecule to form

an internal ______ or ______

Formation of Pyranose Ring

Structure

Pyranose Ring (cont’d)

Glucose and other 6 carbon aldose sugars form six

member rings. Some sugars form five member

rings.

Five and six member rings are stable because therings’ angles are close to the bond angles of carbon

(109.5°). Four and seven member rings are not.

Six member rings are called _________; five member

rings are called __________. Ring structures can be depicted using a __________

projection

Pyranose Ring (cont.)

When the pyranose ring is

formed from 6 carbon D-

aldoses, the –OH on the

carbon next to the ring

oxygen can be either

above or below the plane

of the ring resulting in

two _________

α form: the –OH is _____ the plane of the ring

β form: the –OH is ______

the plane of the ring

Pyranose Form of Important Aldoses

Reactions of Sugars

All monosaccharides and many disaccharides can

function as ________ agents, if they have a free

aldehyde or ketone, or a free _________ hydroxyl.

Reducing properties can be used to measure sugars

If the aldehyde or ketone is “tied up” in a chemical

bond, the sugar cannot act as a reducing agent

The most important reaction of reducing sugars in

foods is the ________ reaction, which occurs at hightemperatures between reducing sugars and the free

amine groups of basic amino acids.

Maillard Reaction

Maillard reaction is responsible for formation of __________ pigments that give a ______ color tocertain foods, i.e. bread crust.

Volatile flavor and aroma compounds, including alarge number of volatile aldehydes, are also formed.

_______ is made nutritionally unavailable, and some

reaction products may be mutagenic

Reaction rate is influenced by __ and _____ _______

Fructose

Fructose is the most

important ______ in

Fructose can form

either a five-member or

six-member ring

Chemical Basis of Sweetness

AH,B,X system of sweetness

perception proposed by

Shallenberger

Involves _______ donor and

receptor sites, and a site for

___________ interaction

__________ ring has –OHgroups in the proper

dimensions to interact with

the receptor cells

Relative Sweetness of Sugars and

Alternative Sweeteners:

Sweetener Index

Sucrose 1.00

Glucose 0.70 – 0.80

Galactose 0.32

Fructose 1.40

Maltose 0.30 – 0.50

Lactose 0.20High conv. corn syrup 0.65

Relative Sweetness (contd.)

Sweetener Index

High fructose corn sweetener

90% 1.20 – 1.60

55% >1.00

42% 1.00

Invert sugar >1.00

Sorbitol 0.50

Xylitol 1.00

Cyclamate 30

Aspartame 180-200

Acesulfame –K 200

Saccharin 200-700 (350)

Intestinal Transport of Sugars

Some sugars are absorbed only by slow ________ driven

by _________ __________ differences, other sugars are

actively transported against a concentration gradient.

Only _____________ are transported across the intestinal

wall. Oligosaccharides must be hydrolyzed by enzymes

into the component _______________.

Sugars must form a ________ ring in order to be actively

transported

Sugar Alcohols (Polyols)

Are produced by hydrogenreduction

Sugars alcohols can be used

in ________ ________ foods

Cannot form a ____ structure,and therefore are not activelytransported

Yield ___ – ___ kcal/g vs. __ kcal/g for glucose or sucrose

Water Binding

Hydroxyl groups can _______ with water molecules,reducing the availability of water to microorganisms.Sugars are therefore used as _____________,especially in intermediate moisture foods.

The dimensions of the pyranose ring correspondwell with the dimensions of the water lattice. Oneglucose molecule can bind ___ water molecules

Sugar alcohols do not form a ring, and thus bindless water. May not be as effective as preservatives

Oligosaccharides

_______ (table sugar,

cane sugar, beet sugar)

α–D-glucopyranosyl-β-

D-fructofuranoside

Does not act as a

reducing agent

Easily hydrolyzed by

acid or enzyme

Oligosaccharides (cont.)

Raffinose and Stachyose

Oligosaccharidescontaining glucose,

fructose, and galactosefound in beans, peas, andother legumes

Humans and other

monogastrics lack theenzymes to completelybreak down these sugarsin the small intestine

Prebiotics

Carbohydrates thatpreferentially stimulate thegrowth of beneficial micro-organisms (probiotics) in

the intestinal tract Prebiotics are not broken

down in the small intestine

Oligosaccharides known to

have prebiotic activityinclude various fructo-oligosaccharides such askestose and nystose

Starch

Found in grains, potatoes, tubers and roots

High molecular weight polymer used by plants to

store excess energy.

Composed of two fractions: amylose and

amylopectin

Amylose

Mostly linear polymer of α-glucose linked 1,4. Contains a few

α-1,6 branch points.

Molecular weight ranges from 250,000 to 1 X 106

Long chain forms a helix with hydrogens oriented to the

interior of the helix. Allows formation of clathrates.

Amylopectin

One of the largest polymers in

nature (m. w. = 1 X 108)

A highly branched polymer of α-glucose linked 1, 4 with 4.5% α-

1,6 links

Glucose chains are much

shorter than in amylose,

averaging 20 – 25 glucose

residues

Starch Granules

Amylose and amylopectin

molecules are organized

in microscopic granules

Granules range from 2 –

100 μm in diameter

Granules are highly

organized, with both

amorphous and

crystalline regions

Starch Granules (cont.)

Because of their high degree of organization, starch

granules exhibit birefringence under polarized light

Gelatinization

When starch

granules are

heated in excess

water, they

imbibe water,

swell and lose

their high degree

of organization.

Crystalinity andbirefringence are

Retrogradation and Syneresis

As a starch gel ages, more H-bonds are formed betweenglucose polymer chains and crystallinity returns. This isreferred to as rectrograde and results in gels becomingopaque.

Amylose recrystallizes faster than amylopectin.Amylopectin retrogradation is thought to be involved inbread staling. Emulsifiers can be added to bread to slowstaling.

As a gel ages and interactions between starch chainsincrease, water molecules can be squeezed out of the gel.This is known as syneresis.

P d ti f Hi h F t

Production of High Fructose

Corn Sweetener

Starch is isolated from corn by “wet-milling” process

Starch slurry is cooked, then converted to glucose

syrup by treating with α-amylase and glucoamylaseenzymes

Glucose syrup is treated with immobilized glucose

isomerase enzyme to create an equilibrium mixtureof glucose and fructose (52:48).

Comparison of Sweetener

Composition

Hydrolyzed sucrose (invert sugar):

50% glucose/50% fructose

High fructose corn sweetener:

42% fructose, 52% glucose, 6% maltose andother oligosaccharides of glucose

48% fructose, 39% glucose, 1% sucrose, 12%maltose and other sugars

U.S. Per Capita Consumption of

Caloric Sweeteners

Modified Starches

Starches can be chemically modified to change their

pasting and gel forming properties

Chemical crosslinking and substitution are the mostcommon modifications

Improves stability of starch pastes and slows the

retrogradation process

Cellulose

Glucose units linked by ______ bonds

Linear polymer containing 6,000 – 12,000 glucose

units on average

The most abundant organic chemical on earth. It is a __________ polysaccharide that is a major constituent of the ____ _____ of higher plants

The β 1,4 linkage is resistant to acid hydrolysis andattack by amylases

Hemicelluloses

A group of carbohydrate polymers associated with

the cell walls of higher plants

Polymers made up of sugars including xylose,arabinose, mannose, galactose and glucose

Are sometimes referred to as __________

Pectins

Polymers of ______________ acid, linked α 1,4, thatare the structural materials of plant soft tissues.

Linear polymers with short side chains of other sugars branching from the linear backbone

Pectins can form _____ at low ___

Pectins can be isolated from various sources (mostcommonly apples) and used in making jellies, jams,and fruit preserves.

Water soluble polysaccharides used as __________

and ___________ in foods

Gums are derived from sources including: seaweed,seeds, tree exudate, microbial fermentation,

apple pomace and citrus peel (pectins), and

wood pulp (chemically modified celluloses)

Are typically added to foods at levels ___%

Uses of Gums

___________

Stabilization of ___________

Suspension of _________

Control of _______________

______________

Sources of Gums

Seaweed extracts: Alginates

Carageenan

Seed gums: Locust bean gum

Guar gum

Exudate gums: Gum Arabia (Acacia)

Gum Ghatti

Gum Karaya

Gum Tragacanth

Sources of Gums (cont.)

Microbial gums: Xanthan

Dextran

Gellan

Modified cellulose: Methylcellulose

Carboxymethylcellulose

Hydroxypropylmethylcellulose

Pectins

Dietary Fiber

Composed primarily of __________ polysaccharides fromplants found in the ____ _____. Good sources of fiber are whole grains, fruits and vegetables.

These components are not digested by humans (or anymonogastric). They are resistant to amylase enzymesand acid.

Fiber components are classified as _______ or ________,based on their solubility in ______

Insoluble Fiber

Thought to be important to ______ health. Provides bulk that

contributes to “regularity”. May reduce contact time of

mutagens and carcinogens by decreasing transit time through

the _____. May also act as absorbent.

Components include:

__________

Hemicelluloses and pentosans

______ (polymer of phenolic compounds closely

associated with carbohydrate cell wallcomponents)

Resistant ______ (raw or retrograded)

Soluble Fiber

Composed primarily of _______ and _____ – polysaccharides that are water soluble but nothydrolyzed in the digestive system.

Has been shown to reduce serum ___________ levels, and in some cases serum _______ levels.

________ from oat and barley fiber have been shownto be particularly effective. These are polymers of

glucose containing both β-1,4 and β-1,3 links.

May also exhibit a prebiotic effect

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