Carbohydrates 1

Question 1

what are the 4 major roles of carbohydrates in biology?

Answer 1

1. energy and carbon source (fuel, energy storage, metabolic intermediates)
2. backbone of nucleic acids (ribose in RNA, deoxyribose in DNA)
3. structure elements (cell walls, exoskeletons, connective tissue)
4. cell surface and signaling (glycoproteins, glycolipids)

Question 2

a sugar with a ketone is called a

Answer 2

ketose

Question 3

a sugar with and aldehyde is called an

Answer 3

aldose

Question 4

what are enantiomers?

Answer 4

nonsuperimposable mirror images

Question 5

what are epimers?

Answer 5

sugars that differ at one of several asymmetric carbon atoms, like D-glucose and D-mannose

Question 6

what is monosaccharide isomerization?

Answer 6

interconversion of D-glucose, D-fructose and D-mannose via an enediol. reactions are catalyzed by base or enzyme.

Question 7

what characterises an enediol?

Answer 7

a double bond between two carbons and a hydroxyl group on each of the carbons (OH-C=C-OH)

Question 8

the most stable form of D-glucose is…

Answer 8

the beta-anomer of a pyranose ring in chair formation

Question 9

how is the ring form of fructose formed?

Answer 9

the open form of fructose closes when the C5 hydroxyl attacks the C2 ketone to form an intramolecular hemiketal

Question 10

what are the most common forms of fructose in different environments?

Answer 10

when fructose is connected to other molecules, furanose form is the most common (sucrose). free fructose in solution is most often in pyranose form, preferably as a beta anomer.

Question 11

what is a reducing sugar?

Answer 11

a sugar with a free aldehyde. the aldehyde can be oxidized to carboxylic acid.

Question 12

what is a non-reducing sugar?

Answer 12

one that does not have a free aldehyde, usually because they are bound with a glycosidic bond to their anomeric carbon.

Question 13

what are “-onic” acids, like gluconic acid?

Answer 13

the anomeric carbon is oxidized to a carboxylic acid

Question 14

what are “-uronic” acids, like galacturonic acid?

Answer 14

the terminal carbon (C6) is oxidized to a carboxylic acid

Question 15

what reaction can be used to determine glucose content in a solution?

Answer 15

oxidation of glucose in ring form by glucose oxidase produces H2O2 (hydrogen peroxide) which with a colorless, reduced chromogen and peroxidase oxidizes the chromogen which then becomes colored.

Question 16

hydrogenation of sugars will convert..

Answer 16

aldose or ketose into an alcohol which cannot form a ring. named with the ending “-itol”

Question 17

what is hydrogenolysis of sucrose and why is it performed?

Answer 17

hydrolysis produces glucose and mannose, hydrogenation converts these into sugar alcohols glucitol/sorbitol and mannitol.

Question 18

properties of sorbitol

Answer 18

found in low levels in fruits and berries. acts as a humectant, which means that it is good at binding water (has a high osmotic potential).

Question 19

properties of mannitol

Answer 19

moderately soluble, used for coating on candies, sugar-free chocolate, hard and soft candies. crystallizes easily

Question 20

hydrogenation of D-xylose gives…

Answer 20

D-xylitol. it is nearly as sweet as sucrose but is non-cariogenic. it has a high negative heat of solution which gives a cooling sensation.

Question 21

what does it mean that xylitol is noncariogenic?

Answer 21

it is not metabolized by the microflora of the mouth that produces dental plaque

Question 22

maltose is..

Answer 22

a reducing disaccharide consisting of 2 D-glucose units joined by an alpha-1-4 glycosidic bond. hydrolyzed by maltase and glucoamylase

Question 23

lactose is..

Answer 23

a reducing disaccharide consisting of a D-galactose joined with a beta-1-4 glycosidic bonds to a D-glucose. hydrolyzed by lactase.

Question 24

what is sucrose?

Answer 24

a non-reducing disaccharide of alpha-D-glucose 1-2 linked to beta-D-fructose. they are linked reducing end to reducing end. hydrolyzed by sucrase (and other alpha-glucosidases) and invertase (a beta-fructofuranoside)

Question 25

how does lactose behave in our intestines?

Answer 25

it stimulates intestinal adsorption and retention of calcium, is not digested until it reaches the small intestine where the hydrolytic enzyme lactase is present, membrane-bound in the microvilli.

Question 26

what happens in lactose intolerance?

Answer 26

Lactose remains in the lumen of the small intestine, where it draws fluid into the lumen by osmosis. this fluid produces abdominal distention and cramps. lactose passes into the large intestine where it ferments to lactic acid and other short-chain acids. osmotic pressure caused by these spur fluid retention in intestine lumen. acidic products lower pH and irritate the lining of the colon, leading to increased movement. diarrhea, gas.

Question 27

what is an oligosaccharide?

Answer 27

2-20 sugars joined by glycosidic bonds

Question 28

how is invert sugar produced? what are the advantages of invert sugar?

Answer 28

by hydrolysis of sucrose with mild acid or invertase a glucose/fructose syrup that does not crystallize is produced. since there are more sugar molecules in invert sugar syrup the osmotic potential is higher and the inhibiting effect on microbial growth is stronger.

Question 29

how is invert sugar produced in jam cooking?

Answer 29

heat in mild acid like that naturally present in many berries reduces water and hydrolyzes sucrose. A high temp is needed for hydrolysis but usually around 50 % of sucrose gets hydrolyzed.

Question 30

what are raffinose oligosaccharides?

Answer 30

sucrose with galactose linked alpha-1-6 to the glucose. present in peas, beans, onions etc. raffinoses are not hydrolyzed or absorbed in our small intestine and thus reach the colon where they are fermented by bacteria causing gas formation.

Question 31

what are the two important non-enzymatic browning reactions?

Answer 31

caramelization and maillard reaction. both start at heating above 100 degrees celsius. flavours and brown pigments are generated.

Question 32

what is the first step in both maillard and caramelization?

Answer 32

reversible isomerization of and aldose or ketose sugart to an enediol intermediate which is hydrated.

Question 33

what is the next step in caramelization once the enediol intermediate has been hydrated?

Answer 33

it is dehydrated due to spontaneous H2O removal from molecules due to moisture loss, to produce a 3,4 dideoxyosone (ketone groups on C1 and C2 and a double bond between C3 & C4). this can cyclize, then is dehydrated and then forms HMF, hydroxymethyl furfural which can further polymerize into other compounds.

Question 34

to make a caramel ingredient..

Answer 34

sucrose is heated with 4 different solutions, to create class 1-4. 1: acid or alkali, catalytic amounts (whisky). 2: sulfite. (whisky) 3: ammonia. (beer) 4: ammonia plus sulfite (soft drinks of the cola type)

Question 35

what is the next step in the Maillard reaction once the enediol intermediate has been hydrated?

Answer 35

a double bond moves to create a 3-deoxyosone (ketones on C1 and C2, no oxygen on C3). this compound is very reactive and continues into the strecker degradation.

Question 36

The maillard reaction is..

Answer 36

a complex series of reactions starting with a reducing sugar and an amine at high temperature. the reaction is favoured at low water contents. may occur as an adverse effect when producing powdered egg and milk.

Question 37

how much lysine and arginine is commonly lost in baked and roasted foods?

Answer 37

15-40 %

Question 38

all melanoidins contain

Answer 38

aromatic rings and conjugated double bonds (which give the color)

Question 39

what is mutarotation?

Answer 39

interconversion between the tautomeric forms of sugar (alpha and beta) in solution. circle opens and closes in new formation

Question 40

what is an aglycon?

Answer 40

the group bonded to a sugar with a glycosidic bond

Question 41

how can monosaccharide ring formation be prevented?

Answer 41

by addition of a chemical group to the aldehydic group, or exhaustive substitution of primary hydroxyl groups

Question 42

what are the 3 primary stages of the maillard reaction?

Answer 42

1. condensation of the aldehyde group of a reducing sugar with amine (NH2) forming a schiff base, followed by reactions forming Amadori product. 2. rearrangement, dehydration, decomposition and/or reaction of Amadori intermediates to form furfural compounds, reductones, strecker degradation products. 3. reaction of maillard intermediary products to form heterocyclic compounds and red/brown to black high-molecular weight melanoidin pigments.

Question 43

the Maillard reaction results in…

Answer 43

flavours and colour

Question 44

the maillard reaction is favored at…

Answer 44

low water contents

Question 45

what are the two possible pathways of the maillard reaction after formation of the amadori compound?

Answer 45

hydrolysis of amino group occurs in acidic conditions and after a series of reactions form hydroxymethyl furfural (HMF). in neutral or alkaline conditions, the amadori compound is de-aminated and forms a 1-deoxyosone, a highly reactive alpha-dicarbonyl which then is dehydrated to form maltol and isomaltol. typically both pathways will occur unless the conditions are extremely acidic or alkaline.

Question 46

maltol and isomaltol are produced in the maillard reaction, what are their functions?

Answer 46

they are flavour compounds in many cooked foods.

Question 47

in the maillard reaction, strecker degradation generates the most important…

Answer 47

volatile flavour and aroma compounds!

Question 48

what happens in strecker degradation (maillard)?

Answer 48

alpha-dicarbonyl molecules and amino acids condensate in heat. the resulting product is then cleaved in different steps and sometimes cyclized. volatile aldehydes formed gives smell of bread and biscuits while pyrazines and heterocyclic compounds gives flavour in chocolate and roast meat.

Question 49

about 95 % of molecules entering maillard reaction end up as…

Answer 49

melanoidin polymers (color pigments). large molecules that are relatively brittle when cooled down, adds texture.

Question 50

reaction rate of the maillard reactions is maximum in..

Answer 50

slightly acidic medium for reaction with amines, slightly basic medium for reaction with amino acids (since protonation of amino group reduces its activity)

Question 51

what Aw gives highest reaction rate in maillard?

Answer 51

0,6-0,7

Question 52

what are some potential adverse effects of the maillard reactions?

Answer 52

in food containing protein and reducing sugar exposed to slightly elevated temperature and prolonged storage, the maillard reaction doesn’t go all the way to coloured end-products and may be undetected. glycosylated amino acids are not metabolized in humans and are “nutritionally lost”. protein surface blocked by glycosylation may block protease digestion.

Question 53

what are some potential adverse effects of the maillard reactions in heat treatment of milk?

Answer 53

pasteurization or UHT causes modest loss of lysine (<2%). aggressive heating such as sterilization, evaporation or roller drying however can result in as much as 25% of lysine becoming unavailable. at 37 C, a mixture of glucose and casein can lose 70% of its lysine content.

Question 54

in what type of products can acrylamide be found?

Answer 54

products that have been heated to high temperatures, at least 120 C. frying, baking, puffing, roasting. not detected in unheated or even boiled foodstuffs, because the temp in boiling foods doesn’t go above 100 C.

Question 55

what are the required reactants for formation of acrylamide?

Answer 55

a reducing sugar and the alpha-amino group of free L-asparagine, found in high levels in ex. potatoes.