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Food chemistry and food physics > Carbohydrates 1 > Flashcards

Carbohydrates 1 Flashcards

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1
Q

what are the 4 major roles of carbohydrates in biology?

A
  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)
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2
Q

a sugar with a ketone is called a

A

ketose

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3
Q

a sugar with and aldehyde is called an

A

aldose

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4
Q

what are enantiomers?

A

nonsuperimposable mirror images

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5
Q

what are epimers?

A

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

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6
Q

what is monosaccharide isomerization?

A

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

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7
Q

what characterises an enediol?

A

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

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8
Q

the most stable form of D-glucose is…

A

the beta-anomer of a pyranose ring in chair formation

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9
Q

how is the ring form of fructose formed?

A

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

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10
Q

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

A

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.

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11
Q

what is a reducing sugar?

A

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

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12
Q

what is a non-reducing sugar?

A

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

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13
Q

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

A

the anomeric carbon is oxidized to a carboxylic acid

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14
Q

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

A

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

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15
Q

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

A

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.

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16
Q

hydrogenation of sugars will convert..

A

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

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17
Q

what is hydrogenolysis of sucrose and why is it performed?

A

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

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18
Q

properties of sorbitol

A

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).

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19
Q

properties of mannitol

A

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

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20
Q

hydrogenation of D-xylose gives…

A

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.

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21
Q

what does it mean that xylitol is noncariogenic?

A

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

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22
Q

maltose is..

A

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

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23
Q

lactose is..

A

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

24
Q

what is sucrose?

A

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)

25
Q

how does lactose behave in our intestines?

A

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.

26
Q

what happens in lactose intolerance?

A

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.

27
Q

what is an oligosaccharide?

A

2-20 sugars joined by glycosidic bonds

28
Q

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

A

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.

29
Q

how is invert sugar produced in jam cooking?

A

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.

30
Q

what are raffinose oligosaccharides?

A

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.

31
Q

what are the two important non-enzymatic browning reactions?

A

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

32
Q

what is the first step in both maillard and caramelization?

A

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

33
Q

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

A

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.

34
Q

to make a caramel ingredient..

A

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)

35
Q

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

A

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.

36
Q

The maillard reaction is..

A

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.

37
Q

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

A

15-40 %

38
Q

all melanoidins contain

A

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

39
Q

what is mutarotation?

A

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

40
Q

what is an aglycon?

A

the group bonded to a sugar with a glycosidic bond

41
Q

how can monosaccharide ring formation be prevented?

A

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

42
Q

what are the 3 primary stages of the maillard reaction?

A
  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.
43
Q

the Maillard reaction results in…

A

flavours and colour

44
Q

the maillard reaction is favored at…

A

low water contents

45
Q

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

A

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.

46
Q

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

A

they are flavour compounds in many cooked foods.

47
Q

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

A

volatile flavour and aroma compounds!

48
Q

what happens in strecker degradation (maillard)?

A

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.

49
Q

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

A

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

50
Q

reaction rate of the maillard reactions is maximum in..

A

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

51
Q

what Aw gives highest reaction rate in maillard?

A

0,6-0,7

52
Q

what are some potential adverse effects of the maillard reactions?

A

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.

53
Q

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

A

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.

54
Q

in what type of products can acrylamide be found?

A

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.

55
Q

what are the required reactants for formation of acrylamide?

A

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

Food chemistry and food physics (12 decks)

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