C2. Dehydration, browning rxns, caramelization, sweetness

Question 1

Dehydration reactions are significant to food industry bc they are the basis of (2) reactions

Answer 1

• Caramelization (color and flavor)
• Maillard reaction

Question 2

best conditions (temp + pH) for dehydration reactions? but can still happen at ?

Answer 2

• concentrated sugar solutions are heated at temps above 130°C and under very acidic conditions (pH < 2.2), dehydration reactions become prevalent
• can still happen, but very slowly at temps below 100°C and in dilute solutions

Question 3

as water evaporates from solution (dehydration reaction), 2 things take place, leading to breakdown of what into what?

Answer 3

• molecular (internal) dehydration and oxidative fission take place
• leading to breakdown of carbon chain into smaller units (flavor compounds)

Question 4

2 ways that water can be eliminated from sugar molecules?
- which one is more predominant?

Answer 4

1. loss of water from 2 sugar molecules by reaction of their hydroxyl groups (R-OH + OH-R –> ROR (polymer) + H2O) –> results in random polymerization (generally not predominant reaction)
2. elimination of water from within a sugar molecule, leading to formation of highly unsaturated ring structures

Question 5

common intermediate formed during caramelization reactions by dehydration is ?

Answer 5

hydroxymethylfurfural (HMF)

Question 6

fragmentation reactions of highly unsaturated ring structures produce various compounds (3 characteristics)
- 3 examples

Answer 6

• low-molecular weight
• unsaturated
• reactive
• acetol, diacetyl, acetoin

Question 7

many of highly unsaturated ring structures and their breakdown products (3) can _____________

Answer 7

breakdown products like short-chain aldehydes, ketones, carboxylic acids)
- can polymerize

Question 8

extensive dehydration reactions eventually lead to formation of an amorphous, _________ ________ pigment called _________

Answer 8

• charged brown pigment
• caramel

Question 9

browning reactions can be classified into 2 categories:

Answer 9

1. enzymatic (oxidative) browning: when fruits/veggies turn brown when left on table
2. non-oxidative browning/non-enzymatic browning

Question 10

enzymatic/oxidative browning:
- related to carbohydrates?
- associated with browning of ________ tissues
- what enzymes oxidizes what into what?

Answer 10

• not related to carbs!
• plant tissues
• most commonly polyphenol oxidase (PPO) oxidizes mono or dihydroxy phenols into quinones

Question 11

quinones are quite reactive/unreactive and can react ___-_________ to _________ into a ________ pigment

Answer 11

• reactive
• non-enzymatically to polymerize into a brown pigment

Question 12

what other enzymes (apart from polyphenol oxidase) are indirectly associated with browning? (2)
- these can produce/supply reactive _______ compounds that can undergo ___-_________ _________

Answer 12

• ascorbic acid oxidase and lipoxygenase
• carbonyl compounds –> non-enzymatic browning

Question 13

2 types of non-oxidative browning
- caramelization vs Maillard?

Answer 13

1. sugar-sugar reactions (caramelization)
2. sugar-amine reactions (Maillard reactions)

Question 14

caramelization reactions occurs when any concentrated sugar solution is ________

Answer 14

heated

Question 15

caramelization is used commercially to produce (2)
- each is generally made from what?

Answer 15

• caramel flavor/syrup –> made from concentrated sucrose solutions
• caramel color –> generally made from glucose syrup (more reducing)

Question 16

which 2 reactions produce what type of polymers that are very dark in color? (high tinctorial power)

Answer 16

• dehydration and polymerization
• produce high-molecular weight water-soluble polymers

Question 17

caramel colors produced in what foods (5)

Answer 17

baking, beer, soft drinks, whiskey, soy sauce…

Question 18

Maillard reaction involves reaction of what with what (generally (2))

Answer 18

• of reducing sugars with primary amines (generally amino acids and proteins: free amino groups on side chains (ie: e-amino group of lysine))

Question 19

which aromas are associated with Maillard reactions? (4 ish)

Answer 19

1. coffee flavor, color and aroma (formed during roasting)
2. popcorn aroma/flavor
3. roasted peanuts, cashews
4. baked bread aroma

Question 20

condensation of primary amine and reducing sugar form a _______ ______ –> followed by formation of an _________ compound

Answer 20

• Schiff base
• Amadori compound

Question 21

what is a key intermediate in Maillard reaction, comparable to _____ in caramelization

Answer 21

Amadori compound!
- comparable to HMF

Question 22

converting Schiff base to amadori compound is termed what?

Answer 22

amadori rearrangement

Question 23

can the amadori compound lose water from within the molecule?

Answer 23

yes

Question 24

at higher temps, what are the major reactions following amadori rearrangement?
- key difference with caramelization?

Answer 24

dehydration and fragmentation (much like caramelization)
- key difference: there is an amine group attached to sugar –> end products tend to contain nitrogen in their structure

Question 25

fragmentation produces short/long chain unsat/sat _______ and _______ compounds that can ________ and/or be released as ________ into the environment (aroma)

Answer 25

- short-chain unsat carbonyl and amino compounds - can polymerize or be released as volatiles

Question 26

net result of reactions of amadori compound? (2)

Answer 26

brown pigments and volatile compounds containing nitrogen are formed

Question 27

Strecker degradation is a subcomponent of the _________ reaction - involves reaction of what with what produced by either _______ or _________ reactions

Answer 27

- Maillard reactions - reaction of free amino aicds with the dicarbonyl intermediates produced by Maillard or caramelization reactions

Question 28

products formed during strecker degradation? (4) which are compounds that contribute to the characteristic aroma of ________ foods

Answer 28

- CO2, ammonia, formaldehyde and pyrazines - roasted foods

Question 29

common strecker aldehydes? (3 + what kind of aroma)

Answer 29

- ethanal --> fruity, sweet aroma - methylpropanal (malty) - 2-phenylethanal (flowery, honey like aroma)

Question 30

condensation of 2 _________ may yield pyrazine derivatives that are also powerful ______ compounds

Answer 30

- aminoketones - aroma

Question 31

what reaction is used commercially to produce artificial flavor compounds? - how? - how are these flavor compounds isolated?

Answer 31

- Strecker degradation - reacting sugars and selected amino acids under proper conditions --> can produce variety of flavors such as chocolate, bread, caramel, almond - by distillation and then added to food products

Question 32

in summary, which 3 reactions represent important group of reactions associated with food color and flavor?

Answer 32

- caramelization - Maillard reaction - Strecker degradation

Question 33

why is evaluation of sweetness quite difficult? (2) - how to evaluate?

Answer 33

- because it is a subjective response + cannot be measured objectively by an instrument - sensory evaluation! by controling as many variables as possible and using statistical methods to remove individual variations

Question 34

which sugar is used as reference for sweetness? why?

Answer 34

sucrose! because it is the most abundant

Question 35

rank relative sweetness from lowest to sweetest: - xylose - lactose - invert sugar - sucrose - glucose - maltose - fructose

Answer 35

- lactose 39 - maltose 46 - invert sugar 65 - xylose 67 - glucose 69 - sucrose 100 - fructose 114

Question 36

what is an invert sugar?

Answer 36

sucrose hydrolyzed to a mixture of glucose and fructose --> optical rotation is inverted (ie from 100 to -100)

Question 37

3 variables involved in how sweetness if perceived 1. increase temp = decrease/increase sweetness --> attributed to what? 2. some people can differentiate ______ of sweetness (vs _______ in sweetness) 3. relative sweetness of sugar is changes in presence of (4)

Answer 37

1. increase temp = decrease sweetness --> attributed to change in relative concentrations of open-chain, a, and b forms which differ in sweetness 2. type of sweetness (vs difference in sweetness) while others can't... 3. salts, acids, others sugars, and a host of other components (ie alcohol tend to heighten sweetness)

Question 38

what is the synergistic effect of sugars?

Answer 38

sugars can become sweeter when mixed together than when tasted separately

Question 39

sweetening power is related to sugar __________

Answer 39

solubility (proportional ish to sweetness determined by sensory eval ishh)

Question 40

when is solubility a very important criterion? (2 ex)

Answer 40

when high sugar concentrations are required to inhibit microbial growth (jams, jellies)

Question 41

what happens if solubility of sugar is too low?

Answer 41

sugar can crystallize, making the product texture grainy

Question 42

limitations in solubility may be encountered in sweetened condensed or evaporated milk and ice cream, owing to what?

Answer 42

relatively low solubility of lactose (milk sugar)

Question 43

common method to avoid sugar crystallization?

Answer 43

mix 2 sugars together --> inhibits their crystallization

Question 44

sugars can also be used as a preservative against ______ and ________ _______, and to help in formation of _______ _____ in jams and jellies - how do sugars do that?

Answer 44

- mold and bacterial growth - pectin gels - sugars effectively bind water, making it unavailable for microorganisms due to the lowering of water activity of the system

Question 45

other uses of sugars: - to facilitate _______ and ______ production during baking (Maillard reaction) --> if not desired, minimize by using ________ - to help ______ development in meat curing --> _________ sugars prevent ________ of meat pigments - as (3)

Answer 45

- browning and flavor production --> minimize browning by using sucrose - to help color development --> reducing sugars prevent oxidation of meat pigments - as humectants, flavor binding agents and viscosity enhancers

Question 46

sugar alcohols are used extensively in food industry as (2)

Answer 46

- bulking agents and viscosity enhancers in artificially sweetened products to provide "body" - as water-binding agents to plasticize intermediate moisture foods and reduce the water activity of food system without adding excessive sweetness

Question 47

sugar alcohols are absorbed by _________ diffusion, thereby contributing to more/fewer calories than sugars - they are greatly/poorly metabolized by microbial flora in mouth --> do/do not contribute to dental caries (which are caused how?)

Answer 47

- passive - fewer calories than sugars - poorly metabolized --> do not contribute to caries - caused by acids produced by microflora form fermentation of sugars

Question 48

most sugar alcohols (2 ex) are more/not as sweet as sugars - except which one? which has comparable sweetness to _________

Answer 48

- sorbitol and mannitol - not as sweet as sugars - xylitol = exception --> comparable to sweetness of sucrose

Question 49

xylitol is produced from what? - how is it produced (3)

Answer 49

- produced from xylan, a polymer of xylose which can be widely found in nature (birch tree chips, almond and pecan shells, corn cobs) - xylan is hydrolyzed to xylose and hydrogenated using a nickel catalyst to produce xylitol OR yeasts convert xylose to xylitol by fermentation OR enzymatic conversion of glucose to xylitol

Question 50

difference between D-xylose and D-xylitol?

Answer 50

- xylose --> 5C --> OH on one end and COOH on one end = aldehyde - xylitol --> 5C --> OH on both ends