C2. Dehydration, browning rxns, caramelization, sweetness Flashcards
Dehydration reactions are significant to food industry bc they are the basis of (2) reactions
- Caramelization (color and flavor)
- Maillard reaction
best conditions (temp + pH) for dehydration reactions? but can still happen at ?
- 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
as water evaporates from solution (dehydration reaction), 2 things take place, leading to breakdown of what into what?
- molecular (internal) dehydration and oxidative fission take place
- leading to breakdown of carbon chain into smaller units (flavor compounds)
2 ways that water can be eliminated from sugar molecules?
- which one is more predominant?
- 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)
- elimination of water from within a sugar molecule, leading to formation of highly unsaturated ring structures
common intermediate formed during caramelization reactions by dehydration is ?
hydroxymethylfurfural (HMF)
fragmentation reactions of highly unsaturated ring structures produce various compounds (3 characteristics)
- 3 examples
- low-molecular weight
- unsaturated
- reactive
- acetol, diacetyl, acetoin
many of highly unsaturated ring structures and their breakdown products (3) can _____________
breakdown products like short-chain aldehydes, ketones, carboxylic acids)
- can polymerize
extensive dehydration reactions eventually lead to formation of an amorphous, _________ ________ pigment called _________
- charged brown pigment
- caramel
browning reactions can be classified into 2 categories:
- enzymatic (oxidative) browning: when fruits/veggies turn brown when left on table
- non-oxidative browning/non-enzymatic browning
enzymatic/oxidative browning:
- related to carbohydrates?
- associated with browning of ________ tissues
- what enzymes oxidizes what into what?
- not related to carbs!
- plant tissues
- most commonly polyphenol oxidase (PPO) oxidizes mono or dihydroxy phenols into quinones
quinones are quite reactive/unreactive and can react ___-_________ to _________ into a ________ pigment
- reactive
- non-enzymatically to polymerize into a brown pigment
what other enzymes (apart from polyphenol oxidase) are indirectly associated with browning? (2)
- these can produce/supply reactive _______ compounds that can undergo ___-_________ _________
- ascorbic acid oxidase and lipoxygenase
- carbonyl compounds –> non-enzymatic browning
2 types of non-oxidative browning
- caramelization vs Maillard?
- sugar-sugar reactions (caramelization)
- sugar-amine reactions (Maillard reactions)
caramelization reactions occurs when any concentrated sugar solution is ________
heated
caramelization is used commercially to produce (2)
- each is generally made from what?
- caramel flavor/syrup –> made from concentrated sucrose solutions
- caramel color –> generally made from glucose syrup (more reducing)
which 2 reactions produce what type of polymers that are very dark in color? (high tinctorial power)
- dehydration and polymerization
- produce high-molecular weight water-soluble polymers
caramel colors produced in what foods (5)
baking, beer, soft drinks, whiskey, soy sauce…
Maillard reaction involves reaction of what with what (generally (2))
- of reducing sugars with primary amines (generally amino acids and proteins: free amino groups on side chains (ie: e-amino group of lysine))
which aromas are associated with Maillard reactions? (4 ish)
- coffee flavor, color and aroma (formed during roasting)
- popcorn aroma/flavor
- roasted peanuts, cashews
- baked bread aroma
condensation of primary amine and reducing sugar form a _______ ______ –> followed by formation of an _________ compound
- Schiff base
- Amadori compound
what is a key intermediate in Maillard reaction, comparable to _____ in caramelization
Amadori compound!
- comparable to HMF
converting Schiff base to amadori compound is termed what?
amadori rearrangement
can the amadori compound lose water from within the molecule?
yes
at higher temps, what are the major reactions following amadori rearrangement?
- key difference with caramelization?
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
fragmentation produces short/long chain unsat/sat _______ and _______ compounds that can ________ and/or be released as ________ into the environment (aroma)
- short-chain unsat carbonyl and amino compounds
- can polymerize or be released as volatiles
net result of reactions of amadori compound? (2)
brown pigments and volatile compounds containing nitrogen are formed
Strecker degradation is a subcomponent of the _________ reaction
- involves reaction of what with what produced by either _______ or _________ reactions
- Maillard reactions
- reaction of free amino aicds with the dicarbonyl intermediates produced by Maillard or caramelization reactions
products formed during strecker degradation? (4) which are compounds that contribute to the characteristic aroma of ________ foods
- CO2, ammonia, formaldehyde and pyrazines
- roasted foods
common strecker aldehydes? (3 + what kind of aroma)
- ethanal –> fruity, sweet aroma
- methylpropanal (malty)
- 2-phenylethanal (flowery, honey like aroma)
condensation of 2 _________ may yield pyrazine derivatives that are also powerful ______ compounds
- aminoketones
- aroma
what reaction is used commercially to produce artificial flavor compounds?
- how?
- how are these flavor compounds isolated?
- 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
in summary, which 3 reactions represent important group of reactions associated with food color and flavor?
- caramelization
- Maillard reaction
- Strecker degradation
why is evaluation of sweetness quite difficult? (2)
- how to evaluate?
- 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
which sugar is used as reference for sweetness? why?
sucrose! because it is the most abundant
rank relative sweetness from lowest to sweetest:
- xylose
- lactose
- invert sugar
- sucrose
- glucose
- maltose
- fructose
- lactose 39
- maltose 46
- invert sugar 65
- xylose 67
- glucose 69
- sucrose 100
- fructose 114
what is an invert sugar?
sucrose hydrolyzed to a mixture of glucose and fructose –> optical rotation is inverted (ie from 100 to -100)
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)
- increase temp = decrease sweetness –> attributed to change in relative concentrations of open-chain, a, and b forms which differ in sweetness
- type of sweetness (vs difference in sweetness) while others can’t…
- salts, acids, others sugars, and a host of other components (ie alcohol tend to heighten sweetness)
what is the synergistic effect of sugars?
sugars can become sweeter when mixed together than when tasted separately
sweetening power is related to sugar __________
solubility (proportional ish to sweetness determined by sensory eval ishh)
when is solubility a very important criterion? (2 ex)
when high sugar concentrations are required to inhibit microbial growth (jams, jellies)
what happens if solubility of sugar is too low?
sugar can crystallize, making the product texture grainy
limitations in solubility may be encountered in sweetened condensed or evaporated milk and ice cream, owing to what?
relatively low solubility of lactose (milk sugar)
common method to avoid sugar crystallization?
mix 2 sugars together –> inhibits their crystallization
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?
- 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
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)
- 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
sugar alcohols are used extensively in food industry as (2)
- 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
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?)
- passive
- fewer calories than sugars
- poorly metabolized –> do not contribute to caries
- caused by acids produced by microflora form fermentation of sugars
most sugar alcohols (2 ex) are more/not as sweet as sugars
- except which one? which has comparable sweetness to _________
- sorbitol and mannitol
- not as sweet as sugars
- xylitol = exception –> comparable to sweetness of sucrose
xylitol is produced from what?
- how is it produced (3)
- 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
difference between D-xylose and D-xylitol?
- xylose –> 5C –> OH on one end and COOH on one end = aldehyde
- xylitol –> 5C –> OH on both ends
