Food Chem 17
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In what conditions are sugars relatively stable?
Relatively stable in water at neutral pH, at room temperature
What happens if sugars are inw ater outside of the neutral range (on either side) or with strong heating?
- Elonization may take place: Double bonds become 'mobile'; monosaccharides may change from one form to another
- Acid or base catalysed, with heat speeding up the overall rate
What is Elonization?
double bonds become 'mobile'; monosaccharides may change from one from to another
What is the general reaction of enolization?
What is an alkaline reaction of sugars?
- Starting with concentrated solution of pure glucose, regluzed under alkaline conditions
- At the end, a variety of different epimers
What are the steps for aldose to ketose conversion of enolization with base?
- 1. Remove the alpha proton
- 2. reprotonate on O
- 3. Reprotonate the O o C2
- 4. Reporonater o C1
What does base catalyzed enolization induce?
- Induces 'wandering' of the double bond
- Epimerization of both aldo and keto sugars
- Conversio from aldoses to ketoses
- Results in complex misture, even when starting with a single sugar
Is base catalyzed enolizatio reversible?
Only if temperature is not too high
What happens in base catalyzed enolization if temperature is high?
- Water evaporates
- Dehydration and oxidative scission
- Carbohydrates breakdown into smaller units
What is the general reaction of sugars in acid catalysis?
- Results similar to base catalysis, but overall tendency is conversion of ketoses to aldoses
- ORganic acids, pH range 3-6
What is the pH range of organic acids?
What happens in dehydration reactions of sugars?
- After loos of free wter, sugar molecules will lose water from within their molecular structure
- Basis for carmellizatio (flavor and color) as well as Maillard reaction
- Common intermediate is HMF (hydroxymethyl furfural)
What is a common intermediate of dehydration reactions?
HMF- hydroxymethyl furfural
What is the basis for carmellization and Maillard reactions?
What is the general procedure of sugar dehydration starting with 70% glucose solution?
- Free water lost, concentration goes up, as well as temperature
- Water eliminated internlly: 2 possible pathways
- pH will affect reaction
What are the 2 possible pathways of water loss in a sugar dehydration reaction?
- Loss of H2O between 2 glucose units (random dimerization; not predominant)
- Loss of H2o from one sugar molecule (highly reactive HMF)
How does pH affect sugar dehydration reactions?
Low pH minimizes enolization prior to dehydration
How is carmel formed?
- Many unsaturated cyclic products and their breakdown products, which may polymerize via the aldol condensation reaction
- Formation of amorphous brown pigments called carmel color
Which reaction is prefered under basic conditions?
- Enolization reaction is more prevalent
- dehydration limited in early stages of the reaction
What are the end products under basic conditions?
- Fragmentation of sugar molecule prevalent
- End products tend to be acidic
- If not properly buffered, will eventally promote dehydration
Do acidic or basic conditions yield brown colors of pigments?
Both basic and acidic conditions yield brown colors or pigments
What is browning often associated with?
Headed products or natural decay processes
What are the two categories of browning reactions?
- Oxidative or enzymatic browning
- Non-oxidative of nonenzymatic browning
What is enzymative browning?
- Associated with plant tissue browning
- Enzyme: as polyphenol oxidase
- Oxidation of hydroxyphenols into quinones
- Quinones may react further and eventually polymerizeinto brown compounds
What is the end product of non-enzymatic browning?
End product attributes depend on type of sugar and reaction conditions
What is the difference between carmellization and maillard reactions?
- Carmellization= sugar-sugar
- Maillard reactions= sugar-amine
How are carmel flavors formed from glucose?
- From concentrated sucrose solution:
- Hydrolysis to glycose and fructose
- Ring opening: equilibration of both anomers
- Enolization and epimerization
- Dehydration: some polymerization
What is the result of polymerization of unsaturated and reactive carbonyl compounds in a carmellization reaction?
Viscous brown solution with typical carmel flavors
What happens to a carmelization reaction upon cooling?
May crystallize. or form amorphous candy
How is carmel color formed from a glucose syrup?
- Heat treatment with dilute H2SO4
- Acid partially neutralized with NH3 and basic salts as the reaction progresses
- Conditions which promote sugar fragmentation and prevent development of bitterness
- Ammonia has double role of neutralizing agent, and cross linker promoting polymerization
- Few sugars are left intact
- Dehydration and polymerization are main reaction
- High molecular weight products
- Water soluble
- Very dark--> high tinctorial power
What is the role of H2SO4 in carmel color production?
- Sulfite plays a role in reducing the isoelectric point of the polymer formed
- Polymers with charged SO3- groups incorporated: remain charged at pH as low as 2.5
- Carmel polymers will thus remain in solution in the low pHs encountered in carboanted beverages
What do maillard reactions require?
Reducing sugars and primary amines
What are some examples of miallard reactions?
- Roasted peanuts
- Baked bread aroma
What can the maillard reaction be considered with respect to carmellization?
The maillard rection can be considered a nitrogen version of the carmellization reaction
How are maillard reactions used to produce flavor constituents reminiscent of specific products?
- Often by exploiting a particular sub-reaction called strecker degredation:
- reactio of free amino acids with dicarbonyl intermediates coming from either the maillard reaction of from carmellization
What is strecker degredation?
Reaction of fre amino acids with dicarbonyl intermediates coming from either the maillard reaction of from carmellization
What are produced from strecker reactions?
A wide variety of 'artificial' flavor/aroma compounds may be produced, by careful selection of sugars and amino acids
What is the downside of maillard reaction?
- Acrylamide formation- considered a compound of high concern
- Present in french fries, potato chips, coffee, prune juice
- Asparagine+carbs: natural pathway for prouction of acrylamide
What is the reference sugar for relative sweetness?
What is the general order of relatie sweetness?
- Invert sugar-65
- Xylose- 67
- Fructose 114
How does temperature affect percieved sweentess?
- Sweetness tends to decrease with increased temperature
- Change in relative concentration of the various forms
- Open chain or cyclic alpha and beta
What are some variables ins weetness perception?
- Tends to decrease with increased temperature
- Personal perception: distinguishing sucrose from glucose for example
- Presence of other components (salt, acids, alcohol)
- Synergy- Mixtures of different sugars may taste sweeter than the sum of individuals
What is the best way to evaluate sweentess?
When are sugar solubilities important?
Important when making jams and jellies, where high concentrations are required to inhibit microbial growth (lower water activity)
What happens if solubility is too low?
If solubility os low, sugar may crystallize out- textyre will be gainy or sandy
How is crystallization avoided?
- Crystallization may be avoided by mixing two sugars together
- Crystal lattie much more difficult to form where more than one sugar is present
How is inversion acquired?
Inversion may be enzymatic, or by heating in the presenc of ascorbic acid
How does caramilk avoid crystallization?
Uses a mixture of sugrose, glucose and fructose
What are some other uses for solubility of sugars?
- Coloor development in cured meats
- Reducing conditions (reducing sugars) prevent oxidation of meat pigments
What is a sugar alcohol?
- Sugar here the carbonyl group has been reduced to an alcohol
- Similar to lipid hydrogenation
Where are sugar acohols used?
- Reduced caloric contribution; ot as sweet as sugars
- Also as water binding agent with reduced sweetnening effet
- Humectnat to bind water and plasticize intermediate moisture foods
- Bulking agend and viscosity enhancers with artificial sweeteners
What are some common sugar alcohols?
- Glucitol (relative sweetness= 51)
- Mannitol (relative sweetness= 54)
What is xylitol?
A very sweet sugar alcohol
What is xylitol similar to in terms of sweetness?
Similar to sucrose in terms of sweetness
What is important about xylitol?
- Poorly metabolizxed y microbial flora in the mouth
- Helps prevent dental caries
- Unique in that it provides both a sweetening sensation and cooling sensation
What is the sugar form of xylitol?
How is xylose naturaly found?
- Widely foun d in nature as xylan, a polymer of xylose
- Plant sources: birch tree chips, almont and pecan shells, corn cobs
How is xylitol produced?
Xylan hydrolyzed to xylose; xylose hydrogenated in xylitol
How is xylitol produced?
- Xylitol may be produced by action of yeast: Fermenting xylose into xylitol
- Or enzymativ conversion of glucose to xylitol
How is xylitol used in canada?
Only in chewing gum in canada
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