Food Chem 17

Relatively stable in water at neutral pH, at room temperature

• 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

double bonds become 'mobile'; monosaccharides may change from one from to another

• Starting with concentrated solution of pure glucose, regluzed under alkaline conditions
• At the end, a variety of different epimers

• 1. Remove the alpha proton
• 2. reprotonate on O
• 3. Reprotonate the O o C2
• 4. Reporonater o C1

• 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

Only if temperature is not too high

• Water evaporates
• Dehydration and oxidative scission
• Carbohydrates breakdown into smaller units
• flavors

• Results similar to base catalysis, but overall tendency is conversion of ketoses to aldoses
• ORganic acids, pH range 3-6

pH 3-6

• 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)

HMF- hydroxymethyl furfural

Dehydration reactions

• Free water lost, concentration goes up, as well as temperature
• Water eliminated internlly: 2 possible pathways
• pH will affect reaction

• Loss of H2O between 2 glucose units (random dimerization; not predominant)
• Loss of H2o from one sugar molecule (highly reactive HMF)

Low pH minimizes enolization prior to dehydration

• Many unsaturated cyclic products and their breakdown products, which may polymerize via the aldol condensation reaction
• Formation of amorphous brown pigments called carmel color

• Enolization reaction is more prevalent
• dehydration limited in early stages of the reaction

• Fragmentation of sugar molecule prevalent
• End products tend to be acidic
• If not properly buffered, will eventally promote dehydration

Both basic and acidic conditions yield brown colors or pigments

Headed products or natural decay processes

• Oxidative or enzymatic browning
• Non-oxidative of nonenzymatic browning

• Associated with plant tissue browning
• Enzyme: as polyphenol oxidase
• Oxidation of hydroxyphenols into quinones
• Quinones may react further and eventually polymerizeinto brown compounds

End product attributes depend on type of sugar and reaction conditions

• Carmellization= sugar-sugar
• Maillard reactions= sugar-amine

• From concentrated sucrose solution:
• Hydrolysis to glycose and fructose
• Ring opening: equilibration of both anomers
• Enolization and epimerization
• Dehydration: some polymerization

Viscous brown solution with typical carmel flavors

May crystallize. or form amorphous candy

• 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

• 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

Reducing sugars and primary amines

• Coffee
• Popcorn
• Roasted peanuts
• Baked bread aroma

The maillard rection can be considered a nitrogen version of the carmellization reaction

• 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

Reaction of fre amino acids with dicarbonyl intermediates coming from either the maillard reaction of from carmellization

A wide variety of 'artificial' flavor/aroma compounds may be produced, by careful selection of sugars and amino acids

• 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

sucrose?

• Lactose-39
• Maltose-46
• Invert sugar-65
• Xylose- 67
• Glucose-69
• Sucrose-100
• Fructose 114

• Sweetness tends to decrease with increased temperature
• Change in relative concentration of the various forms
• Open chain or cyclic alpha and beta

• 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

Taste panels

Important when making jams and jellies, where high concentrations are required to inhibit microbial growth (lower water activity)

If solubility os low, sugar may crystallize out- textyre will be gainy or sandy

• Crystallization may be avoided by mixing two sugars together
• Crystal lattie much more difficult to form where more than one sugar is present

Inversion may be enzymatic, or by heating in the presenc of ascorbic acid

Uses a mixture of sugrose, glucose and fructose

• Coloor development in cured meats
• Reducing conditions (reducing sugars) prevent oxidation of meat pigments

• Sugar here the carbonyl group has been reduced to an alcohol
• Similar to lipid hydrogenation

• 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

• Glucitol (relative sweetness= 51)
• Mannitol (relative sweetness= 54)

A very sweet sugar alcohol

Similar to sucrose in terms of sweetness

• Poorly metabolizxed y microbial flora in the mouth
• Helps prevent dental caries
• Unique in that it provides both a sweetening sensation and cooling sensation

xylose

• Widely foun d in nature as xylan, a polymer of xylose
• Plant sources: birch tree chips, almont and pecan shells, corn cobs

Xylan hydrolyzed to xylose; xylose hydrogenated in xylitol

• Xylitol may be produced by action of yeast: Fermenting xylose into xylitol
• Or enzymativ conversion of glucose to xylitol

Only in chewing gum in canada