Foodchem Lecture 18
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How is glucose obtained?
By hydrolysis of something like corn starch
What is special about the sensation of sweetness?
it is universally accepted
How can sweetness be evaluated?
Usually by sensory evaluation
Why is it so difficult to evaluate sweetness?
It is a subjective response
What are some variables to sensory methods?
- Psychological variability
- whether one smokes
What is the ranking order of sweetness?
- Lactose 39
- Maltose 46
- Invert 65
- Xylose 67
- Glucose 69
- Sucrose 100
- Fructose 114
What is the reference compound for sugar sweetness?
What are some variables involved in how the sweetness response is perceived?
- A) Sweetness tends to decrease with increasing temp
- B) Sweetness is not always perceived the same way by different people - some people can readily tell that glucose has been substituted for sucrose, not so much by the sweetness intensity but by the type of sweetness
- C) The relative sweetness of sugar changes in presence of other components such as salts, acids, other sugars, etc.
- D) Sugars often become sweeter when mxed together than when tasted separately, thus having a synergistic effect
Why does sweetness tend to decrease with increasing temp?
Attributed to the change in the relative concentrations of the open chain forms, alpha and beta forms which differ in sweetness
Alcohol tends to do what to sweetness?
Heighten the sensation
What does it mean that sugar has a synergisitic effect?
Sugars often become sweeter when mixed together than when tasted separately
What does formulation of sugars in a food product have to be based on?
What is the sweetening power of sugars strongly related to?
What is the saturation point for some sugars?
- Lactose 20%
- Maltose 40%
- Glucose 60%
- Sucrose 70%
- Fructose 80%
What is solubility also very important for making?
Making jams and jellies where high sugar is required to inhibit microbial growth (decreasing water activity)
If sugar solubility is low, what can happen?
Sugars can crystallize out of solution and make the product grainy and sandy
In which products is the low solubility of lactose a problem?
- Sweetened condensed milk
- Ice cream
What is a common means of avoiding or minimizing sugar crystallization?
Mixing 2 sugars together
Why do impure or mixed sugar systems inhibit crystallization?
- (Secret to caramilk)
- Enzymatic conversion of sucrose to invert sugar (sucrose/glucose/fructose)
Why are sugars often used in meat curing?
- Help color development
- Provide reducing conditions, which prevent oxidation of meat pigments
How are sugar alcohols used in the food industry?
Lower caloric contribution relative to sugars (diet products) and as water binding agents, but with a reduced sweetening effect
How are sugar alcohols produced commercially?
- By hydrogenation, where the aldehyde group is reduced to a hydroxyl
- Process much like hydrogenation of fats/oils to remove double bonds
Why do sugar alcohols contribute fewer calories relative to the amount consumed?
Absorption is by passive diffusion
What is a humectant?
Water binding agent, plasticizes intermediate moisture foods
Why are sugar alcohols used as humectants?
- Bind water and plasticize intermediate moisture foods
- Reduces the water activity of the food system without inducing excessive sweetness in the productr
What are the more common sugar alcohols?
Glucitol and Mannitol
What are the relative sweetnesses of glucitol and mannitol?
51 and 54 respectively
What are glucitol and mannitol used extensively as?
Bulking agents and viscosity enhancers in the presence of artificial sweeteners to provide ''body'' which is otherwise lacking
What is the sugar alcohol sweetness exception?
Xylitol - very sweet
Why is xylitol a possible better sucrose alternative?
- Similar in sweetness to sucrose, but poorly metabolized by the microbial flora of the mouth
- This provides a unique approach to controlling dental carries which are caused by the acids produced by the fermentation of sugars - limited action when xylitol is the sweetener
What is the other sensation (besides sweetness) that xylitol provides?
Describe the structure of xylitol
- A pentitol
- A normal metabolic carbohydrate intermediate
- Its sugar form is widely found in nature as a polymer of xylose
Where is xylitol present in nature?
In most plant sources - birch tree chips, other hardwood chips, almond and pecan shells, cottonseed hulls, corn cobs
How much xylan do the plant sources of xylitol contain?
about 20-25% of their dry weight is xylan which can be hydrolyzed to xylose and hydrogenated using a nickle catalyst to produce xylitol
What are some other approaches to producing xylitol besides hydrolyzing xylan and then hydrogenating the xylose?
- 1. Use yeasts to convert xylose to xylitol by fermentation
- 2. Enzymatic conversion of glucose to xylitol
Where is xylitol used extensively in food products?
What is the only food product in Canada that xylitol has been allowed as a sweetener?
What are the 3 most common polysaccharides?
Starch, cellulose, pectin
What are the seed gums used in food?
guar gum, locust bean gum
What are the plant exudate gums used in food?
gum arabic, gum tragacanth
What are the seaweed gums used in food?
What are the microbial gums used in food?
dextran gum, xanthan gum
What are polysaccharide properties a function of?
- Their sugar makeup
- The presence of unique functional groups
- Their chain length and degree of branching
- Modifications made to the base structure
What is the most abundant form of stored carbohydrate?
Cellulose - structural component of plants and is indigestible
What is the second most common source of carbohydrate?
Starch - digestible, and our main source of dietary carbohydrate
Is starch present in plants?
Yes, commonly used as an energy store in seeds and tubers for plant germination
What are some main uses for starch that has been isolted from plant sources and used as a functional ingredient?
- As a thickening or texture modifying agent
- As a stabilizing agent
- As an extender/filler
- As a source of sugars and dextrins
In what form is starch found in most plant sources?
As hard, anhydrous, compact, water insoluble granules
Starch is made up exclusively of what?
Starch consists of how many fractions?
What are the 2 fractions of starch?
- 1. Amylose - long chain linear polymer composed of D-glucose linked by alpha-1,4 glycosidic bonds
- 2. Amylopectin - a branched polymer composed of D-glucose which has alpha-1,4 linkages in its linear portions and has alpha-1,6 bonds present at its branch points
Alpha-1,4 glycosyl linkages lead to which structure?
A helical polymer
How are the ratios of amylose and amylopectin determined in plants?
Genetically predetermined ratios
Waxy maize starch is made up of what?
almost 100% amylopectin
Regular corn starch is made up of what?
About 28% amylose and 72% amylopectin
High amylose corn hybrids are made up of what?
About 70% amylose
What are high amylose corn hybrids used for?
Specialty purposes (film forming properties, or as biodegradable plastics)
How are starch granules isolated from plant sources?
- Wet milling
- Centrifugated out of solution
What is the result of starch granule isolation?
Dry white powder composed of discrete starch granules
What is each granule of the dry white starch powder composed of?
Anhydrous amylose and amylopectin tightly bound and held together by intermolecular hydrogen bonding between the polymer chains
What is the definition of gelatinization?
Heating in the presence of water disrupts the hydrogen bonds between chains, hydration leads to transition from an anhydrous structured crystalline granule to the formation of an amorphous, colloidal solution and/or gel
What are the steps of gelatinization?
- Gradual imbibing of water and swelling of the granule facilitated by heat and hydrogen bonding of water with the polymer in the starch granule interstices
- Increasing kinetic energy breaks the sugar-OH hydrogen bonds replacing them with sugar-OH to water-OH hydrogen bonds
- Thus the starch granule slowly imbibes water and swells
- Under plane polarized light, the birefringent pattern of the starch granule slowly disappears
- Loss of granular integrity generally occurs between 60-70°C with the temp of gelatinizaion being dependent on the starch source
- When the granules have swollen extensively (100x original size) the product becomes pasty and the viscosity of the solution increases drastically
- If this solution is stirred, the granules will break up and disperse to form a colloidal solution
- If the concentration of starch is high and is allowed to cool it will form a gel
- At lower concentrations one may obtain a viscouse solution rather than a gel
What does the final product of gelatinization a function of?
- Concentration of starch
- Its amylose:amylopectin ratio
- The average molecular weight (chain length) of the polymers
- Rate of cooling
- Presence of other components in solution
What is amylose like in solution?
Linear polymer which tends to form long thin helical filament-like structures in solution
Is the solubility of amylose high or low? Why?
Very low solubility as it readily hydrogen bonds to neighboring polymers to form large aggregates which precipitate out of solution
Is amylose useful on its own in food systems?
No as it is effectively insoluble
What does amylopectin look like?
Highly branched water dispersible macromolecule
What is amylopectin like in solution?
- Forms a viscous solution
- Branching produces a tangled net-like web in solution
When amylose and amylopectin are mixed together in solution, what happens?
Amylose acts as a cross linking agent between amylopectin webs which produces a 3D gel that traps water within its structure
When does a gel form?
- When the kinetic energy of the system is reduced sufficientl to allow random hydrogen bonds to reform
- The extensively hydrogen bonded starch molecules trap water within the matrix formed, producing a gel
How are starch gels affected by time?
Tend to become firmer with time - generally an undesirable behavior in food systems
What is retrogradation in starch gels due to?
Continued hydrogen bond reformation over time
What are the 2 problems which arise with starch gels?
- 1. the texture of the gel changes over time which is generally undesirable
- 2. syneresis may occur, where a gel tightens up so much that it starts to exude the trapped water
What does retrogradation do to starch?
- Gel becomes more opaque with time - can also be undesirable
- Responsible for the staling of bread and stiffening of pie fillings
What kind of gel does normal corn starch make?
- 28% amylose
- Makes rigid opaque gel
What kind of gel does waxy maize make?
- Almost devoid of amylose
- Will not form gel at all
What kind of gel do potato and tapioca make?
- About 23% and 28% amylose
- Form reasonably stable solutions and gels relative to retrogradation
- Tend to maintain gel clarity and do not stiffen appreciably with time
Why do potato and tapioca starches hold up so well to retrogradation?
- Hydrogen bonds continue to develop, but at a much slower rate
- Attributed to the higher average molecular weight of potato and tapioca starched having longer chain length making it more difficult to line up for hydrogen bonding as a function of time
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