Foodchem Lecture 20
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What happens if you add glucose isomerase to D-glucose?
You make D-fructose
Why is D-fructose desirable?
Had about 1.3x the sweetening power of glucose and is considered somewhat sweeter than sucrose
Is making fructose from glucose easy/cheap?
Advances in enzyme technology have made this ultimate conversion economical and it is now used extensively
Glucose isomeras is used to convert high dextrose equivlent syrups to what?
about 45% D-fructose
Is there a commercial use for isolating pure fructose from the system?
Not really, except in special applications such as pharmaceuticals and fine chemicals
What are high fructose corn syrups very similar in composition to?
What is invert sugar?
An equimolar mixture of glucose and fructose commonly produced by the acid hydrolysis of sucrose
What is high fructose a direct competitor for due to its similarity in composition to invert sugar?
- Sucrose - especially in the soft drink sector
- (The reason for this direct comparison is that sucrose is commonly ''inverted'' to stabilize its sweetness, especially if incorporated into acidic products, a classic acidic application being the sweetening of soft drinks)
What is another reason that inversion of sugar is done, besides stabilization of sweetness?
Assist in prevention of crystallization of sucrose when used at high concentrations in food systems
Why convert sucrose to invert?
- The alpha-1 and beta-2 glycosidic linkage in sucrose is acid labile and readily hydrolyzed by heat and/or acid (over time)
- For acidic beverages (mostly soft drinks) the resukt is a change in sweetness over time
- This can be avoided by inverting sucrose prior to its addition to the beverage so its sweetness is stable, which avoids sweetness changes in the product over time
Does high fructose corn syrup have sweetness stability?
Yes - high fructose corn syrup effectively has this property built into the base product and does not require the inversion step needed for glucose
What are some other reasons HFCS is valued?
- 1. Intrinsically sweeter than glucose
- 2. More soluble than glucose
- 3. Does not crystallize readily (so good for frozen products) as it is a sugar mixture already
- 4. The syrup also becomes sweeter as temperature decreases
What has HFCS done to the economics of the sweetener industry?
Lowered demand on sucrose, so affected the supply side (Cuba, Philippines, Hawaii)
What is the annual world starch production estimated at?
- 12 million tons
- Canada produces about 2% of the total
What country is the largest producer of corn starch?
What product also derived from corn is now competing against this commodity (corn) and driving up its price?
What are the main sources of starch in the food industry?
- Texture modification
- Gel formation
- Raw material as a syrup and sweetener
- Stabilizer for emulsions
What are cross-linked starches used for in acidic foods?
To minimize hydrolysis as a result of heat treatment
Why is starch used as a dusting agent?
To preven sticking
Where are pyroconverted starches used extensively?
In the confectionary industry
How is cellulose different from starch?
- Made up of D-glucose like starch, but has beta-1,4 glycosidic linkages making it indigestible for humans but not ruminants
- (vs alpha-1,4 glycosidic linkages)
Cellulose is strictly which shape?
Linear, no branched counterpart
In what way is cellulose like amylose?
Exists as fiber-like bundles which have extensive crystalline regions - highly organized by hydrogen bonding
What are the structural components of plants?
Is cellulose durable?
- Yes, think about stability of paper and how long it takes wood to rot
- Used in textile industry as a fiber (rayon or viscose)
Is it possible to use microbial enzymes to make glucose form cellulose?
Yes, but no one has successfully applied them on a large scale to commercially produce glucose
Are there any uses to cellulose?
- Largely useless in food, functionally and nutritionally
- But modified celluloses are useful and have important functional roles
What is microcrystalline cellulose?
- Can be used as a bulking agent without contributing calories
- Also used extensivle to convert liquid products into free flowing powders by adsorption
- (Honey, cheese, syrups, etc. can be converted into free flowing powders by mixing with microcrystalline cellulose and then dispersing the coated powder into food products)
What is carboxymethyl cellulose (CMC)?
- A methylated derivative of pure cellulose formed by hydrating cellulose in a strongly basic solution and reacting cellulose with sodium chloroacetate
- The net reaction is the replacement of an OH group with a carboxymethyl on the polymer
What does the CMC process produce?
A more soluble charged polymer that is dispersible and capable of contributing viscosity to a solution as well as forming weak gels
How can you manipulate CMC gels?
Based on the average molecular weight of the polymer and the extent of derivatization
What does the presence of carboxyl groups mean in carboxymethylcellulose (CMC)?
Makes it susceptible to pH changes and may precipitate out of solution around its isoelectric point
What happens if the pH is above the isoelectric point in CMC?
The carboxyl groups repel the other polymers
What does the presence of ionized carboxyl groups allow one to do?
Use divalent ions to crosslink the polymers to form a gel
How is CMC used commercially?
CMC has found a wide application in the food industry as a viscosity enhancing agen hich contributes no calories
Why is CMC useful for ice cream?
Provides a smooth texture and inhibits ice crystal growth during storage (which causes graininess)
What are the other commercial cellulose ether derivatives (besides CMC)?
Hydroxypropyl and methyl cellulose
Are the other commercial cellulose ether derivatives affected by pH or salts?
No, because they are non-ionic
When these cellulose ether derivatives are present in sufficient concentration what can they form?
Heat setting gels which associate by hydrogen bonding
What are pectic substances?
Are polysaccharides associated with the middle lamella found between plant cell walls
What do pectins do within plants?
Act as binders to hold the plant cells together
What is pectin used for commercially?
Used extensively as a gelling agent for the preparation of jams and jellies
Where do the properties of pectic substances come from?
Complex and non-homogeneous and their properties depend on the plant source
What are the usual sources of pectin?
Typically citrus peel or apple pomace extracts
What is pectin largely made up of?
Polygalacturonic acid, a complex polysaccharide, which can be extracted from plant material
What is the basic unit of polygalacturonic acid?
Galacturoic acid, an acid analog of galactose, with its carboxyl group being either the free acid (or its salt) or esterified to a methyl group
How are pectic substances classified?
Based on their relative degree of methylation (DM)
What are the three categories of pectic substances?
- 1. pectic acids
- 2. pectinic acids
- 3. pectin
What are pectic acids?
Polymers of predominantly polygalacturonic acid, free of methyl ester groups
What are pectinic acids?
Polymers of polygalacturonic acid with some degree of methylation (low methoxy pectin)
What are pectins?
Polymers of polygalacturonic acid with >50% of the carboxyl groups methylated (regular --> high methoxy pectins)
What does the degree of methylation of pectin depend on?
- The source
- The method of extraction
What does the degree of methylation of pectin have an influence on?
On the functional properties of the polysaccharide
How is pectin used in jams and jellies?
In combination with sugar and acid, these form a gel, which in combination reduces water activity and pH to inhibit microbial growth after a mild thermal process
Normally, how many carboxyl groups of pectin are methylated?
What is the ideal pH range for pectin to form a suitable gel?
What must be present for pectin to form a gel?
Sugar, acid, and water
Why does pectin form a gel under the right conditions?
The repulsive charge of the residual carboxyl groups is reduced, while the sugar competes for water of hydration allowing the polymers to associate in a 3D manner to form a gel
Can pectin form a gel without sugar?
Yes, but it is uneconomical as large amount are required
How is most pectin obtained?
As a by-product of the citrus and apple juice processing (orange/grapefruit peels or apple pomace) being extracted using dilute acid
Why is acid extraction required for pectin?
Because the methyl groups are labile and are readily lost at neutral or basic pH
What does the degree of methylation of pectin determine?
Both quality and price of pectin
What is the typical use for pectins with a DM of about 50?
Serve well for jellies (no suspended material) where the rate of gel setting is of no major consequence
What is the typical use for pectins with a DM of >74?
For jams, which contain particulate matter, a rapid setting or high methoxy pectin is usually required
What is different about high methoxy pectin?
- Requires special extraction procedures and will set at higher temperatures
- Useful in preventing particulate material from settling out (more expensive)
What are low methoxy pectins useful in the manufacturing of?
Diet jams and jellies
How are very low methoxy pectins produced?
By controlled hydrolysis of the methyl group using either alkaline conditions or using the enzyme pectin methyl esterase
How is a gel produced in very low methoxy pectins?
Much like in CMC, a gel can be produced using divalent ions to crosslink the polymers at the carboxyl groups
What's required for low methoxy pectins to produce a gel?
- No sugar needed
- pH range is much broader
What is required for diet (low sugar) jams/jellies to be self stable?
Antimicrobial agents (like benzoic acid or proprionic acids) are generally required to compensate for the missing osmotic (water activity) reduction hurdle when sugar is not used in the product
Where do seed gums come from?
Seeds of some plants contain unique polysaccharides called gums rather than starch as their germination energy source
What are the 2 important seed gums?
Guar gum and locust bean gum
Both important seed gums are what?
- Galactomannans (polymers of galactose and mannose)
- Important ancillary hydrocolloids used in foods
What is guar gum derived from?
From a leguminous plant which is widely grown in India and Pakistan
What is the ratio of mannose to galactose in guar gum?
How long has guar gum been on the market?
Been produced on a commercial scle for the food industry for >30 years and used locally for millennia
What is guar gum made up of?
A linear chain of mannose (beta-1,4) with D-galactose branches interspersed roughly every other mannos unit (alpha-1,6 linkage)
What is the most important property of guar gum?
- Is its ability to hydrate very rapidly in cold water and form a very viscous solution
- This is particularly useful property in assisting in the rehydration of dry powders used in ''instant products'' - ex. instant soup (rehydrating agent) by including it in the formulation
What is the structure of locust bean gum like?
- Another galactomannan polysaccharide which also has beta-1,4 glycosidic linkages for the mannose main chain, with galactose branches present approximately every fourth or fifth mannose unit (~4-5:1)
- Somewhat similar to guar, except that it requires more heating to go into solution
What is locust bean gum used extensively for in the food industry?
- Used in ice cream to impart a smooth meltdown and freeze/thaw resistance
- Also used to produce a softer dough in the baking industry
- Guar gum and locust bean gum are largely inerchangeable from a functional standpoint and used accordingly depending on price and availability
What are the general properties of guar an locust bean gum?
- 1. Viscosity does not alter much with pH
- 2. Stability and rheology are not affected much by processing
- 3. These gums are compatible with starches, cellulose and other gums
- Often used to subtly modify the texture of other gums or starches
How are plant exudate gums obtained?
- By wounding specific tropical trees and shrubs and collecting the exudate
- Excluded from USA embargo of sudanese products
What are the main exudate gums?
- Gum arabic, ghatti, karaya, and tragacanth
- All these gums are all charged heteropolysaccharides
What is the backbone structure of gum arabic?
C1-C3 linkage with branches at C6
Why is gum arabic charged?
- Due to the presence of glucuronic acid (COO-)
- The distinguishing feature of gum arabic that is extremely soluble in water - can make up to 50% solutions (W/W)
What are the uses of gum arabic?
- As a flavor carrier/dispersant
- Commonly used to encapsulate citrus essential oil flavor emulsions into a water-dispersible powder by spray drying
- Also extensively used to inhibit sugar crystallization and as a thickener in candies, jellies, and chewing gums
- Is also used as a foam stabilizer in beer
Which functional polysaccharides can be extracted from seaweeds?
- 1. Irish moss - Carrageenan
- 2. Red algae - Agar
- 3. Giant kelp (brown algae) - Alginates
What function do these functional polysaccharides have in seaweed?
- Same function as pectin in plants - hold cells into place
- These polysaccharides are the ''pectins of the sea''
What is carrageenan composed of?
Composed of D and L-galactose and along with 3,6 anhydro-D-galactose units with some of the galactose moieties being sulfonated
What does the degree of sulfonation depend on in carrageenan?
Depends on the source and the method of extraction (again much like pectin)
What are the 2 fractions of carrageenan?
Linear and branched
What does carrageenan have the unique ability to do?
To complex with milk proteins to form a very light gel
What do the unique properties of carrageenan allow it to be used as?
- Used extensively to suspend insoluble chocolate solids in chocolate milk
- Also used in ice cream to minimize drip
- In research often used to separate proteins by size exclusion column chromatography
What seaweed species is agar from?
What is the unique property that agar has?
Unique ability to form a thermo-reversible gel around 40°C
What is agar best known for?
Its use in making bacteriological media due to the hysteresis between its gel setting and melting temperatures
Agar dissolves at what temperature? Sets at what temperature?
- Dissolves 95-100°C
- Sets at 40°C
How are the unique agar gel setting and melting temperatures a benefit?
Avoids thermal shock to the bacteria, but entraps them
What else is agar used extensively as (in the food industry)?
As a gelling agent and binding agent for bakery products, confections, meat and fish products
What type of seaweed are alginates from?
Brown algae (giant kelp)
What are alginates made up of?
Made up of largely linear polymers of D-mannuronic and L-glucuronic acids
What does the presence of carboxyl groups in alginates make them sensitive to?
To pH and ionic strength of a food system
How are alginates extracted?
Using base to convert them into their sodium salts so that they will be readily soluble
How can gels be produced in alginates?
Using divalent ions such as calcium
What are alginates commonly used as?
- A bacteriological medium
- No heat required, just crosslink with CaCl2
What are slimes?
- Extracellular polysaccharides
- Many microorganisms produce them
- A wide variety have been isolated and investigated - some very useful ones have been discovered
What are dextrans?
- Type of microbial gum
- Extra-cellular polysaccharide produced by leuconostoc mesenteroides made up of glucose
How is dextran commercially produced by?
The fermentation of sucrose
How does dextran differ from starch?
- Because the glucose units are linked together by alpha-1,6 rather than alpha-1,4 linkages and uses alpha-1,3 linkages for branching
- Also, not an organized crystalline granule, but is excreted as an amorphous polymer
Is dextran easy to digest?
Largely non-nutritive - difficult to digest
How can the chain length and degree of branching of dextran be controlled?
Can be controlled genetically
What are the commercial uses of dextran?
Dough conditioner, ice cream stabilizer, and providing protective films on meat and fish products
What is xanthan gum?
Complex heteropolysaccharide produced by xanthomonas campestris made up of D-glucose, D-mannose and D-galacturonic acid present in a ~3:3:2 ratio
What are the major properties of xanthan gum?
- 1. Excellent heat stability - almost no change in viscosity with temperature
- 2. Viscosity is highly affected by shear, but returns immediately after shear is removed (pseudoplastic)
- 3. Viscosity is practically independent of pH - very useful in highly acidic foods
- 4. Will form a gel with locust bean gum in very low concentrations
What is xanthan gum useful for?
In production of instant puddings or gels without the need for heat
What is needed for the organism xanthamonous campestris to produce xanthan gum?
It can produce xanthan gum from a variety of cheap carbohydrate substrates
What is the dominant polysaccharide or gum available for food use?
How does one decide which polysaccharide provides the characteristics which best fits the product and pricing?
One generally just has to try a number of different ones
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