lecture 7-cell wall/cellulose

Question 1

• what do cell walls do in plant cells?

Answer 1

• provide sturcture
  
  • provide tensile strength
  • protect against mechanical and osmotic stress
  • allows for trugor pressure to develop which is what lets water from soil come up into the plant tissues

Question 2

• what is the structure of cell walls?

Answer 2

is made of three main compnents cellulose, liginin and hemicellulose.

Question 3

• describe the parts of the cell wall: liginin, cellulose and hemicellulose

Answer 3

cellulose: the main component and is a polysachride made up of D-glucose units (B-1,4)

hemicellulose: is made up of different sugars with xylose and arabinose being the main ones 

ligin: is not a carbohydrate techinically but because of its close association with dietary fiber component it affects physiological effects and is therefore classified as a dietary fiber. It is a organic polymer of phenol sub units

Question 4

• what does liginin do?

Answer 4

it acts as a binder of cellulose fibers to add strength and stiffness to the plant cell and will inhbit the cell wall digestion of polysachrides.

Question 5

• whats special about the cell walls of cereals

Answer 5

have beta-D-glucans

Question 6

• whats the function in food of cell walls?

Answer 6

• provide a barrier to cellular contents meaning there is a slow penetaration of water and therefore swelling of starch granules
  
  • cell wall fragments absorb water and oil allowing food to be moist
  • in whole grain bread the bran will disrupt gluten network making a more coarse product

Question 7

• describe the role of cellulolytic microganisms.

Answer 7

These microgoranisms from bacteria and fungal produce enzymes which can degraded cellulose by hydrolysis either through aerobic degradtion which will produce cellobiase and glucose or by anaerobic degradation which will make glucose and cellodextrins. The fate of both of these products will be to go into fermantion. From aerobic conditions carbon dioxide, hydrogen and organic acids will be produced and from anaerobic conditions methane and carbon dioxide will be produced.

Question 8

• explain how comerical cellulose is prepared

Answer 8

1. hot sodium hydroxide an alkali treatment is used to extract cellulose from wood pulp
   
   2. further treatment is done to remover other polysachride s and liginin
   3. further purification is done by sodium hypochlorite

Question 9

• how is the quality of cellulose measured?

Answer 9

by the amount of alpha cellulose which is insoluble in alkali solution, alpha is the purest form and you want more of it

Question 10

• how is powdered cellulose obtained

Answer 10

purified cellulose is miled to reduce the size.

Question 11

• what is the use of powdered celulose

Answer 11

large particles are used for providing bulk as they absorb more water and they are non-caloric

small particles are used for producing products with a smoother texture

often added to cheese that is grated or shredded to prevent caking

Question 12

• how is microcrystalinine cellulose made

Answer 12

it is made by partially depolymerizing cellulose through partial hydrolysis. Which will then be grinded to reduce paritcle size and purified.

Question 13

• what are the functions of microcrystaline cellulose

Answer 13

1. natural source of dietary fiber
   
   2. non-caloric bulking agent
   3. anti-caking agent
   4. emulsifer
   5. fat substitute

Question 14

• what do both MCC and powdered cellulose have in common?

Answer 14

insoluble in water and does not gel

Question 15

• what are the three substuations that can occur on modified cellulose?

Answer 15

1. carboxymethylcellulose
   
   2. methylcellulose
   3. hydroxypropcellulose

Question 16

• what is the theoritical range for degree of substituation and why

Answer 16

0-3 because there are only 3 available hydroxyl groups per monomer at position 2,3 and 6

Question 17

• How do you calculate substitutions?

Answer 17

of substitutions over number of glucose units

Question 18

• what is an ether molecule and are they in pectin or cellulose

Answer 18

ether is R-O-R which is in modified cellulose and pectin has esters

Question 19

• what do the properties of cellulose derivatives depend on?

Answer 19

1. degree of substituation so the amount of esterfication
   
   2. what the substituent group
   3. the average molecular weight of the derivatized cellulose molecules
   4. distrubtion of substieunt groups on 2,3 or 6th position
   5. distrubtion of the substient groups on the cellulose chain like if they are clustered or random

Question 20

• describe the carboxymethyl cellulose (CMC)

Answer 20

It is produced by reacting cellulose with chloroacetic acid, introducing carboxymethyl groups into the cellulose chain. This modification significantly alters the properties of cellulose, making CMC soluble in both hot and cold water it has a DS of 0.7-0.8.

- The presence of carboxylate groups in CMC makes it anionic, which contributes to its effectiveness in stabilizing and thickening by repulsion between negatively charged groups along the polymer chain.

Question 21

• describe what increases water solubility of CMC

Answer 21

decreased DP and increased carboxymethyl substitution

Question 22

• describe the viscosity of CMC

Answer 22

is increased with increased DP and increased concentrations. It has thixotropic behaviour.

Question 23

• how are the rheological propertiers of CMC affected by pH

Answer 23

at a pH between 3 adn 4 repulsion beyween the molecules will decrease because the hydrogen will remove the negative charge which caused repulsions which will allow for more assocations to be made and increase the viscosity

below 3 though percepitation of aggeration will hapen 

above 11 hydrolysis will happen and viscosity will decrease over time

Question 24

• what are the functions of CMC

Answer 24

• thickeners
  
  • protien stabilizer

Question 25

• how are methylcellylose and hydroxypropylcellulose made and what is the result ?

Answer 25

methyl: by treating with methyl chloride creating methyl ether groups with a DS of 1.1-1.2

hydroxy: by treating with propylene oxide and methyl chlodride creating bulky hydrophobic side chains with a DS of 0.02-0.3

Question 26

• Give the probable explanation for thermal gelation of HPMC (Hydroxypropyl methylcellulose) solutions

Answer 26

HPMC has hydrophobic (C-C-C) and hydrophilic regions (cellulose from OH). At lower tempertures the hydrophilic interactions dominate so it can dissolve in water but at certain temperature there are suffienct hydrophobic interactions that allow for crosslinks to form and a gel to form . Cooling will cause the reverse to happen and it will go back to liquid form

Question 27

• describe the properties of MC and HPMC aka MC and how they are useful

Answer 27

• they disperse easily in cold water because they are fully soluble as the hydrophobic bulky sidechains prevent intermolecular associations which is useful for food products that are heated to prevent oil from entering deep fried layer. so they protect
  
  • they are surfactants as they reduce the surface tensions bewteen two phases as either an emsulifer between oil and water and liquid and gas as a foaming agent.

Question 28

• what are MCs used for?

Answer 28

• in bakery products because of film formation
  
  • in deep fried foods to prevent oil absorbtion
  • salad dressings as an emuslifer and stabilzer
  • whipped toppings asa foam stabilzer

Question 29

• what are emulsions

Answer 29

fine dispersion of minute droplets of one liquid which is not soluble in another

Question 30

• how do substances like MC stabilze emuslifcation

Answer 30

1. by thickening and therefore increasing the viscosity they will create kinetic stabilzation by slowing down the rate of collisions between droplets preventing coalescene
   
   2. by inhibiting coalescene by creating a protective coating on droplets

Question 31

• what are foams

Answer 31

basically emulsions but it is an air bubble instead of an oil droplet that needs stabilzation