Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

0013 Endoplasmic reticulum The endoplasmic reticulum

is actually a series of double-membrane channels dis-

tributed throughout the cytoplasm. Ribosomes may

be attached to the membranes and are then referred

to as the rough endoplasmic reticulum. Without

ribosomes attached, it is called smooth endoplasmic

reticulum. This organelle serves several important

functions in the cell, among which are involvement

in the synthesis and storage of molecules, providing a

system of channels for distribution and transport of

materials throughout the cell, release of calcium ions

into the cytosol that initiate contraction in muscle

cells, and providing some structural support for

the cell.

0014 Golgi complex The Golgi complex consists of flat-

tened membranous sacs stacked upon each other with

expanded areas at their ends. The main function of

this organelle is in the sorting and packaging of vari-

ous molecules, particularly proteins, for distribution

to various parts of the cell. The Golgi complex is

particularly extensive in cells with high secretory

activities.

0015 Mitochondria Mitochondria are double mem-

brane–bound organelles that may have a variety of

shapes. The inner of the two membranes is thrown

into folds or plates, called cristae. This arrangement

provides a large surface area for chemical reactions

to take place. Many enzymes involved in energy-

releasing reactions are located on the cristae. These

reactions are collectively known as cellular respir-

ation and include the reactions of the tricarboxylic

acid cycle and the electron transport chain. Since

much of the cell’s energy is produced through these

reactions, mitochondria are sometimes called the

‘powerhouse’ of the cell. Cells that have a high energy

expenditure, e.g., muscle, liver, and kidney tubule

cells, have large numbers of mitochondria. The fact

that mitochondria have some structural similarities

with bacteria, have their own DNA, and are capable

of reproducing themselves has led some scientists to

speculate that mitochondria may have evolved from

bacteria. According to this idea, the bacteria were

incorporated into the cell during evolution.

0016 Lysosomes Lysosomes are membrane-bound, spher-

ical structures containing powerful digestive enzymes

and are formed from vesicles budding off the Golgi

complex. These enzymes are capable of digesting bac-

teria and other solid matter that may be engulfed by a

cell. Leucocytes, specifically neutrophils and mono-

cytes that engulf bacteria and other foreign particles

(a process known as phagocytosis), contain large

numbers of lysosomes.

0017Cytoskeleton Helping to maintain the shape of the

cell and supporting various organelles within the cell

is a scaffolding-like assembly of filaments and tubules

called the cytoskeleton. The filaments and tubules are

composed of proteins similar to those found in the

contractile machinery of muscle. They are rod-shaped

structures and vary in length and thickness. Microtu-

bules average about 24 nm in diameter and may also

provide channels for the transport of materials within

the cell. Microfilaments are about 6 nm in diameter

and may also play a special role in movement of cells,

e.g., movement of phagocytes. Intermediate fila-

ments, diameters ranging from 8 to 12 nm, have also

been discovered inside cells.

Plasma Membrane

0018The plasma membrane (Figure 3) is composed mainly

of lipid molecules. The lipid is arranged into two

layers, the hydrophobic tail regions of the lipid mol-

ecules pointing to the inside of the bilipid layer, and

the hydrophilic heads of the lipid pointing to the

outside. Proteins and carbohydrates float in this lipid

membrane. The plasma membrane facilitates contact

and communication with other cells, mediates

the entry and exit of materials into and out of the

cell, and is the site of many important biochemical

reactions.

0019The plasma membrane is selectively permeable: it

will allow the transit of some substances, but not of

others, and furthermore, some substances are allowed

across the membrane more readily than others (differ-

entially permeable). There are very specific ‘channels’

in the membrane that allow the passage of specific

substances. These ‘channels’ are in fact proteins.

0020All of these properties of the plasma membrane

contribute to the differences that are seen in the com-

position of the fluid inside the cell (intracellular fluid)

compared with that outside (extracellular fluid). A

very important difference to note in terms of ion

concentrations is that of potassium and sodium ions.

Intracellular fluid has a much higher concentration of

potassium ions than the extracellular fluid. The re-

verse is true of sodium ions. It is extremely important

to the function of all cells in the body that these

concentration differences (or gradients) are main-

tained. At times, the concentration differences will

be disturbed. The role of certain proteins found in

the plasma membrane is to act as ion pumps to restore

the original concentration gradients. Of particular

importance is the so-called sodium/potassium pump.

When the concentration gradient of sodium and

potassium ions is decreased, these pumps expend

energy to pump sodium ions out of the cell and at

the same time pump potassium ions into the cell. The

importance of this mechanism to normal cell function

996 CELLS

is markedly demonstrated by the nerve cell (the

neuron). In order to conduct electrical impulses,

sodium ions must enter the cell and potassium ions

move out. The sodium/potassium pump works vigor-

ously in the neuron to try to maintain the original

gradients. Indeed, the neuron expends 80% of its

energy on this pump.

0021 These properties of the plasma membrane contrib-

ute not only to chemical differences between the

inside and the outside of the cell but also to an elec-

trical difference. A small voltage (of the order of tens

of millivolts) can be measured across the plasma

membrane. The inside of the cell is electrically

negative with respect to the outside. This electrical

difference, or membrane potential as it is called, is

particularly important when considering the func-

tions of neurons and muscle cells, both of which can

conduct electrical impulses.

0022 The cell membrane in some cells is thrown into

many small folds that project from the bulk of the

cell and are called microvilli. These greatly increase

the surface area of the cell and provide a much larger

surface for the absorption of materials across the cell

membrane. Such specializations are seen in cells

lining the small intestine and in tubule cells in the

kidneys. In some cells, similar types of projections of

the plasma membrane are seen with underlying con-

tractile machinery of the cytoplasm. These form cilia.

The cilia move in unison, providing a sweeping

motion that moves material over the surface of the

cell. Cells bearing cilia line many of the larger respira-

tory passages, and the mucus produced here traps

dust and foreign particles and is swept up to the

throat region by this mucociliary escalator.

Requirement of Cells

0023For cells to carry out their normal functions properly,

certain conditions need to apply. For human and most

animal cells, environmental conditions such as tem-

perature, pH, and ionic strength of solutions bathing

the cell need to be kept fairly constant. Many of the

body’s mechanisms are geared towards this homeo-

static control. Cells also need to be supplied with

nutrients. They need the basic building blocks to

manufacture complex organic molecules, such as

amino acids for the synthesis of proteins. They need

fuels to supply the substrates for generating energy,

and oxygen to oxidize the fuels and allow energy to

be released. The fuel requirements of some cells are

very exacting. While several types of fuel molecules

may be available to cells, neurons need to use glucose

almost exclusively. If the glucose cannot be supplied

to these cells, the brain will stop functioning properly.

Such is the importance of glucose to neurons that

many body mechanisms operate to maintain ad-

equate levels of glucose in the blood at all times.

Other fuels used by cells are fatty acids and, to a

limited extent, proteins. When glucose is in short

supply, many cells turn to a greater utilization of

fatty acids to derive energy. (See Fatty Acids: Metab-

olism; Protein: Synthesis and Turnover.)

Importance of Specialized Cells

0024While all cells are able to (and need to) carry out

certain basic functions, such as protein synthesis and

energy metabolism, many cells develop special intra-

cellular machinery to carry out specific functions. The

Bilipid

layer

Head

Tail

Channel

Integral

protein

Cholesterol

Glycoprotein

Glycolipid

Carbohydrate

Peripheral

protein

fig0003 Figure 3 Arrangement of molecules in the plasma membrane. Reproduced from Cells, Encyclopaedia of Food Science, Food Technol-

ogy and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

CELLS 997

complex behavior of animals is possible only because

of this cellular specialization. All somatic cells (body

cells, as opposed to the gametes, or sex cells) contain

the full complement of genetic information. With

cellular specialization, however, the capacity to de-

velop in a variety of ways (sometimes called cellular

plasticity) is sacrificed. In some cases, cells become so

constrained and devoted to their special task in the

body that they even lose the ability to divide and

provide more of the same type. Such is the case with

neurons; these cells make up the main controlling and

information-transfer system of the body. Neurons

detect various environmental stimuli and pass this

information on to centers in the nervous system that

interpret this information. Having determined the

appropriate response, the nervous system sends mes-

sages to the muscles and glands of the body to effect

this response. All this is achieved by transmitting

electrical signals around the body. When neurons

are injured and die, they are not replaced. Muscle

cells, too, are incapable of dividing. These cells devote

their energies to building contractile machinery

within them. When these cells are activated, they

contract, bringing about movement of various

organs, limbs, etc., of the body. Adipocytes, or fat

cells, are specialized to store large amounts of fat to

such an extent that the nucleus of the cell becomes

confined to a small part of the cell squashed against

the cell membrane. It is thought that mature adipo-

cytes do not divide. Erythrocytes are packed with the

protein, hemoglobin, which is responsible for trans-

porting the blood gases, particularly oxygen. It

enhances the oxygen-carrying capacity of the blood

about 60-fold. Human erythrocytes are peculiar cells

in that they have no nucleus, losing their nucleus

during maturation. They have a limited lifespan of

about 120 days. Under normal circumstances human

erythrocytes are produced in the body at a rate of

2 10

per second. Hepatocytes are specialized cells

of the liver. These cells contain large amounts of

specific enzymes involved in the metabolism and de-

toxification of many different molecules in the body.

These are just some of the many specialized cells

in the body; there are numerous others, including

osteocytes (bone cells), endocrine cells (hormone-

producing cells), and chondrocytes (cartilage cells).

(See Adipose Tissue: Structure and Function of White

Adipose Tissue; Structure and Function of Brown

Adipose Tissue.)

See also: Adipose Tissue: Structure and Function of

White Adipose Tissue; Structure and Function of Brown

Adipose Tissue; Exercise: Muscle; Fatty Acids:

Metabolism; Glucose: Function and Metabolism; Fats:

Classification; Nucleic Acids: Physiology; Protein:

Synthesis and Turnover

Further Reading

de Duve C (1984) A Guided Tour of the Living Cell, vols.

1 and 2. New York: Scientific American Library.

Fawcett DW (1981) The Cell, 2nd edn. Philadelphia, PA:

WB Saunders.

Ganong WF (1999) Review of Medical Physiology, 19th

edn. Upper Saddle River: Prentice Hall.

Keeton WT and Gould JL (1986) Biological Science, 4th

edn. New York: WW Norton and Co.

Martini FH (1998) Fundamentals of Anatomy and Physi-

ology, 4th edn. Upper Saddle River: Prentice Hall.

Reid RA and Leech RM (1980) Biochemistry and Structure

of Cell Organelles. Glasgow: Blackie.

Tribe MA, Morgan AJ and Whittaker PA (1981) The Evo-

lution of Eukaryotic Cells. London: Edward Arnold.

CELLULOSE

M T Holtzapple, Texas A & M University, TX, USA

Background

0001 Cellulose is the world’s most abundant biological

material. An estimated 10

tonnes are produced an-

nually, a daily production of about 45 kg per person.

Because of its rigidity, cellulose provides structure to

plants. It is almost always associated with other com-

ponents as a composite material termed ‘lignocellu-

lose.’ (See Hemicelluloses; Lignin.)

0002The most-studied celluloses are from cotton, ramie,

Valonia algae and Acetobacter xylinum bacteria.

Structure

Polymer Structure

0003Cellulose is a polymer of the monomer glucose (see

Figure 1a). The free-aldehyde glucose conformation is

very unstable, so it cyclizes into a six-membered pyra-

nose ring. (In aqueous solutions at 25



C, only about

0.02% of the glucose is in the free aldehyde form.) The

C1 hydroxyl of the cyclic ring can be in the equatorial b

998 CELLULOSE

position or the axial a position. The b position is

favored thermodynamically and accounts for 62% of

the glucose with the remaining 38% in the a position.

0004 Cellulose is a linear, unbranched polymer of anhy-

droglucose joined with ether linkages between C1

and C4 (see Figure 1b). Cellulose is polymerized

from b-glucose, whereas starch (see Figure 1c)is

polymerized from a-glucose. In the rightward anhy-

droglucose unit, C1 retains its hydroxyl group, so it

may potentially form free-aldehyde glucose, which

has reducing power. Hence, this terminus is called

the ‘reducing end.’

0005b-Linked cellulose has dramatically different

properties than a -linked starch. Starch is helical,

(a)

(b)

(c)

CHOH

COH

1.038 nm

COH

Nonreducing

end

Nonreducing

end

Anhydrocellobiose

DP = 2n + 2 (500 −15 000)

DP = n + 2 (100−3000)

Reducing

end

Reducing

end

COH

Anhydroglucose

0.42 nm

fig0001 Figure 1 (a) Glucose, (b) cellulose, and (c) amylose starch. Reproduced from Cellulose, Encyclopaedia of Food Science, Food Tech-

nology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

CELLULOSE 999

water-soluble, and easily hydrolyzed by enzymes,

whereas cellulose is planar, water-insoluble, and diffi-

cult to hydrolyze. Whereas starch is a widely utilized

animal food, cellulose is used only by a select few,

ruminants being the most prominent example. In

starch, the repeating unit is anhydroglucose. For

cellulose, anhydrocellobiose is the repeating unit

because adjacent anhydroglucoses are rotated 180



with respect to their neighbors. This rotation causes

cellulose to be highly symmetrical, because each side

of the chain has an equal number of hydroxyl groups,

whereas starch is unsymmetrical. The cellulose degree

of polymerization (DP) ranges from 500 to 15 000.

When fully stretched, a single cellulose molecule

could extend as long as 7 mm. Amylose starch has a

lower DP (100–3000) and could extend as long as

1 mm if fully stretched. Cellulose is completely

unbranched, whereas amylopectin starch has

branches at C6. (See Starch: Structure, Properties,

and Determination.)

Crystalline Structure

0006The cellulose hydrogen atoms are all in the axial

position, whereas the hydroxyl groups are all equa-

torial. These equatorial hydroxyl groups can hydro-

gen-bond with their nearest neighbors, allowing

cellulose to crystallize. The monoclinic crystalline

unit cell for cellulose I (native cellulose) is shown in

Figure 2. The hydrogen bonds run in the a direction

and are medium-strength (15 kcal mol

1

). In the c

direction, the structure is held by weak van der

Waals forces (8 kcal mol

1

). Covalent bonds run in

the b direction and give cellulose its strength (50 kcal

mol

1

). A continuous cellulose strand is about four to

COH

c = 0.786 nm

a = 0.817 nm

b = 1.038 nm

b = 83 8

fig0002 Figure 2 Parallel cellulose I unit cell. Reproduced from Cellulose, Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

1000 CELLULOSE

five times stronger than steel with the same cross-

section. Cellulose I is parallel; that is, all the cellulose

molecules run in the same direction from nonredu-

cing to reducing ends (see Figure 3a).

0007 Native cellulose (cellulose I) can be converted to

other crystalline forms. Cellulose II is formed by (1)

treating cellulose with sodium hydroxide (merceriza-

tion), (2) precipitating from solutions of alkali/salt

(e.g., cuprammonium hydroxide), or (3) removing

the added functional groups from cellulose deriva-

tives (i.e., regenerated cellulose). Cellophane and

rayon are both forms of cellulose II. Table 1 shows

that the unit cell dimensions are slightly expanded in

the c direction and compressed in the a direction. Of

course, the b direction is essentially the same, because

it is the covalent bond. Cellulose II is the most

thermodynamically stable form of cellulose because

it can always be produced from cellulose I, but not

vice versa. The stability may result from hydrogen

bonds extending in the c direction, which normally

has only van der Waals bonds. There is general agree-

ment that cellulose II is antiparallel (see Figure 3b)

with three to four anhydroglucose moieties required

to make the bend. Precipitation of cellulose II from

solution appears to favor the antiparallel conform-

ation, as occurs with many synthetic polymers.

0008Cellulose III is formed by soaking cellulose in cold

(about 80



C) liquid anhydrous ammonia, which

is subsequently removed by evaporation. Cellulose I

is transformed into cellulose III

and cellulose II is

transformed into cellulose III

. When rehydrated,

cellulose III reverts back to its original form.

0009Cellulose IV is formed by soaking cellulose in hot

(about 200



C) glycerol, with subsequent removal by

washing with 2-propanol and water. Cellulose I is

transformed to cellulose IV

, and cellulose II is trans-

formed to cellulose IV

0010Native cellulose (Figure 3a) forms crystalline

regions (40% bacteria, 60% cotton, 70% Valonia)

interspersed with amorphous regions. The amorph-

ous regions are more porous than the crystalline

regions, allowing water or dyes to penetrate and in-

creasing the reactivity to acid or enzymatic hydroly-

sis. When purified cellulose fibers are subjected to

dilute acid hydrolysis, the amorphous regions select-

ively hydrolyze, leaving the more resistant crystalline

regions, which have a ‘levelling-off DP’ of 100–300 in

the case of cotton.

Cellular Structure

0011Young plant cells have only the primary wall to resist

osmotic pressure. This wall is composed principally

of hemicellulose and pectin with small amounts of

cellulose and protein and is sufficiently flexible to

accommodate cell growth. When cell growth ceases,

the secondary wall is formed in three layers (S

and S

) with S

the thickest (see Figure 4). (See Pro-

tein: Chemistry.)

0012All cell wall layers are composed of 7–30-nm-

diameter microfibrils, which are visible using an elec-

tron microscope. In the primary cell wall, the fibrils

are randomly oriented. In the S

layer, the microfibrils

are oriented helically, with each successive layer alter-

nating between right-handed and left-handed helices.

In the S

layer, which provides much of the plant

strength, the microfibrils are arranged in steep right-

handed helices nearly oriented along the cell axis. The

layer is another shallow helix. All higher plants

(softwoods, hardwoods, herbaceous) are thought to

have cell structures similar to that shown in Figure 4.

0013The plant cell diameters range from 15 to 80 mm,

depending on the species and time of year. Spring cell

30−60nm

~15 nm

(b)

Crystalline

Amorphous

(a)

fig0003 Figure 3 (a) Cellulose I parallel and (b) cellulose II antiparallel

structures. Reproduced from Cellulose, Encyclopaedia of Food

Science, Food Technology and Nutrition, Macrae R, Robinson RK

and Sadler MJ (eds), 1993, Academic Press.

tbl0001 Table 1 Unit cell dimensions of cellulose I and II

Cellulose a (nm) b (nm) c (nm) b(



)

I 0.817 1.038 0.786 83.0

II 0.801 1.036 0.904 62.9

From Blackwell J, Kurz D, Su M-Y and Lee DM (1987) X-ray studies of the

structure of cellulose complexes. In: Atalla RH (ed.) The Structures of

Cellulose. ACS Symposium Series, No. 340, pp. 199–213. Washington, DC:

American Chemical Society.

CELLULOSE 1001

diameters are larger than summer diameters, giving

rise to the familiar growth rings in trees. The wall

thickness is typically about 2–5 mm, with thicker

walls formed in the summer. Softwood cells are

about 3–4 mm long, which makes them particularly

useful for paper pulp, whereas hardwood cells tend to

be shorter (0.7–2 mm long). Cotton cells suitable for

textiles are about 25 mm long.

0014 Microfibrils are composed of elementary fibrils of

pure cellulose embedded in a matrix of hemicellulose.

The fringes of the elementary fibrils are paracrystal-

line and intermingle with tightly adsorbed hemicellu-

lose. (An exception to this is cotton in which the

fringes contain only cellulose.) In Fengel’s model of

the microfibril, the elementary fibrils are clustered

into four 4 4 arrays. The lignification process

occurs late in cell life, so lignin is located primarily

on the microfibril exterior, where it covalently bonds

to the hemicellulose. Lignification is initiated in the

middle lamella where lignin constitutes about 70% of

0.786 nm

0.817 nm

Unit cell

3−10 nm

15−80 mm

30−60 nm

3−5 nm

7−30 nm

Elementary

fibril

Hemicellulose

Lignin

Microfibril

Secondary

wall

Primary wall

Middle lamella

Compound

middle

lamella

fig0004 Figure 4 Schematic of plant cell, microfibril, and elementary fibril. Reproduced from Cellulose, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

1002 CELLULOSE

the wall. The wall interior is less lignified, with only

about 15% lignin. The hemicellulose content is fairly

constant (20–30%), with cellulose the remaining 10–

50% of the wall. The cells in fruit pulp are composed

mainly of primary walls containing approximately

34% pectin, 24% hemicellulose, 23% cellulose, and

19% protein.

Properties

Physical

0015 Table 2 shows the physical properties of cellulose and

cellulosic materials. The listed heat of combustion is

the ‘higher heat’ (i.e., the combustion water is 25



liquid). Cellulose is stable up to 300



0016 The DP for various types of cellulose is shown in

Table 3. For cotton, the DP may be as high as 15 000.

Cellulose is insoluble in water because of the strong

hydrogen bonding in its crystal lattice. However, it is

soluble in a number of solvents, including concen-

trated acids (e.g., 85% phosphoric, 72% sulfuric,

and 40% hydrochloric acids) and inorganic salt solu-

tions (e.g., cuprammonium hydroxide, cadoxen (i.e.,

cadmium ions and ethylenediamine)). The viscosity of

cellulose dissolved in salts is often used for molecular

weight determinations.

Chemical

0017 Cellulose may be hydrolyzed to glucose by acids or

enzymes. Acid hydrolysis produces a number of deg-

radation products, such as 5-hydroxymethylfurfural,

formic acid, levulinic acid, formaldehyde, furfural,

and resins. Enzymatic glucose production requires a

cellulase system containing endo-cellulase (to pro-

duce nonreducing ends from the chain interior),

exo-cellulase (to produce cellobiose generally from

the nonreducing end), and cellobiase (to hydrolyze

cellobiose to glucose). Up to about 10% of cellulose

is enzymatically digested by microbes in the human

large intestine, so it is not completely noncaloric.

0018Cellulose is fairly stable to base, provided that

oxygen is excluded. The reaction terminates after

approximately 50 anhydroglucose units are ‘peeled

off’ from the cellulose reducing end; d-glucoisosac-

charinate is the soluble product. Under severe condi-

tions (e.g., 1 M sodium hydroxide, 170



C), alkaline

hydrolysis readily occurs.

0019Cellulose is more stable to oxidants than lignin, a

property exploited in pulp bleaching and analytical

methods, which selectively oxidize lignin. However,

oxidants (e.g., chromic acid, permanganate, and

hypochlorite) can damage cellulose by cleaving the

chain or by inserting carbonyl functional groups.

0020The three cellulose hydroxyl groups are very

reactive, allowing ready formation of the cellulose

derivatives described later.

Specialty Celluloses

0021Microcrystalline cellulose (Avicel, FMC Corporation)

is prepared by acid hydrolysis of cellulose using 2 M

hydrochloric acid at 105



C for 15 min. The highly

reactive amorphous regions selectively hydrolyze,

releasing the crystallites, which are subsequently

tbl0002 Table 2 Physical properties of cellulose and cellulosic materials

Property Compressed cellulose Paper Pine Oak

Specific gravity

1.47 0.70–1.15 0.43–0.67 0.64–0.87

Heat capacity (kJ kg

1

)

1.3 1.3 2.8 2.4

Heat of combustion (MJ kg

1

)

17.6 17.6 20.4 19.2

Thermal conductivity (W m

1

)

With grain 0.13 0.35

Across grain 0.13 0.15 0.21

Tensile strength (MPa)

With grain 62 99

Across grain 2.8 5.5

Water diffusion constant (cm

1

)

5.3  10

7

1.9  10

7

From Baumeister T, Avallone EA and Baumeister T, III (1978) Mark’s Standard Handbook for Mechanical Engineers, 8th edn. New York: McGraw-Hill.

From Graboski M and Bain R (1981) Properties of biomass relevant to gasification. In: Reed TB (ed.) Biomass Gasification: Principles and Technology,

pp. 41–71. Park Ridge: Noyes Data.

From Summitt R and Sliker A (1980) CRC Handbook of Materials Science: Wood, vol. IV, pp. 26–27. Boca Raton, FL: CRC Press.

tbl0003Table 3 Cellulose polymer length

Material Degree of polymerization Molecular weight

Native cellulose 3500–10 000 600 000–1 500 000

Chemical cottons 500–3000 80 000–500 000

Wood pulps 500–2100 80 000–340 000

Rayon filaments 350–450 57 000–73 000

From Hamilton JK and Mitchell RL (1964) Cellulose. In: Standen A (ed.)

Kirk^Othmer Encyclopedia of Chemical Technology, 2nd edn., vol. 4, pp.

593–616. New York: Wiley.

CELLULOSE 1003

mechanically dispersed. Aqueous suspensions of

microcrystalline cellulose have constant viscosities

over a wide temperature range, are heat-stable, and

have good mouth-feel properties. Avicel is used to

extend starches, stabilize foams, and control ice crys-

tal formation. Avicel has found widespread accept-

ance in the food industry for meringue, whipped

toppings, confections, and ice cream, and is also used

as a binder in pharmaceutical tablets and in cosmetics.

0022 Bacterial cellulose (Cellulon, Weyerhaeuser Co.) is

produced by selected strains of Acetobacter xylinum,

which maintain their ability to produce cellulose in

agitated submerged fermentors. The cellulose fibers

are about 0.1 mm in diameter, which is substantially

smaller than softwood pulp fibres (about 30 mm

diameter). Cellulon is a potential noncaloric food

thickener or texturizer.

Modified Celluloses

0023 Alkali cellulose is prepared by soaking cellulose

in concentrated sodium hydroxide (> 14%) in the ‘mer-

cerization’ process invented by John Mercer in 1844.

The sodium ions are incorporated into the cellulose

structure according to the following reaction (eqn (1)):

cell

OH þ NaOH ! R

cell

ONa þ H

Alkali cellulose

ð1Þ

The cellulose structure swells, allowing easy penetra-

tion by dyes or reagents for the manufacture of cellu-

lose derivatives.

0024 Cellulose xanthate is formed by reacting alkali

cellulose with carbon disulfide (eqn (2)):

cell

ONa þ CS

! R

cell

SNa:

Cellulose xanthate

ð2Þ

0025 Regenerated cellulose is made by dissolving cellulose

xanthate in 4–7% sodium hydroxide and contacting

with aqueous sulfuric acid. These steps convert the

cellulose xanthate back into cellulose, which may be

spun into viscose rayon or cast into films. The fibers

are used in textiles (artificial silk), tyre cords, and V

belts. The films are used in packaging (Cellophane) or

sausage casings. Weiner casings (70% regenerated

cellulose, 12% glycerol, and 18% water) are peeled

away after the meat emulsion is cooked. Hemp paper

casings (23% paper, 46% regenerated cellulose, 21%

glycerol, and 10% water) are used in bologna, salami,

pepperoni, summer sausage, and liverwurst.

0026 Cellulose hydroxyl moieties are highly reactive,

allowing a variety of esters and ethers to be manufac-

tured. Because each anhydroglucose has three hydroxyl

groups, the maximum degree of substitution (DS) is

three. Purified wood pulp or cotton linters (short fibers)

are the industrial sources of ‘chemical cellulose.’

Cellulose Ethers

0027Sodium carboxymethyl cellulose (CMC) is formed by

reacting sodium chloroacetate with alkali cellulose

(eqn (3)):

cell

ONa þ CICH

ONa !

cell

OCH

ONa þ NaCl:

CMC

ð3Þ

Commercially available CMC has a DS range of

0.38–1.4, with 0.65–0.85 more common. The nega-

tively charged carboxyl group makes CMC soluble in

both hot and cold water. The solution viscosity de-

creases as the temperature increases. CMC is ‘gener-

ally recognized as safe’ and is used as a thickener in

many foods such as cheese, frozen desserts, and salad

dressings. It is not metabolized, so it is used in low-

calorie foods.

0028Methyl cellulose is formed by reacting alkali cellu-

lose with methyl chloride (eqn (4)):

cell

ONa þ ClCH

! R

cell

OCH

þ NaCl:

Methyl cellulose

ð4Þ

Methyl cellulose (DS 1.8) solutions form a firm gel

when heated to 50–55



C and return to solution when

cooled. Methyl cellulose is added to salad dressings,

jams and preserves, soda water, and meat patties as a

binder.

0029Ethyl cellulose is produced by reacting alkali cellu-

lose with ethyl chloride. In commercial products, the

DS ranges from 2.0 to 2.6. It is water-insoluble and

may be incorporated into inks used for marking foods

and in vitamin tablet binders.

00302-Hydroxypropyl methyl cellulose is formed by

reacting alkali cellulose with mixtures of methyl

chloride and 2-hydroxypropyl chloride. It forms gels

like methyl cellulose, but has a higher gelation tem-

perature. It may be used as an emulsifier, film former,

stabilizer, or thickener in foods such as salad dress-

ings, sherbet, pie fillings, fried foods, whipped top-

pings, breading batters, and baked goods.

00312-Hydroxyethyl cellulose (HEC) is produced by

reacting cellulose with ethylene oxide using a sodium

hydroxide catalyst at 30–35



C for about 4h (eqn (5)):

cell

OH R

cell

OCH

HEC (5)

OH.

⫹

Because the side-chain also has a hydroxyl group,

ethylene oxide can continue to react and form a

1004 CELLULOSE

side-chain with several units. HEC is soluble in both

hot and cold water. The solution viscosity decreases

as the temperature increases. HEC is not permitted as

a direct food additive, but it may be used in food-

packaging adhesives and coatings.

0032 2-Hydroxypropyl cellulose is produced using pro-

pylene oxide, rather than the ethylene oxide used for

HEC. It has a thermal gel point, like methyl cellulose,

and is used in food coatings and glazings.

Cellulose Esters

0033 Cellulose acetate is the most important cellulose ester.

The cellulose is first ‘activated’ in aqueous acetic acid

to ensure uniform acetylation. It is then dehydrated

and reacted with acetic anhydride using a catalyst

(e.g., sulfuric acid) in a solvent (e.g., anhydrous acetic

acid) (eqn (6)).

cell

OH þ CH

cell

þCH

OH:

Cellulose acetate

ð6Þ

The resulting cellulose triacetate (DS 3) product

may be subsequently acid-hydrolyzed to lower

the DS. Cellulose triacetate is water-insoluble and

hydrophobic, whereas cellulose monoacetate is water-

soluble. Cellulose acetate is used in fibers, plastics,

photographic films, lacquers and reverse osmosis or

dialysis membranes.

0034 Other esters (e.g., cellulose formate, cellulose pro-

pionate, cellulose butyrate) may be formed, but they

do not have the widespread commercial applications

of cellulose acetate. Also, mixed esters (e.g., cellulose

acetate propionate, cellulose acetate butyrate, cellu-

lose acetate phthalate) may be produced.

0035 Cellulose nitrate is made by reacting cellulose with

nitric acid/sulfuric acid for 20–30 min. The acids are

then removed by washing with water. The water must

be removed with great care because dry cellulose

nitrate is extremely explosive. It is often shipped

wetted with water or alcohol. Highly nitrated (DS

2.4–2.6) forms are used as explosives (gun cotton).

Less nitrated forms (DS 2.1–2.3) find applications in

plastics, films, and inks.

Composition of Cellulosic Materials

0036 Table 4 shows the composition of many cellulosic

materials. The cellulose content in vegetables ranges

from 1 to 21%; fruits range from 0.6 to 4.2%; seeds

range from about 2 to 12%; agricultural residues

range from 31 to 59%; wood ranges from 41 to

53%; flax and hemp have about 70% cellulose; and

cotton (the purest natural cellulose) has about 95%

cellulose. The free sugars shown in Table 4 are the

sum of glucose, fructose, and sucrose. The ‘crude

fiber’ is reported as the sum of cellulose and lignin

in the case of the seeds. The indigestible portion of

some fruits, called ‘dietary fiber,’ is reported as

the sum of cellulose, lignin and hemicellulose. (See

Dietary Fiber: Properties and Sources; Fructose;

Sucrose: Properties and Determination.)

Cellulose Isolation and Analysis

Gravimetric Methods

0037Van Soest procedure The raw plant material is con-

tacted with dilute acid and washed with solvents to

remove starch, pectin, hemicellulose, fats, oils,

protein, free sugars, and soluble minerals. The resi-

due, called ‘acid detergent fiber,’ contains cellulose,

lignin, and insoluble minerals (mainly silica).

0038Air-dried plant material (1 g), ground to particles of

less than 1 mm, is placed in a beaker with 100 ml of

acid-detergent solution (49 g l

1

,20gl

1

cetyl

trimethylammonium bromide). Decahydronaphtha-

lene (2 ml) is added, and the contents are slowly

boiled for 1 h while the reflux condenser maintains a

constant liquid volume in the beaker. Then, the

beaker contents are filtered through a tared Gooch

crucible, washed with boiling water, acetone, and

hexane (optional). The residual acid detergent fiber

(ADF) is dried at 100



C and weighed.

0039The cellulose content in ADF can be measured by

two methods: (1) cellulose removal with acid or (2)

lignin removal by oxidation.

0040Method 1 The ADF prepared above is placed in a

tared Gooch crucible containing an equal volume of

asbestos as a filter aid. The crucible contents are

contacted with room-temperature 72% sulfuric acid

for 3 h and then thoroughly washed with hot water.

The sample is dried at 100



C for 8 h and weighed.

The weight loss corresponds to the cellulose content.

0041Method 2 The ADF (0.5–1.0 g) is placed in a tared

Gooch crucible. A saturated potassium permanganate

solution (50 g l

1

KMnO

, 0.05 g l

1

AgSO

)and

buffer solution (6 g l

1

Fe(NO

)

9H

O, 0.15 g l

1

AgNO

, 500 ml l

1

glacial acetic acid, 5 g l

1

potas-

sium acetate, 400 ml l

1

t-butyl alcohol) are mixed in

a 2:1 ratio (by volume). This mixture (25 ml) is added

to the crucible for 90 min at room temperature to

oxidize the lignin in the ADF. The spent reagent is

then removed by vacuum filtration. The crucible con-

tents are soaked for 5 min in demineralizing solution

CELLULOSE 1005