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

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filtration/cleaning cycles. Many of today’s routine

commercial applications of membrane separation

technology were only dreams two decades ago.

0024 The cost for many types of membranes has dropped

significantly in the past 10 years, e.g., from $800 to

$350 for similarly sized spiral wound modules.

Energy efficiencies have improved partially due to

lower pressure differential required across the mem-

branes, higher throughput and more sophisticated

computer controls. These efficiencies, coupled with

lower cost membranes, have made membrane separ-

ation processes not only more cost-effective but also

more dependable and easier to maintain and operate.

0025 Membrane separation technology is finding mul-

tiple industrial applications not only in food process-

ing but also in process water and plant effluent

treatment. Most food processors are very aware that

water management has become a major consideration

to an efficient operation. Effluent discharge require-

ments have become more stringent, dependable sup-

plies of process water can no longer be taken for

granted, and both effluent disposal and water supply

costs have increased.

Ozone: Antimicrobial Agent

0026 Ozone (CAS No. 10028-15-6) is a gas at ambient and

refrigerated temperatures and is partially soluble in

water. In the stratosphere, UV and lightening generate

ozone naturally from atmospheric oxygen; ozone is

generated industrially by passing oxygen through a

high-voltage electric corona discharge field.

0027 Ozone (O

) is a molecule composed of three

oxygen atoms, in contrast to ordinary oxygen (O

which has only two oxygen atoms. Because of its

atomic structure, ozone is an unstable gas that

quickly decomposes into ordinary oxygen, especially

in water. This tendency to decompose naturally makes

ozone an important chemical. As ozone changes into

oxygen, the extra oxygen atom splits off from each

ozone molecule. These free oxygen atoms have two

important characteristics: they are toxic to microor-

ganisms, and they oxidize many chemical compounds,

usually changing them into nontoxic substances.

0028Ozonation is used extensively for sanitizing bottled

water, and may become widely used as an antimicro-

bial agent in processing foods for human consump-

tion. Ozone is a powerful oxidant, can be dissolved in

water, and can be used effectively to kill harmful

microorganisms such as Listeria, E. coli, Salmonella,

and other pathogenic bacteria, viruses, fungi, and cyst

organisms such as Giardia. Ozone degrades to oxygen

in a few minutes and thus leaves no chemical residue

like chlorine, iodine, or other common disinfectants.

Current food uses in the USA include aqueous ozone

for cleaning sausage curing racks, washing apples,

garlic cloves, strawberries, and fresh-cut produce in-

cluding celery and lettuce; gaseous ozone is used in

dry storage of onions, garlic, potatoes, and citrus

products.

0029A schematic of a typical corona discharge system

used to generate ozonated water from dry air or

oxygen is shown in Figure 3. Figure 4 shows an

ozone system installed in a food processing plant. In

the foreground is a pressure swing absorption (PSA)

air dryer. The sealed cabinet in the corner houses the

ozone generator; the cabinet to the right contains the

electronic operating controls for the system.

Filtration spectrum

Nano filtration

Reverse osmosis

Micro filtration

Ultra filtration

Coarse filtration

101

0.1

0.01

0..001

0.0001

Salts

Metal ions

Alcohols

Sugars

Proteins

Enzymes

Bacteria Yeast

Emulsified

oils

Suspended solids

Giardia

cyst

microns

Paint

pigments

300

1,000 300,000

MWCO (Dalton)

Whey protein

Screening

Viruses

fig0002 Figure 2 Membrane filtration spectrum. Drawing courtesy of J. Strasser, Process & Equipment Technology.

POWER SUPPLIES/Use of Electricity in Food Technology 4715

Freeze Concentration

0030 Separation is an important step in every food process-

ing operation. Evaporators heated with steam pro-

duced from coal or gas fired boilers is the dominant

separation process used in the food industry. Freeze

concentration (FC) technology is an electrically

driven alternative for conventional evaporation tech-

nologies. The dairy industry is the food industry’s

largest user of energy for evaporation. FC offers both

energy and economic savings to dairy processors using

this innovative freeze concentrate technology. In add-

ition, the food processing industry and the environ-

ment benefit by shifting from fossil fuels to more

environmentally clean electrical energy.

0031FC is a process for removing water from food by

crystallizing the water and mechanically separating

ice crystals from the unfrozen liquid medium. The FC

concentration process has been installed in more than

100 locations throughout the world. It has helped

improve the quality and reduce the operating costs

for concentrating orange juice, grapefruit juice, man-

darin juice, coffee extract, vinegar, and ice beer, and

to remove toxic organic residues from problematic

process waste streams in the chemical and refining

industries.

0032FC stemmed from fundamental research at the

University of Eindhoven in 1960, and the first proto-

types were developed by Grasso/Grenco in The

Netherlands from 1970 to 1975. Early commercial

applications were mainly for the production of sol-

uble coffee in 1975 and orange juice in 1980. Pion-

eering work by the Electric Power Research Institute

(EPRI), the Dairy Research Foundation (DRF), and

Niro Process Technology successfully applied the

freeze concentration process to milk and milk prod-

ucts. Freeze concentration yields milk products with a

remarkably improved product quality, no heat

degradation, lower operating costs, reduced waste

burden, and a sustained continuous operation.

0033Freeze concentration provides several unique ad-

vantages over traditional thermal concentration tech-

nologies. FC products have a better product quality

with no heat degradation and are clearly superior in

taste, color, and functionality compared with other

commercial concentrates. In addition, the improved

operating efficiencies reduce the processors’ bottom

line cost. Freezing consumes 80 kcal (335 kJ) per kilo-

gram of water made into ice versus 526 kcal (2200 kJ)

Generator

(corona discharge 5

−15 kV)

Contactor

Tank

Ozonated water

to process use

Recirculation

loop

Pump

Cold

water

supply

Dry air

oxygen

supply

Ozone

injection

Total power usage:

2 to 8 kWh/454g. ozone

(1/3 of it for ozone generation)

fig0003 Figure 3 Typical system for generating ozonated water.

fig0004 Figure 4 Ozone system installed in a grain-processing plant.

Photo courtesy of RGF Environmental Systems.

4716 POWER SUPPLIES/Use of Electricity in Food Technology

per kilogram of water converted to steam. The

bottom-line additional cost of FC concentrate is no

more than 6 cents per kilogram of milk solids higher

than the cost of the best thermal evaporators of $2.05

to 2.20 kg

1

0034 How freeze concentration works The process

works by pumping the milk through a scraped surface

heat exchanger to form crystals. It is then fed to the

recrystallizer, where the small crystals are mixed with

a population of larger ice crystals. The larger crystals

are grown through a ripening process. A continuous

supply of small crystals from the scraped surface heat

exchangers provides the fuel for this ripening growth.

The ice crystals need to be removed to complete the

concentration step. The wash column provides the

most efficient method to remove these ice crystals

and minimize the loss of product. Figure 5 provides

a simplified schematic of the process.

0035 The ripening process takes advantage of the equi-

librium temperature at which crystals form. The equi-

librium temperature of the solution is when crystals

neither grow nor melt. When placed in the same

concentration solution, small crystals have a slightly

lower equilibrium temperature than larger crystals.

The bulk solution temperature of a mixture of small

and large crystals will be somewhere between the

equilibrium temperature of the small and large crys-

tals. The smaller crystals will be in a warmer environ-

ment and will melt, and the larger crystals will be in a

colder environment and will grow. The heat of crys-

tallization is exactly balanced by melting and growing

crystals. The driving forces are therefore very small,

producing spherical crystals, as shown in Figure 6.

0036Once crystals are formed in the solution, the

remaining liquid is concentrated in the solute(s) con-

tained in the original feed because a portion of the

original water is now in the form of ice crystals. The

ice crystals need to be removed to complete the con-

centration step. The wash column provides the most

efficient method to remove these ice crystals and

minimize the loss of product.

Other Developing Electrotechnologies

0037Ultra-high-pressure processing Pressure can kill

microorganisms in food products at room tempera-

ture with little or no damage to the food product. The

most promising results to date have been with acidic

liquid foods such as orange juice. Ultra-high-pressure

processing, also called high-pressure processing

(HPP) can extend the shelf-life of food products with-

out the application of heat or chemical treatment.

0038In HPP, food products are exposed to pressures in

the range of 103 422–620 000 kPa for a few minutes.

Spores and common food enzymes appear to tolerate

high pressures and are not destroyed.

0039Pulsed electric fields Pulsed Electric Field (PEF) is a

unique nonthermal method of inactivating microor-

ganisms, including many of the common food

pathogens, without heating the product to the usual

pasteurization temperatures. The destruction or

inactivation of the microorganism is achieved by the

breakdown of the microorganism’s cell membranes

during exposure to electric fields.

0040PEF applies multiple short pulses (1 ms each) of

high-intensity electric fields (20–50 kV cm

1

) be-

tween two electrodes. The food product being treated

flows between the two electrodes and is exposed to

the electric field. The degree of treatment is adjusted

to the characteristics of the food product being pro-

cessed by:

0041exposure time (related to flow rate and fluid

volume of the electrode chamber);

(scraped surface heat

exchanger)

Crystal

(Wash column)

Crystal ripening

(recrystalizer)

Milk

Heat

Concentrated milk

Heat

Water

Crystal formation

fig0005 Figure 5 Overview of the freeze concentration process.

Crystal

ripening

fig0006Figure 6 Ripening of crystals.

POWER SUPPLIES/Use of Electricity in Food Technology 4717

.0042 frequency of the pulses;

0043 intensity (kV cm

1

) of the electric field; and

0044 shape of the pulse wave (rate of build and decay of

the electrical intensity).

Even though this technology has been applied only

recently in the USA, it was described in German

Patent 1,237,541 issued in 1960. PEF can be an ef-

fective pasteurization process, but the FDA has ex-

pressed concerns about the consistency of some of the

microbial destruction data. These apparent inconsist-

encies may be due to the effect of the growth phase,

type of organism, media, and pH. PEF presently is not

recommended as a food sterilization method. Promis-

ing test results have been obtained with liquid eggs,

milk, emulsions, juices, rice wine, some sauces, fer-

mented products, and water.

See also: Convenience Foods; Crystallization: Basic

Principles; Effluents from Food Processing: On-Site

Processing of Waste; Filtration of Liquids; Freezing:

Operations; Irradiation of Foods: Applications;

Membrane Techniques: Applications of Reverse

Osmosis; Pasteurization: Pasteurization of Liquid

Products; Radioactivity in Food

Further Reading

A Century of Food Science (2000) Chicago, IL: Institute of

Food Technologists.

Castellari M, Arfelli G, Riponi C and Amati A (2000) High

hydrostatic pressure treatments for beer stabilization.

Journal of Food Science 65(6): 975–977.

Gerber J (1998) The electric farm. EPRI Journal 23(3):

8–17.

Graham DM (1997) Use of ozone for food processing. Food

Technology 51(6): 72–75.

EPRI (1993) Electroconductive (Ohmic) Heating for Con-

tinuous Sterilization of Solid–Liquid Food Mixtures.

Palo Alto, CA: EPRI.

EPRI (1997) Pulsed Electric Field Technology. Palo Alto,

CA: EPRI.

EPRI (1999) Resource Guide for Food Irradiation. Palo

Alto, CA: EPRI.

EPRI (2000) Ozone and UV for Grain Milling Systems.

Palo Alto, CA: EPRI.

EPRI (2000) Food Industry 2000: Food Processing Oppor-

tunities, Challenges, New Technology Applications.

Palo Alto, CA: EPRI.

EPRI (2001) Freeze Concentration of Dairy Products:

Phase 3 and Phase 4. Palo Alto, CA: EPRI.

Kim JG and Yousef AE (2000) Inactivation kinetics of

foodborne spoilage and pathogenic bacteria by ozone.

Journal of Food Science 65(3): 521–528.

McDonald CJ, Lloyd SW, Vitale MA, Petersson K and

Innings F (2000) Effect of pulsed electric fields on micro-

organisms in orange juice using electric field strengths

of 30 and 50 kV/cm. Journal of Food Science 65(6):

984–989.

Marcotte M, Trigui M, Tatiboquet J and Ramaswamy HS

(2000) An ultrasonic method for assessing the residence

time distribution of particlate foods during ohmic

heating. Journal of Food Science 65(7): 1181–1186.

Marquez VO, Mittal GS and Griffiths MW (1997) Journal

of Food Science 62(2): 399–401.

Moore T (1994) A separable feast. EPRI Journal 19(6):

17–23.

Saldo J, Sendra E and Guamis B (2000) High hydrostatic

pressure for accelerating ripening of goat’s milk cheese:

proteolysis and texture. Journal of Food Science 65(4):

636–640.

Vega-Mercado JR, Powers GV, Barbosa-Canovas G

and Swanson BG (1995) Plasmin inactivation with

pulsed electric fields. Journal of Food Science 60(5):

1143–1146.

Prader–Willi Syndrome See Developmental Disabilities and Nutritional Aspects: Down Syndrome;

Prader–Willi Syndrome

Prawns See Shellfish: Characteristics of Crustacea; Commercially Important Crustacea; Characteristics of

Molluscs; Commercially Important Molluscs; Contamination and Spoilage of Molluscs and Crustaceans;

Aquaculture of Commercially Important Molluscs and Crustaceans

4718 POWER SUPPLIES/Use of Electricity in Food Technology

PREBIOTICS

M B Roberfroid, Universite

Catholique de Louvain,

Louvain-la-Neuve, Belgium

Background

0001 The microbial flora that live symbiotically within the

large intestine represent an essential element of the

physiology of most mammals. The flora constitute a

complex ecosystem that needs to be correctly fed

in order to maintain a well-balanced composition in

which the health-promoting bacterial species quanti-

tatively predominate over the potentially harmful

species. Foods that resist hydrolysis by the digestive

enzymes and are not absorbed in the upper part of

the gastrointestinal tract, including the small intes-

tine, become substrates for the colonic microflora.

These dietary components pass into the large bowel,

where most of the indigenous intestinal microflora

are located in a healthy human individual. A wide

variety of dietary carbohydrates, especially resistant

starch, dietary fiber, some polyols and nondigestible

oligosaccharides, have such characteristics, and they

provide quantitatively the majority of colonic food,

i.e., dietary components available for bacterial fer-

mentation in the colon. The colonic fermentation

of such ‘malabsorbed,‘ ‘non-digestible,‘ or ‘resistant‘

carbohydrates (oligosaccharides and polysaccharides)

plays a role in salvaging part of the energy of these

dietary components, in controlling transit time, stool

bulking, and stool frequency, in influencing nutrient,

especially mineral, bioavailability, in producing

short-chain fatty acids that are known to play physio-

logical roles such as control of mucosal motility and

epithelial cell proliferation, or in modulating immune

activity and endocrine functions. Amongst colonic

foods, it has been shown recently that some compon-

ents are especially beneficial to health. These com-

pounds have been called ‘prebiotics,‘ defined as

‘nondigestible food ingredients that beneficially affect

the host by selectively stimulating the growth and/or

activity of one or a limited number of bacteria in the

colon.‘ Prebiotics are thus colonic foods, but they are

more than simply malabsorbed, nondigestible, or

resistant carbohydrates; when they reach the large

bowel, they have specific effects that promote the

growth of advantageous species of bacteria to the

detriment of adverse species. The inclusion of prebio-

tics in the diet can lead ultimately to a marked change

in the composition of the colonic microflora, e.g., by

selectively stimulating the growth of bacteria that are

generally recognized as being beneficial for health and,

at the same time, reducing the number of potentially

harmful bacteria. The most efficient prebiotics identi-

fied today stimulate the growth of bifidobacteria, and

sometimes lactobacilli, whilst reducing the numbers

and activities of potentially pathogenic organisms.

(See Bifidobacteria in Foods; Lactic Acid Bacteria.)

0002The contents of the human gut are not readily

accessible for microbiological analysis; therefore,

demonstration of changes in the composition of the

fecal microflora is often used as a surrogate marker

for the prebiotic effect. However, for such a demon-

stration to be convincing, it is critical that as many

components of the fecal microbiota as possible are

measured. These should include at least bacteroides,

bifidobacteria, clostridia, eubacteria, Gram-positive

cocci, coliforms, lactobacilli, total aerobes, and total

anaerobes. Simple stimulation of growth of bifido-

bacteria and/or lactobacilli is insufficient to substan-

tiate a prebiotic property without determining the

effects on other fecal microorganisms, since it is the

selectivity of effect that determines classification as

a prebiotic. Clearly, studies using pure bacterial cul-

tures are of very limited, if any, value in this respect,

unless they are supported by mixed culture work in a

well-validated set-up. But the ultimate proof must

come from human studies in which correctly collected

and stored fecal samples are analyzed for their

composition in terms of bacterial species that are

further well characterized using classical techniques

combined with a conventional microbiological ap-

proach towards identification and/or modern mo-

lecular genotyping methods. Indeed, it is the effect

of the prebiotic in a competitive ecological environ-

ment that is important.

The Prebiotics

0003At present, the food components for which a prebio-

tic effect has been reported are nondigestible carbo-

hydrates that consist of mixtures of oligomers of

different chain lengths and are characterized by the

average number of osyl moieties, referred to as the

‘degree of polymerization‘ (DP). They have been clas-

sified as ‘nondigestible oligosaccharides,‘ or NDOs,

the osidic bond of which is in a spatial configuration

that confers resistance to the hydrolytic activities of

the digestive enzymes of the upper gut. But once they

reach the large bowel, the NDOs are fermented by at

least some of the colonic bacteria. This fermentation

produces gases which are excreted, and short-chain

fatty acids, which may be absorbed by the host and

PREBIOTICS 4719

thus represent a source of energy. The NDOs affect

the growth and, ultimately, the selective proliferation

of these bacteria. Currently, there are few data on the

relative efficiencies of different prebiotic oligosac-

charides, or on their selectivity at a species, or even

genus, level; however, the newly developed method-

ologies based on hybridization with specific rRNA

probes will help us to make progress in that direction.

It is often the case that a prebiotic effect is claimed for

certain NDOs, or other dietary carbohydrates, with-

out a full and careful investigation of their fermenta-

tion profile. Such data can never be accepted as a

proof for a prebiotic attribute. (See Carbohydrates:

Classification and Properties.)

0004 Among the nondigestible oligosaccharides, those

composed primarily of fructose units occupy a

leading position in food science. Fructooligosacchar-

ide is used as a generic name for all nondigestible

oligosaccharides composed mainly of fructose.

Strictly, however, the linear b-(2–1)-fructans are

inulin-type fructans, which are different from levans,

which are b-(2–6), often branched, fructans. The

inulin-type fructans are by far the most extensively

studied compounds and are clear market-leader

prebiotics. They are composed of b-d-fructofuranose

moities attached by b-2–1 linkages. The first mono-

mer of the chain is either an a-d-glucopyranosyl or a

b-d-fructopyranosyl residue. They constitute a series

of homologous oligosaccharides derived from sucrose

and may be represented by the formula GF

(where G ¼ glucose and F ¼ fructose). The natur-

ally occurring NDOs, which are extracted from

the roots of the chicory plant (Cichorium intybus),

are a mixture of either GF

(a-d-glucopyrano-

syl-[b-d-furanosyl]

n1

-d-fructofuranoside) or GF

(b-d-fructopyranosyl-[b-d-fructofuranosyl]

n1

d-fructofuranoside) molecules, with the number of

fructose units varying from 2 to 60–65 units. As

food ingredients, they are available as native inulin

(inulin ST, average DP 10) and high-molecular-mass

inulin (inulin HP, average DP 20), enzymatically

hydrolyzed inulin (oligofructose, average DP 4), and

a mixture of inulin HP and oligofructose (synergy 1),

all of which occur naturally in a variety of food plants

such as garlic, onion, asparagus, artichoke, banana,

and wheat. Oligofructose can be produced by enzym-

atic conversion of sucrose to give a mixture composed

of only GF

,GF

(average DP 3.8), sucrose,

glucose, and fructose. The galactooligosaccharides,

or transgalactooligosaccharides, are a potentially im-

portant class of prebiotics; they are produced indus-

trially from lactose by transglycosylation reactions

and consist of galactosyl derivatives of lactose with

b1–3 and b1–6 linkages. The purported bifidogenic

(i.e., stimulation of growth of bifidobacteria) nature

of fructooligosaccharides and galactooligosacchar-

ides is explained, at least in part, by the linkage

specificity of the bifidobacterium b-fructosidase and

b-galactosidase enzymes, respectively. These enzymes

are cell-bound.

0005The glucose-based maltooligosaccharides and

xylooligosaccharides are candidate prebiotics; how-

ever, specific bacterial enzymes for the degradation of

these molecules have not been identified.

Malabsorption of the Nondigestible

Oligosaccharides

0006The b configuration of the anomeric C-2 in the fruc-

tose moieties of the inulin-type fructans as well as in

the galactose monomer subunits of galactooligosac-

charides explains why they are resistant to hydrolysis

by human digestive enzymes (a-glucosidase, maltase-

isomaltase, and sucrase), which are mostly specific

for a-osidic linkages. In normal physiological con-

ditions, the inulin-type fructans and the galactooli-

gosaccharides are resistant to acid hydrolysis in

the stomach. The most convincing data have been

obtained from human intervention studies with ileos-

tomy volunteers. These studies show that 86–88% of

the ingested dose (10–30 g) of inulin or oligofructose

is recovered in the ileostomy effluent, supporting the

claim that these carbohydrates are practically non-

digestible in the small intestine of man. Using an in-

tubation technique in human volunteers, it has been

concluded that fructooligosaccharides are malab-

sorbed in the human small intestine (89% recovery).

The small, but still significant, loss of fructooligo-

saccharides in the upper part of the gastrointestinal

tract could be due to fermentation by the microbial

population colonizing the ileum, especially in ileos-

tomy patients, and/or to hydrolysis of the lowest-

molecular-mass oligomers such as the enzymatically

synthesized oligomers. In a recent review of the mal-

absorption characteristics of inulin-type fructans, the

authors concluded that ‘inulin and oligofructose pass

through the small bowel without degradation and

(furthermore) without influencing the absorption of

nutrients and minerals especially calcium, magnesium

and iron.‘

0007There are fewer data on the resistance of other

oligosaccharides to digestion in the human upper

gastrointestinal tract than for the inulin-type fruc-

tans. The available evidence comes predominantly

from in vitro experiments or is based on hydrogen

production and exhalation as an indirect marker of

colonic fermentation, or on stimulation of growth

of specific fecal microorganisms in animal models.

No in vivo human data are available. Thus, the

nondigestibility of isomaltooligosaccharides, soybean

4720 PREBIOTICS

oligosaccharides, galactooligosaccharides, palatinose

condensates, or xylooligosacchairdes in vivo remains

to be demonstrated.

Fermentation in the Large Bowel: the

Prebiotic Effect

0008 The large bowel is by far the most heavily colonized

segment of the human gastrointestinal tract, with up

to 10

(mostly anaerobic) bacteria per gram of gut

content. These bacteria belong to a wide variety of

genera, species, and strains. Through the process of

fermentation, these colonic bacteria produce a wide

variety of metabolites, among which the short-chain

fatty acids represent salvage of part of the energy of

malabsorbed food components, especially malab-

sorbed carbohydrates, and they play important sys-

temic physiological roles.

0009 Evidence for the fermentation of inulin-type fruc-

tans by bacteria colonizing the large bowel has come

from in vitro and in vivo studies. At nutritional doses

(up to 20–40 g per day), these malabsorbed carbo-

hydrates are fermented quantitatively and are not

excreted in the feces; the products of fermentation

are gases and short-chain fatty acids, mainly acetate,

butyrate, and propionate. Compared with most other

malabsorbed carbohydrates (e.g., resistant starch and

dietary fiber), the colonic fermentation of inulin-type

fructans is accompanied by a significant change in the

composition of the colonic microbiota due to select-

ive proliferation of bifidobacteria and a concomitant

reduction in the number of other bacteria, like bacter-

oides, fusobacteria, and pathogenic clostridia. On the

basis of the results of well-designed human studies

that have shown significant changes in the compos-

ition of human fecal flora, it can be concluded that

inulin-type fructans (5–15 g per day for a few weeks)

are prebiotic. But even though some studies showed a

significant reduction in the number of pathogenic

clostridia, the health benefits (e.g., reducing the risk

of intestinal infections) of such a change in the com-

position of the colonic microbiota have yet to be

established. A recent report has shown that oligofruc-

tose (daily dose of 6 g (3 2 g)) had no therapeutic

value in patients with irritable bowel syndrome. But

in an experimental model of necrotizing enterocolitis

in quails, data have been reported that support the

hypothesis that oligofructose might prevent over-

growth of the bacteria known to play a role in this

pathology in preterm neonates.

0010 For the other NDOs, studies in vivo have been

performed with doses ranging from 3 to 15 g per

day, given for periods of 1, 2, or 3 weeks. For soybean

oligosaccharides, a dose of 10 g given twice daily

for 3 weeks significantly increased the number of

bifidobacteria, whilst slightly decreasing clostridia

counts. A dose of 3 g per day increased bifidobacteria,

bacteroides, and eubacteria. For the galactooligosac-

charides, an increase in bifidobacteria and lactobacilli

in response to doses ranging from 3 to 10 g per day

has been reported. Similarly, a daily dose (5 or 10 g)

of galactosylsucrose stimulated the growth of bifido-

bacteria after 1 and 2 weeks of ingestion. A dose of

isomaltooligosaccharide of 13.5 g per day for 2 weeks

significantly increased the number of bifidobacteria

in adult and elderly volunteers. Early results suggest

that platinose condensate may stimulate the growth

of bifidobacteria. For all these NDOs, except the

galactooligosaccharides, only a single human inter-

vention study has been performed, and this will

need to be repeated before any prebiotic effect can

be substantiated. Moreover, as discussed above, great

care should be taken to quantify the component

species of the fecal microflora and to identify changes

in its composition.

Physiological Effects in the

Gastrointestinal Tract

0011The fermentation of prebiotics in the colon has a

series of consequences that affect large-bowel physi-

ology. Firstly, it contributes to the production of

short-chain fatty acids, thus creating a more acidic

environment, which is favorable for the development

of bacteria like bifidobacteria or lactobacilli but

unfavorable for the growth of potentially pathogenic

species like Clostridium spp. or Escherichia coli. Fur-

thermore, in this acidic environment, ammonia and

amines become protonated and are thus much less

absorbable and hence more readily excreted. Sec-

ondly, it leads to a proliferation of colonic bacteria,

which increases fecal mass and thus contributes to a

beneficial bulking effect and a regularization of stool

production. As a consequence of these two large-

bowel processes (i.e., production of acids and prolif-

eration of bacteria), only part of the metabolizable

energy of the NDOs is salvaged. As compared with

absorbed carbohydrates (e.g., nonresistant starch

or sucrose), NDOs represent, weight for weight, a

lower energy value for the host. Caloric values of

1–2.1 kcal g

1

(4.1–8.8 kJ g

1

), i.e., 25–50% of the

caloric value of sucrose, have been reported, espe-

cially for inulin-type fructans. But, as stated recently

by a group of European experts, ‘all carbohydrates

that are more or less completely fermented in the

human colon should be given a caloric value of

1.5 kcal g

1

(6.3 kJ g

1

).‘ In fact, the daily intake of

these carbohydrates is likely to remain small, prob-

ably often not representing more than 5% of total

daily energy intake. Therefore, it is not justifiable to

PREBIOTICS 4721

spend a great deal of effort in trying to give, for such

carbohydrates, a precise calorie value, the determin-

ation of which will depend on the protocol used and

probably also on the diet in which they are included.

0012 Another physiological consequence of the con-

sumption of NDOs that has been reported recently

for the inulin-type fructans is an increased bioavail-

ability of Ca

2þ

. Such an effect has been studied exten-

sively in rat and hamster. These studies have led to the

conclusion that, most probably because of their

malabsorption and colonic fermentation, these food

ingredients facilitate Ca

2þ

absorption from the large

bowel compartment, thus complementing the process

that takes place in the small bowel. A change in

colonic pH, production of short-chain fatty acids,

and increase in mucosal concentration of the protein

calbindin in the colon have been proposed as hypoth-

eses to explain that effect. Besides increased Ca

2þ

bioavailability, it has been shown both in rat and in

hamster that feeding fructooligosaccharides increases

2þ

concentration and improves the structure and

density of the bones. More recently, three human

trials (two in adolescents and one in adults) have

shown that supplementing the diet every day with

16.8 g of oligofructose, 8 g of a mixture of oligofruc-

tose and high-molecular-mass inulin, or 40 g of inulin

significantly increases the apparent absorption of

2þ

by 10–12%. One of these studies used measure-

ment of Ca

2þ

balance, and two used a double stable

isotope technique.

0013 NDOs, because they are malabsorbed and fer-

mented in the colon, may be considered to be part

of the dietary fiber complex. In particular, it has been

shown that inulin-type fructans have a fecal bulking

effect that is comparable on a weight-for-weight basis

with that of soluble fiber such as pectin. Moreover, an

internationally validated method derived from the

AOAC method for dietary fiber analysis has been

developed to quantify inulin and oligofructose in

plants and food products. For the purpose of food

labeling, the NDOs are thus classified as dietary fiber

in most countries. (See Dietary Fiber: Properties and

Sources; Determination; Physiological Effects.)

Prebiotics and the Risk of Colon Cancer

0014 Over the last two years, experimental reports have

been published that repeatedly demonstrate that

feeding inulin-type fructans to rats previously treated

with a colon carcinogen (i.e., dimethylhydrazine or

azoxymethane) reduces the incidence of the so-called

aberrant crypt foci in the colon. In one of these stud-

ies, the synbiotic approach that combines oligofruc-

tose (prebiotic) and bifidobacteria (probiotic) was

reported to be more active than either the prebiotic

or the probiotic alone. Even though still experimen-

tal, these data suggest that inulin-type fructans might

play a role in reducing the risk of developing

preneoplastic lesions and possibly cancer in the

colon. Moreover, such an effect might not be limited

to the inulin-type fructans. Indeed nondigestible

oligosaccharides may exert anti-carcinogenic effects

first because they have been shown to beneficially

affect certain biomarkers known to be associated

with cancer risk (e.g. increase in apoptotis in colonic

mucosa, reduction of bacterial b-glucuronidase and

nitrate reductase, pH, and conversion of a dietary

carcinogen to its genotoxic metabolite in caecal con-

tents or feces of NDOs fed rats and human volunteers

respectively), and second because they stimulate the

growth of lactic acid bacteria for which evidence of

anti-genotoxic and anti-carcinogenic effects have

been reported. (See Colon: Cancer of the Colon.)

Conclusion: Prebiotics, what Benefit(s) for

Human Health?

0015Prebiotics have nutritional properties that, according

to the present state of knowledge, derive mainly from

resistance to the hydrolytic activities in the upper part

of the gastrointestinal tract of monogastric organ-

isms followed by extensive fermentation in the large

bowel, which leads to significant and possibly health-

beneficial changes in the composition of the colonic

microbiota. The gastrointestinal target functions that

are associated with a balanced microflora together

with an optimal gut-associated lymphoid tissue

(GALT) are relevant to the state of well-being and

health and to the reduction of the risk of diseases.

The colonic microflora is a complex ecosystem, the

functions of which are a consequence of the combined

action of the microbes that, besides interacting with

the GALT, contribute to salvage of nutrient energy

and yield metabolic end products like the short-chain

fatty acids (SCFAs) that play a role in cell differenti-

ation, cell proliferation, and metabolic regulatory

processes. It is generally assumed that the group of

potentially health-promoting bacteria includes prin-

cipally bifidobacteria, lactobacilli, eubacteria, and

bacteroides, which are, and possibly should remain,

the most important genera in humans. Changes in the

composition of the fecal flora, a recognized surrogate

marker for the residual colonic microbiota, can be

considered as a marker, both indicator and factor, of

large-bowel functions. They might play a role in gas-

trointestinal infections and diarrhea, constipation,

irritable bowel syndrome, inflammatory bowel dis-

eases, and colorectal cancer. Probiotics (e.g., lactoba-

cilli or bifidobacteria), prebiotics (like chicory inulin

and its hydrolysate oligofructose), and synbiotics (a

4722 PREBIOTICS

combination of probiotics and prebiotics) are recent

concepts in nutrition that are being used to support

the development of functional foods targeted towards

gut functions. Their effects may include:

0016 stimulation of the activity of the GALT (e.g.,

increased IgA response, production of cytokines,

etc.);

0017 reduction of the duration of episodes of rotavirus

infection;

0018 change in the composition of the fecal flora to

reach/maintain a composition in which bifido-

bacteria and/or lactobacilli become predominant

in number, a situation that is considered optimal;

0019 increase in fecal mass (stool bulking) and stool

frequency;

0020 increase in calcium bioavailability via colonic

absorption (e.g., inulin).

By reference to the recently published European

consensus on scientific concepts of functional foods,

prebiotics, especially inulin-type fructans (but also

synbiotics) are good candidates to be recognized as

functional food ingredients for which claims shall

become authorized. Such claims should relate to

enhanced gastrointestinal functions (composition of

colonic flora, bulking effect and bowel habit, Ca

2þ

bioavailability) or risk of developing a disease like

colon cancer.

See also: Carbohydrates: Digestion, Absorption, and

Metabolism; Colon: Cancer of the Colon