Antoxidantes En Frutas Y Vegetales

Review

Antioxidants in fruits and vegetables ± the millennium's

health

Charanjit Kaur1 & Harish C. Kapoor2*

1 Division of Fruits and Horticultural Technology and

2 Division of Biochemistry, Indian Agricultural Research Institute, New Delhi-110012, India

(Received 5 October 2000; Accepted in revised form 1 June 2001)

Summary Some of the most exciting research in the last decade has been the discovery of a group of

nutrients, which have protective e€ects against cell oxidation. These naturally occurring

compounds impart bright colour to fruits and vegetables and act as antioxidants in the

body by scavenging harmful free radicals, which are implicated in most degenerative

diseases. Epidemiological studies have established a positive correlation between the intake

of fruits and vegetables and prevention of diseases like atheroscelerosis, cancer, diabetes,

arthritis and also ageing. So pronounced has been their e€ect on ageing that they have been

called `fountains of youth'. Fruits and vegetables have thus had conferred on them the

status of `functional foods', capable of promoting good health and preventing or

alleviating diseases. Phenolic ¯avonoids, lycopene, carotenoids and glucosinolates are

among the most thoroughly studied antioxidants. The present review highlights the

potential of fruits and vegetables rich in antioxidants, their health bene®ts and the e€ect of

processing on the bioavailability of these nutrients. The paper also reviews some of the

important methods used to determine the antioxidant activity.

Keywords Antioxidant activity, carotenoids, ¯avonoids, free radicals, phenols, processing methods, vitamins.

Introduction

In recent years human health has assumed an

unprecedentedly important status. Increased

interests in nutrition, ®tness and beauty have

exaggerated concerns over diet and human health.

A new diet-health paradigm is evolving which

places more emphasis on the positive aspects of

diet. Foods have now assumed the status of `func-

tional' foods, which should be capable of provi-

ding additional physiological bene®t, such as

preventing or delaying onset of chronic diseases,

as well as meeting basic nutritional requirements.

Nutritional studies are now concentrating on

examining foods for their protective and disease

preventing potential (Nicoli et al., 1999) instead of

negative attributes such as micro-organism count,

adulterants, fatty acids and inorganic pollutant

concentration. Recently phytochemicals in fruits

and vegetables have attracted a great deal of

attention mainly concentrated on their role in

preventing diseases caused as a result of oxidative

stress. Oxidative stress, which releases free oxygen

radicals in the body, has been implicated in a

number of disorders including cardiovascular

malfunction, cataracts, cancers, rheumatism and

many other auto-immune diseases besides ageing.

These phytochemicals act as antioxidants, scav-

enge free radicals and act as saviours of the cell.

Epidemiological studies have consistently shown

that there is a clear signi®cant positive associations

between intake of fruits and vegetables and

reduced rate of heart diseases mortality, common

cancers and other degenerative diseases as well as

ageing (Steinmetz & Potter, 1996; Garcia-Closas

et al., 1999; Joseph et al., 1999; Dillard &

*Correspondent: e-mail: hck_bioc@yahoo.com

International Journal of Food Science and Technology 2001, 36, 703±725 703

Ó 2001 Blackwell Science Ltd

German, 2000; Prior & Cao, 2000; Wargovich,

2000). The strongest evidence is related to reduced

risk of cancers of the mouth and pharynx,

oesophagus, lung, stomach and colon. The avail-

able data also gives strong support to a protective

role for fruits and vegetables in protection against

pancreas, bladder and breast cancer (American

Institute of Cancer Research., 1997). This is

attributed to the fact that these foods may provide

an optimal mix of phytochemicals such as natural

antioxidants, ®bres and other biotic compounds.

Free radicals and antioxidants

Free radicals

It is a paradox that oxygen, which is considered as

essential for life, is also reported to be toxic. Its

toxicity is because of the process that unleashes the

free radicals. The term free radical seems to appear

a lot lately in everything, from vitamin brochures

to cosmetic advertisements. Free radicals are

unstable highly reactive and energized molecules

having unpaired electrons. Examples of oxygen

derived free radicals include super oxide (O2

±),

hydroxyl (OH±), hydroperoxyl (HOO±), peroxyl

(ROO±) and alkoxyl (RO±) radicals. Other com-

mon reactive oxygen species (ROS) produced in

the body include nitric oxide (NO±) and the

peroxynitrite anion (ONOO±) (Prior & Cao,

2000). Free radicals react quickly with other

compounds, trying to capture the electrons needed

to gain stability. Generally free radicals attack the

nearest stable molecules, `stealing' its electrons.

When the molecule that has been attacked and

loses its electron it becomes a free radical itself,

beginning a chain reaction. Once the process is

started, it can cascade, initiating lipid peroxidation

which results in destabilization and disintegration

of the cell membranes or oxidation of other

cellular components like proteins and DNA,

®nally resulting in the disruption of cells (Halliwell

et al., 1995). Oxidation caused by free radicals sets

reduced capabilities to combat ageing and serious

illness, including cancer, kidney damage, athero-

scelrosis and heart diseases (Ames, 1983).

Some free radicals arise normally during meta-

bolism. Sometimes the body's immune system's

cells purposefully create them to neutralize viruses

and bacteria. However environmental factors such

as pollution, radiation, cigarette smoke and

herbicides can also generate free radicals. Thus

free radicals on one hand can produce bene®cial

e€ects but can also induce harmful oxidation and

cause serious cellular damages, if generated in

excess.

Antioxidants

To deal with the free radicals or so called ROS, the

body is equipped with an e€ective defence system

which includes various enzymes and high and low

molecular weight antioxidants. Antioxidants neut-

ralize free radicals by donating one of their own

electrons, ending the electron-stealing reaction.

The antioxidants do not themselves become free

radicals by donating electrons because they are

stable in either form. These act as scavengers and

play the housekeeper's role by mopping up free

radicals before they get a chance to create havoc in

a body. Thus they may well be de®ned as the

substances that are capable of quenching or

stabilizing free radicals.

Antioxidants have also been suggested to have a

well de®ned role as preservatives. These have been

de®ned by the US Food and Drug Administration

(FDA) as substances used to preserve food by

retarding deterioration, rancidity or discoloration

caused by oxidation (Dziezak, 1986). Lipid peroxi-

dation is an important deteriorative reaction of

foods during processing and storage. Toxic sub-

stances formed by lipid peroxidation may lead to

adverse e€ects such as carcinogenesis, cell DNA

mutagenesis and ageing. Antioxidants therefore,

according to their mode of action, have also been

classi®ed as the compounds that terminate the free

radical chain in lipid oxidation by donating

electrons or hydrogen to fat containing a free

radical and to the formation of a complex between

the fat chain and a free radical. Antioxidants stop

the free radical chain of oxidative reactions

by contributing hydrogen from the phenolic

hydroxyl groups, themselves forming stable free

radicals that do not initiate or propagate further

oxidation of lipids (Free Radical Terminators).

Some of the important synthetic antioxidants of

this class are butylated hydroxyanisole (BHA),

butylated hydroxy toluene (BHT), tert- butylhy-

droquinone (TBHQ), propyl gallate (PG) and

tocopherols.

704 Antioxidants in fruits and vegetables C. Kaur & H. C. Kapoor

International Journal of Food Science and Technology 2001, 36, 703±725 Ó 2001 Blackwell Science Ltd

Not all antioxidant activity is conferred by free-

radical interceptors. Reducing agents that func-

tion by transferring hydrogen atoms have also

been categorized as oxygen scavengers. Some of

these are ascorbyl palmitate, sulphites, ascorbic

acid, glucose oxidase and erythorbic acid. To be

e€ective in foods these must be added during

manufacturing or to the ®nished

...