J Agric Food Chem. 2009 (Apr 8); 57 (7): 2614-2622 ~ FULL TEXT
Stephen M. Boue, Thomas E. Cleveland, Carol Carter-Wientjes, Betty Y. Shih,
Deepak Bhatnagar, John M. McLachlan, Matthew E. Burow
Southern Regional Research Center,
Agricultural Research Service,
U.S. Department of Agriculture,
New Orleans, Louisiana 70179, USA.
Functional foods have been a developing area of food science research for the past decade. Many foods are derived from plants that naturally contain compounds beneficial to human health and can often prevent certain diseases. Plants containing phytochemicals with potent anticancer and antioxidant activities have spurred development of many new functional foods. This has led to the creation of functional foods to target health problems such as obesity and inflammation. More recent research into the use of plant phytoalexins as nutritional components has opened up a new area of food science.
Phytoalexins are produced by plants in response to stress, fungal attack, or elicitor treatment and are often antifungal or antibacterial compounds. Although phytoalexins have been investigated for their possible role in plant defense, until recently they have gone unexplored as nutritional components in human foods. These underutilized plant compounds may possess key beneficial properties including antioxidant activity, anti-inflammation activity, cholesterol-lowering ability, and even anticancer activity.
For these reasons, phytoalexin-enriched foods would be classified as functional foods. These phytoalexin-enriched functional foods would benefit the consumer by providing "health-enhanced" food choices and would also benefit many underutilized crops that may produce phytoalexins that may not have been considered to be beneficial health-promoting foods.
From the FULL TEXT Article:
There has been recent, increasing interest to identify phytochemicals
or plant compounds with health-promoting activities.
In vitro screening assays to identify these bioactive
compounds cover a broad area of research including antioxidant,
anticancer, antiobesity, cholesterol-lowering, and many other
activities. Often, successful characterization of a phytochemical
can lead to the development of new food products or supplements
with health-promoting activities. Over the past decade
these foods have been labeled functional foods, which are
generally accepted as foods that naturally contain especially
healthy qualities. Supplements containing health-promoting
activity are referred to as neutraceuticals.
Plants produce a diverse array of over 100,000 low molecular
weight natural products known as secondary metabolites. 
These secondary metabolites differ from the components of
primary metabolism because they are generally considered not
essential to basic plant metabolic processes. Most of these
compounds are derived from various plant pathways, including
the isoprenoid, phenylpropanoid, alkaloid, or fatty acid/
polyketide pathways. One group of important secondary metabolites
is the flavonoid group. Flavonoids are ubiquitous in
many plants and provide utility for the plant as flower pigments
to attract pollinating insects, UV protectants, signal molecules
to symbionts, and defense against pathogens. Onions, apples,
and grapes are examples of foods that naturally contain
flavonoids which also contribute to high antioxidant activity.
Isoflavones are a subclass of flavonoids and are the primary
constitutive secondary metabolites found only in legumes.
Important health-promoting activities have been linked to
legume consumption, including reduced risk of various
cancers [2–7] and coronary heart disease. [2–7] The only
legume to contain nutritionally relevant amounts of isoflavones
is soybean. Genistein, daidzein, and glycitein, along with their
respective – and malonyl glycosides, are the predominant
isoflavones in soybean. Many soy foods and supplements that
are considered to be functional foods have high concentrations
of the constitutive isoflavones daidzein and genistein.
Isoflavones belong to another class of compounds now
becoming important to nutritionists called phytoalexins. Phytoalexins
are low molecular weight antimicrobial compounds
that are synthesized de novo and accumulate in plants in
response to infection or stress due to wounding, freezing,
ultraviolet light exposure, and exposure to microorganisms. [8–12] Phytoalexin biosynthesis can be manipulated by application
of abiotic (nonliving) or biotic (living) factors that
stress the plant into producing or releasing greater phytoalexin
concentrations. [8–13] Antifungal, antimicrobial, and antioxidant
activities are some of the beneficial activities of phytoalexins
that help to enhance the survival of the soybean plant or
seed during stress induction. 
Phytoalexins have been well documented in the field of plant
defense. Much research has been conducted on the elicitation
process, and specific elicitors have been discovered. [8–13]
However, only recently are phytoalexins being explored as
nutritional components and a source for development of healthpromoting
food products. These underutilized plant compounds
could hold previously unknown potential for antioxidant activity,
anti-inflammation activity, cholesterol-lowering ability, and even
anticancer activity. For these reasons, an updated system is
proposed whereby foods containing enhanced or elicited phytoalexins
are considered to be phytoalexin-enriched functional
foods. Phytoalexin-enriched foods would benefit the consumer
by providing health-enhanced food choices and would also
benefit many underutilized crops that may produce phytoalexins
that may not have been considered to be a beneficial healthpromoting
Functional Foods Cover a Broad Spectrum.
foods are foods that provide health benefits beyond basic
nutrition due to certain physiologically active components. Many
of these foods may help in disease prevention, reduce the risk
of disease, or enhance health. As consumer interest has shifted
toward achieving and maintaining good health, interest in
functional foods has increased dramatically. Consumers are
seeking a greater number of healthy functional foods such as
those listed in Table 1.
Red Wine and Resveratrol.
The “French paradox” showed
the incidence of coronary heart disease is relatively low in
southern France despite high dietary intake of saturated fats and
has been attributed in part to the consumption of red wine.
Epidemiological studies found that low incidence of coronary
heart disease among wine-drinking populations correlated with
resveratrol present in wine. [14, 15] Resveratrol is found in
grapes, peanuts, and herbal plants. [14, 15] Resveratrol is found
at high concentrations in the skin of red grapes and is a
constituent of red wine. Red wine contains between 0.2 and
5.8 mg/L, depending on the grape variety, whereas white wine
has much less. Red wine is fermented with the skins, allowing
the wine to absorb the resveratrol, whereas white wine is
fermented after the skin has been removed. Resveratrol has
antioxidant, anti-inflammation, and anticancer properties. [16–19]
Resveratrol has also been shown to extend the life span of
several short-living species of animals  and, more recently,
has been shown to improve the health and survival of mice. 
These studies on the health benefits of resveratrol spurred
consumer interest in red wines, supplements containing resveratrol,
and other functional foods containing resveratrol.
Green Tea and Catechins.
Tea is the second most consumed
beverage in the world, after water. Freshly brewed green, black,
oolong, and decaffeinated teas all seem able to promote health.
Most of the research focus has been on the polyphenolic
constituents of tea, particularly green tea. Green tea extracts
have shown anticarcinogenic  and antimutagenic 
activities. Epidemiological studies linking cancer chemopreventive
effects to green tea have produced both positive
correlations  and inconclusive results.  However, the
consumption of three or more cups of green tea per day was
associated with decreased recurrence of breast cancer in
Japanese women.  Also, there is evidence that green tea
components have cancer chemopreventive effects [27, 28] and
may reduce the risk of cardiovascular disease. [2, 8] Much of
the beneficial effects of green tea have been attributed to its
high antioxidant activity [29, 30] due mostly to the catechins
epigallocatechin gallate, epicatechin gallate, epigallocatechin,
and epicatechin. [30–32]
Legumes and Isoflavones.
The observation that animal diets
of clover and soy affected reproduction led to the discovery of
phytochemicals with estrogenic activity. [33, 34] Since then,
more than 300 plants have reportedly caused estrogenic
responses in animals, resulting in efforts to identify estrogenic
compounds in animal and human food products. [35, 40] The
estrogenic phytochemicals, which include isoflavones, lignans,
phytostilbenes, and enterolactones, appear to primarily function
by binding to and activating the estrogen receptor, at 100–1000
greater concentrations than 17β-estradiol. [41–43] Phytoestrogens are found in a variety of plants, including fruits and
vegetables, but are most abundant in leguminous plants.
Legumes are present in most diets throughout the world, and
many other parts of the plant are edible in addition to the seeds.
The legume that has attracted the most attention is the soybean,
which contains high concentrations of the isoflavones daidzein
and genistein (Figure 1) that are responsible for many of soy’s
health benefits. [35–40]
The observation that soy isoflavones can function as 17β-estradiol agonists is consistent with the observed health benefits
of soy foods such as decreased incidence of osteoporosis and
cardiovascular disease. [35–40, 44–46]
However, the similar
decrease in risk of breast cancer would indicate a potential
antiestrogenic activity of soy isoflavones. [43–46] Consistent
with this information, certain phytochemicals have been reported
to exert antiestrogenic effects at higher concentrations. 
Studies have shown that phytoestrogens may prevent cancer [35–40, 44–46], act as antioxidants [47–49], and lower serum
cholesterol.  Isoflavones act as anticancer agents through
several mechanisms, one of which may be an ability to function
as antioxidants. Isoflavones can inhibit free radical formation [47, 49], reduce lipid oxidation , and stimulate antioxidant enzymes.  That isoflavones can act as antioxidants
is due to an ability to form delocalized unpaired electrons,
stabilizing the formed phenoxyl radical after reaction with lipid
Fermentation Effects on Soy Isoflavones.
have been extensively studied because of their potential to
promote human health. Growing evidence shows that isoflavones
function as antioxidants and free radical scavengers. [47, 48, 55–60]
Naim et al.  observed that the number of hydroxyl
groups in the isoflavone nucleus positively correlated with
antioxidative capacity and that the aglycones had higher
antioxidant activities than their glycosides. Other research has
demonstrated that the malonyl isoflavones possess strong
antioxidant activities, but are very unstable during storage. 
Much of this research suggests that techniques to increase the
aglycone isoflavones in soy would increase antioxidant activities.
One method to increase the concentration of the aglycone
form of isoflavones in soy foods is through the use of
fermentation. Many different soy foods are made from fermented
soybeans using different strains of bacteria and fungi. Miso,
tempe, and soy sauce are all popular foods in Asia and produced
from fermented soybeans. The higher levels of isoflavone
aglycones that are produced contribute to higher antioxidant
activities [62–64]; however, in each study the soybean seeds
were initially steamed at high temperatures (typically 120 °C),
which prevented further production of phytoalexins during the
fermentation process. High-temperature steps applied before
fermentation irreversibly denature the majority of enzyme
proteins within the seed including those in the isoflavonoid
pathway and lyse cells, thus disrupting the biochemical signaling
necessary for phytoalexin production. Heating the soy followed
by fermentation could affect the levels and health-promoting
effects of the isoflavones.
Organic Foods Contain Altered Levels of Secondary Metabolites.
One method that could increase the production
of secondary metabolites, particularly flavonoids, in plants is
through organic culturing techniques instead of conventional
production. Conventional agricultural practices utilize many
chemical substances such as fertilizers, herbicides, and insecticides that both increase and decrease production of the
polyphenolic compounds in plants which may be beneficial to
health. [65, 66] Without the use of many protective fungicides
and pesticides, plants grown using organic agricultural practices
are left vulnerable to more insect and plant pathogen attack,
which could alter their concentrations of secondary metabolites.
The production of polyphenolics can occur in the edible portion
of vegetable and fruit plants, particularly the skin or outer
surface, thus contributing to health effects of the food.
Several researchers have demonstrated differences in nutrients
and polyphenolics in foods prepared from organic versus
conventionally grown crops. Micronutrients in tomatoes are
influenced by growing conditions, and several papers have
compared their effects on microconstituents. Significantly higher
levels of quercetin (30%), kaempferol (17%), and ascorbic acid
(26%) were found in organically grown Burbank tomatoes (fresh
weight basis), and significantly higher levels of kaempferol
(20%) were found in a second tomato variety, Ropreco. 
Caris-Veyrat et al.  found that organically grown tomatoes
had higher vitamin C, carotenoid, and polyphenol contents
(except for chlorogenic acid) based on fresh matter when
compared with conventionally grown tomatoes. However, a
study by Rossi et al.  found organic tomatoes contained
more salicylic acid but less vitamin C and lycopene versus
Besides vegetables, many fruits are affected by growing
conditions. Carbonaro et al.  demonstrated a parallel increase
in polyphenol content and polyphenol oxidase activity of organic
peaches and pears when compared to conventionally grown
peaches. Ascorbic acid and citric acids were higher in organic
peaches, and R-tocopherol was increased in organic pears. 
The authors concluded that organic cultivation practices improved
the antioxidant defense system of the plant. A study
comparing organic and conventional grapefruit demonstrated
that the organic fruit contained higher levels of ascorbic acid
and naringin, but lower levels of lycopene.  Also, organic
grape juice (white and purple) showed statistically higher values
of total polyphenols and resveratrol compared to conventional
grape juices from both. 
PHYTOALEXIN-ENRICHED FUNCTIONAL FOODS
Generation of Phytoalexin-Enriched Foods.
were first defined as plant secondary metabolites with antimicrobial
activity that were synthesized de novo and functioned
as the basis of a disease resistance mechanism.  In 1980, a
new working definition assigned phytoalexins as low molecular
weight, antimicrobial compounds that are both synthesized and
accumulated in plants after exposure to microorganisms. 
This new definition excluded antibiotic compounds that are
present in plant tissues prior to microbial infection. J. W.
Mansfield coined these pre-existing compounds phytoanticipins
and defined them as low molecular weight, antimicrobial
compounds that are present in plants before challenge by
microorganisms or are released after infection solely from preexisting
constituents.  This distinction between phytoanticipins
and phytoalexins helps to distinguish plant compounds
and is based solely on how these compounds are produced.
Therefore, resveratrol in grapes and daidzein in soy would be
both phytoanticipins (Figure 2) and phytoalexins depending on
how they were produced. Other compounds such as the
glyceollins in soy would be labeled strictly as phytoalexins. [8–12]
Most functional foods would be defined as foods with healthpromoting
activities based on phytoanticipin content (Figure 2). These functional foods are based on pre-existing plant
compounds or processing methods that convert these pre-existing
compounds to different forms. Phytoalexin-enriched foods would
be defined as foods with health-promoting activities based on
phytoalexins and would be a subclass of functional foods. Thus,
a phytoalexin-enriched food could contain phytoanticipins, but
its health-promoting activity would be due in part to its
phytoalexin content. These phytoalexins could be generated by
numerous methods using biotic and abiotic elicitors and other
stress-inducing techniques both preharvest and postharvest
(Figure 3). This change would help clarify future uncertainties
in the labeling of foods and supplements containing
Elicitor Treatments Enhance Resveratrol Content of Grapes.
Several studies have demonstrated that elicitor treatment
can increase resveratrol concentrations of postharvest
grapes. [76–80] The ability of UV irradiation to elicit the
phytoalexin resveratrol was first demonstrated using leaf disks
and immature berries.  Further experiments using UV
irradiation showed that mature grape berries produced higher
concentrations of resveratrol 48 h after UV light exposure. 
The potential of UV irradiation to create a resveratrol-enriched
table grape was also demonstrated by Cantos et al. [79, 80] A serving of irradiated grapes (unpeeled) could supply the
resveratrol content equivalent to three glasses of red wine (ɣ
mg of resveratrol per glass). In both the title and conclusion of
the publication, the authors ask if this is a new “functional”
fruit.  By the definitions created here we believe that this
resveratrol-enriched fruit points to a new subclass of healthpromoting
functional foods that can be engineered by inducing
higher concentrations of targeted phytoalexins.
Elicitor Treatments Enrich Antioxidant and Antiestrogenic Activities of Soy Extracts.
Research in the area of plant
defense over the past several decades has fostered identification
of many phytoalexins throughout a vast number of plant
species. [8, 13] Of particular interest to many groups were the
isoflavone phytoalexins produced by soybean and other
legumes. [8–13, 81–86]
The glyceollins (I, II, and III) are the
predominant soybean phytoalexins with antimicrobial activity
against numerous soybean pathogens. [8–13] Recent research
from our laboratory has shown that the glyceollins have
antiestrogenic and anticancer activities. [82–84] Further work
has shown that extracts from elicitor-treated soybeans have
higher antioxidant activities when compared to untreated
controls.  Soy isoflavone phytoalexins, long known only
as plant defensive antimicrobials, are now being viewed as
beneficial plant compounds that can be considered alongside
other soy isoflavones when anticancer activity and other healthpromoting
properties are evaluated (Figure 4).
soy glyceollins are currently not present in any commercial soy
food products, the potential exists for a new subclass within
functional foods that could be termed phytoalexin-enriched
foods. Phytoalexin-enriched foods are defined as any food
prepared from plant material that contains higher concentrations
or de novo levels of phytoalexins resulting from elicitor
treatment. Elicitor treatments range from biotic elicitors such
as microorganisms (Aspergillus sojae, Aspergillus oryzae, and
Rhizopus oligosporus), microorganism cell wall extracts, and
carbohydrates to abiotic elicitors including UV induction
and wounding (cutting). Recently, Feng et al.  showed that
black soybeans germinated under fungal stress with food grade
R. oligosporus could be utilized for the production of soy milk
and soy yogurt containing glyceollins and oxylipins. Also,
germination of black soybean with R. oligosporus for 3 days
was sufficient to reduce stachyose and raffinose (which cause
flatulence) by 92 and 80%, respectively. This research serves
as proof of principle that phytoalexin-enriched foods or foods
containing phytoalexins can support a niche in food research.
Induction of Phytoalexins Preharvest.
Most work inducing
phytoalexins in food products is done postharvest (Figure 3).
The phytoalexin resveratrol is induced to higher concentrations
in grapes postharvest using several elicitors. [76–80] Most
research inducing glyceollins and other soy phytoalexins is done
postharvest using seeds, pods, cotyledons, or hypocotyls.
However, preharvest techniques are being developed to induce
phytoalexins in plants.
Resveratrol concentrations can be increased preharvest using
several different techniques. [88, 89] Jeandet et al. observed
3–5-fold increased concentrations of resveratrol in grapes
harvested under conditions that encouraged the development
of gray mold caused by Botrytis cinerea.  Iriti et al. (88)
observed increased polyphenolic and resveratrol contents in
preharvest grapes using the plant activator benzothiazole (BTH,
0.3 mM). Field treatments with BTH induced resistance against
gray mold caused by B. cinerea and caused a 110% increase in
the phytoalexin resveratrol. These experiments demonstrate the
potential for preharvest techniques to produce a resveratrolenriched
Besides inducing resveratrol in grapes, other experiments have
demonstrated the potential for the preharvest induction of
phytoalexins. Greenhouse experiments demonstrated that seed
and foliar applications of biotic elicitors increased total isoflavone
concentrations in soybean.  Total isoflavone (genistein,
daidzein, and glycitein) increases ranged from 16 to 93% in
the mature seed using a foliar application of chitosan; however,
the phytoalexin glyceollin was not detected. Glyceollins can
be induced in soybean seedling tissues using UV light in the
living plant. [81, 92] The preharvest application of biotic elicitors
to the soybean plant induces the phytoalexins genistein and
daidzein within the seed (systemic), but is not expected to induce
glyceollin in the mature seed that is protected by the soybean
pod. Other preharvest techniques need to be developed to induce
glyceollin in soybean, including the production of a glyceollinenriched
Cell Culture and Hydroponics Produce Phytoalexins.
cell cultures have long shown that phytoalexins can be produced
using elicitors. [93–96] Cell cultures and hydroponics using
either elicitors or electrical current could also be utilized to
enhance phytoalexins in different plants. Recently, VanEtten
et al. showed that pisatin could be produced at higher concentrations
using both elicitors and electrical stimulation applied to
the developing root systems of hydroponically grown pea
seedlings.  They found that exposing pea plants to certain
sublethal doses of electric current produced 13 times higher
amounts of pisatin than plants that were not exposed to
electricity. The researchers observed similar increases in plant
chemicals produced by a variety of other plants when exposed
to electricity. There were no adverse effects on the plants.
Likewise, similar treatments could be applied to root systems
of soybean and other legume seedlings during hydroponic
growth to elicit production of phytoalexins with possible health
Most functional foods rely on constitutive plant compounds
or phytoanticipins to provide health-promoting benefits. Recent
trends have shown increased interest by consumers in many
health-promoting foods and supplements. Organically grown
foods have been reported to contain higher levels of healthpromoting
compounds due to exposure to “naturally” occurring
challenges from plant pests that induce defensive compounds
(phytoalexins) that may have additional health benefits. It is
tempting to speculate that in modern agriculture we are limiting
at least to some extent the production of health-promoting
compounds in our diets that may be present at higher levels in
organically grown foods or have been at higher levels in foods
grown before the advent of modern agricultural pest control.
We propose a new area within functional food research called
phytoalexin-enriched foods that utilize induced plant compounds
or phytoalexins created either pre- or postharvest that have been
traditionally viewed only as plant defensive compounds, but
have beneficial health effects. Research from our laboratory and
others has shown that many plants can produce higher levels
of beneficial compounds under conditions of stress or elicitor
treatment. By employing the plant’s own enzyme factory, many
of these compounds can be produced at increased levels and
readily incorporated into food products.
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