Carotenoids
Carotenoids
are compounds constituted by eight isoprenoid units (ip). The ip units are
joined in a head-to-tail pattern, but the order is inverted at the molecule center
(Delgaldo and Lopez, 2003). Carotenoids
are naturally occurring tetraterpene pigments widely distributed through out
the living world (Zdzlaslaw, 2002). Lycopene (C40 H56) is
considered the first colored carotenoid in the biosynthesis of many other natural
carotenoids and it has a linear structure (Delgaldo and
Lopez, 2003). Lycopene is the carotenoid that gives the red color to tomatoes
and watermelons.
Moreover, it is also common to find acyclic, cyclic, and shortened carotenoids,
among others. Carotenoids have very important
biological activities and their use as food and feed today is recommended
largely due to their vitamin A and antioxidant activities important in
maintaining life
(Delgaldo and Lopez, 2003; Zdzislaw, 2002). Due to their ability to quench singlet oxygen and trap peroxyl
radicals, carotenoids have been described as excellent antioxidants. In
addition, an inverse association between the ingestion of carotenoid-containing
fruits and vegetables and the risk of certain forms of chronic diseases has
been suggested (Østerlie and Lerfall, 2005). Carotenoids
of the higher plants are found in roots, stems, leaves, flowers and fruits
giving them the red, yellow, and orange colors. In higher plants, carotenoids
are found in plastids (Delgaldo and Lopez, 2003). They are in the chloroplast
of photosynthetic tissues and they are found in chromoplasts. The majority of
the carotenoids found in plant tissues is β-carotene (Zdzislaw, 2002). Zdzislaw (2002) defines β-carotene as an
asymmetrical molecule of 40 carbon atoms, consisting of 8 isoprene units having
11 conjugated double bonds and 2 β-ionone rings. Animals derive
carotenoid pigments by consumption of carotenoid-containing plant materials.
The name carotenoid has been derived from the major pigment of the carrot (Zdzislaw, 2002).
Fig 7: Structural Formulae of different types of
carotenes (Zdzislaw,
2002).
The color of carotenoid pigments is as
a result of the presence of a system of conjugated double bonds. A minimum of
seven conjugated double bonds is required for the yellow color to appear. The
increase of double bonds results into a shift of the major adsorption bands to
the longer wavelengths, and the hue of carotenoids become more red (Zdzislaw, 2002). Because of the
highly conjugated double-bond system, carotenoids show ultraviolet and visible
absorption spectrum characteristics. According to Zdzislaw (2002), for most carotenoids, three peaks or two peaks
and a shoulder, are absorbed in the range of 400–500 nm. The absorption maxima
and molecular extinction values are significantly affected by the solvent used.
In unprocessed plants, usually all-trans double-bond configurations
occur, but cis isomers of each
carotenoid are also possible. Processing and storage cause isomerization of
carotenoids in foods and affect the color. The deepest color is due to
compounds with all-trans configurations.
So, increasing the number of cis bonds
results into gradual lightening of the color. This is because cis isomers absorb less strongly than
the all-trans isomer, and peak
at 330–340 nm (Zdzislaw, 2002).
Due to the presence of asymmetric
carbon atoms, many carotenoids have chiral centers. However, natural
carotenoids exist only in one of the possible enantiomeric forms, because the
biosynthesis is enantiomere selective.
Carotenoid classification.
By their structural elements
-Carotenes; carotenoids constituted by
carbon and hydrogen e.g. α-carotene, β-carotene and β-cryptoxanthin.
-Xanthophylls; constituted by
carbon, hydrogen and additionally oxygen. For example lutein, zeaxanthin,
violaxanthin, neoxanthin, and flucoxanthin.
By their functionality.
-Primary; carotenoids required for
the photosynthetic process e.g. β-carotene, lutein, zeaxanthin, violaxanthin,
neoxanthin, and antheraxanthin.
-Secondary; their presence is not
directly related with plant survival.
Carotenoids as food colors
Natural carotenoid extracts have
been used as food colorants for centuries. The most commonly used natural carotenoid
extracts for foodstuffs are annatto, paprika, and saffron. Many others sources,
including alfalfa, carrot, and tomato, citrus peel, and palm oil, are also
utilized (Delgado and Lopez, 2003). In food industries, carotenoids are
utilized mainly to color alcoholic beverages, soft drinks, fresh meat, cheese,
cereals, salad dressings, canned fruits and vegetables, chewing gum, yoghurt,
ice-cream, milk, margarine, pasta, canned fish, bread, potato derived products,
sauces, fruit juices, instant soups, oil, butter, etc.
Tomatoes (Lycopersicon esculentum)
Tomatoes have a high content of
carotenoids with lycopene being the main compound making up 80 to 90% of the
total carotenoid, followed by β-carotene (Delgado and
Lopez, 2003). The lycopene content in tomatoes increases with ripeness. New
tomato varieties with high and improved content have been developed and lycopene
products have begun to be commercialized (Zdzislaw, 2002). However, according to Delgado and Lopez (2003), other
modifications are required to use tomato extract as colorant because of its
strong flavor. Adding lycopene from natural sources to minced meat could lead
to a meat product with different taste, better color and with a well,
documented health benefit and a new functional food would be developed and may
be natural lycopene could replace nitrite added minced meat(Østerlie and
Lerfall, 2005).
Processing and stability
In processing or storage of colored
food, carotenoids are sensitive to treatments. High and short times are
preferred conditions during processing of carotenoid-containing foods ( Fennema
1996; DeMan, 1999; Delgado and
Lopez, 2003). Drying (uncontrolled)
leads to carotenoid degradation due to the presence of free radicals. In tomatoes,
the dehydration process leads to isomerization but it has been observed that
osmotic treatment does not affect the isomeric profile of lycopene (Delgado,
and Lopez, 2003). Freezing causes
little changes in carotene content. Blanching increases carotenoid content
relative to the raw tissue due to the inactivation of lipoxygenase which is
known to catalyze oxidative decomposition of carotenoids. Carotenoids are extremely
lipophilic compounds that are almost insoluble in water. In aqueous
surroundings they tend to form aggregates or adhere to surfaces (Zdzislaw, 2002; Fennema 1996; DeMan, 1999; Delgado
and Lopez, 2003)
Anthocyanins
Anthocyanins are flavonoids with a characteristic C6 C3C6
carbon skeleton making them the most commonly distributed
pigment group in plants kingdom (Deman, 1999; Fennema, 1996; Zdzislaw, 2002).
Anthocyanins occur in all higher
plants, mostly in flowers and fruits but also in leaves, stems, and roots. In
these parts they are found predominantly in outer cell layer and account for a
wide range of colors including blue, purple, pink, orange, red, magenta and
violet in the plant kingdom (http://www.food-info.net/uk/colour/anthocyanin.htm;
DeMan, 1999).
In food plants, the main sources of anthocyanins are berries, such as
blackberries, grapes, blueberries, and some vegetables, such as egg-plants (aubergine), red cabbage and avocado.
Other sources include oranges, elderberry, olives, red onion, fig, sweet
potato, mango, Hibiscus spp (musayi
plant) and purple corn. They play a definite role in attracting animals in
pollination and seed dispersal. Accordng to Zdzislaw (2002), they may also have
a role in the mechanism of plant resistance to insect attack.
The structure of anthocyanins is
based on a C15 skeleton consisting of a chromane ring bearing a
second aromatic ring. The cyclic structures are arranged in the pattern
C-6-C-3-C-6. Anthocyanins structure is complemented by one or more sugar
molecules bonded at different hydroxylated position of the basic structure (Delgado
and Lopez, 2003).
All anthocyanins are based on a single basic core structure, the
flavyllium ion (Zdzislaw, 2002).
Fig 8; Flavylium cation (Zdzislaw, 2002).
These side groups can be a hydrogen
atom (H), a hydroxide (OH) or a methoxy-group (OCH) depending on the considered
pigment. According to Zdzislaw (2002), from about 20 known naturally occurring
anthocyanidins, only 6 occur most frequently in plants and these include:
pelargonidin, cyanidin, peonidin, delphinidin, petunidin, and malvidin.
Classification
Anthocyanins are glycosides of polyhydroxy and polymetoxy
derivatives of 2-phenylbenzopyrylium or flavylium cation. They show high
diversity in nature but all are based on a reduced number of basic
anthocyanidin structure. Anthocyanin diversity is associated with the number of
sugars found in nature but glycosylated anthocyanins are formed with glucose,
rhamnose, xylose, galactose, arabinose, and fructose (Deman, 1999; Fennema,
1996; Zdzislaw, 2002).
The chemical combinations of these
sugars with organic acids to produce acylated anthocyanins also increase
diversity. Differences between individual anthocyanins are: the number of
hydroxyl groups in the molecule; the degree of methylation of the hydroxyl
groups; the nature, number, and position of glycosylation; and the nature and
number of aromatic or aliphatic acids attached to the glucosyl residue.
Substitution of the hydroxyl and
methoxyl groups affects the color of the anthocyanins. Color is also affected
by the number of hydroxyl and methoxyl groups. If more hydroxyl groups are
present, then the color goes towards bluish shade, and redness is increased if
more methoxyl groups are present (http://en.wikipedia.org/wiki/Anthocyanin;
Zdzislaw, 2002)
Chemically anthocyanins are subdivided into the; sugar-free
anthocyanidine aglycons and anthocyanin glycosides.
Anthocyanins as food colors.
The anthocyanins constitute a large family of differently colored
compounds and occur in countless mixtures in practically all parts of higher
plants. They are of great economic importance as fruit pigments and thus are
used to color fruit juices, jams, wines, some beverages, canned fruit, fruit
syrups, yogurt, and other products. They are used as food additive with
European Union number E163.
In the USA, the grape color extracts were approved for use in non beverage
foods, where as grape skin extract (enocyanin) is permitted in beverages. (DeMan,
1999; Delgado and Lopez, 2003)
Processing and stability
Anthocyanins are water soluble strong colors and have been used to
color food since historical times (Delgado and Lopez 2003). Extracts of berries
have been used to color drinks, pastries and other foods. However, some
drawbacks in the use of anthocyanins as food colors exist.
Anthocyanins
are water soluble and pH dependent which restricts their use. For example the
color of red cabbage is enhanced with the addition of vinegar or other acid. On
the other hand, when cooked in aluminum pans which cause a more alkaline
environment, the color changes to purple and blue (http://www.food-info.net/uk/colour/anthocyanin.htm).
The color is
also susceptible towards temperature, oxygen, UV-light and different
co-factors. Temperatures destroy the flavylium ion, and thus cause loss of
color. Temperature also causes maillard reactions, in
which the sugar residues in the anthocyanins may be involved. Light may have a
similar effect (Delgado and Lopez 2003; Zdzislaw 2002). Oxygen may destroy the anthocyanins, as do other oxidizing
reagents, such as peroxides and vitamin C. Many other components in plants and
foods may interact with the anthocyanins either destroying, changing or
increasing the color. Quinones in apples for example, enhance the degradation
of anthocyanins whereas the addition of sugar to strawberries stabilizes the
color. (Delgado and Lopez, 2003; http://www.food-info.net/uk/colour/anthocyanin.htm)
All these factors limit the use of anthocyanins in foods. Some loss
of color during storage may be prevented by storing at low temperatures, in
dark containers or under oxygen-free packaging (Deman, 1999; Fennema, 1996;
Zdzislaw, 2002; Delgado and Lopez 2003).
In practice the
pure colors are very hard to obtain and most often (crude) extracts are used as
food colors. Grape peel (E163 (i)), and black currant extract (E163 (iii)) are
the most widely used anthocyanin mixtures in foods. (http://www.food-info.net/uk/colour/anthocyanin.htm)
Betalains
Betalains are immonium derivatives
of betalamic acid, with a general formula based on the protonated 1, 2, 4, 7,
7-pentasubstituted 1, 7- diazaheptamethin system (Delgado and Lopez, 2003). Betalains occur in centrospermae, mainly
in red beets, but also in some cactus fruits and mushrooms (Zdzislaw,
2002; Deman, 1996). Zdzislaw, (2002) reported
that they consist of red-violet betacyanins (λmax ~ 540 nm) and yellow
betaxanthins (λmax ~ 480 nm). The major betacyanin is betanin, glucoside of
betanidin, which accounts for 75–95% of the total pigments of beets. The other
red pigments are isobetanin (C-15 epimer of betanin), prebetanin, and
isoprebetanin. According to Deman (1996), the latter two are sulfate monoesters
of betanin and isobetanin, respectively. Unlike anthocyanins, betanins cannot
be hydrolyzed to aglycone by acid hydrolysis without degradation
(Zdzislaw, 2002).
The major yellow pigments are vulgaxanthin I and
II. High betalain content in beet root, on average 1% of the total solids,
makes this vegetable a valuable source of the food colorant (Deman,
1996; Zdzislaw, 2002).
Fig 9: Molecular structures of betanin
and isobetanin and the major yellow pigments, vulgaxanthin I and II (Zdzislaw, 2002).
Betalains
are found in different plant organs, and are accumulated in cell vacuoles,
mainly in epidermal and sub epidermal tissues (Delgado and Lopez, 2003). Some accumulate in plant
stalks such as in the roots of red beet. Betalains are also present
in the higher fungi Amanita, Hygrocybe, and Hygrosporus (Zdzislaw, 2002).
Classification:
Betalains are commonly classified
into two based on their structural characteristics;
(i)
Betacyanins (red-purple)
(ii)
Betaxanthins (yellow)
Each group of pigment is
characterized by specific R1-N-R2 moieties. R1 and R2 groups can be
hydrogen, aromatic group or another substituent. Betalain color is attributable
to the resonating double bonds. A large number of betaxanthins can be formed
with the same dihydropyridine moiety, by conjugation with several amine
compounds such as amino acids. The diversity of betacyanins is associated with
the combination of the basic structures (betanidin and isobetanidin) with
different glycosyl and acyl groups attached by the hydroxyl groups at positions
5 and 6. The most common glycosyl moiety is glucose and the most common acyl
groups are sulfuric, malonic, citric and caffeic acids.
Betalains as food color.
According to Delgado and Lopez
(2003), betalains have been in use as food colorants at least since the return
of the 20th century. Diaz et al., (2006)
reported that there is a growing propensity to substitute synthetic colorants
with natural pigments in the food industry. This is further confirmed by the
red beet betacyanins being approved for use as a food additive in the United
States of America (No. 1600), and in the European Union (E-162). Commercially,
the red beet betacyanins are exempt from batch certification (http://www.food-info.net/uk/colour).
Processing and stability
The most-studied betalains are found
in red beets (Beta vulgaris) in
which the main betacyanins are betanin and isobetanin (Delgado and Lopez,
2003). Betalains stability is affected by temperature, pH, oxygen, light, and
aqueous activity (Diaz et al., 2006).
According to Delgado and Lopez (2003), enzyme degradation is also an important
factor that must be considered when a betalain pigment product is to be
processed.
Zdzislaw (2000), reviewed that color
stability of betanin solution is strongly influenced by pH and heating.
Betanins are stable at pH values of 4–6, but thermostability is greatest between
pH values of 4 and 5. As a result of betanin degradation cyclo-DOPA and
betalamic acid are formed. This reaction is reversible.
Fig 10: Degradation of betanin (Zdzislaw, 2002)
Light and air have a degrading
effect on betanin according to Zdzislaw
(2002). These effects are cumulative, but some protection may be offered
by antioxidants such as ascorbic acid. Small amounts of metallic ions increase
the rate of betanin degradation. Therefore a chelating agent can stabilize the
color. Many protein systems present in food products also have some protective
effect (Zdzislaw, 2002).
Freeze-drying is the best method to
dry pigments which are sensitive to high temperatures (Diaz et al.,
2006).
On market, Beetroot red (E 162) is
available as liquid beetroot concentrate and as beetroot concentrate powders.
According to Zdzislaw(2002) and Delgado
and Lopez(2003), it is suitable
for products of relatively short shelf life, which do not undergo as severe
heat treatment such as meat and Soya protein products, ice cream, and gelatin
desserts.
Note that all
references used in all postings related to the topic of sausages, meat and meat
product colorings will be posted in the last article about this topic
About the author
Mr.
Sempiri Geoffery, the author of this article graduated from
Makerere University with a Bsc In
Food Science and Technology Degree in January, 2011.
