Biosynthesis of
Plant-derived flavor
compounds
By Dudsadee Uttapap
Biosynthesis of
plant-derived flavor compounds
References
1.
“Flavor Chemistry and Technology”, H.B. Heath, G. Reineccius, 1986.
2.
“Flavor Chemistry”, D.B. Min, http://class.fst.ohio-state.edu/fst820/default.htm
3.
“Biosynthesis of plant-derived flavor compounds”, The Plant
Journal (2008) 54, 712–732
4.
“Plant Biochemistry” http://www.uky.edu/~dhild/biochem/lecture.html
Flavor compounds
Flavor molecules constitute a heterogeneous
group of compounds, with straight-chain,
branched-chain, aromatic and heteroaromatic
backbones
bearing diverse chemical groups such as
hydroxyl, carbonyl, carboxyl, ester, lactone,
amine, and thiol functions. More than 700 flavor
chemicals have been identified and catalogued
Chemical synthesis VS Biosynthesis
Most commercial flavorants are ‘nature identical’, which
means that they are the chemical equivalent of natural
flavors but are chemically synthesized, mostly from
petroleum-derived precursors
Bioproduction, including the extraction from natural sources,
de novo microbial processes (fermentation), and bioconversion
of natural precursors using micro-organisms or isolated
enzymes
Biological functions of plant volatiles
“associated with defensive and attractive roles”
Compounds emitted by flowers most probably serve to attract and guide
pollinators
volatiles might also protect the carbohydrate- rich nectar by inhibiting
microbial growth.
vegetative plant tissue release volatiles following herbivore damage.
Some of these substances attract arthropods that prey upon or
parasitize the herbivores.
Volatiles also act as direct repellents or toxicants for herbivores and
pathogens.
In fruits, volatile emission and accumulation facilitate seed dispersal by
animals and insects.
vegetative tissues often produce and release many of the
volatiles after their cells are disrupted. These volatile flavor
compounds may exhibit anti-microbial activity.
Aromatic compounds responsible for odor
and flavor of fruits comprise;
Alcohols R-OH
Carbonyls RR-CO-R’
Acids
CHO
R-COO-R’
Esters
Lactones
O
R-COOH Phenols
R
CO
Estimated world
consumption of
selected aroma
chemicals in flavor
and fragrance
compositions
CHO
OCH3
OH
Calvin cycle
Amino acid synthesis
N enters roots as NO3- or NH4+. The NH4+ is incorporated into amino
acids in roots and leaves and the amino acids accumulate in proteins. The
main if not sole function of some proteins is to provide a store of amino
Amino acid synthesis
Glycolysis
isoprenoid biosynthesis proceeds either via the
"classical" or most well studied, mevalonate pathway
(cytosolic) (for the synthesis of sterols, sesquiterpenes,
triterpenoids)
or via the non-mevalonate (1-deoxy-D-xylulose-5phosphate, DXP) pathway for plastidic isoprenoids
(carotenoids, phytol [side-chain of chlorophylls], plastoquinone,
isoprene, monoterpenes and diterpenes).
Biosynthesis of flavors in vegetables and fruits
Fruit flavors
are formed during brief ripening period
Vegetable flavors
develop when tissue damage occurs
(Intact vegetable generally contains few
volatiles)
BIOGENESIS OF FRUIT AROMA
develops entirely during ripening period of plant
Minute quantities of lipids, CHO, protein (amino
acids) are enzymatically converted to volatile
flavors.
FRUIT FLAVOR COMPOUNDS
Apple
n-hexanal, ethyl butyrate, 1-propyl propionate,
1-butyl acetate, trans-2-hexenal, ethyl 2methylbutyrate, 2-methylbutyl acetate, 1hexanol, hexen-1-ol, trans-2-hexen-1-ol, hexyl
acetate, Esters; alcohols; aldehydes; ketone;
acids; including hexanal; ethyl 2-methyl butyrate
Banana
alcohols; esters, including amyl acetate, isoamyl
acetate, butyl butyrate, amyl butyrate
Peach
Ethyl acetate, dimethyl disulfide, cis-3-hexenyl
acetate, methyl octanoate, ethyl octanoate, 6pentyl alpha pyrone, gamma decalactone
Lipids Polysaccharide Proteins/Enzymes
Malonyl CoA
Fatty acid
Acetyl-CoA
Aliphatic
Acids
Alcohols
Esters
Carbonyls
lactones
Carbohydrate
Pyruvate
Acetyl CoA
Lignins
Amino acid
Shikimic acid
Terpene
Mevalonyl CoA
Terpenes
Sesquiterpenes
Hydrocarbons
Alcohols
Carbonyls
monoterpenes
Cinnamic acid
Methyl-Branched
Alcohols
Acids
Esters
carbonyls
Biosynthesis of fruit volatiles
Aromatic
Alcohols
Acids
Esters
carbonyls
Flavorants from carbohydrate metabolism
Only a limited number of natural volatiles
originate directly from carbohydrates without
prior degradation of the carbon skeleton.
Furanones and pyrones
“fruit constituents”
Furanones and pyrones
Carbohydrate-derived flavor molecules, including 4-hydroxy-2,5-dimethyl-3(2H)furanone (furaneol), 2,5-dimethyl-4-methoxy-3(2H)-furanone (methoxyfuraneol), 4hydroxy-5-methyl-3(2H)-furanone (norfuraneol), 2-ethyl-4-hydroxy-5-methyl-3(2H)furanone (homofuraneol), 4-hydroxy-2-methylene-5-methyl-3(2H)- furanone (HMMF)
and 3-hydroxy-2-methyl-4H-pyran-4-on (maltol).
Flavorants from carbohydrate metabolism
Glucose (6C)
2 Pyruvate (3C)
CO2
-O2
-O2
Glycolysis
+O2
Ethanol
Lactate
TCA Cycle
Pyruvic acid
CH3COCOOH
Acetic acid
CH3COOH
Acetyl CoA
CH3COSCoA
+ CO2
Malonyl CoA
HOOCCH2COSCoA
Malonic Acid
HOOCCH2COOH
Flavorants from carbohydrate metabolism
“the most interesting is terpene biosynthesis”
Terpenoids are enzymatically synthesized from acetyl CoA and
pyruvate provided by the carbohydrate pools in plastids and the
cytoplasm.
Terpenoids constitute one of the most diverse families of
natural products, with over 40 000 different structures of
terpenoids
Many of the terpenoids produced are non-volatile and are
involved in important plant processes such as membrane
structure (sterols), photosynthesis (chlorophyll side chains,
carotenoids), redox chemistry (quinones) and growth regulation
(gibberellins, abscisic acid, brassinosteroids)
Important plant-derived volatile terpenoids.
Biosynthesis of Terpenes
“isoprene is derived from acetyl-CoA”
Classification of Terpenes
Apocarotenoid formation
Carotenoid substrates are oxidatively cleaved to yield the apocarotenoid derivatives
(right).
Some of the volatile organic compounds in wine come from the grape's skin, or
exocarp, while others come from the grape's flesh, or mesocarp. Organic acids give
wine its tartness, and sugars give it sweetness. Terpenes provide floral or fruity
flavors. Norisoprenoids impart a honeylike character. Thiols are the sulfur-based
compounds behind complex wine aromas such as guava, passionfruit or grapefruit —
but when thiols go wrong, they can make a wine taste "funky."
Lipids
metabolic pathway for lipid biosynthesis plays
a significant role in flavor formation.
Oxidation via lipoxygenase
Alpha-, Beta-oxidation
products; acids, alcohols, diketones, ketones, esters of these compounds.
Oxidation via Lipoxygenase
Lipoxygenase activity is believed to be the
major source of volatiles in plants.
Lipoxygenase enzymes (dioxygenase) catalyze reactions
between O2 and polyunsaturated fatty acids
Substrate: unsaturated fatty acid (linoleic and linolenic acids).
Major products: volatile C6 and C9 aldehydes and alcohols
Linolenic acid-derived flavor molecules.
AAT, alcohol acyl CoA transferase; ADH, alcohol dehydrogenase; AER, alkenal
oxidoreductase; AOC, allene oxide cyclase; AOS, allene oxide synthase; HPL,
hydroperoxide lyase; JMT, jasmonate methyltransferase; LOX, lipoxygenase; OPR, 12oxo-phytodienoic acid reductase; 3Z,2E-EI, 3Z,2E-enal isomerase.
Fatty acid precursors (Tomato)
- and -oxidation of fatty acids
the specific pathways in plants are not well understood
Palmitoyl-CoA (16:0)
+ Acetyl-CoA
Myristoyl-CoA (14:0)
Formation of pear flavors via beta-oxidation
Lactones
Amino Acid Metabolism
Amino acid metabolism yields short chain aliphatic and
aromatic alcohols, acids, carbonyls and esters
They are the primary source of branched chain
aliphatic flavor compounds
their pathways have been barely analyzed in plants.
amino acid precursors
(Tomato)
Biosynthesis of amino acid-derived
flavor compounds
(a) Catabolism of branched-chain amino acids leading to methyl branched flavor compounds, and
(b) postulated biosynthesis of sotolon. Formation of aldehyde (a) from amino acids requires the
removal of both carboxyl and amino groups. The sequence of these removals is not fully known
and could be the opposite to that shown or aldehyde could be formed in one step by aldehyde
synthase
Starting amino acids: Tyrosine and phenylalanine products:
phenolic/spicy in character
Shikimic acid formation
Vegetable Flavors
Vegetable flavors
flavor again arises from major metabolic processes e.g. Lipids, CHO & amino acids.
the precursors, enzymes and end flavors are quite
different from fruits.
The role or importance of S compounds to
vegetable flavor is quite significant.
Carbohydrate
Fatty acid
Amino acid
Nonvolatile Precursors
Linoleic, Linolenic acid Thioglucosinolates
Cysteine-sulfoxides
Methyl-methionine
Precursor-splitting Enzymes
Lypoxygenase
Carbonyls
Alcohols
Oxo-acids
Thioglucosidases
Isothiocyanates
Nitriles
S C O
Thiocyanates
C-S-lyases
Polysulides
Alkylthosulfinates
Formation of flavor in vegetables
None (Heating)
CH3-S-CH3
Vegetable Flavor Categories
Genus Allium
Enzymes produce volatiles from derivatives
of cysteine (sulfoxides)
Genus Brassica
Enzymes produce volatiles from glucosinolates
Alliaceous vegetables
garlic (Allium sativum L.)
onion (Allium cepa L.)
chive (Allium schoenoprasum L.)
leek (Allium porrum L.)
Characteristic flavors
not exist in the bulb before processing
are produced when the cellular tissues are
ruptured by cutting or chewing
flavor is produced very rapidly by the action
of an enzyme on the odorless precursors which
coexist in the cells
Onion and Garlic Flavor
Enzymatic reaction of cysteine derivative
GLUCOSINOLATES
Glucosinolate precursors are important to the
flavor of both the Brassica and Cruciferae family
Cruciferae family includes radish, horseradish,
mustard.
Hydrolysis of the glucosinolate
glucosinolate
thioglucosidase
thiocyanate, nitrile, or isothiocyanate
& glucose
Natural carbon pools for the production
of flavor compounds, and the pathways
Mevalonic acid
Acetate
Mevalonic acid
Isoprene
Shikimic acid
Pyruvate + Erythrose phosphate
Flavorants from carbohydrate metabolism
“the most interesting is terpene biosynthesis”
most of essential oils get flavor from terpenoids (10 carbon)
Limonene - a monoterpene hydrocarbon - is the major terpene in
many or most citrus products. Orange > 95% of the essential oil is
limonene,
lemon ~ 65% limonene, yet is of little flavor significance.
Citral - oxygenated monoterpene - seldom comprises > 2% of
the essential oil of lemon - largely carries the lemon flavor.
1 Methane
11 Undecane
21 Henicosane
31 Hentriacontane
2 Ethane
12 Dodecane
22 Docosane
32 Dotriacontane
3 Propane
13 Tridecane
23 Tricosane
33 Tritriacontane
4 Butane
14 Tetradecane
24 Tetracosane
40 Tetracontane
5 Pentane
15 Pentadecane
25 Pentacosane
50 Pentacontane
6 Hexane
16 Hexadecane
26 Hexacosane
60 Hexacontane
7 Heptane
17 Heptadecane
27 Heptacosane
70 Heptacontane
8 Octane
18 Octadecane
28 Octacosane
80 Octacontane
9 Nonane
19 Nonadecane
29 Nonacosane
90 Nonacontane
30 Triacontane
100 Hectane
10 Decane
20 Icosane


Isoamyl acetate, a strong fruity odor described as
similar to banana or pear
2-Methyl-butyl acetate has a strong apple scent