Biochemistry of fruit ripening

INTRODUCTION
Ripening is the final stage of development of a fruit,
which involves series of physiological and biochemical
events leading to changes in colour, flavour, aroma
and texture.
On ripening, the fruit becomes both attractive and
tasty.

INTRODUCTION
Ripening is a well regulated genetically programmed
phase of fruit development that involves activation and
inactivation of various genes leading to various
biochemical and physiological changes within the tissue.
The biochemical aspect of fruit ripening is highly
diverse with marked variations in metabolism in
different fruits.

Major Biochemical changes during
ripening
Changes in carbohydrate composition
Change in pigmentation
Flesh softening and textural change
Formation of aroma volatiles
Accumulation of organic acids with flavor
development

Ethylene – Ripening hormone
Ethylene has been established as a ripening hormone.
However, ethylene is not a universal ripening hormone since some
fruits do not respond to ethylene treatment.
Onset of ripening is associated not only with a rise in the ability to
biosynthesize ethylene but also a marked increase in ethylene
responsiveness.
Fruits are broadly classified as climacteric or non-climacteric,
depending on respiration peak and ethylene production during ripening.

Climacteric fruits
Climacteric fruits are characterized by transient increase
in both ethylene synthesis and respiration at an early
stage of ripening.
The peak of ethylene production rate is proportional to
the peak respiration rate.
Tomato, banana mango, apple, jack fruit, papaya,
plum and avocado are examples of climacteric fruit.

Non-climacteric fruits
Non-climacteric fruits do not show any increase in
respiration and ethylene synthesis during ripening.
In fact, they show decline in respiration rate and ethylene
production throughout the ripening process.
Eg. Citrus fruits, cherry, cucumber, grape, lemon, orange,
pineapple, strawberry, etc.
These fruits need to stay on the tree or vine until ready to eat
in order to have their desired eating characteristics

Biosynthesis of
Ethylene
Methylthioadenosine +

Cell wall degradation
Modification of cell wall determines the fruit texture
and ripening characteristics.
The plant primary cell wall consists of a network of
strong, rigid cellulose microfibrils held together by the
hemicelluloses (composed mainly of neutral sugars) and
the pectins (rich in galacturonic acid), together with
smaller amounts of structural proteins.

The middle lamella present between cells is composed
mainly of pectins. It provide attachment to neighbouring
cells.
Fruit softening involves alterations to various cell wall
components such as cellulose, pectin and hemicellulose.
 Solubilization of pectin
The cell wall swells and shows an increased affinity for
water

Loss of galactose from pectins (especially of a
galactan that is tightly associated with the cellulose
microfibrils)
De-esterification of some pectins.
Depolymerisation of the hemicellulosic
polysaccharide xyloglucan, which is associated with a
reduction in cell wall strength

Depolymerisation of pectin, which is associated with dissolution
of the middle lamella and reduced intercellular adhesion.
Thus pectin solubilisation of the middle lamella results in a great
reduction in intercellular adhesion.
The primary walls are also weakened by the various changes
that have occurred, and cells easily rupture when bitten or
chewed, releasing the cell contents as juice.

Modifications of Cell wall
Cellulase hydrolyses cellulose molecules and causes cell
wall loosening.
β-Galactosidase causes the removal of terminal galactose
residues from β-D-galactosides. This causes reduction in
the polymer length of hemicelluloses and pectin.
Enzymes such as pectin methylesterase, polygalacturonase
and pectate lyase solubilize and degrade pectins.

Modifications of Cell wall
It is found that polygalacturonases are responsible for the major cell
wall disassembly during fruit ripening in many fruits and is
induced by ethylene.
Both endo and exo forms of polygalacturonase are present in fruits.
The combined action of these enzymes causes dissolution of the
cell wall fibrillar network and degradation of cell wall
components leading to fruit softening.

Conversion of starch to simple sugars
The conversion of starch to sugars is the most remarkable chemical
change occurring in many fruits during ripening.
In banana, starch content decreases from 20% in unripe fruits to
about 2% in fully ripe fruit. The concentrations of maltose, sucrose,
fructose and glucose increases during ripening.
During early stages of ripening sucrose predominates, but later
glucose and fructose contents are higher.
Since sucrose is utilized for metabolic activity after degradation of
starch, its concentration decreases during the later stages of ripening
process.

Changes in pigments
During ripening the amount of chlorophyll decreases
as it is broken down by the enzyme chlorophyllase.
Other pigments such as Carotenes, Xanthophylls,
anthocyanins and anthoxanthins are synthesized
which gives the specific colours to fruits during
ripening.

Changes in pigments
In yellow ripe banana α-carotene, β-carotene and lutein are present.
The green to red colour change in tomato is due to transition of
chloroplast to chromoplast with degradation of chlorophyll and
accumulation of lycopene, carotenes and xanthophylls.
Apples and plums- Cyanidin (anthocyanin); Grapes- Malvidin;
Strawberries- Pelargonidin

Changes in flavor and aroma
 Increase in simple sugars – give sweetness
 Decrease in organic acids
 Decrease in phenolics – minimize astringency
 Increase in volatiles – produce characteristic aroma
 Taste of fruits during ripening is associated with
changes in acid and sugar contents.

Acidity of fruit decreases during ripening
The most common organic acids are Malate and
Citrate vary with different fruits.
In banana flesh, malic, citric and oxalic acid are present.
As the fruit ripens, malic acid content increases, while
oxalic acid decreases.

Grapes - Malic and tartaric acids; Apple- malic acid;
During ripening period, metabolism changes to catabolism
of primary metabolites with formation of aroma volatile
terpenoids, aldehydes, esters and alcohols.
Methyl butyrate, Methyl butanoate, Ethyl butyrate, Ethyl
butanoate, Isoamyl acetate, limonene.

Outlines
 Increase in rate of respiration
 Hormonal changes – Ethylene
 Changes in chlorophyll content
 Synthesis of carotenoids, anthocyanin,
Xanthophyll
 Increase in activity of enzymes
 Synthesis of volatile compounds
 Synthesis of simple sugars.

Biochemistry of fruit ripening

Biochemistry of fruit ripening

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Biochemistry of fruit ripening