Physiological and biochemical changes during fruit ripening
ABSTRACT
Fruit ripening is a complex and highly regulated physiological and biochemical process
transforming mature but unripe fruit into a palatable and nutritionally rich product. This
process involves intricate texture, colour, aroma, and flavour changes, driven by hormonal
signalling and metabolic shifts. Ethylene, a key plant hormone, plays a crucial role in
ripening, particularly in climacteric fruits, by triggering a cascade of gene expression changes
that regulate softening, sugar accumulation, pigment formation, and volatile compound
synthesis (Giovannoni, 2004; Klee & Giovannoni, 2011). During ripening, starch reserves are
hydrolyzed into simpler sugars such as glucose, fructose, and sucrose, enhancing sweetness
(Beaudry, 1999). Cell wall degradation is facilitated by enzymes such as polygalacturonase,
pectin methylesterase, and cellulase, leading to fruit softening (Brummell & Harpster, 2001).
Chlorophyll breakdown and the synthesis of carotenoids, anthocyanins, and flavonoids
contribute to the characteristic colour changes (Barry & Giovannoni, 2007). Concurrently,
increased respiration rates and shifts in mitochondrial metabolism support the heightened
energy demands of ripening (Prasanna et al., 2007). The biosynthesis of volatile organic
compounds (VOCs) enhances fruit aroma and flavour, primarily through lipid and amino acid
metabolism (Goff & Klee, 2006). Additionally, phenolic metabolism undergoes significant
alterations, influencing antioxidant properties and postharvest quality (Liu et al., 2015).
Understanding these physiological and biochemical mechanisms is crucial for improving
postharvest storage, extending shelf life, and optimizing fruit quality.
Keywords: Fruit ripening, Ethylene, Sugar metabolism, Cell wall degradation, Respiration,
Aroma compounds, Postharvest quality.
,Introduction:
Fruit ripening is a highly coordinated, genetically programmed, and irreversible phenomenon
involving a series of physiological, biochemical, and organoleptic changes that lead to the
development of soft and edible ripe fruit with desirable quality attributes. A wide spectrum of
biochemical changes such as increased respiration, chlorophyll degradation, biosynthesis of
carotenoids, anthocyanins, essential oils, and flavour and aroma components, increased
activity of cell wall-degrading enzymes, and a transient increase in ethylene production are
some of the major changes involved during fruit ripening (Brady, 1987). The colour change
during fruit ripening is due to the unmasking of previously present pigments by degradation
of chlorophyll dismantling of the photosynthetic apparatus and synthesis of different types of
anthocyanins and their accumulation in vacuoles, and accumulation of carotenoids such as β-
carotene, xanthophyll esters, xanthophylls, and lycopene (Tucker and Grierson, 1987; Lizada,
1993). The increase in flavour and aroma during fruit ripening is attributed to the production
of a complex mixture of volatile compounds such as ocimene and myrcene (Lizada, 1993),
and the degradation of bitter principles, flavonoids, tannins, and related compounds (Tucker
and Grierson, 1987). The taste development is due to a general increase in sweetness, which
is the result of increased gluconeogenesis, hydrolysis of polysaccharides, especially starch,
decreased acidity, and accumulation of sugars and organic acids resulting in an excellent
sugar/acid blend (Lizada, 1993; Grierson et al., 1981; Selvaraj et al., 1989). The metabolic
changes during fruit ripening include an increase in biosynthesis and evolution of the
ripening hormone, ethylene (Yang and Hoffman, 1984), an increase in respiration mediated
by mitochondrial enzymes, especially oxidases and de novo synthesis of enzymes catalyzing
ripening-specific changes (Tucker and Grierson, 1987). Alteration of cell structure involves
changes in cell wall thickness, permeability of plasma membrane, hydration of cell wall,
decrease in the structural integrity, and increase in intracellular spaces (Tucker and Grierson,
1987; Redgwell et al., 1997). The major textural changes resulting in the softening of fruit are
due to enzyme-mediated alterations in the structure and composition of a cell wall, partial or
complete solubilization of cell wall polysaccharides such as pectins and cellulose (Tucker and
Grierson, 1987), and hydrolysis of starch and other storage polysaccharides (Selvaraj et al.,
1989; Fuchs et al., 1980). During the ripening process, there is a change in respiration rate
and biosynthesis and evolution of the ripening hormone ethylene. Based on their respiratory
pattern and ethylene biosynthesis during ripening, harvested fruits can be further classified as
climacteric and non-climacteric types (Table 1). Climacteric is defined as a period in the
, ontogeny of fruit during which a series of biochemical changes are initiated by autocatalytic
production of ethylene making the change from growth to senescence and involving an
increase in respiration leading to ripening of fruit. This phenomenon was first observed by
Kidd and West in 1925. (Paul et al., 2012).
Respiration
Respiration is the process of breaking stored organic reserves (carbohydrates, proteins and
fats) into simpler molecules with the release of energy in the form of ATP (Fonseca et al.,
2002). Living cells always respire to get energy. The cells consume O2 and produce CO2
during respiration. Therefore, cellular respiration is an energy-releasing process in biological
systems. Food materials (carbohydrates, protein and lipids) use O2 for liberating energy with
the concomitant release of CO2. The ratio of moles of CO2 evolution per mole of O2
consumption is called the respiratory quotient (RQ). It also indicates the types of substrates
used in respiration.
The RQ value of sucrose is 1, fat is less than 1, and organic acid is greater than 1 (Saltveit,
1999). During respiration, a major part (about 57 %) of the energy is dissipated in the form of
heat, which is popularly known as vital heat or heat of respiration, which contributes to an
increase in the temperature of the commodity further. Three distinct phases are identified in
the respiratory pathway: (i) Hydrolysis breakdown of polysaccharides into simple sugars, (ii)
Glycolysis-oxidation of sugars into pyruvic acid, and (iii) Citric acid cycle-aerobic
transformation of pyruvate and other organic acids into carbon dioxide, water, and energy
and mitochondrial electron transport chain (Haard 1995; Wills et al., 1998). The rate of
respiration varies with the commodity and its stage of maturity. Generally, the rate of
respiration possesses an inverse relationship with the shelf-life of the fresh produce (Saltveit
1999). With progressive senescence, the stored food material gets exhausted by respiration
for providing energy and maintaining the living status of the commodities. This loss results in
reduced energy value, loss of flavour, quality, and salable dry weight, which is especially
important for commodities destined for dehydration. The rate of respiration is mainly affected
by temperature, atmospheric composition, and mechanical damage (Siddiq, 2012).
Respiration is the major process responsible for the deterioration of fresh produce and the
respiratory metabolic processes double for every 10ºC rise in temperature (Fagundes et al.,
2013). The rate of respiration could be significantly reduced by altering the oxygen and/or
carbon dioxide content around the fruit which ultimately increases their storage life.
ABSTRACT
Fruit ripening is a complex and highly regulated physiological and biochemical process
transforming mature but unripe fruit into a palatable and nutritionally rich product. This
process involves intricate texture, colour, aroma, and flavour changes, driven by hormonal
signalling and metabolic shifts. Ethylene, a key plant hormone, plays a crucial role in
ripening, particularly in climacteric fruits, by triggering a cascade of gene expression changes
that regulate softening, sugar accumulation, pigment formation, and volatile compound
synthesis (Giovannoni, 2004; Klee & Giovannoni, 2011). During ripening, starch reserves are
hydrolyzed into simpler sugars such as glucose, fructose, and sucrose, enhancing sweetness
(Beaudry, 1999). Cell wall degradation is facilitated by enzymes such as polygalacturonase,
pectin methylesterase, and cellulase, leading to fruit softening (Brummell & Harpster, 2001).
Chlorophyll breakdown and the synthesis of carotenoids, anthocyanins, and flavonoids
contribute to the characteristic colour changes (Barry & Giovannoni, 2007). Concurrently,
increased respiration rates and shifts in mitochondrial metabolism support the heightened
energy demands of ripening (Prasanna et al., 2007). The biosynthesis of volatile organic
compounds (VOCs) enhances fruit aroma and flavour, primarily through lipid and amino acid
metabolism (Goff & Klee, 2006). Additionally, phenolic metabolism undergoes significant
alterations, influencing antioxidant properties and postharvest quality (Liu et al., 2015).
Understanding these physiological and biochemical mechanisms is crucial for improving
postharvest storage, extending shelf life, and optimizing fruit quality.
Keywords: Fruit ripening, Ethylene, Sugar metabolism, Cell wall degradation, Respiration,
Aroma compounds, Postharvest quality.
,Introduction:
Fruit ripening is a highly coordinated, genetically programmed, and irreversible phenomenon
involving a series of physiological, biochemical, and organoleptic changes that lead to the
development of soft and edible ripe fruit with desirable quality attributes. A wide spectrum of
biochemical changes such as increased respiration, chlorophyll degradation, biosynthesis of
carotenoids, anthocyanins, essential oils, and flavour and aroma components, increased
activity of cell wall-degrading enzymes, and a transient increase in ethylene production are
some of the major changes involved during fruit ripening (Brady, 1987). The colour change
during fruit ripening is due to the unmasking of previously present pigments by degradation
of chlorophyll dismantling of the photosynthetic apparatus and synthesis of different types of
anthocyanins and their accumulation in vacuoles, and accumulation of carotenoids such as β-
carotene, xanthophyll esters, xanthophylls, and lycopene (Tucker and Grierson, 1987; Lizada,
1993). The increase in flavour and aroma during fruit ripening is attributed to the production
of a complex mixture of volatile compounds such as ocimene and myrcene (Lizada, 1993),
and the degradation of bitter principles, flavonoids, tannins, and related compounds (Tucker
and Grierson, 1987). The taste development is due to a general increase in sweetness, which
is the result of increased gluconeogenesis, hydrolysis of polysaccharides, especially starch,
decreased acidity, and accumulation of sugars and organic acids resulting in an excellent
sugar/acid blend (Lizada, 1993; Grierson et al., 1981; Selvaraj et al., 1989). The metabolic
changes during fruit ripening include an increase in biosynthesis and evolution of the
ripening hormone, ethylene (Yang and Hoffman, 1984), an increase in respiration mediated
by mitochondrial enzymes, especially oxidases and de novo synthesis of enzymes catalyzing
ripening-specific changes (Tucker and Grierson, 1987). Alteration of cell structure involves
changes in cell wall thickness, permeability of plasma membrane, hydration of cell wall,
decrease in the structural integrity, and increase in intracellular spaces (Tucker and Grierson,
1987; Redgwell et al., 1997). The major textural changes resulting in the softening of fruit are
due to enzyme-mediated alterations in the structure and composition of a cell wall, partial or
complete solubilization of cell wall polysaccharides such as pectins and cellulose (Tucker and
Grierson, 1987), and hydrolysis of starch and other storage polysaccharides (Selvaraj et al.,
1989; Fuchs et al., 1980). During the ripening process, there is a change in respiration rate
and biosynthesis and evolution of the ripening hormone ethylene. Based on their respiratory
pattern and ethylene biosynthesis during ripening, harvested fruits can be further classified as
climacteric and non-climacteric types (Table 1). Climacteric is defined as a period in the
, ontogeny of fruit during which a series of biochemical changes are initiated by autocatalytic
production of ethylene making the change from growth to senescence and involving an
increase in respiration leading to ripening of fruit. This phenomenon was first observed by
Kidd and West in 1925. (Paul et al., 2012).
Respiration
Respiration is the process of breaking stored organic reserves (carbohydrates, proteins and
fats) into simpler molecules with the release of energy in the form of ATP (Fonseca et al.,
2002). Living cells always respire to get energy. The cells consume O2 and produce CO2
during respiration. Therefore, cellular respiration is an energy-releasing process in biological
systems. Food materials (carbohydrates, protein and lipids) use O2 for liberating energy with
the concomitant release of CO2. The ratio of moles of CO2 evolution per mole of O2
consumption is called the respiratory quotient (RQ). It also indicates the types of substrates
used in respiration.
The RQ value of sucrose is 1, fat is less than 1, and organic acid is greater than 1 (Saltveit,
1999). During respiration, a major part (about 57 %) of the energy is dissipated in the form of
heat, which is popularly known as vital heat or heat of respiration, which contributes to an
increase in the temperature of the commodity further. Three distinct phases are identified in
the respiratory pathway: (i) Hydrolysis breakdown of polysaccharides into simple sugars, (ii)
Glycolysis-oxidation of sugars into pyruvic acid, and (iii) Citric acid cycle-aerobic
transformation of pyruvate and other organic acids into carbon dioxide, water, and energy
and mitochondrial electron transport chain (Haard 1995; Wills et al., 1998). The rate of
respiration varies with the commodity and its stage of maturity. Generally, the rate of
respiration possesses an inverse relationship with the shelf-life of the fresh produce (Saltveit
1999). With progressive senescence, the stored food material gets exhausted by respiration
for providing energy and maintaining the living status of the commodities. This loss results in
reduced energy value, loss of flavour, quality, and salable dry weight, which is especially
important for commodities destined for dehydration. The rate of respiration is mainly affected
by temperature, atmospheric composition, and mechanical damage (Siddiq, 2012).
Respiration is the major process responsible for the deterioration of fresh produce and the
respiratory metabolic processes double for every 10ºC rise in temperature (Fagundes et al.,
2013). The rate of respiration could be significantly reduced by altering the oxygen and/or
carbon dioxide content around the fruit which ultimately increases their storage life.
