Morphological and Chemical Changes in Fruits: From Ripening to Senescence

Fruits go through a remarkable transformation as they move from ripening to senescence. The green, firm mango on your kitchen counter gradually turns golden-yellow and soft – and eventually, if left too long, starts to decay. These changes aren’t random. They are driven by a tightly regulated series of morphological and chemical processes that determine everything from colour and texture to flavour and nutritional value. Understanding these changes is essential not just for food scientists but also for anyone involved in post-harvest management and storage.

What happens during fruit ripening? 🔗

Fruit ripening can be understood as the sum of all cellular and chemical changes that occur after a fruit reaches its full size and before it begins to deteriorate. It involves softening, sweetening, pigment development, and the production of characteristic flavours and aromas. These changes serve a biological purpose – making the fruit attractive to animals that aid in seed dispersal.

Fruits are broadly classified into two categories based on their ripening behaviour. Climacteric fruits – such as tomatoes, bananas, and apples – show a sharp increase in ethylene production and a respiratory peak at the onset of ripening. Non-climacteric fruits – like citrus, grapes, and strawberries – do not exhibit this dramatic burst of ethylene or respiration rate during ripening. This distinction has practical implications for how these fruits are harvested, stored, and transported.

Colour changes: chlorophyll degradation and pigment synthesis 🔗

One of the most obvious signs of ripening is the change in fruit colour. This transformation is driven by two simultaneous processes: the breakdown of chlorophyll (the green pigment) and the synthesis of new pigments.

Chlorophyll breakdown 🔗

As a fruit matures, chlorophyll – the pigment responsible for its green appearance – is progressively degraded. During this process, the chloroplasts (organelles that carry out photosynthesis) are converted into chromoplasts, which are specialised for pigment storage. As chlorophyll disappears, the thylakoid membranes within the chloroplast disintegrate and the internal membrane system is restructured, allowing carotenoids to accumulate. Genes related to photosynthesis are simultaneously downregulated during this transition.

Some fruits, however, retain their chlorophyll even when ripe. Green-skinned apple varieties like Granny Smith and the flesh of green kiwifruit are well-known exceptions to this general pattern.

Carotenoid and anthocyanin accumulation 🔗

As chlorophyll fades, other pigments take centre stage. Carotenoids produce the yellow, orange, and red colours seen in fruits like mangoes, papayas, and tomatoes. These pigments are synthesised within the newly formed chromoplasts and their production is regulated by ethylene signalling and light exposure. Research published in BMC Plant Biology confirms that as a fruit ripens, a dynamic colour shift from green to yellow to red occurs due to the interplay between chlorophyll loss and new pigment biosynthesis.

Anthocyanins, on the other hand, are water-soluble pigments stored in the cell vacuoles that give red, purple, and blue hues to fruits such as grapes, blueberries, cherries, and pomegranates. In many fruits, anthocyanin production is triggered only after chlorophyll and carotenoid degradation have already begun. Environmental factors like light intensity, temperature, and even orchard management practices such as pruning and bagging can strongly influence fruit colouration.

Textural changes: softening through starch and pectin breakdown 🔗

Texture is one of the most important quality attributes of a fruit, and it changes significantly during ripening. Two key processes drive this softening: the conversion of starch into sugars and the degradation of cell wall pectins.

Starch-to-sugar conversion 🔗

In many fruits, starch serves as the primary carbohydrate reserve during development. As ripening begins, this starch is hydrolysed into simpler sugars – glucose, fructose, and sucrose – by enzymes such as β-amylase (BAM) and α-amylase (AMY). In bananas, for instance, starch accounts for about 20-25% of the pulp weight in unripe fruit, but this drops to just 1-2% in ripe fruit as sugars accumulate. This conversion is responsible for both the increased sweetness and the softer texture of ripe fruit.

Pectin degradation and cell wall disassembly 🔗

Pectins are complex polysaccharides that provide structural support to plant cell walls and hold cells together via the middle lamella. During ripening, a coordinated set of cell wall-degrading enzymes dismantles this structure. The main culprits include:

Polygalacturonase (PG) – breaks down the galacturonic acid backbone of pectins. Pectin methylesterase (PME) – removes methyl groups from pectin, making it accessible to PG. Pectate lyase (PL) – cleaves pectin chains through a β-elimination mechanism. β-Galactosidase – removes galactose side chains from pectin.

These enzymes work cooperatively. PME first de-esterifies pectin, and then PG and PL break down the resulting demethylated chains. This leads to the dissolution of the middle lamella, reduced cell-to-cell adhesion, and the softening that consumers perceive when they bite into a ripe peach or tomato. In addition, expansin proteins loosen the bonds between cellulose and hemicellulose, increasing wall porosity and allowing even more enzymatic access.

Flavour development: changes in organic acids and sugars 🔗

The flavour of a fruit is determined by a careful balance of sugars, organic acids, and volatile compounds. During ripening, each of these components shifts in concentration.

Organic acid metabolism 🔗

The concentration of organic acids – particularly malic acid and citric acid – generally decreases as a fruit ripens. These acids are consumed through the tricarboxylic acid (TCA) cycle during cellular respiration. As acidity drops, the perceived sweetness of the fruit increases even if sugar content remains stable. In bananas, malic acid levels actually rise during ripening, while oxalic acid is metabolised – showing that acid dynamics vary across species.

Volatile compound production 🔗

Ripening also triggers the production of a complex mixture of volatile compounds – esters, aldehydes, alcohols, and terpenes – that contribute to each fruit’s characteristic aroma. For example, compounds like ocimene and myrcene are produced during ripening, while bitter-tasting flavonoids and tannins are degraded, making the fruit more palatable.

Lipid changes 🔗

Lipid metabolism plays a supporting role in flavour development. Membrane lipids are gradually broken down during ripening and senescence, releasing fatty acid precursors that are converted into volatile aroma compounds. This lipid degradation also affects membrane integrity, contributing to the overall decline in cellular structure as the fruit ages.

Structural changes during senescence 🔗

Senescence is the final stage of fruit development – a period when the chemical pathways that build and maintain the fruit give way to degradative processes that lead to tissue aging and death. While ripening makes a fruit attractive and edible, senescence marks its decline.

Cell wall and membrane breakdown 🔗

During senescence, the cell wall degradation that began during ripening intensifies. Enzymes like cellulases and hemicellulases continue breaking down structural carbohydrates, resulting in mushy texture and eventual tissue collapse. The loss of membrane integrity allows cell contents to leak, accelerating decay.

Mitochondrial degradation 🔗

Mitochondria – the organelles responsible for energy production via cellular respiration – undergo progressive breakdown during senescence. As mitochondrial function declines, the fruit’s metabolic capacity drops, further accelerating the aging process. This decline is closely linked to a surge in reactive oxygen species (ROS), which damage cellular components and speed up deterioration.

Nutritional decline 🔗

Overripe and senescent fruits show a measurable loss of vitamins and antioxidants. Vitamin C content, for instance, drops significantly in overripe fruit. The balance of sugars and acids shifts further, often resulting in off-flavours – fruits may taste fermented or excessively sweet.

The role of ethylene in ripening and senescence 🔗

Ethylene – a gaseous plant hormone – plays a central role in coordinating ripening, particularly in climacteric fruits. It is synthesised from the amino acid methionine through a pathway involving two key enzymes: ACC synthase (ACS) and ACC oxidase (ACO). Once produced, ethylene binds to specific receptors and triggers a signalling cascade that activates ripening-associated gene expression.

In non-climacteric fruits, abscisic acid (ABA) and sugars appear to be the dominant regulators of ripening rather than ethylene. However, even non-climacteric fruits respond to exogenous ethylene – for instance, it accelerates chlorophyll degradation in citrus and speeds up senescence in most fruits.

Recent research has highlighted the role of nitric oxide (NO) as a natural antagonist of ethylene. Studies show that NO treatment can significantly extend the shelf life of fruits and vegetables by suppressing ethylene formation and delaying the onset of senescence.

Practical applications: extending shelf life 🔗

Understanding the morphological and chemical changes during ripening and senescence provides the scientific basis for effective post-harvest management strategies.

Cold storage 🔗

Lowering temperature slows enzymatic activity and delays both ripening and senescence. Fruits like apples, grapes, and kiwis benefit significantly from refrigerated storage. However, some tropical and subtropical fruits are sensitive to chilling injury, which can cause surface pitting and internal browning.

Controlled atmosphere (CA) storage 🔗

In controlled atmosphere storage, oxygen levels are reduced and carbon dioxide levels are increased to slow respiration and ethylene production. This technique is widely used commercially for apples, bananas, and avocados. Liquid nitrogen and compressed nitrogen gas are sometimes used to displace ambient air from storage facilities.

Ethylene management 🔗

Compounds like 1-methylcyclopropene (1-MCP) block ethylene receptors and are used commercially to delay ripening. Conversely, it is recommended to store high-ethylene-producing climacteric fruits separately from non-climacteric fruits and vegetables to prevent premature senescence in the latter.

Calcium treatment 🔗

Calcium plays a role in maintaining cell wall firmness by forming cross-links between pectin chains. Post-harvest calcium chloride treatments have been shown to reduce pectin degradation and delay softening in fruits like apples and strawberries.

Why does this matter? 🔗

Post-harvest losses account for a significant portion of global food waste, especially in developing countries where cold chain infrastructure is limited. A solid understanding of what drives ripening and senescence at the molecular level helps develop targeted, cost-effective interventions – from simple temperature management to advanced biotechnological approaches like modifying ethylene biosynthesis genes.

For students and professionals in food science and agriculture, these concepts form the foundation for improving fruit quality from farm to fork – ensuring that produce reaches consumers in the best possible condition.

What do you think? How might advances in controlled atmosphere storage and ethylene-blocking technologies reshape the way fresh produce is managed in tropical regions where cold chain infrastructure is lacking? And do you think genetic modification of ripening-related genes is an acceptable approach to reducing post-harvest losses, or should we focus on non-biotechnological solutions?

References

1. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/fruit-ripening
2. https://pubs.acs.org/doi/10.1021/acs.jafc.9b01954
3. https://bmcplantbiol.biomedcentral.com/articles/10.1186/s12870-021-03411-w
4. https://www.rroij.com/open-access/biochemical-physiological-and-horticultural-perspectives-of-fruit-colour-pigmentation-a-review.php?aid=33838
5. https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.00615/full
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC6529986/
7. https://scialert.net/fulltext/?doi=ajbkr.2017.1.23
8. https://www.britannica.com/topic/fruit-processing/Maturation-and-spoilage
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC8350513/