Structural Changes in Fruits During Growth and Ripening: What Happens Inside?

When you pick up a ripe mango or bite into a soft, juicy peach, you’re experiencing the end result of a series of complex structural transformations that occurred inside the fruit over days and weeks. These changes happen at the cellular level – involving cell walls, membranes, pigment-containing organelles, and the fruit’s outer surface. Understanding what happens structurally inside fruits during growth and ripening is essential for farmers, food scientists, and anyone involved in the postharvest supply chain.

How fruit development sets the stage
Cell wall softening: the most noticeable change
Role of pectin in texture
Changes in plasmalemma permeability
Changes in intercellular space
Chloroplast to chromoplast transition: the colour shift
Pigments involved in colour change
Cuticle deposition increases during ripening
Reduction of epidermal hairs (trichomes)
Why these structural changes matter for agriculture
A quick summary of key structural changes

How fruit development sets the stage

Fruit development begins after pollination and fertilization. The ovary wall starts to grow rapidly through two main processes: cell division (an increase in the number of cells) and cell expansion (an increase in the size of individual cells). During this growth phase, cells are tightly packed together, intercellular spaces are minimal, and cell walls are thick and rigid. The fruit at this point is firm, often hard, and typically green in colour due to the presence of chloroplasts in its cells.

As the fruit transitions from growth to ripening, the focus shifts from building structure to reorganising and breaking it down in a controlled, genetically programmed way. This shift is triggered largely by hormonal signals, particularly ethylene in climacteric fruits (like bananas and tomatoes) and abscisic acid in non-climacteric ones (like strawberries and grapes).

Cell wall softening: the most noticeable change

The most significant structural change during ripening is the softening of fruit tissue, which is primarily caused by modifications to the cell wall. In unripe fruits, cell walls are composed of cellulose microfibrils embedded in a matrix of pectins and hemicelluloses. These components work together to provide rigidity and structural support.

As ripening begins, a coordinated set of enzymes gets to work disassembling this structure. The main players include:

Pectinases (including polygalacturonase and pectin methylesterase) – these enzymes break down pectin, the polysaccharide that acts as a “glue” holding adjacent cell walls together in the middle lamella. In unripe fruits, pectin molecules are extensively cross-linked, often through calcium bridges, forming a rigid gel-like structure. Ripening enzymes systematically break these bonds, converting insoluble pectin into soluble forms.

Hemicellulases (such as xyloglucan endotransglucosylase/hydrolases) – these target xyloglucan, the main hemicellulose in fruit cell walls, loosening the cross-links between cellulose microfibrils.

Expansins – these are non-enzymatic proteins that loosen cell walls by disrupting hydrogen bonds between cellulose and hemicellulose, allowing the wall to stretch and become more pliable. Research on tomato fruit has shown that polygalacturonase and expansin proteins work together to disassemble the cell wall polysaccharide network during ripening.

The combined action of these enzymes and proteins reduces cell wall thickness, weakens the connections between neighbouring cells, and transforms what was once a firm, hard fruit into a soft, tender one. This is why a green tomato gradually becomes easy to squeeze as it turns red.

Role of pectin in texture

Pectin deserves special attention because it is the most abundant polysaccharide in the middle lamella – the layer that bonds neighbouring cells. According to research published in Trends in Food Science & Technology, the degradation of pectin during ripening reduces its molecular size, complexity, and degree of branching. This directly weakens intercellular adhesion. When you bite into a ripe fruit and feel juice release easily, that is the result of weakened cell-to-cell bonds making cells rupture easily under pressure.

Changes in plasmalemma permeability

The plasmalemma (also called the plasma membrane) is the semipermeable membrane surrounding each plant cell. In young, unripe fruit, the plasmalemma tightly controls what enters and exits the cell, maintaining the osmotic balance responsible for turgor pressure – a key contributor to fruit firmness.

During ripening, the permeability of the plasmalemma increases significantly. This has several important consequences. First, enzymes that were previously kept separate from their substrates by membrane compartmentalisation come into contact, accelerating biochemical reactions. Second, increased membrane permeability allows volatile aromatic compounds to move more freely, contributing to the characteristic aroma and flavour of ripe fruits. Third, as membranes become leakier, cells gradually lose turgor. This loss of turgor pressure works together with cell wall degradation to produce the soft, sometimes slightly wrinkled texture of very ripe fruits.

An electron microscopy study on oranges published in the Journal of Ultrastructure Research observed distinct structural changes in the plasmalemma during ripening, including invaginations, vesicle formation, and the development of circlet structures that appeared to function as mechanisms for transporting material into cells.

Changes in intercellular space

Young, growing fruits have tightly packed cells with very little space between them. This compact arrangement contributes to the dense, firm texture of unripe fruits. As ripening progresses, intercellular spaces increase – sometimes dramatically.

This happens for two main reasons. First, the degradation of pectin in the middle lamella weakens the bonds holding cells together, causing them to separate slightly and create air pockets. Research on strawberry fruit development found that cell separation begins early and increases throughout ripening, with the expanding apoplastic space closely tracking the overall progression of fruit development.

Second, differential cell expansion during ripening means that neighbouring cells may not grow uniformly, creating additional gaps between them.

These intercellular spaces serve several functions: they facilitate gas exchange (important for the respiratory activity of the fruit), allow for the accumulation of volatile compounds responsible for aroma, and contribute to changes in texture. For example, the extensive intercellular air spaces in apples are what give them their characteristic crisp, snapping texture when bitten.

Chloroplast to chromoplast transition: the colour shift

One of the most visible structural changes during fruit ripening is the transformation of chloroplasts (green, photosynthetically active organelles) into chromoplasts (organelles specialised for pigment storage). This transition is responsible for the colour change that signals ripeness – green to red in tomatoes, green to yellow in bananas, green to orange in citrus fruits.

At the structural level, this conversion involves significant internal remodelling. The thylakoid membranes inside the chloroplast – where photosynthesis takes place – are progressively dismantled. Chlorophyll is degraded, and photosynthetic gene expression is downregulated. Simultaneously, carotenoid pigments (such as lycopene in tomatoes and beta-carotene in mangoes) accumulate in large quantities within newly formed membrane structures inside the chromoplast.

Research using confocal microscopy on tomato fruit confirmed that chromoplasts arise directly from pre-existing chloroplasts. At the “breaker” stage of ripening, intermediate plastids were observed containing both chlorophyll and carotenoid pigments, and the transition was found to be synchronous within individual cells.

In citrus fruits, this process is often referred to as “colour break.” Studies on Satsuma mandarin have shown that the chloroplast-to-chromoplast conversion in the fruit peel is stimulated by sucrose accumulation and regulated through ethylene signalling, while gibberellic acid can delay it.

Pigments involved in colour change

The colour change during ripening is not driven by carotenoids alone. In many fruits, anthocyanins – water-soluble pigments stored in cell vacuoles – are responsible for red, purple, and blue hues. Grapes, blueberries, and cherries are prime examples. According to a review in BMC Plant Biology, the synthesis, degradation, and retention of these pigments are regulated by a combination of hormonal, genetic, and environmental factors. The interplay between declining chlorophyll and rising carotenoid or anthocyanin levels determines the final colour of the ripe fruit.

Cuticle deposition increases during ripening

The cuticle is a waxy, lipid-based layer that coats the outer surface of the fruit epidermis. It serves as a barrier against water loss, UV radiation, and pathogen entry. During ripening, cuticle deposition generally increases, making the fruit surface smoother and more protective.

Research on cashew pseudofruit development showed that while the outer epidermal cell wall thickness decreased during later ripening stages, the cuticle layer continued to thicken – rising from about 1.2 micrometres in early development to over 3.2 micrometres at full maturity. A similar pattern has been documented in tomatoes, where cuticle biosynthesis and thickness increase continuously until ripening is complete.

The cuticle is primarily composed of cutin (a polyester of fatty acids) intertwined with cell wall polysaccharides, along with surface waxes that can be either epicuticular (on the surface) or intracuticular (embedded within the cutin matrix). In tomato fruit, flavonoid compounds like naringenin chalcone also accumulate in the cuticle during ripening, contributing to fruit surface properties and colour.

Reduction of epidermal hairs (trichomes)

Trichomes, or epidermal hairs, are hair-like structures found on the surface of many young fruits. They serve protective roles – including defence against herbivores, UV shielding, and reduction of water loss. However, as ripening progresses, these trichomes typically reduce in number or disappear entirely.

This is especially noticeable in peaches, where the fuzzy surface of young fruit becomes noticeably smoother as it ripens. In tomatoes, fruit trichomes are abundant on undisturbed developing fruit but are largely absent after harvest on mature specimens.

The reduction of trichomes during ripening may serve an ecological purpose: a smoother fruit surface facilitates the release of volatile aromatic compounds that attract animals and aid in seed dispersal. Research has also revealed that trichome development and cuticle formation are genetically interlinked – changes in genes controlling one process frequently affect the other. This complex interplay means that as cuticle deposition ramps up during ripening, trichome density often decreases simultaneously.

Why these structural changes matter for agriculture

Understanding the structural changes during ripening has direct, practical applications in agriculture and the food industry.

Harvest timing: Farmers use knowledge of cell wall softening and colour changes to determine the optimal point for harvesting. Picking too early gives fruits that may never properly ripen off the plant; picking too late results in fruits that are too soft to transport without damage.

Storage technology: Controlled atmosphere storage works by slowing down enzyme activity that drives cell wall breakdown. By regulating oxygen, carbon dioxide, and ethylene levels around stored fruit, the rate of softening and other ripening-associated structural changes can be significantly reduced. Temperature management further complements this by controlling the speed of biochemical reactions.

Food processing: Understanding pectin breakdown is essential for industries producing jams, jellies, and fruit juices, where pectin content determines gel formation and product consistency. Knowledge of colour change timing helps processors decide when to harvest fruits intended for products where appearance matters.

Genetic research: Scientists are studying how specific genes control ripening-related structural changes. For example, research on the CHLORAD proteolytic pathway in tomatoes has shown that manipulating certain ubiquitin ligase genes can either accelerate or delay ripening, opening possibilities for developing fruit varieties with longer shelf life.

A quick summary of key structural changes

To bring everything together, here are the main structural transformations that occur inside a fruit as it ripens: cell wall components (pectin, hemicellulose) are broken down by enzymes, reducing wall thickness and causing tissue softening; plasmalemma permeability increases, leading to turgor loss and enhanced enzyme-substrate interactions; intercellular spaces expand as the middle lamella degrades and cells separate; chloroplasts convert into chromoplasts, replacing green chlorophyll with colourful carotenoids; cuticle thickness increases, providing a stronger protective barrier on the fruit surface; and epidermal trichomes reduce or disappear, giving the fruit a smoother appearance.

Each of these changes is interconnected – cell wall degradation enables cell separation, membrane changes accelerate enzymatic reactions that drive further softening, and hormonal signals coordinate the timing of colour change with textural transformation.

What do you think? Given that excessive softening is one of the biggest causes of postharvest food waste, how might a deeper understanding of these cellular-level changes help develop fruit varieties that stay firmer for longer without sacrificing flavour? And if genetic tools could slow down specific structural changes during ripening, which change would have the greatest impact on reducing food loss in the supply chain?

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References

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Table of Contents

How fruit development sets the stage 🔗

Cell wall softening: the most noticeable change 🔗

Role of pectin in texture 🔗

Changes in plasmalemma permeability 🔗

Changes in intercellular space 🔗

Chloroplast to chromoplast transition: the colour shift 🔗

Pigments involved in colour change 🔗

Cuticle deposition increases during ripening 🔗

Reduction of epidermal hairs (trichomes) 🔗

Why these structural changes matter for agriculture 🔗

A quick summary of key structural changes 🔗