UNIT 1.3.4

Enzymes in Action: Fruit Ripening & Browning

Applying enzyme biochemistry to real-world horticulture

🎯 After this unit, you will be able to:

Explain the enzymatic changes during fruit ripening
Describe the biochemistry of enzymatic browning
Apply methods to control ripening and browning
Connect enzyme concepts to post-harvest management

🍎 From Biochemistry to Your Kitchen

Two of the most visible and economically important enzymatic processes in horticulture are fruit ripening and enzymatic browning. Understanding these processes allows growers, shippers, and consumers to manage fruit quality from harvest to table.

Key insight: Both processes are essential for fruit quality (ripening makes fruit edible) but also cause losses (over-ripening and browning reduce shelf life and appeal). The goal is to manage, not eliminate, these enzymatic activities.

🍌

Fruit Ripening

Controlled enzymatic changes that make fruit palatable, sweet, and attractive.

Key enzymes:

Amylase - starch → sugars
Polygalacturonase - softens fruit
Pectin methylesterase - texture changes
Chlorophyllase - color change (green loss)

🍎

Enzymatic Browning

Rapid discoloration when fruits/vegetables are cut or bruised.

Key enzyme:

Polyphenol oxidase (PPO)
Also: Peroxidase (secondary role)

Substrates: Phenolic compounds

Products: Brown melanins

🍌 Part 1: The Biochemistry of Fruit Ripening

Ripening is a programmed series of biochemical changes controlled by the plant hormone ethylene and involving multiple enzymes.

Key Enzymatic Changes During Ripening

Change	Enzyme(s) involved	Effect
Starch → Sugar	Amylase, invertase	Sweetness increases (bananas, mangoes, apples)
Cell wall softening	Polygalacturonase, pectin methylesterase, cellulase	Fruit becomes softer, easier to eat
Color change	Chlorophyllase (breaks down chlorophyll); enzymes for pigment synthesis	Green fades; carotenoids (yellow/orange) and anthocyanins (red/blue) appear
Aroma production	Lipoxygenase, alcohol acyltransferase	Volatile compounds create characteristic fruit smells
Acid decline	Various metabolic enzymes	Tartness decreases, sugar/acid ratio improves

🍌 [Diagram: Biochemical changes during banana ripening - starch to sugar, chlorophyll breakdown — to be inserted]

Climacteric vs. Non-Climacteric Fruits (Review)

As you learned in Unit 1.2.2, fruits are classified by their ripening behavior:

🍌 Climacteric

Ripen after harvest, triggered by ethylene. Starch converts to sugars.

Examples: Banana, tomato, apple, mango, avocado

🍇 Non-climacteric

Only ripen on plant. No starch conversion after harvest.

Examples: Grape, citrus, strawberry, cherry

🍌 Case Study: Banana Ripening Management

Bananas are harvested green and firm (starchy) and ripened at destination. Commercial ripening involves:

Ethylene treatment: 100-150 ppm ethylene for 24-48 hours triggers enzyme production
Temperature control: 14-18°C for slow, even ripening; higher temperatures speed ripening but risk uneven quality
Humidity: 90-95% prevents water loss
1-MCP treatment: If ripening needs to be delayed, 1-methylcyclopropene blocks ethylene receptors, inhibiting all ripening enzymes

The enzyme connection: Ethylene triggers gene expression for ripening enzymes (amylase, polygalacturonase, etc.). By controlling ethylene and temperature, shippers deliver bananas at exactly the right stage.

🍌 Did you know? A single gene controls the conversion of starch to sugar in bananas. Some wild bananas lack this gene and remain starchy even when ripe—like plantains, which are cooked as vegetables.

🍎 Part 2: Enzymatic Browning

When you slice an apple, potato, or banana, it turns brown within minutes. This is enzymatic browning, a major cause of quality loss in fresh-cut produce.

The Biochemistry of Browning

🧪 [Diagram: Browning reaction - PPO + phenolic compound + O₂ → melanin — to be inserted]

The reaction requires three components:

Polyphenol oxidase (PPO) — the enzyme
Phenolic compounds — substrates (e.g., chlorogenic acid, tyrosine, catechol)
Oxygen — from the air

When cells are damaged (cut, bruised), PPO and phenolics mix with oxygen, and the enzyme catalyzes the oxidation of phenolics to quinones, which then polymerize into brown melanins.

Factors Affecting Browning Rate

Factor	Effect	Example
PPO activity level	Varieties with higher PPO brown faster	Some apple varieties brown faster than others
Phenolic content	More phenolics = more browning potential	Potatoes with higher phenolics darken more
pH	PPO optimum pH 6-7; acidic pH inhibits	Lemon juice (pH 2) prevents browning
Temperature	Heat denatures PPO; cold slows it	Blanching stops browning; refrigeration slows
Oxygen availability	No oxygen = no browning	Vacuum packaging prevents browning

🛡️ Methods to Prevent Enzymatic Browning

Each prevention method targets one of the three requirements:

🍋

Acid treatment — Lower pH (lemon juice, citric acid, ascorbic acid) denatures PPO or chelates copper at active site

❄️

Refrigeration — Slows enzyme activity (but doesn't stop it)

🔥

Blanching — Heat denatures PPO permanently (used for vegetables before freezing)

💧

Water immersion — Limits oxygen exposure (but only temporary)

🧂

Sulfites — Chemical inhibitors (effective but can cause allergic reactions; restricted in many countries)

🧪

Ascorbic acid (vitamin C) — Reduces quinones back to phenolics before they polymerize; also lowers pH

🌊

Vacuum packaging — Removes oxygen

🍎 Case Study: Commercial Fresh-Cut Apple Slices

Pre-cut apple slices in grocery stores don't turn brown. How?

Commercial processors use a combination of methods:

Variety selection: Choose apple varieties naturally low in PPO or phenolics (e.g., some cultivars brown much slower)
Acid dip: Slices are dipped in a solution of ascorbic acid (vitamin C) and calcium ascorbate. Ascorbic acid prevents browning chemically; calcium maintains firmness.
Modified atmosphere packaging: Bags are flushed with nitrogen or have low oxygen permeability to limit oxygen exposure.
Cold chain: Constant refrigeration slows any remaining enzyme activity.

This combination allows fresh-cut apples to stay attractive for 2-3 weeks.

📊 Comparison: Ripening vs. Browning

Feature	Fruit Ripening	Enzymatic Browning
Purpose	Programmed development for seed dispersal	Defense response? (Maybe antimicrobial)
Desirable?	Yes—makes fruit edible	Usually no—quality loss
Key enzyme(s)	Amylase, polygalacturonase, etc.	Polyphenol oxidase (PPO)
Trigger	Ethylene (in climacteric fruits)	Cell damage (cutting, bruising)
Oxygen required?	No	Yes
Control methods	Ethylene management, 1-MCP, temperature	Acid, antioxidants, blanching, oxygen exclusion

🇪🇹 Local Examples: Ethiopian Horticulture

Mango Ripening

Ethiopian mangoes are often harvested mature-green and ripened for local markets or export. Understanding the starch-to-sugar conversion (amylase activity) helps determine optimal harvest time. Mangoes for export may be treated with hot water (to kill fruit flies) which also affects surface enzymes.

Enset (Kocho) Processing

Enset fermentation involves microbial enzymes, but plant enzymes also play a role in the initial breakdown of starch and cell walls. The texture of kocho depends on the balance of enzymatic activities during fermentation.

Potato Storage

In Ethiopian highlands, potatoes are stored at cool temperatures. However, too-cold storage can cause "cold-induced sweetening" (amylase converts starch to sugar), leading to dark chips when fried (due to Maillard reaction, not browning). This is a major quality issue for processors.

🥔 Did you know? Potato varieties bred for chip-making have lower reducing sugar content and amylase that's less active at cold temperatures, preventing dark chips.

📌 Unit Summary

Fruit ripening involves multiple enzymes: amylase (starch→sugar), polygalacturonase (softening), chlorophyllase (color change)
Controlled by ethylene in climacteric fruits; managed via temperature, ethylene, and 1-MCP
Enzymatic browning requires PPO + phenolics + O₂; causes quality loss in fresh-cut produce
Browning can be prevented by targeting any of the three requirements: acid (pH), heat (denature), oxygen exclusion, antioxidants

Reflection question: Think about a fruit or vegetable common in your area that browns quickly when cut (e.g., banana, potato, apple). Based on what you've learned, what practical methods could you recommend to a street food vendor or household to minimize browning without expensive equipment?

📌 Key terms introduced

Polyphenol oxidase (PPO) Polygalacturonase Amylase Chlorophyllase Melanin Quinones 1-MCP Ascorbic acid Blanching

✅ Check your understanding

Name three enzymes involved in fruit ripening and what each does.
Why do bananas harvested green eventually become sweet, even off the plant?
What three components are necessary for enzymatic browning?
Explain how lemon juice prevents browning at the biochemical level.
A potato chip manufacturer complains that some potato varieties produce dark chips. What enzyme-related factor might be responsible?

Discuss your answers in the course forum.

Plant Biochemistry for Horticulture · HORT 202 · Dilla University · Last updated March 2026