FRUIT RIPENING CONDITIONS

Ilyana Causing & Sharmaine Ylanan

Introduction

• Fruits can be classified into two groups: • Climacteric and Non-climacteric

• Biale and Barcus published measurements of the respiration rate of some fruits • They classified them into Climacteric, Non-climacteric and Indeterminate

• Respiration rate of non-climacteric fruit and vegetables tends to decrease during development • Ripening involves physical and chemical changes in the fruit• This occur after the fruit is at full maturity

Introduction

• Immature fruit may be harvested and exposed to postharvest conditions that are conductive to ripening• Temperature, Gas content in the atmosphere, and Humidity

• Initiation of ripening occurs when the threshold level of ethylene reached the cells of the fruit• Can occur naturally during maturity or if the fruit undergoes stress

Changes During Fruit Ripening

• Color• Texture• Carbohydrates• Acids• Phenolic Compounds• Flavor and Aroma• Toxicity

Color

• External Color – most change in fruit ripening• Change in color in fruit ripening is associated with breakdown of

chlorophyll with carotenoid levels remains constant• Color changes is used as a rough guide to the stage of ripeness • Used in Bananas

• Used commercially in the form of color-matching charts

Examples: Color

In Cherry Tomatoes• Total chlorophyll level was reduced from 5490 μg per 100 g fresh

weight in green fruit to 119 μg per 100 g fresh weight in dark-red fruit• At the same time - degradation process, lycopene, carotenes and

xanthophylls are synthesized• giving the fruit its characteristic color, usually red

• The optimum temperature for color development is 24°C• 30°C and above lycopene is not formed

In Banana and Plantains• The pigments in the peel of are chlorophylls, carotenoids and

xanthophylls• Cavendish banana fail to de-green completely when they are ripened

at 25°C and above• Result: bananas are ripe in every other respect but remains green• Physiological disorder of Cavendish bananas called ‘pulpa crema’ or ‘yellow

pulp’ where chlorophyll in the skin is not fully broken down

• In Plantains - chlorophyll destruction can occur even at 35°C

Examples: Color

Texture

• Fruits soften during ripening• Softening is due to the breakdown of starch and other non-pectic

polysaccharides in the pulp• This reduce cellular rigidity

• Change in the moisture status contributes to the ease with the detaching of the peel from the pulp• There are changes in pectic polymers during ripening

Texture

• There are major changes in pectic polymers during ripening• Neutral sugars (Galactose)• Some loss of arabinose – major components of the cell wall neutral pectin• Losses of acidic pectin

• Solubility of these polyuronides increases and hav shown to depolymerized• Solubilization of non-soluble pection to water-soluble pectin influence

the texture of Japanese pears.

Texture

• Stow and Genge measured the cell wall strength of apples using osmotic techniques• They found that cell walls do not weaken during fruit softening• Softening results from loss of cell-to-cell cohesion• Soluble pectin content of the apples do not correlate with fruit firmness• They also suggested removal of ethylene from store could slow softening

once started

• Cellulase is involved in softening during ripening of avocado fruit

Texture

• Genetic engineering – produced fruit that do not soften normally but market was restricted• In ripening of bananas and plantains • Ratio of mass of pulp – mass of peel increases which makes the peel easily

detach from the pulp• This could be used to measure the fruit’s ripeness

• Softening of bananas during ripening appears to be associated with two or three processes

1. Breakdown of starch to form sugars since starch granules could have a structural function in the cells2. Breakdown of the cell walls due to the solubilization of pectic substances and even the breakdown of cellulose• increased activity of cellulase during banana ripening

3. Movement of water from the peel of the banana to its pulp during ripening• affect the turgidity of the skin - enhanced by transpirational losses

Examples: Texture

Carbohydrates

• In Climacteric fruit there is increase in starch content during development• Hydrolysis of starch to simple sugars is a chemical change during

ripening• Early part of ripening – Sucrose is the predominant sugar• Later stage – Glucose and Fructose predominates• Proportion of different sugars is related to the stage in the respiratory

climacteric of the fruit

Carbohydrates

• Starch is broken down to sucrose – by action of sucrose phosphate synthetase• Non-reducing sugar from sucrose – by acid hydrolysis• Starch-sugar conversion is influenced by harvest maturity

Avocados• Starch occurs in the plastids of unripe avocados but reduces to

undetectable levels when ripeKiwi• Starch was hydrolyzed to glucose and fructose and to a lesser degree

to sucrose during ripeningMangoes• Starch content was completely hydrolyzed to sugar during ripening• glucose, fructose and sucrose• reduction in the level of sucrose is due to it being used by the fruit for

metabolic activity after all the starch had been hydrolyzed

Examples: Carbohydrate

Acids

• Organic acids also influence the overall fruit flavor• Acids help form the desirable sugar-acid balance – for pleasant taste• Acidity of fruit decreases during ripening• The most common organic acids: Malate and Citrate• But vary with different fruits

Banana and Plantains• Acidic in with a pulp pH below 4.5• Main acids in bananas were citric, malic and oxalic acid• Levels of these acids normally increase during ripening

• Titratable acidity in bananas increased during ripening at 20°C and then decreased

Examples: Acids

Phenolic Compounds

• Phenolic (i.e. Tannins) are polymerized to insoluble compounds• It reduce astringency in the ripe banana fruit

• Tannins most important phenolic compound• give fruit an astringency taste

• As fruit ripens astringency decreases• Tannins form polymers due to change in structure

• Phenolics are responsible for the oxidative browning reaction in immature fruit• Polyphenol oxidase – responsible for this reaction

Bananas and Plantain • Can contain high levels of phenolic compounds, especially in the peel• Tannins are polymerized to insoluble compounds - reduction in

astringency in the ripe banana fruitCarabao Mango • Decrease in total phenolic content during ripening

Examples: Phenolic Compounds

Flavor and Aroma

• Flavor – a subtle and complex perception • Combination of taste, smell, and texture/mouth feel

• Ripening• Increase in simple sugars – give sweetness• Decrease in organic acids • Decrease in phenolics – minimize astringency• Increase in volatiles – produce characteristic flacor

• Aroma – in ripe fruit is due to production of a complex mixture of individual volatile components

Apples and Pears• butyl ethanoate, 2-methylbutyl ethanoate and hexyl ethanoate • typical flavor and aroma compounds synthesized during ripening

Tomatoes• More than 400 substances have been shown to contribute to the odor

of tomatoes

Examples: Flavor and Aroma

Toxicity

• Toxins may exist in unripe fruit which reduce in ripening

Tomatoes• at the green stage of maturity contain a toxic alkaloid called solanine• Decreases during ripening

Ackee fruit• contain the toxin hypoglycin in the arils• reduces as the fruit matures

Examples: Toxicity

Controlled Atmosphere Storage

• Levels of in the environment of climacteric fruit can affect ripening rate• Controlled Atmosphere – suppress production of ethylene in fruit

Banana• High CO2 and low O2 delayed the high production of ethylene

associated with the initiation of ripening• application of exogenous ethylene reverse this effect

• Bananas could be ripened in atmospheres of reduced O2 (low as 1%) but peel failed to de-green

Design of Ripening Rooms

• Primary requirements• Have good temperature control system• Have good and effective air circulation• Gas tight• Have good system for introducing fresh air

Design of Ripening Rooms

• Air circulation is important to prevent accumulation of • In bananas the boxes are lined with polyethylene film and stacked on pallets

• Air circulation systems are largely convectional• Air is blown through the cooler and then across the top just below the ceiling• Cool air falls by convection through the boxes of the fruit and then recirculated

• Inflatable air bags are used for better air circulation• Good ventilation – important for successful fruit ripening• During initiation of ripening (24 hours) no fresh air is introduced ony

ethylene gas

Design of Ripening Rooms

• If rooms are not frequently ventilated ripening can be delayed or abnormal ripening occur• Gas tight – ensure threshold levels of ethylene are maintained• The room is metal-lined with mastic between joints or use Gas-tight paint to

ensure no gas can pass through

• The room should have high humidity (90-95%)• Rooms are fitted with humidification device – spinning disc humidifier

Ethylene on Ripening

• Change in physiology of climacteric fruit from maturation to ripening is initiated when cellular quantities of ethylene reach a threshold level• High levels of in stores could compete with ethylene for binding sites

in fruits• accumulation in the intercellular spaces of fruit acts as an ethylene

antagonist

• Increase in ethylene synthesis is followed by changes in the fruits physiology, texture and composition

Ethylene on Ripening

• Threshold levels of ethylene will be reached naturally at fruit maturity• Can also arise by the fruit being put under stress during production

(water shortage, infection by disease-causing organisms, mechanical damage and exposing fruit to low humidity)

Sources of Ethylene

• Ethylene Application Methods• Liquid• Large Gas Cylinders• Small Gas Cylinders• Ethylene Generators

Ethylene Application Method

• There are several sources of ethylene that can be used in fruit ripening and de-greening• The source and the method selected for applying ethylene to fruit

depends on: • cost• convenience • safety factors

Liquid• Ethrel or Etheph on (2- chloroethylphosphonic acid) is used as a

source of ethylene. Ethrel is hydrolyzed in plant tissue to produce ethylene, phosphate and chloride• Ethylene can also be released from Ethrel by mixing it with a base

such as sodium hydroxide• Ethrel ‘C’ - will release 93 g from 1 litre or 74.4 litres of ethylene gas per litre

of Ethrel• Used initiate fruit to ripen by placing containers of Ethrel in a gas-tight room

containing the fruit and then adding the base to the containers

Large Gas Cylinders

• Ethylene is available in large steel cylinders where it is stored under pressure• Use of large cylinders of the pure gas is discouraged –Ethylene is

highly flammable• Dilution with nitrogen – allow margin for error • Application: Volume of the room should have been previously

calculated and the volume of ethylene introduced calculated with a flow meter and a stop-watch

Small Gas Cylinders

• Lecture tubes – steel cylinders that contain 35 L of ethylene• Types• a. Has a cover which, when it is punctured, releases all the gas

inside - commonly used for initiating fruit to ripen commerciallyb. Can be fitted with a metering device to allow for slow and

controlled release of the gas• Application: Calculate the volume of the ripening room and the

release the gas from the correct number of cylinders to achieve the correct concentration of ethylene required for ripening or de-greening

Ethylene Generators

• Devices that are placed in ripening rooms• A liquid is poured into them and they are connected to an electrical power

source, and they produce ethylene over a protracted period

• Way of generating ethylene would be to heat ethanol in a controlled way in the presence of a copper catalyst• Application: Calculate the volume of the store and place the correct

number of generators in the store to provide the required ethylene concentration

Alternative Gases: Acetylene

• Produced in less developed countries throughout the world using calcium carbide • cheaper than ethylene sources and easier to apply in simple ripening rooms

• Pure calcium carbide then 1 kg would produce 300 L of acetylene gas• gas is released when the calcium carbide is exposed to moisture

• High humidity reacts with the calcium carbide, giving a slow release of acetylene• Large quantities of acetylene are required - small amounts of calcium

carbide can be dropped carefully into large buckets of water.

Simple Methods: Fruit Ripening

•Smoke•Damage•Fruit Generation

Smoke

• Smoky fire is lit in the ripening room• Produce various gases, including acetylene, ethylene and carbon

monoxide – initiate ripening

Damage

• Wounding the banana bunch stalks or even the fruit may produce ethylene in response to the wound• Other methods, includes cutting, scraping or ‘pinching’ papaya, chico

or avocado, which can hasten ripening

Fruit Generation• Fruit that are ripening and thus giving out ethylene can be placed in

an air-tight room with green fruit• Room should need to be frequently ventilated to ensure there was no

build-up of (inhibits the effect of ethylene)