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ISSN 1447-428X
Volume 12, Issue 1
Term 1 2007
»In this issue
» Making a difference
» Red or green tomatoes - teaching activity
» Career profile
» Aussie kids measure millipede mayhem
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Victorian Essential Learning Standards
Use of these learning and teaching activities may contribute to achievement of the Standards. It primarily addresses the Science and Thinking Processes learning focus at Levels 3, 4, and 5.


Red or green tomatoes
   

LandLearn would like to gratefully acknowledge the input of Dr Rod Jones of DPI Knoxfield with this activity.

Duration and setting
1 hour plus follow up time in the classroom.

Summary
This activity allows students to undertake a simple experiment to explore the chemical process involved in ripening fruit.

Student outcomes
Students will be able to:
» investigate the role of ethylene gas in ripening fruit
» explore the role other fruit plays in the ripening process
» plan, conduct and present results of a scientific investigation.

Background information
Did you know that fresh tomatoes purchased at the supermarket are rarely "fresh"? Tomatoes and other fruit are often picked while green and transported to storage sheds where they are stored in ventilated conditions until ethylene gas is added to make them ripen and turn red. Because of this artificial ripening process they often do not have the same flavour as a vine ripened tomato and won't keep very long on a supermarket shelf or in the kitchen.

Ethylene gas (H2C=CH2) is a colourless gas with a faint smell that occurs naturally and is also created by human sources such as combustion engines. The largest natural producers of ethylene are plants. Ethylene is manufactured and released by rapidly growing tissues in roots, senescing (ageing) flowers, and ripening and rotting fruit. Most fruit and vegetables produce ethylene in small amounts however certain fruits produce larger quantities of ethylene and ripen rapidly and uniformly when exposed to an external source of ethylene. Ethylene affects the growth, development, ripening, and senescence of all plants.

Fruits and vegetables contain receptors that bond with ethylene gas and after the gas binds to these sites, they quickly ripen. Ethylene actually destroys chlorophyll thus removing the green colour from the fruit or vegetable allowing other pigments such as red and yellow carotenoids to be revealed. This is why ripening produce usually changes colour.

Examples of ethylene producing plants include apples, avocados, bananas, melons and pears (see table 1). Ethylene-sensitive plants that have low ethylene production include broccoli, cabbage, cauliflower, lettuce and other leafy greens. Storing ethylene-sensitive plants with ethylene-producing plants can spoil fresh produce quickly.

Table 1: Examples of ethylene production levels in selected fruit and vegetables

High ethylene producing
Low ethylene producing
Apples, pears, passionfruit apricot, nectarine, peach mango, avocado, banana, cantaloupe
Cherry, blackberry, strawberry lettuce, cucumber, spinach lemon, orange, mandarin water melon, grape, tomato (med-low)

Adapted from Jenny Jobling 'Postharvest Ethylene: Effects of ethylene on fresh produce'. Accessed at www.postharvest.com.au > Postharvest information sheets > Postharvest Ethylene: A critical factor in quality management

Apple growers take advantage of the ripening effects of ethylene by picking fruit before it is ripe, holding it under controlled gas conditions to minimise the ethylene concentration, and then exposing the fruit to ethylene just before taking it to market. This enables growers to provide consumers with newly ripened Australian-grown apples in spring and summer, even though apples naturally ripen in autumn.

It is possible to apply these principles to ripening fruit by placing them in a paper or plastic bag (sealed plastic bags are more air tight) with another already ripe high ethylene-producing fruit (see table above). The ethylene produced by the fruit is contained in the bag and increases in concentration therefore stimulating ripening. If left too long this can lead to spoiling. A similar effect of ethylene concentration can occur in refrigerators and shipping containers.

Understanding the role of ethylene in ripening fruit is important for the horticulture industry. For example, picking tomatoes when they are green reduces the amount of time fruit spends hanging on plants where birds, insects, disease or the weather may damage them. Fruit is also less prone to damage during transport when green and can be ripened when the consumer demand is present. Withholding ethylene from fruit and vegetables allows producers, handlers and sellers to spread produce availability between seasons and meet demand from consumers. In addition, removing ethylene from storage and shipping containers slows spoilage and reduces waste.

Materials
» Two brown paper bags
» Two mature green tomatoes or unripe avocados (same degree of ripening - try a green grocer or vegetable market)
» Two ripe bananas

The activity

For levels 3-4
1 Using table 2 ask students to predict whether they think certain fruits will ripen when placed in a bag with other fruits, record the predictions. Ask students to identify reasons for their points of view.
2 For the experiment, place a tomato in a bag with two ripe bananas. In the second bag (the control), place only a tomato.
3 Have students observe the tomatoes until one or both of them ripens and record observational data.
4 Discuss as a class or investigate in groups, the effect of the ethylene gas on the rate of ripening of the tomatoes.
5 Revisit original predictions: were they proved or disproved? What factors led to a change in student thinking?
6 Some fruits give off more ethylene than others. Ask students to design an experiment to investigate which fruits release off ethylene and which don't.

Table 2 Observations table
Prediction
Day
Bag with one tomato (control)
Bag with one tomato and two bananas (test sample)
  Day 0    
  Day 1    
  Day 2    
  Day 3    
  Day 4    

For level 5
7 Ask students to design an experiment to test the effects of ethylene on fruit based on some of the information in the Background Information and explain how they arrived at their design. (Consider the types of thinking that benefit the process of designing scientific experiments).
8 Conduct tests using different variables eg: fruit position - light, dark, room temperature, fridge or freezer, length of time exposed to ethylene or the effect of different fruit and vegetables on each other (see Table 1 for high and low ethylene producing fruit).
9 Investigate the saying 'it only takes one rotten apple'. What effect does an over-ripe apple has on a bag of apples?

Website resources
Effects of ethylene on fresh produce http://www.postharvest.com.au/EthylenePDF.PDF

Related LandLearn activities
Grow and Gobble activity booklet available on LandLearn Resource Booklets CD. Activities include - 'Capturing wild yeast', 'How much starch is in my bread?', 'How much vitamin C can you see?', 'Browning apples', 'Curds and whey', 'Making ice cream'.

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Image: : ripe tomatoes

image: : avocados

Image: bananas
image: : paper bags

Image: red and green tomatoes

 
For more information please contact the LandLearn Team: landlearn.program@dpi.vic.gov.au - Ph. (03) 5482 0453
This document was reviewed 21 August, 2007