Testing the Amount Of Juice, The Acid
And The Sugar Levels In Citrus Fruit
A chemistry experiment for secondary schools students
Contents
Teacher Information ............................................................................................................................. 2
Testing the Amount Of Juice, The Acid And The Sugar Levels In Citrus Fruit ......................... 3
Background ...................................................................................................................................... 3
Testing Fruit ..................................................................................................................................... 3
Percent Juice Content ........................................................................................................... 3
Brix (total soluble solids): .................................................................................................... 3
Acid (titratable acidity ......................................................................................................... 3
Experiment 1: Calculating the Percentage of juice in the fruit ..................................................... 5
Equipment: ....................................................................................................................................... 5
Procedure ......................................................................................................................................... 5
1. Collection of fruit sample: .......................................................................................................... 5
2. Determining percentage juice content: ....................................................................................... 5
3. Questions:.................................................................................................................................... 6
Experiment 2. Determining soluble solids content (% sugar or °Brix) ........................................ 7
Equipment ........................................................................................................................................ 7
Procedure ......................................................................................................................................... 7
Questions:......................................................................................................................................... 8
Experiment 3. Determining acidity (citric acid content) ................................................................ 9
Equipment ........................................................................................................................................ 9
Part 1: Procedure Titration ............................................................................................................. 9
Part 2: Procedure Determining sugar/acid ratio: ........................................................................... 10
Questions ........................................................................................................................................ 10
Research activities that could be done by students ....................................................................... 12
References ......................................................................................................................................... 12
Teacher Information
Target audience
• Year 11 and 12 Chemistry
Class size
• ~28 students. It is suggested students work in teams of three to four.
Time
•
Experiment to operate over a double lesson
Learning Outcomes
At the end of this experiment, students will be expected to understand;
- Calculating juice percentages in citrus
- Determining citrus solid contents
- Using refractometers
- Understanding titrations
Acknowledgements
This practical activity was developed by teachers from the Sunraysia area who took part in a
tour of the Citrus Packaging Section of the Mildura Fruit Company in Mildura. Our thanks
to Bill Robinson – Grower Services Manager, Mara Milner – Scientist/Food technologist
and Tony Filipi – Grower Services Rep who provided the teachers with the information
about the science involved in the citrus packing industry.
Testing the Amount Of Juice, The Acid And The Sugar Levels In Citrus Fruit
Background
The Mildura Region (also known as Sunraysia and part of the wider Murray Valley) is one of
Australia’s prime areas for growing quality citrus fruits. The irrigation-based horticultural area
produces approximately 21% each of Australia’s total wine grape crush, 20% of citrus production
and almost 100% of Australia’s dried vine fruit production (www.growmilduraregion.com.au
2009). The high quality fruits that are produced by growers in the region are utilised in the
following way:
-
44% is exported (to the United States, Hong Kong, Malaysia, Japan, Singapore, New
Zealand, Indonesia and Taiwan).
22% is sold as fresh fruit on the domestic market; and
34% is processed into juice.
Testing Fruit
Before citrus fruit such as oranges, lemons, grapefruit and mandarins are picked the juice of the
fruit is tested to see that it meets certain specifications. There are further tests done once the fruit is
picked. For example, when each truckload of oranges is delivered to the citrus plant, such as the
Mildura Packing Company, a sample of oranges is taken, the fruit is juiced and the juice evaluated
to see if it meets certain standards for taste. Specifically, the tests looks at certain fruit maturity
requirements such as the percent juice content, the measure of solids or sugars in the juice (or the
‘°Brix’ measure) and the total acid level of the fruit ‘Acid, Ratio’..
-
Percent Juice Content: Juice content is an important measure of internal quality. Under
or over-ripe fruit tend to be less juicy, which directly affects eating quality. The juice
content is determined by weighing components of the whole fruit and the juice.
-
Brix (total soluble solids): In the citrus industry this is a measure of the total soluble
solids in the juice. These soluble solids are primarily sugars; sucrose, fructose, and
glucose. Citric acid and minerals in the juice also contribute to the soluble solids. Brix is
reported as "degrees Brix" and is equivalent to percentage. For example, a juice which is
12 degrees Brix has 12% total soluble solids. The National standards state the minimum
brix sugar/ acid ratio for navel oranges is 10:1.
-
Acid (titratable acidity): This is a measure of the total acid present in a juice. The
predominant acid naturally occurring in orange juice is citric acid. There are also small
amounts of malic acid and tartaric acid present. The amount of acid present in the juice
is reported as percent citric acid. It needs to be noted that the total acid cannot be
measured by pH because the acids concerned are “weak acids” and not completely
ionized. The acid content must be measured using a titration with sodium hydroxide.
These tests are an easy but essential measure to ensure the consumer received an end product, fresh
orange juice, that meets a continuous standard, taste and quality. Students can conduct the
following tests in the classroom to determine if their fruit makes the grade!
Experiment 1: Calculating the Percentage of juice in the fruit
The test to determine the ‘Percent Juice Content’ is important to determine the quality of the fruit.
If to fruit is under or over-ripe there can be less juice, which affects eating quality and the amount
of money a farmer will receive for their crop.
The following procedure can be used to determine the Percent Juice Content.
Equipment:
- scales
- Fruit juicer, juice only - no rind
- Fine plastic strainer
- 100 mL conical flask
- 1 to 2 L jug
Procedure
1. Collection of fruit sample:
A minimum of three fruits, representative of the variety to
be harvested, must be collected. This does not mean
selecting the biggest and best fruit to test. Select fruit that
are of similar size and colour to as those that you would
pick for market.
2. Determining percentage juice content:
a) Weigh the three representative fruit and record the combined weight in grams.
b) Weigh the empty jug and record the weight in grams.
c) Cut the fruit in halves and extract the juice from the three sample fruit using a juicer (this
will ensure you collect as much juice as possible from the fruit).
d) Strain the juice into the weighed jug.
e) Weigh the juice and record the weight in grams.
f) Calculate the percentage of juice.
Example Calculation
Gross fruit weight
Gross juice weight
Jug weight
=
=
=
600 g
450 g
150 g
Gross Juice weight - jug weight
eg 450 g - 150 g
=
=
net juice weight
300 g
Juice weight net x 100
Fruit weight
=
% juice
300 x 100
600
=
50%
3. Questions:
1) What was the percentage juice you received for your sample?.........................................................
………………………………………………………………………………………………………....
2) Refer to the Industry Standards Chart (provided in appendix 1) and determine if these samples
meet industry requirements? Comment …………………………………………................................
…………………………………………………………………………………………………………
…………………………………………………………………………………………………………
3) Why is it important you collect a sample of fruit to test that represents what you will be
harvesting?…………………………………………………………………………………………….
…………………………………………………………………………………………………………
…………………………………………………………………………………………………………
4) What conditions on a farm might affect the levels of juice in a fruit?..............................................
…………………………………………………………………………………………………………
…………………………………………………………………………………………………………
…………………………………………………………………………………………………………
Experiment 2. Determining soluble solids content (% sugar or °Brix)
Determining the total soluble solids (Brix) is an important measurement taken in a wide range of
crops. In the citrus industry this is a measure of the total soluble solids in the juice. These soluble
solids are primarily sugars; sucrose, fructose, and glucose.
How is there sugar in the fruit? As the flesh of fruit forms it deposits nutrients as starch that, as the
fruit ripens, transform to sugars. The percentage sugar, measured in degrees Brix (oBrix), indicates
the sweetness of the fruit by measuring the number of soluble solids in the juice (source
/www.agric.wa.gov.au/PC_93298.html).
Citric acid and minerals in the juice also contribute to the soluble solids.
The ratio of the Brix to total acid reading must be above a certain tolerance level. Oranges need the
sugar level for sweetness and the acid for a tart taste and preservation of the fruit.
The following procedure will allow you to calculate the Brix.
Equipment
1. refractometer (can be purchased at reasonable prices on ebay)
2. fresh orange juice
Procedure
1. Collect a refractometer from your teacher (these should be handled carefully as they are
expensive!)
2. Ensure the refractometer prism surface is clean and dry.
3. Place a small amount of fresh juice (a couple of drops is sufficient) onto the prism of the
refractometer.
4. Look through the eyepiece while pointing the prism in the direction of good light (not
directly at the sun).
5. Focus and take the reading of where the base of the blue colour sits on the scale and record
the % percentage sugar (°Brix).
6. Clean the refractometer immediately with a damp tissue, and dry thoroughly.
Questions:
1) What was your Brix reading? ……………………………………………………………..
2) On the picture of the fruit, label where you think the nutrients are deposited first (i.e. top of the
fruit or bottom)?
Experiment 3. Determining acidity (citric acid content)
Citric acid, and small amounts of malic and tartaric acid, give citrus its tartness and unique taste.
The levels of acid are at the highest concentrations early in the season and they decrease as the fruit
mature.
The amount of acid present in the juice is reported as percent citric acid. To calculate this value we
use a titration with sodium hydroxide.
Equipment
-
50 mL burette
Burette stand and clamp
10 mL pipette and pump
1 L volumetric flask
0.1 M sodium hydroxide (NaOH)
1% Phenolphthalein indicator
100 mL conical flask
Pipette
Distilled water
Fresh Juice
Gloves, safety glasses and lab coat (ensure all students are wearing)
Part 1: Procedure Titration
1. Pipette 10 mL of juice into a clean conical flask
2. Clean pipette immediately
3. Pipette 10 mL of distilled water into the conical
flask containing the juice
4. Add six drops of phenolphthalein indicator to flask
5. Carefully swish mixture
6. Fill the burette with 0.1 M sodium hydroxide
solution.
7. Open burette tap and allow a trickle of sodium
hydroxide NaOH to run into a beaker. This is to
ensure no air is in the burette prior to titration
8. Refill the burette, making sure that it reads zero at
the top of the scale.
9. Hold the conical flask containing the juice mix under the
burette and while swirling, slowly add the sodium hydroxide to
the juice.
10. Keep adding sodium hydroxide to the flask while swirling
until the solution just starts to change colour to pink/purple.
This is the end point and you should now record how much
sodium hydroxide you have added to the flask.
11. Multiply the volume of sodium hydroxide added by 0.064 to
get the value of the acid (in grams per 100ml)
Note the end point of the titration may be very difficult to tell at the start until you get used to the
procedure. If you look closely you will see the juice mix slowly lighten in colour and then change to
a green colour. This is the point just before the end of the titration and a few extra drops will see the
solution change colour to pink. If you go past this point, and the solution becomes a deep
purple/orange, you have gone too far.
Example Calculation of acid content.
Amount of 0.1 M sodium hydroxide added
Millilitres of 0.1M sodium hydroxide x 0.064
=
=
24.2 mL
citric acid concentration (g per 100 mL)
e.g. 24.2 x 0.064
=
1.55 g per 100 mL citric acid.
Part 2: Procedure Determining sugar/acid ratio:
“The sugar-acid ratio contributes to the unique flavour of citrus. At the beginning of the ripening
process the sugar-acid ratio is low, because of low sugar content and high fruit acid content—this
makes the fruit taste sour. During the ripening process the fruit acids are degraded, the sugar content
increases and the sugaracid ratio achieves a higher value”
(http://www.agric.wa.gov.au/PC_93298.html).
To determine the sugar / acid ratio you need to divide the sugar concentration (°Brix) by the citric
acid concentration.
Example Calculation
Sugar concentration
Citric Acid concentration
Sugar concentration (°Brix)
Citric acid concentration
=
=
=
15.2°Brix
1.55 g per 100mL
X:1 sugar/acid ratio
eg
=
9.8:1 sugar/acid ratio
15.2 (°Brix)
1.55 (g per 100 mL)
Questions
1) Describe the results from your titration and indicate what these results mean for the taste of the
juice? …………………………………………………………………………………………………
…………………………………………………………………………………………………………
…………………………………………………………………………………………………………
2) What was your sugar / acid ratio?....................................................................................................
INDUSTRY STANDARDS
The following shows the industry standards for citrus fruits in some states of Australia. These
values can vary in different states.
Table 1. Total soluble solids (°Brix) to acid ratio Fruit Type Total soluble solids (°Brix) to acid
ratio
Type of Fruit
Mandarins
Oranges (navel only)
Oranges (other than navel)
Grapefruit and pummelos
Hybrids of any species
Acid/Brix ratio
8 to 1
9 to 1
8 to 1
5.5 to 1
8 to 1
Table 2. Total soluble solid determinations (°Brix) Fruit Type Total soluble solids (°Brix)
Type of Fruit
Mandarins
Oranges
Grapefruit and pummelos
Hybrids of any species
°Brix at 20oC
8
8
8
8
Table 3. Minimum juice content Fruit Type Minimum juice content (%)
Type of fruit
Lemons
Mandarins
Oranges
Grapefruit and pummelos
Limes
Hybrids of any species
Minimum % juice content
25
28
33
33
33
33
Research activities that could be done by students
These activity could be repeated with different types of citrus fruits.
Numerous investigations could be carried out such as the effect of
age of the fruit.
temperature of storeage
other gases such as CO2 on the acid/brix ratio.
Other ideas include:
1. Find out how frozen concentrated orange juice is made and why the brix is higher than
normal orange juice and acid content of the concentrate is.
2. Some oranges early in the season have the desired Acid/Brix ratio but are still green. The
packaging shed can change the colour from green to orange in a process called
“Degreening” by treating the oranges with a chemical. Research how this is done.
3. In the packaging process the oranges must be cleaned and sterilized to prevent the growth of
bacteria and other organisms. Research what the chemicals used for this process are and
how they work.
4. Citrus fruit is also waxed to improve the appearance and help protect the fruit. Research
what these waxes are made of.
References
Government of Western Australia, Department of Agriculture and Food (2009)
http://www.agric.wa.gov.au/PC_93298.html Viewed on the 22 July 2009
Grow Mildura Region Web Site (2009) http://www.growmilduraregion.com.au/citrus.php
Viewed on the 22 July 2009