Testing the Amount Of Juice, The Acid And The Sugar Levels In Citrus Fruit

Transcription

1 Testing the Amount Of Juice, The Acid And The Sugar Levels In Citrus Fruit A chemistry experiment for secondary schools students

2 Contents Teacher Information... 2 Testing the Amount Of Juice, The Acid And The Sugar Levels In Citrus Fruit... 3 Background... 3 Testing Fruit Percent Juice Content Brix (total soluble solids): Acid (titratable acidity... 3 Experiment 1: Calculating the Percentage of juice in the fruit... 5 Equipment:... 5 Procedure Collection of fruit sample: Determining percentage juice content: 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: Questions Research activities that could be done by students References 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.

3 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 ( 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

4 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!

5 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 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 = 600 g Gross juice weight = 450 g Jug weight = 150 g

6 Gross Juice weight - jug weight = net juice weight eg 450 g g = 300 g Juice weight net x 100 = % juice Fruit weight 300 x 100 = 50% Questions: 1) What was the percentage juice you received for your sample? ) 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?...

7 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 ( o Brix), indicates the sweetness of the fruit by measuring the number of soluble solids in the juice (source / 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.

8 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)?

9 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 M sodium hydroxide (NaOH) - 1% Phenolphthalein indicator 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 to get the value of the acid (in grams per 100ml)

10 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 = 24.2 ml Millilitres of 0.1M sodium hydroxide x = citric acid concentration (g per 100 ml) e.g x = 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 ( To determine the sugar / acid ratio you need to divide the sugar concentration ( Brix) by the citric acid concentration. Example Calculation Sugar concentration = 15.2 Brix Citric Acid concentration = 1.55 g per 100mL Sugar concentration ( Brix) = X:1 sugar/acid ratio Citric acid concentration eg 15.2 ( Brix) = 9.8:1 sugar/acid ratio 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?...

11 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 Acid/Brix ratio Mandarins 8 to 1 Oranges (navel only) 9 to 1 Oranges (other than navel) 8 to 1 Grapefruit and pummelos 5.5 to 1 Hybrids of any species 8 to 1 Table 2. Total soluble solid determinations ( Brix) Fruit Type Total soluble solids ( Brix) Type of Fruit Brix at 20 o C Mandarins 8 Oranges 8 Grapefruit and pummelos 8 Hybrids of any species 8 Table 3. Minimum juice content Fruit Type Minimum juice content (%) Type of fruit Minimum % juice content Lemons 25 Mandarins 28 Oranges 33 Grapefruit and pummelos 33 Limes 33 Hybrids of any species 33

12 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 CO 2 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) Viewed on the 22 July 2009 Grow Mildura Region Web Site (2009) Viewed on the 22 July 2009