January 2010 Primefact 980

F O R P R O F I T A B L E , A D A P T I V E A N D S U S T A I N A B L E P R I M A R Y I N D U S T R I E S

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Citrus maturity testingSandra Hardy

Industry Leader, Citrus Industry & Investment, Gosford Primary Industries Institute

Graeme Sanderson

Research Horticulturalist, Industry & Investment, Dareton

Introduction

Some markets require that citrus fruits meet specific maturity standards before being sold. These standards are most commonly described as a ratio of sugar, expressed as total soluble solids (TSS) to acid and sometimes percent juice. Maturity testing is undertaken on a sample of 1020 pieces of fruit representative of the fruit ready to harvest. The more fruit tested the more accurate the test results.

Juice content (% juice). This is the quantity of juice in the fruit, expressed as percent juice. The juice content of fruit increases as they mature. However when fruit are over-mature their juice content often decreases.

Total soluble solids (TSS) or Degrees Brix ( Brix). TSS refers to the total amount of soluble constituents of the juice. These are mainly sugars, with smaller amounts of organic acids, vitamins, proteins, free amino acids, essential oils and glucosides. Approximately 85% of the total soluble solids of citrus fruit are sugars so TSS is an excellent guide to the sugar content of fruit. Fruit sugar levels generally increase as the fruit matures; however levels can decrease when fruit are over-mature.

TSS content also forms the basis of payment for fruit by some juice processors in a number of countries, especially where the trade in juice is

based on frozen concentrate. The lower the TSS content of fruit the lower the yield of concentrate produced from it. TSS is often expressed as kg/tonne.

TSS or Brix can be calculated using either a Brix scale hydrometer or a refractometer. A Brix hydrometer measures specific gravity and is calibrated to read directly in units of sugar concentration degrees Brix (Brix) at a temperature of 20C. One degree of Brix is the concentration of a cane sugar solution containing 1 g of sugar/100 g of solution. When the hydrometer is floating in a sample of juice the point on the scale corresponding with the juice surface is the soluble solids content in Brix. The hydrometer sinks or floats in response to changes in density. The volume of a substance expands with a rise in temperature and contracts with a fall in temperature, so the density of the juice is also affected by temperature. The Brix reading requires temperature correction because the hydrometer is calibrated to read true only at 20C. Temperature is corrected by a factor in a standardised table (Table 1). A hydrometer cannot be used if juice temperature is higher than 37.5C.

Refractometers are quick and easy to use, especially when there are lots of samples to be tested. A refractometer is an optical instrument that measures the amount of light refracted in a liquid. There are different types of refractometers, handheld (analog and digital) and digital. Most digital refractometers have automatic temperature compensation for a specified temperature range (e.g. 1030C) so there is no need to correct the Brix reading for temperature. For analog refractometers the reading has to be corrected for temperature. All refractometers need to be calibrated. Distilled or deionised water is used to set the baseline reading to zero this should be done regularly throughout testing. Additionally, testing solutions of a known Brix level (i.e. 10 and

30) can also be made up or purchased to more accurately calibrate the instrument.

Factors affecting TSS

Rootstock rough lemon rootstock generally has lower TSS content, acidity and juice content compared with citrange and trifoliata rootstocks.

Variety/scions there can be differences between varieties (e.g. Murcott mandarin is a high Brix variety).

Fruit maturity sugar levels generally increase as fruit mature, but can decline if fruit become over-mature. The main changes in maturity are associated with a decline in acidity so time of harvest is important.

Seasonal effects there can be year to year differences in fruit maturity and TSS, largely due to climatic conditions.

Climate can affect both sugar and acid content. Sugar and acid levels are generally lower in hot climates compared to cooler regions.

Nutrition general affects of nitrogen (N), phosphorous (P) and potassium (K) on internal fruit quality. High levels of N increase juice acidity and slightly increase % soluble solids and decrease % juice, TSS/acid ratio, TSS/tonne. High levels of P increase % juice content, TSS/acid ratio and TSS/tonne and decrease juice acidity and slightly decrease % soluble solids. High levels of K increase juice acidity and decrease % juice and TSS/acid ratio.

Fruit size and load generally small fruit have higher TSS content than large fruit. A heavy crop (on year) generally has a higher TSS content than a light crop (off year).

Tree age fruit from older trees generally has a higher TSS than fruit from young trees.

Position of fruit on tree fruit in the shade (inside the canopy) generally has lower TSS content than fruit in full sun.

Differences between trees a result of many factors including soil type, nutrition, aspect etc.

Growing regions maturity differences mainly as a result of climate but can also be because of regional differences in variety, rootstock, soil type etc.

Acid (citric) content. The citric acid content of fruit is highest early in the season and decreases as fruit mature. Fruit acidity is determined by a procedure known as titration. This test involves adding a few drops of indicator solution (phenolphthalein) to 10 ml of juice and then measuring the volume of

sodium hydroxide required to neutralise the solution which is indicated by a colour change (persistent pink) in the solution. The more citric acid the juice contains the more sodium hydroxide required to neutralise it. Titratable acidity is measured in ml.

Brix (sugar):Acid ratio. Immature fruit normally have a low sugar:acid ratio because of low sugar and high acid levels, making fruit taste sour. As fruit mature and ripen carbohydrates are converted into sugars, increasing the sugar content whilst the acids diminish, resulting in a higher sugar:acid ratio.

Methods

Maturity testing involves measuring the weight of the fruit sample, the weight of the juice from the fruit sample, the percent total soluble solids of the juice (expressed as Brix) and the titratable acidity of the juice. From these measurements you can then calculate: total soluble solids (TSS); % acid; % juice and the sugar:acid ratio.

Collect the fruit sample harvest fruit from all sides of selected trees (including the tops and inside of tree). The sample should be representative of all fruit sizes and of the fruit to be harvested (i.e. if selective picking then only sample fruit ready for harvest). A minimum of 10 fruit should be used for testing, but 20 fruit is better.

Determining juice content

1. The fruit sample should be a minimum of 10 fruit.

2. Place the container that fruit will be weighed in onto the scales and tare to zero.

3. Place fruit into the container and weigh the fruit sample. Record weight in grams.

4. Extract and strain the juice through a fine sieve from the fruit.

5. Place the container that will hold the juice on the scales and tare to zero.

6. Pour the juice into the container.

7. Weigh the juice and record weight in grams.

8. % juice = juice weight fruit weight x 100.

Determining Brix using a hydrometer

1. The fruit sample should be a minimum of 10 fruit.

2. Cut each fruit in half (at right angles to fruit axis).

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3. Juice fruit and strain through a fine sieve (12 mm). Put aside some juice (10 ml) for the titration (acidity) test.

4. Pour juice into a measuring cylinder until it overflows the lip which helps with removing bubbles and froth. Air bubbles will affect the reading. The diameter of the measuring cylinder should be at least 10 mm bigger than the diameter of the hydrometer bulb.

5. Let the juice stand for 5 minutes so the solids in suspension settle out.

6. Record the temperature of the juice (immerse a thermometer in the juice for 1 minute).

7. Gently lower the hydrometer (graduated from at least 515 Brix) into the measuring cylinder, releasing with a slight swirl.

8. When the hydrometer has stabilised (1 minute), read the value on the scale and record.

9. If the juice temperature is not 20C correct the reading for temperature using the factors in Table 1. This is the Brix reading.

10. Between samples rinse all equipment in clean water and allow to dry.

Determining Brix using a refractometer

There are two types, hand-held manual analog refractometers and digital refractometers. The reading from a hand-held analog refractometer needs to be corrected for temperature. Most digital refractometers have automatic temperature compensation for a specified temperature range

(e.g. 1030C) so there is no need to correct the Brix reading for temperature.

1. Follow steps 13 as for hydrometer.

2. Place a few drops of juice onto the stage of the refractometer and take the Brix reading and record.

3. If the refractometer is not temperature compensated then correct the reading and record the final figure this is the Brix reading.

4. Between samples clean the refractometer with distilled water and dry.

5. For hand-held refractometers, periodically (i.e. every ten readings) check that the baseline is set to zero.

Determining acidity (titration)

1. Put 10 ml (for lemons you can use 5 ml) of the juice into a conical flask.

2. Add 5 drops of phenolphthalein solution (indicator).

3. Fill a burette with 50 ml of 0.1% sodium hydroxide.

4. Slowly add the sodium hydroxide solution drop by drop to the flask and swirl, until the colour goes a persistent pink for at least 30 seconds (~pH 8.2).

5. Record the amount of sodium hydroxide solution used in ml.

6. Acidity = 0.064 (or 0.128 for lemons when using 5 ml of juice) x ml of sodium hydroxide used.

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Determining Brix using a hydrometer:

Step 7. Gently lower the hydrometer into the measuring cylinder, releasing with a slight swirl.

Step 8. When the hydrometer has stabilised read the value on the scale.

Determining Brix using a refractometer:

Step 2: Place a few drops of juice onto the stage of the refractometer and take the Brix reading and record.

Determining acidity: Step 1. Place 10 mls juice in flask

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Step 2. Add 5 drops of phenolphthalein solution

Step 4. Slowly add sodium hydroxide solution until colour change

Formulas

% juice = juice weight (g) fruit weight (g) x 100

% Acid (citric) = titratable acidity (ml) x 0.064* (factor for 10 ml juice)

*For lemons if using 5 ml of juice the multiplication factor is 0.128.

TSS or % sugar = Brix

TSS (kg) per tonne = % juice x temperature corrected Brix 10

Sugar (TSS): Acid ratio = temperature corrected Brix acidity (% citric acid)

References

Tasting the Difference The importance of citrus juice yield and quality. NSW Department of Agriculture. Proceedings of a seminar in Griffith, 1988.

Tugwell, BL & Robinson, JB, Soluble Solids Content in the Juice of Valencia oranges.

El-Zeftawi, B, Sarooshi, R, Gallasch P, Treeby M 1982, Factors affecting total soluble solids of oranges used for processing, Department of Agriculture, Victoria.

Howard LJ & Wade N, A Procedural manual for the Maturity testing of some fruits by NSW Department of Agriculture Plant Diseases Act Inspectors, NSW Department of Agriculture

Citrus Maturity Test Record SheetDate:Variety: Block:No. of Fruit = 20% Juice

A. Fruit Weight (g) g

B. Juice Weight (g) g% Juice calculationjuice weight fruit weight x 100

B A x 100

C. % Juice = %% Acid

D. Amount of sodium hydroxide used (ml) to neutralise juice = ml

Acidity calculation0.064 x ml sodium hydroxide

0.064* x D (*Figure for 10 ml of juice)

E. % Acid = %

1. Brix (Soluble Solids) using hydrometer Juice temperature (C) = C

Hydrometer reading = Brix

Temperature corrected hydrometer reading

If juice temperature not 20C then add or subtract correction factor from Table 1=

F. Brix =

2. Brix (Soluble Solids) using temperature compensating digital refractometer

F. Refractometer reading = Brix3. Brix (Soluble Solids/Sugar) using analog refractometer

Juice temperature (C) = C

Refractometer reading = Brix

Temperature corrected refractometer reading

If juice temperature not 20C then add or subtract correction factor from Table 1=

F. Brix = BrixBrix: Acid ratioBrix: Acid ratio calculationBrix % acid

F E

G. Brix: Acid ratio = :

TSS(kg) per tonneTSS calculation% juice x Brix 10

C x F 10

TSS = kg/tonne

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Table 1 Temperature correction table for Brix to standard temperature 20CTemperature C Correction factor Brix Temperature C Correction factor Brix

10 0.45 25.5 +0.3011 0.40 26 +0.3512 0.40 26.5 +0.3513 0.35 27 +0.40

14 0.30 27.5 +0.4015 0.30 28 +0.45

15.5 0.25 28.5 +0.50

16 0.25 29 +0.5516.5 0.20 29.5 +0.55

17 0.15 30 +0.60

17.5 0.10 30.5 +0.65

18 0.10 31 +0.65

18.5 0.05 31.5 +0.70

19 0.05 32 +0.75

19.5 0 32.5 +0.7520 0 33 +0.80

20.5 +0.05 33.5 +0.85

21 +0.05 34 +0.9021.5 +0.10 34.5 +0.90

22 +0.10 35 +0.9522.5 +0.15 35.5 +1.00

23 +0.15 36 +1.05

23.5 +0.20 36.5 +1.10

24 +0.20 37 +1.1024.5 +0.25 37.5 +0.15

25 +0.25

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State of New South Wales through Department of Industry and Investment (Industry & Investment NSW) 2010. you may copy, distribute and otherwise freely deal with this publication for any purpose, provided that you attribute Industry & Investment NSW as the owner.

ISSN 1832-6668

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Disclaimer: The information contained in this publication is based on knowledge and understanding at the time of writing (January 2010). However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of State of New South Wales through Industry & Investment NSW or the users independent adviser.

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