Methods for Determining the Quality of Citrus Juice - ACS Symposium

13 Methods for Determining the Quality of Citrus Juice JOSEPH W. MCALLISTER

Downloaded by TUFTS UNIV on November 30, 2015 | http://pubs.acs.org Publication Date: December 15, 1980 | doi: 10.1021/bk-1980-0143.ch013

U.S. Department of Agriculture, P.O. Box 860, Winter Haven, F L 33880

In the thirty years from the middle of the 1930's to the middle of the 1960's the world experienced a growth in citrus processing and marketing that is hardly rivaled by any other food product. There was a slow steady increase in processed citrus from the middle 30's until shortly after World War II, when frozen concentrated orange juice was developed and successfully marketed. With the advent of frozen citrus concentrates the product acquired a new name--"The Cinderella Product," and it was. The manufacture of this product resulted in phenomenal growth; it did not occur easily or smoothly. Quality control methodology was being developed and constantly changed to keep up with advancing processing technology (1). Today quality control technology is advanced, sophisticated and adequate to assure high quality under most conditions for all processed cultivars and their by-products (2). In the middle 1970's the marketing of citrus concentrates started to change. In earlier years the producer shipped the retail product from his plant directly to the market. Now more and more bulk citrus concentrates are being shipped to widespread markets for remanufacture locally. Bulk concentrates are now manufactured and stored at a high degree of solids in large capacity tank farms awaiting shipment within the United States in tank trucks or internationally in 55 gallon drums. With this change in marketing, more emphasis is being put on developing methods to determine the composition and quality of the end product. METHODS FOR DETERMINING QUALITY Brix Determination Introduction. Degrees Brix is a term used to designate the percent by weight of dissolved sugar in a solution. In citrus juices it is used to indicate the percent of soluble solids contained in the juice. This is one of the more important This chapter not subject to U.S. copyright. Published 1980 American Chemical Society In Citrus Nutrition and Quality; Nagy, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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CITRUS NUTRITION AND QUALITY

determinants of q u a l i t y . The B r i x of a j u i c e i s used as a f a c t o r i n determining m a t u r i t y of f r u i t , to e s t a b l i s h v a r i o u s grades of q u a l i t y , and i s important i n p r i c i n g of the commodity. Soluble s o l i d s may be determined e i t h e r with a hydrometer or r e f r a c t o metrically. Hydrometer readings c o r r e c t e d f o r temperature w i l l g i v e a c l o s e r reading to the true s o l u b l e s o l i d s content of a j u i c e than a refractometer. Because c i t r i c a c i d depresses refractometer readings, a c o r r e c t i o n f o r the amount of a c i d must be added. Refractometer readings must a l s o be c o r r e c t e d f o r temperature (3). I t has been shown (4) (5) (6) that many other c o r r e c t i o n s must be made to the refractometer to make the readings more comparable to the true s o l u b l e s o l i d s content of citrus juices. Corrected refractometer readings on s i n g l e s t r e n g t h j u i c e s w i l l u s u a l l y read 0.1 to 0.3 degrees B r i x higher than on c o r r e c t e d hydrometer readings. B r i x by Refractometer. A. Equipment. (a) Refractometer with a B r i x s c a l e c a l i b r a t e d at 20°C. B. Procedure. (a) Consult i n s t r u c t i o n manual s u p p l i e d with instrument f o r procedures covering methods of c a l i b r a t i o n , reading and care. (b) Clean and dry prisms using d i s t i l l e d water and s o f t tissue. (c) Place sample on prisms with a wood or rubber a p p l i c a t o r , making sure there are no i n c l u d e d p a r t i c l e s of pulp or a i r bubbles. Allow time f o r instrument and sample temperature e q u a l i z a t i o n . (d) Make reading, apply temperature and a c i d c o r r e c t i o n s from Table I and Table I I . C. Example. I f the refractometer reading i s 44.8° B r i x , the a c i d content of the sample i s 2.5% and the r e f r a c t ometer temperature i s 22°C the c a l c u l a t i o n s are as follows : 44.8° .49 .16 45.45°

reading from refractometer c o r r e c t i o n f o r 2.5% a c i d temperature c o r r e c t i o n f o r 22°C Corrected B r i x

B r i x by Hydrometer. A. Equipment. (a) Hydrometer c a l i b r a t e d 5 to 15° B r i x at 20°C i n tenths of degrees. The hydrometer should c o n t a i n an i n t e r n a l thermometer with a temperature s c a l e i n degrees B r i x . (b) Hydrometer c y l i n d e r ^ inch l a r g e r i n diameter than the body of the hydrometer.

In Citrus Nutrition and Quality; Nagy, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


0.08 0.16 0.24 0.32 0.40 0.48 0.56 0.64

0.08 0.16 0.24 0.32 0.40 0.48 0.56 0.64

0.08 0.16 0.24 0.31 0.40 0.48 0.56 0.64

0.08 0.15 0.23 0.31 0.40 0.48 0.56 0.64

0.08 0.15 0.23 0.31 0.39 0.47 0.55 0.63

0.08 0.15 0.23 0.30 0.38 0.46 0.55 0.63

0.07 0.15 0.22 0.30 0.38 0.45 0.54 0.62

0.07 0.14 0.22 0.29 0.37 0.44 0.53 0.61

0.07 0.14 0.21 0.28 0.36

0.43 0.52 0.60

0.07 0.13 0.20 0.27 0.35

0.42 0.50 0.57

0.06 0.13 0.19 0.26 0.33

0.40 0.48 0.56

26 27 28

Add to the % sucrose

21 22 23 24 25

0.40 0.32 0.24 0.16 0.08 0.39 0.31 0.23 0.16 0.08 0.38 0.30 0.23 0.15 0.08 0.37 0.30 0.22 0.15 0.08

0.35 0.28 0.21 0.14 0.07

0.34 0.28 0.21 0.14 0.07

0.34 0.27 0.21 0.14 0.07

0.33 0.26 0.20 0.14 0.07

0.31 0.25 0.19 0.13 0.06

0.29 0.24 0.18 0.13 0.06

0.27 0.22 0.17 0.12 0.06

15 16 17 18 19

0.79 0.71 0.63 0.55 0.48 0.76 0.69 0.61 0.54 0.46 0.74 0.67 0.60 0.53 0.45

0.72 0.65 0.58 0.51 0.44

0.50 0.46 0.42 0.37 0.33

10 11 12 13 14

0.68 0.62 0.56 0.49 0.42

0.66 0.60 0.54 0.48 0.41

0.64 0.58 0.52 0.46 0.40

0.61 0.55 0.50 0.44 0.39

0.58 0.53 0.48 0.42 0.37

0.54 0.49 0.45 0.40 0.35

0

Temp. °C

70

60

50

15

10

5

40

% Sucrose 30 25 20 Subtract from the % sucrose

TABLE I TEMPERATURE CORRECTION TABLE (2) For refractometer readings made a t temperatures other than 20°C


to

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Η

« ο >

294


TABLE I I


CORRECTION TABLE FOR TOTAL SOLUBLE SOLIDS DETERMINED BY MEANS OF THE REFRACTOMETER IN SUCROSE SOLUTIONS CONTAINING CITRIC ACID (2)

% Anhy Citric Acid

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5

Add (%)

0.11 0.21 0.31 0.40 0.50 0.60 0.68 0.78 0.88 0.98 1.08 1.17 1.27 1.37 1.47 1.57 1.67 1.77 1.87 1.97 2.07 2.17 2.27 2.37 2.47

% Anhy Citric Acid

13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0

Add (%)

2.55 2.65 2.73 2.83 2.92 3.01 3.10 3.19 3.28 3.37 3.47 3.55 3.64 3.73 3.82 3.91 4.00 4.08 4.17 4.26 4.34 4.43 4.52 4.59 4.69

% Anhy Citric Acid

25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 30.5 31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0 35.5 36.0 36.5


Add

4.77 4.86 4.93 5.02 5.11 5.19 5.26 5.36 5.45 5.52 5.60 5.69 5.77 5.85 5.94 6.02 6.10 6.19 6.27 6.35 6.44 6.52 6.60

13.

MCALLISTER

B.

Quality

of

Citrus

Juice

295

Procedure. (a) F i l l c l e a n dry hydrometer c y l i n d e r to overflowing with deaerated j u i c e (see t e x t f o r deareation method). (b) Lower hydrometer i n t o j u i c e g i v i n g the stem a s l i g h t s p i n . Allow time f o r hydrometer to s t a b i l i z e and temperature e q u a l i z a t i o n . (c) Read hydrometer across the l i q u i d l e v e l to the bottom of the meniscus. (d) Remove hydrometer, read degrees B r i x c o r r e c t i o n f o r temperature, and apply to o r i g i n a l reading to obtain corrected Brix.


Deaeration I n t r o d u c t i o n . A i r becomes incorporated i n t o j u i c e during processing or upon r e c o n s t i t u t i o n of concentrates. The i n c o r porated a i r must be removed i f the B r i x of the j u i c e i s to be determined by hydrometer. I t i s e s s e n t i a l to deaerate j u i c e when determining the c o l o r with a c o l o r i m e t e r . The c o l o r i s s l i g h t l y l e s s a f f e c t e d by a i r when v i s u a l comparisons are made. Equipment. Assemble as i n Figure 1. A. Vacuum source, such as a small e l e c t r i c a l pump. B. 5000 ml wide mouth f l a s k . C.

Hoses, 3 hole rubber stopper, tubing, clamps.

Procedure. A.

A l l clamps c l o s e d , draw a vacuum on l a r g e a s p i r a t o r flask. B. Place i n t a k e hose i n t o raw j u i c e sample, open clamp and draw the j u i c e up i n t o the f l a s k to remove a i r bubbles. C. Shut o f f vacuum source, break vacuum by l e a v i n g raw j u i c e sample clamp open. D. Open second clamp and allow deaerated j u i c e to flow i n t o a container. A c i d Determination I n t r o d u c t i o n . C i t r i c a c i d i n c i t r u s j u i c e s may be d e t e r mined according to the Methods of A n a l y s i s as given i n the AOAC (7). However, a n a l y s t s who run a l a r g e number of t e s t s d a i l y have a l t e r e d the method to speed up the t i t r a t i o n and make c a l c u l a t i o n s e a s i e r . One of these a l t e r a t i o n s i s to use 0.3125 Ν sodium hydroxide r a t h e r than 0.1 Ν a l k a l i . The use of the higher n o r m a l i t y a l k a l i i s d e s i r a b l e e s p e c i a l l y when t i t r a t ing samples of high d e n s i t y concentrates. I t i s almost impera t i v e when t i t r a t i n g lemon concentrates. The a c i d content i n concentrated c i t r u s j u i c e s i s determin ed as the t o t a l t i t r a t a b l e a c i d c a l c u l a t e d as anhydrous c i t r i c



296


Figure 1.

Deaeration apparatus


13.

MCALLISTER

Quality

a c i d and i s expressed of concentrate).

of Citrus

Juice

297

as "percent by weight" (grams per 100 grams


Reagents. A. Sodium hydroxide s o l u t i o n 0.3125 Ν (12.5 g per l i t e r sodium hydroxide reagent grade. Standardize against potassium a c i d p h t h a l a t e ) . B. Phenolphthalein i n d i c a t o r s o l u t i o n - 1% s o l u t i o n i n n e u t r a l i s o p r o p y l a l c o h o l . This i s prepared by d i s s o l v ing 1.0 gram phenolphthalein powder i n 100 ml of 50% isopropyl alcohol. Equipment. A. Gram s c a l e (accuracy ί 0.05 g ) . Β. 250 ml Erlenmeyer f l a s k C. 50 ml b u r e t t e D. Magnetic s t i r r e r ( o p t i o n a l ) E. pH meter ( o p t i o n a l ) Procedure. A. Weigh a 10 g sample of concentrate i n t o a 250 ml Erlenmeyer f l a s k . B. Add approximately 25 ml of d i s t i l l e d water and 1.0 ml phenolphthalein s o l u t i o n . I f a magnetic s t i r r e r i s used, a d d i t i o n a l d i s t i l l e d water may be needed. C. T i t r a t e with 0.3125 Ν sodium hydroxide s o l u t i o n to a d e f i n i t e pink c o l o r (end p o i n t ) , which holds f o r approximately 25 seconds or pH 8.2. As the end point approacheSjthe flavonones present i n c i t r u s j u i c e s w i l l turn yellow. Calculation. A. M u l t i p l y the number of m i l l i l i t e r s of standard a l k a l i used i n the t i t r a t i o n by 0.2. The r e s u l t w i l l be per cent by weight of anhydrous c i t r i c a c i d by weight of concentrate. Example: 18.9 ml of standard a l k a l i X 0.2 = 3.78% anhydrous c i t r i c a c i d . The a c i d content i n s i n g l e s t r e n g t h c i t r u s j u i c e s i s determined the same way as i n the procedure given above f o r concentrates u s i n g a weighed sample. However, a l a r g e r sample should be used. Using a 20-gram sample of s i n g l e strength the number of m i l l i l i t e r s of standard a l k a l i should be m u l t i p l i e d by 0.1 to give the percent c i t r i c a c i d . Example: 9.7 ml of standard a l k a l i X0.1 = .97%. Many a n a l y s t s p r e f e r to measure a given volume of s i n g l e strength j u i c e , i n s t e a d of weighing i t , then c a l c u l a t e the r e s u l t s on a weight to weight percentage. Example: Using a l a r g e opening 25 ml p i p e t t e , t r a n s f e r a sample i n t o a 250 ml Erlenmeyer f l a s k and t i t r a t e to an end point the same as f o r concentrates. To c a l c u l a t e grams c i t r i c a c i d per 100 ml of j u i c e , m u l t i p l y the



298

m i l l i l i t e r s of a l k a l i by .08 to get grams c i t r i c a c i d per 100 ml of j u i c e - weight/volume. The percent by weight can now be found using the f o l l o w i n g c a l c u l a t i o n : a c i d wt/vol. S p e c i f i c g r a v i t y of j u i c e Example:

a c i d 0.98 g/100 Sp.gr. 1.050

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Methods for Determining the Quality of Citrus Juice - ACS Symposium

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