CITRUS AND SUBTROPICAL FRUITS

Presented via Trans Texas Videoconference Network

Cross listed as:

PLSS 6390 (Texas A&M University-Kingsville)
PLSS 4390 (Texas A&M University-Kingsville)
HORT 422 (Texas A&M University-College Station)

Fall 2004

LAB EXERCISE NO. 6: INTERNAL FRUIT QUALITY AND STATE MATURITY LAWS

OBJECTIVES:

1) To acquaint the students with laboratory procedures used to determine fruit quality.

2) To familiarize students with the state citrus quality standards.

I. Maturity Standards

State governments in citrus producing regions have established maturity laws requiring that citrus fruits meet certain quality standards before they can be shipped or sold in order to prevent the shipment of immature and unpalatable fruit. The laws differ slightly from state to state, but they recognize the same factors as criteria for quality.

The first such laws in Texas were established in 1927 , and have been revised several times, each time becoming more stringent. Texas now has the highest standards for fruit quality of all the citrus producing areas. These laws protect the consumer by assuring that only high quality fruit is offered for sale on the fresh market. They also protect the producer by insuring customer confidence in the fruit offered for sale, and by promoting an orderly procession of mature, quality fruit to the market place.

The U.S. government accepts the standards set by the states , as well as state testing and control procedures. Thus fruit shipped as interstate commerce does not come under the scrutiny of the Pure Food and Drug Act. In general, these laws have resulted in better quality fruit, more stable prices, and more orderly marketing than pre-maturity state laws.

Unlike many fresh fruits, citrus are non-climacteric, in that they do not undergo a rise in respiration and continue to ripen after removal from the tree. In climacteric fruits such as the pear, starchy reserves are converted to sugars after harvest, thus becoming more palatable. Avocado fruits have a classic climacteric rise in respiration (Beal, 1950) that only begins after harvest which allows harvest and shipping of hard fruit which soften after they are in retail channels or have been purchased by the consumer. With citrus, fruit quality remains stable when held in storage for some time. Thus, even though citrus fruits will retain their quality in storage they will never get any sweeter or have a higher juice content than what they have when they are picked from the tree. Therefore, it is imperative to make sure that they are palatable when harvested. Grapefruit havested from a tree in December is not as sweet as fruit from the same tree harvested in March.

Also unlike other fruits, the maturity of citrus cannot be determined by the surface color of the fruit. A citrus fruit whose skin is green may be ready for harvest if its internal quality is good. Loss of chlorophyll of citrus peel is positively correlated to cool temperatures. Maturity in citrus must, therefore, be determined by standards other than external appearance. Factors which are easily determined are: 1) brix, a measure of soluble solids, which is almost all sugars, 2) acid content , 3) the brix/acid ratio, and 4) the juice content. Other factors of maturity, such as organic acids, esters, and glucosides, which play a major role in the development of flavor, are not easily measured. Since the brix/acid ratio changes during maturation, with sugars increasing and acid decreasing, it and juice content are used to establish fruit quality.

II. Exercise:

A. Lab Procedures.

1. Juice content: for a given size fruit, a given species, and a given cultivar, fruit quality increases with an increase in juice content.

a. Take ten fruits and determine their size by measuring and recording their diameters and height. Calculate and record their diameter/height ratio and their weight.

b. Cut the fruit in half and extract the juice with a Sunkist power reamer. Strain the juice through a coarse sieve.

c. Transfer the juice to 1000 ml graduated cylinders. Record the volume of juice.

d. Calculate the average volume of juice per fruit and record.

e. Calculate the number of gallons of juice per standard box of 1 3/5 bushels by: 1) using the average diameter of the fruit to determine the average number of fruit per box, (see the attached charts), 2) multiplying this number by the average juice volume per fruit, 3) dividing the product by 3784 (the number of ml per gallon). Record this number.

2. Brix (total soluble solids: a measurement of sugars and organic acids, and other soluble materials.

a. Fill cylinder with juice, gently place hydrometer in juice, and leave hydormeter floating for two minutes without letting it touch the sides of the cylinder.

b. Read the hydrometer at the level of the juice, record the measurement.

c. Adjust the readings for temperature (see attached sheets).

d. Place one drop of juice in the refractometer. Record the measurement.

e. Measure the brix content of your favorite soft drink and record.

3. Total titratable acid: juice sourness is measured by titrating the juice with NaOH. The reading is reported as anhydrous citric acid, despite the fact that there are other acids also involved here.

a. Using a 20 ml pipette, transfer a 20ml aliquot of clear juice to a 250 ml Erlenmeyer flask.

b. Titrate to an end point of 8.1 to 8.2, using 0.3N Na0H. Record the amount of NaOH used.

c. Calculate the percent acid by using the following equation:
 

Method for Computing % Anhydrous Citric Acid

Anhydrous Citric acid=  A x N x equivalent wt (Citric acid) x 100
                                                                   ml of sample
A= ml of NaoH used to neutralize juice to pH 8.1

N= Normality of NaoH
Equivalent wt of citric acid can be calculated based on the molecular wt and considering 3 COOH in citric acid.
Molcular wt of citric acid (C6 H8 O7)  = 192.13
Equivalent wt of citric acid= 192.13/3= 64.04

Equation

Acid                 +            Base  ------------          Salt           +              Water
C6 H8 O7                                       NaoH                  C6 H5 ONa 3                                H20
Citric acid                        Sodium Hydroxide       Sodium Citrate
 

4. Brix/acid ratio: This is one of the first indices used as a measure of maturity. It is simply the quotient of the percent total titratable acids divided into the Brix. Alone it is a poor measure of maturity, since a great variety of Brix and acid readings would produce the same ratio. A fruit low in acid would produce a high ratio, however that low acid fruit is insipid in taste, especially in oranges. A change in acid affects the ratio more than does a change in Brix.

a. Calculate the Brix/acid ratio of the fruit juice and record.

B. Record values and determine maturity of all samples analyzed in the lab.

C. Answer these questions:

1. What is the primary organic acid found in most citrus fruits? What is the primary organic acid found in sweet limes?

2. How do early season fruit and late season fruit differ in their Brix/acid ratios?

Citrus Maturity Requirements

Texas Department of Agriculture

Subtract from





Add to 
Brix reading





Brix reading
Temp. oC Grapefruit



Temp.
10o - 0.32 Minimum Total
Solids to Acid
20o - 0.00
11 - 0.31 Solids Percent
Minimum Ratio
21 - 0.04
12 - 0.29 9.0------------------------7.2 to 1  22 - 0.10
13 - 0.26 10.0------------------------7.0 to 1 23 - 0.16
14 - 0.24 11.0------------------------6.8 to 1 24 - 0.21
15 - 0.20 11.5------------------------6.5 to 1 25 - 0.27
16 - 0.17





26 - 0.33
17 - 0.13





27 - 0.40
18 - 0.09 Oranges Color Add Requirements
28 - 0.46
19 - 0.05 Minimum Total
Solids to Acid
29 - 0.54
20 - 0.00 Solids Percent
Minimum Ratio
30 - 0.61


8.5---------------------10.0 to 1
31 - 0.71


9.0-----------------------9.0 to 1
32 - 0.79


All higher solids 9.0 to 1
33 - 0.86








34 - 0.94
Juice content requirements a minimum of 4 1/2 gallons to Standard 35 - 1.02
packed box of one and three fifths (1-3/5th) bushels capacity.




GRAPEFRUIT SIZES

AND 

JUICE REQUIREMENTS IN C.C

SEEDLESS:












SIZE DIAM. 1 FRT. 2 FRT. 3 FRT. 4 FRT. 5 FRT.







126 3-1/2 150 300 450 600 750
96 3-3/4 180 360 540 720 900
80 4- 195 390 585 780 975
70 4-1/8 200 400 600 800 1000
64 4-1/4 205 410 615 820 1025
54 4-1/2 220 440 660 880 1100
45 4-3/4 235 470 705 940 1175
36 5- 250 500 750 1000 1250
28 5-1/4 265 530 795 1060 1325







SEEDED:












126 3-1/2 140 280 420 560 700
96 3-3/4 165 330 495 660 825
80 4- 175 350 525 700 875
70 4-1/8 180 360 540 720 900
64 4-1/4 200 400 600 800 1000
54 4-1/2 220 440 660 880 1100
46 4-3/4 235 470 705 940 1175
36 5- 250 500 750 1000 1250
28 5-1/4 265 530 795 1060 1325
ORANGE SIZES AND JUICE REQUIREMENTS IN C.C.









SIZE
DIAM.

MIN


MAX

1 FRT.

2 FRT.

3 FRT.

4 FRT.

5 FRT.








96 3-6/16 3-11/16 177.4 354.8 532.8 709.6 887.0
126 3-3/16 3-8/16 135.2 270.4 405.6 540.8 676.0
150 3 3-4/16 113.6 227.2 340.8 454.4 568.0
176 2-14/16 3-2/16 96.8 193.6 290.4 387.2 484.0
200 2-12/16 3 85.2 170.4 255.6 340.8 426.0
216 2-10/16 2-14/16 78.9 157.8 236.7 315.6 394.5
250 2-8/16 2-12/16 68.1 136.2 204.3 272.4 340.5
288 2-6/16 2-10/16 59.1 118.2 177.3 236.4 295.5

Beal - climacteric of avocados.