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Group Title: Lake Alfred AREC research report - University of Florida Agricultural Research and Education Center ; CS-74-1
Title: A proposed method for estimating avocado fruit maturity
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Permanent Link: http://ufdc.ufl.edu/UF00072465/00001
 Material Information
Title: A proposed method for estimating avocado fruit maturity
Series Title: Lake Alfred AREC research report
Physical Description: 11 p. : ill. ; 22 cm.
Language: English
Creator: Barmore, Charles Rice, 1942-
Rouse, A. H
Agricultural Research and Education Center (Lake Alfred, Fla.)
Publisher: Institute of Food and Agricultural Sciences, University of Florida, Agricultural Research and Education Center
Place of Publication: Lake Alfred FL
Publication Date: 1974
 Subjects
Subject: Avocado -- Ripening -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: Charles. R. Barmore, A.H. Rouse.
General Note: Cover title.
General Note: "Presented at the Subtropical Fruits Institute, Homestead, Fla., February 20, 1974."
 Record Information
Bibliographic ID: UF00072465
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 76805543

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Full Text

*--*
Lake Alfred AREC Research Report-CS74-1
2/19/74-CRB-1 O










Presented at the Subtropical Fruits Institute, Homestead, Fla.




February 20, 1974



A PROPOSED METHOD FOR ESTIMATING AVOCADO FRUIT MATURITY


- '- r~r


CHARLES R. BARMORE

A. H. ROUSE


INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, UNIVERSITY OF FLORIDA,

AGRICULTURAL RESEARCH AND EDUCATION CENTER, LAKE ALFRED










INTRODUCTION

Methods for estimating avocado fruit maturity have been extensively

investigated. At present, the most reliable method for estimating maturity

of a particular cultivar (variety) grown in Florida is based on field

observations during fruit growth and its typical growth characteristics

as observed over several years.

Recently, the authors noted that an enzymatic test could possibly be

used to aid in the estimation of avocado fruit maturity. This test is being

proposed as a supplement to the present method, thereby, strengthening its

validity.

The following discussion examines the basis for the test, its validity,

method of analysis, and how it can supplement the present method of estimating

avocado fruit maturity. A cooperative study is proposed between the Avocado

Administration Committee (AAC) and the authors to determine the feasibility

of using this method by the AAC.

Basis for Test

Softening is the most obvious change associated with ripening of

avocados. This is brought about by the action of pectolytic enzymes on

the pectic constituents of the cell walls of fruits. Isolation and

characterization of these enzymes in avocado and other fruits are well

documented.

Two enzymes of particular interest associated with the softening process

are PECTINESTERASE (PE) and POLYGALACTURONASE (PG). PG activity does not

become measurable until after harvest, increasing to a maximum when the

fruit becomes soft. In contrast, PE activity is highest in immature fruit,

decreasing as the fruit matures. After harvest, PE activity continues to










decrease, reaching a low plateau when softening is complete. The activity

of PE and PG as the fruit matures and ripens is shown in Fig. 1. Because


100


80


0 60

X
J 40

20
20


A S 0 N D 0 2 4 6 8 10
MONTH DAYS


Fig. 1. A stylized graph showing

the changes in PE and PC during

fruit maturation and ripening of

a typical avocado fruit. PE x---x,

PG o---o.

of this particular pattern of PE activity during fruit maturation of a given

variety, this enzyme could possibly be used as an indicator for estimating

avocado fruit maturity. Information from our laboratory and others indicates

that at maturity a specific PE value is reached. A graphic representation

of the relationship between fruit maturation and the PE activity during this










period is shown in Fig. 2. This work was reported by Drs. Zaubermann and

Schiffmann-Nadel of the Volcani Institute of Agricultural Research, Israel.

Their work also supports our concept of using PE as a measure of maturity.


J J A S
MONTH


0 N D


Fig. 2. The activity of PE in 'Fuerte'

avocado fruit at various stages of

development. (From: Zaubermann and

Schiffmann-Nadel, 1972. Plant Physiol.

49: 864-865.)

Validity of the test is based on the fact that there is a relationship

between PE activity and days required to soften. The latter method is

presently used to check maturity when in question. We have conducted

numerous tests during the past 2 seasons for the purpose of determining

the accuracy of using PE to estimate softening time. With standardized


250


210

m
170

-I
130 .o
W


MATURE


1













conditions, we are able to predict softening time within + 1 day. Deviation

from the actual time required is due to the fact that definition of "soft"

is inexact.

Additional support for the test is given in Fig. 3. Three sizes of

'Haile' fruit having an average weight to length ratio of 366 g/13 cm,

422 g/13.3 cm and 530 g/14.3 cm were analyzed for PE activity. The latter

group of fruit was mature and being shipped commercially. Theoretically,

fruit of this cultivar having a PE value of 30 could be considered mature.

At present, size restriction would be necessary to ensure that only such

fruit were harvested.
120 HAILE
7-31-73

100


0 80

X
U J 6 0 \


40

MATURE -
2 0 -
24 28 32 36 40

WEIGHT/LENGTH

Fig. 3. Relationship between dimension
(expressed as weight in grams divided by
length in centimeter) and PE activity of
'Haile' avocado fruit.

Pectinesterase Analysis

Description. The method for PE determination is designed to measure

the rate of demethylation of pectin, thus freeing the carboxyl (acidic)

groups which can be titrated with an alkali. In essence, the higher the











PE activity the greater will be the rate of demethylation, thus requiring

an increasing amount of alkali to maintain the pH of the reaction mixture

at pH 7.5.

Equipment and reagent needs for PE analysis are listed below. All

materials can be purchased through most scientific supply companies.

Reagents

1. Pectin NF (Sunkist Growers pure citrus pectin).

2. Sodium chloride, reagent grade.

3. Sodium chloride, 2% solution.

4. Sodium hydroxide, 0.05 N.

5. Sodium hydroxide, 0.25 N.

6. Buffer solution, pH 7.0.

7. Toluene.

Equipment

1. pH meter with Automatic titrator.

2. Sorvall Omni-mixer.

3. Precision stirrer with rheostat and s.s. stirring rod

or variable speed magnetic stirrer with teflon covered

stirring bar.

4. Mettler balance, P 2000.

5. Food chopper.

6. Miscellaneous:

a) Buret, 25 ml.

b) Beakers, 150 ml, 1000 ml.

c) Graduated cylinder, 50 ml.

d) Mohr pipette, 5 ml.

e) Volumetric flask, 1000 ml.


--k-










f) Glass stoppered bottles (3), 1000 ml.

g) Plastic wash bottle, 500 ml.

h) Plastic reagent bottle, 500 ml.

i) Timer.

j) Spatula.

k) Drop bottle, 30 ml.

Representative sample. The sampling procedure is a very important part

of the overall analysis. This procedure must be standardized. Fruit to be

analyzed must be representative in both size and weight of those fruit to

be evaluated for maturity. In addition, the number of fruit sampled must

be sufficient that the PE value obtained is statistically valid. We have

arbitrarily selected a 10 fruit sample, but the actual number should be

determined from a testing program.

Sample preparation. Tissue preparation consists of grinding each fruit

individually in a food chopper and taking a specific aliquot from each

sample. The aliquots are combined and thoroughly mixed. This composite

sample can be stored frozen at-100F or below for at least 1 month without

.significant loss in PE activity.

PE analysis. Directions for the preparation of the solution and

analysis of PE are as follows:

Preparation of Solutions

1. 17 pectin-salt solution--Mix 10 g of citrus pectin with 11.7 g

sodium chloride and slowly add to 800 ml distilled water with

agitation. After completely dispersed make to 1 liter. Add 6

drops of toluenu.

2. 27. sodium chloride solution--Dissolve 20 g sodium chloride in

distilled water and make up to 1 liter.










3. 0.05 N and 0.25 N sodium hydroxide--Dissolve standard strength

ampule to volume for proper normality.

PE Analysis

1. From the composite sample, weigh 100 g of chopped avocado and

add 100 g 2% sodium chloride(salt) solution.

2. Homogenize the mixture for 5 minutes.

3. Weigh 10 g sample into a 150 ml beaker, add 50 ml 1% pectin-salt

solution and place under stirrer on pH meter.

4. Adjust to pH 7.8 with 0.25 N sodium hydroxide.

5. When pH returns to 7.5, begin timer (set for 15 minutes).

6. As pH drops below 7.5, add 0.05 N sodium hydroxide solution

dropwise from buret at a rate that will maintain pH 7.5.

7. The pectinesterase activity is read from the chart depending

on the quantity of 0.05 N sodium hydroxide solution used, or the

quantity of sodium hydroxide used is multiplied by the factor

6.7 to give units of PE activity for the sample.

The total time required for the analysis is approximately 20 minutes.

The analysis is automatic once the reaction has been started. Several samples

can be run simultaneously.

Calculation of PE value. The PE value is calculated from the number

of mlof alkali required to maintain the pH of the reaction mixture at

7.5. Activity of the enzyme is expressed as the number of PE units X 104.

This value can be obtained directly from a table or formula as shown in

Table 1.

Establishing Maturity Requiremonts

The proposed method is not suggested as a replacement for the present

method, but as a supplement. The PE value, once determined through a






-9-

Tabl.e .' Chart for the determination of pectinesterase activity.



(1 ml 0.05 N NaOH = 0.00067 X 104 = 6.7 PE units)
Reaction Time: 15 min. *10 gram Sample
PE X 104 ml 0.05 N NaOH PE X 104 ml 0.05 N NaOH PE X 104 ml 0.05 N NaOH

6.7 1.0 23.3 3.5 40.0 6.0
7.3 1.1 24.0 3.6 40.7 6.1
8.0 1.2 24.7 3.7 41.3 6.2
8.7 1.3 25.3 3.8 42.0 6.3
9.3 1.4 26.0 3.9 42.7 6.4
10.0 1.5 26.7 4.0 43.3 6.5
10.7 1.6 27.3 4.1 44.0 6.6
11.3 1.7 28.0 4.2 44.7 6.7
12.0 1.8 28.7 4.3 45.3 6.8
12.7 1.9 29.3 4.4 46.0 6.9
13.3 2.0 30.0 4.5 46.7 7.0
14.0 2.1 30.7 4.6 47.3 7.1
14.7 2.2 31.3 4.7 .48.0 7.2
15.3 2.3 32.0 4.8 48.7 7.3
16.0 2.4 32.7 4.9 49.3 7.4
16.7 2.5 33.3 5.0 50.0 7.5
17.3 2.6 34.0 5.1 50.7 7.6
18.0 2.7 34.7 5.2 51.3 7.7
18.7 2.8 35.3 5.3 52.0 7.8
19.3 2.9 36.0 5.4 52.7 7.9
20.0 3.0 36.7 5.5 53.3 8.0
20.7 3.1 37.3 5.6 54.0 8.1
21.3 3.2 38.0 5.7 54.7 8.2
22.0 3.3 38.7 5.8 55.3 8.3
22.7 3.4 39.3 5.9 56.0 8.4


*Sample consists


of a 50:50 mixture of 2% sodium chloride plus avocado tissue by weigh










testing program for each cultivar, would be used to determine the actual

date a fruit of a specific size would be mature.

The date of harvest proposed early in the season would be determined

by the present criteria. However, as this date is approached, PE activity

would be determined for the larger fruit in order to judge when they are

actually mature. The date that the fruit reaches maturity could be on,

before, or even after the proposed picking date. This would be valid

since PE is a measure of the number of days required to soften which now

can be determined only after the first suitable picking date is past. Use

of such a maturity standard is shown in Table 2. In this example, the proposed

picking date for 'Lula' for 1974 would be October 1. Fruit at this picking

must be 14 oz in weight with a width of 3-3/8 in. or greater. If on September

29 the PE was down to 45, fruit of the above size would be considered as

mature and allowed to be harvested. Adjustment in size restriction could

be made at any time. If PE was still above 45, picking would be delayed.

Similarly, picking might be advanced if PE was significantly below 45

prior to October 1.

Table 2. Example of the maturity standards

for showing the relationship of PE activity

for several avocado cultivars.
Minimum
Proposed Fruit size Maximum
picking Weight Width PE
Variety date (oz) (in.) Units X 104

Lula Oct. 1 14 3-3/8 45

Booth 8 Sept. 15 13 3-1/4 75

Haile July 31 17 3-9/16 30

Waldin Aug. 7 15 3-9/16 90






-11-



Conclusion

The use of PE as an indicator of maturity has several merits in that

it is relatively a simple and quick test, it is a measure of softening

time, the test is easily standardized, and it would supplement the present

method. We emphasize that to date the method has been tested only in

the laboratory.

Our purpose in presenting the method to the Avocado Administration

Committee and growers is to develop a joint effort among those concerned

in examining the feasibility for commercial use. A testing program of

at least 2 years would be required.




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