California Avocado Society 1991 Yearbook 75:71-85

 

Relationship Between Physiological Maturity and Percent Dry Matter of Avocados

 

 

Clifford Ranney

Avocado grower; retired president, Cal-compack Foods, Inc., Santa Ana, California; B.S., Chemical Engineering, Yale University

 

Introduction

In the transition from a California maturity standard for avocados governed by percent oil content, as estimated by the "Halowax Method", to one based on percent dry matter, there have been many questions raised as to how accurately either depicts actual physiological maturity. There was also a feeling in the industry that the relationship between physiological maturity and percent dry matter might vary enough from year to year to require a change every season in the minimum percent dry matter required for harvest. To ascertain if percent dry matter could be used as an indicator for maturity, four items needed to be accomplished:

1. Develop a definition for acceptably mature avocados based on the physiological maturity of the fruit, eliminating the subjectivity of taste to the maximum extent possible.

2. Determine if this definition of maturity can be correlated with percent dry matter by variety.

3. If there is a measurable correlation, determine if the relationship varies from year to year; and if so, by how much.

4. If there is a yearly change, establish if it is large enough to preclude using a constant value of percent dry matter from year to year.

With this information, it would be possible to adjust the minimum maturity standards to accurately represent the same level of physiological maturity for each variety. It was also hoped that these data would be of benefit in promulgating improved standards, methods, and regulations concerning avocado maturity.

Results of a preliminary study made during the 1985-86 "maturity season" strongly indicated that a measurable relationship existed between physiological maturity and the percent dry matter of the pulp. The objective of this study was to define and quantify this relationship. These results also indicated that establishing the relationship between percent dry matter and maturity each year prior to harvest was not practical, since the time needed to soften and score the fruit was such that if a delay in harvest was indicated, it would not be known until after harvest had started. Given this problem, it was hoped that the relationship between percent dry matter and physiological maturity would be constant enough from year to year that yearly testing would be unnecessary.

Since there was strong industry interest in the preseason establishment of harvest dates by size and variety, the study was designed to collect data that would be helpful in the future development of such a program.

Materials and Methods

Since the then current procedure for the determination of percent dry matter used by the State of California entailed a sample of five avocados, a sample size of five fruit was chosen for this study.

The general approach was to match, as closely as possible, five pairs of fruit, all falling within a narrow weight range; determine the percent dry matter individually on one set of five; and assign a "maturity value" to the other set. When the second set reached an acceptable level of physiological maturity, the average percent dry matter of the first set could be considered the appropriate minimum value for the variety and year in question.

The sampling periods were chosen in an attempt to encompass a broad maturity range, from unmarketable to fully marketable.

Each sample site (variety, district) was sampled every two weeks until past the point of full marketability, or until harvested. Only good commercial groves were chosen as sample sites, and any obviously stressed or diseased areas were avoided. For each sampling period, approximately 25 fruit were picked from 25 different trees in one general area of each grove. Careful consideration was given to fruit size. It was attempted to pick all fruit as close to a single size (48 or 40) as possible, since in a given grove percent dry matter will generally vary with size.

The fruit was brought back to the Avocado Inspection Service office in Escondido and held until the next day for testing.

Each site sample was inspected, and any damaged or obviously over-or under-sized fruit was discarded. Using the remaining fruit, five pairs were chosen, with each pair matched as closely as possible in size, color, and shape. One set of five (Group A, 1-5) was tested for dry matter, and the other set of five (Group B, 1-5) was allowed to soften and was then scored for maturity.

Each fruit was weighed (gram weights) the day following harvest.

Dry Matter was run on the individual fruit of Group A immediately after weighing.

Group B was placed on a ventilated rack in such a manner that the fruit did not touch and was held at 20°Celsius (68 °F) and 80% Relative Humidity.

Percent Dry Matter

Percent Dry Matter is defined as the average percent dry matter of five avocados determined by the procedure listed in 1408.3 of the Administrative Code - Title 3 of California.

In an effort to improve the test's reproducibility, the following changes were made in the official procedure:

1. Opposing eighths from each fruit were used as the sample instead of slices from the face of a single half.

2. The sample was chopped to approximately 3 mm. in a food processor instead of using individual slices.

3. The sample was dried for 25 minutes and then to constant weight at 5 minute intervals instead of 10 minutes and 1 minute intervals.

Maturity Value

Maturity Value is defined as the average of the external and internal panel scores of the five fruit in question. (The panel consisted of eight persons during the course of this study, with a core group of five persons present at 95 percent of the sessions.)

Scoring was done when the sample was determined to be soft enough to use, or in the case of immature fruit, when it appeared that no further softening would take place. Each fruit was individually scored externally by each member of the panel; the fruit was then halved, and scored internally. Upon completion, each panelist noted the overall marketability of the sample as a whole and expressed it as "almost marketable", "just marketable", or "marketable". A consensus of the panel was used to describe the sample. When the entire panel felt the sample was marketable, an asterisk (*) was placed beside the "marketable" designation.

As could be expected, given the inherent fruit-to-fruit variation, the five avocados did not soften uniformly. To compensate for this, and to allow scoring to be done at one time, when a fruit was within one day of scoring, it was placed under refrigeration at 5 degrees Celsius (41 °F) until the day before a scoring session, when it was returned to the storage rack. In this manner, all fruit was at the same temperature for scoring and were reasonably uniform in their softness. The number of days that a given fruit had been under refrigeration was noted, and the assumption was made that it had not softened during that time.

The definitions used for scoring were as follows:

EXTERNAL

5= Uniformly soft, with no more than slight shriveling.

4 = Generally soft, with some shriveling.

3 = Not completely soft, with or without noticeable shriveling or noticeably shriveled.

2 = Firm, with or without heavy shriveling or heavily shriveled.

1 = Hard to the touch or rubbery with prune-like shriveling.

INTERNAL

5= Uniformly soft throughout, with creamy smooth texture.

4 = Generally soft, with some evidence of firm or grainy texture or slight wateriness.

3 = Some firmness or resistance entailing less than 25 %, and/or signs of excessive wateriness.

2 = Rubbery, and/or hard spots more than 25% and less than 50%.

1= Rubbery, and/hard spots more than 50%.

Results and Discussion

Figures 1a through 1d are graphical representations for Bacon, Fuerte, Hass, and Zutano of the average Percent Dry Matter and average Maturity Value by sample period for the three crop years that comprised the entire period of the study. Only data representing Maturity Values of 3.0 and above (from the scoring tables supra) have been included in the graphical presentations. The values below 3.0 are of no economic importance, and the linear correlation is improved by their omission.

The graphs confirm the early indication that a measurable relationship between dry matter and physiological maturity exists.

The linear relationships developed for each variety were analyzed for yearly variation. There were no significant differences in slopes for all varieties. There was only one difference between years for adjusted Maturity Value, and that was in Hass, 1986-87 being different at the 5% level from 1987-88 and 1988-89. The difference was not large enough to preclude the use of an average value for the three years. This difference was not confirmed by the other varieties, suggesting causes other than yearly weather variation—most likely, site variation, since the same sites were not used every year.

With no strong indication of differences due to yearly variations, a single equation was developed for each variety (Table 5).

Figure 1a

Figure 1b

Figure 1c

Figure 1d

 

Figures 2a and 2b show the same year comparison between the North Coastal and South Coastal Districts for Bacon and Hass.

The comparisons between the North Coastal and South Coastal Districts were analyzed for statistical differences. There was no difference in slopes between districts for either variety, and a significant but small difference in adjusted Maturity Value for the Bacon which was not confirmed by the Hass.

The conclusion was made that no meaningful difference exists between the two districts as to the level of percent dry matter required for a given maturity value for a given variety. This allows a single standard to be used for both districts.

 

Figure 2a

Figure 2b

 

It was not possible to collect adequate data for Central Valley Zutano since, in most years, harvest was completed before full marketability levels were attained.

Table 1 lists the Average Maturity Values by variety for each level of marketability taken from the tables in the Appendix.