Determining Maturity in Whole Avocados
Bob Bergh, Junji
Kumamoto
Botany and Plant Sciences,
Paul Chen
Agricultural
Engineering,
Introduction
Unlike many fruits, avocado maturity does not
coincide with ripeness. An avocado becomes ripe—softens, and so can be
eaten—only after it has been picked from the tree, requiring several days at
room temperature. This delay adds to the problem of determining proper fruit
maturity.
Avocados do change in appearance as they approach
maturity. The skin loses some of its glossiness, becoming duller; surface russeting increases; varieties that turn black on ripening
may become partly so on the tree. When cut open on picking, a mature fruit will
slice smoothly, without a crackling sound or rough cut surface. Also, the seed
coat will usually be thin and brown instead of fleshy and white.
The ultimate test is picking a fruit and letting it
ripen. Mature avocado varieties vary widely in pleasantness of flavor, but an
immature fruit of any variety will likely shrivel as it ripens and have a
watery, bland, or even "grassy" flavor.
However, visual observation and personal taste
testing is too subjective when legal standards are necessary to keep immature
fruit off the commercial market. For decades, the accepted objective measure of
lawful maturity was fruit oil percentage. The rather arbitrary legal oil
minimum for all varieties in
A significant improvement was the replacing of direct
oil percentage with determination of dry matter. Because the sum of fruit oil
plus water commonly remains nearly constant as fruit develops, one need only
determine the water content, such as by drying a fruit slice in a microwave
oven, to obtain the oil percentage by subtraction. Thus, any grower can now
determine the relative oil level of his own fruit (Lee, 1982; Lee et al., 1983). The final step in
commercial application is to correlate calculated values with eating quality
for each important variety.
NMR Method
NMR, nuclear magnetic resonance, has been used for
some time to measure the quantity of liquid oil in a substance, such as by a
Newport Analyser Mark III (Lee, 1982). It does this
by passing radio-frequency energy through the substance, thereby exciting
hydrogen atoms which emit radio-frequency signals that are measurable by a
spectroscope to give an oil content reading. A problem has been that the
substance must be completely dry, since otherwise the hydrogen atoms in water
will confound the oil measurement (Lee, 1982).
Recent advances in chemical-physical research have produced
NMR spectrometer procedures for a number of specialized applications. For
example, they are now being used to image internal human structures for medical
diagnoses. And now, because the signals from oil and water hydrogen atoms decay
at different rates, it is possible to obtain oil-enhanced images of undried avocado slices (Fig. 1). Note how the image becomes
brighter with advancing maturity.
Figure 1. NMR imaging of
fresh avocado slices from four fruits. Slices 1(1) and 4 (B) were from immature
fruits, 2 (3) was nearly mature, and number 3 (6) was
mature: the image brightens with maturity.
The procedure is extremely complex. It involves the
theory of nuclear-magnetic spin systems under the influence of magnetic fields
and pulses of radio-frequency energy, with the resulting rates of signal decay
yielding measurements by means of unimolecular
kinetics.
NMR Oil
Analysis of Entire Fruit
The new method is now being used to evaluate the
internal quality of various types of intact fruits (Chen et al., 1989). Toward the ultimate goal of determining maturity in
intact avocados, preliminary signal scans have been run (Fig. 2). This
illustrates that the method is clearly sensitive enough to show differences in
oil percentages between different parts of the same fruit.
Figure 2. NMR imaging of an intact avocado. The fruit was then cut
open and analyzed: the upper left region had 2.1% more dry matter than the
lower right region.
These new techniques have the major advantages of
simplicity of operation, speed of determination, and accuracy of measurement.
The chief disadvantage is high equipment cost, but efforts are being made to
develop new low-cost equipment for agricultural applications.
References
Lee, Seung-Koo. 1982. Methods for percent oil analysis of avocado
fruit.
Lee, S. K., R. E. Young, P. M. Schiffman,
and C. W. Coggins, Jr. 1983. Maturity studies of avocado fruit based on picking
dates and dry weight. Jour. Amer. Soc. Hort. Sci. 108: 390-394.
Chen,
P., M. J. McCarthy, R. Kauten. 1989. NMR for internal
quality evaluation of fruits and vegetables. Trans. Amer. Soc. Agr. Engineers 32: 1747-1753.