Proceedings of The World Avocado Congress III, 1995 277
- 284
TOWARDS IMPROVED MATURITY STANDARDS FOR 'FUERTE'
AVOCADO (Persea americana Mill.) FRUIT IN A COOL SUBTROPICAL CLIMATE
C. Kaiser and B.N.
Wolstenholme J. Levin
Department of Horticultural Science Medical Research Council
University of Natal
Private Bag X01, Scottsville Private Bag
X385
Pietermaritzburg, 3209 Pretoria, 0001
Abstract
Fruit maturity is a key issue
for harvesting and post-harvest handling and avocado fruit are no exception.
Physiological maturity may be defined as that stage of development at which the
fruit, once detached from the tree, will ripen and result in a product
desirable for eating. Immature fruit are known to be bland and likely to
shrivel as they ripen and in extreme cases the fruit may not even soften. In
addition, 'Fuerte' avocado fruit have been shown to be more prone to
post-harvest physiological disorders especially later in the season, than are
'Hass' fruit. Consequently, it is imperative that maturity standards, both
minimum and maximum be set for these fruit. Presently, in South Africa the lipid
and reciprocal moisture percentages are used to determine minimum fruit
maturity. The current investigation examined lipid accumulation, fruit firmness
and pectin methylesterase (PME) activity over two seasons and found that PME
activity was too variable between and within replications in both 1994 (sed =
0.005) and 1995 (sed = 0.011). Lipid content proved to be the most accurate
marker and 'Fuerte' fruit should have been picked at the latest for export on
Everdon Estate, Howick at lipid concentrations of about 16.5 % on a fresh mass
basis or 60 % on a dry mass basis.
1. Introduction
'Fuerte' avocados are one of
the main Californian cultivars grown in South Africa for export to European
markets. Harvesting of 'Fuerte' fruit begins in the hotter Northern Province in
February and continues until July in the cooler Kwazulu-Natal midlands. The
Californian maturity standard has been adopted by the South African industry
and fruit may not be picked before the lipid percentages have reached a minimum
of 8% on a fresh mass basis. Although some cultivars e.g. "Hass" may
be left to hang on the tree for extended periods of time without subsequent
deterioration of fruit quality (Kaiser and Wolstenholme, 1994), others may not
be suited to delayed harvesting as fruit senescence while on the tree is
probably a limiting factor. The fact that 'Fuerte' avocado fruit especially
late in the season are more prone to post-harvest physiological disorders than
are 'Hass' fruit (Witney et al., 1990) is supporting evidence for this.
In the past, the only
reliable maturity standard for avocado fruit was found to be total lipid
content (Eaks and Sinclair, 1978) and this is still the case today (Kaiser,
1994).
Since the total lipids and
moisture content are reciprocal and sum to a constant for any one cultivar
(Swarts, 1976) the moisture content is still used extensively by the South
African industry as a maturity standard. However, the moisture content varies
according to the prevailing orchard conditions. Consequently, for accurate readings,
lipid concentrations should be determined on a dry mass basis. On a different
note however, Zauberman and Schiffmann-Nadel (1972) examined pectin
methylesterase activity (PME), the enzyme responsible for the initiation of the
ripening process, in, 'Fuerte' fruit at various stages of development and
ripening and found that PME activity on the day of harvest decreased with an
increase in the stage of fruit development. In younger fruit, PME activity
decreased rapidly while in mature fruit, PME activity decreased moderately and
they suggested from these data that PME activity may be a possible maturity
indicator. Apparently however, no further work has been done in this regard.
Consequently, the present study was undertaken to determine whether PME activity
is a suitable maturity marker for 'Fuerte' fruit.
2. Materials and Methods
During 1994, 232 'Fuerte'
fruit of count 16 (236g to 265g) from well managed orchards on the farm Everdon
Estate (30º16ºE and 29º27ºS) near Howick, a cool subtropical area were
harvested on a weekly basis between 17 May 1994 and 6 July 1994. Sixteen fruit
were sampled on the day of harvest and fruit firmness, fruit and seed masses
were recorded. The moisture content of the fruit flesh was determined
gravimetrically and lipid percentages were determined using a soxhlet apparatus
and plotted over time (figure 1). In addition, PME activity of radial sections
was monitored by modification of the method developed by Hagerman and Austin
(1986). Meanwhile, 80 fruit were placed in the laboratory and allowed to ripen
at room temperature (21ºC). Eight fruit were sampled on each of the subsequent
10 days of storage and fruit firmness, fruit and seed masses, and PME activity
were recorded. The change in PME activity was analyzed statistically and
plotted over time (figure 2). The remaining fruit were stored at four different
temperature regimes. These storage regimes were identical to treatments 2,4,6
and 9 (table 1) of Donkin et al. (1995). During each week of storage, 8 fruit
were removed from each treatment and fruit firmness recorded. These values were
averaged for each week and the data plotted over time.
During 1995, 144 'Fuerte'
fruit of count 16 were harvested at the same site on a weekly basis between 23
May 1995 and 26 June 1995. Sixteen fruit were sampled on the day of harvest and
fruit firmness, fruit and seed masses were recorded. The moisture content and lipid
percentages were determined as above and the results plotted over time (figure
3). In addition, 80 fruit were placed in the laboratory and allowed to ripen at
room temperature (21ºC), and 8 fruit were sampled on each of the subsequent 10
days of storage. Again the change in PME activity was analyzed statistically
and plotted over time (figure 4). The remaining fruit were again stored at the
same temperature regimes (treatments 2,4,6 and 9) and the firmness of 16 fruit
was recorded each week. These weekly values were averaged and the data plotted
over time.
3. Results and Discussion
During 1994, lipid
percentages increased over the season from about 11 % on a fresh mass basis (or
53% on a dry mass basis) on 17 May 1994 to slightly more than 20% on a fresh
mass basis (or about 65% on a dry mass basis) on 6 July 1994 with some minor
fluctuations between these times (figure 1). During 1995, a similar trend was
seen where lipid percentages increased over the season from about 12 % on a
fresh mass basis (or 54.5 % on a dry mass basis) on 23 May 1995 to slightly
more than 19 % on a fresh mass basis (or about 64 % on a dry mass basis) on 27
June 1995 (figure 4).
In most instances during both
1994 and 1995, PME activity usually increased the day after harvest but then
declined steadily while the fruit was ripening. No definite trends in initial
activity nor the rate of change of activity could be observed across either
season. Consequently, PME activity is not a suitable maturity marker. Two
definite decreases in initial activity were however, observed between 17 May
1994 and I June 1994 in the first instance and 7 June 1994 and 6 July 1994 in
the second instance (figure 2). Two similar trends were observed during 1995
between 23 May 1995 and 13 June 1995 in the first instance and 20 June 1995 and
26 June 1995. PME activity was thus modeled against fruit firmness using a
simple linear regression model. The relationship between fruit firmness and PME
activity was highly significant (P < 0.0001). Consequently, firmness
definitely increased with decreasing PME activity however, the r-value was only
22.8%, which means that PME activity only describes 23% of the variability in
fruit firmness. This implies that PME activity is not a very good marker for
fruit softening either.
Of the four temperature
trials, treatment 6 (5.5ºC, 5.5ºC, 5.5ºC, 5.5ºC) the industry norm, proved the
best in both seasons where fruit softness was concerned. None of the fruit
receiving this treatment exceeded the maximum 35 kPa firmometer readings after
4 weeks of storage (figure 5). During the 1994 season however, fruit from
treatment 6, harvested after 21 June 1994 were beginning to soften after 4
weeks in storage but still had an acceptable firmness. Indeed, some of the
fruit harvested in Kwazulu-Natal left Durban harbour on Vessel 906 on 25 June
1994, and arrived in Europe soft. It appears that the optimum harvesting date
for 'Fuerte' fruit in the Kwazulu-Natal midlands was up to 21 June 1994.
Indeed, this was confirmed by Donkin et al. (1995) who found that there was a
definite increase in physiological and pathological disorders after this date.
Fruit harvested at that time had a lipid content of about 57% on a dry mass
basis (or 15% on a fresh mass basis). A similar trend was seen in 1995 when
post-harvest physiological problems were seen in several of the fruit cut after
20 June 1995. Here, the lipid content was about 60% on a dry mass basis (or
16.5 % on a fresh mass basis).
Fruit from
treatments 2, 4 and 9 were only firm after four weeks of cold storage for the
first two weeks of harvest in 1994. Whereas during 1995, fruit which underwent
these same treatments, had an acceptable firmness after until the last week of
harvest but was significantly higher than treatment 6 (sed= 0.997).
Consequently, treatment 6 resulted in the firmest fruit after four weeks of
cold storage.
4. Conclusions
'Fuerte' fruit
harvested in the Kwazulu-Natal midlands at Everdon Estate,' Howick during the
1994 and 1995 seasons stored best in regular atmospheric cold storage at a
constant 5.5'C over 4 weeks when compared to other stepped-up or stepped-down
temperature regimes where fruit softness was concerned. The experimental fruit
stored at this regime in South Africa were still relatively firm even when
harvested after 21 June 1994 however, some of the commercial consignments of
fruit (on Vessels 906 and 907), which underwent similar storage conditions
arriving in Europe 4 weeks after that date were soft. During 1995 however, all
fruit were firm after 4 weeks at a constant 5.5ºC. For the first three weeks of
storage during 1994 (17 May 1994 until 01 June 1994) and the first 5 weeks of
storage during 1995 (23 May 1995 until 20 June 1995) however, there were no
marked differences in fruit firmness where treatments 2,4,6 and 9 were
concerned. Consequently, it is recommended that fruit are stored a constant
5.5ºC for 4 weeks to reach the European markets firm.
Lipid content, still the most
reliable maturity standard for avocados to date, plateaued on 21 June 1994 at
approximately 58% on a dry mass basis (or 16% on a fresh mass basis). After
that time it declined to about 57 % on a dry mass basis (or 15 % on a fresh
mass basis) by 29 June 1994. This together with the fact that physiological
browning disorders of the mesocarp (Donkin et al. 1995) and distal-end browning
(Kaiser et al., 1995) increased after 21 June 1994 indicates a maximum fruit
maturity date. In 1995, a similar trend was seen where post-harvest
physiological disorders appeared in fruit harvested after 20 June 1995. Here,
fruit lipid content was 60% on a dry mass basis (or 16.5 % on a fresh mass
basis). Consequently, based on these results it is recommended that 'Fuerte'
fruit from Everdon Estate, Howick should all be harvested when lipid
concentrations reach a maximum of 60% on a dry mass basis (or 16.5% on a fresh
mass basis). During 1994 and 1995, this coincided with the last week of June.
Analysis of PME activity
showed two definite decreases in initial activity in both 1994 and 1995
(figures 2 and 4) however they were not coincidental. Consequently, PME
activity is not a good fruit maturity marker. Finally, modeling of PME activity
showed that PME activity only describes 23% of the variability in fruit
firmness.
5. Literature Cited
Bergh, B., Kumamoto,J., and
Chen, P., 1989. Determining maturity in whole avocados. Calif. Avo. Soc. Yearb.
73:173-176.
Donkin, D.J., Mans, C.C.,
Slabbert, M.J., Levin, J., and Wolstenholme, B.N., 1995.
"Stepped-down" storage temperature regimes for 'Fuerte' fruit grown
in the Natal midlands: do they reduce the incidence of physiological disorders?
S. Afr. Avo. Grs' Assn Yearb. 18 (In press).
Eaks, I., and Sinclair, W.B.,
1978. Pectin and related constituents in avocado fruit ontogeny, cold storage
and ripening. J. Amer. Soc. Hort. Sci. 103:846-849. Hagerman, A.E., and Austin,
P.J., 1986. Continuous spectrophotometric assay for plant pectin methyl esterase.
J. Agric. Food Chem. 34:440-444.
Kaiser, C., 1994. Evaluation
of maturity standards in avocado fruit. Subtropica 15:18- 20.
Kaiser, C., and
Wolstenholme, B.N., 1994. Aspects of delayed harvest of 'Hass' avocado (Persea
americana Mill.) fruit in a cool subtropical climate I. Fruit lipid and
fatty acid accumulation. J. Hort. Sci. 69:437-445
Kaiser, C., Boschoff, M., Mans, C.C., Donkin, D.J., and Slabbert, M.J., 1995. Distal-end browning ('Bolverkleuring') of Tuerte' fruit in the Kwazulu-Natal midlands. S. Afr. Avo. Grs' Assn Yearb. 18 (In press).
Swarts, D.H., 1978. The
no-nonsense determination of oil content for avocados. Citrus and Subtrop. Res.
Inst. Info. Bul. 42:4.
Witney, G.W., Hofman, PJ, and Wolstenholme, B.N.,
1990. Effect of cultivar, tree
vigour and fruit position on calcium accumulation in
avocado fruits. Scientia Hort. 44:269-278.
Young R.E., and Lee, S.K.,
1978. Avocado fruit maturity. Calif. Avo. Soc. Yearb. 62:51-57.
Zauberman, G., and
Schiffmann-Nadel, M., 1972. Pectin methylesterase and polygalacturonase in
avocado fruit at various stages of development. Plant Physiol. 49:864-865.