Fruit harvest and conditioning

Hass avocado fruits were harvested from three farms in the Department of Antioquia (Colombia) (table 1) in three harvest maturity stages, i.e. at 23 % ± 1.5 % (early harvest), 26 % ± 1.5 % (intermediate harvest) and 29 % ± 1.5 % (late harvest), taking into account the commercial export window that ranges from 23 % to 30 %.

Table 1 Location and climatic conditions of the avocado farms studied (2016) 

Source: Elaborated by the authors

During harvesting, the experimental lots were monitored taking at random five fruits in each farm and establishing dry matter content (Association of Official Analytical Chemists [AOAC], 1999). Once fruits reached the desired maturity stage (table 2), 32 were collected for each harvest maturity index in each farm (96 per farm and 288 in total). Fruits were examined to verify their compliance with export requirements regarding physical damage and presence of pests.

Table 2 Dry matter content from different Hass avocado harvest times 

Average for n = 5.

Source: Elaborated by the authors

Later, these were coded and taken to the postharvest laboratory of Centro de Investigación La Selva (Rionegro, Antioquia) where they were conditioned according to commercial criteria: disinfection with prochloraz (active component: 0.05 % imidazole w/v) was carried out for 30 seconds and packed in commercial corrugated cardboard boxes.

Storage and ripening

Fruits were stored for different time periods: one control (0 weeks) and three treatments of 3, 4 and 5 weeks of storage (8 fruits per storage time) in climatic chambers (Memmert hpp 110 at 5 °C and 90 % relative humidity), simulating the commercial export process. Then, fruits were ripened in climatic chambers (Memmert hpp 110 at 20 °C and 90 % relative humidity).

Periodic inspections of all samples were carried out to suspend ripening when fruits reached their consumption ripeness, taking into account the scales defined by the international avocado quality manual (^{White, Woolf, Hofman, & Arpaia, 2009}), the purple color of the epidermis and a moderate deformity to soft touch.

Physicochemical parameters

Physicochemical parameter analyses were carried out when fruits reached consumption ripeness. Each fruit was divided into halves, the seed and the skin were removed, and the pulp was homogenized. Tests were performed in duplicate for each analysis.

Dry matter was established according to the AOAC (1999) method (Stove Binder FD 115 UL) through convective drying at 105 °C for 24 hours or until achieving a constant weight.

To measure pH and acidity (percentage of tartaric acid) a potentiometer (Accumet Basic Cole-Parmer AB 15) was used following the AOAC protocols (1998) and the Ecuadorian technical standard (Ecuadorian Standardization Institute [INEN], 2009), respectively.

Regarding soluble solids (°Bx), the methodology published by ^{Maftoonazad and Ramaswamy (2008)} was used, in which an avocado pulp solution with distilled water was prepared in a 1:3 ratio, which was centrifuged (centrifuge Jouan G4.11) at 3,000 r. p. m. for 15 minutes, and then the measurement was made to the supernatant using a digital refractometer (Atago PAL-BX / RI).

Statistical analysis

A factorial design of 3 (maturity indices) by 3 (localities) by 4 (storage times) was used to compare means and establish the degree of difference among treatments; moreover, the significant differences and the Dunnett tests were carried out, the latter to establish differences among the control and the treatments for storage time. Analyses were performed using the Statgraphics Centurion xvi program with a 95 % confidence.

Soluble solids (Brix degrees)

A range between 5.07 and 7.26 °Bx was observed (table 4), and these results are in accordance with those obtained by ^{Burdon et al. (2007)} and ^{Buelvas-Salgado, Patiño-Gómez and Cano-Salazar (2012)} .

Table 4 Physicochemical parameters of avocado cv. Hass at the consumer maturity stage 

Averages in parameters with different letters indicate significant differences (p ≤ 0.05; n = 32). Eight fruits were studied per treatment at 0, 3, 4 and 5 weeks of storage.

Source: Elaborated by the authors

The °Bx tended to increase in the in early maturity stages, except at Bella Vista farm, where there was a decrease in mature fruits and previously stored fruits for four and five weeks under refrigeration conditions (figure 1a). A similar trend was observed in avocados harvested with intermediate and late maturity indices (figures 1b and 1c), but at the El Banco and La Escondida farms, sugars decreased in avocados subjected to five weeks of refrigeration (figures 1b and 1c).

Source: Elaborated by the authors

Figure 1 Behavior of physicochemical variables (°Bx, pH and acidity) of avocado cv. Hass in consumer maturity stage from different locations, harvest indexes and storage times. BV: Bella Vista; EB: El Banco; LE: La Escondida. Vertical bars represent standard deviation (n = 8). 

Previous behaviors could be due to different processes and biochemical changes that occur in fruits simultaneously. An increase in Brix degrees is related to the conversion of polysaccharides and organic acids into sugars or short-chained acids (^{Caparrotta et al., 2015}; ^{Vinha, Moreira, & Ferreira, 2013}).

In this regard, ^{Buelvas-Salgado et al. (2012)} reported increases in °Bx during maturation. This was due to transpiration processes in which the fruit has a lower amount of water, and at the same time, a higher concentration of sugars due to the respiration phenomenon, both originated by the climacteric behavior of the avocado. However, Buelvas-Salgado et al. (2012) did not evaluate the effect of storage in °Bx.

According to the results obtained in our study, we can affirm that storing avocados for long periods (five weeks in this case), causes a decrease in °Bx. This may be associated with higher sugar degradation rates, which occur in greater proportion during the fruit ripening stage, when these compounds are used as a source of carbon and energy for the pro duction of ethylene, enzymatic processes and color changes in the skin (from green to purple or black) (^{Liu, Robinson, Madore, Witney, & Arpaia, 1999}).

Likewise, the locality had a significant influence on the °Bx (table 3). In consumer ripening stage, ^{Vinha et al. (2013)} characterized Hass avocado of Portuguese origin and reported values of 6.68 °Bx. On the other hand, ^{Henríquez, Patiño and Salazar (2012)} mentioned a lower value of 5.51 °Bx for the Colombian Hass avocado. Furthermore, ^{Márquez, Yepes, Sánchez and Osorio (2014)} found even lower values of 2.45 and 2.0 °Bx in four farms in the Department of Antioquia (Colombia) located in the municipalities of El Carmen de Viboral and El Retiro. These differences may be due to the origin of the plant material, soil type, climatic conditions, and preharvest and postharvest management.

In addition, ^{Burdon et al. (2007)} found that the °Bx of Hass avocados from the same farm, but evaluated in different harvest years, were not similar. These differences were attributed to competitive growth aspects of the trees (blooming, root activity and increase in number of shoots, among others) regarding the higher or lower load that the fruit tree had, i.e. the more fruits the trees had, less carbohydrates were available for each, and vice versa. Moreover, these differences were also credited to specific site conditions, such as temperature or rainfall.

Hydrogenionic potential (pH)

pH showed values between 6.58 and 7.14 (table 4), similar to the results reported by ^{Henríquez et al. (2012)} . In general, an increase in fruit pH was observed between week 0 and 3 of storage (figures 1d, 1e and 1f), and their maximum values were reached after the third week in early and intermediate harvest. From this point onwards, a decrease was observed.

^{Buelvas-Salgado et al. (2012)} indicated that in their results the pH increases in the fruit maturity stage until approaching neutrality, since most of the maximum values were equal or close to that point. On the other hand, during late harvest period, and when there are long storage times, i.e. periods of four and five weeks, a significant decrease is noticed (figure 1f).

pH behavior is associated to the organic acid content present in the fruit, since during maturation these tend to decrease as they are consumed at different metabolic cycles. In addition, many act as precursors of volatile substances during this stage (^{Márquez et al., 2014}). Consequently, there is an inverse relation-ship between organic acid content and pH (^{Maftoonazad, & Ramaswamy, 2008}).

In our study, no consistent trends were observed, possibly due to the conditions to which fruits were subjected. However, we must mention that, in general, pH had a tendency to increase with longer cooling and maturation times, contrary to what happened with acidity, which generally tended to decrease.

Titratable acidity

Regarding acidity, this variable had values ranging from 9.24 % to 19.47 % (table 4), and treatments subjected to storage behaved in a stable manner or without significant differences. This was mainly found in fruits harvested with intermediate and late maturity indexes (figure 1h and 1i), except for some treatments, such as those of the Bella Vista farm in intermediate maturity stages (figure 1h), where there was an increase in refrigerated fruits for three weeks.

As mentioned above, overall, the titratable acidity tends to decrease during fruit storage and maturation (figures 1g, 1h and 1i a behavior that agrees with what was observed by ^{Buelvas-Salgado et al. (2012)} .

This decrease is attributed to the consumption of organic acids in different metabolic cycles of the fruit, including tartaric acid, which predominates in avocado; Moreover, acids and carbohydrates are used to provide the energy that the fruit requires during the ripening process (^{Caparrotta et al., 2015}).

In quality terms, physicochemical variables serve as inspection parameters. In the literature, pH and Brix degrees have been reported as quality indicators (^{Özdemir et al., 2009}; ^{Vinha et al., 2013}). However, it is important to point out that it is necessary to carry out studies in other producing regions, and to correlate these variables with different quality criteria, such as sensory, microbiological and metabolic analyzes.