Weight loss

The weight of the fruits was recorded with an ALSEP EY-2200 digital scale, until the fruits reached consumption maturity. The percentage of weight loss was calculated according to equation 1.

Firmness

It was determined with a texture meter (Wagner Force Five model FDV30) with conical tip of 7 mm in diameter, the measurements were made every 2 days. Values were reported in Newton (N).

Fruit color

The color change was determined in the epicarp and mesocarp, using a HunterLab reflection colorimeter (Reston Virginia model D25-PC2) with a D65 lighting system and an observer angle of 2°, with the values obtained (L, a and b), the luminosity (L), °Hue (equation 2) and chroma (equation 3) were calculated, every two days until the fruits reached consumption maturity (^{Lancaster et al., 1997}).

Shelf life

Shelf life was established considering the number of days in which the fruits of each physiological state reached consumption maturity, without presenting physical damage or contamination by microorganisms.

Ethylene and CO₂ concentration

These variables were only determined in avocado fruits in the changing physiological state, evaluations were made daily by means of a gas chromatograph (Perkin Elmer Clarus) according to the headspace method ^{Tovar et al. (2011)}, a random sample of 6 fruits was taken, with which 3 experimental units (repetitions) of two fruits each were established, which were placed in 2 L containers hermetically closed for 1 hour.

A 1 mL sample of the gas was taken from the headspace and injected into a gas chromatograph (Perkin Elmer Clarus), with a TG-Bond Q 30 m x 0.32 mm x 10μ column of Thermo Fisher Scientific, which was simultaneously connected to a flame ionization detector (FID) and a thermal conductivity detector (TCD). The operating conditions were: column temperature 150 °C, FID 180 °C and TCD 180 °C, a standard of CO₂ with 498.6 μL/mol and of ethylene with 100 μL/mol, both of INFRA^®, was used. Respiration data were reported as mL CO₂ kg^-1 h^-1 and ethylene concentration in μL C₂H₄ kg^-1 h^-1.

Experimental design

For the analysis of data, in fruits stored at 22 °C, three repetitions (two fruits as an experimental unit) were considered, and the split plot model was applied, the evaluation of the physiological states corresponds to the large plot and the evaluation days to the small plot, an analysis of variance was performed, and the means were compared with Tukey’s test (p≤ 0.05). The statistical program SAS^® System Version 9 for Microsoft^® Windows^® was used.

Weight loss

Authors such as ^{Magaña et al. (2013)} mention that the weight loss of fruits is associated with water loss, when fruits are harvested and exposed to room temperature for several days, this loss may also depend on the intrinsic and extrinsic characteristics of the product, which could be related to the vapour pressure deficit between its surface and the atmosphere in which they are. The weight loss of the fruit during the initial stages of maturation influences the maturation time, incidence and severity of ripe rots, ^{Dixon et al. (2022)}.

Of the three physiological states evaluated, the fruits in black physiological state, Figure 1, presented higher percentage of weight loss (8.10%) when reaching consumption maturity (day 4), while ^{Herrera et al. (2017)} reported weight losses of 8.3% for ‘Hass’ avocados and 6.6% for cv. Mendez stored at room temperature.

Figure 1 Quality variables during the maturation of fruits of ‘Lonjas’ avocado harvested in three physiological states (green, changing and black). Values with different letters on each evaluation day (0, 2 and 4) are statistically different (Tukey; p≤ 0.05). 

Firmness

A proper prediction of firmness in avocado fruits is important, since it provides indirect access to the levels of maturity, fruit quality and probable storage time, the fruits are harvested green and hard to the touch; however, they soften. The rate of softening is moderate at first, increases later and stops at maturity, ^{Mishra et al. (2021)}.

As expected, the firmness decreased, Figure 1, as time passed and as the fruit matured, the green state presented the greatest firmness (9.6 N) on day 0 of its evaluation, followed by the black and changing physiological state (6.3, 5.9N); nevertheless, on day 4, the fruits in green and changing state presented slightly higher values than black (0.32 N), ^{Rámos et al. (2021)} reported firmness values below 5 N (1.3-2.7 N) in fruits identified as ‘CA-1’ and ‘CA-4’ but higher than those presented in fruits of ‘Lonjas’ avocado, this quality attribute has been reported very little in native materials.

Fruit color

When L*y °Hue decrease significantly, the green coloration decreases and the skin turns black, then, maturation occurs regularly, and the fruits reach consumption maturity. In fruits of ‘Méndez’ and ‘Hass’ avocado, the fruits reached consumption maturity between 8 and 12 days after harvest (^{Herrera et al., 2017}).

In the fruits of ‘Lonjas’ avocado, the luminosity of the epicarp on day 4 (consumption maturity) presented the lowest values (Figure 1) with respect to days 0 and 2, for the green physiological state a value of 13.7 was reported, for changing 13.9 and for black 12.8, in the same way, values of 26.9 and 29.7 in L for external color were reported in fruits identified as ‘CA-1’ and ‘CA3’ in fruits stored at 25 °C ( ^{Ramos et al., 2021}).

The peel of the fruits of native avocados generally presents colors with hues ranging from purple to dark due to the presence of anthocyanins, pigments with high antioxidant activity, ^{Ramos et al. (2021)}. In evaluations of Hue, which externally indicates the dark coloration, there were higher values in the changing and black state (236.9, 241.3) of ‘Lonjas’ fruits.

The pulp has a consistency very characteristic of avocado fruits, similar to butter due to its high lipid content (up to 23% of the total weight of the pulp) and the color has yellowish hues in the center and greenish on the outside, ^{Corrales and Méndez (2020)}. The luminosity of the pulp was found in the ranges of 20 to 30, obtaining the highest values on day 2, in the case of the green and black physiological states (31.8 and 31.9), ^{Ramos et al. (2021)} reported L values in pulp between 55.0 and 61.72 for native fruits; in the case of chroma in ‘Lonjas’ avocado, the values increased until reaching the fourth day, with the changing physiological state being the one who presented the highest value on day 4 (21.9).

Considering that day 4 is when the fruits reached consumption maturity, it can be observed, Figure 1, that in the firmness parameter, the green state (0.32 N) is significantly different from black (0.22 N), in % of weight loss the black state (8.1%) is significantly different from the green state (6.83%), in luminosity of the epicarp the changing state (13.9) is significantly different from black (12.8), in Hue the black state (236.9 Hue) is significantly different from green (227.1 Hue), while for chroma, changing (21.9) is significantly different from green and black (15.3 and 16.3).

The results, Table 2, show how fruits in different physiological states influence shelf life, with lower weight losses in the green physiological state and higher values of luminosity of the mesocarp and chroma in the black physiological state.

Ethylene and CO₂ in fruits with changing physiological state

In climacteric fruits such as avocado, respiration increases very quickly, coinciding with increases in ethylene production, which stimulates the maturation and development of organoleptic characteristics, ^{Azcón and Talón (2013)}. The highest concentration of ethylene in fruits stored at 22 °C (Figure 2) occurred on days 2 and 3 (87.8, 92.8 μL C₂H₄ kg^-1 h^-1), while for the case of CO₂, on day 3 (84.4, mL CO₂ kg^-1 h^-1).

Figure 2 Respiratory intensity and ethylene production in fruits of ‘Lonjas’ avocado in changing physiological state, stored at 22 ± 2 °C. 

So far there are no reports regarding changes in ethylene and CO₂ concentration throughout maturation in native materials, nevertheless, in ‘Hass’ and ‘Méndez’ avocados evaluated by ^{Herrera et al. (2017)}, stored at room temperature, the climacteric was reached from 7 to 8 d after harvest (harvested fruits with higher dry matter presented greater ethylene production and greater respiration speed), obtaining values of 10-100 μL C₂H₄ kg^-1 h^-1 and 10-50 mL CO₂ kg^-1 h^-1 in relation to the dry matter content.

Shelf life

The fruits of ‘Lonjas’ avocado in the states: green, changing and black had a shelf life of 4 days. On the other hand, due to the refrigeration treatment used in ‘Lonjas’ fruits in changing physiological state, the fruits reached a shelf life of up to 11 days; since the fruits were stored for 9 days at 13 °C, then stored at 22 °C and reached consumption maturity after 2 days. These results coincide with those reported by ^{Acosta et al. (2013)}, who mention that the shelf life of native avocado fruits varies from 10.3 to 11 days and that low values ranging from 4 to 6.3 days can also be found.

Quality of fruits stored at 13 °C

Storage temperature in Hass avocado has greater influence on weight loss and firmness, ^{Benítez et al. (2021)}, as well as on the change in color, ^{Osuna et al. (2017)}. The higher the maturation temperature, the shorter the time to reach consumption maturity. At a storage temperature of 6 °C, color and firmness are maintained at acceptable levels, without external damage and with five days of shelf life after storage, ^{Osuna et al. (2017)}. At 7 °C for 20 days combined with maturation at 20 °C for 6 days, fruits with homogeneous maturation, less internal damage and acceptable weight loss can be obtained (^{Benítez et al., 2021}).

The fruits of ‘Lonjas’ avocado in changing state evaluated at the exit from the refrigeration treatment presented firmness values of 0.7 ± 0.1 N, percentage of weight loss of 3.4 ± 0.1%, luminosity and chroma in epicarp of 33.8 ± 1.2 and 11.9 ± 0.5 respectively, as well as luminosity and Hue in mesocarp of 17.3 ± 0.6 and 204.5 ± 33.2, the values were different from those obtained when reaching consumption maturity (two days after their exist from refrigeration and storage at 22 °C), firmness 0.3 ± 0.1 N, percentage of weight loss of 7.7 ± 0.1%, luminosity and chroma in epicarp with values of 39.5 ± 1.6 and 7.7 ± 0.1, as well as luminosity and Hue in mesocarp of 13.8 ± 0.0 and 247.7 ± 4.6, respectively.

Concentration of ethylene and CO₂ in fruits stored at 13 °C

There is currently not enough research describing changes in ethylene and CO₂ concentration in native avocado fruits, as reported by ^{Osuna et al. (2017)}. Avocado is a climacteric fruit with a high respiration rate (80 to 300 mg CO₂ kg^-1 h^-1 at 20 °C ) and high ethylene production (>100 μL C₂H₄ kg^-1 h^-1 at 20 °C), making it difficult to preserve and market to distant markets. One of the most common techniques to prolong shelf life and maintain avocado fruit quality is the use of refrigeration. The avocado is a fruit sensitive to cold damage when subjected to low temperatures for prolonged times.

The main symptoms of cold damage manifest as faint brown to dark brown zones in the mesocarp (^{Román et al., 2002}). The principle for using refrigeration is that it slows down the physiological processes that lead to fruit maturation, ^{Osuna et al. (2017)}, in this sense and in relation to respiration and ethylene production, the fruits in changing state stored for 9 days at 13 °C presented, at their exit, concentrations of 74.64 ± 12.67 μL C₂H₄ kg^-1 h^-1 and 49.33 ± 5.21 mL CO₂ kg^-1 h^-1, one day after their storage at 22 °C values of 33.09 ± 2.99 μL C₂H₄ kg^-1 h^-1 and 46.98 ± 3.36 mL CO₂ kg^-1 h^-1. When reaching consumption maturity, the presence of ethylene and CO₂ was minimally quantifiable, so these values are not reported.