Dry material

Using a microwave oven according to ^{Lee and Coggins (1982)}. The 10 g of pulp were obtained by longitudinally cutting thin strips of the fruit with a homemade potato peeler, the strips were placed in Petri dishes and dehydrated in a microwave oven for 6 to 10 min until constant weight was obtained. The percentage of dry matter was calculated with the following formula: (fresh weight-dry weight/fresh weight) X 100.

Weightloss

By portable digital scale with capacity of 2 000 g and approximation of 0.1 g (Ohaus corp Florham Park, NJ). Twenty fruits were scored per treatment, which were weighed periodically throughout the evaluation stage. The difference in weight and its relation to the initial weight was expressed as percentage weight loss.

Turn color of shell

It was quantified as the percentage of fruits that turned in the color of their shell from green to black (at least 75% of the shell tonality).

Firmness

It was determined with a Chatillon Model DFE-050 penetrometer (Ametek Instruments, Largo, FL), adapted with a 10 mm diameter cylindrical punch; the measurements were taken in the equatorial region of the whole fruit. A 5 mm section of the shell was removed to expose the pulp and the punch was inserted at 4 mm depth at a speed of 180 mm min-1. Data were expressed in Newtons (N).

Cold damage based on hedonic scale (^{Corrales and Tlapa-Rangel, 1999}).

External: 1= no damage; 2= 1 to 25% of damaged surface; 3= 26 to 50% of damaged surface; 4= 51 to 75% of damaged surface and 5> 76% of damaged surface with respect to the total area of the fruit.

Internal: 1= no damage; 2= light damage (<25% of damaged pulp); 3= moderate (26 to 50% of damaged pulp); 4= severe (51 to 75% of damaged pulp) and 5= very severe (> 76% of damaged pulp).

The data were analyzed completely randomly with 2x3x7 factorial arrangement with the following factors: A. Production cycle: 2015 and 2016; B. Production region: Nayarit, Michoacan and Jalisco. C. Treatments (7): combination of temperatures and weeks of storage. For the weight loss variable, 20 repetitions were used (fruits), while for the rest of variables eight repetitions were used. Analysis of variance and test of means (Waller-Duncan p≤ 0.0) with SAS statistical package (^{SAS, 2002}).

Weight loss

The weight loss by effect of the factors under study is illustrated in Figure 1. Regarding the cycle effect (Figure 1A), it was observed that the fruits harvested in 2015 accumulated to maturity of consumption a greater loss (7.8%) that was statistically different from those harvested in 2016 (6.5%). This can be correlated with MS content, since fruits with higher MS content (2015= 27.8%) have higher metabolic activity than fruits with lower MS (2016= 26.3%) [^{Erikson et al., 1971}]. Regarding the effect by state or region (Figure 1B), the accumulated weight loss was higher in the fruits harvested in Nayarit and Jalisco (7.6% for both) and was statistically different from the fruits of Michoacán (6.3%), which may be correlated with the degree of roughness of the shell.

Figure 1 Weight loss (%) of avocado ‘Hass’ fruits subjected to different temperatures and transfer simulation times during two cycles in three locations. Each point represents the mean of 20 observations ± the standard error. 

The fruits of Nayarit showed the greatest roughness, those of Jalisco intermedia and those of Michoacán were the smoothest. However, no references were found regarding the roughness effect on weight loss. The factors with the greatest impact on this variable were the temperatures and storage time (Figure 1C). Fruits stored at 4 °C for three weeks at the end of refrigeration showed a loss of only 1.8%, while those at 6 and 8 °C had a weight loss of 2.4%, whereas the control fruits stored under marketing simulation (22 ± 2 °C; 70 ± 10% HR) showed 5.6%.

Regarding the weight loss of the fruits stored for four weeks, no significant differences between 4, 6 and 8 °C were detected, showing losses of 3 to 3.2%. On the other hand, at maturity of consumption a very marked difference was observed between any of the treatments in refrigeration versus those maintained under market simulation. The fruits under refrigeration presented accumulated losses of 6.6 to 7%, whereas the cores reached 9.9%. This is most likely due to higher temperature, higher respiration rate, and ethylene production (^{Morris, 1982}).

Turn of shell color

The change of shell color and the softening of the fruits are the most visible characteristics in the ripening process of avocado ‘Hass’. The green-to-black peel color change is due to a decrease in chlorophyll content followed by an increase in anthocyanins (^{Cox et al., 2004}). Table 1 shows that fruit harvested in 2015 had a higher percentage of fruits with a color reversal, which, as already mentioned, may be correlated with the higher MS content (27.8%) compared to 2016 (26.3%) and probably due to the fact that the former have higher metabolic activity due to the higher MS content (^{Erikson et al., 1971}).

In relation to the effect of the locality or region, it was observed that Jalisco fruits showed the highest percentage of color turn at the end of refrigeration or consumption, which probably could be due to the different growth conditions (^{López and Cajuste-Bomtemps, 1999}). Distant markets such as Europe and Japan require that the avocado ‘Hass’ fruit arrive green, but ready for sale (dark color) within four to five days of arrival. It was notorious that temperatures of 4 and 6 °C for three or four weeks and those of 8 °C for three weeks maintained that condition.

However, the temperature of 8 °C was no longer able to keep all the fruits green at four weeks of storage since it showed 10% of fruits that turned color, which were statistically equal to the control under simulation of marketing. It is indisputable that at higher temperatures, greater color change, which may be correlated with higher respiration rate and ethylene production (^{Morris, 1982}).

Firmness of pulp

Softening is the most characteristic feature of the avocado ‘Hass’ fruit maturation process and together with the shell color turn is considered a benchmark to evaluate potential post-harvest life (^{Ochoa et al., 2009}). Our results show that firmness was affected by the three factors under study, with temperature and storage time being the most influential (Figure 2). Regarding the cycle effect (Figure 2A), the fruits harvested in 2015 at the end of refrigeration showed more firmness than those of 2016; however, this difference was not significant to consumption.

Figure 2 Pulp firmness (N) of avocado ‘Hass’ fruits subjected to different temperatures and transfer simulation times during two cycles in three locations. Each point represents the mean of eight observations ± the standard error. 

Something similar was observed in the effect by state or region (Figure 2B), where at the end of refrigeration the fruits of Nayarit showed greater firmness compared to those of Michoacán or Jalisco, which could be due to the fruits of Nayarit were processed the day of the harvest, while those of Michoacán and Jalisco were processed 24 hours after the harvest; however, there were no significant differences in consumption. The factor of greatest impact on firmness were temperature and storage time (Figure 2C). At the end of refrigeration the differences were very marked, the fruits that were kept at a lower temperature (4 °C for three or four weeks and 6 °C for three weeks) presented a high firmness, although those that remained in storage at 6 °C for four weeks decreased almost 50% of their initial firmness, however, still within the acceptable commercial range.

Remember that importers demand fruit that arrives green and firm to markets that require three to four weeks of transportation. The fruits stored at 4 °C fulfilled this condition, but caused external cold damage, so it is suggested to use the temperature of 6 °C, as it was completely satisfactory for three weeks of storage and although its effectiveness decreased for four weeks, still remained at commercially acceptable levels, and above all, did not cause external cold damage. No significant differences were detected between treatments, although the temperature of 4 °C provided up to six shelf life days after three or four weeks of storage, while 6 °C induced only five days (within the range required by the importer) and 8 °C only four days of shelf after three weeks of storage and was insufficient for four weeks.

External cold damage

The effect of climate and degree of shell roughness did not affect this variable since it was again observed that the factor with the most impact were temperature and storage time (Figure 3). The external cold damage could only be evaluated at the end of refrigeration because at maturity of consumption the damage was masked by the color turn of the shell from green to black. It was observed that as the temperature decreases and the storage time increases, the external cold damage was greater (Figure 3C). The greatest damage was observed in fruits stored at 4 °C for three or four weeks, followed by fruits stored at 6 °C, although much lower. Fruits stored at 8 °C for three or four weeks and controls at 22 ° C showed no external cold damage.

Figure 3 External damage of avocado ‘Hass’ fruit subjected to different temperatures and transfer simulation times during two cycles in three locations. Each point represents the mean of eight observations ± the standard error. 

The literature mentions that cold damage occurs by storing the fruit at temperatures below 6 °C and above the freezing point, especially when the cold storage time is prolonged (^{Yahía 2001}, ^{Arpaia 2005}). The damage occurs at the level of the cell membrane affecting its structural integrity. It is believed that low temperatures modify the distribution of lipids in the membrane causing gelled and other fluid areas, because saturated fatty acids tend to solidify more easily because of the low temperatures.

The polyphenoloxidase enzyme is responsible for browning by oxidizing phenols in quinones and melanin (^{Chaplin et al., 1982}; Lee and Young, 1984; ^{Trejo et al., 1992}). The results of this research suggest that the optimum storage temperature is 6 °C because although at four weeks reduced firmness, it maintained the green color of shell and provided five days of shelf life, thus meeting the requirements of importers.

Internal cold damage

The internal cold damage was practically negligible (Table 2) and only manifested slightly for fruits kept at 4 °C for four weeks of storage. Internal cold damage usually occurs at temperatures below 4 °C and for more than four weeks of storage (^{Yahía, 2001}; ^{Arpaia, 2005}).

The effect of climate and degree of hull roughness did not influence the refrigeration tolerance of avocado ‘Hass’ fruits, although the degree of hull rugosity affected weight loss. The factor of greatest impact were the temperature and storage time. At higher temperature, greater weight loss and turn color; at lower temperature, greater external damage in fruits. At 4 °C the fruit showed longer shelf life with high firmness and no color change, but with external damage. The best storage temperature was 6 °C, as it maintained color and firmness at acceptable levels, without external damage and with five days shelf life after storage. The temperature of 8 °C reached for three weeks of simulation of transfer and insufficient for markets of four weeks.