Study 1. Mechanical damage in avocados during harvesting and transport to the baler

The mechanical damages in avocado were quantified from the harvest to the pre-packing in an export packing machine, in Uruapan, Michoacán. The study was carried out from October 1 to 18, 2013. Seven fruit cutters certified for export (Tingambato, Leones, Sal 3, Volcan, La 18, Sal 1 and Valle) were evaluated in seven orchards with registry of the Ministry of Agriculture, Livestock, Fisheries and Food (SAGARPA, for its acronym in Spanish) of the municipalities of Tacambaro, Uruapan, Tancítaro, Ziracuaretiro and Periban, in the state of Michoacan.

The process of harvest, collection and transport to the packaged was divided into 5 stages (Table 1). The crews started harvesting at about 8:00 am. Each group consisted of 11 people, of whom three random cutters were selected. The workday lasted between 5-6 h, for which evaluations 1, 3 and 5 h were performed after the beginning of the fruit cut. The assessment began at each hour evaluated with the measurement of the fruits in stage 1 (Table 1), the carton where the fruits were placed (stage 2) was marked to follow up in the later stages (stage 3 to 5).

The evaluation consisted of counting the total fruits of the box and the fruits with mechanical damages. To avoid contamination of the fruit (contact with the soil), the evaluation was carried out on a tarpaulin of 2 m2 . Each mechanical damage found in the fruits was marked with a circle and the number of the stage, to avoid double the count of damages.The mechanical damages evaluated were: 1) tearing. Mechanical damage in the area of the peduncle or any part of the fruit, caused by the scissors, the harvest hook or broken plastic boxes. Not deep wound that causes rupture of the epidermis; 2) cutting. Mechanical damage in any part of the fruit caused by harvesting hook or broken plastic boxes, characterized by being deep wounds with a length greater than 1 cm; 3) bruising. Injury caused by striking or compressing a part of the fruit without causing rupture of the epidermis (skin) giving a dark and elongated coloration; and 4) unattended. Fall of the peduncle caused by the harvester or the harvest. In addition to the mechanical damages, the total number of boxes harvested in the day, number of boxes in each hour harvested, as well as the number of fruits in each box evaluated were also registered.

Study 2. Evaluation of external and mechanical damages in two harvest periods

In an avocado packer for export, in Uruapan, Michoacán, sampling was carried out on lots of fruits that arrived at the packing house at two different harvest times. The first sampling was carried out in October 2015, in fruits with an average harvest maturity of 24-26% dry matter (MS) in the pulp, in 11 plots from 11 orchards in different locations. The second sampling was performed in April 2016, in fruits with an average harvest maturity of 32-34% MS in the pulp, in 11 lots from 11 orchards. Simple random sampling was used, 200 fruits were taken from each batch.

The presence or absence of external damages (scab, thrips, chicken pox, nail and sunburn) and mechanical damages (bruises, scrapes, cuts and peduncle fall) were evaluated in each fruit. evaluated the degree of maturity with a subjective scale of 5 categories (1= forest green, 2= fruit turning black, 25%, 3= fruit turning black, 50%, 4= fruit turned black, 75%, and 5= 100% black fruit) (^{Herrera et al., 2017}). The data were expressed as percentage of fruits with external or mechanical damages, by caliber and degree of maturity.

Study 3. Calculation of the sample sizes to estimate the external and mechanical damages of fruit of ‘Hass’

With the data obtained in study sample 2, the proportion of fruits with each damage (external or mechanical) was estimated by dividing the number of damaged fruits between the sample size used (n= 200). With the values of p, the proportion (q) of fruit without damage was calculated in each study period (q = 1-p). The sample size was calculated with the expression n= [(Zα/2)2 pq]/β2], where Zα/2 is the critical value of the standard normal distribution with a significance level α= 0.05 and has a value of 1.96 and β is the maximum permissible error in terms of proportion (^{Montgomery, 2004}). With the above data the sample size of each mechanical or external damage was calculated.

Study 4. Effect of mechanical damage on postharvest

In order to evaluate the effect of the mechanical damages, avocado ‘Hass’ fruits with 24-26% of dry matter in the pulp were sampled, free of physical damages, with supreme quality (Secretaria de ^{Economia, 2006}), between 210-250 g. Four groups of fruits were made, to which mechanical damages were induced: group 1. Bruising (compression); group 2. Friction (friction); group 3. Punctures (cuts) and group 4. Without stalk (unclogged). As a control, a group of fruits without mechanical damage was included. After induction, the fruits were stored at room temperature (22 ° C ±2 °C and 85 ±10% relative humidity) until consumption maturity. During storage and consumption maturity, weight loss (%), days at maturity of consumption, external quality, dehydration, vascular obscuration, basal decay scales proposed by (^{White et al., 2005}) and day of detachment of peduncle.

Study 1. Mechanical damage in avocados during harvesting and transport to the baler

Study 2. Evaluation of external and mechanical damages in two harvest periods.

Study 3. Estimation of sample sizes of external and mechanical damages of ‘Hass’

The sample sizes calculated with the proportions of external and mechanical damages obtained in study 2, for each of the damages are shown in Table 7. The largest sample sizes, estimated for October crops, corresponded to damages by rubella, bruises and chicken pox with 374, 372 and 369 fruits per harvested batch, whereas, for April crops, the largest sample sizes were for rust, peduncle fall and bruises with 382,340 and 329 fruits per harvested batch.

Therefore, the sampling procedure suggested for the inspection and evaluation of external and mechanical damages in the avocado packer for export would be as follows: 1. Collect the maximum number of fruits indicated in Table 7; 2. The fruits should be collected with a simple random sampling, using the double-blind method, avoiding visual contact with the fruits; 3. Inspect each fruit until it reaches the number of fruits required for each external and mechanical damage. For example, in samplings for October crops, when harvesting the fruit 100 is stopped evaluating cuts and continuing with the remaining damages, by extracting the evaluated fruit 185, it will stop evaluating the nail and continue with the rest of the damages until reaching the variable rash with the maximum number of fruits; and 4.Capture the data and estimate the percentages of fruits with mechanical and external damages in the fruits of each batch that arrive to the packer for selection and packaging for export. With the data obtained, the packer can estimate the quality of each batch and can determine how to run the fruit in the machinery and distribution of the fruits with mechanical and external damages in the packing boxes.

A different and smaller sample size can be selected than those obtained in Table 7, sacrificing precision, reliability and maximum permissible error when estimating the proportions of the damages to the fruit.

Study 4. Effect of post-harvest mechanical damage

Analysis of variance for the quality variables in fruit maturity showed significant statistical differences between induced mechanical damages (p< 0.05). Fruit friction caused the greatest weight loss (21.1%), the rest of the induced mechanical damages and the control lost between 13 and 15% of weight. The fruits that took the most time to reach maturity of consumption were fruits with bruises, cuts and the witness, all with 13 days; this indicates that these mechanical damages had no effect on the ripening processes of the fruit. However, fruit with friction reached the maturity of consumption 11.6 days after harvest, two days before the rest of the treatments, being this mechanical damage that accelerates the time from maturity to consumption.

The damage due to the lack of peduncle as well as the control showed the lowest external quality of the fruit with values of 1.1 and 1.3, respectively, while friction damage obtained a value of 2.1, higher than those mentioned.

Dehydration showed no significant difference between treatments and did not exceed 5% as a result of these treatments. Friction and bruising significantly affected 20 and 12%, respectively, vascular darkening (Table 8). The basal rot was more affected in fruits with frictions and fruits without peduncle. The control fruits and those with bruises took 12 days to detach the peduncle. The fruits with punctures (10 days) and frictions (9 days) were, two or three days earlier in the peduncle detachment. This indicates that fruits with broken or damaged cells have lower quality at maturity of consumption.