Biological material

Avocado fruit from Nayarit, Michoacán and Jalisco was used. The climate of Nayarit is subhumid warm with an average temperature of 22 ºC, that of Michoacán is semi warm, sub -humid of group C, with an average annual temperature of 18 ºC. The climate of Jalisco is temperate and presents an average annual temperature between 12 ºC and 18 ºC (Table 1). The fruit was harvested in two consecutive years as indicated in Table 1 for each of the production sites.

In each production site, three lots of fruits were harvested in a green state at physiological maturity with at least 21.5% of dry matter and sent to the Functional Food laboratory of the Bajío Experimental Field located in Celaya, Guanajuato. In the laboratory, the 16 fruits of each of the three batches or replicates of each production site were divided into two groups. The first group was frozen at -82 °C, lyophilized and stored at -20 °C until analysis which will be identified hereafter as “green” fruit. The second group of fruits, “mature” fruit was stored at room temperature 12 to 15 days until the peel presented the characteristic black color of maturity for consumption which will be identified hereafter as “mature fruit”.

Dry material

The dry matter was determined by microwave oven according to ^{Lee and Coggins (1982)}. The 10 g of pulp were obtained by cutting the fruit lengthwise with a potato peel, the slices were placed in Petri dishes and dried in a microwave oven for 6 to 10 minutes until constant weight was obtained. The percentage of dry matter was calculated with the following formula: (fresh weight-dry weight/fresh weight)_*100.

Oil content

The oil content in the samples was determined using ^{AOAC method 920.85 (2000)}. The oil content was reported on a wet basis taking into account the moisture level presented by the fruit before being lyophilized.

Fatty acid profile

The fatty acid profile was obtained using the ^{AOCS method (2013)} using a mass-coupled gas chromatograph (Agilent Technologies, Inc. Santa Clara CA, USA, Models 6890N and 5973). An HP-88 column (100 m x 0.25 mm ID, 0.2 µm, 250 °C, 1 µL injection volume) was used, with hydrogen as carrier gas A and helium as carrier gas B; with constant flow of 2 mL min-1 and oven conditions A: 120 °C for 1 min, 10 °C min-1 175 °C for 10 min, 5 °Cmin-1 at 210 °C for 5 min, 5 °C min-1 at 230 °C for 5 min; and oven B: 175 °C for 10 min, 3 °C min-1 at 220 °C for 5 min.

Statistic analysis

A completely randomized experimental design was used, with the factors of variation being maturity and the year of harvest. A variance analysis was performed for each of the variables evaluated using the statistical package SAS for Windows V9.2 (^{SAS, 2008}). The comparison of means to determine statistical differences between all factors of variation and dry matter, oil contents, dry matter-oil ratio and acid degrees was performed with the Tukey test, with a level of significance p= 0.05 Duncan’s multiple range test was also performed with significance level p= 0.05 to identify significance between a specific fatty acid and the variation factors. Correlations were also made between the fatty acid profile and dry matter with different levels of significance.

Dry material

The dry matter content of avocado presented statistical differences between localities, maturity stage and year of harvest (Table 2). In harvest 1 (October 2015 - February 2016), all samples of mature fruit have a higher content of dry matter; on the other hand, the green and ripe fruit from the state of Jalisco contains a greater proportion of dry matter compared to the fruit of Nayarit and Michoacán. The fruit of the crop 2 presented the same behavior with respect to the mature fruit; however, there were no statistical differences in the dry matter content between localities when the levels present in the fruit of the three localities were compared.

Oil content

The oil content of the mature fruit from Nayarit, Michoacán and Jalisco was higher compared to the green fruit, in the harvest years (Table 2) . The oil content of the mature fruit harvested in harvest 1 was 3% in Nayarit and 13% in Jalisco; while in harvest 2 it was 4% and 7% in Nayarit and Jalisco, respectively. On the other hand, the oil content was up to 29% higher in the mature fruit from the state of Jalisco collected in both dates when compared to the oil content in the fruit of the rest of the localities (Table 2), this result suggests that the environmental conditions of Jalisco promote a greater accumulation of oil in the fruit.

It has been reported that as the mature avocado fruit, the oil content and the sensorial acceptance increase, but the moisture content decreases (^{Osuna et al., 2010}). The oil content as well as the effect of the locality on the oil content in the fruit were similar to that reported by ^{Pedro and Velásquez (2015)}. These authors indicate that the avocado ‘Hass’ harvested in Colombia presents from 12.9 to 17.6% of oil in mature fruit. Likewise, they report significant differences in oil content in fruits harvested in five different locations.

Dry matter/oil ratio

It has been reported that the dry matter content is directly proportional to the oil content, so the dry matter content can be used as an index of maturity (^{Lee et al., 1983}). In this sense, the correlation coefficient calculated between the dry matter content and the oil content for fruits of crop 1 and 2 were 0.87 and 0.71, respectively. This result agrees with that reported by ^{Cerdas et al. (2014)} for ‘Hass’ avocado, who point out that at 9% dry matter levels the oil content is only 4%, when the dry matter increases 20%, the oil content rises to 16%.

Fatty acids

The fatty acid profile of crop 1 and crop 2 are shown in Table 3 and 4. Where oleic acid was highest in all production sites and maturity stages with a content of 45.1% (Nayarit green fruit) to 59.7% (ripe fruit of Jalisco) at harvest 1 and at harvest 2 from 43.4% (Nayarit green fruit) to 56.7% (mature fruit from Michoacán). It has been reported that the content of oleic acid for avocado ‘Hass’ from Chile ranges from 57 to 61%, from Spain from 54 to 60% and from Peru from 40 to 47% (^{Donetti and Terry, 2014}).

These authors hypothesized that oleic acid may be a marker of origin; in view of the fact that the content of oleic acid for the fruits reported in this study ranges from 43.1 to 59.7%, then the Mexican avocado analyzed in this study can be placed in the middle of the fruit of Peru and that of Spain could serve as a marker of origin, at least for the three localities of production considered in this work.

On the other hand, palmitic acid was reduced by an average of 29% when it passed from green to mature in the fruits of the production sites in the avocado harvested in harvest 1 (Table 3). The same behavior was observed with palmitoleic acid from the same crop, which was reduced by 8% in fruits from Michoacán and 2% in Jalisco. In fruits from Nayarit increased 2% from green to mature in crop 1 (Table 3).

The same behavior was observed during harvest 2 (Table 4); however, in Nayarit the reduction of palmitic acid from green to mature was slightly higher (30%) compared to crop 1; while palmitoleic acid was reduced in the fruit of Michoacán and Jalisco 13%, being higher when compared to the fruit of the same locality of the crop 1. The reduction of the saturated fatty acids is highly desirable since both saturated fatty acids contribute to raising blood cholesterol in humans.

With respect to the oleic unsaturated fatty acid, it can be seen in Tables 3 and 4 that in all cases it increased significantly when the fruit went from the green to the mature state. In harvest 1 a level of 21% (fruit of Jalisco) was reached, while in crop 2, up to 21% in fruit from Michoacán. Linoleic acid was reduced by maturity in all analyzed samples from all localities and harvest. On the other hand, the content of palmitic and palmitoleic acid was lower at a higher altitude as can be seen in Tables 1, 3 and 4 except for the palmitoleic acid content of the green and ripe fruit from Jalisco at harvest 1. While that the content of oleic acid was higher under the agroclimatic conditions of Michoacán (semi- warm, sub-humid) and Jalisco (temperate). With regard to linoleic acid, the content of this acid was reduced 38% in crop 1 and 12% in crop 2 when passing the fruit from green to mature from Jalisco (Tables 3 and 4) . Changes in the composition of fatty acids due to maturity may be related to metabolism during fruit respiration in the tree (^{Meigh and Hulme, 1965}).

Figure 1 shows the effect of the harvest, planting location and maturity stage on the level of fatty acids and dry matter content. As can be observed, the palmitic, oleic and dry matter acids were not affected by Harvest; i.e. fruits harvested at the end of 2015 and early 2016 had the same content of these fatty acids as those harvested at the end of 2016. On the other hand, the environment has a significant effect on oleic acid content; the fruit harvested in Jalisco presents up to 10 to 14% more than the fruit from Nayarit and Michoacán (Figure 1). A similar behavior in the content of oleic acid occurs in the state of maturity; when the fruit is ripe ready for consumption, the avocado contains 14% more oleic acid than the green fruit.

Figure 1 Multi-range tests for the content of fatty acids (g 100 g-1 oil) and dry matter (%); a) harvest (n= 18),b) locality (n= 12) and c) (n= 18). Palm= palmitic; Palmito= palmitoleico; Linol= linoleic; MtSe= dry matter. Averages with equal letters in each fatty acid or dry matter content are not statistically similar (Duncan, 0.05). 

The degree of unsaturation of an oil is determined by the amount of oleic acid it contains, which is important from the point of view of health (^{Pedro and Velásquez, 2015}). It can be seen that in all samples analyzed, oleic acid is the predominant monoisaturated acid in avocado. It has been reported that oleic acid is the predominant fatty acid in avocado ‘Hass’ grown in Australia, Malaysia (^{Yanty et al., 2011}) and Colombia (^{Pedro and Velásquez, 2015}).However, the presence of oleic acid in the majority form is not the only factor desirable from the point of view of health. It has been shown that the greater the oleic/linoleic (O/L) ratio in avocado oil, the better the effect on health, particularly on cardiovascular problems (^{Hwang et al., 1997}).

Linoleic acid reduces the growth of human tumor cells (^{Maggiora et al., 2004}). So the presence of these unsaturated fatty acids in the “Hass” avocado has important significance on the nutrition and health of the general public. In this sense, all the mature samples analyzed showed an O/L ratio greater than 1, the mature fruit harvested in Michoacán and Jalisco in harvest 1 presented an O/L ratio of 5.9 and 5.7, respectively, whereas the mature fruit of the same localities collected in crop 2, has an O/L ratio of 4.9 and 3.6, respectively (Tables 3 and 4). The levels in the O/L ratio reported in this paper are higher than those reported by Donetty and Terry (2012) (2 - 3) for Hass avocado from South Africa and Peru and higher than those reported by ^{Pedro and Velásquez (2015)} for avocados grown in Colombia (2.78-5.1).

The relationship between fruit maturity and fatty acid content in avocado ‘Hass’ is an important factor to consider at the time of harvest, particularly when it is desired to relate to improvement in health (^{Pedro and Velásquez, 2015}). Since avocado does not require any postharvest processing, any attempt to make the fruit present the best quality in the fatty acid profile is vital if it is to be certified for its contribution to the good health of the consuming public. The content of oleic acid had a negative correlation with palmitic acid (p< 0.0001), while the dry matter level presented a positive correlation with palmitoleic (p< 0.02) and oleic (p< 0.01) acids and a negative correlation with linoleic acid (0.011) (Table 5).