Introduction

A lot of apple that is grown is stored for later marketing. From the same, a fraction is stored in refrigeration without atmosphere modification (RA), which keeps the fruit available to the market for a short period (^{Knee, 1993}), while another fraction is stored under refrigeration with controlled atmosphere (AC), to maintain the attributes of quality (firmness, color, acidity, etc.) for a longer time, compared to air-cooled storage (RA) ^{Karder (1986)}; ^{Dixon and Hewett, (2000)}. The storage with low concentrations of oxygen causes a reduction and modification of volatile compounds synthesized by fruits (^{López et al., 1998}; ^{López et al., 2000}; ^{Fellman et al., 2003}; ^{Echeverría et al., 2004}) which, at the end of the storage period, may result in apples having a satisfactory appearance and firmness, but with loss of their typical taste because the volatile compounds are of great importance, since they influence the characteristic flavor of each fruit.

In the apple, about 400 volatile compounds (CV) have been identified (^{Forney et al., 2009}). The synthesis of these compounds may be affected by several factors, including storage. The present study was carried out with apple ‛Red Delicious’, which is one of the varieties that are most cultivated in Chihuahua, Mexico (^{SAGARPA, 2015}). Because apples grown in this state are notable for their taste (^{Bismark 2002}), it is important to determine the impact of the storage period on RA and AC, on production of CV.

It was selected for this study, tree with 35-year-old, of the commercial orchard located in Cuauhtemoc, Chihuahua, Mexico (28° 23’ 51.43” north latitude, 106° 49’ 05.79” longitude west, at 2 062 m). The apples were harvested at day 176 after full bloom, when the internal ethylene concentration (CIE) was 0.3 mg L^-1. The harvested apples were stored at 0 °C in AC (2% O₂ and 3% CO₂), RA (21% O₂) and AC plus 7 days of RA (AC + RA). The composition and concentration of CV were evaluated at the time of harvest and after 1, 3, 5 and 7 months of storage. The CIE was determined according to ^{Salas et al. (2011)}. The CIE average was calculated from the analysis of 10 apples per variety. The concentration of CV was determined by gas chromatography coupled to mass spectrometry (Varian Saturn 2100D GC/MS), using solid phase microextraction (SPME) technique (^{Salas et al., 2011}).

For the statistical analysis of the data, we used a mixed model design, with storage conditions, period of storage and replication as fixed factors, and replication nested in the treatment as a random factor, was performed using statistical software SAS version 9 (SAS Institute, Cary, NC). The comparison of means by LSM p≤ 0.05 was performed.

In this study 44 volatile compounds (9 aldehydes: butanal, 2-methyl butanal, pentanal, cis 3-hexenal, hexanal, 2-hexenal, benzaldehyde, nonanal and decanal. 10 alcohols: 1-butanol, 2-methyl-1-propanol, 2 3-hexene-1- ol (Z), 2-hexen-1-ol (E), 1-hexanol, 1-heptanol, 2-ethyl- 1-hexanol and 1-octanol and 25 esters: ethyl acetate, ethyl propanoate, n-propyl acetate, methyl butanoate, 2-methyl propyl acetate, methyl 2-methyl butanoate, ethyl butanoate, propyl propanoate, butyl acetate, ethyl 2-methyl butanoate, 2-methyl Butyl acetate, ethyl pentanoate, butyl propanoate, pentyl acetate, 3-methyl buten-1-ol acetate, butyl butanoate, 3-hexen-1-ol acetate, hexyl acetate, butyl 2-methyl butanoate, propyl hexanoate, hexyl propanoate , Hexyl butyrate, ethyl octanoate, hexyl 2-methyl butyrate, hexyl hexanoate) were identified and quantified during storage of ‛Red Delicious’ apple. The most abundant CVs and their evolution are shown in Table 1.

Table 1 Most abundant volatile compounds (mg L^-1) in ‛Red Delicious’ apple under different storage conditions.  

The highest CV production is observed after the first month of storage in RA and AC. After 3 and 5 months of storage in RA, the concentration of CV remains high, in contrast to the concentration of CV in apple stored in the same period in CA, showing 49% and 59% respectively, less than in apple stored in RA (Figure 1).

Figure 1 Aldehydes, alcohols, esters and total volatile compounds in “Red Delicious” apple stored under different conditions. Values represent the mean of three replicates and the vertical bars represent ±ES. Means with different letters are significantly different p≤ 0.05 (LSM).  

At the time of harvest, 2-hexenal, cis-3-hexenal and 2-methyl-1-butanol comprise 91% of the total CV contained in the fruit. These aldehydes have a“green/acute” and “as-leaf” sensory description respectively (^{Rizzolo et al., 1989}; ^{Defilippi et al., 2009}) and 2-methyl butanol as “fruity” (^{Dimick and Hoskin, 1981}). Whereas, after the first month of RA storage, apples mainly produced 2-methyl 1-butanol, butyl acetate, 2-methyl butyl acetate, 2-hexenal and cis-3-hexenal, these compounds accounted for 95% of the total CV.

The apples stored in AC mainly produced 2-methyl 1-butanol, ethyl acetate, 2-hexenal, ethyl butanoate and ethyl 2-methyl butanoate, representing 87% of the total CV. The apples with the AC+RA treatment had the same compounds as those stored in RA: 2-methyl 1-butanol, 2-methyl butyl acetate, butyl acetate, cis-3-hexenal and 2-hexenal, representing 89% of the total CV. The results of apple stored with the AC+AR treatment showed a significant decrease in the concentrations of ethyl acetate, ethyl butanoate and ethyl 2-methyl butanoate, reducing the concentration of these compounds by approximately 99% compared to apples stored in AC. There is also a recovery in the concentration of compounds such as butyl acetate and 2-methyl butyl acetate of 86 and 65% respectively.

This result suggests that the AC+RA treatment was not adequate for the production of volatile compounds to increase in the post-AC storage period; this may be due to a residual effect, on CV production, after storage of AC apples. These results indicate a likely relationship between inhibition of respiration and alteration of the production of volatile compounds especially after storage under AC conditions that inhibit overdriving. ^{Streif and Bangerth (1988)} reported residual respiratory depression and decreased ethylene production as a result of long-term AC storage. For what is suggested, the evaluation of different post-storage periods in controlled atmospheres at different temperatures.

After the third month of storage the apples in RA mainly produce butyl acetate, 2-methyl 1-butanol, 2-methyl butyl acetate, 2-hexenal and 1-hexanol which represents 96% of the total CV, whereas apples in AC, mainly produce 2-methyl-1-butanol, 2-hexenal, cis-3-hexenal, butyl acetate and hexanal, which comprise 94%. And apples with the AC+RA treatment mainly produce 2-methyl 1-butanol, cis-3-hexenal, 2-methyl butyl acetate, 2-hexenal, 1-hexanol and butyl acetate (Table 1).

These results show that storage in AC generated a reduction in the concentration of volatile compounds in apple “Red Delicious”; this reduction in CV production during AC storage has been reported in other varieties such as como ‛Redchief Delicious’ and ‘Jonagold’ apples (^{Fellman et al., 2003} and ^{Saquet et al., 2003}).

In addition to the above, the results showed that the CV profile of “Red Delicious” apples stored in AC, is different from that of apples stored in RA. After the first month of storage, mainly esters were produced, which has been described as “ethereal fruit” and “sweet fruit”; Such as butyl acetate and 2-methyl butyl acetate (^{Yahia 1994}). For its part, apples stored in AC produce high concentrations of ethyl esters such as ethyl acetate, ethyl butanoate and ethyl 2-methyl butanoate, which have been described as “brandy”, “intense” and “odor spicy”, respectively (Yahia, 1994). In particular, ethyl acetate is an indicator of possible generation of undesirable odors during storage in CA (^{Lara et al., 2006}), induced by anaerobic respiration in the fruit, reported by ^{Mattheis et al. (1991)}.

As can be seen, there is a decrease in the synthesis of butyl acetate in “Red Delicious” apples stored in AC, this compound is considered one of the main volatile compounds that contribute to the aroma and flavor of most varieties of apple (^{Fellman et al., 2000}). The same decrease in the concentration of butyl acetate has been observed in other varieties stored in CA, such as: Golden Delicious”, “Redchief Delicious, “Royal Gala”, “Pink Lady” (^{Fellman et al., 2003}; ^{Moya-León et al., 2007}; ^{López et al., 2007}; ^{Harb et al., 2008})

Conclusions

The storage in controlled atmospheres exerts a significant influence on the profile and on the total production of volatile compounds of the “Red Delicious” apple aroma stored after 3 months. In order to provide a better flavor quality, for a longer period of time, regulations for storage conditions of controlled atmospheres should be evaluated. On the other hand, the low response to the post-storage treatment in controlled atmosphere, to achieve an increase in the production of volatile compounds, in this study, may be due to the temperature. It is suggested to evaluate different periods and temperature after storage in a controlled atmosphere.