Effects of Film Mulching on Aroma Components of Pineapple Fruits

The aim of this study was to test the effects of film mulching treatment on aroma components of pineapple fruits. In this present study, the pineapple fruits of treatment and control (open field) were harvested twice (in winter and summer, respectively) to detect the aroma components. The results indicated that esters and alkenes were the main aroma components of both treatment and control pineapple fruits. There were more kinds of esters with higher total relative contents detected in the treatment fruits both harvested in winter and summer. There were fewer kinds of alkenes with lower total relative contents detected in the treatment fruits harvested in summer. There were equal kinds of alkenes detected in both treatment and control fruits harvested in winter. But the total relative content of alkenes of treatment fruits harvested in winter was lower than that of control. There was one ester containing sulfur, propanoic acid, 3-(methylthio)-, methyl ester with relative content of 1.40%, detected in the treatment fruits harvested in winter. No ester containing sulfur was detected in the control fruits harvested in winter. In the treatment fruits harvested in summer, two esters containing sulfur, propanoic acid, 3-(methylthio)-, methyl ester (2.60%) and 3-(methylthio) propanoic acid ethyl ester (1.46%) were detected. There was only one ester containing sulfur, propanoic acid, 3-(methylthio)-, methyl ester (3.01%) was detected in the control fruits harvested in summer.


Introduction
Pineapple [Ananas comosus (L.) Merr.], cultivated worldwide all around the tropical and subtropical regions for local consumption and international export, is the most important representative of the Bromeliaceae family.The pineapple fruit is consumed freshly by many people and used as source of essential mineral elements, nutrients and vitamins.Pineapple is an important fresh fruit, which is widely cultivated in a few provinces of southern China.According to the newly online databases from Ministry of Agriculture of P. R. China, China is one of the leading producers of pineapple with area 53500 ha and yields 933633 t in 2008.However, in most pineapple production areas of China, the growth of pineapple plant is retarded due to the seasonal drought and shortage of water during autumn-winter-spring seasons.
Film mulching is an important practice which is wildly used in crop production.Film mulching can modify the microclimate, reduce water evaporation from soil, and maintain the soil humidity (Tarara, 2000;Bu et al., 2006;Zhang et al., 2010).Historically, vegetable growers have employed plastic films for mulching to reduce the growth of weeds, soil erosion from wind or water, leaching of fertilizers, especially on light, sandy soils, and the development of plant diseases coming from the soil.Black polyethylene is the most widely used due to its excellent properties and low cost.It can improve the farming soil temperature, activate the soil nutrition and increase the nutrient availability of soil (Li et al., 2007).And it can efficiently inhibit the growth of weeds, promote the plants growth, and increased the yields and quality of crops (Mahmood et al., 2002;Sarker et al., 2003;Aniekwe et al., 2004;Bukun et al., 2005;Bhadauria & Kumar, 2006;Islam et al., 2007;Novak et al., 2007;Liu et al., 2009).So film mulching plays an important role in crop production, especially the arid and semi-arid crops (Zhang et al., 2007;Chakraborty et al., 2008;Wang et al., 2009;Zhang et al., 2010).But, there were also some reports indicated that film mulching had some drawbacks, such as speeding up the senescence of plants and depletion of soil nutrition (Wang et al., 2004;Zhai et al., 2008;Zhang et al., 2008).
Previous studies on pineapple plants indicated that film mulching could improve the soil ecological condition and promote the growth of pineapple plants.Liu et al. (2010) reported that film mulching could availably maintain water content of soil of pineapple orchard, and improve the ecological condition of soil near the pineapple roots.Film mulching could increase the water content and the organic matters in the soil, and increase the number of microorganism and enzyme activities of soil such as urease, acid phosphatase, catalase in autumn and winter.Liu et al. (2008) also reported that film mulching could increase some physiological properties, such as contents of chlorophyll, content of soluble sugar and protein of pineapple leaves and roots.However, the previous studies have neglected to investigate the effects of film mulching on the quality especially on fragrance of pineapple fruits.And few investigations have been focused on the effects of film mulching on aroma components of other fruits in previous reports.So in this study, we aim to test the aroma components of pineapple fruits affectted by film mulching.

Materials
In this study, the materials, pineapple cultivar 'Yueyinaoka' were growing in a commercial orchard located in zengcheng city (113°29′4"E~113°59′44"E, 23°4′42"N~23°37′20"N) Guangzhou, Guangdong, China.The study was designed with two treatments.One was black film mulching, and another was open field (control).Each treatment has area of about 200 m 2 (2 m×100 m).The pineapple plants were cultivated with planting density of 37500 plants/ha.The fertilizer managements for treatment and control plants were alike.The pineapple plants were induced for flowers with ethylene (500×).The sample fruits were harvested twice, on December 15, 2009 (winter) and August 31, 2010 (summer), respectively.The sample fruits harvested in winter were induced with ethylene for flowers in Jun.2009, and those harvested in summer were induced in Mar.2010.Each time, 3 fruits of treatment and control were harvested randomly from the treatment and control area, respectively.All the sample fruits for detection were sent to lab immediately after harvest.

Methods
Aroma components were analyzed by SPME/GC-MS.Two hundred and fifty grams of fresh pulp from the 3 fruits, after being grinded evenly, were removed into a 500 mL headspace vial.After sealing the vial, the pulp was extracted for 30 min using a solid-phase micro-extraction fiber (CAR/PDMS 75 μm).And then desorption for 3 min at 220 ℃ and sampling.

GC-MS conditions
Aroma components were analyzed with Trace GC-MS analysis made by Finnigan.The MS apparatus was equipped with HP-1 column (Length 30 m, Diameter 0.25 mm, Film thickness 0.1 μm).Column temperature maintained at 45 ℃ for 1.0 min.After that, the temperature was increased to 100 ℃ at a rate of 3 ℃• min -1 , then to 180℃ at a rate of 10 ℃• min -1 .At last, the column temperature was maintained at the final temperature for 5 min.He (99.99%, 1.0 mL•min -1 ) was used as the carrier gas.The MS was operated with an EI ion source with temperature of 170 ℃, electron energy 70 eV, and photomultiplier tube voltage of 350 V. Mass scanning range was 35~335 amu.
Aroma components were tentatively identified by comparing their mass spectrum with the mass spectra of NIST library.The relative content of each component was determined by their peak area.

Total ionic chromatogram of aroma components of pineapple fruits of treatment and control
The GC-MS total ionic chromatograms of aroma components of treatment and control fruits were presented in Fig. 1 and Fig. 2. The aroma components were ascertained by searching NIST, and the relative contents were counted based on the apex area.

Effects of film mulching on aroma components of pineapple fruits
In this present study, besides one kind of heterocycle was detected in the treatment fruits harvested in winter, esters and alkenes were the main aroma components of pineapple fruits harvested in both winter and summer (Table 1), which agreed with previous reports (Preston et al., 2003;Elss et al., 2005).
Previous researches indicated that film mulching can increase the fruit quality (Tachibana et al., 2009).Singh et al. (2007) reported that film mulching increased TSS content, acidity and ascorbic acid content of strawberry fruits.And also, Loughrin & Kasperbauer (2003) reported that film mulching increased the aroma compound concentration of fresh basil leaves, and for most aroma compounds, the highest concentrations were obtained from the leaves grown over black film.In this present study, the results indicated that there were more ester and fewer alkene components detected in the treatment fruits than control.Also film mulching treatment increased the total relative content of ester components and decreased the alkene components of pineapple fruits harvested in both winter and summer.Preston et al.(2003) reported that the esters containing sulfur such as propanoic acid, 3-(methylthio)-, methyl ester and 3-(methylthio) propanoic acid ethyl ester were the characteristic aroma components of pineapple fruits.In this study, there was one ester containing sulfur, propanoic acid, 3-(methylthio)-, methyl ester with relative content of 1.40%, detected in the film mulching fruits harvested in winter, and no ester containing sulfur was detected in the control fruits harvested in winter.In the film mulching treatment fruits harvested in summer, two esters containing sulfur, propanoic acid, 3-(methylthio)-, methyl ester (2.60%) and 3-(methylthio) propanoic acid ethyl ester (1.46%) were detected.There was only one ester containing sulfur, propanoic acid, 3-(methylthio)-, methyl ester (3.01%) was detected in the control fruits harvested in summer.From the above, this present study indicated that film mulching treatment increased the relative contents of characteristic aroma components of pineapple fruits.

Conclusion
In this study, esters and alkenes were the main aroma components of pineapple fruits.Film mulching treatment increased the number of ester aroma components of pineapple fruits harvested in winter and summer, and decreased the number of alkene aroma components harvested in summer.Film mulching increased the total relative content of ester components and decreased the alkene components of pineapple fruits harvested in winter and summer.Also film mulching increased the characteristic aroma components of pineapple fruits harvested in winter and summer.

Figure 1 .
Figure 1.Total GC-MS ionic chromatogram of aroma components of treatment and control fruits harvested in winter

Table 1 .
Aroma components and relative contents of treatment and control pineapple fruits harvested in winter and summer Note: -not found or not exist

Table 2 .
Aromatic categories and relative contents of film mulching treatment and control pineapple fruits harvested in winter and summer