Characterization of aroma-active compounds in delicious apples juices by gas chromatography-mass spectrometry (GC-MS), gas chromatography–olfactometry (GC-O) and odor activity value (OAV)

Volatile aroma compounds of delicious apple juice in three cultivars were obtained by gas chromatography-mass spectrometry (GC-MS), gas chromatography-olfactometry (GC-O), and GC-flame photometric detection (FPD). Quantitatively, the major volatiles of the delicious apple juice were detected by GC-MS, such as esters, alcohols, aldehydes. In addition, GC-O and OAV were used to determine the aroma-active compounds in fruit. Amongst these compounds, ethyl 2-methylbutanoate (47-229), butyl 2-methylbutanoate (8-208), (E)-2-hexenal (25-120), butyl propanoate (14-54), methyl 2-methylbutanoate (28-41), ethyl hexanoate (4-32), ethyl butanoate (5-17) showed high OAVs in three delicious apple juices, which contributed greatly to the aroma of delicious apple juice. Beside those compounds, methanethiol (OAV: 1.1-1.6), dimethyl sulfide (OAV: 2.5-3.6), methional (OAV: 4.2-11.7) and 2-(methylthio)ethanol (OAV: 1.2-1.9) also presented relatively high OAVs. Finally, four compounds (ethyl 2-methylbutanoate, ethyl octanoate, ethyl butanoate and ethyl hexanoate) were selected to investigate the possible interactions occurring in the delicious apple juice. The resultants demonstrated that those aroma volatile compounds can decrease threshold of the solution to dissimilar degrees before and after their addition.

volume was 0.1mL. The solution was diluted with water in multiples of 2. Ten volume 154 gradients were used. In this experiment, sigmoid curve was employed to determine 155 detection threshold (Lytra et al., 2013). Detection threshold is defined as the 156 corresponding volume when the correct detection probability was 50%. The change of 157 sensory intensity was also evaluated before and after the addition of ethyl 2-158 methylbutanoate, ethyl hexanoate, ethyl butanoate, and ethyl octanoate.  165 Before extraction, several parametres in SPME should be investigated, such as 166 sample amount, extraction time and extraction temperature. The amount of volatile 167 compounds absorbed on SPME fibers might be related to the amount of samples. 168 Normally, the larger the sample amount, the more volatile compounds absorbed by the 169 extraction fiber. However, too many samples could weaken volatilization of volatile 170 compounds. At the same time, the adsorption of fibers tends to be saturated, which was 171 not conducive to the extraction of volatile compounds (Rocha et al. 2001). In this 172 experiment, sample amounts of 3, 4, 5 and 6 g were investigated. The results showed 173 that the optimum extraction conditions was 5 g. respectively. The results demonstrated that the optimum extraction time was 45 minutes.

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Extraction temperature was also an important factor in SPME experiment. Volatile 181 compounds in fruits are particularly sensitive to temperature. It is easy to produce 182 various by-products during high temperature, especially sulfur compounds (Chung-183 May & Wang., 2000). Therefore, the temperature of 25, 30 and 35 °C were investigated.

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The results suggested that the volatile compounds increased with the extraction 185 temperature elevated. After comprehensive consideration, the extraction temperature 186 should be 30 °C. Based on these observations, the optimum conditions of HS-SPME 187 were determined, i.e. extraction time 45 minutes, sample volume 5 g and extraction 188 temperature 30 °C.

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The results of the GC-O analysis were listed in Table 1 Table 2, a total of nine sulfur compounds were identified in two 223 dissimilar columns by FPD. The calibration curves, determination coefficients, 224 validation range, LOD (μg/kg) and LOQ (μg/kg) were established in this experiment.

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According to the previous investigations, the contributions of compounds to global 231 aroma was not determined by content, but by OAV of compounds. Normally, the 232 compounds were considered as contributors to aroma with OAV above 1 (Guth, 1997).

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Dimethyl sulfide (DMS) was a volatile sulfur-containing compound, which was 237 widely found in foods (Fatima et al., 2010;Mestres et al., 2000). DMS was considered 238 as "asparagus, corn and molasses" aroma, which was beneficial to aroma profile of food   305 Sensory evaluation was performed by organoleptic assessments of quality of 306 three kinds of delicious apple juices using seven descriptors, including "wax", "fruity", 307 "sour", "green", "sweet", "sulfur" and "floral". ANOVA was employed to ensure 308 reliability of sensory data. As seen from Table 5, the significant intensity differences  Table 5, no significant interaction between panellist×samples was also 313 found. Thus, the sensory data was valid.

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The intensities of "wax", "sweet" and "green" in Y1 sample were greatly higher 315 than other two samples. The compounds closely related to "wax" descriptor were 2-316 methylpropanol, heptanol, hexanol and 2-methylbutanol. As described in GC-O, β-317 damascenone was the most important contributor to "sweet" descriptor in delicious 318 apple juices. The conclusion was consistent with other research which demonstrated 319 that β-damascenone considered as "sweet" descriptor in jujub (Zhu & Xiao., 2018). 320 Similarly, (E)-2-hexenal and hexanal contributed greatly to "green" note in delicious 321 apple juices. The Y2 sample was accompanied by the "fruity" and "floral" descriptors.

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As described in GC-O, ester compounds were responsible for "fruity" descriptor, which     Figure. 1. The aroma profiles of three delicious apple juices obtained from the Y1, Y2 and Y3 samples. The sensorial parameters indicated with an (*) are significantly Figure 2. The comparing of detection probability of reconstitution solution before and after adding ethyl 2-methyl butanoate, ethyl butanoate, ethyl hexanoate and ethyl