Original papers
Effect of filter cloths on the relationship between fatty acids and ethyl esters during automatic pressing of sake
2021 Volume 27 Issue 3 Pages 417-428
Details
2021 Volume 27 Issue 3 Pages 417-428
Japanese sake is manufactured via the moromi (fermentation mash)-filtration process using filter cloths. We measured volatile compounds accumulated in used filter cloths and detected many esters, including fatty acid ethyl esters (FAEEs), suggesting that filter cloths adsorb esters from sake. The profile of volatile compounds trapped in filter cloths differed depending on the material. Moreover, we demonstrated that medium and long-chain FAEEs, but not fatty acids (FAs), were decreased in the initial period of practical automatic pressing. These results indicated that medium and long-chain FAEEs are likely to be adsorbed to filter cloths made of polyethylene terephthalate, which is generally used in automatic pressing. The ratio of FAs to FAEEs with 8 and 10 carbons increased after automatic pressing. This is possibly one reason why organoleptic properties are changed after moromi-filtration.
Japanese sake is a traditional alcoholic beverage containing various flavor compounds, most of which are mainly produced by yeast during mash fermentation. After fermentation, the moromi (fermentation mash) is separated into sake and sake cake using filter cloths made of polyethylene terephthalate (PET), polyvinyl chloride (PVC), or cotton. It is known that organoleptic properties in sake change through the moromi-filtration process, especially automatic pressing. Some flavor compounds could diffuse into the air or adsorb to the sake cake and other materials such as the filter cloth (Yoshizawa, 1966a; Yoshizawa et al., 1966b; Ito, 1987) and activated carbon (Nagai et al., 2015). Although filter cloths are washed after moromi-filtration for reuse, once certain compounds, especially those consisting of hydrophobic compounds, are trapped in the filter cloths, they tend to accumulate. However, the types of compounds that accumulate in the filter cloths are unclear.
In automatic pressing, sake is filtrated using layered boards covered with filter cloths. The automatic pressing process is divided into three steps; the initial, middle, and final periods. In the initial period, the moromi is filtrated with no or low pressure and the sake cake layer is very thin. Thus, the sake cake is considered to have few effects on the adsorption of some flavor compounds in the initial period of the automatic pressing process. Pressure is gradually increased during the middle period, and high pressure is maintained throughout the final period; and the sake cake layer grows as the pressure increases. Although it is known that the flavor compounds in sake obtained from each period are different, they are mixed to produce sake as a final product. The main factors that affect flavor compounds in sake, including the sake cake, filter cloths, and pressure, change in each period of the automatic pressing process, and the effect of each factor on flavor compounds in sake has not been clarified by analysis of mixed sake. Therefore, only a few studies have reported on the factors which affect flavor compounds in sake (Yamashita et al., 1983; Nagai et al., 2020).
It has been reported that the contents of high-volatility compounds such as ethyl acetate, 1-propanol, 2-methyl-1-propanol (isobutanol), and 3-methyl-1-butanol (isoamyl alcohol) in sake were not changed before and after moromi-filtration (Iino and Watanabe, 1989). Moreover, it has also been reported that most fatty acid ethyl esters (FAEEs) with greater than 8 carbons were adsorbed to the sake cake through automatic pressing but major alcohols and acetates were not (Yamashita et al., 1983). At the time of those reports, the contents of FAEEs with greater than 8 carbons in sake were relatively low compared to more recent sake production. In addition, the influence of filter cloths on esters was not taken into consideration in the reports. These long-chain FAEEs are considered to be adsorbed to the sake cake. However, contemporary sake has large amounts of fatty acids (FAs) and FA esters because of the recent trend for many flavor compounds produced by yeast. Therefore, it is necessary to consider the influence of filter cloths on these flavor compounds.
The addition of medium-chain FAs (4–10 carbons) degraded the organoleptic properties of sake (Yamane et al., 1997). Further, Takahashi et al. reported on the organoleptic properties imparted by medium-chain FAs (especially hexanoic acid and octanoic acid) to ginjo using simultaneous analysis of medium-chain FAs and FAEEs (Takahashi and Goto-Yamamoto, 2011; Takahashi et al., 2014). Moreover, it has been reported that the contents of FAs do not change during automatic pressing; however, except for hexanoic acid and octanoic acid, FAs were not detected in the sake examined (Kuribayashi et al., 2012). The balance between FAs and FAEEs is very important for the organoleptic properties of sake. Recently, Nagai et al. reported that FAs in sake increased with increasing moromi-filtration pressure (Nagai et al., 2020). However, it remains unclear how the amounts of FAs and FAEEs and their balance are altered by filter cloths during the moromi-filtration process.
We attempted to measure volatile compounds trapped in used filter cloths and showed that many esters were detected in the used filter cloths. This indicated that many esters adsorbed to and remained in the used filter cloths. Therefore, we focused on analyzing changes in the relationship between FAs and FAEEs before and after practical automatic pressing.
Sample preparation For the analysis of used filter cloths, 18 filter cloths were provided from sake breweries in Okayama Prefecture. These had been used many times for moromi-filtration of various types of sake, washed with detergents and warm water (under 50 °C), dried in the shade, and stored. Although the manufacturers or model numbers of the filter cloths are unclear, we confirmed the materials of the filter cloths by Fourier transform infrared spectroscopy. Six, eight, and four filter cloths were made of cotton, PET, and PVC, respectively. Each cloth was cut into 5 cm squares, and further cut into 0.5 × 2.5 cm rectangles and placed in vials. For the analysis of sake, sake samples before and after practical automatic pressing were provided by sake breweries in Okayama Prefecture. These sake samples were separated from moromi using different cloths in the analysis of used filter cloths. The samples were obtained immediately before practical automatic pressing or in the initial period of practical automatic pressing at a low temperature. Sake samples before practical automatic pressing (moromi) were centrifuged at 4 500 × g for 15 min to eliminate solids. As shown in Table 1, 4 types of sake, daiginjo,jummai-ginjo,honjozo, and ordinary sake, were used.
| Sample | Material (polishing rate) | Yeast | Addition of alcohol | Concentration of ethanol before pressing (%) | Brix before pressing (%) | Percentage of sake cake to total rice weight (%) |
|---|---|---|---|---|---|---|
| Daiginjo | ||||||
| 1 | Yamadanishiki (35 %) |
K1801/K901 | + | 17.1 | 12.4 | 45.5 |
| 2 | Yamadanishiki (35 %) |
K1801/K901 | + | 16.0 | 12.0 | 54.3 |
| Jummai-ginjo | ||||||
| 1 | Omachi (60 %) |
K1801 | − | 17.8 | 12.3 | 45.3 |
| Honjozo | ||||||
| 1 | Natsuhikari (65%) |
Ginjo type | + | 18.0 | 12.8 | 29.2 |
| 2 | Akebono (70 %) |
K1401 | + | 20.1 | 13.0 | 29.0 |
| Ordinary sake | ||||||
| 1 | Gohyakumangoku (70 %) |
K7 | + | 17.0 | 14.3 | 30.0 |
Analysis of volatile compounds in filter cloths Sampling from the rectangular cloths in the vials by dynamic headspace (DHS) was conducted with a GERSTEL MPS2 autosampler equipped with a DHS module (GERSTEL, Mülheim an der Ruhr, Germany). Volatile compounds were trapped on the Tenax TA tube by DHS sampling at 80 °C. The tube was desorbed with the thermal desorption unit (TDU) and concentrated in the pre-cooled cooled injection system (CIS) 4 programmed temperature vaporizing (PTV) inlet. Compounds trapped in CIS4 were injected onto the analytical column (Agilent DB1, 30 m × 0.25 mm i.d., 1 µm film thickness; Agilent, Santa Clara, CA, USA). The injection to a gas chromatograph (GC, Agilent 7890B; Agilent) and subsequent mass-spectrometry (MS, Agilent 5977B; Agilent) were performed with the low split option (GERSTEL). The oven temperature program began at 40 °C for 8 min, was increased to 100 °C at 5 °C/min, further increased to 260 °C at 10 °C/min and then maintained for 14 min. Helium was used as a carrier gas at a constant flow rate of 2 mL/min. Mass spectra were scanned from m/z 29 to 350 under the following conditions: electron impact energy, 70 eV; ion source temperature, 230 °C; mass quadrupole temperature, 150 °C. Detected compounds were identified by matching with the NIST14.L spectrum database, and their retention indices (RI) were calculated using C7–C24 n-alkanes. GC-MS analysis was performed 3 times per sample. All data sets were processed with automated mass spectral deconvolution and identification system v.2.72 (Agilent). Entities were created with Mass Profiler Professional v.14.9 (Agilent) from the obtained compounds. Toluene-D8 was used as an internal standard.
Analysis of volatile compounds in sake Sampling from 100 µL of sake in a vial by the 2 step multi-volatile method combined with DHS and full evaporation DHS (FEDHS) was conducted with a GERSTEL MPS2 autosampler equipped with a DHS module (GERSTEL). High-volatility compounds were trapped on the Carbopack B&X tube by the first DHS sampling at 25 °C. A subsequent dry purge of the trap results in the breakthrough of ethanol. Low-volatility compounds were trapped on the Tenax TA tube by the second FEDHS sampling at 80 °C. The two tubes were sequentially desorbed with TDU in reverse order of the DHS sampling and concentrated in the pre-cooled CIS4 PTV inlet. Compounds trapped in CIS4 were injected onto the analytical column (Agilent DB1, 30 m × 0.25 mm i.d., 1 µm film thickness; Agilent). The analysis condition of GC-MS and identification method of compounds were similar to the analysis of filter cloths. Methyl hexanoate was used as an internal standard. For semi-quantification of each compound, individual ion chromatograms were extracted from the total ion chromatogram using the selected ions shown in Table 2. The content of each compound was estimated from the proportion of the selected ion intensity to the total intensity with NIST MS search v.2.2. GC-MS analysis was performed 3 times per sample.
| Abbreviation | Selected ion (m/z) | Relative proportion to total ion intensity | |
|---|---|---|---|
| FA | |||
| Acetic acid | C2:0 | 60 | 0.223 |
| Propanoic acid | C3:0 | 74 | 0.147 |
| Butanoic acid | C4:0 | 60 | 0.399 |
| Hexanoic acid | C6:0 | 60 | 0.307 |
| Octanoic acid | C8:0 | 60 | 0.189 |
| Nonanoic acid | C9:0 | 60 | 0.177 |
| Decanoic acid | C10:0 | 60 | 0.143 |
| Dodecanoic acid | C12:0 | 60 | 0.105 |
| Tetradecanoic acid | C14:0 | 60 | 0.085 |
| Hexadecanoic acid | C16:0 | 60 | 0.088 |
| Octadecanoic acid | C18:0 | 60 | 0.070 |
| FAEE | |||
| Ethyl acetate | C2:0EE | 88 | 0.036 |
| Ethyl propanoate | C3:0EE | 57 | 0.220 |
| Ethyl butanoate | C4:0EE | 88 | 0.091 |
| Ethyl hexanoate | C6:0EE | 88 | 0.114 |
| Ethyl octanoate | C8:0EE | 88 | 0.221 |
| Ethyl decanoate | C10:0EE | 88 | 0.221 |
| Ethyl dodecanoate | C12:0EE | 88 | 0.201 |
| Ethyl tetradecanoate | C14:0EE | 88 | 0.233 |
| Ethyl hexadecanoate | C16:0EE | 88 | 0.179 |
| Ethyl octadecanoate | C18:0EE | 88 | 0.168 |
| Ethyl oleate | C18:1EE | 88 | 0.030 |
| Ethyl linoleate | C18:2EE | 67 | 0.081 |
| Other | |||
| Ethyl 3-hydroxybutanoate | 88 | 0.069 | |
| n-Propyl acetate | 43 | 0.521 | |
| 2-Methylpropyl acetate | 43 | 0.331 | |
| 3-Methylbutyl acetate | 43 | 0.281 | |
| 2-Hydroxyethyl propanoate | 45 | 0.685 | |
| 2-Phenylethyl acetate | 104 | 0.399 | |
| 2-Phenylethyl hexanoate | 104 | 0.291 | |
| Internal standard | |||
| Methyl hexanoate | 74 | 0.263 |
Statistical analysis Hierarchical cluster analysis with Euclidean distance and Ward's method was performed using Mass Profiler Professional v.14.9 (Agilent). Data for sake are presented as mean ± standard deviation (SD). Statistical analysis for volatile components in sake was performed using a Student's paired t-test.
Volatile compounds trapped in filter cloths We initially focused on the effect of filter cloths on changes in volatile compounds during the moromi-filtration process; thus, volatile compounds adsorbed to the used filter cloths were measured. Since the filter cloths used in this study were washed, we presumed that compounds that strongly adsorbed to the used filter cloths would be detected. In addition, we considered that volatile compounds that adsorbed to the sake cake were negligible, as the sake cake was removed from the filter cloths by washing. Compounds volatilized from the used filter cloths were analyzed, and more than 400 components in total were detected. Among them, 71 components with high intensity were selected, followed by the selection of 58 components showing significant differences among any components. Moreover, 40 of the components identified by RI or the NIST library were selected as components trapped in the used filter cloths and further analyzed. As a result of cluster analysis, samples were classified according to the cloth materials and a Euclidian distance of 100 was used to divide the samples into three clusters, PET, PVC, and cotton (Fig. 1). The Euclidian distance between the clusters of PET and PVC was close, while the cluster of cotton was obviously distant. This indicated that the profile of volatile compounds trapped in the used filter cloths was different depending on the material. A greater amount and variety of volatile compounds were trapped in PET and PVC than in cotton. Of the 40 components analyzed by cluster analysis, 18 esters, 7 aldehydes, 5 benzene derivatives, 5 alkanes, 4 ketones, and 1 alcohol were found. Among them, esters tended to be detected more often and with higher intensity than the other compounds. Moreover, among esters, FAEEs with 6, 8, 10, 12, 14, and 16 carbons were detected with higher intensity in PET and PVC than in cotton. 3-Methylbutyl acetate, an aroma of ginjo, was also detected with higher intensity in PET and PVC than in cotton. On the other hand, among aldehydes, nonanal and hexanal were the primary aldehydes in all cloths. Toluene, a benzene derivative, was detected with high intensity in PET and PVC but not in cotton. Aldehydes and benzene derivatives in the filter cloths could be generated due to some unknown reaction during washing, drying, or storage but not from the moromi-filtration process, since few of these compounds are contained in sake. Medium and long-chain FAEEs, which have hydrophobic properties, would be more easily adsorbed to PET and PVC than other compounds during the moromi-filtration process, since many amounts of medium and long-chain FAEEs were contained in PET and PVC, although some of them were also contained in cotton. In automatic pressing, PET is generally used as a filter cloth. Therefore, manufacturers should consider the influence of PET on FAEEs in sake during the moromi-filtration process.
Heatmap of hierarchical cluster analysis of volatile compounds adsorbed in filter cloths. Oneway ANOVA statistically analyzed volatile compounds in filter cloths and only those which were significantly different at p < 0.05 were represented. Row and column represent filter cloths and volatile compounds, respectively. Dendrogram scales indicate Euclidean distance. Color scale (log2) represents the relative intensity of each volatile compound against internal standard, with red indicating high intensity and blue indicating low intensity.
Volatile compounds in sake before and after automatic pressing We observed that many volatile compounds were trapped and adsorbed by PET compared with the other used filter cloths; therefore, volatile compounds in 4 types of sake obtained before and after practical automatic pressing with a PET membrane were analyzed using the FEDHS method. The FEDHS method enables us to simultaneously measure from low- to high-volatility compounds. In automatic pressing, pressure is not initially applied, but increases gradually. Accompanying the pressure increase, the sake cake layer gradually thickened. Therefore, we analyzed sake in the initial period of practical automatic pressing to eliminate the effect of the sake cake as much as possible. FAs and FAEEs were semi-quantitatively evaluated by the relative areas of their ion chromatograms against internal standards. As shown in Tables 3–6, many esters such as long-chain FAEEs (up to 18 carbons) were detected. In addition, FAs (2–18 carbons) were also detected despite their hydrophobic and low volatility properties. Even-numbered FAs and FAEEs were detected in most samples. However, the amounts of FAs and FAEEs with 12, 14, and 18 carbons were relatively low compared to other FAs and FAEEs. Ethyl oleate and ethyl linoleate were also detected, while oleic acid and linoleic acid were not detected because of the low content. FAEEs with 2 carbons were detected with low intensity, although sake is generally considered to contain abundant FAEEs with 2 carbons. Among odd-numbered FAs, FAs with 3 and 9 carbons were detected, while FAs with 5 and 7 carbons were detected with little intensity (Tables 3–6). Among odd-numbered FAEEs, FAEEs with 3 carbons were detected, while FAEEs with 5, 7, and 9 carbons were detected with little intensity (Tables 3–6). FAs and FAEEs with 11, 13, 15, and 17 carbons were nearly absent in sake. As shown in Table 3, many FAs and FAEEs were contained in daiginjo; especially, FAs and FAEEs with 6 carbons were significantly abundant, as reported previously (Ichikawa et al., 1991; Aritomi et al., 2004). Most of the volatile compounds analyzed in this study tended to decrease after practical automatic pressing. FAEEs including ethyl oleate and ethyl linoleate decreased after practical automatic pressing. The residue ratio of FAEEs after practical automatic pressing showed that FAEEs with more long carbon chains showed a more noticeable tendency to decrease. In contrast, FAs were not decreased compared with FAEEs, although there were some exceptions. Other esters, including branched-chain FAEEs except for 2-phenylethyl hexanoate, were not drastically decreased compared with FAEEs (Table 3). Jummai-ginjo also contained many FAs and FAEEs, but not as much as daiginjo (Table 4). In contrast to daiginjo, there was little change in volatile compounds of jummai-ginjo after practical automatic pressing. Contents of FAs and FAEEs in honjozo tended to be low compared with those in daiginjo and jummai-ginjo (Table 5). FAEEs with greater than 8 carbons, including ethyl oleate and ethyl linoleate, significantly decreased after automatic pressing. The residue ratio of FAEEs after practical automatic pressing showed a similar tendency to daiginjo. The contents of FAs and FAEEs in ordinary sake also tended to be low compared with those in other types of sake (Table 6). The contents of ethyl octadecanoate, ethyl oleate, and ethyl linoleate in ordinary sake were obviously higher than in other types of sake. FAEEs with greater than 10 carbons were significantly decreased after automatic pressing. In contrast to the other types of sake, FAs in ordinary sake tended to be decreased after automatic pressing, although they showed lower contents than in the other types of sake. The ratios of medium and long-chain FAs to FAEEs were calculated. As a result, in any sake samples except ordinary sake, the ratios of FAs to FAEEs with 8 and 10 carbons increased after automatic pressing.
| 1 | 2 | ||||||
|---|---|---|---|---|---|---|---|
| before | after | Residue (%) | before | after | Residue (%) | ||
| FA | |||||||
| C2:0 | 100 ± 17 | 110 ± 5.0 | 111.0 | 240 ± 10 | 220 ± 28.0 | 91.0 | |
| C3:0 | 13 ± 1.5 | 10 ± 1.4 | 78.9 | 21 ± 0.22 | 28 ± 2.9 | 123.3 | |
| C4:0 | 17 ± 1.4 | 13 ± 1.8 | 77.4 | 23 ± 2.2 | 16 ± 4.3 | 67.3 | |
| C6:0 | 610 ± 73 | 440 ± 77 | 72.8 | 460 ± 40 | 540 ± 58 | 117.5 | |
| C8:0 | 190 ± 22 | 120 ± 19 | 64.1 | 140 ± 9.5 | 170 ± 18 | 118.2 | |
| C9:0 | 65 ± 8.3 | 47 ± 6.6 | 72.0 | 9.2 ± 1.1 | 12 ± 2.2 | 132.5 | |
| C10:0 | 29 ± 2.8 | 10 ± 1.9 * | 35.5 | 12 ± 1.9 | 7.7 ± 1.7 | 62.5 | |
| C12:0 | 2.7 ± 0.33 | 1.1 ± 0.19 * | 39.4 | 0.31 ± 0.11 | 0.17 ± 0.065 | 53.4 | |
| C14:0 | 4.7 ± 1.4 | 0.66 ± 0.28 | 14.1 | 0.27 ± 0.031 | 0.26 ± 0.10 | 97.4 | |
| C16:0 | 35 ± 19 | 6.1 ± 2.6 | 17.3 | 1.7 ± 1.0 | 0.41 ± 0.13 | 24.5 | |
| C18:0 | 1.1 ± 0.39 | N.D. | N.D. | N.D. | |||
| FAEE | |||||||
| C2:0EE | 0.88 ± 0.12 | 1.0 ± 0.059 | 109.4 | 14 ± 6.2 | 3.6 ± 2.2 | 25.8 | |
| C3:0EE | 9.3 ± 1.0 | 7.1 ± 0.67 | 76.5 | 16 ± 1.0 | 11 ± 0.68 ** | 68.9 | |
| C4:0EE | 20 ± 2.4 | 16 ± 1.8 | 78.6 | 25 ± 1.3 | 18 ± 0.84 ** | 72.9 | |
| C6:0EE | 260 ± 34 | 180 ± 11 | 68.2 | 250 ± 3.0 | 180 ± 4.1 ** | 71.9 | |
| C8:0EE | 59 ± 7.7 | 22 ± 0.81 * | 36.5 | 37 ± 4.1 | 22 ± 1.5 * | 58.4 | |
| C10:0EE | 13 ± 1.5 | 1.9 ± 0.073 * | 14.6 | 6.5 ± 1.1 | 3.3 ± 0.32 * | 49.9 | |
| C12:0EE | 1.8 ± 0.33 | 0.75 ± 0.26 | 40.8 | 1.3 ± 0.028 | 0.89 ± 0.13 * | 67.2 | |
| C14:0EE | 2.2 ± 0.70 | 0.60 ± 0.33 | 27.7 | 7.1 ± 0.39 | 3.8 ± 0.43 ** | 53.4 | |
| C16:0EE | 18 ± 7.0 | 4.1 ± 1.3 | 23.3 | 31 ± 1.7 | 7.7 ± 1.3 ** | 24.6 | |
| C18:0EE | 1.5 ± 0.35 | 0.26 ± 0.057 * | 17.8 | 1.3 ± 0.12 | 0.52 ± 0.088 * | 40.0 | |
| C18:1EE | 6.4 ± 1.7 | 0.63 ± 0.033 * | 9.9 | 1.8 ± 0.75 | 0.83 ± 0.20 | 46.3 | |
| C18:2EE | 3.7 ± 0.90 | 0.48 ± 0.058 * | 13.1 | 0.33 ± 0.15 | 0.23 ± 0.038 | 68.5 | |
| Other | |||||||
| Ethyl 3-hydroxybutanoate | 6.5 ± 0.66 | 6.6 ± 0.14 | 101.3 | 24 ± 1.1 | 24 ± 1.7 | 99.7 | |
| n-Propyl acetate | 1.1 ± 0.11 | 0.85 ± 0.087 | 79.0 | 2.8 ± 0.28 | 1.6 ± 0.22 ** | 59.4 | |
| 2-Methylpropyl acetate | 3.3 ± 0.30 | 2.9 ± 0.27 | 87.2 | 7.0 ± 0.25 | 4.4 ± 0.22 ** | 62.5 | |
| 3-Methylbutyl acetate | 34 ± 4.7 | 27 ± 2.2 | 79.3 | 59 ± 1.3 | 40 ± 0.54 ** | 68.8 | |
| 2-Hydroxyethyl propanoate | 32 ± 1.9 | 27 ± 2.5 | 85.2 | 100 ± 4.0 | 101 ± 22 | 100.7 | |
| 2-Phenylethyl acetate | 36 ± 3.3 | 28 ± 1.7 * | 77.9 | 73 ± 4.5 | 70 ± 3.8 | 95.8 | |
| 2-Phenylethyl hexanoate | 10 ± 0.81 | 1.5 ± 0.26 ** | 16.0 | 6.1 ± 0.46 | 3.1 ± 0.78 * | 49.8 | |
Data are presented as mean ± SD by × 10−2.
N.D. means not detected.
| 1 | ||||
|---|---|---|---|---|
| before | after | Residue (%) | ||
| FA | ||||
| C2:0 | 29 ± 12 | 27 ± 4.9 | 90.6 | |
| C3:0 | 1.9 ± 0.34 | 2.2 ± 0.69 | 117.4 | |
| C4:0 | 3.7 ± 1.3 | 4.8 ± 0.92 | 128.3 | |
| C6:0 | 270 ± 25 | 320 ± 50 | 116.7 | |
| C8:0 | 29 ± 17 | 41 ± 8.9 | 144.5 | |
| C9:0 | 3.8 ± 2.7 | 5.6 ± 2.0 | ||
| C10:0 | 1.0 ± 1.1 | 1.4 ± 1.1 | 144.3 | |
| C12:0 | N.D. | N.D. | ||
| C14:0 | N.D. | N.D. | ||
| C16:0 | 0.46 ± 0.18 | 0.51 ± 0.15 | 111.5 | |
| C18:0 | N.D. | N.D. | ||
| C2:0EE | 1.2 ± 0.015 | 1.5 ± 0.16 | 125.8 | |
| C3:0EE | 15 ± 1.4 | 16 ± 2.7 | 109.9 | |
| C4:0EE | 22 ± 1.5 | 24 ± 4.1 | 111.2 | |
| C6:0EE | 199 ± 6.9 | 190 ± 28 | 97.5 | |
| C8:0EE | 40 ± 3.2 | 33 ± 5.9 | 82.2 | |
| C10:0EE | 8.2 ± 0.52 | 5.5 ± 1.0 | 67.1 | |
| C12:0EE | 1.0 ± 0.25 | 0.68 ± 0.038 | 70.3 | |
| C14:0EE | 0.46 ± 0.067 | 0.38 ± 0.031 | 82.2 | |
| C16:0EE | 3.9 ± 0.69 | 2.8 ± 0.080 | 71.7 | |
| C18:0EE | 0.16 ± 0.017 | N.D. | ||
| C18:1EE | 0.57 ± 0.11 | 0.46 ± 0.10 | 81.3 | |
| C18:2EE | 0.24 ± 0.017 | 0.37 ± 0.033 | 155.8 | |
| Other | ||||
| Ethyl 3-hydroxybutanoate | 18 ± 0.12 | 20 ± 2.3 | 108.0 | |
| n-Propyl acetate | 1.4 ± 0.12 | 1.7 ± 0.36 | 116.7 | |
| 2-Methylpropyl acetate | 2.7 ± 0.60 | 2.7 ± 0.61 | 99.7 | |
| 3-Methylbutyl acetate | 33 ± 1.4 | 34 ± 6.2 | 105.7 | |
| 2-Hydroxyethyl propanoate | 58 ± 0.61 | 62 ± 8.1 | 107.1 | |
| 2-Phenylethyl acetate | 42 ± 1.9 | 39 ± 5.7 | 93.1 | |
| 2-Phenylethyl hexanoate | 5.9 ± 0.11 | 3.0 ± 0.44 | 50.3 | |
Data are presented as mean ± SD by × 10−2.
N.D. means not detected.
| 1 | 2 | ||||||
|---|---|---|---|---|---|---|---|
| before | after | Residue (%) | before | after | Residue (%) | ||
| FA | |||||||
| C2:0 | 110 ± 9.5 | 100 ± 12 | 92.5 | 71 ± 14 | 81 ± 40 | 115.1 | |
| C3:0 | 5.3 ± 0.53 | 5.1 ± 0.93 | 97.3 | 7.0 ± 0.82 | 7.9 ± 3.3 | 113.0 | |
| C4:0 | 30 ± 2.8 | 36 ± 2.4 | 121.3 | 5.7 ± 1.3 | 6.9 ± 4.8 | 121.2 | |
| C6:0 | 92 ± 2.4 | 93 ± 5.6 | 101.5 | 220 ± 43 | 180 ± 57 | 81.0 | |
| C8:0 | 63 ± 4.8 | 60 ± 4.0 | 95.1 | 57 ± 7.9 | 45 ± 13 | 79.7 | |
| C9:0 | 29 ± 1.2 | 41 ± 10 | 140.2 | 16 ± 3.7 | 16 ± 5.1 | 96.5 | |
| C10:0 | 7.2 ± 1.3 | 5.5 ± 0.58 | 76.1 | 7.4 ± 0.88 | 6.1 ± 1.6 | 82.2 | |
| C12:0 | 0.27 ± 0.058 | 0.20 ± 0.019 | 75.1 | 0.11 ± 0.086 | 0.22 ± 0.076 | 197.5 | |
| C14:0 | 0.24 ± 0.076 | 0.069 ± 0.061 | 28.8 | N.D. | N.D. | ||
| C16:0 | 1.2 ± 1.1 | 0.87 ± 0.72 | 71.3 | 0.52 ± 0.18 | 0.48 ± 0.28 | 92.1 | |
| C18:0 | N.D. | N.D. | N.D. | N.D. | |||
| FAEE | |||||||
| C2:0EE | 2.9 ± 0.40 | 2.4 ± 0.085 | 83.7 | 2.4 ± 0.57 | 5.9 ± 1.7 | 248.3 | |
| C3:0EE | 11 ± 0.30 | 11 ± 0.48 | 98.5 | 11 ± 0.41 | 15 ± 1.4 | 128.5 | |
| C4:0EE | 32 ± 2.7 | 34 ± 0.84 | 108.1 | 15 ± 0.67 | 19 ± 2.6 | 124.5 | |
| C6:0EE | 56 ± 0.39 | 68 ± 1.4 | 122.0 | 110 ± 1.1 | 130 ± 1.1 | 116.4 | |
| C8:0EE | 28 ± 0.71 | 19 ± 0.95 * | 69.3 | 37 ± 0.81 | 25 ± 0.43 | 67.3 | |
| C10:0EE | 5.6 ± 0.19 | 2.2 ± 0.15 ** | 38.5 | 23 ± 1.6 | 6.1 ± 0.92 ** | 26.5 | |
| C12:0EE | 1.4 ± 0.22 | 0.44 ± 0.065 * | 31.8 | 8.2 ± 0.76 | 1.2 ± 0.28 ** | 14.0 | |
| C14:0EE | 1.8 ± 0.34 | 0.19 ± 0.0073 * | 10.8 | 8.0 ± 0.36 | 1.5 ± 0.35 ** | 19.4 | |
| C16:0EE | 29 ± 7.3 | 1.2 ± 0.15 * | 4.1 | 55 ± 7.1 | 18 ± 5.5 * | 33.0 | |
| C18:0EE | 2.0 ± 0.88 | 0.065 ± 0.0090 | 3.2 | 8.0 ± 1.5 | 1.8 ± 0.62 * | 21.9 | |
| C18:1EE | 3.0 ± 1.2 | N.D. | 39 ± 6.4 | 9.2 ± 3.4 * | 23.5 | ||
| C18:2EE | 1.2 ± 0.50 | N.D. | 9.3 ± 1.4 | 0.75 ± 0.18 * | 8.1 | ||
| Other | |||||||
| Ethyl 3-hydroxybutanoate | 53 ± 0.82 | 58 ± 1.1 | 109.4 | 18 ± 2.7 | 18 ± 5.1 | 101.3 | |
| n-Propyl acetate | 2.4 ± 0.25 | 2.5 ± 0.13 | 108.2 | 2.0 ± 0.052 | 2.4 ± 0.34 | 120.6 | |
| 2-Methylpropyl acetate | 6.0 ± 0.46 | 6.3 ± 0.23 | 104.1 | 3.5 ± 0.18 | 4.3 ± 0.35 | 123.0 | |
| 3-Methylbutyl acetate | 62 ± 1.6 | 61 ± 0.73 | 99.0 | 37 ± 1.2 | 43 ± 5.5 | 115.8 | |
| 2-Hydroxyethyl propanoate | 57 ± 2.3 | 63 ± 0.68 | 110.8 | 150 ± 21 | 150 ± 37 | 99.1 | |
| 2-Phenylethyl acetate | 81 ± 1.7 | 80 ± 1.3 | 98.4 | 110 ± 15 | 110 ± 33 | 99.1 | |
| 2-Phenylethyl hexanoate | 0.59 ± 0.14 | 0.66 ± 0.029 | 111.1 | 6.9 ± 1.1 | 3.0 ± 0.83 | 43.6 | |
Data are presented as mean ± SD by × 10−2.
N.D. means not detected.
| 1 | ||||
|---|---|---|---|---|
| before | after | Residue (%) | ||
| FA | ||||
| C2:0 | 70 ± 24 | 49 ± 16 | 70.4 | |
| C3:0 | 12 ± 1.7 | 6.5 ± 0.64 * | 53.4 | |
| C4:0 | 22 ± 3.1 | 17 ± 1.9 * | 74.1 | |
| C6:0 | 51 ± 5.6 | 47 ± 2.7 | 91.4 | |
| C8:0 | 43 ± 3.7 | 35 ± 3.8 ** | 80.7 | |
| C9:0 | 7.7 ± 1.1 | 6.7 ± 0.87 | 87.0 | |
| C10:0 | 3.0 ± 0.63 | 0.70 ± 0.46 ** | 23.0 | |
| C12:0 | N.D. | N.D. | ||
| C14:0 | N.D. | N.D. | ||
| C16:0 | 0.79 ± 0.54 | N.D. | ||
| C18:0 | N.D. | N.D. | ||
| FAEE | ||||
| C2:0EE | 3.6 ± 0.84 | 1.0 ± 0.095 * | 27.9 | |
| C3:0EE | 2.2 ± 0.80 | 2.8 ± 0.17 | 127.5 | |
| C4:0EE | 11 ± 4.9 | 16 ± 1.7 | 146.1 | |
| C6:0EE | 19 ± 7.8 | 24 ± 0.93 | 128.9 | |
| C8:0EE | 12 ± 4.8 | 9.9 ± 0.39 | 83.0 | |
| C10:0EE | 6.0 ± 1.1 | 1.6 ± 0.041 * | 26.5 | |
| C12:0EE | 2.6 ± 0.17 | 0.44 ± 0.014 ** | 16.6 | |
| C14:0EE | 4.3 ± 0.53 | 0.69 ± 0.11 ** | 16.1 | |
| C16:0EE | 73 ± 12 | 4.4 ± 1.1 ** | 6.0 | |
| C18:0EE | 6.6 ± 1.2 | N.D. | ||
| C18:1EE | 58 ± 8.7 | 0.64 ± 0.19 ** | 1.1 | |
| C18:2EE | 58 ± 7.5 | 0.78 ± 0.11 ** | 1.4 | |
| Other | ||||
| Ethyl 3-hydroxybutanoate | 44 ± 3.8 | 37 ± 2.5 | 83.5 | |
| n-Propyl acetate | 1.8 ± 0.44 | 2.1 ± 0.10 | 119.0 | |
| 2-Methylpropyl acetate | 8.8 ± 3.3 | 14 ± 0.44 | 162.4 | |
| 3-Methylbutyl acetate | 59 ± 23 | 88 ± 1.7 | 148.8 | |
| 2-Hydroxyethyl propanoate | 55 ± 2.8 | 56 ± 2.8 | 101.1 | |
| 2-Phenylethyl acetate | 120 ± 6.6 | 100 ± 4.6 * | 87.2 | |
| 2-Phenylethyl hexanoate | 0.40 ± 0.038 | 0.26 ± 0.019 * | 66.6 | |
Data are presented as mean ± SD by × 10−2.
N.D. means not detected.
We showed that several esters, including FAEEs, were trapped by and accumulated in used filter cloths. This suggested that volatile compounds in sake are decreased by adsorption to filter cloths, although it is possible that volatile compounds in sake are also adsorbed to the sake cake or eluted from the sake cake by moromi-filtration pressure. Among the filter cloths, PET and PVC trapped larger amounts of volatile compounds than cotton. Generally, PET is used in automatic pressing. Therefore, we analyzed a broad range of volatile compounds in sake before and after practical automatic pressing to confirm that volatile compounds detected in used filter cloths were decreased in sake after practical automatic pressing. In any sake analyzed in this study, FAEEs with greater than 8 carbons showed a tendency to be decreased after automatic pressing through the PET membrane, but FAs did not. Although FAs with 16 carbons in daiginjo were decreased after practical automatic pressing, one of the causes was considered to be the high amount of FAs with 16 carbons compared with that in the other types of sake. We also obtained data showing that more FAEEs with 8 and 10 carbons tended to be decreased than FAs with the same carbon number; in this experiment, the PET membrane was soaked in sake to confirm the adsorption levels of FAs and FAEEs in sake (data not shown). Ito demonstrated that 36% and 10% of FAEEs with 6 carbons were adsorbed to PET and cotton, respectively (Ito, 1987). Moreover, it was also reported that esters in shochu were adsorbed to polyethylene (Yonemoto, 2017). These reports correspond with our results showing medium and long-chain FAEEs in sake were decreased during practical automatic pressing using a PET membrane.
Washed filter cloths were assessed in this study. Nevertheless, many volatile compounds derived from sake were detected, suggesting that the filter cloths trapped and strongly adsorbed volatile compounds from the sake. However, these volatile compounds were not considered to be eluted in subsequent filtered sake samples, as there was no change in volatile compounds following rewashing of the cloths by ethanol or detergents at 40 °C (data not shown).
The volatile compounds adsorbed in sake cake could not be analyzed, as we were unable to obtain sake cake samples halfway through the automatic pressing process. Although we cannot speculate on the balance of compounds before and after the automatic pressing process as a whole, we considered that the sake cake had a negligible influence on flavor compounds in this study, as we compared the volatile compounds in sake before automatic pressing and after the initial automatic pressing. Therefore, the several fold decreases in volatile compounds after the initial automatic pressing in this study were considered to be due to adsorption to the filter cloths. Estimating the approximate amounts based on internal standards, FAEEs such as FAEEs with 6, 8, and 10 carbons remained on the PET membrane in the order of 0.1–1.0 mg per one board of the automatic pressing in this study. In the initial period of automatic pressing, the amounts of compounds lost through the filter cloth were larger than those remaining in the PET membrane, suggesting that the PET membrane has additional capacity for adsorption.
In alcohol-added sake such as daiginjo, more FAEEs tended to be decreased than in non-alcohol-added sake, i.e., jummai-ginjo in this study. Since the addition of alcohol to sake before moromi-filtration has the effect of holding the flavor compounds in sake and desorbing flavor compounds from the sake cake (Ito, 1987), more inclusion of flavor compounds in sake by adding alcohol might provide the flavor compounds greater opportunity for contact with the cloths. On the other hand, adsorption of flavor compounds to the filter cloths was considered to be influenced by the pH of sake. However, in this study, the pH of sake was not considered to be a factor, since the pH of sake samples in this study did not differ. In the initial period of automatic pressing, it seemed that the amount of sake cake does not affect the organoleptic properties. Since the sake cake solid would be sparse in the initial period of automatic pressing (the timepoint where the sake samples used in this study were obtained), flavor compounds in sake were considered to contact the PET membrane directly. From the middle period of automatic pressing, the sake cake layer increases due to accumulation of the solid on the PET membrane surface, meaning that flavor compounds in the sake contact the PET membrane after the sake cake layer. Therefore, the effect of the sake cake on flavor compounds is considered to increase after the middle period of automatic pressing. Although the main factors that affect flavor change during the whole of automatic pressing, we cannot ignore the influence of the PET membrane on flavor compounds.
FAs give a different sensory impression compared with FAEEs having a similar carbon number. For example, FAEEs with 6, 8, and 10 carbons, which are major aroma constituents in more recently produced sake, impart a highly favorable impression, while FAs with the same carbon number give an unpleasant impression (Yamane et al., 1997). In addition, FAEEs with 12, 14, 16, and 18 carbons give a roundish and waxy sense, while FAs with the same carbon number give an oily sense, according to the Aroma Office 2D database (GERSTEL). FAs give an unpleasant impression compared with FAEEs; therefore, a more unpleasant impression might be given when the ratio of FAs to FAEEs in sake increases. The sensory gaps between FAs and FAEEs with 6, 8, and 10 carbons are larger than those between FAs and FAEEs with 12, 14, 16, and 18 carbons. In any of the sake analyzed in this study except ordinary sake, the ratio of FAs to FAEEs with 8 and 10 carbons increased after automatic pressing through the PET membrane. This is one of possible reasons why changes in organoleptic properties are observed after moromi-filtration.
It is acknowledged that the organoleptic properties of sake are changed after the moromi-filtration process. This study showed that filter cloths used during the moromi-filtration process such as PET membranes tend to adsorb volatile compounds, including long-chain FAEEs. The loss of FAEEs through PET membranes is one of the important causes of undesirable changes in the organoleptic properties of sake after automatic pressing. The moromi-filtration process using PET membranes should be performed more carefully to maintain the quality of sake.
Acknowledgements We greatly appreciate Okayama Sake Brewers Association for kindly providing the filter cloths and sake samples.
