Carbon Stable Isotope Analysis for Glucose in Sake: Simple Freeze-Dried Sake Can Substitute for Glucose Following HPLC Isolation

Carbon stable isotopes of glucose during the degradation of rice by the koji fungus Aspergillus oryzae

Glucose, a key component of traditional Japanese fermented foods, is derived from rice starch via saccharification by hydrolytic enzymes produced by Aspergillus oryzae. The δ 13C value of glucose reflects that of its rice source. However, the influence of saccharification parameters (glucose concentration, degradation temperature, and reaction time) on glucose δ 13C values is unclear. Here, we investigated the influence of saccharification on the δ 13C value of glucose. Our experiments showed a significant difference in the δ 1³C value of glucose (-27.0 ± 0.1 ‰) obtained from saccharification compared to the ingredient rice (-27.1 ± 0.1 ‰) and remaining solid residue (-27.1 ± 0.1 ‰); however, it did not differ significantly from those of rice koji (-27.0 ± 0.1 ‰) and steamed rice (-27.1 ± 0.1 ‰), despite all values being within 0.1 ‰. Notably, glucose concentration, degradation temperature, and reaction time did not significantly affect glucose δ 13C values. These findings demonstrate the remarkable preservation of glucose δ 13C values. The δ 13C values remain aligned with the original δ 13C value of the rice, even with up to 60 % degradation during A. oryzae saccharification. This persistence of the δ 13C value throughout the process offers a potential tool for authenticating the origin of rice-fermented beverages based on the δ 13C value of their glucose component.

Carbon stable isotopic compositions of glucose and ethanol in sake after simultaneous saccharification and fermentation processes

The carbon stable isotopic composition of glucose is transferred to that of ethanol during the simultaneous saccharification and fermentation processes of sake production. However, there is limited information regarding carbon isotope discrimination between the ingredient rice and the sake components. Our fermentation experiments show that the carbon stable isotopic composition of rice is intermediate between those of glucose and ethanol in sake and do not differ significantly from those of rice koji and sake lees. Carbon isotope discrimination from rice to ethanol and from glucose to ethanol is 0.9 ± 0.1‰ (mean ± standard deviation, n = 18) and 1.9 ± 0.2‰, respectively. This is approximately half of the isotope discrimination observed in grape wine due to the saccharification process during sake manufacture. Carbon isotope discrimination from ingredient rice to sake components provides valuable insights for the manufacturing process and the authentication of sake.

Prediction method for determining the carbon stable isotopic composition of berry sugars in the original must of Chardonnay wines

The carbon stable isotopic composition, as indicated by the δ¹³C value, of wine ethanol is inherited from berry sugars, but little is known about the variation in sugar δ¹³C values of Japanese grapes relative to overseas grapes. This study found a large variation in sugar δ¹³C values of Chardonnay grapes grown in Japan (-27.2 ± 0.9‰, mean ± standard deviation, n = 33), with sugar δ¹³C values depending on the δ¹³C values and content of monosaccharides. After complete fermentation, the carbon isotope discrimination between berry sugars and wine ethanol was 1.5 ± 0.1‰. Ethanol δ¹³C values and carbon isotope discrimination enabled prediction of sugar δ¹³C values in the original must. Imported wines had higher sugar δ¹³C values than those of wines made from Japanese grapes, suggesting drier overseas viticulture conditions. The determination of sugar δ¹³C values in grape berries provides valuable information for viticulture and wine authentication.

Carbon isotope ratio of organic acids in sake and wine by solid-phase extraction combined with LC/IRMS

We developed an analytical procedure for determining the δ¹³C values of organic acids in sake and wine using solid-phase extraction combined with liquid chromatography/isotope ratio mass spectrometry (LC/IRMS). First, the solid-phase extraction (SPE) procedure was performed and various tests were conducted to extract organic acids from alcoholic beverages using the simulated sake sample. Under the optimal SPE procedure, high recovery rates (96-118%) and good accuracies (≤ 0.7‰) were thus achieved for the simulated sake and wine samples. Next, we determined the δ¹³C of organic acid (tartaric acid, malic acid, lactic acid, succinic acid) in 9 sake and 11 wine samples. Finally, the δ¹³C values of lactic acid in nine sake samples suggested that lactic acid had been added during the brewing process. The high correlation between the δ¹³C values of tartaric acid and malic acid in 11 wine samples was consistent with their common source, grapes. This analytical method may help to identify when organic acids have been added to sake and wine and to elucidate the process of organic acid production therein. Graphical abstract.

Long-term responses to climate change of the carbon and oxygen stable isotopic compositions and gelatinization temperature of rice

The stable isotopic compositions of carbon (δ¹³C) and oxygen (δ¹⁸O) in rice are often used to confirm its authenticity. The gelatinization temperature is a crucial factor in alcoholic fermentation. However, little is known about the isotopic and thermal responses of rice to climate change. We show that in sake rice grown annually in the same paddy field from 1994 to 2013, the δ¹³C (-27.4‰ to -25.9‰) and δ¹⁸O values (20.4‰ to 27.0‰) correlated negatively with the mean daily minimum air temperature and precipitation, and positively with the hours of sunshine during grain-filling. In contrast, of the air temperatures tested, the gelatinization temperature of the rice grains (63.2°C to 70.0 °C) correlated positively and most strongly with the mean daily mean air temperature. Thus, we identified the mean daily minimum temperature during grain-filling, a nocturnal temperature, as a major factor affecting the isotopic and thermal variations in rice grains.

Compound Specific Carbon Isotope Analysis in Sake by LC/IRMS and Brewers' Alcohol Proportion

Sake is a traditional Japanese alcohol. Nowadays, the consumption for Sake is increasing in worldwide and its popularity is growing. However, there are act of fraudulence by additional brewers’ alcohol and sugar. Therefore, a method is needed to find illegal fraud on label. In this work, we analyzed the δ¹³C values of the ethanol (δ¹³C_eth) and glucose (δ¹³C_glu) in Sake by liquid chromatography combined with isotope ratio mass spectrometry for the first time. Further, we developed the criteria using δ¹³C_eth and δ¹³C_glu to check brewers’ alcohol and sugar. In addition, there are some sake categories (Ginjyo and Futsu-shu) allowed to additional brewers’ alcohol up to legally determined percentage. The experimental additions of brewers’ alcohol from a C4 plant were conducted to Junmai, as sake by C3 plants. There was a strong correlation (R = 0.98, P < 0.05) between the percentage of added brewers’ alcohol and the δ¹³C values. We developed the method using the relationship for calculating percentage of brewers’ alcohol for the first time and estimated the percentage for commercial sake. Further, the price of sake was found to be inversely related to the percentage of brewers’ alcohol in the sake.

Increase in the oxygen stable isotopic composition of water in wine with low ethanol yield

The stable isotopic composition of oxygen (δ¹⁸O) in wine is often analysed to determine the geographic origin of the wine and the amount of water dilution. However, little is known regarding the effects of two major winemaking techniques (the addition of acid (acidification) and sugar (chaptalization)) on the δ¹⁸O value of water in wine. Here we show that acidification and chaptalization have minor direct effects on the δ¹⁸O value but indirect effects based on the ethanol yield, which causes isotopic variation of up to 0.6‰. During fermentation, δ¹⁸O values increase at low ethanol yields, suggesting that yeast release water with a high δ¹⁸O value into wine when consuming sugars. Additionally, the ethanol yield is negatively correlated with the consumption of amino acids by the yeast, indicating that yeast growth decreases the ethanol yield. We therefore identify ethanol yield, which is decreased by the consumption of sugars by yeast for non-alcohol-fermentation processes as a potential factor leading to variations in the δ¹⁸O value of water during the winemaking process.

Carbon stable isotopic compositions of citric acid and malic acid in Japanese apricot liqueur decrease as the fruit ripens

The carbon stable isotopic composition (δ¹³C) is often analyzed to quantify the addition of acidulants to Japanese apricot liqueur, but little is known about the variation in the δ¹³C values of the main organic acids arising from differences in the ripeness of Japanese apricots. We show that in Japanese apricot liqueur prepared using fruits at different stages of ripeness, the δ¹³C values of citric acid and malic acid ranged from -25.1‰ to -23.7‰ and from -22.3‰ to -19.7‰, respectively, and the δ¹³C values decreased as the fruit ripened. The average δ¹³C value of citric acid from liqueurs was 0.7‰ higher than that from fresh fruits, whereas the δ¹³C values of malic acid showed no isotope discrimination. The variation in δ¹³C values of the main organic acids in Japanese apricot liqueurs will help detect acidulant addition and control authenticity.