Chinese liquor has a compound aroma with esters as the main body, using koji and liquor mother as saccharification starter culture, using starch, that is, sugar as raw materials, through cooking, saccharification, fermentation, distillation, aging and blending to make various types of liquor. Strictly speaking, a blend of edible alcohol and spices cannot be considered liquor. Baijiu is mainly concentrated in the upper reaches of the Yangtze River and the Chishui River Basin in Guizhou Renhuai and Yibin in Sichuan, and the Luzhou triangle in Sichuan has the world's largest and best quality distilled liquor production areas, namely Mao, Wu and Lu, the three famous liquors in China, and its liquor industry cluster carries half of China's liquor. Chinese liquor can be subdivided into the following flavor types: sauce flavor type, light flavor type, strong flavor type, Laobai dry flavor type, rice flavor type, phoenix flavor type, combined flavor type, Dong fragrance type, and other flavor types.
As one of the world's famous distilled spirits, baijiu is very popular among people because of its special flavor and taste, and its quality has also received widespread attention. The total amount of esters is actually the sum of esters and organic acids in liquor. Organic acids and esters in liquor are important aroma and flavor substances, and organic acids and esters and alcohols will undergo a reversible transformation reaction under natural conditions, which is conducive to making the product quality and flavor stable and the taste more mellow within a certain period of time. The total amount of ester can reflect the change of the quality of liquor during storage, which is a key indicator in the detection of liquor.
At present, the detection of acids, esters, alcohols and other substances in liquor or spirits is mainly measured by gas chromatography or acid-base titration, and the above methods have the shortcomings of long analysis cycle, cumbersome operation, single detection, and environmental pollution. As a fast and efficient analysis method, near-infrared spectroscopy analysis technology has the advantages of simple operation, rapid analysis, non-destructive testing of samples, good repeatability of results, environmental protection and pollution-free, etc., which has important significance and role in reducing costs, improving efficiency and protecting the environment. In recent years, it has been gradually applied to the traditional Chinese liquor brewing industry, and has played an important role in the quantitative analysis of liquor quality testing components. Near-infrared spectroscopy was used to detect and study the organic acids in spirits, and a rapid detection model of total esters in liquor was established by near-infrared spectroscopy, and a near-infrared prediction model of typical alcohols, caproic acid and acetic acid in liquor-based liquor was constructed by near-infrared technology.
The organic molecules in liquor have hydrogen-containing groups such as O-H, N-H, C-H, etc., and the frequency doubling and combined frequency absorption regions of hydrogen-containing groups are mainly concentrated in 700nm to 2500nm, that is, the near-infrared spectral region. Then, the spectrum can be preprocessed and optimized to reduce or eliminate the influence of other non-target factors on the spectrum.
The Bruker MPA II near-infrared spectrometer in Germany was used to establish a near-infrared prediction model of the total amount of esters in liquor, which could achieve the purpose of rapid detection of the total amount of esters and realize real-time monitoring of production. The main steps in setting up a model are:
1. According to the national standard GB/T10345-2022 determination method of the total amount of esters in the liquor sample, the total amount of esters in the modeling set and the verification set liquor samples were determined. The principle is to neutralize the free acid in the sample with alkali, then add a certain amount of alkali, heat and reflux to saponify the ester, and neutralize the remaining alkali with acid. The total amount of esters is derived by calculating the total consumption of bases.
2. Near-infrared spectroscopy of liquor samples: Bruker MPA II Fourier transform near-infrared spectroscopy was used to collect the spectral images of liquor samples. Half an hour after the instrument was warmed up, the spectra were collected in a quartz cuvette with a 1 mm pathlength and a Bruker MPA II Fourier transform NIR spectrometer and OPUS analysis software. The resolution was set to 8 cm-1, the number of scans was 16, and each sample was collected 3 times, and the resulting average spectrum was used for analysis.
3. Spectral data preprocessing and modeling: In order to improve the effective information of the collected spectra and improve the reliability and stability of the model, some interference factors that lead to baseline drift and spectral non-repetition in the acquisition process are processed, and appropriate spectral regions are selected to establish the model. The automatic functions of the OPUS analysis software can be used to select the optimal spectral region, band, and optimal pretreatment method for the test model.
The model can be established by cross-testing the liquor samples in the modeling set, and the model quality is evaluated by the predicted value of the model and the coefficient of determination R2 and the cross-test standard deviation (RMSEV) of the chemical truth value determined by the national standard method. Finally, the model was externally verified by independent validation set samples, and the results of the model calculation were compared with the chemical values measured by the national standard method to verify the reliability of the model.
The near-infrared model of the total amount of esters in the finished liquor was established, which could provide a basis for the rapid determination of the total amount of esters in liquor. This inspection model can be used in an off-line environment, such as a laboratory, or in-line in the production process to achieve real-time monitoring of the total amount of esters on the production line. Near-infrared spectroscopy has many advantages in testing, which can provide new ideas for rapid analysis in the liquor industry. At present, near-infrared spectroscopy analysis technology is used to detect the moisture, acidity, starch, residual sugar, total ester and alcohol content of distillery, total acid, total ester, alcohol content, ethyl caproate, ethyl lactate, ethyl acetate, ethyl butyrate and methanol in finished wine, and raw materials such as moisture, crude protein, crude fat, crude ash, crude fiber, starch content, neutral detergent fiber, acid detergent fiber, fatty acid value, net energy, digestive energy, total sulfur amino acids and other indicators in the crushed corn kernels.