| In China, using Daqu to initiate the solid fermentations for the production of Chinese liquor is the key technology which is a major difference from the other famous distilled spirits in the world. Daqu is a type of grain Qu, which is mainly made from raw wheat and by natural inoculation. Daqu is the major source of microorganisms, enzymes, flavour compounds and their precursors in Chinese liquor process. Its quality is directly related to the yield and the quality of Chinese liquor. Medium and high temperature Daqu which were produced by Luzhou Laojiao Company located in Luzhou China was studied here. Characteristic flavours, physicochemical indexes and enzymes of medium and high temperature Daqu were analyzed. Aroma-producing microorganisms were screened from Daqu and the relationship between the changes of microbial population and the aroma formation was studied. This article also preliminary study the safety problems of Daqu. The main research contents are as follows:Volatile compounds in Daqu samples were extracted by headspace solid phase microextraction and analyzed by gas chromatography-mass spectrum (GC-MS). Comparing volatiles with the quality of Daqu by sensory analysis, electronic noses (EN) method was adopted to analyse Daqu with GC-MS. The method is quick, simple and less sample consumption. Most volatiles of Daqu can be extracted by this method, and the repeatability and recovery of some compounds were satisfactory. A total of 75 volatile compounds were characterized by GC-MS, including alcohols, esters, acids, aromatic compounds, phenols, furans, nitrogen-containing compounds, aldehydes and ketones. By a principal component analysis (PCA), the ten Daqu samples could be classified into three groups according to their origins, and in particular, the production technologies. In this case, Daqu samples fermented by middle temperature procedure formed a clear group; Daqu samples fermented by high temperature procedure formed two different groups. Daqu also be analyzed by electronic noses (EN) method, different samples of daqu has certain response to electronic nose. As can be seen from the radar map, the trend of common and high quality of medium temperature Daqu and common quality of high temperature Daqu were similar, but the trend of high quality of high temperature Daqu was different. Odorants of Daqu extracted by solvent-assisted flavour evaporation (SAFE) were investigated by gas chromatography-olfactometry (GC-O) system with two olfactometric methods: detection frequency (DF) method and aroma extract dilution analysis (AEDA). Based on the results of the two olfactometric method (FD and DF), the aroma of medium temperature Daqu could be primarily attributed to hexanal, unknown (no.u2), phenylacetaldehyde and 4-ethyl guaiacol; the aroma of high temperature Daqu could be primarily attributed to hexanal, 2,6-dimethylpyrazine, nonanal, 1-octene-3-ol, phenylacetaldehyde, 4-ethyl guaiacol and unknown (no.u2, u3, u5 and u7).Based on sensory properties, the relationship of microorganisms, physicochemical indexes and enzymes of different quality of Daqu were analyzed. The results show that the numbers of microorganisms in good clothing Daqu were more than the numbers of microorganisms in poor clothing Daqu, especially the number of moulds. The activities of most enzymes and esterifying capability in commom quality Daqu were lower than those in high quality Daqu, but the esterifying power of breaking was lower in high quality Daqu than commom quality Daqu, which would be good for producing volatile compounds. Besides, fermentation ability and total free amino acids of high quality Daqu was higher than that of commom quality Daqu too. The changes of microorganisms, physicochemical indexes, enzymes, volatile compounds and sensory during Daqu fermentation and storage were studied. The water content and humidity decreased gradually during fermentation and then remained stable during storage and the temperature rose rapidly as fermentation progressed during the first 3 days and peaked on the seventh day. The evolutions of physicochemical indexes, enzymes, flavour and sensory was related to microbial growth. The total microbial population including bacillus, yeast and mould proliferated rapidly at the preliminary stage of fermentation. With the rising of temperature and declining of water content and humidity, the environment was no longer suitable for the growth of microorganisms in medium temperature Daqu. Bacillus species have a better ability than other microorganism to survive under high water content and high temperature in high temperature Daqu. There were two peak hours of producing acidity in Daqu, and the acidity decreased as the the numbers of microorganisms decreased. A total of 46 volatile compounds were identified and measured in the Daqu samples using HS-SPME-GC-MS. The sum of the nitrogen-containing compounds showed a slowly increase from the first day to the third day and followed by a sharp increase from the third day to the ninth day of fermentation, and then decreased gradually. The evolution of each group of volatile compounds during fermentation and storage of Daqu had a relationship with the change of physical index and microflora and determined the aroma intensity of Daqu. This made it possible to optimize the productive technology of Daqu on the basis of the contents of volatile compounds. The aroma intensity of fruity, flowery and clove increased on the first five days and then decreased gradually. The result was in line with the change of aroma content and showed that fermentation period had significant impacts on the aroma score.A total of 93 moulds, 112 yeasts and 45 bacilli were screened and identified using different culture during during Daqu fermentation and storage. After primary screening and secondary screening, ZY-1 and GY-3 were identified as Issatchenkia orientalis and Candida xylopsoci through ITS sequence analysis, ZL-1 and GL-4 were identified as Bacillus subtilis and Bacillus licheniformis through 16SrDNA sequence analysis. The aroma-producing conditions of ZL-1 and GL-4 were optimized. ZL-1 and GL-4 have a great influence in promoting brown of medium through model Maillard reaction, and the brown reached a maximum on 6th day. The changes of total free amino acid, the activity of proteases and reducing sugar during fermentation were studied. Compared with the fermentation products after sterilization and inactivation of enzyme by high temperature, the fermentation products after sterilization by membrane filtration smelled stronger sauce aroma. The volatile compounds of ZL-1 and GL-4 fermentation product were analyzed by GC-MS. 2,3-butanedione, 2,3,5-trimethyl pyrazine, 2,3,5,6-Tetramethylpyrazine and guaiacol could have a relationship with characteristic flavour of Daqu. The volatile compounds of ZY-1 and GY-3 fermentation product were esters, alcohols and ketones which could have a relationship with aroma-producing of Daqu. Brewing experiment was carried out using different quality of Daqu. The results indicated that the total content of esters, free amino acids and acids in liquor produced by high quality Daqu were better than those in liquor produced by common quality Daqu. The total aldehydes and solid content had little difference in the different quality Daqu, but the alcohol content had features obviously difference from each other. The volatile compounds of high quality Daqu was higher than those of common quality Daqu. Numbers of the volatile compounds producing by angel yeast were less than those producing by Daqu, which reflects the importance of Daqu in producing aroma during liquor production. Four typical toxin producing by moulds were tested by HPLC which had a satisfactory repeatability and recovery. The content of four typical toxin producing by moulds did not exceed national or international standard. Because of the complexity of the special traditional techniques of Chinese liquor, there is little chance to bring toxin producing by moulds to Chinese liquor.
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