Volatile acid refers to the sum of various low-boiling acids and their derivatives distilled from wine under certain conditions. It is generally composed of two parts: one part is produced by alcohol fermentation (normal); the other part is 4^-0. 7g/L。 Bacteria parasitic (generic), the total value is generally between 0. 4 ^ -0. 7g / L. The increase in volatile acid content is due to bacterial infestation or the occurrence of “hop†wines. As the volatile acid content increases, the sour taste of the wine gradually becomes stronger, gradually occupying the main taste and suppressing other taste characteristics of the wine. If the content of volatile acid in the wine exceeds 0.8 g/L, the wine will completely lose the nutritional and taste characteristics of the wine in a relatively short period of time. The content of volatile acid has a great influence on the flavor, taste and quality of the wine, so its content is an important indicator of wine quality control. For this reason, it is of great significance to develop an accurate and rapid method for determining the volatile acid content of wine. X - The measured volatile acid content in the sample (calculated as acetic acid), g/L Related equipment: recorder
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The main method for the determination of volatile acids in wine is the national standard method. This method adopts electric heating distillation and manual titration, which makes the measurement result more discrete and time-consuming. Sheng Hui et al. used the automatic wine analyzer to quickly and accurately determine the volatile acids in wine. However, because of the high price and professionalism of the instrument, it was difficult to popularize it.
Automatic nitrogen determination instrument is commonly used in various types of testing laboratories. It is mainly used for testing nitrogen content in materials, foods, and feeds. The degree of automation of the instrument is high, and the precision and accuracy of the measurement results are all relatively good. In this paper, the method of rapid and accurate determination of volatile acid content in wine was developed for the first time using an automatic nitrogen determination instrument. The method was applied to the determination of volatile acid content in real samples of wine and satisfactory results were obtained.
2 experimental section
2.1 Instruments and reagents The B-324 automatic nitrogen analyzer is equipped with a Mettler DL50 titrator (Mettler, Switzerland); an all-glass steam distillation unit.
Glacial acetic acid: pure grade, 99. 5 0 o -100. 5 0 0; tartaric acid solution: 2000; phenolphthalein indicator solution: 10 g/L, prepared according to GB/T 603; sodium hydroxide standard titration solution: c (NaOH ) = 0. 02mol/L, according to GB/T 601 preparation and calibration, and accurate dilution.
2.2 Experimental method Pipette 20mL sample (V) and lOmL tartaric acid solution in a distillation tube, distilled on a distillation apparatus for 6min lOs, collected about 100 persons 2mL distillate, directly on the Mettler DL50 automatic titrator with 0. 02mo1/ L NaOH standard solution titrate to the end point, read the consumed volume, calculate the volatile acid content by the following formula: cXV, X69.0/V type
C—molar concentration of substance in sodium hydroxide standard titration solution, mol/L
V—volume mL of consumed sodium hydroxide standard titration solution
60. 0 - Quality of acetic acid in grams equivalent to 1.00 mL of sodium hydroxide standard solution (NaOH) = 1. 000 mol/L]
V - sample volume, mLo
3 Results and Discussion
3.1 Determination of the time of the distillate The volume of the distillate has an effect on the precision and the recovery of the determination result. For a simple glass distillation apparatus, because the steam generation efficiency is difficult to be constant, it is often determined by the amount of the effluent volume whether or not the organic acid in the sample is completely distilled. The national standard specifies that 100-105 mL of distillate should be collected. However, for the automatic nitrogen determination apparatus distillation apparatus, the distillation efficiency is high, and can be controlled by setting, and after the distillation efficiency and the distillation time are fixed at the same time, the distillation volume can be relatively fixed. To do this, it is necessary to select the appropriate distillation time by experimentation. Figure 1 shows the spike recovery of wine at different distillation times when the distillation efficiency is fixed at 100%. 
As can be seen from Figure 1, the recovery rate increases in proportion to the increase in distillation time. At a distillation volume of 103 mL, the recovery was only 4400, indicating that it was impossible to completely evaporate the volatile acid in a relatively short time. However, when the distillation volume reaches 201 mL, the recovery rate is 94.900, which is ideal. This may be due to the high steam generation efficiency of the automatic nitrogen analyzer. However, the larger effluent volume is disadvantageous for titrating low levels of volatile acids, so the effluent volume must be controlled from the lower distillation efficiency while achieving a better recovery. The test results of the best distillation conditions are shown in Table 1. When the distillation efficiency is set to 5500 and the distillation time is 6min10s, the recovery rate of the spiked standard can reach 94.200^97.7. Therefore, this experiment selected this condition as the best distillation conditions for automatic nitrogen determination.
3.2 Distillation Droplet Pretreatment In the literature method, before titrating with a sodium hydroxide standard solution, a distilling apparatus is used to heat the distillate to boiling and then titrate for a long time. This experiment compared the effect of the distillate on the determination of volatile acid content in wine with or without heating. The results are shown in Table 2. From the data in the table, it can be seen that titration directly with a Mettler DL50 automatic titrator after distillation using an automatic nitrogen determination instrument can also achieve better recovery and precision. Therefore, after distillation in this experiment, the lixiviated liquid was directly titrated. 
3. 3 Determination of the sample The automatic determination of the volatile acid in the wine under the optimal distillation conditions was used to determine the volatile acid in the wine and compared with the national standard method Chuan. The results are shown in Table 3. As can be seen from the table, the determination of volatile acid content in wine using automatic determination of nitrogen, the relative standard deviation of five parallel determination results was 0.1700, and the recovery rate was in the range of 93.000^99.3%. At the same time, compared with the method of [2], the degree of agreement is better. 
4 Conclusions Based on the experimental results, comparing the above two methods, we found that the automatic nitrogen determination method has the following characteristics compared with the literature method (GB/T 15038-1994):
(1) Since the literature method directly uses the electric furnace to heat, the temperature is not easy to control, resulting in non-uniform steam flow, and the volatile acid distills out evenly, resulting in large fluctuations in the measurement results; while the automatic nitrogen analyzer, the steam is generated by the built-in steaming bag, The steam flow rate is stable, volatile acids are distilled out evenly, and the measurement results are of good precision.
(2) Literature method In the process of titration operation, the introduction of human error is larger; the automatic nitrogen determination instrument uses an automatic titrator; the minimum titration volume is 0.005 mL, and the precision and accuracy of the results are good.
(3) Because there is no heat preservation equipment in the literature method, the heat loss is large, the steam efficiency is low, and the airtightness is difficult to control; the automatic titration method is automatically controlled from cleaning, distillation, and titration, and the device has good airtightness, good heat insulation, and steam loss. Less, save energy.
(4) The phenolphthalein is used as an indicator in the literature method. The operator's human factors greatly affect the titration error. The automatic nitrogen determination instrument adopts the principle of potentiometric titration. The sudden jump at the measurement point and the titration termination are all controlled by the instrument. The error is small. High accuracy.
In short, the use of automatic nitrogen determination of volatile acid content in wine has the advantages of simple, quick and accurate operation. The actual sample determination results show that this method can fully meet the needs of wine enterprises in production control and food safety quality control.