Fermentation is under the action of yeast, the sugar in grape skin juice generates alcohol and carbon dioxide. The pre-fermentation process of red wine is that the skins and juices are mixed together, and yeast has been inoculated in the juice when the grapes are broken. Because there is yeast in the frost on the grape skin, it is not necessary to add yeast to the homemade wine when fermenting. The optimum fermentation temperature is 15~25℃, which should not exceed 35℃, but it is easier to dissipate heat when fermenting in a small container, which can generally not exceed 32℃. After the peel juice is put into the container, it usually takes one day to start fermentation. At first, the liquid level was calm. At this time, there are weak carbon dioxide bubbles, indicating that yeast has begun to reproduce. After 2~3 days, a large amount of carbon dioxide is released, and the skin residue floats to form a hat shape. When you taste the fruit juice, the sweetness gradually decreases and the taste of wine gradually increases. During fermentation, floating grape skins should be pressed into the juice twice a day with sterilized chopsticks. This can prevent the grape skin from being moldy and sour, and at the same time, the pigment on the skin can be immersed in the juice. Yeast is facultative anaerobic and produces alcohol when oxygen is insufficient, so it should be kept sealed to make fermentation more vigorous. After the climax, the fermentation momentum began to weaken. At this time, sugar can be added, which is dissolved with wine instead of water. ..