sources of bitterness and astringency in wines. Its content and quality are one of the important factors in evaluating the quality of red wine. Sugar Just as people use sugar or salt when making preserved foods, sugar is also a preservative for wine. The sugar lowers the water activity of the food, meaning that less water is available for the bacteria, stunting its growth. Salt is used as a preservative in the same way. Alcohol Alcohol is used as a preservative especially in spirits such as sherry. Because common alcohol can have the same problems as acid when used as preservatives, the lack of acidity does not play a role of maintaining quality. Chemical preservatives Acid Acid is widely used as a preservative. In red wine, Sulfurous anhydride or sulfur dioxide (SO2), in particular, is the most frequently used source. Acid protects wine from bacteria by regulating the pH value, which affects the growth and vitality of yeast during fermentation. Acidity also directly affects color, balance and taste. For example, sulfur dioxide, the most widely used preservative by wine merchants, has been used as a cleaning agent for centuries. Sulphur dioxide has a pungent taste that people dislike, and its use has been controversial because of its disease-causing properties (see section on the controversy). Sorbic acid, which acts like sulfites, has recently been approved for use in European Community countries, but only under very limited conditions. It must be effective in wines with an alcohol concentration of 12%. Apart from sulfur dioxide, which is used most frequently, other chemicals such as benzoic acid, diethyl pyrocarbonate, parabens, pimaricin and sorbic acid are also used as preservatives in wine, but the potential threat or side effects of these chemicals are still to be investigated, so they are not difficult to be used in large quantities for the time being. Weak acid Compared with other preservatives, weak acids are characterized by their antiseptic effect by slowing down the growth and reproduction cycle of microorganisms rather than killing them. Weak acids inhibit microbial growth by dissociating protons from cells. Although the chemical formula of different weak acids is different, they all play a more obvious inhibitory effect in the environment with a low pH value; that is, the more acidic the environment, the greater the inhibitory effect. This practice was first discovered by John Evelyn in 1670, who used sulfur dioxide from burning sulfur