# Thai Rice Vinegars: Production and Biological Properties
## 1 Introduction  
Vinegar has been widely used as a flavoring and cooking ingredient in foods for thousands of years. Various materials such as fruits, rice and whole grains are used for vinegar production via the solid-state fermentation process [1]. There are many advantages of vinegar, especially in medicinal applications, such as anti-infective, antidiabetic and cardiovascular-protective effects, digestive system assistance, appetite stimulation, regulation of blood pressure and recovery from exhaustion. Moreover, the antioxidative activity and antitumor promoting effects are found in the extracts of vinegar. Therefore, vinegar has been consumed for medicinal use and health benefits [2; 3; 4]. Rice vinegar is a traditional vinegar and has been widely used as a food seasoning for a long time in Asian countries that can be made from species of rice by solid-state fermentation. Rice vinegar is available in \"white\" (light yellow), red and black varieties [3; 5]. Rice vinegar is classified by types of rice materials. White rice vinegar or polished rice vinegar has been known as Komesu in Japan, is colorless, has a mild acidity and has a rather smooth flavor. Some varieties of rice vinegar are sweetened or otherwise seasonedwith spices or other added flavorings [6]. In Southern China, Zhenjiang aromatic vinegar produced from sticky rice is the most famous variety. Red rice vinegar is traditionally used in China, also known as Fujian red rice vinegar. Red rice vinegar is produced from red yeast rice or red koji by _Monascus_ sp. [7]. Black rice vinegar, also known as Kurosu in Japan, is produced from unpolished rice with rice germ and bran. Kurosu is dark in color and contains more nutrients such as amino acids and vitamins than Komesu. As Kurosu is rich in nutrients, it is widely used in beverages for its health benefits [2; 6]. Another type of black rice vinegar derived from black glutinous rice is very popular in China, as well as in other East Asian countries. Black vinegar is characterized as a healthy drink because it has many reported health benefits [4]. Solid-state fermentation of rice vinegar consists of four continuous steps; the first step is Koji preparation. Koji is produced by inoculation of _Aspergillus oryzae_ on the steamed rice to produce enzymes and other metabolites for rice vinegar fermentation [5; 8]. This step is also called saccharification, which converts starch into \(\alpha\)-amylase and sugar such as glucose and other oligosaccharides. The released sugar from saccharification is converted to alcohol (ethanol) by _Saccharomyces cerevisiae_ during alcohol fermentation. This is the second step of rice vinegar fermentation. The third step is acetic acid fermentation, in which ethanol is oxidized to acetic acid by _Aceobacter_ sp. After the formation of acetic acid, the maturation or ripening process is done to store the vinegar product at a low temperature for 2-3 months (depending on the type of vinegar). Biological activity studies of vinegar refer to the total phenolic compounds and phytochemical contents of plant materials. For example, balsamic vinegar is a traditional vinegar from Italy that is made from grape or grape wine. Antioxidant compounds (phenolic acids, flavanols, polymeric tannins and melanoidins) are found in traditional balsamic vinegars [9]. Japanese rice vinegars consist of dihydroferulic acid (DFA), dihydrofinapic acid (DSA), ferulic acid and sinapic acid, which are phenolic compounds [10]. Many kinds of Chinese vinegars are made from various materials, such as Shanxi vinegar (made from sorghum), Zhenjiang vinegar (made from sticky rice), Sichuan bran vinegar (made from 108 types of medicinal herbs) and Jiangzhe rose vinegar (made from blossoming rose and rice) [7; 11]. In Thailand, there are many varieties of nutrient-rich rice in all parts of the country that can be used as a material for nutrient-rich rice vinegar production. White or polished rice is popularly consumed.